5. SITE 7361

Shipboard Scientific Party2

HOLE 736A

Date occupied: 25 December 1987

Date departed: 28 December 1987

Time on hole: 2 days, 4 hr, 45 min

Position: 49°24.12'S, 71°39.61'E

Bottom felt (rig floor: m, drill-pipe measurement): 639.5

Distance between rig floor and sea level (m): 10.5

Water depth (drill-pipe measurement from sea level, m): 629.0

Total depth (rig floor, m): 891.8

Penetration (m): 252.3

Number of cores: 30

Total length of cored section (m): 250.3

Total core recovered (m): 147.05

Core recovery (°7o): 58

Oldest sediment cored:Depth sub-bottom (m): 242.6Nature: diatom oozeEarliest age: late PlioceneMeasured velocity (km/s): 1.53

HOLE 736B

Date occupied: 28 December 1987

Date departed: 28 December 1987

Time on hole: 1 hr, 30 min

Position: 49°24.12'S, 71°39.61'E

Bottom felt (rig floor: m, drill-pipe measurement): 638.6

Distance between rig floor and sea level (m): 10.5

Water depth (drill-pipe measurement from sea level, m): 628.1

Total depth (rig floor, m): 666.0

Penetration (m): 27.4

Number of cores: 3

Total length of cored section (m): 27.4

Total core recovered (m): 27.81

Core recovery (%): 101

Oldest sediment cored:Depth sub-bottom (m): 27.4Nature: diatom ooze, diatom ooze with volcanic detritus, and sandEarliest age: QuaternaryMeasured velocity (km/s): 1.44

1 Barron, J., Larsen, B., et al., 1989. Proc. ODP, Init. Repts., 119: CollegeStation, TX (Ocean Drilling Program).

2 Shipboard Scientific Party is as given in the list of Participants preceding thecontents.

HOLE 736C

Date occupied: 28 December 1987

Date departed: 29 December 1987

Time on hole: 1 day, 2 hr, 15 min

Position: 49°24.16'S, 71°39.65'E

Bottom felt (rig floor: m, drill-pipe measurement): 639.5

Distance between rig floor and sea level (m): 10.5

Water depth (drill-pipe measurement from sea level, m): 629.0

Total depth (rig floor, m): 1010.5

Penetration (m): 371.0

Number of cores: 18

Total length of cored section (m): 371.0

Total core recovered (m): 77.41

Core recovery (%): 20

Oldest sediment cored:Depth sub-bottom (m): 371.0Nature: diatom oozeEarliest age: late early PlioceneMeasured velocity (km/s): 1.46

Principal results: Site 736 (49°24.12'S, 71°39.61'E; target Site KHP-1)is on the northern part of the Kerguelen-Heard Plateau in a waterdepth of 629 m. A 371-m-thick continuous section of upper Plioceneand Quaternary diatomaceous ooze with a varying component ofvolcanic ice-rafted and/or detrital material was cored in three holesat the site.

Coring commenced on 26 December 1987 in Hole 736A with theadvanced piston corer (APC). With the exception of the interval be-tween 119.5 and 146.4 m below seafloor (mbsf), which was coredwith the extended core barrel (XCB), APC coring was completed inthe upper 165.4 m of the sediment column with an average recoveryof 94%. Below that level, XCB coring in Hole 736A continued until242.6 mbsf, where poor recovery and drilling characteristics resultedin the decision to switch to rotary core barrel (RCB) coring. Hole736B was a shallow APC site that duplicated coring in the upper 27m of Hole 736A in order to allow a more complete recovery of theuppermost Quaternary. In Hole 736C, RCB coring commenced at206.9 mbsf and continued to a total depth of 371 mbsf on 29 Decem-ber. Recovery was variable, ranging from no recovery to full recoveryand averaging 47%.

Two lithologic units were recognized at Site 736. The upper 257 mof the section consists of diatomaceous ooze containing scatteredpebbles and gravels that appear to be ice rafted. The age of the baseof this unit is approximately 2.6 Ma according to diatom stratigra-phy, so the base of the unit probably is tied with the onset of ice raft-ing in the area. Approximately this same upper unit (0-260 mbsf) ischaracterized on the seismic lines by closely spaced strong reflectors.Regionally, considerable channeling and thickness variations are as-sociated with this upper unit, which appears to be unit SI ofMunschy and Schlich (1987). Below 257 mbsf diatomaceous oozescontinue (Unit II) without appreciable detrital content to the base ofthe hole (371 mbsf).

Diatoms and radiolarians are common and consistent in the Site736 sediments, and standard Antarctic zonations can be applied.Coring at the site was terminated in the diatom Nitzschia inter-frigidaria/Coscinodiscus vulnificus Zone (3.1-3.4 Ma). Carbonate

123

SITE 736

is sparse and sporadic in its occurrence. Biostratigraphy indicatesthat the section is complete, with sediment-accumulation rates rang-ing between 54 and 140 m/m.y. Unfortunately, the magnetic signalof the sediments is weak, and no reversal stratigraphy could be es-tablished.

No gas was found in Site 736 sediments, and total organic car-bon is low.

BACKGROUND AND OBJECTIVES

Paleoceanography

The Antarctic Circumpolar Current (ACC) is a major sur-face current that is driven by strong westerly winds and circlesthe Antarctic continent between the latitudes of 40° and 65°S.Barker and Burrell (1982) suggested that the ACC first devel-oped in the earliest Miocene and that progressive polar cooling,which typified the late Cenozoic, resulted from a successivestrengthening of the ACC and increasing thermal isolation ofAntarctica. Near the axis of the ACC, the Antarctic Conver-gence, or Polar Front, separates subantarctic waters to the northfrom antarctic waters to the south (Gordon, 1971). The Antarc-tic Convergence also approximates the boundary between cal-careous oozes to the north and biosiliceous oozes to the south(Barker and Burrell, 1982). The biosiliceous oozes, which circleAntarctica in a broad band between about 45° and 65°S, are themajor pelagic sediment type that typifies the modern SouthernOcean.

Site 736 (49°24.12'S, 71°39.61'E), the northern site of theKerguelen Plateau paleoceanographic transect, lies beneath themodern Antarctic Convergence in 629 m of water; the conver-gence, however, may migrate seasonally through as much as 4°of latitude according to Gordon (1971). A major objective ofpaleoceanographic studies at Site 736 was to trace the movementof the Antarctic Convergence through time, both with sedimenttypes and with micro fossil assemblages. We believe that climaticfluctuations during the Neogene should be reflected by north-ward and southward displacement of the Antarctic Convergenceacross Site 736, resulting in the alternating appearance of ant-arctic and temperate assemblages in the sediments.

Kemp et al. (1973) used sedimentary evidence from Deep SeaDrilling Project (DSDP) Leg 28 to suggest that the AntarcticConvergence moved northward to its present location at the be-ginning of the Pliocene. Similarly, Brewster's (1980) argumentthat biosiliceous sediment-accumulation rates in the SouthernOcean increased dramatically in the earliest Pliocene is furtherevidence supporting a northward movement of the convergenceand expansion of the Antarctic surface-water mass. Ciesielskiand Grinstead (1986) provided more detail on numerous shiftsin position of the Antarctic Convergence (Polar Front) in thesouthwest Atlantic during the middle to late Pliocene. We hopedto carry out similar studies at Site 736.

Site 736's location on the Kerguelen Plateau, above the re-gional carbonate compensation depth (CCD), suggests that car-bonate should also be present in the Neogene sediments to berecovered there. Nearby emergent areas on Kerguelen and Heardislands also are likely to contribute detrital sediments and scat-tered ash. Indeed, piston cores collected by the French researchvessel Marion Dufresne reveal that Neogene sediments are dia-tomaceous muds with consistent, but variable, detrital compo-nents of volcanic origin and minor amounts of carbonate (R.Schlich and L. Leclaire, pers. comm., 1987).

Regional Geology and GeophysicsThe Kerguelen-Heard Plateau has been studied by Ameri-

can, French, Australian, and Soviet research ships (see refer-ences in Munschy and Schlich, 1987). The data collected consistof bathymetry, seismic reflection, sonobuoy wide-angle reflec-

tion and refraction, dredges, piston cores, gravimetry, and mag-netics. The recent article by Munschy and Schlich (1987) sum-marized the present knowledge of the area (see "Introduction"chapter, this volume).

Munschy and Schlich (1987) divided the Neogene sequenceon the Kerguelen-Heard Plateau into two units, SI and S2. Theyounger sequence, SI, exists only in the central part of the pla-teau and shows considerable variations in thickness. Unit S2,the older Neogene sequence, is found over the entire plateauand also shows significant variations in thickness. Site 736 waschosen on French seismic line MD 26-10 at shotpoint 2300(Munschy and Schlich, 1987) in order to maximize the thicknessof Neogene sequences SI and S2.

Interpretation of the seismic stratigraphy suggests that asmuch as 910 m of Neogene section (units SI and S2) lies above aregional middle Tertiary unconformity (discordance A ofMunschy and Schlich, 1987) at Site 736. Discordance A is a ma-jor event in the sedimentary section, which probably marks a hi-atus separating the middle Eocene carbonates from the lowerMiocene diatomaceous muds or clastic deposits, according toMunschy and Schlich (1987). Munschy and Schlich (1987) inter-pret discordance A to be the time of uplift and rifting of thecombined Kerguelen-Heard Plateau and Broken Ridge. Thisevent, therefore, should separate the pre-rifting from breakupand post-breakup sequences; presumably, the area of Site 736was subaerially exposed between the middle Eocene and theearly Miocene.

ObjectivesThe expanded Neogene section expected at Site 736 should

be an excellent stratigraphic section of high-resolution biostrati-graphic and paleoceanographic studies. In addition, a secondobjective at Site 736 was to date the reflector at 910 mbsf, whichshould correspond with discordance A of Munschy and Schlich(1987). Inasmuch as seismic units SI and S2 show considerableregional variations in thickness, we planned to study the sedi-ment properties in order to determine depositional processes.Recovery of the pre-rift Eocene carbonates could provide an im-portant northern point on the Ocean Drilling Program (ODP)Legs 119 to 120 north-south paleoceanographic transect for thepre-Neogene. Additionally, these sediments should compare wellwith Eocene sediments to be cored on Broken Ridge duringODP Leg 121.

SITE GEOPHYSICSSite 736 was proposed based on multichannel seismic-reflec-

tion (MCS) lines collected by Marion Dufresne (MD 26-10, shot-point 2300, and MD 26-04) and supplemented by single-channelseismic-reflection data from USNS Eltanin collected in 1971.On approach to the site (ODP line 119-01), a 30-nmi-long geo-physical survey line with standard navigation, seismic, and mag-netic equipment (see "Explanatory Notes" chapter, this volume)was recorded by JOIDES Resolution parallel to MD 26-04 andacross Eltanin lines 26 and 15 from 49°5.45'S, 71°27.0'E to49°33.05'S, 71°45.0'E. The survey line passed over the targetsite (Fig. 1), providing a good tie of drilling data to regionalgeophysical surveys.

A sonobuoy was deployed about 1 km before the target siteand was recorded for 20 km to the end of the survey line. A bea-con was dropped about 2.5 km beyond the target site to avoiddrilling into a subsurface anticlinal structure. At the end of thesurvey line, the ship turned to a reciprocal course and returnedto the beacon. The final choice for Site 736 (49°24.12'S,71°39.61'E) was about 1.5 km south of the target site (1 kmfrom the beacon) to avoid possible buried channel deposits wherethe beacon was dropped. A second beacon was dropped to markSite 736.

124

SITE 736

49°00'S -

50°00 70°00'E 71°00'

Figure 1. Index map of seismic lines recorded during site surveys for Site 736.

Sonobuoy Data

The sonobuoy was deployed 1 km from the proposed site toacquire velocity data using wide-angle reflection and refractionarrivals through the shallow sedimentary section directly "belowthe site (Fig. 2). Strong direct, reflection, and refraction arrivalswere recorded over the first 6 km of the sonobuoy station. Theserecorded arrivals provided good estimates of interval and refrac-tion velocities for the sedimentary section and acoustic base-ment (refraction velocity only) (Fig. 3). Velocities computed fromthe sonobuoy were used prior to drilling to estimate depths forseismic-reflection horizons beneath Site 736. This preliminaryvelocity-depth information was used extensively for guiding thedrilling strategy (Table 1).

Seismic-reflection data across Site 736 indicate that the shal-low sedimentary section (above 1.9 s; EE, Fig. 3) contains anupper well-layered unit and a lower acoustically-transparent unitwith scattered reflectors. The sonobuoy data, however, showstrong continuous reflections throughout both units, therebyproviding good reflections (AA to FF) for interval-velocity esti-mates. Interval velocities are low (1.45-1.79 km/s) within bothunits (Fig. 4). The velocity increase with depth is probably dueto burial compaction of the predominately diatomaceous sedi-mentary section. The absence of identifiable refraction arrivalsfrom horizons AA and BB, as well as the large horizontal offsetto slow (1.5 and 1.6 km/s) refraction arrivals (probably fromlayers CC and DD; Table 1), also suggests that velocities are lessthan 1.8 km/s in the upper part of the sedimentary section (i.e.,above at least 846 mbsf, horizon EE).

Abrupt velocity increases occur at both horizons EE and FF,which are associated with linear refraction arrivals with veloci-ties of 2.96 and 3.35 km/s, respectively. These horizons werenot reached by drilling at Site 736; however, the velocities aresimilar to those for sedimentary rocks drilled at Site 737 (see

"Site Geophysics" section, "Site 737" chapter, this volume). AtSite 737, calcareous claystone and chert-limestone were recov-ered from units with distinct refraction velocities of 2.52 and3.7-3.9 km/s (sonobuoy units DD-EE and EE-FF, respectively).

Refraction velocities for likely igneous or massive volcanicrocks (e.g., greater than 4.5 km/s) were not recorded in the firstsonobuoy at Site 736; however, they were recorded at Site 737,where 794 m of 3.7-3.9-km/s rocks overlies likely igneous base-ment (4.51 km/s). The approximate total thickness of strataabove horizon FF at Site 736 is 2037 m (Table 1). If rocks be-tween horizon FF and igneous basement are of equivalent thick-ness to those at Site 737 (e.g., 794 m), then the total sedimentarysection at Site 736 would be 2785 m. This thickness is probablya minimum value, because the seismic-reflection data betweenSites 736 and 737 indicate that the sedimentary section thickenstoward Site 737.

The sonobuoy interval velocities are similar to those deter-mined from routine analysis of stacking velocities for MCS lineMD 26-10 near Site 736. The sonobuoy data provide more com-plete velocity information than MCS data because the greaterhorizontal offset of the sonobuoy (up to 5 km) provides addi-tional refraction-velocity information concerning velocity gradi-ents and velocity discontinuities.

OPERATIONSThe expedition began on 14 December 1987 at Port Louis,

Mauritius.

Mauritius Port CallLeg 119 began with the first mooring line at Berth 5, Port

Louis Harbor, at 0615 hr, 14 December. The major port call ac-tivities were logistical with large shipments of ODP cores andspecial drilling/coring equipment from Leg 118 to be off-loaded.The oncoming shipments for the Antarctic voyage were larger

125

SITE 736

0—

Sonobuoy 1 (near Site 736) Sonobuoy 1 (near Site 736)

0.8-

2 km

Figure 2. Vertical-incident seismic and sonobuoy seismic record for so-nobuoy station 1. Site 736 is about 2 km south of where the sonobuoywas deployed. See Figure 1 for location.

2 km

Figure 3. Vertical-incident seismic and sonobuoy seismic record for Site736 showing wide-angle reflection and refraction interpretations. Let-ters denote layers in Table 1. See Figure 1 for location.

than normal, and a considerable amount of freight was back-loaded from local storage in Mauritius. Other work items in-cluded the annual American Bureau of Shipping inspection forthe ship; repair of number 7 engine and number 2 crane; repairof cracks in the thruster wells; loading of drill water, lube oil,and 900 metric tons of fuel; and UDI and ODP crew changes.

On the morning of 18 December, the vessel had to shift twoberths down the wharf to accommodate another ship. Loadingthen continued until minutes before the last line was cast off at1730 hr.

Mauritius to Site 736Generally good weather prevailed during the scheduled eight-

day transit to Kerguelen. Winds were light to moderate and usu-ally from astern. With the help of a following current, an aver-age speed of 11.7 kt was achieved for the first four days. On 22December the Southern Ocean greeted JOIDES Resolution withbeam winds gusting to 45 kt. The swell built to 7 m, and the ves-sel experienced rolls to 20°. Speed dropped briefly below 10 kt,but conditions moderated after about a day. Following windsthen returned and assisted the ship into colder waters beforethey shifted to the south and impeded progress somewhat.

The drill site, which is about 40 nmi east of Kerguelen Is-land, was approached from the north, and seismic profilinggear was streamed about 18 nmi north of the site. Speed was re-duced to 6 kt to improve record quality. At about 2 nmi north ofthe site, a sonobuoy was deployed for a seismic-refraction veloc-

Table 1. Preliminary results for sonobuoy 1, Site 736.

Horizona

AABBCCDDEEFF

Vrms(km/s)

1.4651.4631.4791.4961.5662.159

Vint(km/s)

1.451.561.561.793.22—

Ti(s)

0.871.041.241.531.912.65

Zi(km)

0.6280.7510.9071.1331.4742.665

Vrfr(km/s)

nrnr

b1.5b1.62.963.35

To(s)

—0.350.681.571.88

Note: Vrms = rms velocity for horizon; Vint = interval velocitycomputed using Vrms, Ti, and Dix equation (e.g., Vint be-tween horizons EE and FF is 3.22 km/s); Ti = vertical inci-dence reflection time (two-way) to horizon; Zi = total depthfrom sea level to horizon computed from Vint and Ti (waterdepth = 628 m); Vrfr = refraction velocity associated withhorizon (e.g., Vrfr at top of layer CC-DD is 1.5 km/s); To =intercept time for refractor associated with horizon; nr = norefractor observed.

a Letters of horizons do not correlate between drill sites and donot correspond to prior stratigraphic analyses.Refractors assumed to be associated with the horizon.

ity profile. A recoverable positioning beacon was launched onthe first pass over the location. The profile was then extendedbeyond the drop point before the vessel returned on a reciprocalcourse and recrossed the beacon location. The profiling gearwas recovered, and the vessel took station on the beacon. Aspreparations were made for the pipe trip, the drilling position

126

SITE 736

'Vrfr=i.50

Vrtr=1.60

Vrfr=2.96

I Vint=3.22

— Vrfr=3.35

0.5 km

Vertical-incident seismic data showing sonobuoy interval and refraction velocities, Site 736.

was refined by offsetting the ship 1000 m to the north andlaunching a second beacon.

The total transit/survey distance of 1943 nmi was covered atan average speed of 11.0 kt.

Site 736—Kerguelen-Heard Plateau

Hole 736A

Operations began slowly, with temperatures of 3° to 4°C andwinds gusting to 45 kt. Bottom-hole assemblies (BHA) wereconstructed, and all components were measured and checkedfor internal clearance. After an electrical problem in the topdrive was resolved, Hole 736A was spudded at 1800 hr on 26December 1987 at a depth of 629 m by drill-pipe measurement.

APC cores were taken from the seafloor to 117.5 mbsf (Table2) before pull-out force reached 60,000 lb. Extreme vessel mo-tion resulted in short APC stroke, failed core liners, and shearedovershot pins. On one occasion the coring line failed about 20m above the coring assembly. It was successfully recovered from

the drill string, along with the full core barrel, using a wireline-deployed barbed spear. Oriented APC and heatflow probe runswere only partially successful because of vessel motion condi-tions.

Three XCB cores (119-736A-17X through 119-736A-19X,117.5 to 146.4 mbsf) were then attempted, but recovery was poorin the soft diatom ooze. The APC corer was redeployed, andtwo more successful APC cores (119-736A-20H through 119-736A-21H, 146.4 to 165.4 mbsf) were recovered before the sys-tem survived a 130,000-lb pull-out. XCB coring continued, withrecovery of only about 20%, to 252.3 mbsf. Minor amounts ofbasalt and pumice gravel continued to fall from an uphole sourceand to be recovered at the top of each core, but the gravel didnot adversely affect operations.

Because the ultimate coring objective at Site 736 was in ex-cess of 900 mbsf and hard sedimentary rocks were expected,Hole 736A was terminated to change the BHA. That step hadbeen planned for deeper in the section, but the low core recov-ery rendered further XCB coring a waste of time.

127

SITE 736

Table 2. Coring summary, Site 736.

DateCore (Dec.no. 1987) Time

Depth

top bottom(mbsf)

Length

cored recovered(m) (m)

Recovery

119-736A-

1H2H3H4H5H6H7H8H9H10H11H12H13H14H15H16H17X18X19X20H21H22X23X24X25X26X27X28X29X30X

119-736B-

1H2H3H

119-736C-

1W2R3R4R5R6R7R8R9R10R11R12R13R14R15R16R17R18R

(Coring)

(Washing)

26

26

26

26

26

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

27

28

28

28

28

28

29

29

29

29

29

29

29

29

29

29

29

29

29

29

29

29

29

29

1810

1850

2100

2145

2300

0000

0045

0115

0145

0230

0300

0615

0700

0830

0915

0950

1100

1115

1300

1405

1600

1755

1830

1900

1930

2015

2240

2315

0045

0130

0925

0950

1010

0030

0100

0130

0320

0405

0445

0515

0550

0630

0700

0735

0805 h

0835

0910

0945

1010

1045

1130

0.0

9.0

18.5

28.0

33.0

38.0

43.0

48.0

53.0

59.0

68.5

73.5

79.5

89.0

98.5

108.0

119.5

127.1

136.8

146.4

155.9

165.4

175.0

184.6

194.4

203.9

213.5

223.2

232.9

242.6

0.0

8.4

17.9

0.0

206.9

216.5

226.0

235.7

245.4

255.0

264.7

274.4

284.0

293.7

303.4

313.1

322.8

332.4

342.0

351.6

361.3

9.0

18.5

28.0

33.0

38.0

43.0

48.0

53.0

59.0

68.5

73.5

79.5

89.0

98.5

108.0

117.5

127.1

136.8

146.4

155.9

165.4

175.0

184.6

194.4

203.9

213.5

223.2

232.9

242.6

252.3

8.4

17.9

27.4

206.9

216.5

226.0

235.7

245.4

255.0

264.7

274.4

284.0

293.7

303.4

313.1

322.8

332.4

342.0

351.6

361.3

371.0

9.09.59.55.05.05.05.05.06.09.55.06.09.59.59.59.57.69.79.69.59.59.69.69.89.59.69.79.79.79.7

250.3

8.411.09.5

28.9

206.99.69.59.79.79.69.79.79.69.79.79.79.79.69.69.69.79.7

164.1

206.9

371.0

8.955.316.314.965.204.525.215.325.997.354.785.969.769.879.479.701.380.871.648.409.060.653.362.190.045.751.260.003.220.57

147.05

8.339.869.62

27.81

0.103.030.000.236.270.102.362.859.869.666.430.579.564.499.745.511.824.83

77.31

0.10

77.41

99.455.966.499.2

104.090.4

104.2106.499.877.395.699.3

103.0104.099.7

102.018.19.0

17.188.495.3

6.835.022.3

0.459.913.00.0

33.25.9

99.189.6

101.0

31.50.02.4

64.61.0

24.329.4

103.099.666.3

5.998.546.8

101.057.418.749.8

Hole 736BBecause of adverse weather and a core liner failure, the up-

permost sediments were incompletely recovered in Hole 736A.A second effort was made to recover the valuable material lostwhen the bit cleared the seafloor after abandonment of Hole

736A. Three quick APC cores were taken from the seafloor (wa-ter depth of 628 m) to a depth of 28.9 mbsf with a recovery of27.8 m. Under much-improved weather conditions, completecore recovery was achieved. A round trip was then made for theRCB BHA.

Hole 736CFollowing the pipe trip, the soft upper sediments were washed

to a depth of 207 mbsf. The hole was flushed with drilling mud,and a successful heatflow-probe measurement was taken.

Continuous RCB coring then began, with results equivalentto those with the XCB through the repeated interval. The sedi-ment became somewhat firmer with depth, and the recoveryrate improved to about 65%.

When 371 m of fairly uniform ooze had been penetrated, re-vealing an extremely high rate of sedimentation (-140 m/m.y.),the drilling objectives of the site were re-evaluated (see "Back-ground and Objectives" section, "Site 737" chapter). We deter-mined that the desired scientific results could be achieved moreefficiently by terminating operations at Site 736 and moving tonearby target Site KHP-3, where the younger sediment sectionwas thinner or absent.

Hole 736C was filled with weighted mud and abandoned.The drill string was recovered, and the drillship departed Site736 at 1630 hr on 29 December.

LITHOSTRATIGRAPHY AND SEDIMENTOLOGYThe sediment sequence drilled at Site 736 has been divided

into two lithologic units: Unit I is a diatom ooze containing sub-stantial basaltic volcanic debris and more than 0.5% organiccarbon; Unit II is a diatom ooze containing minor feldspathicand pumiceous detritus, nannofossil ooze, and less than 0.4%organic carbon. The divisions are summarized in Table 3 andFigure 5. Smear slide compositional data for the units are in-cluded in the barrel sheets. In addition to examination of thesmear slides and of the mineralogy of the sand fraction as ob-served through the binocular microscope, six samples were ana-lyzed by X-ray diffraction (XRD). Carbonate analyses were con-ducted on the core-catcher samples ("Inorganic Geochemistry"section, this chapter). Grain-size analysis was made using a La-sentech Lab-Tec 100 scanning-laser particle-size analyzer ("Ex-planatory Notes" chapter). Coarse-sediment fractions were ex-amined visually.

Drilling DeformationMuch of the core was disturbed during coring. The sediment

is soft to sightly firm; the latter was affected by closely spaced,concave-up bowed partings. A particular problem experiencedat this site was drill hole cave-in contamination of the upperpart of the APC cores. Discrimination of these artifact gradedgravels from in-situ, natural graded gravel was based on criteriadescribed in the "Explanatory Notes" chapter. Contaminatedintervals occurred in Sections 119-736A-4H-1, 0-30 cm, 119-736A-5H-1, 0-57 cm, 119-736A-6H-1, 0-75 cm, 119-736A-7H-1,0-50 cm, 119-736A-8H-1, 0-37 cm, 119-736A-9H-1, 0-35 cm,119-736A-10H-1, 0-7150 cm, 119-736A-11H-1, 0-37 cm, 119-736A-12H-1, 16-90 cm, 119-736A-13H-1, 0-57 cm, 119-736A-14H-1, 0-55 cm, 119-736A-15H-1, 0-57 cm, 119-736A-16H-1,0-12 cm, 119-736A-17X-1, 0-18 cm, 119-736A-18X-1, 0-47 cm,119-736A-18X-CC, 0-41 cm, 119-736A-19X-1, 0-151 cm, 119-736A-19X-CC, 0-14 cm, 119-736A-20H-1, 0-56 cm, 119-736A-21H-1, 0-32 cm, 119-736A-22X-1, 0-39 cm, 119-736A-22X-CC,0-26 cm, 119-736A-24X-1, 0-66 cm, 119-736A-27X-1, 0-10 cm,119-736A-29X-1, 0-4 cm, 119-736A-30X, 0-57 cm, 119-736C-2R-1, 0-12 cm, 119-736C-4R-1, 0-12 cm, (?)119-736C-13R-1,0-10 cm, and (?)119-736C-18R-1, 0—15 cm. However, no agree-ment was reached on whether Sections 119-736A-6H-1, 75-90

128

SITE 736

Table 3. Summary of lithologic units, Site 736.

Age UnitDepth(mbsf)

Thickness(m)

Coreinterval

Quaternary- Ilate Pliocene

late Pliocene

0-257.1

257.1-366.1

257.1

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cm, and 119-736A-7H-1, 0-50 cm, are downfall material or in-situ gravels. A problem that remains in regarding all these ascontaminants is that their source layers were not sampled atshallower levels in the hole, despite almost complete recovery inthe initial cores.

The soupy nature of the sediment throughout Hole 736C is aresult of remobilization during rotary drilling. Drilling distur-bance data are summarized in the barrel sheets. Otherwise,drilling deformation features are similar to those encountered atother ODP sites.

Unit I

Lithostratigraphy

Diatom ooze with minor volcanic debris

Core 119-736-1H through Sample 119-736A-6H-1, 75 cm; depth,0-257.1 mbsf.

Core 119-736B-1R through Section 119-736B-3H-CC; depth, 0-27.4 mbsf.

Core 119-736C-2R through Sample 119-736C-7R-2, 61 cm; depth,0-257.1 mbsf.

Age: Quaternary-late Pliocene.

Unit I is composed of diatom oozes with varied degrees ofincorporated volcanic detritus, minor tuffs, and basaltic gravelsrecovered mainly in the form of downhole cave-in contamina-tion.

The unit consists of olive-colored (e.g., 5Y 4/3) diatom ooze,interstratified with darker olive gray (e.g., 5Y 4/2) to gray-black(e.g., 5Y 3/1) diatom ooze. The darker varieties bear dissemi-nated volcanic silt, sand, gravel, and clay. The amounts of vol-canic detritus incorporated range up to 15%; the colors grayblack (5Y 3/1, 3/2, 2.5/1, and 2.5/2), olive gray (5Y 4/1, 4/2,5/1, and 5/2), and olive (5Y 4/3, 4/4, 5/3, 5/4, and 5/6) repre-sent diatom oozes with high, moderate, and low volcanic detri-tal content, respectively. Figure 5 illustrates trends in color tonesthrough Unit I, suggesting that volcanic incorporation tends toincrease to the top of the sequence. The base of Unit I is takenas the lowermost olive gray (5Y 4/2) or darker olive diatom ooze(257.1 mbsf at 27 cm in Section 119-736C-7R-2; see Fig. 5) sig-nifying the onset of volcanic contribution to the diatom ooze.

Grain-size analyses of the oozes indicate that all but a few ofthe pure and impure diatom oozes are of silty texture withl%-7% sand, 82%-88% silt, 11%-17% clay fraction, and 10-14-µm mean grain size. The clay-size fraction contains at leastsome broken diatom material. The silt and sand fractions con-sist largely of entire or fragmented diatoms. Organic carboncontents are between 0.5% and 1.5%, whereas carbonate valuesare generally 0%-4%, ranging to 10% (see "Organic Geochem-istry" section, this chapter; Fig. 6). From smear slide examina-tions 75%-100% of the sediments consists of diatoms, 0%-20% is clay minerals, and 0%-12% is volcanic glass. Other mi-nor components include nannofossils and foraminifers.

For the most part, the diatom ooze sediments are thick bed-ded (usually 0.5 to 4 m) and internally homogeneous, apart

129

SITE 736

from a few slight and vague color changes. Bedding relationsbetween the color tone variations are not clear. Most lithologicboundaries are diffuse, irregular, and/or wispy, and many aresteeply inclined. Most are not marked by any appreciable changein grain-size texture. Evidence exists for substantial penecon-temporaneous or post-depositional sediment disruption. Softintraclasts up to 7 cm long and wispy trains of light- or dark-toned diatom ooze are commonly included in ooze of anothertone (and thus volcanic content). Some intraclasts show internalbedding in the form of sandy laminae (e.g., core photograph of119-736A-8H-3, 110-120 cm; 52 mbsf), and such laminae maybe inclined, bowed, and disrupted. Rare intraclasts of volcanic,sandy, laminated foraminiferal diatom ooze are also seen (e.g.,Section 119-736A-4H-1, 74 cm; 30.2 mbsf).

Bioturbation structures are rarely seen and difficult to iden-tify, apart from wispy segregates in the disturbed, impure, dia-tom oozes. The sole macroscopic benthic faunal remains detectedwere a scaphopod shell at 50 cm in Section 119-736A-2H-3 (12.5mbsf) and shell fragments in Section 119-736A-14H-2 (89 mbsf).

Nonsorted units of mixed gravel, sand, and diatom ooze at-tain thicknesses of 40-60 cm (Cores 119-736A-3H to 118-736A-5H, 119-736A-7H to 119-736A-16H, and 119-736B-1H to119-637B-3H; 18.5-108 mbsf) Typically they have colors fromblack to very dark olive gray (5Y 2.3/2 to 5Y 3/1). The coarsestcomponents consist of subangular to subrounded basalt andpumice at granule size (usually 2-8 mm), whereas sand compo-nents consist of glass, pumice, feldspar, and basaltic lithic frag-ments. The estimated percentages of gravel plus sand in suchunits is up to 20%. Rare clasts of altered volcanics (up to 15 mmin size) and reddish (?)sandstone have been noted. Several exam-ples show sharp upper and lower bedding contacts and coarse-tail grading. In one instance (Section 119-736A-7H-3, 75 cm; 47mbsf), volcanic-derived granules are in the lower part of a 56-cm-thick unit that has crude grading at its base. In Section 119-736A-10H-5 (64 mbsf) 10-20-cm granules and a 2-cm basaltclast have accumulated at the base of a graded unit. Intraclastsof finer textured impure ooze are also commonly incorporated(e.g., at 60 cm in Section 119-736A-12H-3).

Pebbles and granules of basalt and pumice occur as isolatedclasts (lonestones) in most of the color variations of the diatomooze. The largest appears to reach a size of 4.5 cm (Section 119-736A-4H-1, 80 cm; 29 mbsf), but most are <2 cm. Although itis conceivable that some were incorporated in the soft sedimentduring drilling, many are obviously in undisturbed positions. Afew of the pebbles (e.g., Section 119-736A-1H-4, 136-142 cm;5.9 mbsf) show faceting, striations, or bullet-nose shapes. Allsuch pebbles are of basalt or pumice, and together they amountto < 1% of the core's split surface.

Well-sorted sediments are volumetrically insignificant in thein-situ core recovery. Where present, they are in the form of tufflayers (the highly disturbed volcanic gravels are described else-where in this section). Olive gray (5Y 3/2) to black (5Y 2.5/1)volcanic ash and lapilli tuffs are present in beds only a few cen-timeters thick in Sections 119-736A-3H-1, 119-736A-3H-2, 119-736A-6H-2, 119-736A-8H-2, and 119-736B-3H. They seem to berestricted to stratigraphic levels at approximately 18-20, 40, and50 mbsf. The sand/gravel components are up to 7 mm in size;consist of pumice, glass, feldspar, and (?)quartz; and range fromvery well sorted to poorly sorted. Significantly, no diatom oozeis incorporated as matrix. The degree and form of internal sizegrading is variable, from none (e.g. Section 119-736A-3H-1, 49-56 cm) to normal and reverse grading in basalt (glass) and pum-ice sand, respectively. A fine example of this grading occurs at128-134 cm in Section 119-736A-3H-1 (20 mbsf) where silt-sandvitric ash tuff fines upward from a sharp bed base and well-rounded pumice granules coarsen toward the sharp upper-bedboundary. Most pumice fragments are very well rounded. Thebed boundaries are usually sharp, but not noticeably erosional.

Unit II

Diatom ooze with minor nannofossil ooze

Samples 119-736C-7R-2, 27 cm, to 119-736C-18R-4, 30 cm; depth,257.1-366.1 mbsf.

Age: late Pliocene.

Unit II consists almost entirely of diatom ooze containingonly traces (<1%) of volcaniclastic and terrigenous material,but with an environmentally significant 7-cm-thick nannofossilooze at 61-68 cm in Section 119-736C-7R-2.

The diatom ooze is olive (5Y 4/3 and 5Y 4.4) to olive gray(5Y 5/4 and 5Y 5/3) and consists of 80% silt-size and 20% clay-size particles, with a mean size of 7 µm. Smear slides show a mi-nor content of feldspathic and quartz silt, ash fragments, andglass. The nature of this siliciclastic component is quite differ-ent from that in the Unit I oozes, principally in the lack of pum-ice and basalt fragments and in the presence of feldspars andglass. At about 293 mbsf the unit contains traces of authigenicdolomite (Section 119-736C-10R-7, 30-60 cm). The diatom oozeis homogeneous, with no sedimentary structures in evidence. Asoupy structure in parts may be due to the drilling disturbanceof sediment with a high water content. Small pebbles, amount-ing to < 1% of the core, are dispersed throughout; the largest is~ 2 cm in diameter. They are mainly subangular to subroundedand are commonly faceted with some striations.

The nannofossil ooze is pale olive (5Y 6/2), lacks clastic com-ponents, and is free of diatoms. Grain-size studies reveal 2%sand, 76% silt, 22% clay, and a 9.6-µm mean size. No internalstructures are apparent, but the upper and lower boundaries areabrupt.

Interpretation of Depositional EnvironmentThe sedimentary sequence at Site 736 is dominated by two

distinct components: a biogenic component characterized by si-liceous diatom oozes and a volcanogenic one consisting of amixture of basaltic gravel and pumice, plus sand- and silt-sizedvolcanic glass, lithics, and minerals. The supply of volcanic de-tritus began in the late Pliocene, but reached a climax only latein the Quaternary.

Diatom OozesThroughout the time represented by the sequence, deposition

of diatoms continued almost without interruption and withoutsignificant reworking or disaggregation by bottom currents. Therecovered cores of pure diatom ooze almost totally lack sedi-mentary structures (Unit II and parts of Unit I). Accumulationwas therefore well below wave base, probably in bathymetricconditions similar to those of today. Bioturbation, although evi-dent in a few places, does not appear to have been a majorprocess.

WfsThe thin, well-sorted lapilli- and ash-tuff horizons that are

free of diatom ooze matrix appear to represent submarine ash-fall tuffs. Grain-size sorting is an important feature that is nor-mal with mineral and glass grains, but reversed in the case ofpumice grains. Upper and lower stratal boundaries are sharpand apparently not erosional. No other form of sediment struc-ture was detected. The absence of diatom ooze, where that li-thology lies above and below, implies that emplacement was notacross the seafloor (density flow), but directly through the watercolumn.

Volcanic Detritus Incorporated in Diatom OozeAs illustrated in Figure 5 and described in this section, the

diatom oozes in Unit I contain substantial components of dis-seminated volcanogenic gravel, sand, silt, and clay. The volcaniccontent is not uniformly distributed, but is concentrated in cer-

130

SITE 736

tain strata, in sandy intraclasts of diatom ooze, in units of grav-elly/sandy diatom ooze, and as isolated clasts (lonestones). Thedata are presently not sufficient to define the origins and modesof emplacement of the detritus, but plausible hypotheses in-clude: (1) erosion and transport on Kerguelen and Heard islandsby glacial and fluvial processes, (2) distribution over the Ker-guelen Plateau by ice rafting, and (3) distribution (or redistribu-tion) in the form of gravity flows.

The well-sorted gravels that lie at the tops of cores as(1) downhole cave-in contamination, (2) gravels that are in placebut disturbed by the drilling, and (3) in-situ gravel deposits areparticularly difficult to interpret. Frohlich (1983), Leclaire (1983),and Schlich and Leclaire (1981, 1983) documented similar grav-els forming a surficial or barely buried apron over the KerguelenPlateau near Kerguelen Island. Although the Leg 119 ShipboardScientific Party agreed that most core top gravels were downholecontamination derived from these near-surface gravels, somesedimentologists regarded the gravels of Sections 119-736A-1Hand 119-736A-7H-1 as in situ but drilling disturbed.

The origin of the gravel material, however, is reasonably inevidence. All lithologies can be related to the Kerguelen andHeard islands' volcanic centers and are, therefore, of local deri-vation. The extent of glacial processes in erosion and subse-quent gravel transport and emplacement is not precisely knownbut is evidenced in the occurrence of striated, bullet-nosed, anddistinctively faceted clasts, which imply transport at some stageat the base of a wet-based glacier. Furthermore, some lone-stones that can be interpreted as ice-rafted debris occur in allcolor variations of diatom ooze (implying that they are indepen-dent of the volcanic sand-silt-clay supply) and are most com-monly associated with high contents of this finer material. Un-fortunately, a lack of fine bedding in the core prevents the rec-ognition of true dropstone structures. Kerguelen and Heardislands are currently glaciated, and their geomorphology sug-gests more extensive glaciation in the past (Nougier, 1970), ex-tending into marine environments and involving the calving oftidewater glaciers. According to this hypothesis, the gravels foundat Site 736, whether at the core tops, disseminated in the impurediatom ooze, or in the form of lonestones, were emplaced asice-rafted debris.

In many of the units of diatom ooze with disseminatedgravel-sand-silt-clay volcanic material, sediment textures andstructures imply some transport and emplacement by density-flow processes. We observed clear instances of sharp basal con-tacts with crude size grading of the gravel and coarse sand com-ponents, incorporation of soft intraclasts of volcanic-bearingooze and relatively pure ooze, apparent post-depositional dis-turbance of bedding relations with irregular and locally highlyinclined boundaries, and diffuse and wispy lithologic bounda-ries. As already noted, bioturbation was not an important pro-cess in this regard. Identification of the volcanic-bearing diatomoozes as density-flow deposits of a type described as a "disor-ganized turbidite" (Stow and Piper, 1984) would explain thepreceding features. The physical behavior of such sediments isnot well known, but it may be speculated that they represent im-mature states of density-current flow (before size grading) orshort-lived turbidity currents. The behavior of diatom ooze indensity flows is not well documented. A density-flow origin isconsistent with the high observed sedimentation rates (see "Sed-imentation Rates" section, this chapter) and local Quaternaryseafloor gradients.

BIOSTRATIGRAPHYAn apparently biostratigraphically continuous 371-m-thick

sequence of upper Pliocene to Pleistocene diatom oozes withminor volcaniclastics was recovered at shallow-water (629 m)Site 736 on the northern Kerguelen Plateau.

Recovery, lithology, biostratigraphic ages and correspondingbiozones, the abundance and preservation of the various fossilgroups, and some additional sedimentologic information gainedfrom the analyses of all core-catcher samples available are sum-marized in Figure 6. Biostratigraphic age assignments of this ne-ritic section are based almost exclusively on diatoms. No usefulpaleomagnetic data are recoverable.

Abundant, well-preserved, and highly diverse diatom assem-blages dominate the biogenous components throughout the re-covered interval. Radiolarians are abundant and well preserved.Low coarse-fraction percentages consist of mostly terrigenous,detrital particles, with some foraminifers and abundant largediatoms. Low-diversity subpolar planktonic foraminifers decreasein abundance below the Pliocene/Pleistocene boundary. Mostlywell-preserved benthic foraminifers are rare to common through-out the recovered interval. Most core-catcher samples are barrenof calcareous nannofossils. A few moderately etched, monospe-cific, subpolar nannofossil assemblages are present in a gravellyhorizon at 60 mbsf and between 200 and 250 mbsf. Low-diver-sity silicoflagellate assemblages occur throughout the recoveredsequence. The section is devoid of organic-walled microfossils.

Calcareous NannofossilsCalcareous nannofossils are either absent or very rare in all

core-catcher samples investigated from Site 736. Mud-line Sam-ple 119-736A-1H, 0-2 cm, contained rare, well-preserved Gephy-rocapsa oceanica. In the upper 250 m of the sedimentary sec-tion only moderately to strongly etched Coccolithus pelagicuswere encountered in Samples 119-736A-9H-CC (rare), 119-736A-25X-CC (rare), 119-736A-27X-CC (few), 119-736A-29X-CC (com-mon), 119-736A-30H-CC(rare), 119-736C-4R-CC(rare), 119-736C-5R-CC (few), 119-736C-6R-CC (rare), and 119-736C-7R-CC(few). Sample 119-736C-7R-2, 66 cm, from a thin layer (about5 cm thick) of calcareous nannofossil-diatom ooze at 257 mbsfcontained a well-preserved monospecific assemblage of Reticu-lofenestra perplexa (= Dictyococites antarcticus Haq). Sample119-736C-8R-CC contained strongly etched C. pelagicus (few)and Gephyrocapsa sp. (rare), and a few, moderately well-pre-served Reticulofenestra perplexa were found in Sample 119-736C-9R-CC.

These nannofossils are of limited biostratigraphic value. Theupper 250 m section can be assigned to the Pliocene throughPleistocene, and the interval from 250 to 280 mbsf is probablyPliocene in age.

The absence or scarcity of calcareous nannofossils in the up-per Neogene diatom oozes on Kerguelen Plateau seems to bedue to lack of supply instead of preservation.

ForaminifersThe 371 m of lower Pliocene through Quaternary sediments

recovered at Site 736 is carbonate poor, mainly consisting of sili-ceous oozes with interspersed volcanogenic clasts, basaltic grav-els, and glacial dropstones. Foraminifers were studied from themud-line sample (0-2 cm) and all core-catcher samples fromHoles 736A, 736B, and 736C. The occurrence of a few stainedspecimens (Rose Bengal) in the mud-line sample indicates only asmall loss of the surface layer through the drilling operation.The relative abundance of species from the >150-µm fractionwas estimated for each sample (see "Explanatory Notes" chap-ter). The abundance of foraminifers varies downcore and showsa changing planktonic/benthic ratio. Below a depth of 313 mbsf(Core 119-736C-12R) no foraminifers were found. The recov-ered assemblages show typical features of the Antarctic region:low diversity and lacking age-diagnostic species. Test preserva-tion is good down to Core 119-736A-17X (118 mbsf), where cal-cite encrustation becomes apparent on some planktonic fora-minifers. The benthic specimens are generally well preservedthroughout the range of their occurrence.

131

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Figure 6 (continued).

in low abundance to a depth of 223 mbsf, Globorotαliα crαssα-formis, found in low to moderate abundance in Cores 119-736A-9H, 119-637A-21H, 119-736A-22X, and 119-736C-6R, andseveral specimens of Globigerinoides quadrilobatus, found inCores 119-736A-21H, 119-736A-25X, and 119-736C-6R. Of thesespecies, G. crassaformis provides the best age control, as it has areported range of late early Pliocene (upper Zone N19) to Holo-cene (Kennett and Srinivasan, 1983). The other taxa have beenreported from late Miocene age or older sediments.

Benthic Foraminifers

Benthic foraminifers are dominated by calcareous speciesthroughout the entire sequence. The assemblage is characterizedby a relatively low diversity and only a few dominant species.The abundant taxa include Cassidulina oblonga, Bulimina acu-leata, Sphaeroidina bulloides, Melonis barleanum, Anguloger-ina earlandi, Pullenia bulloides, and Globobulimina pacifica.The fluctuation of these dominant species marks the changes in

133

SITE 736

the assemblage through time that may indicate changes in bot-tom-water conditions.

The Quaternary and upper Pliocene interval above Sample119-736C-9R-CC is characterized by an alternating dominanceof C. oblonga, B. aculeata, A. earlandi, S. bulloides, and M.barleanum. Accompanying species with a less dominant occur-rence include Angulogerina angulosa, Cassidulina laevigata, Pul-lenia subcarinata, Stainforthia complanata, P. bulloides, Boli-vina earlandi, Globocassidulina subglobosa, and several rarespecies.

Within the interval from Cores 119-736C-9R through 119-736C-11R, the assemblage changes to a fauna dominated by G.pacifica and Buliminella elongata. Both species are absent inthe sequence above Sample 119-736C-9R-CC. Benthic foramini-fers are rare or completely absent from the siliceous ooze se-quence that spans the Quaternary/Pliocene boundary.

Agglutinated species are represented only in the mud-line sam-ple by single specimens of Recurvoides scitulus, Reticulophrag-mium trullissatum, Trochammina globigeriniformis, and Textu-laria earlandi. Martinotiella antarctica is preserved in the fossilrecord but was rarely found downcore. This species uses sili-ceous cement and is reported to occur predominantly in diato-maceous sediments (Echols, 1971). M. antarctica is also foundin downcore sediments at Site 695 in the Weddell Sea (Ship-board Scientific Party, 1988). The sparse agglutinated fauna atthis site contrasts with a study of surface samples on the westernslope of the Kerguelen Ridge, west of Head Island, where an ag-glutinated component of 21% (at 790 m depth) was found (Lin-denberg and Auras, 1984). The calcareous component in thesurface study yields a high abundance of A. earlandi and B.aculeata, which compares with the Holocene fauna of Site 736.

Interpretation of changes in the benthic foraminifer assem-blage will depend on results from oxygen and carbon isotopicanalyses of the planktonic species Neogloboquadrina pachy-derma, as benthic taxa are too sparse. These results may definethe effect of the lateral shifts of the Polar Front on the deep-wa-ter environment.

DiatomsDiatoms are abundant in all core-catcher samples examined

from Site 736. Preservation is generally good with sporadic sam-ples exhibiting moderate preservation. The lowermost samples(119-736C-17X-CC and 119-736C-18X-CC) have poor preserva-tion. The majority of the species present are characteristic ofthe Southern Ocean; however, some species present are moretypical of the middle latitudes.

Stratigraphic indicators are common and allow recognitionof the Southern Ocean diatom zonation of Ciesielski (1983).Zonal boundaries defined by Ciesielski (1983) on the last occur-rence of Nitzschia weaveri, Nitzschia interfrigidaria, and Cos-miodiscus insignis are recognized in this study at the last com-mon occurrence rather than at the last occurrence of these spe-cies.

The stratigraphic sequence recovered from Site 736 is Plio-cene and Quaternary in age and can be divided into nine diatomzones. The last occurrence of Actinocyclus ingens in Sample119-736A-4H-CC and the absence of this species in Sample 119-736B-3H-CC allows placement of Cores 119-736A-1H through119-736A-3H and 119-736B-1H through 119-736B-3H into theThalassiosira lentigenosa Zone (previously the Coscinodiscus len-tiginosus Zone) of Ciesielski (1983). Thus, these intervals havean estimated age younger than 0.62 Ma. The assemblage presentin these intervals is dominated by Nitzschia kerguelensis andEucampia antarctica.

The last occurrence of Rhizosolenia barboi in Sample 119-736A-15H-CC suggests that the interval from 119-736A-4H-CCthrough 119-736A-14H-CC is equivalent to the Coscinodiscuselliptopora/Actinocyclus ingens Zone of Ciesielski (1983). How-ever, placement of this zonal boundary is tentative because R.barboi has a sporadic occurrence in the uppermost part of itsstratigraphic range. Supporting the zonal assignment is the oc-currence of Coscinodiscus elliptopora in Cores 119-736A-8Hthrough 119-736A-16H. The last occurrence of Coscinodiscuskolbei between Cores 119-736A-18X and 119-736A-20H definesthe base of the C. elliptopora/A. ingens Zone. This zonal inter-val is dominated by N. kerguelensis, T. lentigenosa, and A. in-gens.

Cores 119-736A-15H through 119-736A-18X (no core was re-covered from 119-736A-19X) are assigned to the Rhizosoleniabarboi/Nitzschia kerguelensis Zone, which is defined as the in-terval from the last occurrence of C. kolbei to the last occur-rence of R. barboi. This interval is characterized by N. ker-guelensis, R. barboi, A. ingens, T. lentigenosa, Thalassiosiragracilis, and Thalassionema sp.

The last occurrence of C. kolbei in Sample 119-736A-20H-CCand the last occurrence of Coscinodiscus vulnificus in Sample119-736A-25X-CC allow Cores 119-736A-20H through 119-736A-24X to be placed into the Coscinodiscus kolbei/Rhizosoleniabarboi Zone. A. ingens, C. kolbei, Nitzschia angulata, N. ker-guelensis, Thalassionema sp., and T. lentigenosa dominate thediatom assemblage in this interval.

The last occurrence of C. vulnificus in Sample 119-736A-25X-CC, the last common occurrence of C. insignis betweenSamples 119-736A-27X and 119-736A-29X-CC, and the last com-mon occurrence of TV. weaveri in Sample 119-736C-7R-CC indi-cate that Cores 119-736A-25X through 119-736A-30X and 119-736C-2R through 119-736C-6R are equivalent to the Coscino-discus vulnificus and Cosmiodiscus insignis Zones. The boundarybetween these zones is tentatively placed between Cores 119-736A-27X and 119-736A-29X and between Cores 119-736C-6Rand 119-736C-7R, respectively. C. insignis, N. angulata, andThalassionema sp. typify this interval.

Core 119-736C-8R is assigned to the Nitzschia weaveri Zonebased on the last common occurrence of N. weaveri in Sample119-736C-7R-CC and the last common occurrence of N. inter-frigidiaria in Sample 119-736C-8R-CC. Actinocyclus karsteni,N. weaveri, C. kolbei, C. insignis, Paralia sulcata, and Thalas-sionema sp. characterize the assemblage in this sample.

The sporadic occurrence of C. vulnificus in the lower part ofits stratigraphic range requires that the N. interfrigidiaria/C.vulnificus Zone be combined with the Nitzschia interfrigidariaZone. The base of this zonal interval is defined as the first oc-currence of TV. weaveri. Cores 119-736C-9R through 119-736C-18Rare assigned to this combined zone based on the occurrence ofTV. interfrigidiaria, Thalassiosira lentigenosa van obovatus, TV.angulata, TV. weaveri, T. gracilis, and rare specimens of Nitz-schia praeinterfrigidaria. Based on the diatom stratigraphy, thebase of Hole 736C is younger than 3.8 Ma.

Radiolarians

Holes 736A and 736BRadiolarians are generally common and well preserved

throughout the cores of Holes 736A and 736B. They are few orrare in Cores 119-736A-1H, 119-736A-3H, 119-736A-4H, 119-736A-20H, and 119-736A-23X through 119-736A-26X. No bar-ren samples were found. Sediments range in age from Quater-nary to late early Pliocene.

134

SITE 736

QuaternaryCompared to the pelagic faunas described in subantarctic

sediments (Chen, 1975; Weaver, 1983), the Pleistocene radiolar-ian assemblage from Holes 736A and 736B contains very fewspecies (15 to 20). The absence of many stratigraphic markerssuch as Stylatractus universus, Saturnalis circularis, and Phor-mostichoartus pitomorphus, which appear to be deep-living pe-lagic radiolarian species, reflects eutrophic conditions. This as-sociation of radiolarians and the great abundance of diatoms isindicative of local upwelling conditions. The subantarctic zona-tion cannot be used, and Zones NR1 through NR4 are describedas the "Antarctissa denticulata Interval." Sample 119-736A-1H-CC contains a typical upper Quaternary Antarctic radiolar-ian assemblage that can be assigned to the NR1 Zone by compari-son with previously studied Kerguelen Plateau material (Caulet,1986). Common species include Antarctissa denticulata, Spongo-trochus glacialis, Theocalyptra bicornis, Theocalyptra davisi-ana, Saccospyris antarctica, and Triceraspyris antarctica. Sam-ple 119-736A-7H-CC contains Pterocanium trilobum and fallswithin the upper part of the NR3-4 Zone (= S. universus Zone;Weaver, 1983). The last appearance of Antarctissa ewingi occursin Sample 119-736A-8H-1, 53-55 cm.

Pliocene

The last occurrence of Clathrocyclas bicornis in Sample 119-736A-20H-1, 63-65 cm, places this sample in Zone NR5. Radio-larians are rare and apparently winnowed in Sample 119-736A-20H-CC. Sample 119-736A-21H-CC contains a typical late Pli-ocene age fauna with C. bicornis, Antarctissa ewingi, T.davisiana, and very few A. denticulata. This assemblage is foundin Samples 119-736A-21H-CC through 119-736A-27X-CC, andthe interval falls within the upper Pliocene Zone NR5. First oc-currence of Lithelius nautiloides is observed in Sample 119-736A-20H-CC. The first occurrence of T. davisiana is in Sample 119-736A-23X-1, 35-37 cm. This datum occurs within the upperGauss Chronozone and has an estimated age between 2.6 and2.8 Ma (Weaver, 1983).

Samples 119-736A-29X-CC and 119-736A-30X-CC are up-per lower Pliocene. The last occurrences of Desmospyris spon-giosa and Pseudocubus vema in Sample 119-736A-29X-1, 53-55 cm, mark the top of the NR6 Zone. The radiolarian assem-blage is well preserved, but neither warm water- nor deep-waterspecies are reported.

Hole 736C

Radiolarians are well preserved in all cores from Hole 736C.Sample 119-736C-2R-CC contains a Quaternary assemblage thatcan be assigned to the "A. denticulata Interval." Samples 119-736C-4R-CC through 119-736C-18R-CC are upper lower Plio-cene, and can be assigned to the NR6 Zone. The last occurrenceof Theocorys redondoensis occurs in Sample 119-736C-11R-CC.This species is common throughout the hole.

PalynologyTwenty-six core-catcher samples from Hole 736A (Cores 119-

736A-1H through 119-736A-16H, 119-736A-18X, 119-736A-20Hthrough 119-736A-26X, 119-736A-28X, and 119-736A-30X) wereprocessed for palynomorphs. Only one species of dinoflagellatecyst, Tectacodinium pellitum, was recorded. This form, whichoccurred only in very low abundance in five core-catcher sam-ples (Cores 119-736A-2H through 119-736A-4H, 119-736A-6H,and 119-736A-14H) has a long stratigraphic range (Paleocene-Pleistocene). Its first appearance downhole in the second coreof Hole 736A is probably consistent with a Pleistocene age forthis part of the succession.

Other OrganismsAn arm of a live ophiuroid (R. Hessler, pers. comm., 1988)

was found in Sample 119-736A-1H-1, 80 cm. Sporadic molluskshells (mostly scaphopods) were preserved in several cores.

PALEOMAGNETICSThree holes were drilled at Site 736. The sediments from

Holes 736A and 736B were collected using the APC coupled tothe Multishot core-orientation tool. Thus, Cores 119-736A-5Hthrough 119-736A-21H and 119-736B-1H through 119-736B-3Hwere oriented and are potentially important for paleomagneticstratigraphy. The Multishot orientations as well as the magneticdeclination correction (-52°W) were used to reorient sampledeclinations into the corrected Earth's magnetic field coordi-nates. Results for Hole 736A are shown in Figure 7; however,because of the unstable natural remanent magnetization (NRM),the results cannot be used to define the record of the polarityhistory here. Future shorebased demagnetization experimentsare necessary.

Because of the lack of liquid helium aboard ship at the timeof drilling, the cryogenic magnetometer could not be used tomeasure whole cores. Instead, discrete paleomagnetic sampleswere collected and measured on the Molspin spinner magneto-meter. We collected 287 samples from Hole 736A, 35 orientedsamples from Hole 736B, and 31 samples from Hole 736C (ro-tary coring). In washing down to the desired drilling depths,however, the muds from Hole 736C were liquified, causing de-formation of the samples. Thus, only limited samples were col-lected from this hole.

The NRM directions and intensities from these samples areplotted stratigraphically in Figures 7 through 9. All of the sam-ples collected from Site 736 are green diatomaceous oozes, withmagnetic intensities that are quite low (90 to 0.05 mA/m). Thelow magnetic intensities reflect the reduced magnetic minerals inthe sediments. The results of the paleomagnetic measurementsare extremely scattered; therefore, polarity designations cannotreliably be made on the basis of these data.

Limited shipboard three-axis alternating field (AF) demag-netization was carried out on a pilot set of representative sam-ples from the cores. A broad spectrum of stability behavior canbe observed in the demagnetization studies. Only one samplestudied displayed moderate magnetic stability (see Fig. 10). Theresults of the demagnetization of Sample 119-736A-9H-2, 38-40cm, show that after demagnetization at 2 mT, the sample direc-tion stabilized with a reversed direction and moderate paleolati-tude (positive inclination) at demagnetization levels between 4and 20 mT. An orthogonal plot of the pilot sample data showsthat a strong secondary component of magnetization affectsmost of the samples. Usually much of this component can be re-moved during the first demagnetization step. Sample 119-736A-6H-2, 85-87 cm, lost 80% of the normalized intensity after asingle demagnetization step at 2 mT. Other samples (e.g., Sam-ples 119-736A-6H-3, 110-112 cm, and 119-736A-9H-2, 69-71cm) display higher magnetic coercivities but fail to stabilize indirection with demagnetization in fields up to 30 mT.

The results of the initial shipboard magnetic measurementsof the sediments from Site 736 indicate that the sediments fromHoles 736A, 736B, and 736C are characterized by magnetic car-riers that are not stably magnetized. The sediments are weaklymagnetized, making initial measurements difficult. Further de-tailed demagnetization in a low-ambient magnetic field environ-ment may succeed in identifying some samples with stable mag-netization that could be used to reliably determine magnetic po-larity. However, the initial investigations completed in the locallystrong magnetic fields of the drillship were not successful in

135

SITE 736

50

100

8-

150

200

250

,%

V . •'

.'?:.*

90 180 270 360-90 -30 30 90Declination (degrees) Inclination (degrees)

O~1 1 10 102 103 104

Intensity (mA/m)

Figure 7. Stratigraphic plot of declination, inclination, and intensity for samples from Hole 736A. NRM val-ues are plotted as circles, and data for samples demagnetized using an alternating field (peak value 30 mT) areplotted as squares.

u

25

50

75

1 1 ' 4

—

—

. i .

r——

. i .

•

0 90 180 270 360-90 -30 30 90 10"2 1 102

Declination (degrees) Inclination (degrees) intensity (rnA/m)

Figure 8. Stratigraphic plot of declination, inclination, and intensity for samples from Hole 736B.

136

SITE 736

200

225

« 250

275

300

325

^

• •_ •

-20 90 180 270 360 -90 -30 30 90 10Declination (degrees) Inclination (degrees) Intensity (mA/m)

Figure 9. Stratigraphic plot of declination, inclination, and intensity for samples from Hole 736C.

10*

identifying the stable components of magnetization or the mag-netic polarity sequence.

Magnetic SusceptibilityAll recovered sections of Hole 736A were measured using the

Bartington Instruments M.S.I susceptibility meter at the 0.1sensitivity and low-frequency (0.47-kHz) settings (Fig. 11). Thecores were measured at 20-cm intervals. The volume susceptibil-ity varies between 0.2 × 10~6 and 889.3 × 10~6 cgs units. TheNRM intensity and susceptibility values in Figure 11 have astrong correlation. The frequent spikes in the susceptibility mea-surements are probably the result of rust within the cores.

SEDIMENTATION RATESBiostratigraphic control at Site 736 relies on both diatom and

radiolarian stratigraphies ("Biostratigraphy" section, this chap-ter). The stratigraphic constraints used to construct the sedi-mentation-accumulation rate curve are summarized in Table 4.With the exception of the first occurrence of the radiolarian Cy-cladophora davisiana, all of the events fall on a dog-legged lin-ear plot. Because the first occurrence of C. davisiana suggestsan age older than that suggested by the other events, we assumethat the range of C. davisiana is ecologically controlled at thissite.

Two main segments characterize the sediment-accumulationrate curve shown in Figure 12.

1. Between 0 and about 80 mbsf, the mean sedimentationrate is estimated to be 54 m/m.y. This rate is based on the lastoccurrence of the diatom Actinocyclus ingens, assuming a con-tinuous stratigraphic sequence and an age of 0 Ma for the top-most sediment.

2. Between about 80 and 370 mbsf, the numerous diatomevents allow the calculation of a mean sedimentation rate ofabout 140 m/m.y. The change in the sedimentation rate is placedat 80 mbsf; however, such a placement is arbitrary because it isbased on one biostratigraphic event and assumes stratigraphiccontinuity. The base of the hole has an extrapolated age of3.5 Ma (latest early Pliocene).

INORGANIC GEOCHEMISTRYA total of 14 whole-round 5-cm-long minicores were ob-

tained from Holes 736A and 736C for the purpose of intersti-

tial-water chemical analyses (see Table 5). All of the sedimentintervals sampled for interstitial waters are composed of diato-maceous ooze with varying amounts of volcanogenic material(see "Lithostratigraphy and Sedimentology" section, this chap-ter). Most of the minicores contained <5% calcium carbonateand < 1% organic carbon. The porosities and water contents ofthe samples are high, ranging from around 50% to 90% andfrom 40% to 80%, respectively (see "Physical Properties" sec-tion, this chapter). The four interstitial-water samples taken fromHole 736C are from RCB cores, and as these sediments are un-consolidated, RCB coring resulted in a great deal of sedimentdisturbance; the interstitial waters from Hole 736C are undoubt-edly mixtures of drilling fluids and primary pore fluids. Thesesamples are included in the Site 736 data set and constitute thefour deepest data points in the following figures. These data willnot be considered in this report.

The high sedimentation rates, shallow water depth, and highcontent of biogenic sediment encountered at Site 736 suggestthat these sediments might be good high-latitude, open-oceananalogs of some hemipelagic sequences deposited under highlyproductive surface waters along continental margins (e.g., Perumargin, Southern California borderland, Gulf of California,etc.). The interstitial-water chemistry at Site 736 is, however,quite different than that encountered in other areas of high bio-logical productivity (Gieskes et al., 1982). No major redoxboundaries were identified from the interstitial-water chemicalanalyses at Site 736. Little, if any, variation as a function ofdepth below the seafloor occurs in salinity, chloride, calcium,magnesium, pH, alkalinity, and sulfate in Hole 736A. There isno evidence for significant levels of sulfate reduction within thesediment column at Site 736 based on the interstitial-water datafrom Hole 736A. The maintenance of sulfate concentrationsnear standard seawater values (28.9 mmol/L for the IAPSOstandard) to 200 mbsf requires the maintenance or continual re-plenishment of dissolved oxygen concentrations during sedimentburial.

Phosphate, silica, and ammonium are the only chemical spe-cies that show significant variations in their interstitial-waterconcentrations as a function of depth in Hole 736A. Dissolvedphosphate concentrations are about five times greater than thosein average seawater near the sediment/water interface and de-crease with depth whereas ammonium concentrations increaseas a function of depth. These trends are indicative of microbial

137

SITE 736

(0 to 25 mT>Scale: 3.00 mA/m per div

E. up

(0 to 15 mT)Scale: 0.90 mA/m per div

E. up

W, down W. down

(0 to 30 mT)Scale: 0.90 mA/m per div

E, up

(0 to 20 mT)Scale.- 7.00 mA/m per div

ε, up

• Horizontal+ vertical

1 s

W, down W, down

Figure 10. Orthogonal plots of sample demagnetization vectors for pilot samples from Hole 736A. A. Sample 119-736A-9H-2, 69-71 cm. B.Sample 119-736A-6H-3, 110-112 cm. C. Sample 119-736A-6H-2, 85-87 cm. D. Sample 119-736A-9H-2, 38-40 cm.

degradation of organic matter within the sediment column, butnot at sufficiently high levels to exhaust the dissolved oxygen inthe interstitial waters. The highest phosphate and ammoniumconcentrations measured at Site 736 are quite low in comparisonto those encountered in sediments deposited below highly pro-ductive surface waters (e.g., Peru margin; Suess, von Huene, etal., 1988). Much of the organic matter associated with these bi-ogenic sediments may have been lost, or rendered unreadable,prior to deposition at Site 736.

Methods

All sediment minicores were squeezed for interstitial watersimmediately after retrieval from the seafloor. All of the analyseswere completed within five days of coring. Each of the sampleswas analyzed for salinity, chloride, calcium, magnesium, sul-fate, silica, pH, alkalinity, sulfate, ammonium, and phosphate.Details of the analytical methods used can be found in the "Ex-planatory Notes" chapter.

138

SITE 736

Volume magnetic susceptibility (1O~6 cgs)- 2 0 0 200 600 1000

100 — —

150 — —

200 — —

250lO~1 1 1 0 1 0 2 103 104

Intensity <mA/m)

Figure 11. Stratigraphic plot of magnetic susceptibility (plotted as linetrend) with NRM discrete sample intensity measurements (solid circles)superimposed. There is good agreement between susceptibility and mag-netic intensity.

Table 4. Biostratigraphic markers and their esti-mated ages, Site 736.

EventaDepth(mbsf)

Age(Ma)

L Actinocyclus ingensL Rhizosolenia barboiL Coscinodiscus kolbeiT Clathrocyclas bicornisL Coscinodiscus vulnißcusLC Cosmiodiscus insignisLC Nitzschia weaveriLC Nitzschia interfrigidariaF Cycladophora davisianaF N. weaveriF Pseudocubus vema

28.0-33.098.5-108.0

136.8-155.9127.1-147.0194.3-203.9232.9-242.6255.9-264.7264.7-274.4175.0-194.3>371.O>371.0

0.621.581.891.6-1.82.222.492.642.802.6-2.8<3.88<3.9

L = last occurrence; LC = last common occurrence;F = first occurrence.

100 —

200 —

s•

300 —

400

Figure 12. Sediment-accumulation rate curve for Site 736. Refer to Table4 for events.

Results

Salinity and Chloride

Salinity and chloride concentrations remain near standardseawater levels (IAPSO chlorinity is 559 mmol/L) and show lit-tle variation throughout Hole 736A (0 to 252.3 mbsf) (Fig. 13).

Calcium and Magnesium

Magnesium and calcium concentrations do not change sig-nificantly as a function of depth in Hole 736A (Fig. 13). Theconcentrations of calcium and magnesium in all of the Site 736interstitial-water samples are close to standard seawater values(10.55 and 54.0 mmol/L, respectively, in IAPSO). The low mag-nesium to calcium ratios (Fig. 13) in all of the samples rule outthe possibility of significant authigenic dolomite formation.

Silica

The sediments cored in Hole 736A are rich in biogenic silica(opal-A), resulting in high dissolved silica concentrations in allof the analyzed interstitial-water samples (Fig. 13). Average sea-water (about 89 µmol/L silicic acid; Stumm and Morgan, 1970)is greatly undersaturated with respect to amorphous silica(Krauskopf, 1956). Dissolved silica concentrations in Hole 736Aincrease with depth to 1130 µmol/L at 234.35 mbsf. Thus, sig-nificant silica dissolution is occurring in Hole 736A.

pH, Alkalinity, and SulfateAfter increases in the first 23 m of the sediment section, pH,

alkalinity, and sulfate all show little variation with depth to thebottom of Hole 736A (Fig. 13). The increase in sulfate concen-tration downward from Samples 119-736A-1H-4, 145-150 cm,to 119-736A-3H-3, 145-150 cm, is suspect because it is difficult

139

SITE 736

Table 5. Interstitial-water geochemical data for Site 736.

Core, section,interval (cm)

119-736A-

1H-4, 145-1503H-3, 145-1506H-2, 145-1509H-2, 145-15013H-4, 145-15016H-4, 145-15020H-4, 145-15023X-2, 145-15026X-3, 145-15029X-1, 145-150

119-736C-

8R-1, 145-15011R-3, 145-15014R-2, 145-15018R-2, 145-150

Depth(mbsf)

5.9522.9540.9555.9585.45

113.95152.35177.95208.35234.35

266.15298.15325.75364.25

Salinity(g/kg)

35353535363636363636

36343534

Chloride(mmol/L)

556557562559556560559559562559

560554545554

Calcium(mmol/L)

10.0810.2410.4610.3510.5510.129.989.939.929.84

10.1510.2010.059.62

Magnesium(mmol/L)

51.651.754.053.154.654.053.552.352.553.4

53.452.452.052.2

Mg2 + /Ca2 +

5.125.055.165.135.185.335.365.265.305.43

5.265.145.185.42

Silica(µmol/L)

725767922836862984999990

10011130

1428193918971943

PH

7.37.77.87.67.57.67.77.67.57.5

7.57.37.27.2

Alkalinity(mmol/L)

3.325.004.904.584.204.174.494.264.364.23

4.774.313.993.60

Sulfate(mmol/L)

17.5424.7624.4525.6622.9525.3616.0324.7625.3625.66

24.7624.4524.7625.06

Ammonium(mmol/L)

0.140.280.250.250.310.330.380.410.390.39

0.390.400.380.39

Phosphate(µmol/L)

16.6913.0810.126.385.614.323.802.902.262.26

3.685.484.844.96

to formulate a plausible explanation for sulfate depletion nearthe sediment/water interface and regeneration with increasingdepth. A slight "hydrogen sulfide" odor was noticed in Sample119-736A-16H-4, 145-150 cm, but no significant depletion insulfate concentrations is observable in the data. The presence ofhigh sulfate concentrations near the bottom of Hole 736A indi-cates either low levels of biological oxygen consumption and/orrapid diffusion of dissolved oxygen from the seafloor to rela-tively deep levels below the sediment/water interface.

Alkalinity values are approximately twice the average valueof seawater (Fig. 13), which, considering the constancy of thepH data, indicates the addition of bicarbonate to the pore-watersystem, presumably by means of microbial degradation of or-ganic matter.

Ammonium and Phosphate

Dissolved phosphate concentrations drop from a high of16.69 µmol/L near the sediment/water interface to near averageseawater values (about 5 µmol/L; Stumm and Morgan, 1970) atapproximately 60 mbsf. This trend indicates that significantamounts of phosphate are produced near the sediment/waterinterface and that either phosphatic minerals are forming in thetop 100 mbsf and/or phosphate is being lost to the water col-umn.

Interstitial-water ammonium concentrations are significantlyhigher than average seawater values and increase with depth inHole 736A (Fig. 13). The ammonium data imply that microbialdegradation of organic matter continues to the bottom of Hole736A, apparently under aerobic conditions.

ORGANIC GEOCHEMISTRYRoutine organic geochemical analyses were run on the first

core from each hole and every third core where recovery permit-ted. The mud-line cores from Holes 736A and 736B were ana-lyzed for comparison. The organic geochemistry program forSite 736 included (1) monitoring of hydrocarbon gases, (2) de-termining total organic carbon, and (3) characterizing organicmatter. The methods and instruments used are described in the"Explanatory Notes" chapter.

Hydrocarbon Gases

Only the small-vial headspace procedure was used becauseno gas pockets were found at Site 736. Analysis was performedfor approximately every 30 m of core to determine the concen-trations of methane (Q), ethane (C2), and propane (C3) dissolvedin interstitial water and adsorbed on particles. C2 or C3 was notdetected in any samples from Site 736. Core 119-736A-1H has

245 ppm (by volume) methane. All of the rest of the sampleswere at or near the 3-ppm mean level for laboratory air.

The absence of gas suggests that methanogenic bacteria wereinactive during sedimentation at this site. Methanogens are an-aerobes that are inhibited by sulfate ions (Claypool and Kven-volden, 1983). Interstitial-water analysis showed that sulfate wasrelatively constant at approximately 24 mmol/L (see "InorganicGeochemistry" section). This level was sufficient to inhibit theanaerobes.

Carbon AnalysisTotal carbon and inorganic carbon were measured on the

squeeze-cake samples left over from the interstitial-water study(see "Inorganic Geochemistry" section). The difference is thetotal organic carbon (TOC) (see "Explanatory Notes" chapter).Inorganic carbon was measured at several intervals from recov-ered core-catcher samples (Table 6 and Figs. 14 and 15). The lowinorganic carbon at Site 736 coincides with the absence of cal-careous organisms.

Organic carbon is high to moderate for diatomaceous sedi-ments. The top 200 m of sediment has about 1% TOC. Below200 mbsf, the values fall to 0.5% and steadily decreasedownhole.

Rock-Eval PyrolysisRock-Eval pyrolysis was also performed on the squeeze cakes

left over from interstitial-water studies. The parameters mea-sured and calculated are listed in Table 7 and shown in Figures16-18.

BIOLOGY AND OCEANOGRAPHY

Phytoplankton: Living, Holocene, and PleistoceneDiatoms

Study of living species can add to the knowledge of fossil as-semblages by clarifying which components of an existing eco-system have been preserved in a death assemblage and by show-ing which life history stages of one species have been fossilized.On the other hand, the study of fossil assemblages can give di-rect evidence of evolutionary patterns for the systematist, plusprovide a record of cyclic environmental phenomena. This in-terdisciplinary study considered both assemblies.

A small (10-cm mouth diameter) net with 20-µm mesh wasdeployed for living phytoplankton for 5 to 7 min per day at1000 hr off the stern of the drillship in the effluent from thethrusters. Surface temperatures of 3.5°, 3.5°, 3.7°, and 4.0°Cwere recorded during the four sampling periods. Winds were 37,

140

100

200

300

400

\

1

1 ' I ' V ~ 1 i y i

i • i

~

i • i

w • i ^

30 35 40 500 550 600 0 12 0 30 60 0Salinity Chloride Calcium Magnesium Mg2+/Ca2*

8 0 1000 2000 6Silica

(µmol/L)

7 8pH

0 2 4 6 0 10 20 30 0 0.2 0.4Alkalinity Sulfate Ammonium

Ig/kg) (mmol/L) (mmoi/L) (mmol/L) (µmol/L) (mmol/L) (mmol/L) (mmoi/L)

Figure 13. Salinity, chloride, calcium, magnesium, Mg 2 + /Ca 2 + ratio, silica, pH, alkalinity, sulfate, ammonium, and phosphate interstitial-water profiles, Site 736.

0 10 20Phosphate

(µmol/L)

SITE 736

Table 6. Total carbon, inorganic carbon, organic carbon, and car-bonate carbon, Site 736.

Sample(interval in cm)

736A-1H-4, 145-150736B-1H-CC, 0-1736B-1H-CC, 5-6736A-1H-CC, 0-1736A-2H-CC, 0-1736B-2H-CC, 0-1736A-3H-3, 145-150736A-3H-CC, 0-1736B-3H-CC, 0-1736A-4H-CC, 0-1736A-5H-CC, 0-1736A-6H-2, 145-150736A-6H-CC, 0-1736A-7H-CC, 0-1736A-8H-CC, 0-1736A-9H-2, 145-150736A-9H-CC, 0-1736A-10H-CC, 0-1736A-11H-CC, 0-1736A-12H-CC, 0-1736A-13H-4, 145-150736A-13H-CC, 0-1736A-14H-CC, 0-1736A-15H-CC, 0-1736A-16H-4, 145-150736A-16H-CC, 0-1736A-17X-CC, 0-1736A-18X-CC, 0-1736A-20H-4, 145-150736A-20H-CC, 0-1736A-21H-CC, 0-1736A-22X-CC, 0-1736A-23X-2, 145-150736A-23X-CC, 0-1736A-24X-CC, 0-1736A-25X-CC, 0-1736A-26X-3, 145-150736A-26X-CC, 0-1736C-2R-CC, 0-1736A-27-CC, 0-1736C-4R-CC, 0-1736A-29X-1, 145-150736A-29X-CC, 0-1736C-5R-CC, 0-1736A-30X-CC, 0-1736C-6R-CC, 0-1736C-7R-CC, 0-1736C-8R-1, 145-150736C-8R-CC, 0-1736C-9R-4, 75-77736C-9R-CC, 0-1736C-10R-CC, 0-1736C-11R-3, 145-150736C-11R-CC, 0-1736C-12R-CC, 0-1736C-13R-CC, 0-1736C-14R-2, 145-150736C-14R-CC, 0-1736C-15R-CC, 0-1736C-16R-CC, 0-1736C-17R-CC, 0-1736C-18R-2, 145-150736C-18R-CC, 0-1

Depth(mbsf)

5.958.198.248.83

14.1318.0722.9524.6327.3732.6138.0540.9542.2747.9253.0955.9558.7666.1473.0079.1685.4589.2098.80

107.81113.95117.50120.71127.57152.35154.60164.69165.80177.95178.20186.47194.40208.35209.40209.87214.48226.18234.35235.84241.92242.60245.40257.26266.15267.49279.65284.21293.60298.15300.03303.91322.60325.75327.22342.08347.45353.36364.25366.07

Totalcarbon

(%)

1.10

0.91

0.76

1.49

1.22

0.61

1.03

1.28

1.13

0.52

1.24

0.36

1.15

0.49

Inorganiccarbon

(%)

0.480.790.230.110.430.210.040.180.090.040.020.140.121.270.040.060.640.061.170.600.320.310.270.010.040.170.090.150.010.020.730.070.120.200.080.250.030.020.010.370.500.010.610.680.550.900.140.840.840.040.200.040.040.050.040.030.800.230.630.380.100.200.14

Organiccarbon

(%)

0.62

0.87

0.62

1.43

0.90

0.57

1.02

1.16

1.10

0.51

0.40

0.32

0.35

0.29

CaCO3

(%)

4.06.61.90.93.61.80.31.50.80.30.21.21.0

10.60.30.55.30.59.85.02.72.62.30.10.31.40.81.30.10.26.10.61.01.70.72.10.30.20.13.14.20.15.15.74.67.51.27.07.00.31.70.30.30.40.30.36.71.95.33.20.81.71.2

27, 28, and 21 kt, respectively. The subantarctic summer florawas diverse, with long chains of vegetative cells and no restingspores noted (Table 8). The flora was increasing daily over thefour-day period, as indicated by increasing phytoplankton abun-dance in the net.

In addition, dinoflagellates, a cold-water silicoflagellate (Di-stephanus speculum), gelatinous colonies of the prymnesiophytePhaeocystis, and an acantharian were noted in the living com-munity. Of particular interest are copepod fecal pellets, crowded

100 —

200 —

3 0 0

4 0 0 . I . I .

i

. I . I .0.2 0.6 1 JO 1 .4 0.2 0.6 1 .0 1 .4

total o r g a n i c

Figure 14. Total carbon (CtoUd)and organic carbon (Corganic) from squeeze-cake and core-catcher sediment samples, Site 736.

100 -s

2 0 0

8-

3 0 0

0 2 4 6 8 1 0 0 0.4 0.8 1.24 0 0

Figure 15. Percent calcium carbonate and inorganic carbon from squeeze-cake and core-catcher sediment samples, Site 736.

142

SITE 736

Table 7. Rock-Eval summary, Site 736.

Core, section,interval (cm)

Depth(mbsf)

Weight(g)

SI(mg HC/g)

S2(mg HC/g)

S3(mg CO2/g)

Productivityindex S2/S3

Pyrolysizedcarbon

(0.08 [SI + S2])TOC(wt%)

Hydrogenindex

(mg HC/g Corg)

Oxygenindex

(mg CO2 /g Corg)

119-736A-

1H-4, 145-1503H-3, 145-1506H-2, 145-1509H-2, 145-15013H-4, 145-15016H-4, 145-15020H-4, 145-15023X-2, 145-15026X-3, 145-15029X-1, 145-150

119-736C-

8R-1, 145-150

5.9522.9540.9555.9585.45113.95152.35177.95208.35234.35

266.15

98.396.397.999.797.3100.496.596.3106.978.8

102.0

398400458393404395401403390397

400

0.360.740.400.550.760.430.560.960.750.29

0.27

0.511.180.691.172.001.281.642.851.411.14

0.32

0.951.051.970.450.890.191.170.790.730.31

0.85

0.420.390.370.320.280.250.250.250.350.20

0.47

0.531.120.352.602.246.731.403.601.933.67

0.37

0.070.160.090.140.230.140.180.310.180.11

0.04

0.620.870.621.430.900.571.021.161.100.51

0.40

8213611182

222225161246128224

80

153121318319933115686661

213

50

100

8-150

2 0 0

250

, I ,» l , I •

i T • r r r r i

T I • I • I • I • I • L i • i ,0.2 0.6 1 .0 0 1 2 3 0 0.5 1 .0 1 .5 2.0 0 1 2 3 4 5 6 7 380 400 420 440460 0.4 0.8 1 .2 1 .6

SI (mq HC/g) S2 (mg HC/g) S3 (mg C02/g) S2/S3 Tmax (°C) T O C ( w t % )

Figure 16. Comparison of Rock-Eval parameters SI, S2, S3, S2/S3, Tmax, and TOC from Site 736. TOC was determined as the difference betweentotal carbon and inorganic carbon.

with pennate diatom species that sink to the sediment more rap-idly than individual cells. In most cases, a few pennate diatomsin the pellets were still pigmented and appeared alive. In a fewcases, the pellets were crowded with reproducing cells.

Sediment was analyzed for diatom species from the mud lineand from core catchers from Cores 119-736A-1H through 119-736A-17X and 119-736B-1H through 119-736B-3H. Additionalspecies not found in the living plankton are listed in Table 9. Asediment assemblage has been averaged over time (depending onsedimentation rate, in this case estimated at 80 m/m.y.) andover space (depending on currents) in contrast to the short time/space scale represented by the samples of the living phytoplank-

ton. Thus, some species found in the fossil assemblage analyzedfrom the Holocene and Pleistocene sediments may grow in otherseasons and were not found in these summer samples.

Easily dominating both the living plankton and the assem-blage in the sediment was the pennate diatom genus Nitzschia.In the sediment, the dominant species was Nitzschia kerguelen-sis from the group of species of the genus with ribbon-shapedcolonies; in the plankton, the dominant form was needle-shapedNitzschia species from the group forming colonies with overlap-ping tips. Also well represented with several species was the cen-tric diatom genus Thalassiosira. Chaetoceros was well representedin the plankton; in the sediment, Chaetoceros neglectus resting

143

SITE 736

50

100

200

250

*• I • I • I I • I I •0 0.1 0.2 0.3 0.2 0.3 0.4 0.5 100 150 200 250 0 100 200 300Pyrolysized carbon Productivity index Hydrogen index Oxygen index

Figure 17. Comparison of Rock-Eval parameters pyrolysized carbon, productivity index, hydrogen in-dex, and oxygen index, Site 736.

spores were present, in addition to a few resting spores from anunknown species of Chaetoceros and from Chaetoceros radi-cans.

Evidently frustules from only certain selected species bothreach the ocean floor and are preserved in the fossil assemblage.The proportion of empty cells has not been found to increaseappreciably with depth in the euphotic zone of the Weddell Sea(Fryxell and Kendrick, 1988). As the numbers of living cellsgenerally decrease with depth, the numbers of empty (dead)cells also decrease. Apparently no steady rain of cells reachesthe sediment. The heavily silicified resting spores or vegetativethecae seen in the sediment could well be deposited in pulses fol-lowing periods of abundant growth. Fecal pellets sink rapidlyout of the water column and could also be a major contributorto sediments.

PHYSICAL PROPERTIESThe objectives of the physical-properties program at Site 736

were (1) to add to the limited data base of geotechnical andother physical properties of high-latitude sediments (the site issouth of the Antarctic Convergence), particularly those con-taining abundant biogenic silica, and (2) to provide referencedata for the geophysical and stratigraphic evaluation of the sedi-mentary section at the site.

Physical properties evaluated were (1) index properties, (2) un-drained shear strength, (3) thermal conductivity, and (4) com-pressional-wave velocity. General techniques and laboratory pro-cedures are discussed in the "Explanatory Notes" chapter.

Three holes (736A, 736B, and 736C) were drilled at the sitewith APC, XCB, and RCB. The degree of core disturbance, asobserved both visually and in the physical properties, increasedslightly from APC to XCB cores and significantly in RCB cores.

All physical-properties data, except temperature, are presentedin Figure 19 and Tables 10 through 14. Temperature measure-ments are presented in Figures 20 and 21 and Table 15.

Index PropertiesValues of water content, porosity, bulk density, and grain

density can be subdivided into two geotechnical units, Gl andG2, corresponding to a change from APC/XCB to RCB coringfrom Holes 736A and 736B to Hole 736C. Hence, the division ismainly a function of core disturbance. The upper part of thesedimentary section, above 235 mbsf, shows a slight overall de-crease in water content, whereas porosity, bulk density, and graindensity stay relatively constant, despite a large local scatter (thedata have not been filtered for bad data points). This is some-what anomalous, as porosity would be expected to decrease andbulk density to increase with increasing overburden/decreasingwater content. This anomaly is most likely a consequence of thelithology. The sediment is essentially a diatom ooze throughoutthe entire section ("Lithostratigraphy and Sedimentology" sec-tion). Diatom ooze will maintain a relatively open but strongstructure that prevents large downcore changes. An increase inthe content of volcanic fragments toward the top of the core—fragments that mostly consist of pumice—may also contributeto the observed trends.

Generally, the index properties are correlative (Fig. 19 andTable 10). Decreases in water content correspond to intervals ofdecreasing porosity and increasing bulk density. The positivecorrelation between grain and bulk density is unusual in thattypically no correspondence appears between grain density andindex properties. This correlative relationship is probably a con-sequence of the diatomaceous lithology, which is also the causeof the generally low density values. Biogenic silica has a lowdensity of 2.02 g/cm3 (Barker, Kennett, et al., 1988).

144

SITE 736

1000

800

600

400

200

1

// 'I

—iIf

-1f

I /i /Lj11

i!

i1

11 ^;; ^

i

-

-

-

—

Li1 -

1100 200

Oxygen index300

Figure 18. Hydrogen and oxygen indices obtained from Rock-Eval py-rolysis of squeeze-cake sediment samples from Site 736 plotted on a vanKrevelen-like diagram (Tissot and Welte, 1984).

Table 8. Species found in living plankton, Site736.

* Asteromphalus hookeriAsteromphalus parvulus

&Azpeitia tabularisChaetoceros castracaneiChaetoceros constrictusChaetoceros convolutusChaetoceros curvisetusChaetoceros pendulusCorethron inerme

bDactyliosolen antarcticuscEucampia antarctica (two kinds of spiraling chains)Navicula directaNitzschia closteriumNitzschia heimii

dNitzschia kerguelensisNitzschia turgidulaNitzschia turgiduloides

eOdontella weissßogii vegetative cellsPorosira pseudodenticulataRhizosolenia alata (long, straight chains)Rhizosolenia chuniiRhizosolenia cylindrusRhizosolenia hebetata f. semispina (also bidens)Rhizosolenia simplex

aThalassionema nitzschioidesThalassiosira gravida (gelatinous colonies)

^Thalassiosira lentigenosa^Thalassiosira tumidaThalassiosira maculatal

aThalassiothrix antarcticalTropidoneis antarctica

a Also found in sediment.Also found only in the heavily silicified bands insediment.

c Winter stage found in sediment.Three forms found in sediment.

e Resting spores found in sediment.

The lower part of the section, RCB cored in Hole 736C,shows a return to normal, expected trends of decreasing poros-ity and water content with depth and slightly raised but fairlyconstant values of bulk density. This is rather surprising in viewof the fact that visual examination of the split cores revealed ex-tensive disturbance: bedding deformation due to friction alongcore liners, bedding plane separations, and, in the deepest sec-tions, liquefaction of the recovered material.

Bulk density measured by the GRAPE on whole-round coresections shows the same trends as for the gravimetrically deter-mined values. Relatively high values in the upper 50-60 m aredue mostly to the content of volcanic fragments, which were notpresent in the samples measured gravimetrically. Furthermore,the large scatter toward higher values is caused by gravelly ma-terial that fell into the hole and concentrated in the core tops.The most distinct feature in the GRAPE data is the more con-centrated values of Hole 736C (RCB cores), which again areslightly raised relative to the upper part of the section. In gen-eral, the GRAPE values are approximately 0.05 to 0.1 g/cm3

higher than those measured on discrete samples. The higherGRAPE values probably resulted from measuring whole-roundcores, which include coarser material not included in the smalldiscrete samples.

Uπdrained Shear Strength

The variability of undrained shear-strength values above 100mbsf reflects variations in index properties in this interval andshows an overall trend of slightly increasing values with depth.Both the vane shear and the fall cone measurements show thesame trend and local variations (an exception is the high valuesof cone shear strength at 150-170 mbsf that are not apparent inthe vane shear strength), but the values of the fall cone measure-

Table 9. Species found only in thesediment, Site 736.

Actinocyclus actinochilusActinocyclus ingensAsteromphalus hyalinaAzpeitia endoP.Chaetoceros sp. (resting spore)Cocconeis fasciolataCoscinodiscus elliptoporaCoscinodiscus oculoidesCoscinodiscus spp.Nitzschia angulataNitzschia curtaNitzschia obliquecostataNitzschia panduriformisNitzschia ritscheriNitzschia separandaParalia sulcataPorosira glacialis (resting spore)Rouxia isopolica Schrader (extinct)Thalassiosira ambiguaThalassiosira australisThalassiosira gracilis f. gracilisThalassiosira gracilis f. expectaThalassiosira oliveranaThalassiosira scotia (resting spore)Thalassiosira trifultaThalassiosira "sp. 10" sensu SchraderThalassiosira sp. cf. lentiginosa

ments are generally higher than those of the vane shear (Fig. 19and Tables 11 and 12). The scatter is also larger for the formerdevice. A decrease occurs in the shear-strength trend from 110to 210 mbsf. This interval corresponds to the change from APCto RCB coring with poor recovery and probably reflects sampledisturbance. The high values in cone shear-strength values around

145

Hole I Hole I — Compressional-wave velocity736A 736B j S 1 Th«-m«i

I j i g i i iS• t W a . r S M . , =RAPE^ <Sfityu"% , ir,,r'rHOTi,,.nFr^ ,-„„.,,,«,* T-Crcf-. c.n3? r

> > β 8 porosity i%) (g/cm3) <g/cm3) ( k P a ^ ( m / 8 , (m/s) ( C ) (W/m/ C)S α o α L j i o S o 50 100 1 2 3 1 2 0 40 80 120 1000 2000 1000 2000 0 20 40 60 0 1 2 3

300 —

400

G2

AΔ

Δ

A ΔA

AΔ

A• Δ

A

1 I '

A Δ

A Δ

A Δ

A Δ

A Δ

A Δ

A Δ

A Δ

• Δ

1 I ' I ' I i i I r

à '1 I ' I ' . I ' I

, 1 . 1 . 1 , 1 , 1water• Hole 736A* Hole 736BA Hole 736CPorosityo Hole 736AO Hole 736BΔ Hole 736C

Bulk• Hole 736A+ Hole 736BA Hole 736C

Graino Hole 736AO Hole 736BΔ Hole 736C

Hole 736AHole 736BHole 736C

Vane• Hole 736A* Hole 736BA Hole 736C

Coneo Hole 736AO Hole 736BΔ Hole 736C

• Hole 736A* Hole 736BA Hole 736C

Hole 736AHole 736BHole 736C

aData filtered for values <1 .0 and >2.5 g/cm3 and blocked in 0.2-m averages.bOata filtered for signal strengths <15O m/s and blocked in 0.2-m averages.

Figure 19. Physical-property profiles, Site 736.

SITE 736

Table 10. Index properties of water content, porosity, bulk density, dry-bulk density, and grain density, Site 736.

Core, section,interval (cm)

119-736A-

1H-2, 751H-4, 1011H-6, 1112H-4, 1113H-1, 963H-2, 1003H-4, 584H-1, 984H-2, 984H-3, 915H-3, 956H-3, 837H-2, 687H-3, 938H-1, 1378H-2, 1318H-3, 1069H-1, 849H-3, 2010H-3, 651QH-5, 8611H-2, 8112H-2, 7613H-2, 10413H-6, 8614H-2, 8114H-7, 7115H-1, 7715H-3, 7316H-4, 9317X-1, 9620H-2, 12720H-6, 4021H-1, 8121H-2, 10421H-4, 11121H-6, 9423X-2, 1524X-1, 9026X-2, 8626X-4, 7129X-2, 28

119-736B-

1H-3, 762H-3, 772H-CC, 113H-2, 76

119-736C-

8R-1, 739R-2, 7610R-2, 7911R-3, 8312R-1, 2813R-1, 7514R-2, 8316R-3, 9617R-1, 8318R-2, 73

Depth(mbsf)

2.245.508.6013.7319.4520.9923.5728.9730.4731.9036.9241.8245.1746.9049.3650.8052.0553.8356.1962.6265.8570.8075.7582.0387.8591.3098.7099.26101.57112.59120.45149.16154.29156.70158.43161.48164.33176.64185.49206.25209.10234.67

3.7512.1618.1720.15

265.40276.65286.27297.50303.65313.84325.10345.93352.40363.50

Water(%)

79.5963.8661.8678.3556.9856.7458.2358.4961.1676.3971.5875.3866.1770.8670.7640.8449.6645.2846.6964.1766.1367.1465.8872.7347.6368.8362.9051.4067.2160.1267.5166.9966.0065.0770.2568.4169.5468.4437.2769.3270.4767.98

79.6765.9470.1658.21

57.9764.4253.1158.9164.5968.2254.6849.8840.3439.28

Porosity(%)

87.4680.2278.3183.6674.4575.5276.1775.9378.6384.1681.5085.0979.3580.2483.9163.8668.9164.8366.2679.3581.4880.7380.5185.3569.7481.6776.3370.5483.1577.0880.3881.6581.3175.8482.6979.0181.8881.3060.6882.9081.3181.51

88.4780.1884.8376.41

74.8778.7873.7873.3878.2182.4070.8461.7753.7053.22

Bulkdensity(g/cm3)

1.111.331.321.081.401.381.391.351.371.221.201.201.251.20.22.711.511.55.491.331.321.321.371.24.56.26.36.49.29.32.23.21.28.20.24.201.191.211.661.201.201.18

1.171.371.381.43

.33

.27

.30

.30

.251.391.341.261.281.36

Dry-bulkdensity(g/cm3)

0.230.480.510.230.600.600.580.560.530.290.340.300.420.350.361.010.760.850.800.480.450.430.470.340.820.390.500.720.420.530.400.400.440.420.370.380.360.381.040.370.350.38

0.240.470.410.60

0.560.450.610.540.440.440.610.630.770.82

Graindensity(g/cm3)

1.742.302.231.312.212.362.302.252.341.591.721.831.961.642.142.592.262.252.262.152.252.042.142.172.562.011.902.282.412.241.962.192.241.672.011.721.971.992.632.141.802.07

1.912.092.372.34

2.172.052.501.921.962.172.021.621.721.77

150-170 mbsf were measured in two APC cores that were takenwithin the RCB coring session. However, downcore-decreasingshear-strength values have been reported from other siliceoussections (Eldholm, Thiede, et al., 1987); thus, a lithologic causecan not be excluded.

Shear-strength values below about 210 mbsf are very low, re-flecting the disturbance associated with rotary coring in thesesediments. Values of shear strength for Holes 736A and 736Coverlap from about 200 to 225 mbsf. These data suggest that

shear strength, and presumably index properties as well, is littleaffected by XCB recovery (165-242 mbsf, Hole 736A) whereasRCB recovery (207 mbsf to total depth, Hole 736C) results incomplete remolding of the sediment.

Compressional-Wave VelocityCompressional-wave velocities, which were determined with

the Hamilton Frame and P-wave logger, show generally compa-rable but somewhat different values and trends (Fig. 19). Both

148

SITE 736

Table 11. Undrained shear strength fromvane measurements, Site 736.

Table 12. Undrained shear strength fromfall cone measurements, Site 736.

Core, section,interval (cm)

119-736A-

1H-1, 203H-1, 953H-2, 1053H-4, 574H-1, 924H-2, 944H-3, 885H-3, 967H-2, 707H-3, 888H-2, 1288H-3, 1039H-1, 769H-3, 1810H-3, 6710H-5, 8511H-2, 8012H-2, 7513H-2, 10313H-6, 8514H-2, 8014H-7, 7015H-1, 7615H-3, 7016H-4, 9021X-1, 8221X-2, 9921X-4, 10824X-1, 9326X-4, 7327X-1, 5529X-2, 28

119-736B-

1H-3, 752H-3, 762H-CC, 103H-2, 75

119-736C-

2R-2, 805R-1, 1235R-3, 838R-1, 709R-2, 7510R-2, 7711R-3, 8012R-1, 2513R-2, 7414R-2, 8015R-2, 8616R-3, 8317R-1, 8018R-2, 70

Depth(mbsf)

0.2019.4521.0523.5728.9230.4431.8836.9645.2046.8850.7852.0353.7656.1862.6765.8570.8075.7582.0387.8591.3098.7099.26

102.20113.40156.72158.39161.48185.53209.13214.05234.68

3.7512.1618.1720.15

209.20236.93239.53265.40276.65286.27297.50303.65315.34325.10334.76345.83352.40363.50

Undrainedshear strength

(kPa)

6.29.18.77.09.1

15.351.549.7

8.730.014.310.611.69.1

12.89.5

24.25.43.3

11.415.131.032.716.118.611.411.211.414.510.820.715.5

0.61.4

23.46.6

2.11.23.10.43.32.30.85.20.20.20.20.20.20.2

of the instruments measure velocity parallel to bedding. Differ-ences, therefore, are not due to directional anisotropy. P-wave-logger values are characterized by a slight but steady trend to-ward increasing values with depth. No break is apparent in thistrend between Holes 736A and 736C. Because of operationalproblems, Hamilton Frame measurements were not started be-fore 62 mbsf in Hole 736A, and the low values first recordedmay be erroneous (Table 13). However, the data show a distinctincrease with depth to around 200 mbsf. Hamilton Frame databelow 200 mbsf are anomalous and are obviously the result ofsevere core disturbance caused by rotary drilling below this depth.P-wave-logger results seem to be less affected by the disturbance.

Core, section,interval (cm)

119-736A-

1H-2, 651H-4, 1002H-4, 1003H-1, 923H-2, 993H-4, 594H-1, 934H-2, 984H-3, 915H-3, 986H-3, 827H-2, 677H-3, 918H-1, 1398H-2, 1328H-3, 1059H-1, 829H-3, 1610H-3, 6710H-5, 8511H-2, 8012H-2, 7513H-2, 10313H-6, 8514H-2, 8014H-7, 7015H-1, 7615H-3, 7016H-4, 9020H-2, 12820H-6, 3921H-1, 8021H-2, 10321H-4, 11521H-6, 9423X-2, 1024X-1, 9026X-2, 8326X-4, 7127X-1, 6029X-2, 22

119-736B-

1H-3, 752H-3, 762H-CC, 103H-2, 75

119-736C-

2R-2, 835R-1, 1235R-3, 858R-1, 709R-2, 7510R-2, 7711R-3, 8012R-1, 2513R-1, 7414R-2, 8015R-2, 8616R-3, 9317R-1, 80

Depth(mbsf)

2.155.50

13.6319.4220.9923.5928.9330.4831.9136.9841.8245.1746.9149.3950.8252.0553.8256.1662.6765.8570.8075.7582.0387.8591.3098.7099.26

102.20113.40149.18154.29156.70158.43161.55164.34176.60185.50206.23209.11214.10234.62

3.7512.1618.1720.15

209.23236.93239.55265.40276.65286.27297.50303.65313.84325.10334.76345.93352.40

Undrainedshear strength

(kPa)

0.86.1

19.011.013.011.019.015.053.068.012.015.048.016.042.034.026.024.017.018.053.08.9

27.088.042.074.088.048.021.030.0

123.0127.053.074.067.016.022.0

9.039.074.019.0

0.82.7

41.015.0

3.32.34.56.62.74.27.02.52.11.91.30.81.3

However, this may be only apparent, and the consistent trendcould be caused by the partly liquified sediments being com-pacted inside the liner when placed vertically in the GRAPE/P-wave-logger frame. In general, the velocities are low for theentire section. This is most likely a consequence of sedimenttype.

149

SITE 736

Table 13. Compressional-wave velocity deter-mined with the Hamilton Frame, Site 736.

Table 14. Thermal conductivity, Site736.

Core, section,interval (cm)

119-736A-

10H-3, 70-7310H-5, 80-8311H-2, 80-8312H-2, 75-7713H-2, 105-10713H-6, 86-8814H-2, 80-8314H-7, 70-7315H-1, 76-7916H-4, 90-9320H-2, 129-13220H-6, 40-4321H-1, 76-7921H-2, 106-10921H-6, 95-9823X-2, 10-1324X-1, 87-9026X-2, 80-8326X-4, 68-7127X-1, 60-6329X-2, 31-34

119-736B-

1H-3, 75-782H-3, 76-792H-CC, 10-133H-2, 75-78

119-736C-

5R-1, 115-1185R-3, 78-818R-1, 70-739R-2, 75-7810R-2, 77-7912R-1, 25-2813R-1, 74-7614R-2, 80-8315R-2, 86-8916R-3, 80-8317R-1, 80-8318R-2, 70-73

Depth(mbsf)

62.7065.8070.8075.7582.0587.8691.3098.7099.26

113.40149.19154.30156.66158.46164.35176.60185.47206.20209.08214.10234.71

3.7512.1618.1720.15

236.85239.48265.40276.65286.27303.65313.84325.10334.76345.80352.40363.50

Compressional-wavevelocity(m/s)

136312631468134914111305153215341609154014271516154015441566156915841602151615731532

1412140214591440

139814651369142614611500145414661403144514221462

Thermal ConductivityThermal-conductivity values were measured on APC, XCB,

and RCB cores and show high variability that is largest with theRCB cores (Fig. 19 and Table 14). During measurement, thethermal-conductivity instrument showed a relatively large tem-perature drift gradient. Only points where this gradient exceeds1.0°C/min were removed, whereas the instrument manual rec-ommends that values greater than 0.01°C/min should not beused. High drift gradients may explain why many conductivityvalues are lower than 0.59 W/m/°C, the conductivity of waterat ambient conditions, and why the thermal-conductivity dataseemingly show low correspondence to the other physical pa-rameters.

The conductivity values have an apparent bimodal distribu-tion (0.2-0.5 and 0.65-0.85). The lower values are unrealisti-cally low, but the upper values, even though of relatively smallmagnitude, are similar to values recorded at other Leg 119 siteson the Kerguelen Plateau. Bimodal distribution of thermal con-ductivity values has been related to systematic variations in min-eral components (Matsuda and Von Herzen, 1986). However, wesuspect that the lower set of values at Site 736 is erroneousand is due in large part to insufficient laboratory "stabilizationtime" for the cores before measuring the conductivities.

Core, section,interval (cm)

119-736A-

4H-1, 894H-3, 915H-2, 305H-4, 306H-1, 877H-2, 207H-2, 1007H-4, 208H-1, 1009H-1, 809H-3, 8010H-5, 7011H-1, 8012H-2, 7012H-4, 7013H-3, 8013H-6, 8014H-1, 10015H-1, 10021H-1, 10021H-6, 10024X-1, 10026X-4, 5027X-1, 5029X-2, 80

119-736B-

1H-2, 501H-4, 1001H-5, 701H-5, 130

119-736C-

2R-1, 1007R-1, 808R-2, 809R-4, 8010R-3, 8011R-3, 8013R-3, 7514R-2, 7515R-1, 7517R-1, 7518R-3, 50

Depth(mbsf)

28.8931.9134.8037.8038.8744.7045.5047.7049.0053.8056.8065.7069.3075.7078.7083.3087.8090.0099.50

156.90164.40185.60208.90214.00235.20

2.005.506.707.30

207.90255.80267.00279.70287.80297.50316.85325.05333.15352.35364.80

Thermalconductivity(W/m/°C)

0.2150.2010.2410.2570.3930.6600.1840.3220.1970.7920.7010.4870.2510.3490.1910.4070.5150.8070.8160.9050.6270.8030.7330.7600.416

0.8700.5170.5540.381

0.6120.7180.7990.3920.6790.5530.7700.6830.2840.3280.692

Thermal conductivities for poorly consolidated sediments aregreatly dependent on porosity and water content (Lachenbruchand Marshall, 1966). The high porosity values (80%) in the dia-tom oozes at Site 736 probably explain the small conductivityvalues that were not biased by stabilization time (Fig. 19). Thesevalues, like those of the diatomaceous oozes of Site 737, averageabout 0.7 W/m/°C.

Temperature and Heat FlowTemperature measurements were made at five sub-bottom

depths in Hole 736A and at one depth in Hole 736C using theUyeda probe and standard operational procedures (see "Explana-tory Notes" chapter). The results are listed in Table 15 and shownfor each measurement in Figure 20. Temperature equilibrationcurves are given in Figure 21. A summary plot of temperature vs.depth and measured conductivity values for Site 736 is shown inFigure 19.

The temperature measurements at Site 736 are generally good.For some measurements, however, ship surge of up to 4-7 mprobably caused the probe to be extracted and reinserted duringthe measurements, thereby adding irregularities to the tempera-

150

SITE 736

10

0

1

\

On deck

1

TSF

Down

Hole 736A-79.5 mbsf

!

In bottom |

i

1

\

up

|

TSF

TB=

i

1

- 1 .5°C

6.6°C

On deck —

I

20

Z 10 —

Q .

30

20 —

10 —

20 40

20 40

60

— On

Vdeck

1

i/

Down

r ~

In

FHole 736A-135.8

\ T B

\

bottom j

1

mbsf

\

Up

TSF

T B =

J

r

- 1 5°C

1 7 .2°C

On

-

J.deck

-On deck i iDown

i

1Hole 736A

h " ^

in bottom

1

1-164.4 mbsf

i Up i 10 m iBelow mud line

i

i

TS F - 1 .6öC

TM= 3.1°C

TB - 17.2°C

up

1

1

—

i On deck —

120 40 60

Time (min)

Figure 20. Temperature vs. measurement time for six Uyeda probe deployments in Holes 736A and 736C. See Table 15 for critical tem-perature points TSF, and TB. TM = temperature at 10 mbsf.

151

SITE 736

20

£ 10 -

T

— On deck

i

1

- \

T S F V - ^

iDown,

1

i

J

i

1Hole

In bottom

1

1736A-213.5 mbβf

•B

Somewhere?below

I

1

10 mmud

^–belowline

1

TSF

|

1

- 2 .0°C

. 3.1°C• 15.6°C

— • * -

UP

1

—

J

On deck-

|

2 0 4 0 6 0 8 0 100

10

0

1

\

1 / V

At/near bottom of hole— On i Down i not in bottom

αecKi t

1Hole 736C-226

This is

but1 Up i 10 m

1 mud

mbsf

below i Up iline

1

TSF-

™"

On

1

1 .5°C

3.0°C

—

deck

i

2 0 4 0 6 0 8 0

o

πp

era

f

si-

30

20

10

Λ

1

-""V

V *On deck

i

Hole

^ V A

^ ^

Down .

1736A-252.7 mbsf

f " ^ ^ " ^i Hold at 45 m i i

above seafloor Down

in λ

Nf "Out?

in

i

in

J\

bottom

1

In

A TB(?)

Vi Up |10 m

1 mud

1

— • i

below,line

i

T M

1

T M - 3 .

T B - 21

/- '-Up

|

_

I°C

.o°c

—

-

i On deck

12 0 40 60

Time (min)80 100

Figure 20 (continued).

152

SITE 736

Table 15. Temperature measurements for Site 736.

Coreno.

119-736A-

12H18X21H26X30X

119-736C-

3R

Depth(mbsf)

79.5135.8164.4213.5252.7

226.0

Probe insediment (min)

1717212320

0

Probe equilibrationtime (min)

125

1921

8

0

Temperature8

T S F

1.51.51.62.0?

1.5

(°C)

T B

6.2-7.115.0-17.017.915.218.4-20.5

-

1 T S F = lowest temperature immediately prior to probe penetration; T B = equil-ibration temperature in sediment.

2 16α> 15

213 mbsf

0 0.2 0.4 0.6I/time (min"1)

Figure 21. Temperature vs. I/time, Hole 736A.

ture profile. Initial penetration of the probe into the sedimentwas attained for all depths except 226 mbsf in Hole 736C, wherethe probe apparently did not reach, or stay beneath, the sedi-ment interface. Thin (about 0.3-m-thick) gravel layers were com-monly recovered from the core tops directly following each tem-perature measurement. The gravel, derived from layers higher inthe hole, did not prevent penetration but may explain some largefrictional-heating temperature peaks upon insertion.

Temperatures are high in the upper 120 m of Hole 736A, in-creasing from about 1.6°C at the seafloor to 18°C at 164 mbsf.Temperatures then decrease about 3° over the next 50 m and fi-nally increase to 18°-20°C at the deepest measurement (253mbsf). The average temperature gradient in Hole 736A (seafloorto 253 mbsf) is about 70°C/km. The high temperature gradients

for the predominantly diatomaceous sedimentary section at Site736 are probably explained by the low thermal-conductivity val-ues at this site.

The cause of the apparent decrease in temperatures from 164to 215 mbsf is unknown. The equilibrium temperatures appearto be reliable. Increased conductivity values at these depths couldcause smaller temperature gradients; however, conductivity in-creases would not explain a negative temperature gradient. Apossible, but unconfirmed, explanation is heat transfer by circu-lation of pore fluids in the highly porous, and probably permea-ble, diatomaceous sediments.

The heat flow at Site 736 is estimated to be 52.8 W/m2 (1.26HFU), based on an average temperature gradient of 70°C/kmand an average thermal conductivity of 0.75 W/m/°C. The ob-served temperature gradients, although high, may not reflecttrue heat flow because the high sedimentation rates since thelate Pliocene (140 m/m.y.; "Biostratigraphy" section, this chap-ter) would have had a cooling effect. The high sedimentationrates may have decreased temperatures by 20% to 30% (i.e.,true heat flow could be 20% to 25% higher than 58.5 W/m2).

The estimated temperature at the bottom of the deepest holeat Site 736, 736C, based on downward projection of the 78°C/kmtemperature gradient in Hole 736A, is about 28°C at 371 mbsf.

In summary, good temperature measurements were obtainedat five depths within the upper 253 mbsf at Site 736, giving anaverage temperature gradient of 70°C/km and a heat flow of52.8 W/m2 (1.26 HFU).

Discussion

Bulk densities for Site 736 are generally less, whereas watercontents and porosities are usually higher, for a given depth andshow less distinct trends with depth than for sediments at otherhigh-latitude sites (e.g., ODP Sites 643, 689, and 690). Thiscomparison is also true for trends in shear strength and com-pressional-wave velocity and probably results from (1) the morehomogenous nature of the sequence at Site 736; (2) probable,overall higher biogenic silica content; and (3) possibly more ex-tensive core disturbance.

In addition to showing anomalous index-property and shear-strength values and velocities in diatomaceous sediment, coredisturbance in this sediment type is also clearly demonstrated atSite 736. Shear strength is the most obviously affected.

Comparison between different properties in the disturbedcores of Hole 736C shows some evidence of sediment structure.The fact that shear strengths drop to almost zero and the sedi-ment appears soupy—whereas the water content and porositydecrease and bulk density remains constant—probably resultsfrom the diatom ooze having a strong framework that may becompletely disrupted by drilling disturbance; in other words,the sediment liquifies. The apparent changes in index propertiesprobably result from the escape of water from the remoldedcore prior to sampling.

SEISMIC STRATIGRAPHY

Munschy and Schlich (1987) used the seismic section MD 26-10across Site 736 (target Site KHP-1) as an illustration of theirseismic stratigraphy of the area; thus, the seismic correlation isclear (Figs. 22 and 23). Based on the results of the sonobuoy ex-periment (see "Site Geophysics" section, this chapter), the promi-nent unconformity reflector A is situated at approximately 825mbsf. The top of the underlying sequence is locally truncated.According to Munschy and Schlich (1987) the unconformitymarks a hiatus separating the middle Eocene from the lower Mi-ocene and, thus, the pre-rifting sequence from the post break-up sediments, but the results from the site indicate a shorter du-ration of the hiatus. The nature of the unconformity is furtherdiscussed in the "Site 737" chapter.

153

SITE 736

Profile MD 26-1 Orsection 2SSW

2100 2200 2300NNE

2400

5 km

Profile MD26-10:section 2SW

2000 2100 2200 2300 2400

NE

2500

1-

α 2•

3-

4-

Site 736

51 Holocene to upper Pliocene

52 upper Pliocene-middle Miocene

11 upper Oligocene-upper Eocene

I2.I3 middle Eocene to upper Cretaceous

Basement

Figure 22. Multichannel seismic section across Site 736. Seismic line courtesy of R. Schlich; interpretation from Munschy and Schlich (1987).For detailed correlation with drilled section see "Site Geophysics" section. Location of line shown on Figure 1.

The seismic sequence above reflector A is divided into twounits, SI and S2, which Munschy and Schlich (1987) separatedwith reflector Al. Al is at approximately 260 mbsf at Site 736.This corresponds closely with the boundary between lithologicUnits I and II ("Lithostratigraphy and Sedimentology" section).In the multichannel seismic lines Al appears as a fairly strongreflector with great continuity on top of a sheet drape that has avery light appearance on the records. On JOIDES Resolution

single-channel seismic records Al appears as the boundary be-tween an upper sequence (SI) characterized by close parallel re-flections and a lower sequence (S2) that is almost reflection freein its upper part.

The lower unit S2 in the area of Site 736 is characterized by5- to 10-km-wide and 200- to 400-m-high mounds composed ofsheet drape units, which appear as wavy but nearly concordantreflectors. The thickness of the individual sheets varies relatively

154

SITE 736

Site 7361730

I 1807I

I 2000 2059

3.2-

Figure 23. Single-channel seismic section made by JOIDES Resolution across Site 736 from north to northwest.Reflections designated according to Munschy and Schlich (1987). The light layer is marked L. The sequenceabove reflector Al (also called SI) increases in thickness toward the north and fills in a channel? north of themound. Location of line shown on Figure 1.

little horizontally even though the depths to the top of the sheets'surface (paleoseafloors) vary from 200 to 600 mbsf. The base ofthe mounds near reflector A is characterized by a series of down-laps into reflector A, which is formed by the bases of the sheets.Judging from the seismic lines, internal unconformities are notcommon inside of the mounds. The direction of the downlapson the northern part of the plateau is chiefly, but not exclu-sively, toward north and upslope of reflector A. According tothe isopach map of sequence A2 in Munschy and Schlich (1987),the big mound observed at the site is part of two long sedimentridges or drifts along the base of the insular shelf of the Ker-guelen Plateau. The light layer mentioned previously seems toform an 80- to 100-m-thick uniform cover over the mound. Cor-ing at Site 736 has documented that this cover consists of ho-mogenous diatom ooze with almost no admixture of coarse ter-rigenous grains. This is suggestive of uniform pelagic sedimen-tation.

The upper unit SI near Site 736 is characterized by a nearlyparallel band of reflections. Lenticular bodies of low acousticreflectivity are intercalated into this pattern. Coring penetratedone of these bodies between 85 and 110 mbsf, but no distinctivedifferences from the rest of the sedimentary sequences were rec-ognized. However, it may be significant that the diatom oozecorresponding to seismic unit Al contains mostly thin layers ofsand and gravel, which probably cause the acoustic character ofthe sequence. North of the site, the individual reflection bandsincrease in thickness, forming a sequence of sigmoidal progra-dational bodies in the basin or channel north of the mound.Eventually, the channel is filled in by sediment bodies clearlyprograding from the south. The whole of sequence Al and A2is unconformably covered with a relatively thin layer of presum-ably Holocene sediments, forming the relatively level surface ofthe recent bank. The sedimentation rate of this layer was proba-

bly lower than the rest of the sequence at Site 736 (see "Sedi-mentation Rates" section).

ConclusionsThe older sequence above the unconformity (S2) seems to

have been deposited as long sedimentary drift ridges formed bycurrents, which in the northern part of the ridge flowed north-ward along the slope of the Kerguelen insular shelf. The sedi-mentary ridge is locally up to 700 m thick, and no signs of ero-sion have been observed in the upper part. This suggests thatthe water depth was not less than 1000 m above the level of re-flector A at the time of deposition. The covering light layer indi-cates a continuation of the pelagic type of sedimentation and adecrease in the influence of currents in the area.

The upper sequence is also mostly a pelagic diatom ooze, butan additional contribution from glacial-rafted material and pos-sibly sporadic terrigenous mud flows is important. The sedimen-tary structures indicate that sediment transport from a southerlydirection was dominant and that the depocenters were chieflythe local basins in the topography. Possible erosion occurred inthe higher parts of the Kerguelen Plateau (e.g., Site 737), whichis indicative that current transport has increased in importancerelative to that of the previous stage. A decrease in water depth,tectonic tilting of the platform, and change of current patternare possible causes.

SUMMARY AND CONCLUSIONSSite 736 (49°24.12'S, 71°39.61'E) lies on the northern part

of the Kerguelen-Heard Plateau in a water depth of 629 m. The371-m-thick section cored at the site in three holes consists ofupper lower Pliocene to Quaternary diatomaceous ooze withvariable contributions from volcanic debris and very little car-bonate. Sediment-accumulation rates range from 45 m/m.y.

155

SITE 736

above 80 mbsf (about 1.4 Ma) to 140 m/m.y. in the underlyinginterval to the extrapolated basal age of Hole 736C at 3.5 Ma.The sediments cored comprise a high-resolution biosiliceous ref-erence section for the latest Cenozoic located near the present-day location of the Antarctic Convergence.

LithostratigraphyTwo lithologic units are recognized in the sediments recov-

ered from Site 736. The upper 257 m (Unit I) of section coredconsists of diatomaceous ooze containing basaltic volcanic de-bris whereas the lower 114 m (Unit II) of the cored section isdiatom ooze, which includes very little feldspathic detritus, pum-ice, and a thin layer of calcareous nannofossil ooze. These twounits are also recognizable by seismic stratigraphy. The upperunit is characterized by tightly spaced reflections, and the lowerunit displays more diffuse reflections. These seismic units aretraceable regionally, and they probably correspond with seismicunit SI and the upper part of unit S2 of Munschy and Schlich(1987), respectively. The upper unit shows considerable thicken-ing and thinning in the nearby area, and it fills in hollows andcovers the topography of the lower unit. The age of the base ofUnit I is approximately 2.6 Ma according to diatom biostratig-raphy, so the unit corresponds to the latest Cenozoic time of in-creased intensity of Antarctic glaciation. This age, the dispersednature of pebble-sized clasts in the sediments, and the presenceof faceting, striations, or a bullet-nosed shape of some of thepebbles argue for emplacement by ice rafting, possibly from gla-ciers on nearby Kerguelen Island.

Alternatively, some of the diatom ooze with incorporated vol-canic detritus in Unit I may have been deposited by debris flows.Features supporting this mode of deposition include (1) sharpcontacts with underlying purer diatom oozes that show basalsize grading from some of the volcanic-bearing oozes; (2) incor-poration of isolated bodies of volcanic-bearing ooze in otherhorizons, including in relatively pure ooze; (3) apparent post-depositional disturbance of bedding relations with locally sub-vertical boundaries; and (4) partial destruction of lithologicalboundaries, many of which are diffuse or wispy (cuspate). Thesedebris flows could be redeposited glacially derived sedimentsthat had accumulated higher on the plateau and closer to emer-gent areas. However, the upper unit displays a generally pelagic,draping relationship with underlying Unit II.

Biostratigraphy and PaleoclimatologyAll of the cores taken at Site 736 contain common to abun-

dant diatoms and radiolarians, and standard Antarctic zona-tions can be applied. Diatoms and radiolarians are generallywell preserved, and little, if any, reworking of older species isobserved. Deposition appears to have been continuous, and nohiatuses are present. The majority of the diatom species is typi-cal of the Southern Ocean, but species more typical of middlelatitudes are sporadically present, especially in the pre-upper-most Pliocene. Further study is required in order to determinetimes of movement of the Antarctic Convergence back and forthacross the site. The radiolarian assemblage is notable because ofthe relatively low number of species (15 to 20) compared withSouthern Ocean assemblages of equivalent age. This low diver-sity is probably due to the relatively shallow depositional depthof the site and the high amount of upwelling and surface-waterproductivity present.

The scarcity and low diversity of the calcareous nannofossiland foraminiferal assemblages in Site 736 sediments probably,in part, reflect the presence of strong upwelling and surface-wa-ter productivity at the site as well as the overall Antarctic aspectof the assemblages. Essentially no refined calcareous biostrati-graphic zonations could be recognized. Palynomorphs are es-sentially barren from Site 736 sediments.

The magnetic signal of the Site 736 sediments is weak, and amagnetic reversal stratigraphy could not be established.

ChemistryMost of the samples analyzed for interstitial waters contain

<5% calcium carbonate and < 1 % organic carbon. The lowamount of organic carbon could be further evidence of resus-pension of the sediments by debris flows and transportationover short distances. The good quality of preservation of boththe diatoms and the radiolarians and the scarcity of reworkingargue against redeposition and transport by bottom currents overlong distances.

Phosphate, silica, and ammonium are the only chemical spe-cies that show significant variations in their interstitial-waterconcentrations downhole at Site 736. Phosphate and ammoniumdecrease downsection, whereas silica concentrations increase andsuggest the onset of silica diagenesis. Although much higherthan the values found in seawater, dissolved phosphate and am-monium concentrations are considerably less than the valuesfound in sediments beneath highly productive surface waterstypified by upwelling (e.g., Peru margin; Suess, von Huene, etal., 1988).

The absence of hydrocarbon gases and the presence of sul-fate in Site 736 sediment pore waters indicate that methanogenicbacteria have been inactive since the time of deposition. Or-ganic carbon is high to moderate for diatomaceous sediments(1% in the upper 200 m of sediments), but the organic matterpresent is immature. The depositional environment of Site 736has been aerobic, and the kerogen is mostly from varied sources,indicating reworking.

Physical PropertiesThe upper 235 m of the Site 736 sediment column shows an

overall decrease in water content whereas porosity, bulk density,and grain density stay relatively constant. This unusual featureis probably a consequence of the lithology of diatom oozes,which tend to maintain a relatively open but strong structuredowncore. Measurements of shear strength and compressional-wave velocity are strongly affected by coring disturbance causedby XCB and RCB coring below about 110 mbsf, but they showan increase with depth above that horizon.

Heat FlowThe heat flow at Site 736 is estimated to be 52.8 W/m2 (1.26

HFU), based on measured downhole temperatures, which sug-gest a temperature gradient of 70°/km and average thermal con-ductivities of 0.75 W/m/°C. The estimated temperature at thebottom of Hole 736C is 28°C.

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