the cycling and accumulation of biogenic silica and organic carbon in antarctic deep-sea and...

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Marine Chemistry, 35 (1991) 489-502 Elsevier Science Publishers B.Y., Amsterdam 489 The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments D.J. DeMaster", T.M. Nelsona, S.L. Harden" and C.A. Nittrouer'' "Department ofMEAS. North Carolina Slate University. Raleigh. NC 27695, USA "Marine Sciences Research Center, State University ofNew York, Stony Brook, NY 11794, USA (Received 1 October 1990; accepted 8 March 1991) ABSTRACT DeMaster, D.l., Nelson, T.M., Harden, S.L. and Nittrouer, C.A., 1991. The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments. Mal'. Chem., 35: 489-502. Rates of biogenic silica and organic carbon accumulation are reported for Antarctic deep-sea and continental margin deposits. Naturally occurring radionuclides e 26 Ra, 231 Pa, and 230Th) were used to establish rates of sediment accumulation in the rapidly accumulating siliceous deposits beneath the Antarctic Polar Front. The rates were as high as 20-180 em ka -I, and when coupled with biogenic silica and organic carbon measurements, yield accumulation rates as high as 35 rng em-2 year-I for silica and 0.3 mg ern"? yearr ' for organic carbon. Si0 2/organic C weight ratios in Polar Front sedi- ments are typically about 100, although values as high as 300 were measured. Considering that the Si0 2/organic C ratio in Polar Front plankton ranges from 0.5 to 2, the high ratios observed in Polar Front sediments indicate that during settling and burial an enrichment of 50-600-fold occurs in bio- genic silica relative to organic carbon. Rates of sediment accumulation were determined for the continental margin deposits of the Brans- field Strait and the Ross Sea using 210Pb and 14C chronologies. Accumulation rates ranged from 0.02 to 0.5 em year-I. Based on these data and measurements of biogenic silica and organic carbon con- tent, typical rates of accumulation on the continental margin are of the order of 3-12 mg ern"? year- 1 for silica and 0.1-0.8 mg cm "? year " for organic carbon. The Si0 2!organic C weight ratios in these continental margin sediments range from 3 to 32. Comparing the accumulation rate data with esti- mates of annual silica and organic carbon production rates indicates that approximately 25-50% of the gross silica production in surface waters is preserved in the sea-bed, in contrast to less than 5% of the organic carbon. In SOme of the continental margin environments, lateral transport of biogenic material can create local areas where the rate of silica accumulation is equal to as much as 70% of the production in the overlying water column. In the Southern Ocean environments examined in this study, biogenic silica is preferentially preserved in the sedimentary record relative to organic carbon. This trend is consistent with the greater role of Southern Ocean deposits in the global silica cycle as compared with the organic carbon cycle. 0304-4203/91/$03.50 © 1991 Elsevier Science Publishers B.Y. All rights reserved.

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Page 1: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

Marine Chemistry, 35 (1991) 489-502Elsevier Science Publishers B.Y., Amsterdam

489

The cycling and accumulation of biogenic silicaand organic carbon in Antarctic deep-sea and

continental margin environments

D.J. DeMaster", T.M. Nelsona, S.L. Harden" and C.A. Nittrouer''"Department ofMEAS. North Carolina Slate University. Raleigh. NC 27695, USA

"MarineSciences Research Center, State University ofNew York, StonyBrook, NY 11794, USA

(Received 1 October 1990; accepted 8 March 1991)

ABSTRACT

DeMaster, D.l., Nelson, T.M., Harden, S.L. and Nittrouer, C.A., 1991. The cycling and accumulationof biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments.Mal'. Chem., 35: 489-502.

Rates of biogenic silica and organic carbon accumulation are reported for Antarctic deep-sea andcontinental margin deposits. Naturally occurring radionuclides e26Ra, 231 Pa, and 230Th) were usedto establish rates of sediment accumulation in the rapidly accumulating siliceous deposits beneath theAntarctic Polar Front. The rates were as high as 20-180 em ka-I, and when coupled with biogenicsilica and organic carbon measurements, yield accumulation rates as high as 35 rng em-2 year-I forsilica and 0.3 mg ern"? yearr ' for organic carbon. Si02/organic C weight ratios in Polar Front sedi­ments are typically about 100, although values as high as 300 were measured. Considering that theSi02/organic C ratio in Polar Front plankton ranges from 0.5 to 2, the high ratios observed in PolarFront sediments indicate that during settling and burial an enrichment of 50-600-fold occurs in bio­genic silica relative to organic carbon.

Rates ofsediment accumulation were determined for the continental margin deposits of the Brans­field Strait and the Ross Sea using 210Pb and 14C chronologies. Accumulation rates ranged from 0.02to 0.5 em year-I. Based on these data and measurements of biogenic silica and organic carbon con­tent, typical rates of accumulation on the continental margin are of the order of 3-12 mg ern"? year- 1

for silica and 0.1-0.8 mg cm"? year " for organic carbon. The Si02!organic C weight ratios in thesecontinental margin sediments range from 3 to 32. Comparing the accumulation rate data with esti­mates of annual silica and organic carbon production rates indicates that approximately 25-50% ofthe gross silica production in surface waters is preserved in the sea-bed, in contrast to less than 5% ofthe organic carbon. In SOme of the continental margin environments, lateral transport of biogenicmaterial can create local areas where the rate of silica accumulation is equal to as much as 70% of theproduction in the overlying water column. In the Southern Ocean environments examined in thisstudy, biogenic silica is preferentially preserved in the sedimentary record relative to organic carbon.This trend is consistent with the greater role of Southern Ocean deposits in the global silica cycle ascompared with the organic carbon cycle.

0304-4203/91/$03.50 © 1991 Elsevier Science Publishers B.Y. All rights reserved.

Page 2: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

490

INTRODUCTION

DJ. DEMASTER ET AL.

With the development oflarge multi-disciplinary programs such as JGOFS(Joint Global Ocean Flux Study) and WOCE (World Ocean Circulation Ex­periment), research on Antarctic biogeochemical cycles has received in­creased attention. Several investigators (e.g. Sarmiento and Toggweiler, 1984;Labeyrie et al., 1990) have suggested that the biogeochemical cycles and thecirculation in the Southern Ocean play an important role in controlling at­mospheric carbon dioxide levels, which in turn affect global climate. Burialof organic matter removes carbon dioxide from the ocean-atmosphere sys­tem, thus ameliorating possible temperature changes associated with thegreenhouse effect. Sediments in the Southern Ocean account for 50-75% ofthe biogenic silica removal «2-3) x t 0 14 g Si02 year- I) from the marineenvironment (DeMaster, 1981; Ledford-Hoffman et al., 1986). The poten­tial removal of organic carbon in these deposits can be appreciated by multi­plying the biogenic silica removal rate in the Southern Ocean by the organicCjSi02 weight ratio typical of Antarctic phytoplankton (0.5-2; Lisitzin, 1972;Nelson and Gordon, 1982). The product ( 1-6 X 1014 g organic C year- 1 ) rep­resents as much as 30% of the marine organic carbon buried in the entireocean (2 X 1015 g organic C year- I; Romankevich, 1984). However, becauseof the preferential degradation of organic matter relative to biogenic silicaduring settling and burial, Southern Ocean deposits do not account for nearlythis percentage of the organic carbon burial in the ocean. This paper examinesthe Si/organic C fractionation process in two Antarctic environments: therapidly accumulating siliceous sediments beneath the Antarctic Polar Front(water depths 2000-5000 m ), and the continental margin deposits surround­ing Antarctica, specifically in the Bransfield Strait and in the western RossSea (water depths 400-2200 m). Some data on silica contents and organiccarbon contents of Antarctic plankton and sediments exist in the literature(e.g. Lisitzin, 1972). This paper, however, reports recent field measurementsfrom several Antarctic environments and discusses the data in the context ofdifferential degradation of biogenic phases.

SAMPLE COLLECTION AND ANALYTICAL METHODS

Siliceous cores from the South Atlantic Polar Front (Fig. 1A) were chosenbecause sediments in this area have some of the highest accumulation ratesin the Southern Ocean siliceous belt (DeMaster, 1981; Cooke and Hays,1982). Polar Front samples were obtained from the core library at Lamont­Doherty GeologicalObservatory. Sediment samples from the Bransfield Strait(Fig. 1B) were collected during January and February 1986 aboard theUSCGC "Glacier" using box and piston corers. The Ross Sea samples (Fig.

Page 3: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

BIOGENIC SILI CA AND ORGAN IC CARBON CYCLING 491

//

//

AFRICA0"

15" .:1- "/"35 · .. .. . . : : : .

I I 30"

I I -;..-l- /_1- _ + 45" /

...... - RC13-255 -L

15"t\\~

([I]) Rap Idly accumu la llnQ SIliceous ud,menh

- Boundary of SIliceous sed,menls

- - - Pa lo' irani

"-<,

45 "x,

"-,-,r;

Fig. I. Sediment samples were obta ined from three Antarctic field areas including: (A) theSouth Atlantic Polar Front , (B) the Bransfield Stra it, and (C) the Ross Sea. In the BransfieldStra it the cores discussed in the text are circled.

1C) were obtained during January 1990 on board the " Polar Duke" using abox corer and a kasten corer (Kuehl et al., 1985).

Laboratory analyses included measurements of230Th, 226Ra, 14C, and 21°Pbactivitics, as well as determinations ofbiogenic silica and organic carbon con­tent. Polar Front sediments were totally dissolved using a combination ofhy­drochloric, nitric, perchloric, and hydrofluoric acids (DeMaster, 1979) . Nearthe surface of the piston cores, 226Ra activity was determined by the 222Rnemanation technique (Key et al. , 1979) and by the measurement of 21°Pb(via 21OPO). Good agreement between the 226Ra and 2

1°Pb activities indi­cated that no excess 21 °Pb activity existed in the tops of the cores. Because the21°Pb analysis has greater precision than the 222Rn emanation technique, the21 °Pb data were used to calculate radium ingrowth chronologies. A 228Th spikewas used to monitor chemical yield during the isolation of thorium, whichwas performed using ion exchange techniques and TTA (thenoyltrifluoroac­etone) extractions (Ku, 1966). Thorium isotope activities were measured byalpha spectroscopy.

For the Antarctic continental margin sediments, 226Ra activity was deter­mined by gamma counting 125 ml ofwet sediment that was sealed in a plasticPetri dish. Three weeks after sealing the Petri dish , the gamma decay of 214Pb

(350 keY) and 214Bi (608 keY) were measured as indicators oF26Ra activ­ity. 21 °Pb was measured on totally dissolved samples (as described abo ve) bypla ting the activity of its granddaughter, 21Op O, onto silver disks followed byalpha spectroscopy (DeMaster, 1979). 14C chronologies were measured on

Page 4: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

62'

BRANSFIELD STRAIT

62'S

Fig. I. Continued.

.,

~o 50 100I , I

Sca le (km)

Q• Box Core Stations*Piston Core Stations

Bathymetric contoursin mete

B.j>.'-0N

o!­cm3::

~m;l:l

!:j>r

Page 5: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

BIOGENICSILICA AND ORGANIC CARBONCYCLING

1990 ROSS SEA STATIONS

493

X Mooringo Grab Sampla

• Box Core.. Kas ton/Piston Core 7Bo

1600 e

72°

74"

76·

Victoria Land

170"e

Ross Ice Shell

n . ..

.. ..~ .... .... X T

8

180·

c72°

74°

76°

170·W

Fig. 1. Continued.

the organic fraction of the sediments. Carbonate was removed from the sam­ple using a 0.5 N HClleach at room temperature. Approximately 200 g of thetreated sediment was combusted to form COl> which was converted to acet­ylene and then finally to benzene (Noakes et al., 1965). The 14C activity ofthe benzene was determined on a Packard low-level scintillation counter. Bio­genic silica analyses were made using the time series alkaline leach techniquedescribed by DeMaster (1979, 1981), whereas organic carbon determina­tions were made using a Carlo Erba CNS analyzer.

RESULTS AND DISCUSSION

Biogenicsilica and organic carbon cycling beneath theAntarctic Polar Front

Rates of sediment accumulation for the siliceous deposits located beneaththe Antarctic Polar Front were determined using 226Ra ingrowth chronolo­gies. The basis for this technique is the deficiency of 226Ra activity relative to

Page 6: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

494 D.J. DEMASTER ET AL.

(1)

its parent, 230Th, in surface sediments and its subsequent ingrowth towardsecular equilibrium over a period of 5000-8000 years. The ingrowth time ata particular depth (z) can be evaluated from the following equation:

{1- e26RaoF 30ThO) }!

t(z)=ln 1_[ 226Ra(z)j230Th(z)] A

where t is the ingrowth time (years); Rao jThO is the activity ratio at the sed­iment-water interface, 226Ra (z) F30Th (z) is the activity ratio at depth z, andAis the 226Ra decay constant (year- 1). Assumptions used to derive eqn. (1)include that the 226Rao j230Tho at the sediment-water interface be constantover the dating interval and that the 230Th activity does not change signifi-

. 230Th() 230Tho hi h' .cantly during the period of ingrowth (i.e. Z = ; w 1C , given its75000 year half-life, is a reasonable assumption). In slowly accumulatingsediments (8< 1 em ka- 1), the 226Ra ingrowth occurs over a depth range ofapproximately 5 em (e.g. in core A1l76-16; DeMaster et al., 1988). In rapidlyaccumulating sediments (8) 10 em ka- I ), such as beneath the Polar Front,the ingrowth of 226Ra activity occurs over a depth range of 50 ern or more andcan be used to establish age vs, depth relationships on a time scale of 5000­8000 years.

226Raj230Th ingrowth profiles from piston cores RC13-255 and RC11-76are shown in Fig. 2 together with their excess 230Th and excess 231 Pa profiles(from DeMaster, 1979) as well as the radiolarian stratigraphies of Cyclado­phora davisiana (from Cooke and Hays, 1982). In core RC13-255, the 226Raingrowth chronology yields an accumulation rate of 26 em ka - I (on a timescale ofabout 5000 years), which is in good agreement with the Cycladophoradauisiana accumulation rate of 21 em ka -1 (on a time scale of about 15 000years). In this core the radiolarian stratigraphy indicates a dramatic decreasein sediment accumulation rate between the time of the last glacial maximumand the present interglacial. The increase in the excess 23°Th and excess 231 Paspecific activities near the top of the core is consistent with this decrease inmass flux.

In core RC 11-76, the 226Ra ingrowth data yield an accumulation rate of 180em ka - 1, which is significantly greater than the Cycladophora dauisiana rateof 22 em ka - I. Some, but not all, of the discrepancy in accumulation ratesmay result from differences in time scale (i.e, the accumulation rate duringthe past 5000 years may have been much greater than the mean value overthe last 15 000 years). To resolve the discrepancy, accelerator 14C dates onthe organic fraction of the sediment are needed from several intervals of thecore. Based on the 226Ra chronology and measurements of biogenic silica andorganic carbon in the seabed (mean values of 55% and 0.5%, respectively),the accumulation rates for core RC11-76 are 35 mg cm "? year : ' for silicaand 0.3 mg em -2 year- 1 for organic C. This silica accumulation rate exceeds

Page 7: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

BIOGENIC SILICA AND ORGANIC CARBON CYCLING 495

/-+ 15 ky

// 5

26 cm/ky /168

0 / 0E / E

18 ky

P 2 / P 10T / T 32H / H

(m) / (m)/ 37 ky

3 - - - - - - - - - -_/ 15

( I - 228 Ra/ 230 T h )

01 10

EXCESS

001 0.1o cm/ky

21

(dpm/g)

10

20C. dcvisionnSiratigraphy

• 230Th

+ 231Po

0.01o cm/ky

• 230Th

+ 231Pa

j 7cm/ty

7 cm/ky

92 ky +

C. dO'li'ilonaStratigraphy

EXCESS ACTIVITY (dpm/g)

01 10 10

6

22

18 ky

15

20

oEP /0TH

(rn)

10

r" - .... 5,IIIIIIIIII,

IIII1(I

_____ -1

4

5

2

RCII-76

( I - 228 Ra/ 230 T h )

0001 0.1

oEPTH

(rn)

Fig. 2. 226Ra/230Th ingrowth profiles from two South Atlantic Polar Front piston cores. Theadditional radiochemical data (DeMaster, 1979) and the biostratigraphic data (Cooke andHayes, 1982) for these cores are shown for comparison of time scales and accumulation rates.

most estimates of silica production in Polar Front waters (e.g. Nelson andGordon, 1982) with the possible exception of that reported by Lisitzin (1985).Comparison of production and accumulation of silica in this core (i.e. the

Page 8: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

496 DJ. DEMASTER ET AL.

one-dimensional box model approach) is inappropriate because the high ac­cumulation rates observed in core RCll-76 probably result from lateral fo­cusing or mass movement caused by bottom topography and/or deep-watercurrents. The important point is that siliceous Polar Front sediments accu­mulate rapidly and contain approximately 50-60% biogenic silica but only0.2-0.6% organic carbon. The resulting biogenic silica/organic carbon weightratio in these sediments is about 100 (Lisitzin, 1972), although values as highas 300 were measured as part of this study. Plankton at the Polar Front havesilica/organic C weight ratios ranging from about 0.5 (Nelson and Gordon,1982) to two (Lisitzin, 1972); therefore, the preferential loss of organic car­bon relative to biogenic silica during settling and burial enhances the silica/organic C ratio in the seabed by a factor ranging from 50 to 600. The rate ofsediment accumulation beneath the Polar Front may be highly variable spa­tially (Van Bennekom et al., 1988) as a result of topography, currents, vari­able production, or grazing; but a consistent characteristic in all of these sed­iments is the high biogenic silica and low organic carbon content of thedeposits.

Biogenic silicaand organic carbon cyclingin Antarcticcontinental margindeposits

Fine-grained sediments accumulate in the basins of the Bransfield Strait atdepths ranging from 400 to 2200 m. Based on 21°Pb and 14C chronologies(Fig. 3), sediment accumulation rates vary from 0.08 to 0.5 em year- I (Nel­son, 1988; Harden et al., 1991). The good agreement between the 21°Pb and14C accumulation rates indicates that bioturbation has a negligible effect onthe Bransfield Strait radionuclide profiles. The biogenic silica content of thesediments varies from 3 to 17%,with a mean value of about 12%. The organiccarbon content of these deposits ranges from 0.56 to 1.7%,with a mean valueof approximately 1.0%. Nelson (1988) constructed a biogenic silica and anorganic carbon budget for the Bransfield Strait. He reported that the meanbiogenic silica accumulation rate in the basins was 12 mg Si02em-2 year- I,

whereas the mean burial rate for organic carbon was 0.75 mg C em -2 year- I.

Based on calculations of biogenic silica and organic carbon production in sur­face waters taken from field data reported in the literature, Nelson (1988)concluded that 71% of the silica produced in surface waters (a total of 15 mgSi02 em"? year- 1) accumulated in the seabed, whereas only 9% of the or­ganic carbon production (a total of 7.7 g C em -2 year- I ) was preserved inthe sediments. Banahan and Goering (1986) observed similar high preser­vation efficiencies (up to 65% of production) on the Bering Sea shelf.

This high preservation rate for silica in the Bransfield Strait, however, con­flicts with the particle trap data of Gersonde and Wefer (1987), who reporteda 50% decrease in biogenic silica flux between trap depths of 323 and 963 m.

Page 9: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

BIOGENIC SILICA AND ORGANIC CARBON CYCLING 497

Excess Pb-210 (dpm/g) C-14 AGE (ky)

PC-?

s; 0.28 cm/y

100

300

.'.......

Es..c:a

// 2l 200..

BC-S /s ~ 0.34 cmly ....

10 100 0 2 4 6 8 10 12+-"-'-'.L..W.OJ"--'-..............uw..~'--'-T'""" ••••••••....•••••..••••••••••••...•.•• 0 +...o.-l-"'r-"---....-..........-'-"-'--'--'.1

0

10

20E~..c:a 30oilC

40

50

Fig. 3. Depth profiles of excess 210Pb and 14C age (organic carbon fraction) for a box core anda piston core collected from the eastern Bransfield Strait. The agreement in accumulation ratesindicates that bioturbation has a negligible effect on the distribution ofthese naturally occurringradionuclides in the seabed,

Based on particle flux data from particle traps deployed in the Bransfield Straitfor a year, Wefer et al. (1988) estimated the annual biogenic silica flux at 500m depth to equal 6 mg Si02em -2 year- I (2-3 times lower than the Nelson( 1988) value for surface waters). The apparent discrepancy in these data setsis removed if one-half to two-thirds of the biogenic silica produced in thesurface waters dissolves in the upper water column and lateral transport ofsediment (via resuspension and downslope mass movement) enhances thelocal biogenic silica accumulation rates in the Bransfield Strait basins. Sup­portive evidence for the lateral transport of sediment in the study area comesfrom two different sources. First, particle trap studies in the Bransfield Straitreveal higher lithogenic fluxes near the seabed than in midwater depths as aresult of resuspension (which, coupled with the basin topography, can pro­duce gradual downslope movement of particles). Second, the inventories ofexcess 210Pb activity (primarily scavenged by fine-grained sediments) arethree times greater in the basins (270 dpm cm-2) than on the flanks (90 dpmem -2), consistent with the redistribution of sediment and lateral transport.Thus, approximately one-third to one-half of the biogenic silica produced insurface waters accumulates in the sediments below.

The biogenic silica/organic carbon weight ratio in Bransfield Strait sedi­ments ranges from 6.4 to 18. Local plankton have a ratio of about two (Jen­nings et al., 1984); hence, the sediments are enriched in biogenic silica rela­tive to organic carbon by a factor of 3-9 during settling and burial. Wefer etal. (1988) stated that most of the vertical transport of particles is through the

Page 10: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

498 OJ. OEMASTER ET AL.

fecal pellets of krill. This mode of vertical transport plus the decreased waterdepth in the Bransfield Strait may explain why the degradation of organicmatter relative to biogenic silica during settling and burial is reduced in theBransfield Strait (enrichment factor of 3-9) relative to the Polar Front (en­richment factor of 50-600).

Rates of sediment accumulation for southern Ross Sea sediments were de­termined by Ledford-Hoffman et al. (1986) using 2lOPb geochronologies.Values ranged from 0.06 to 0.3 em year-I; however, no piston core 14C sam­ples were available. As part of a multi-disciplinary field program conductedin January 1990, sediment samples for 14C analysis were taken from 43 kastencores and piston cores collected from the central and western Ross Sea. Initialmeasurements have focused on Station 1 (Mooring Site A) sediments be­cause silica uptake rates are available from this site and 21°Pb chronologiesalready exist for this area. 14C measurements on the organic fraction of Sta­tion 1 sediments (KCl12.08) yield a sediment accumulation rate of 0.016em year- 1 (Fig. 4), which is substantially less than the apparent 210Pb accu­mulation rates (0.09 and 0.25 em year-I) determined by Ledford-Hoffmanet al. (1986) for this area. 14C measurements on additional kasten and pistoncores are currently in progress to test the accuracy of the 210Pb accumulationrates; however, these initial results indicate that bioturbation may have af­fected some 210Pb profiles (i.e. the upper 20-30 em of the seabed).

The biogenic silica content of Station 1 sediments is approximately 31%,

s = 16.2 cm/Ky

50

E..c:...c 100Q.CIlC

150

KC112.08200

C·14 Age (Ky)

5 10 15 20 25 30

Fig. 4. Profile of 14C age (organic carbon fraction) for kasten core 112.08 collected from thesouthwestern Ross Sea. At 130 em depth, there is a change in lithology (lower porosity at depthand a color change). This is consistent with the age at 155 em being greater than predicted fromthe 14C data in the upper lOa em.

Page 11: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

BIOGENIC SILICA AND ORGANIC CARBON CYCLING 499

whereas the organic carbon content averages about 1.2%. Comparable bio­genic silica and organic carbon values have been reported for this area byDunbar et al. (1985). These data yield a seabed biogenic silica/organic car­bon ratio of 26, which is 32 times greater than the ratio in surface plankton(0.8). The enhancement of biogenic silica relative to organic carbon in thewestern Ross Sea (enrichment factor of 32) is greater than that observed inthe Bransfield Strait (3-9), but less than that associated with the Polar Front(50-600). The mode of vertical transport for particles in the Ross Sea is notwell known; however, the greater fractionation between silica and carbon(relative to the Bransfield Strait) may suggestthat a greater percentage of thematerial settles as discrete particles and not as fecal pellets (optimizing thetime for microbial activity and organic carbon degradation). The nature ofsettling particles in the Ross Sea is currently being studied as part of the RossSea project by deploying particle traps at three locations (sites A, B, and C;Fig. lC) for two consecutive years (R. Dunbar, personal communication,1990 ).

Based on the 14C accumulation rate and biogenic contents of Station 1 sed­iments, the biogenic silica accumulation rate for this site is 3.5 mg Si02 cm"year- I, whereas the organic carbon rate is 0.14 mg C em- 2 year- I. Duringthe "Polar Duke" cruise, a box core (BC-166) was collected at Station 1. A6.2 em diameter subcore with overlying water was removed from BC-166 andincubated at in situ temperatures with periodic stirring. Nutrient concentra­tions were monitored in the overlying water for a period of 2.5 days (Fig. 5).The increase in nutrient concentrations as a function of time can be used tocalculate nutrient fluxes out of the seabed. Based on the BC-166 data and thepresumption of steady-state conditions, the fluxes from the seabed are equalto 48 /lmol em -2 year " ' for silica, 6.6 /lmol ern-2 year- I for nitrate, and 0.34/lmol cm " year" ' for phosphate. As a crude estimate of seabed organic mat­ter regeneration, the nitrate (plus ammonia) flux and the phosphate flux canbe multiplied by their corresponding Redfield C/N (6.6) and C/P (106)ratios. This calculation shows that the ratio of silica to organic carbon regen­eration in the seabed is about six (by weight), whereas the burial ratio isabout 26. Thus, organic matter is preferentially degraded relative to silicaboth in the seabed and in the water column. Ifthe dissolved silica flux out ofthe seabed (equivalent to 2.9 mg Si02 em-2 year-I) is added to the biogenicsilica accumulation rate (3.5 mg Si02 em- 2 year- 1) the sum is equal to thetotal flux of biogenic silica arriving at the sediment-water interface (6.4 mgSi02 ern -2 year- I ). The most recent estimate of gross silica production atthis site is 12-18 mg cm ? year-I, however, the net silica production in theeuphotic zone is only 6 mg cmr? year-I (Nelson and Smith, 1986; Nelson etal., 1991). Thus, the net production in the euphotic zone approximatelymatches the flux of particulate silica reaching the sediment-water interface.This can occur if no dissolution occurs in the water column below the eu­photic zone, or if lateral transport of suspended sediment carries siliceous

Page 12: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

500 DJ. DEMASTER ET AL.

20 40 60 80TIme (hrs)

y = 51.1329 + 0.683Sx

Flux Core Measurements from Be RS90-166110

100

:r 902-

j 80

70Cii60

500

20 ..,.-------------:1y = 12.2756 + 0.0941X A= 1.00

18

806040

Time (hrs)

20 40 60 80

Time (hrs)

20

y =0.8156 + 0.0046x R =0.98

14

16

120

1.2

1.1

:r2- 1.0$til.c 0.9c.II)0s: 0.8 .Q,

0.70

Fig. 5. Nutrient concentrations from an incubated flux core collected at Station 1 (Mooring SiteA). The slope of the nutrient vs, time plot is directly proportional to the flux from the sea-bed.The data yield fluxes from the sea-bed to the water column of 48 ,umol ern -2 year " ' for silica,6.6 pmol em-2 year- 1 for nitrate, and 0.34 tlmol cm -2 year-I for phosphate.

material into the study area. The lateral flux of sediment is being monitoredat each of the mooring sites by placing Aanderaa current meters, equippedwith transmissometers, 40 ill above the seabed and 10m below the lower

Page 13: The cycling and accumulation of biogenic silica and organic carbon in Antarctic deep-sea and continental margin environments

BIOGENIC SILICAAND ORGANIC CARBONCYCLING 501

particle trap. The relationships between mass flux in the particle traps andthe current speed, the current direction, and the light attenuation, should proveuseful in evaluating the importance of lateral transport in the study area.

ACKNOWLEDGMENTS

This research was presented at the International Symposium on The Bio­geochemistry and the Circulation of Water Masses in the Southern Ocean,which was held in Brest, France. We would like to thank Paul Treguer andBernard Queguiner for their tireless efforts in organizing and running thisproductive symposium. We appreciate the efforts of the Lamont-DohertyGeological Observatory Core Library in sending us Polar Front samples. JohnAnderson (Rice University) kindly provided us with piston core samplesduring the Bransfield Strait cruise. The multi-disciplinary study in the RossSea would not have been possible without the collaborative efforts ofthe otherprincipal investigators, Dave Nelson, Walker Smith, and Rob Dunbar. Weowe special thanks to Robin Pope, Geoffry Pierson, Susan Boehme, and AmyLeventer for their assistance in coring and subsampling during the Ross Seacruise. The pore-water nutrient analyses from the Ross Sea were run by JoeJennings and Paul Treguer. Funding for this research was provided by Na­tional Science Foundation grants DPP-8512514 and DPP-8817209.

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