11. biomarker and pyrolysis geochemistry of organic …

Gradstein, F. M., Ludden, J. N., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 123

11. BIOMARKER AND PYROLYSIS GEOCHEMISTRY OF ORGANIC MATTER FROM ARGO ANDGASCOYNE ABYSSAL PLAIN SEDIMENTS, NORTHEASTERN INDIAN OCEAN1

D. T. Heggie,2 C. J. Boreham,2 and R. E. Summons2

ABSTRACT

The sediments of the Argo and Gascoyne abyssal plains are generally lean in organic matter, are immature, and containhydrocarbons trapped during sediment deposition rather than those generated during sediment catagenesis.

TOC concentrations in the Argo Abyssal Plain Cenozoic sediments are 0.5 wt%, and organic matter appears to befrom mixed marine and reworked, degraded, organic matter sources, with the latter being contributed by turbidity flowsfrom the nearby continental margin. TOC concentrations within the Cenozoic sediments of the Gascoyne Abyssal Plainare mostly undetectable (<O.l wt%).

Biomarker distributions determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) indicate that organic matter extracted from the Lower Cretaceous sediments from both sites is predominantly marinewith varying contributions from terrestrial organic matter. The specific marine biomarker, 24-n-propylcholestane is inrelatively high abundance in all samples. In addition, the relatively high abundance of the 4-methylsteranes with the 23,24-dimethyl side chain (in all samples) indicates significant dinoflagellate contributions and marine organic matter.

The ratios of n-C27/«-C17 reflect relative contributions of marine vs. terrestrial organic matter. TOC, while generallylow at Argo, is relatively high near the Barremian/Aptian boundary (one sample has a TOC of 5.1 wt%) and theAptian/Albian boundary (up to 1.3 wt% TOC), and two samples from the Barremian and Aptian sections contain relativelyhigh proportions of terrestrial organic carbon. TOC values in the Lower Cretaceous sediments from Gascoyne AbyssalPlain are low (<O.l wt%) near the Aptian/Barremian boundary. TOC values are higher in older sediments, with maximain the upper Barremian (1.02 wt%), the Barremian/Hauterivian (0.6 wt%), and Valanginian (1.8 wt%). Sediments fromthe upper Barremian contain higher amounts of terrestrial organic carbon than older sediments.

INTRODUCTION

The formation of the modern Indian Ocean, including the Argoand Gascoyne Abyssal Plains, off northwestern Australia (Fig. 1),began with Late Jurassic to Early Cretaceous rifting along thenorthern and western margins (Ludden, Gradstein et al., 1990).Geohistory analysis of the nearby DSDP Site 261 in the ArgoAbyssal Plain indicates an initial water depth of around 2.5 km.Rapid cooling and subsidence resulted in a water depth near 4000m during the Hauterivian/Barremian. Slower subsidence throughthe Late Cretaceous and Cenozoic resulted in a present-day waterdepth near 5700 m (Ludden, Gradstein et al., 1990). The GascoyneAbyssal Plain (Site 766, Fig. 1) is near the foot of the ExmouthPlateau escarpment. Subsidence and sedimentation considera-tions indicate this site originated at a water depth near 800 m;rapid subsidence in the Early Cretaceous brought the site near toits present water depth of 4000 m.

The sediments at the Argo and Gascoyne abyssal plains beganto be deposited during the Early Cretaceous with the onset ofseafloor spreading. About 935 m of sediment was cored at Site765 (Argo), and approximately 500 m of these sediments areLower Cretaceous calcareous claystones and hemipelagic claysthat accumulated at rates of about 5 meters per million years(ra/Ma) during Valanginian through Barremian time, 14 m/Maduring the Aptian, and 4.2 m/Ma during the Albian (Ludden,Gradstein, et al., 1990). Site 765 has remained close to or belowthe CCD. Some turbidite deposition is evident in the LowerCretaceous sequence. Late Cretaceous and Paleogene sedimen-tation rates were low (<2 m/Ma), and the appearance of siliciclas-

1 Gradstein, F. M., Ludden, J. N., et al., 1992. Proc ODP, Sci. Results, 123:College Station, TX (Ocean Drilling Program).

Bureau of Mineral Resources, Division of Marine Geosciences and PetroleumGeology, P.O. Box 378, Canberra, ACT, 2601, Australia.

tic and calcareous turbidites in the Upper Cretaceous to Paleogenesequence indicates a change from primarily hemipelagic to turbi-dite-dominated sedimentation. Turbidites dominate the Cenozoic;Neogene sedimentation rates were estimated at near 26 m/Ma.

About 467 m of sediment was cored at Gascoyne Abyssal Plain(Site 766), of which approximately 300 m accumulated during theEarly Cretaceous. Sedimentation rates decreased from the Valan-ginian (60 m/Ma) through the Aptian/Albian (3.2 m/Ma). LowerCretaceous sediments are greenish-gray to dark-green and blackclay, sand, and silts tones. A marked change occurs near theAptian/Barremian boundary from deposition of hemipelagic, ter-rigenous, and shallow-water marine sediments to hemipelagic andpelagic calcareous sediments. Site 766 has remained above thecalcite compensation depth (CCD; Ludden, Gradstein, et al.,1990).

This study describes the results of organic biomarker andpyrolysis geochemistry conducted in the Bureau of Mineral Re-sources (BMR) laboratories. These laboratory data have beenintegrated with the Rock-Eval data obtained on board the JOIDESResolution to help elucidate the nature of the depositional envi-ronments of the deep-water Argo and Gascoyne abyssal plain drillsites. Biomarker and combined pyrolysis data were used to inves-tigate: (1) the source (i.e., marine and/or terrestrial) of the organicmatter in the sediments and, (2) the variations in source throughtime. The focus is on the Lower Cretaceous sediments. Rock-Eval, Tmax values, and some biomarker maturity parameters wereused to assess the thermal maturation levels of the organic matter.

METHODSThirty sediment samples selected for analyses at the BMR

were first screened by Rock-Eval pyrolysis. These data are sum-marized in Table 1. Eleven samples were then soxhlet-extractedwith solvent (CHCl3:Me0H:87:13) for 48 hr. Column chroma-tography of the extract on silica gave saturates, aromatics, and thepolar (NSO) fraction (Powell, 1982). Seven saturated hydrocar-

215

D. T. HEGGIE, C. J. BOREHAM, R. E. SUMMONS

114° 120

O260 I

GASCOYNE\

ABYSSAL_J

ODP Leg 123 site

O DSDP site

500 km

20°

24°

Figure 1. Map of the continental margin of northwestern Australia showing drill Sites 765

(Argo Abyssal Plain) and 766 (Gascoyne Abyssal Plain).

Table 1. Rock-Eval pyrolysis data from BMR.

Core, section,interval, (cm)

123-

765B-03H-3, 8-16765C-35R-3, 60-69765C-44R-3, 42-50765C-44R-5, 2-10765C-49R-1,96-104766A-27R-2, 66-68766A-28R-6, 106-109766A-29R-4, 89-95766A-30R-4, 48-54766A-30R-1,140-150766A-31R-1, 58-64766A-31R-2, 145-150766A-32R-4, 132-138766A-33R-3, 42-50766 A-34R-1,69-75766A-36R-1, 122-127766A-37R-1, 59-65766A-38R-3, 98-104766A-39R-2, 114-122766A-40R-3, 44-51766A-41R-4, 82-87766A-42R-4, 100-108766A-43R-4, 97-105766A-43R-4, 114-119766A-44R-6, 5-11766A-45R-4, 98-106766A-46R-6, 50-57766A-47R-4, 95-102766A-48R-4, 16-18766A-49R-3, 47-51

Age

PApApApBBBBBBBBB/HHHHHHHHHHHHHHVVVV

TOC(wt%)

0.540.351.230.400.550.140.300.330.590.610.700.781.020.340.270.250.420.680.530.440.510.510.360.530.491.250.730.680.500.71

'max(°C)

5454944174705413553544543374144164204213853343623793393X3358370371389377410424422403395399

Si

0.350.000.190.000.100.130.070.100.100.040.140.130.1810.050.070.290.240.290.290.330.280.220.220.210.240.330.210.210.150.27

s2

1.531.411.231.231.110.190.270.520.250.610.660.620.770.370.350.510.580.490.580.580.570.540.630.590.801.270.610.650.700.88

PI

1.810.000.130.000.080.410.210.160.000.060.170.180.190.120.170.360.290.370.340.370.330.290.260.260.230.210.260.240.180.24

HI

28340310030820213690

15842

100947975

10913020413872

1091321121061751111631028496

140124

Age: P = Pleistocene, Ap = Aptian, B = Barremian, B/H = Barremian/Hauterivian, VValanginian. Sj, S2 in mg HC/g; PI and HI in mg HC/g organic C.

216

BIOMARKER AND PYROLYSIS GEOCHEMISTRY OF ORGANIC MATTER

bon fractions were then analyzed for a variety of biomarkers bycombined gas chromatography-mass spectrometry (Summons etal., 1988).

TOTAL ORGANIC CARBON DISTRIBUTIONS,PYROLYSIS, AND SOURCE CHARACTERISTICS

Argo Abyssal PlainShipboard total organic carbon (TOC) data from the Argo

Abyssal Plain are summarized in Figure 2. Four additional sam-ples were selected from the Lower Cretaceous sediments of Site765 of Aptian through Barremian age, while one sample was takenfrom the Pleistocene section (BMR Rock-Eval data shown in opencircles). The TOC values from these additional analyses (Table 1)vary between 0.4 and 1.3 wt% and are similar to those found fromthe shipboard analyses.

Argo Abyssal Plain (Cenozoic Sediments)

TOC data from the Cenozoic sediments are highest in the top50 mbsf (up to 1.5 wt%), and decrease with increasing depth,although the data show large concentration differences over shortdistances. These sediments are dominated by turbidite deposition(Ludden, Gradstein, et al., 1990). Calcium carbonate and TOCdata from a single turbidite are shown in Figure 3. Calciumcarbonate data are lowest in the top of the turbidite, probablybecause of dissolution in corrosive bottom waters below the CCD.TOC concentrations vary about tenfold over about 1.5 m of thisturbidite and are lowest at the base of the turbidite in the coarse-grained sediments and highest in the fine-grained sediments to-ward the turbidite top. These large variations within a singleturbidite, over distances of less than 2 m, are comparable to

changes in TOC over the whole Cenozoic section and thus ex-plain, in part, TOC variations in the Cenozoic sediments.

Pore water data from this site (Ludden, Gradstein, et al., 1990)show that sulfate decreases with increasing depth in the sedimentsto near asymptotic concentrations of 6 to 7 mM, representingabout a 70% depletion from bottom-water concentrations, at about300 mbsf. The oxidation and remineralization of buried organicmatter, primarily by seawater metabolites, oxygen, nitrate, andsulfate during early diagenesis (e.g., Bender and Heggie, 1984)explains the trend of decreasing TOC with increasing depth ofburial. The observed TOC distributions with depth in the Ceno-zoic sediments of the Argo Abyssal Plain thus result from thecombined effects of pelagic and frequent turbidite deposition witha diagenetic overprint.

Rock-Eval Data

The combined Rock-Eval data sets from Site 765 werescreened, and those having S2 <0.2 and TOC <0.5 wt% were notconsidered further (the screened Rock-Eval data from shipboardanalyses are summarized in Table 2). A cross plot (Peters, 1986)of the hydrogen index (HI) and oxygen index (OI) of Site 765 datais shown in Figure 4. This plot may be used similarly to a vanKrevelen diagram to differentiate organic matter types. Becauseall samples have TOC <2 wt% (with one exception) and havevariable amounts of calcium carbonate and clay minerals, thepyrolysis yields have probably been suppressed by the mineralmatrix effect (Espitalié et al., 1980; Orr, 1983; Katz, 1983; Peters,1986). This effect results in low, but variable, HI values comparedto results on demineralized organic matter and, hence, introducesartificial differences in organic matter compositions.

The Cenozoic (Fig. 4) samples were from the top 100 mbsf ofsediment and are of Pleistocene age. These samples have variable

0 1

200 -

4001

600-j

f••:f -E BB

800-

?

/////

T0C(wt%)2 3 4

• Pleistocene

Pliocene

Miocene

Oligocene/Eocene/Pa leocene

Late Cretaceous

Albian

*" Aptian

Barremian

Hauterivian-Berriasian

5.

•

7A

6

Ter

tiary

etac

eous

Ear

ly C

r

7/

δ

Figure 2. Downhole TOC profile, Site 765 (shipboard Rock-Eval data = closedsquares; BMR data = open circles; samples extracted for hydrocarbons are shownwith an arrow).

217


CaCθ3(wt%) TOC(wt%)

0 20 40 60 80 100 0.0 0.5 1.0

Figure 3. Calcium carbonate and TOC data from a turbidite.

HI values (150<HI<500) and plot between the Type II/III meanevolution lines (data not plotted have HI<400 and OI>350), sug-gesting predominantly Type II organic matter, with the variationin HI values probably owing to a variable mineral matrix effect.An additional contribution from varying amounts of hydrogen-poor, reworked organic matter is also likely (see discussion in"Maturity" section, this chapter). Shipboard palynological inves-tigations indicated that samples from Miocene/Pliocene mass-flow deposits contained reworked Jurassic dinoflagellates, as wellas spores and pollen (Ludden, Gradstein, et al., 1990). The varia-tion in OI is more problematic. For example, in the Cretaceous

marine Toolebuc Formation (Queensland, Australia), which isType II organic matter (Boreham and Powell, 1987), oxidation (ofthe marine organic matter) results in a decrease in HI with littleincrease in OI, whereas high OI and low HI values were associatedwith terrestrial organic matter. The trend in the Cenozoic samplesin Figure 4 (decreasing HI with increasing OI) also suggests acombination of varying degrees of oxidation associated withmixed marine and terrestrial sources of organic matter. However,in these organic-lean rocks, a major contribution to the amount ofCO2 released is most likely from decomposition of carbonateminerals, which would mask any trend in the data.

Argo Abyssal Plain (Mesozoic Sediments)

TOC are generally low ( wt%) in the Mesozoic section. How-ever, isolated samples of higher TOC were occasionally found.The highest TOC was found in an isolated, thin (1 cm), blacksediment layer (123-765C-45R-4,100-101 cm) near the Barremi-an/Aptian boundary (Fig. 2). This sediment (Thurow, pers.comm., 1991) was finely laminated and not bioturbated (althoughadjacent sediments were bioturbated) and was (under the scanningelectron microscope) almost entirely framboidal pyrite. Theseobservations and other estimates of the pyrite abundance (Heggie,this volume) suggest that this sediment was deposited during abrief period of anoxia in the overlying water (additional samplesfor laboratory analyses, however, were not available from thisinterval). TOC vales near the Aptian/Albian boundary were up to1.3wt%(Fig. 2).

The Mesozoic samples of Site 765, from depths greater than650 mbsf and representing Lower Cretaceous sediments fromAlbian through Valanginian/Berriasian ages, have a low HI (75)and cluster about the Type III mean evolution line. These data

Table 2. Selected shipboard Rock-Eval and calcium carbonate data, Site 765.


123-

765B-1H-1,96-98765B-1H-2, 78-80765A-1H-4, 38-40765B-1H-6, 61-63765A-1H-6, 68-70765B-2H-3, 65-67765B-2H-3, 123-135765B-2H-3, 145-150765B-2H-4, 7-19765B-2H-4, 143-150765B-2H-4, 145-150765B-3H-3, 145-150765B-3H-4, 125-128765B-3H-4, 145-150765B-3H-4, 145-150765B-3H-6, 23-26765B-4H-4, 0-5765B-4H-4, 0-7765B-4H-6, 20-22765B-5H-2, 85-87765B-6H-6, 99-101765B-7H-4, 0-7765B-7H-4, 0-5765B-11H-2, 19-21765B-35X-2, 0-5765C-33R-1, 20-21765C-35R-3, 55-57765C-35R-3, 86-90765C-40R-4, 149-150765C-45R-4, 100-101765C-49R-1, 94-95765C-49R-1, 141-143765C-59R-3, 149-150765C-59R-4, 111-112

Depth(mbsf)

0.962.284.888.118.18

12.9513.5313.7513.8715.2315.2523.2524.5524.7524.7526.5333.0033.0036.2040.4556.2961.9061.9097.69

329.80654.90677.65677.96726.89773.70807.24807.71902.39903.51

CaCO3

(wt%)

13.052.5

0.42.5

72.32.93.40.85.4nd0.30.41.60.30.3

66.10.50.52.4

31.732.769.269.2

0.680.1

0.66.64.70.72.31.11.00.60.5

Si

0.640.380.680.560.420.460.210.380.220.560.450.540.500.820.630.270.460.400.290.140.060.280.340.170.080.110.100.070.130.360.010.020.160.18

S 2

1.471.252.762.191.651.652.272.371.393.523.391.562.593.312.551.394.013.991.780.320.372.011.572.350.300.550.760.320.508.930.430.550.630.66

s3

4.985.151.053.224.473.553.771.794.761.381.282.433.612.351.164.860.942.636.304.574.534.942.771.311.530.352.211.510.752.800.330.510.080.06

TOC(wt%)

0.850.520.610.540.550.740.710.840.840.740.811.030.901.020.920.630.800.780.781.460.830.720.610.620.530.571.080.590.555.120.570.540.510.50

C%

0.170.130.280.220.170.170.200.220.130.340.320.170.250.340.260.130.370.360.170.030.030.190.150.210.030.050.070.030.050.770.030.040.060.07

HI

172240452405300222319282165475418151287324277220501511228

2144

279257379

5696705490

17475

101123132

01

585990172596812479530213566186158235401230126771117337807313545686454211288

612042551365457941512

T'max(°C)

384404490544413517588464544470488407548471504413513468521310417418413474396384408398517412399398373369

PI

0.300.230.200.200.200.220.080.140.140.140.120.260.160.200.200.160.100.090.140.300.140.120.180.070.210.170.120.180.210.040.020.040.210.21

S2/S3

0.290.242.620.680.360.460.601.320.292.552.640.640.711.402.190.284.261.510.280.070.080.400.561.790.191.570.340.210.663.181.301.077.87

11.00

218


600

500-

400-

8

ü 300

200-

100-

TYPE II

TYPE III

0 100 200

01 (mg Cθ2/g carbon)

Figure 4. Diagram of Rock-Eval HI vs. OI of Site 765 Cenozoic samples(open squares), Site 765 Mesozoic samples (closed squares), and Site 766Mesozoic samples (open triangles).

suggest that these samples have a high content of terrestrialorganic matter, although the mineral matrix effect (because of therelatively low calcium carbonate and relatively high clay con-tents) probably results in depressed HI compared to the Cenozoicsamples. The TOC-rich sample (5.1 wt%) had the highest HI, butthe Rock-Eval data from this and adjacent samples did not provideany clues to changes in the type of organic matter being depositedin the sediments.

Gascoyne Abyssal Plain

Shipboard TOC data in sediments from the Gascoyne AbyssalPlain are summarized in Figure 5. TOC data from 25 additionalsamples (open circles) selected from the Lower Cretaceous (Table1) were similar to shipboard data. TOC was mostly undetectablein the Cenozoic and Upper Cretaceous sediments, but increasedin the Lower Cretaceous to concentrations of 1.8 wt% in samplesof Valanginian age. The Rock-Eval data determined aboard theResolution were screened, and all samples having S2<0.2 andTOC <0.5 wt% have been excluded—the remainder are summa-rized in Table 3. The Mesozoic samples (open triangles) areplotted in Figure 4. All Cenozoic samples failed to provide reli-able Rock-Eval parameters and have been excluded. The Creta-ceous samples have HI values of generally <IOO and variable OI,and contain Type III organic matter similar to that of the LowerCretaceous sediments from Argo.

GAS CHROMATOGRAPHY AND SOURCECHARACTERISTICS

Gas chromatography (GC) analyses of the saturates' extract ofall samples showed a homologous series of straight-chained al-kanes ranging from «-C15 to >ΛZ-C30 Typical GC traces from EarlyCretaceous samples (of Aptian age) from Argo (i.e., Sample

(

1

1

1

1

400 -

)

•

1

•

1

•

•OI

•

••

•

• o

0

• •••

•

* •

• rl • o

•

m

m

‰

T0C(wt%)1

Tertiary

Late Cretaceous

Albian

Aptian

• o ^ Barremian

• • o ^

Hauterivian

£*^ Valanginian

•°Ji •

/ / BASALT ' / / / / / / /

2

|

s

I

Ear

Y//Figure 5. Profile of downhole TOC, Site 766 (shipboard Rock-Eval data= closed squares; BMR data = open circles. Samples extracted for hydro-carbons are shown with an arrow).

123-765C-44R-3, 42-50 cm) and a Valanginian sample fromGascoyne (123-766A-48R-4, 16-18 cm) are shown, respectively,in Figures 6A and 6B. These traces represent the end-membercompositions and illustrate the differences between the charac-teristics of the saturated hydrocarbons at each of these two sites.

Three important features are found in the gas chromatograph(FID) traces. The first relates to the relative proportions of short-and long-chain n-alkanes summarized in Table 4. The ratio ofn-C27/n-C17 may be used as an indicator of the relative contribu-tions of terrestrial and marine organic matter respectively (e.g.,Tissot and Welte, 1984). The high n-C27/n-C 1 ratios measured inthe relatively thick (115 m) Aptian (Sample 123-765C-44R-3,42-50 cm) and Barremian (85 m) sections (Sample 123-765C-49R-1, 96-104 cm) from Site 765 indicate high proportions ofterrestrial organic matter. Similarly, the high ratios measured intwo samples from Site 766 (123-766A-30R-4, 48-54 cm and123-766-31R-2, 145-150 cm) in dark greenish-gray to blackupper Hauterivian and Barremian calcareous claystones indicatea higher proportion of terrestrial organic matter in these samplesthan in older sediments from Site 766. These observations areconsistent with palynological shipboard investigations, whichfound an abundance of spores and pollen in the high sedimen-tation rate (14 m/m.y.) Aptian/Albian sections of Site 765 and alsoin the M. Australis Zone (mostly of Barremian age) of Site 766(Ludden, Gradstein, et al., 1990).

The second feature of the chromatograms (Fig. 6) is the highproportion of peaks eluting between each alternative n-alkane(peaks indicated by an asterisk). Their elution positions and massspectra suggest that these are homologous series of branchedalkanes, but at present, little is known about either their precisestructure or their origin. These branched alkanes have differentcharacteristics from the mid-chain monomethyl alkanes that areprominent in many Proterozoic and lower Paleozoic sediments

219


Table 3. Selected shipboard Rock-Eval and calcium carbonate data, Site 766.

100Core, section,interval (cm)

123

766A-30R-1, 140-150766 A-31R-1,67-70766A-31R-2, 143-150766A-32R-4, 138-141766A-32R-4, 143-150766A-37R-1,69-70766A-44R-4, 0-2766A-44R-6, 15-17766A-45R-4, 0-2766A-45R-4, 106-108766A-46R-2, 0-2766A-46R-3, 32-34766A-46R-4, 140-150766A-46R-5, 141-143766A46-R-6, 58-59766A-47R-3, 103-105766A-47R-3, 148-150766A-47R-4, 102-104766A-48R-1, 131-133766A-48R-4, 16-18766A-48R-5, 0-2766A-49R-1,97-98766A-49R-2, 0-2766A-49R-2, 142-143766A-49R-4, 34-35

Depth(mbsf)

279.30288.17290.43303.08303.13346.19417.70420.85427.30428.36434.04435.86438.44439.95440.62446.23446.68447.72453.11456.46457.80462.47463.00464.42466.34

CaCO3

(wt%)

4.14.26.83.32.84.65.14.36.75.15.14.64.36.54.65.44.93.83.63.33.41.02.32.41.8

Si

0.060.000.430.010.090.130.170.010.000.080.000.040.040.060.040.040.010.040.080.050.020.010.040.110.01

s2

0.560.360.900.280.670.370.620.410.281.590.500.480.411.200.390.410.290.360.440.330.330.510.710.510.28

s3

1.491.272.081.000.701.790.851.230.801.570.901.420.861.841.551.751.171.421.010.990.681.010.721.011.29

TOC(wt%)

0.560.550.790.710.750.570.790.620.571.600.810.790.651.530.690.660.510.640.630.500.570.670.750.820.57

C(%)

0.050.030.110.020.060.040.060.030.020.130.040.040.030.100.030.030.020.030.040.030.020.040.060.050.02

HI

10065

11339896478664999616063785662565669665776946249

OI

26623026314093

31410719814098

11117913212022426522922116019811915096

123226

T1max(°C)

436418399414411400412416419423419422402422415416409412401330418426596414413

PI

0.100.000.330.040.120.260.220.020.000.050.000.080.090.050.100.090.030.100.150.130.060.020.050.180.04

S2/S3

0.370.280.430.280.950.200.720.330.351.010.550.330.470.650.250.230.240.250.430.330.480.500.980.500.21

Table 4. Extractable organic matter and gas chromatographyanalyses.

Core, section,interval (cm)

123-

765C-44R-3, 42-50765C-49R-1,96-104

766A-30R-4, 48-54766A-31R-2, 145-150766A-32R-4, 132-138766A-38R-3, 98-104766A-41R-4, 82-87766A-45R-4, 98-106766A-47R-4, 95-102766A-48R-4, 16-18766A-49R-3, 47-51

Age

ApB

BBB/HHHHVVV

Extractableorganic matter

Yield(PPm)a

7585

62182184135323238192137125

mg HC/grock

1.63.5

5.27.88.62.8

21.41.84.29.13.5

Gaschromatography

B-C27/n-C17

1.861.72

1.451.570.751.001.051.170.950.821.37

Pr/Ph

0.651.01

nd1.000.810.801.000.931.331.501.43

• mid-methyl alkanes

lPpm yield of extract per gram rock; Ap = Aptian, B = Barremian, H =Hauterivian, V = Valanginian.

and petroleums (Jackson et al., 1986; Klomp, 1986; Summons,

1987; Summons et al., 1988). These unidentified hydrocarbons

sometimes appear in samples of sedimentary bitumen that have

experienced high degrees of thermal alteration (Summons, un-

publ. data, 1990), or in immature, organically lean, oceanic sedi-

ments. The former, however, is not consistent with other indica-

tors of the low thermal maturity of samples from both sites. Here,

their presence, along with an extensive unresolved complex mul-

tiple (UCM) of hydrocarbons, is consistent with a suite of struc-

tures that no longer resemble their original biogenic precursors

and that might indicate a component of reworked or "background"

organic matter. Gough and Rowland (1990) showed by chemical

degradation that the UCM (hump) owes its appearance to a mul-

tiplicity of unnaturally branched structures. Their high relative

0 10 20 30 40 50 60 70

* mid-methyl alkanes

0 10 20 30 40 50 60 70

Figure 6. Representative GC traces of (A) a sample of Aptian age from

Site 765 and (B) a sample of Valanginian age from Site 766.

220


abundance here would result from the fact that little or no hydro-carbon has been generated by catagenesis of in-situ kerogen.

Finally, the two major acyclic isoprenoid hydrocarbons in allthe samples are pristane and phytane. Empirical observations ofmarginally mature to mature rocks suggest that the ratios ofpristane to phytane vary systematically according to the type ofdepositional environment (Powell and McKirdy, 1973). However,the exact reasons for this are currently subject to some controversy(see reviews by Volkman and Maxwell, 1986; ten Haven et al.,1985, 1987, 1988). The sources of pristane and phytane aremultiple and complex, but as a general rule, Pr/Ph values >3 aretypical of organic matter having predominantly terrestrial sourcesdeposited in, or transported through, an oxic environment. Valuesof less than 1 are typically found in sediments from anoxic marineor hypersaline depositional environments. Because pristane/phy-tane ratios are only reliable indicators of source environment inmarginally mature to mature rocks, the immaturity of all Mesozo-ic samples from both sites (see below) and the fact that Pr/Phratios mostly fall between 1 and 3 preclude a definitive interpre-tation of the ratios summarized in Table 4 and, therefore, ofdepositional environment and occurrences of anoxic/oxic condi-tions (Powell, 1988).

Interpretations of biomarker ratios, such as Pr/Ph and others(see below), to infer the depositional setting and sources of or-ganic carbon must be made cautiously and in consideration ofother known information. In this case, the indicators of lowmaturity tell us that only a minor proportion of the organic carbonhas been converted to hydrocarbon. As such, these hydrocarbonsmay not be representative of the whole suite of organic mattertypes, because thermally stable kerogen will only release diagnos-tic markers at higher levels of maturation. Hence, the source-re-lated biomarkers that we identified can indicate that contributionsof different types exist, but cannot be used to define accuratelythe quantitative relationships of these contributions.

GC-MS AND SOURCE-RELATED BIOMARKERS

Information regarding source type sometimes may be obtainedfrom the relative abundance of C27:C28:C29 sterols or their fossilcounterparts, the steranes (Huang and Meinschein, 1979; Shi etal., 1982). Land plant influxes usually result in a dominance ofC29 desmethylsteranes, whereas in predominantly marine sedi-ments, different types and proportions of algal to zooplanktonsources can result in a wide variety of sterane distributions (Sei-fert and Moldowan, 1978; Mackenzie et al., 1982; Volkman,1986). Geological age also may be an important consideration

(Grantham, 1986; Grantham and Wakefield, 1988) when assess-ing the C2 7/C2 9 parameter. The lower C2 7/C2 9 ratios for the Barre-mian/Hauterivian through Valanginian sediments of Site 766,compared with the Aptian and Barremian sediments of Site 765(Table 5, Col. 1), suggest an increased terrestrial influence in theValanginian sediments of Site 766. However, this is contrary tothe n-alkane distribution (Table 4) and highlights the difficultiesfor attributing specific sources to the desmethylsteranes (Volk-man, 1988). In this case, we suggest that the n-alkane distributionsbetter reflect the terrestrial influxes. The presence and high rela-tive abundance of the specific C 3 0 sterane, 24-n-propylcholestane,is a reliable marine marker (Moldowan et al., 1985, 1990). Itsrelative abundances here (Table 5, Col. 2) are similar, for exam-ple, to the Mesozoic marine Toolebuc Formation (Boreham andPowell, 1987) and other classic marine sediments, such as theMonterey Formation of California, Lower Toarcian of France, andNorth Sea Kimmeridge Oil Shale (Moldowan et al., 1985; 1990;Summons et al., 1987) and, consequently, indicate a major con-tribution of organic matter of marine origin to the Argo andGascoyne abyssal plains.

Methylsteranes are in high abundance relative to their des-methyl counterparts in all samples (Table 5, Col. 3). The 4-methylsubstitution in ring-A is a characteristic of dinoflagellate sterolsand suggests that these organisms are the source of the 4-methyl-steranes (Whithers, 1983; Rubinstein and Albrecht, 1975; Robin-son et al., 1984; Brassell et al., 1987). The presence of the steroidside chain substitution of 24-ethyl, together with four isomers of23,24 dimethyl for the C 3 0 4-methylsteranes (Table 5; Col. 4), isa characteristic of marine sediments (Summons et al., 1987). Thispattern becomes prominent in the middle Triassic and coincideswith the first appearance in marine sediments of dinoflagellatecysts (Thomas et al., 1990). In nonmarine sediments of equivalentage, only the 24-ethyl side-chain substitution pattern is found in4-methylsteranes (Summons et al., 1987). The higher relativeabundance of the 24-ethyl to the 23,24 dimethyl side chain (di-nosterane) (Table 5; Col. 4) in the Aptian and upper Barremiansediments from Argo may reflect subtle compositional differencesbetween dinoflagellate lipid influxes. Two additional series ofmethylsteranes, 2α-methyl and 3ß-methyl ring-A substitution,were observed together in near equal concentrations as their4-methyl isomers (Table 5; Col. 5). Some evidence exists tosuggest that the occurrence of 2- and 3-methylsteranes is associ-ated with bacterial activity during early diagenesis (Summons andCapon, 1988). This appears to be true for the present sample set.The Aptian and upper Barremian sediments from Argo, which

Table 5. Gas chromatography and mass spectrometry source-dependent biomarker parameters.

Core, section,interval (cm)

765C-44R-5, 2-10765C-49R-1, 96-104766A-38R-3, 98-104766A-45R-4, 98-106766A-47R-4, 95-102766A-48R-4, 16-18766A-49R-3, 47-51

C27Stera

C29Ster

0.841.100.530.520.460.620.45

C30Sterb

C29Ster

0.0870.0700.0790.1200.0650.0650.110

^C30Me Sterc

C29Ster

0.510.510.320.440.370.450.48

24-ethyld

23,24DiMe

0.901.210.480.480.630.530.58

2α+3ßMee

4αMe Ster

1.600.770.290.510.560.570.53

Z H o p f

ZSter

5.513.32.34.74.03.84.1

2αMe Hop g

Hop

0.0220.0320.0160.0120.0320.0410.029

29Hh

Hop

0.510.720.800.270.540.450.51

28,3OJHop

0.0240.0230.0510.0240.0470.0480.044

28,30k

29,30

1.12.02.93.23.52.23.0

a (20R)-5α(H),14α(H),17α(H)-cholestaneC(27)/(20R)-24-ethyl-5α(H),14α(H),17α(H)-cholestane(C29).b (20R)-24-propyl-5α(H),14α(H),17α(H)-cholestane(C30)/(20R)-24-ethyl-5α(H),14α(H),17α(H)-cholestane(C29).c (20R)-2α+(20R)-3ß+(20R)-4α-methylsteranes/(20R)-24-ethyl-5α(H),14α(H),17(α)-cholestane(C29).d (20R)-4α-methyl-24-ethyl-5α(H),14α(H),17α(H)-cholestane(C30)/(20R)-4α-methyl-23,24-dimethyl-5α(H)>14α(H),17α(H)-cholestane(C30).e (20R)-2α+(20R)-3ß-methyl-24-ethyl-5α(H),14α(H),17α(H)-cholestane(C3o)/(20R)-4α-methylsteranes(C30).f

8 2α-methyl-17α(H)-hopane(C31)/17α(H),21ß(H)-hopane(C30).h 17α(H),21 ß(H)-norhopane(C29)/17α(H),21 ß(H)-hopane(C30).j 28,30 bisnorhopane(C2g)/17α(H),21ß(H)-hopane(C30).k 28,30 bisnorhopane(C28)/29,30 bisnorhopane(C28).

221


have the highest relative concentration of hopanes, (Table 5; Col.6, an indicator of bacterial activity, see below), also contain moreabundant 2- and 3-methylsteranes relative to 4-methylsteranes(Table 5; Col. 5).

A significant and variable bacterial contribution can be seenfrom the relative abundance of hopanes to steranes (Table 5; Col.6). The Aptian and upper Barremian samples from Argo containthe highest concentration of hopanes relative to the desmethyl-steranes. Sedimentary hopanes are known to be derived fromfunctionalized hopanoids in bacteria (Rohmer et al., 1984; Ouris-son et al., 1987), while hopanoids and hopane polyols having anadditional methyl substitution at C-2 or C-3 in ring-A have beenrecognized in several classes of bacteria, such as methylotrophicbacteria and cyanobacteria (Ourisson et al., 1987). Price et al.(1988) and Summons and Jahnke (1990) observed that high rela-tive abundances of methylhopanes (i.e., 2-methylhopanes) in therange 10% to 20% of corresponding hopane are characteristic ofsediments having high carbonate contents. The relative abun-dances of 2-methylhopanes in the samples from both sites (Table5; Col. 7) are low and in the range 1.2% to 4.1 % of the correspond-ing hopanes. This is consistent with the low carbonate contents,and the ratio of 2-methylhopane/hopane does not vary signifi-cantly over the sample set. The ratio of C29 hopanes/C30 hopanes(Table 5, Col. 8) is also typical of clastic sediments.

Other distinctive hopanes include the C28 species, 28,30-bis-norhopane, commonly found in marine sediments (e.g., Granthamet al., 1980; Curiale and Odermatt, 1989) and 29,30-bisnorhopane(Summons and Powell, 1987). Both these biomarkers are presentin low abundances in all samples (Table 5; Cols. 9 and 10) buttheir presence, in accord with other source parameters, suggestsa marine source for the organic matter. The high relative abun-dances of bacterial biomarkers is a further illustration of theimportance of bacterial contributions to sedimentary organiccarbon irrespective of the type and age of the depositional setting.

MATURITYPyrolysis data are summarized in a plot of Tmax vs. HI (Fig. 7).

The Cenozoic samples appear to separate into two distinct groups:those samples with Tmax <430°C and those greater than 450°C.The former Tmax values suggest that the samples are immature,while the latter indicate mature to over-mature organic matter.However, based upon these limited data, one can see that twocarbon sources are contributing to the TOC in these organic-leansediments. A high proportion of reworked allochthonous organicmatter will manifest itself as a high Tmax value, while the immatureautochthonous component is responsible for the consistently highHI values. Where this latter component is high, a low Tmax also isobserved. Based on the Tmax values, the organic matter in theMesozoic samples from both sites is immature

GC analyses of the saturated hydrocarbons indicate that apronounced odd-to-even predominance exists in the hydrocar-bons greater than C23 in all samples, reflecting the immaturity ofthe sediment with respect to petroleum generation (Tissot andWelte, 1984). The low maturity of the Mesozoic samples, inferredfrom the Rock-Eval data, also is evident from the biomarkermaturity parameters. The original biological configuration (20Rin steranes and 22R in hopanes) is dominant over the more stable(20S and 22S) geological configuration that is present in matureoils and sediments (Tissot and Welte, 1984). Epimer ratios for thesteranes and hopanes (Table 6; Cols. 1 and 2, respectively) are farfrom the equilibrium values that exist within the oil window.Furthermore, the presence of hopanes with the biological17ß(H),21 ß(H) configuration places their maturity well below theonset of oil generation. This configuration is unstable and disap-pears during early thermal evolution of all sediments (ten Havenet al., 1986). The high proportions of moretane 17ß,21α (>10%),

600

500-

400-

Sσ>

O 300-X

X200-

100-

TYPE I

TYPE II

D

α

ü

TYPE III

300 400 500

•max^

Figure 7. Plot of HI vs. Tmax for Site 765 Cenozoic samples (open squares),Site 765 Mesozoic samples (closed squares), and Site 766 Mesozoicsamples (open triangles).

Table 6. Maturation-dependent biomarker parameters.


123-

765C-44R-5, 2-10765C-49R-1, 96-104766A-38R-3, 98-104766A-45R-4, 98-106766A-47R-4, 95-102766A-48R-4, 16-18766A-49R-3, 47-51

20S(%)a

19151414111610

22S(%)b

29133828303630

ßαc

αß

0.160.290.110.090.220.170.13

Tmd

Ts

3.83.70.92.32.71.62.5

a100x(20S)/(20S)+(20R)-24-ethyl-5α(H),14α(H),17α(H)-cholestane(C29).b 100x(22S)/(22S)+(22R)-17α(H),21ß(H)bishomohopane (C32).c 17ß(H),21α(H)/17α(H))21ß(H)-hopane(C30).d 18α(H)-trisnorneohopane/17α(H)/-trisnorhopane(C27).

compared to hopane (17α,21ß(H) (Table 6; Col. 3), and thedominance of Tm over the more stable Ts (Table 6; Col. 4)constitute further evidence of the immature nature of the sedimen-ts. The relative maturity within the sample set is harder to ascer-tain because these maturation parameters are also influenced tovarying degrees by source effects. However, it appears that nosignificant maturity difference exists among samples from the twosites, although petrography of the organic matter (which was notdone) might clarify the question of maturity.

SUMMARYExamination of GC, GC-MS biomarker, and Rock-Eval data

derived from samples analyzed at BMR and Rock-Eval datagathered aboard the Resolution indicates the following about thenature and source of organic matter at these two abyssal plain drillsites:

1. Cenozoic sediments from Site 766 have undetectable TOC(<O. 1 wt%). Cenozoic sediments from Argo have <1.5 wt% TOC,

222


and concentrations tend to decrease downhole (although errati-cally because of turbidite deposition) as organic carbon is remin-eralized via early diagenetic reactions.

2. Rock-Eval pyrolysis of Cenozoic samples from Argo sug-gests that the organic matter is mostly of a marine source, but witha component of reworked and degraded organic matter, probablyderived from the continental margin via turbidite deposition.

3. Biomarker distributions determined by GC and GC-MSanalysis indicate that the organic matter extracted from the sedi-ments (inferred from the high relative abundances of the specificmarine biomarker, 24-N-propylcholestane), at both sites, is pri-marily of a marine source with varying contributions of terrestrialorganic matter (inferred from the ratios of n-C2η/n-Cll).

4. Influxes of dinoflagellates, inferred from the high abun-dances of the 4-methylsteranes with the 23,24 dimethyl sidechain, are significant at both sites.

5. The organic matter in the Lower Cretaceous sediments fromboth sites is immature. This immaturity and the low levels ofextractable hydrocarbons found in these sediments suggest cau-tion when interpreting biomarker data, which may not completelyrepresent the total organic carbon content from the sediments.

6. Lower Cretaceous sediments from Argo are generally leanin organic matter, although a sample near the Barremian/Aptianboundary has a TOC of 5.1 wt% and samples near the Aptian/Al-bian boundary have TOC values up to 1.2 wt%. Sediments fromthe Aptian and upper Barremian contain marine components oforganic matter and relatively high proportions of terrestrial or-ganic matter.

7. Lower Cretaceous sediments from Gascoyne have low TOC(<O. 1 wt%) in the Aptian/Albian and near the Barremian/Aptianboundaries. Maxima in TOC occur in the upper Barremian (1.02wt%), the Barremian/Hauterivian (0.6 wt%), and the Valanginian(1.8 wt%). Sediments from Site 766 contain marine organic mat-ter, and samples from the upper Barremian contain higheramounts of terrestrial organic carbon than older sediments.

8. Influxes of bacteria comprise significant contributions to theorganic matter composition at both sites.

ACKNOWLEDGMENTS

We thank Zoltan Horvath, Janet Hope, and Philip Fletcher forprocessing the samples, for assisting with the analyses, and forcollating the data, and G. W. O'Brien, T. G. Powell, and L.Snowdon for their constructive comments. BMR Editor K. H.Wolf assisted with preparation of the final manuscript. DTH, CJB,and RES publish with the permission of the Executive Director,Bureau of Mineral Resources, Canberra, Australia.

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Date of initial receipt: 27 August 1990Date of acceptance: 22 May 1991Ms 123B-168

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