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Gradstein, F. M., Ludden, J. N., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 123
12. ORGANIC MATTER AND PALEOCHEMISTRY OF CRETACEOUS SEDIMENTS FROM THEARGO AND GASCOYNE ABYSSAL PLAINS, NORTHEASTERN INDIAN OCEAN1
David T. Heggie2
ABSTRACT
The accumulation of organic matter, ferrous and pyrite iron, and the ratios of organic carbon/total sulfur and organic carbon/totalphosphorus in the Lower Cretaceous sediments from the Argo and Gascoyne abyssal plains have been used as indicators of both thesource and reactivity of organic matter in the sediments and the depositional environment.
Total sulfur, used as an indicator of pyrite sulfur, is more abundant in sediments from the Gascoyne Abyssal Plain than in thosefrom the Argo Abyssal Plain. Sulfur positively correlates with TOC at both sites (although poorly at the Argo Abyssal Plain site, R= 0.48), with an extension of the line of best-fit through the origin, indicating that pyrite (TOC<2 wt%) is diagenetic and depositedfrom normal marine conditions. The average ratio of C/S for samples of TOC <2 wt% is 5.4 at Argo Abyssal Plain (compared tothe modern normal marine value of 2.8) indicating deposition of organic matter probably of mixed terrestrial and oxidized marinesources that is unreactive to the sulfate-reducing bacteria. One sample from the Aptian sediments is rich in TOC (5.1 wt%) and hasa C/S ratio of 0.5. The average C/S ratio in Gascoyne Abyssal Plain sediments is 0.8 (R = 0.97), which indicates the formation ofabundant pyrite in addition to burial and preservation of relatively fresh organic matter that is reactive to the sulfate-reducing bacteria.Organic carbon to phosphorus ratios (C/P) in the sediments indicate preferential remobilization of organic carbon over phosphoruswith increasing water depth.
Estimates of the degree of pyritization (DOP) increase with increasing TOC at both sites, indicating iron is not limiting andpyrite is formed diagenetically. The one sample with a TOC content of 5.1 wt%, from the Argo Abyssal Plain near theBarremian-Aptian boundary, is composed mostly of framboidal pyrite, finely laminated and not bioturbated, and hence may havebeen deposited during a brief period of anoxia in the overlying waters.
INTRODUCTION
Drilling data from Site 765 in the Argo Abyssal Plain (present-day water depth, 5725 m; Fig. 1) and Site 766 at the foot of theExmouth Plateau escarpment in the Gascoyne Abyssal Plain(present-day water depth, 3997.5 m; Fig.l) indicate sedimenttypes and total organic carbon (TOC) contents in the Cretaceoussections (Ludden, Gradstein, et al., 1990) that reflect differentdepositional environments in this part of the incipient north-eastern Indian Ocean.
On the Argo Abyssal Plain, -500 m of zeolitic and siliceousclays, calcareous ooze, and turbidites overlie condensed Paleo-gene and Upper Cretaceous sections and -400 m of Lower Creta-ceous sediments. Calcareous and siliciclastic turbidites and radio-larites are evident in these Lower Cretaceous sediments, althoughthe Berriasian through Barremian sections (805-971 m belowseafloor [mbsf]) and the Aptian/Albian sections (591-805 mbsf)are dominated by alternating bands of various thicknesses, overscales of centimeters to meters, of red, brown, green, dark-gray,and black claystones with various degrees of bioturbation (vonRad et al., 1989; Ludden, Gradstein, et al., 1990).
On the Gascoyne Abyssal Plain, -100 m of Cenozoic pelagiccalcareous and siliceous ooze overlie -370 m of Cretaceous sed-iments, of which >300 m were deposited during the Early Creta-ceous. These Lower Cretaceous sediments from the GascoyneAbyssal Plain are dominated by mixtures of greenish-gray/blacksiltstones and sandstones (Valanginian/Barremian), while the Ap-tian through Albian sediments are tan to light-green nannofossil
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.
chalks which are clayey, silicified, and zeolitic (von Rad et al.,1989; Ludden, Gradstein, et al., 1990).
The objectives of this research were (1) to compare and con-trast the Early Cretaceous depositional environments at the twosites, (2) to comment upon the reactivity and source(s) of organicmatter, and (3) to comment upon oxic/anoxic conditions in thesediments and overlying waters. This study was prompted, in part,by (1) the shipboard observations of the patterns of color varia-tions in the sediments at Site 765 (black, dark-green/gray-green,and red claystones) and the probability that these indicated differ-ent abundances of diagenetic iron minerals as a result of variableTOC fluxes, source(s), and reactivity of the organic matter, (2)the obvious contrast between sediments of comparable ages at thetwo sites, and (3) the single occurrence of a thin (1 cm) section ofhigh TOC content (5.1 wt%) in the Aptian sediments of the ArgoAbyssal Plain, possibly representing the record of an isolatedanoxic water column event.
During this study, I examined estimates of the abundances ofsome diagenetic products of organic matter degradation that pro-vide clues to the depositional environment, e.g., sediment TOCsources and the reactivity of the organic matter and bottom wateroxygen conditions. The following estimates are reported on here:(1) total organic carbon (TOC) and total sulfur (an estimate ofpyrite sulfur) contents; (2) organic carbon/total sulfur ratios (here-after shown as C/S); (3) limited iron speciation (ferrous iron andpyrite iron abundances); (4) estimates of DOP (degree of pyriti-zation); (5) total phosphorus contents; and (6) organic carbon/total phosphorus ratios (hereafter reported as C/P) in the sedi-ments.
METHODSCore samples were analyzed for TOC by Rock-Eval pyrolysis
and calcium carbonate by coulometry (Emeis and Kvenvolden,1986). These data are summarized in Ludden, Gradstein, et al.
225
D. T. HEGGIE
114° 120
20°
ODP Leg 123 site
O DSDP site
500 km
23/OWA/130
Figure 1. Map of the study area with the locations of the Argo (Site 765) and Gascoyne (Site 766)abyssal plain drill sites.
(1990). Twenty-nine samples from the Argo (Site 765) and 19from Gascoyne (Site 766) abyssal plains were freeze dried,ground, and analyzed at the Bureau of Mineral Resources (BMR)laboratories by X-ray fluorescence (XRF) for major elements(Norrish and Hutton, 1969). The total sulfur content has been usedto approximate pyrite sulfur and also to stoichiometrically calcu-late pyrite iron. I have assumed that organic sulfur, iron monosul-fides, and oxidized sulfur species are not quantitatively importantin these deep-sea sediments. These assumptions place constraintson the interpretation of the data discussed below.
Ferrous oxide was determined by a modification of the methoddescribed by Shapiro and Brannock (1961), as follows: 0.5 g ofsediment was weighed into a 50-mL platinum crucible and di-gested with H2SO4 (10 mL, 1:3) and HF (5 mL, 40%) for 10 minon a hot plate. The digest was transferred to a 800-mL beakercontaining distilled water, boric acid, and phosphoric acid (10mL, 1:1). The solution was titrated with standard potassium di-chromate solution using a Metrohm E536 potentiograph and com-bined platinum reference electrode. Several international stand-ards (GSP-1, PCC-l,and BCR-1) were also analyzed, andreplicate analyses of BMR standards indicated a precision ofl%-3%. Although standards with sulfides were not analyzed, themethod for FeO contents does not attack pyrite in the samples (G.W. Skyring, T. Donnelly, pers. comra., 1990). The mineralogy of12 samples from the Argo Abyssal Plain was determined by X-raydiffraction (XRD).
RESULTS
Calcium Carbonate and Major Elements
The downhole calcium carbonate results are reported in Lud-den, Gradstein, et al. (1990) and are shown, for both sites, inFigure 2. Erratic variations in the downhole calcium carbonatecontents of the Cenozoic section from Argo Abyssal Plain (Fig.
2A) indicate turbidite-dominated sedimentation below the calcite-compensation depth (Ludden, Gradstein, et al., 1990). Calciumcarbonate contents of Lower Cretaceous sediments are generallylow, although some samples had calcium carbonate approaching60 wt%. Calcareous and siliciclastic turbidites and radiolarites areevident throughout the Cretaceous, but the Lower Cretaceoussection is dominated by alternating bands of various thicknessesof red, brown, green, dark-green, dark-gray, and black claystones(von Rad et al., 1989; Ludden, Gradstein, et al., 1990).
The downhole calcium carbonate profile from Gascoyne Abys-sal Plain (Fig. 2B) indicates generally low (<15 wt%) calciumcarbonate in the Berriasian, Valanginian, and Barremian sedi-ments and a marked increase in calcium carbonate within theAptian sediments. This increase in carbonate suggests a changein sedimentation near the Aptian/Barremian boundary from hemi-pelagic to pelagic conditions; the Lower Cretaceous sediments arecombinations of dark-green calcareous claystones, sandstones,and siltstones, while the Late Cretaceous and Tertiary sedimentsare combinations of calcareous ooze, siliceous clays, and turbi-dites (von Rad et al., 1989; Ludden, Gradstein, et al., 1990).
Sample selection of the Lower Cretaceous sediments fromboth sites, for analyses and consideration of diagenetic products,focussed on sediments that are not significantly diluted by bio-genic components so that iron limitation is not a factor influencingpyrite formation (Raiswell and Berner, 1986). The XRF majorelement data from Sites 765 (Argo Abyssal Plain) and 766 (Gas-coyne Abyssal Plain) are summarized in Tables 1 and 2. Thesamples contain <20 wt% calcium carbonate (estimated from CaOdata). The XRF data show that AI2O3 contents vary between 4 and16 wt% and SiC<2 between 30 and 80 wt%. A crossplot of thesedata (Fig. 3) shows decreasing AI2O3 with increasing S1O2, indi-cating some dilution of the terrigenous aluminosilicate compo-nents by another siliceous (biogenic-marine) phase, probablyradiolarites.
226
ORGANIC MATTER AND PALEOCHEMISTRY OF CRETACEOUS SEDIMENTS
CaCO3(wt%) CaCO3(wt%)40 60 100
Berriasian
WWWWWW
Hauterivian
Valanginian
\w.\\\\\.\ \\Figure 2. A. Calcium carbonate vs. depth (mbsf) for the Argo Abyssal Plain (cross-hatching = basement). B. Calcium carbonatevs. depth (mbsf) for the Gascoyne Abyssal Plain (cross-hatching = basement).
3 0 4 0r
5 0 6 0SiO2 (wt%)
8 0
Figure 3. AI2O3 (wt%) vs. Siθ2 (wt%) for Argo (closed triangles) and Gascoyne(open squares) abyssal plain sediments.
Organic Matter (TOC) in Sediments
The oxidation of organic matter within sediments results in theliberation to pore waters of oxidation/reduction metabolites, achange in the molecular composition of the organic matter, andthe accumulation in the sediments of diagenetic byproductsformed by the reaction of pore-water metabolites with sedimen-tary components (Froelich et al., 1979; Berner, 1981).
A brief description of the sedimentary units, from which sam-ples were collected for this work, together with TOC, TOC/totalS (wt%) ratios, TOC/total P(atom) ratios, iron speciation includ-ing total iron Fe(t), ferrous iron Fell, the ratio FeII/Fe(t), pyriteiron Fe(py) calculated stoichiometrically from total S (Tables 1and 2) and the ratios Fe(py)/Fe(t) and [Fell + Fe(py)]/Fe(t) aresummarized in Tables 3 (Argo Abyssal Plain) and 4 (GascoyneAbyssal Plain).
The TOC data for both sites are summarized in Figures 4A and4B. Although the Lower Cretaceous sediments from both siteswere visually scrutinized at sea for dark, possibly TOC-richsediments, most samples were generally low in TOC. TOC at theArgo Abyssal Plain was highest in the Barremian-Aptian-Albiansections (< 1.1 wt%) near the Aptian/Albian boundary and one thin(1 cm) sediment layer near the Barremian/Aptian boundary had aTOC of 5.1 wt% (Fig. 4A). TOC in the Albian and Aptiansediments from Gascoyne Abyssal Plain (Fig. 4B) were low (<O. 1wt%). Relatively high TOC contents (<0.8 wt%) were measuredin the Barremian dark-green calcareous claystones and Valan-ginian siltstones (1.6 wt%) from Gascoyne Abyssal Plain (Fig.4B). While several of the Valanginian-Hauterivian samples were<0.5 wt% TOC, these were still higher than TOC in the Berria-sian-Hauterivian claystones from the Argo Abyssal Plain (Fig.4A).
The organic carbon accumulation rates in Cretaceous sedi-ments are summarized in Table 5. The Upper Cretaceous sedi-ments from both sites are characterized by low TOC, low sedi-mentation rates, and low organic-carbon accumulation rates. TheLower Cretaceous sediments, in contrast, are characterized byhigher average TOC, high sedimentation rates, and higher or-ganic-carbon accumulation rates.
Ratios of Organic Carbon to Total Sulfur (C/S)
The principal controls on pyrite formation within sediments(Berner, 1984) are (1) the amount, type, and reactivity of organicmatter, (2) the amount of reactive iron minerals, and (3) theavailability of dissolved sulfate. Plots of pyrite sulfur and TOC insediments have been used to distinguish between marine andbrackish-water environments and also between deposition fromnormal marine conditions (those underlying oxygenated bottomwaters where sulfate reduction begins a few centimeters below thesediment/seawater interface) and from an anoxic water column(Berner and Raiswell, 1983; Leventhal, 1983; Berner, 1984).
Sulfate is not limiting in normal marine sediments and, whereiron is not limiting, the accumulation of pyrite is controlled by theamount and reactivity of metabolizable organic carbon. In modernnormal marine sediments there is a good positive correlation
227
D. T. HEGGIE
Table 1. Major element and FeO determinations, Site 765, Argo Abyssal Plain.
Core, section,interval (cm)
123-765C-
30R-4,32-3435R-3, 55-5735R-3,60-694 0 R ^ , 143-14444R-3, 24-2544R-3, 42-5044R-3, 106-10744R-3, 135-13644R-5, 2-1045R-3, 53-554 5 R ^ , 100-10147R-3, 55-5748R-4, 36-3749R-1,94-9549R-1,96-10450R-2, 80-8250R-6,51-5251R-CC, 2-552R-1, 132-13352R-2, 8-1052R-2, 10-1153R-3, 72-7453R-6,123-12555R-2, 82-8457R-7,15-1759R-3, 123-12559R-4, 111-11260R-2, 61-6361R-5, 105-106
Depth(mbsf>
630.64677.65677.74726.83761.74761.95762.56762.86764.53771.73773.70790.95801.76807.24807.28817.90823.61832.52835.82836.08836.10847.82852.83865.32890.55902.13903.51909.61923.95
SiO2
52.9470.5669.0264.7862.6063.5961.9463.2965.2967.4143.9762.5862.2472.8973.3269.8870.3870.3471.0476.5171.1170.4276.6868.3072.3568.8066.6762.4147.61
TiO2
0.780.350.380.850.780.760.770.700.830.600.550.890.670.470.480.840.790.680.630.690.630.500.360.550.430.420.660.550.67
A12O3
15.486.677.324.89
12.2211.8012.4012.0212.0910.406.52
10.7311.079.479.579.218.809.138.539.138.377.965.908.518.789.77
12.3012.08
8.76
Fe2O3
7.281.842.004.896.236.105.956.044.834.15
12.559.545.172.762.794.894.714.294.374.474.195.854.615.875.935.622.168.107.86
FeO MnO MgO(all in wt%)
0.210.540.660.591.031.050.900.950.610.871.400.610.720.560.580.690.610.770.800.690.820.270.180.290.240.280.400.250.00
0.230.020.020.050.150.040.170.070.051.280.800.052.170.030.170.140.040.040.040.070.140.130.090.560.550.040.030.131.46
2.511.231.342.792.402.322.292.392.841.971.242.472.371.821.862.332.122.292.262.352.212.161.582.372.172.382.582.431.96
CaO
2.874.025.020.630.670.600.710.600.640.790.570.671.070.530.580.790.550.540.500.540.530.560.460.970.760.740.740.77
10.77
Na2O K2O
2.221.231.261.47 '1.431.411.411.331.49 '1.300.941.431.271.061.101.201.181.251.131.11
(::::
.12
.07).89.09.14.13.22.10 :
5.14.00.09
1.061.211.162.031.31>.Ol.76.08.83.77.50.55.75.64.78.83.90.63.40.20.56.49.32.89
..880.94 2.57
P2O5
0.110.130.140.110.100.090.120.090.100.130.090.100.110.090.100.230.080.090.070.080.070.100.120.260.130.130.110.190.14
S
0.050.300.380.040.200.250.530.150.040.22
10.160.030.090.240.250.090.060.180.160.160.080.090.100.160.080.070.130.040.03
LOI
12.1912.1111.369.86
10.009.81
10.7910.059.189.11
20.119.07
11.298.577.657.959.058.628.622.309.099.477.849.525.949.31
11.129.08
17.25
Table 2. Major element and FeO determinations, Site 766, Gascoyne Abyssal Plain.
Core, section,interval (cm)
123-766A-
14R-4, 7_817R-6, 105-10628R-2, 25-2829R-2, 44^*630R-4, 4 1 ^ 432R^Φ, 138-14134R-1, 75-7636R-1, 138-13937R-1, 69-7039R-2, 122-12439R-3, 32-3439R-3, 32-3442R-1, 94-9745R^1, 106-10846R-3, 32-3446R-5, 141-14346R-6, 58-5947R-3, 103-10549R-1, 97-9849R-2, 142-143
Depth(mbsf)
128.17161.05260.35270.24282.81303.08317.25337.18346.19367.52368.12368.12394.74428.36435.86439.95440.62446.23462.47464.42
SiO2
49.4230.6158.7460.6258.1064.7459.4058.0758.3955.5754.3355.7848.1156.3058.0954.1456.3855.7052.4658.37
TiO2
0.860.640.450.390.630.660.770.970.951.061.041.051.141.000.900.941.070.950.800.87
A12O3
11.946.819.499.36
11.448.639.23
10.6210.4611.9811.5111.9015.0111.9812.5011.7113.5013.0110.8012.60
Fe2O3
6.934.733.683.684.574.825.806.035.785.186.006.169.574.604.795.574.884.566.995.64
FeO MnO MgO(all in wt%)
0.000.000.580.700.931.011.311.541.521.702.132.145.382.531.531.891.782.256.111.87
0.470.830.020.020.020.010.020.020.020.020.020.020.030.020.010.030.020.020.030.02
4.141.662.302.242.691.942.072.632.522.883.193.263.552.322.312.212.512.477.803.23
CaO
3.7324.46
5.284.923.412.114.642.752.843.264.474.121.813.152.894.223.053.540.891.74
Na2O
:
:;;:
:
.91
.65
.74
.51
.75
.48
.42
.60
.41
.45
.88
.61
.64
.39
.491.471.551.451.541.28
K2O
2.431.401.471.452.022.283.033.863.363.483.253.372.292.332.522.282.612.591.924.11
P2O5
0.340.170.090.120.160.110.160.140.140.140.140.140.090.160.150.170.190.270.120.22
S
0.240.070.410.620.851.160.230.310.730.450.270.270.161.971.102.170.960.870.481.04
LOI
17.5926.9715.7514.3613.4411.0411.9211.4711.8912.8311.7710.1711.2412.2511.7113.1911.5012.3210.059.05
between organic carbon and pyrite sulfur with a line of best-fitextending through the origin with a mean C/S ratio of ~2.8(Leventhal, 1983; Berner and Raiswell, 1983; Berner, 1984;Raiswell and Berner, 1986). However, in fresh or brackish waters,where sulfate may be limiting or organic matter that is unreactiveto the sulfate-reducing bacteria is present, the amount of pyriteformed is less than that formed in marine sediments for compa-rable quantities of organic carbon buried. Hence, C/S ratios arehigher than 2.8, depending upon the salinity and sulfate content
of the overlying water as well as the type and reactivity of theorganic matter buried in the sediments (Berner and Raiswell,1984).
C/S ratios have been used to comment upon depositionalenvironments from other Australian sediments, including the Cre-taceous Toolebuc Formation (Boreham and Powell, 1987), themiddle Cambrian sediments of the Georgina Basin (Donnelly etal., 1988) and the middle Proterozoic Velkerri Formation in theMcArthur Basin (Donnelly and Crick, 1988).
228
ORGANIC MATTER AND PALEOCHEMISTRY OF CRETACEOUS SEDIMENTS
Table 3. TOC and diagenetic products, Site 765, Argo Abyssal Plain.
Core, section,interval (cm)
123-765C-
30R-4, 32-3435R-3, 55-5735R-3, 60-6940R-4, 143-14444R-3, 24-2544R-3, 42-5044R-3, 106-10744R-3, 135-13644R-5, 2-1045R-3, 53-5545R-4,100-10147R-3, 55-5748R-4, 36-3749R-1, 94-9549R-1, 96-10450R-2, 80-8250R-6, 51-5251R-CC, 2-552R-1, 132-13352R-2, 8-1052R-2, 10-1153R-3, 72-7453R-6,123-12555R-2, 82-8457R-7, 15-1759R-3, 123-12559R-4, 111-11260R-2, 61-6361R-5, 105-106
Depth(mbsf)
630.64677.65677.74726.83761.74761.95762.56762.86764.53771.73773.70790.95801.76807.24807.28817.90823.61832.52835.82836.08836.10847.82852.83865.32890.55902.13903.51909.61923.95
Age
AlAAeAeAeAeAeAeAeAeAeAeABBBBBBBBBBHV/HB/VB/VB/VB
Color
orangegreengreengreengray/greengray/greengray/greengray/greengreengray/greenblackredgreengreengreengray/greengray/greengreengreengreengray/greenredredredredredgreenredred
Unit
IVBIVCIVCVAVBVBVBVBVBVBVBVBVBVCVCVCVCVCVCVCVCVCVCVIVIVIIVIIVIIVII
Fe(t)(wt%)
5.261.711.913.885.165.094.864.973.853.589.877.154.182.372.403.963.773.603.683.673.574.303.374.334.344.151.825.865.50
Fell(wt%)
0.160.420.510.460.800.820.700.740.470.681.090.470.560.440.450.540.470.600.620.540.640.210.140.230.190.220.310.190.00
FeII/Fe(t)
0.030.250.270.120.160.160.140.150.120.190.110.070.130.190.190.140.120.170.170.150.180.050.040.050.040.050.170.030.00
Fe(py)(wt%)
0.040.260.330.040.180.220.460.130.040.198.890.030.080.210.220.080.050.160.140.140.070.080.090.140.070.060.110.040.03
Fe(py)/Fe(t)(DOP)
0.010.150.170.010.030.040.100.030.010.050.900.000.020.090.090.020.010.040.040.040.020.020.030.030.020.020.060.010.01
iFe/Fed)
0.040.400.440.130.190.200.240.180.130.241.010.070.150.270.280.160.140.210.210.190.200.070.070.090.060.070.230.040.00
TOC(wt%)
0.001.080.350.440.361.230.460.430.400.245.120.040.050.570.550.090.310.280.330.230.280.020.000.220.010.030.500.020.00
C/S(wt)
3.600.92
11.001.804.920.872.87
10.001.090.501.330.562.382.201.005.171.562.061.443.500.22
1.380.130.433.850.50
C/P(atom)
4.911.482.362.138.082.272.822.361.09
33.620.240.273.743.250.232.291.842.791.702.360.12
0.500.050.142.690.06
Age; Al = Albian, A = Aptian, eA = early Aptian, B = Barremian, V = Valanginian, H = Hauterivian.Fe(t) = total iron, Fell = ferrous iron, Fe(py) = pyrite iron, ZFe = ferrous + pyrite iron, TOC all in wt%. Fe(py)/Fe(t) = degree of pyritization (DOP).Brief descriptions of the subunits from which samples were collected are included below. A complete description can be found in Ludden, Gradstein, et al. (1990).
Subunit IVB: late Albian to late Aptian light colored clays and minor calcareous turbidites. Subunit IVC: late Aptian light to minor dark claystones, lessercalcareous turbidites, and minor radiolarites. Subunit VA: early Aptian dark claystones with minor laminated silty layers. Subunit VB: Barremian to earlyAptian dark claystones with laminated silty layers; color extremely variable but predominantly gray to very dark gray, dark reddish-brown, dark reddish-gray,dusky green or grayish green. Color banding occurs on a cm-dm scale, elongated and flattened mottles indicate diagenetic processes and or bioturbation,scattered occurrences of pyrite, zeolites, and organic debris observed in smear slides. Rhodochrosite sediment and /or microconcretions occur throughout thesubunit. Subunit VC: Barremian reddish-brown and green claystones, lesser rhodochrosite and radiolarites, similar color banding as for Subunit VB, closelyspaced bands of alternating colors are common, probably flattened burrows. Rhodochrosite micronodules and pyrite are sparse. Unit VI: Valanginian toHauterivian reddish-brown chalks (turbidites), lesser claystones and minor radiolarites, calcareous claystones constitute about two-thirds of Unit VI and aretypically reddish-brown and less commonly greenish-gray, locally abundant bioturbation, some rhodochrosite micronodules, and minor amounts of organicmatter and pyrite, glauconite and phosphatic debris evident. Unit VII: Berriasian to Valanginian reddish-brown and green claystone, minor radiolarites andbentonites and brown silty claystones, claystones contain minor silt or sand, rhodochrosite micronodules, radiolarians, organic debris, pyrite, apatite, andfeldspar.
Total Organic Carbon (TOC) and Sulphur (S)
C/S ratios determined on individual samples from both sitesare summarized in Tables 3 and 4 and these data plotted in Figure5. Total sulfur has been used to approximate pyrite sulfur andindividual pyrite sulfur values have not been determined. Somesulfur could be present as organic sulfur, although in these lowTOC sediments organic sulfur would be expected to be small.Saxby (1986), for example, examined sulfur speciation in coresfrom the Eromanga Basin of eastern Australia and his data indi-cate that at <2 wt% TOC, 80%-100% of total sulfur consistentlyoccurred as pyrite. Over a wide range of organic carbon contents,<~IO wt% pyrite sulfur still represented 60%-85% of total sulfur,and organic sulfur contents showed no clear relationship withorganic carbon concentrations. Furthermore, there is no evidenceof sulfate minerals in these deep-sea sediments and residual porewater sulfate in the samples was estimated to be <0.2 wt% totalsulfur. Therefore, total sulfur has been used to approximate pyritesulfur, recognizing that actual pyrite sulfur must be less than thatreported from total S contents.
Undoubtedly, a large error is associated with C/S ratios at lowTOC contents (Raiswell and Berner, 1986). Low TOC (<0.2 wt%)samples from the Argo Abyssal Plain have low total S (<O.l wt%),C/S ratios between <0.3 and 1.4, and occur as red to red/brownsediments (Table 3). Excluding those low TOC samples, thereremain large variations in the C/S ratios of individual samplesfrom all sedimentary units of Site 765. C/S ratios vary between0.5 for the highest TOC sample (5.1 wt%; 773.7 mbsf) of earlyAptian age and 11.0, for another sample (TOC = 0.44 wt%; 726.83mbsf) also of early Aptian age. For those samples with TOC >0.2wt%, the average of individual C/S ratios for the Aptian (sedimen-tary Units IVC, VA, VB) is 3.3+3.6 (12 samples) and for Barre-mian samples (Subunit VC), the average of individual C/S ratiosis 2.2+1.5 (9 samples). TOC and S are poorly correlated in theLower Cretaceous sediments of Site 765; a linear regression of allthat data with TOC<2 wt% (Fig. 5) indicates a mean C/S ratio of5.4 (R = 0.48).
TOC data and C/S ratios from Gascoyne Abyssal Plain aresummarized in Table 4. TOC and total S are plotted in Figure 5.For those samples with TOC>0.2 wt%, the average of individual
229
D. T. HEGGIE
Table 4. TOC and diagenetic products, Site 766, Gascoyne Abyssal Plain.
Core, section,interval (cm)
123-766A-
14R-4, 7_8
17R-6, 105-10628R-2, 25-2829R-2, 44-4630R-4, 41^Φ432R-4, 138-14134R-1, 75-7636R-1, 138-13937R-1, 69-7039R-2, 122-12439R-3, 32-3442R-1, 94-9745R-4, 106-10846R-3, 32-3446R-5, 141-14346R-6, 58-5947R-3, 103-10549R-1, 97-9849R-2, 142-143
Depth(mbsf)
128.17161.05260.35270.24282.81303.08317.25337.18346.19367.52368.12394.74428.36435.86439.95440.62446.23462.47464.42
Age
TAlBBBBHHHHHHHVVVVVV
Color
red/browncreamIt greenIt greenIt greendark greenIt greendark greendark greendark greendark greendark greendark greengreen/graygreen/graygreen/graydark greendark greendark green
Unit
IIBIICIIIAIIIAIIIAIIIAIIIBIIIBIIIBIIIBIIIBIIIBIIIBIIIBIIIBIIIBIIIBIIIBIIIB
Fe(t)(wt%)
4.853.313.033.123.924.165.085.425.234.955.86
10.885.194.545.374.804.949.645.40
Fell(wt%)
0.000.000.450.540.720.781.021.201.181.321.664.181.971.191.471.381.754.751.45
FeII/Fe(t)
0.000.000.150.170.180.190.200.220.230.270.280.380.380.260.270.290.350.490.27
Fe(py)(wt%)
0.210.060.360.540.741.020.200.270.640.390.240.141.720.961.900.840.760.420.91
Fe(py)/Fe(t)(DOP)
0.040.020.120.170.190.250.040.050.120.080.040.010.330.210.350.180.150.040.17
EFe/Fe<t)
0.040.020.270.350.370.430.240.270.350.350.320.400.710.470.630.460.510.540.44
TOC(wt%)
0.010.000.140.390.580.710.130.230.570.460.070.031.600.791.530.690.660.670.82
C/S(wt)
0.040.000.340.630.680.610.570.740.781.020.260.190.810.720.710.720.761.400.79
C/P(atom)
0.02
9.1719.2321.2838.46
4.819.71
23.8119.612.961.97
58.8231.2552.6321.2814.4933.3322.22
Fe(t) = total iron, Fell = ferrous iron, Fe(py) = pyrite iron, ZFe = ferrous+pyrite iron, TOC all in wt%. Fe(py)/Fe(t) = degree of pyritization (DOP).Age: T = Turonian, Al = Albian, B = Barremian, H = Hauterivian, V = Valanginian.Brief descriptions of the subunits from which samples were collected; a complete description can be found in Ludden, Gradstein, et al. (1990). Unit IIB: calcareous
ooze and clays are interlaminated, a distinctively color-banded interval. Unit IIC: dominated by bioturbated zeolitic, clayey nannofossil ooze intercalated withlighter brown nannofossil ooze and chalk. Unit IIIA: claystones with disseminated glauconite, quartz and radiolarians; bioturbation is pervasive, pyrite nodulesoccur sporadically. A large pyrite nodule coated with cubes and pyritohedrons of pyrite was found in this subunit, Section 123-766A-31R-1 at 15-21 cm.Unit IIIB: sandstones and siltstones containing various proportions of altered volcanic lithic grains, glauconite and neritic carbonate bioclasts, dark to verydark greenish-gray sandstones constitute the upper part of Subunit IIB, sandy to clayey siltstones predominate in the lower part. Bioturbation is pervasive,pyrite concretions occur throughout, pyrite is particularly abundant in the lower part of the Subunit in the sandy to clayey siltstones, other components notedare coal fragments, radiolarians, and feldspar.
TOC(wt%)
2 3 4
TOC(wt%)1
£400E
800
J•
id* D
ftp D
Shi1-1
Ö D
T \ \ \ \ \
Pleistocene
Pliocene
Miocene
c~.™.,, OligoceneeocenePaleocene
Late Cretaceous
Albian —
Aptian
D
Barremian
HauterivianBerriasian
v \ \ \ \ \ \
2Q.
200 J
400
B
G•- •D I - D• •
•
Late Cretaceous
Albian.
Aptian
Barremian
Hauterivian
Valanginian
\ \ \ \ \ \ \ \ \ \ \
Figure 4. A. TOC vs. depth (mbsf) Argo Abyssal Plain (Site 765). B. TOC vs. depth (mbsf) Gascoyne Abyssal Plain (Site 766).Open squares = samples analyzed for this study.
C/S ratios for Hauterivian/Valanginian sediments (Unit IIIB) is0.7+0.3 (13 samples) and for Barremian/lower Aptian sediments(Units IIC, IIIA) 0.6±0.2 (5 samples), while a linear regression toall the Site 766 data in Figure 5 indicates a mean C/S of 0.8 (R =0.97).
A comparison of the data from the two sites shows consistentlyhigher quantities of pyrite in Site 766 sediments than in those atSite 765, for comparable quantities of TOC buried. The resultsuggests that a more metabolizable organic matter, reactive to thesulfate-reducing bacteria, is buried in the Gascoyne Abyssal Plain
230
ORGANIC MATTER AND PALEOCHEMISTRY OF CRETACEOUS SEDIMENTS
Table 5. Average organic carbon accumulation rates, Argo andGascoyne abyssal plains.
Core Age
Argo Abyssal Plain
123-765C-
48R-62R48R-33R45R32R-27R26R-23R
Be-BApApAlC-M
Gascoyne Abyssal Plain
123-766A-
33R-44R32R-25R24R-13R12R-10R
V-HBAp-CaCa-M
TOC(wt%)
0.160.345.10.040.03
0.470.39
<O.Ol<O.Ol
Sed. rate(m/m.y.)
6.014144.21.7
60103.21.7
TOC accumulation(g/cm2/m.y.)
1.26.2
92.80.20.1
36.75.1
<O.l<O.l
Al = Albian, Ap = Aptian, B = Barremian, Be = Berriasian, C =Cenomanian, Ca = Campanian, M = Maestrichtian, H = Hauterivian,V = Valanginian.
TOC accumulation rate = average TOC (wt%) × sedimentation rate(m/my) × (1-porosity) × grain density (g/cm3).
The table was assembled using constant sedimentation rate intervals(Ludden, Gradstein, et al., 1990) and the average TOC (calculatedfrom all Rock-Eval data, Ludden, Gradstein, et al., (1990)) in thatinterval. A porosity of 50% and a grain density of 2.6 was used forall calculations.
.̂o -
2.0 --
1.5 -
1.0 -
0.5 -
r
o.o I
D r /
DC?
ss
ss
\&&£^(Aoöeilü-'
×us
s
0 .0 0 .5 1 . 0TOC(wt%)
1 .5 2 .0
Figure 5. TOC (wt%) and S (wt%) from Cretaceous sediments of the Argo (Site765 = closed triangles) and Gascoyne (Site 766 = open squares) abyssal plains.
sediments. Frequent occurrences of pyrite concretions and nod-ules were noted in the Lower Cretaceous sediments from Gas-coyne Abyssal Plain, but only minor to trace amounts of pyritewere scattered throughout the Lower Cretaceous sediments fromthe Argo Abyssal Plain (Ludden, Gradstein, et al. 1990). Plots ofpyrite sulfur vs. organic carbon, with an extension of the line ofbest-fit that passes through the origin, indicate deposition innormal marine sediments (Berner and Raiswell, 1983; Leventhal,1983; Berner, 1984). The plots of Figure 5 (i.e., of TOC<2 wt%)and evidence of bioturbation at both sites (Tables 3 and 4) indicatethat deposition occurred under normal marine conditions (thesmall positive intercept at low TOC concentrations is probablyresidual pore water sulfate <0.2 wt%). Deposition under euxinic
conditions (anoxic water column) results in a non-zero intercepton the S axis (as additional pyrite can be formed in the anoxicwater column) and a poor correlation between TOC and S with,increased deposition of organic carbon (Leventhal, 1983; Bernerand Raiswell, 1983; Berner, 1984; Raiswell and Berner, 1986).The thin (1 cm) interval from the Argo Abyssal Plain with a highTOC (5.1 wt%) and a C/S of 0.5 may have been deposited froman anoxic water column (see below).
CIS, Source, and Reactivity of Organic Matter
The mean C/S ratio of near five from Site 765 sediments (Table3, Fig. 5) is higher than (1) the average of 1.2 for a variety ofCretaceous normal marine shales, (2) the average of 1.8±0.5quoted for Devonian through Tertiary shales (Raiswell andBerner, 1986), and (3) the modern normal marine value of 2.8(Berner, 1982; Berner and Raiswell, 1983; Raiswell and Berner,1986). This value near five for the Lower Cretaceous sedimentsresults from a combination of factors. Site 765 has always been arelatively deep-water marine location with an initial water depthof -2.5 km (Ludden, Gradstein, et al., 1990). Hence sulfatelimitation, because of low salinity (Berner and Raiswell, 1984),cannot be considered as a control on pyrite formation in thesesediments. Furthermore, the sediments are primarily claystoneslow in calcium carbonate (Table 1) with abundant iron and ironlimitation is not a constraint (see below) on pyrite formation(Berner, 1984; Raiswell and Berner, 1986).
Consequently, controls on pyrite formation are limited to thetype and reactivity of organic matter accumulating in the sedi-ments. Increasing proportions of land plant debris in sediments,i.e., increasing proportions of nonmetabolizable organic matterfor the sulfate-reducing bacteria, relative to marine organic matter(Lyons and Gaudette, 1979), result in a decreasing abundance ofpyrite, hence increasing C/S ratios. The reactivity of the organicmatter to the sulfate-reducing bacteria in the sediments, and henceproduction of H2S for pyrite formation, is also reduced as organicmatter is oxidized in the overlying water. Site 765, with an initialwater depth near 2.5 km, rapidly subsided during the Early Cre-taceous to near its present-day water depth (Ludden, Gradstein,et al., 1990). Hence, progressive oxidation of organic carbon as itsettles to abyssal depths would be expected to result in increasingC/S ratios in the sediments as an excess of refractory organicmatter accumulates over pyrite produced from residual metabo-lizable organic matter. Extensive bioturbation of the sediments, aprocess that results in preferential loss of H2S (Berner andWestrich, 1985) also contributes to increasing C/S ratios.
The good C/S correlation, with extension of the line of best-fitthrough the origin for Gascoyne Abyssal Plain (Site 766) sedi-ments (Fig. 5), and a mean C/S ratio of 0.8, indicates carbon isnot limiting in pyrite formation, pyrite is diagenetic, and deposi-tion occurred under normal marine conditions. This value (0.8)for these Lower Cretaceous sediments from the Gascoyne AbyssalPlain, is considerably lower than the mean (C/S = 5.4) found fromArgo Abyssal Plain sediments and is indicative of the burial oforganic matter that is reactive to the sulfate-reducing bacteria,from which abundant pyrite is formed. Site 766 had an inferredinitial water depth around 800 m (Ludden, Gradstein, et al., 1990)and relatively high sedimentation rates during the Early Creta-ceous (Table 5). These factors are consistent with the burial ofreactive organic matter. Because pyrite sulfur was not measureddirectly, a component of non-pyritic sulfur in these samples couldhave resulted in an artificial lowering of C/S ratios. Furthermore,the loss of organic carbon via methane production during catage-nesis (moderate heating of buried organic matter) and/or bacterialactivity during diagenesis contributed to the lowering of C/Sratios in Devonian to Pleistocene sedimentary rocks in contrast tomodern normal marine sediments (Raiswell and Berner, 1986).
231
D. T. HEGGIE
The sediments from this site were immature (Ludden, Gradstein,et al., 1990), excluding the possibility of production of highconcentrations of thermogenic methane. However, methane andother light hydrocarbon gases were detected in these Lower Cre-taceous sediments, -2-3 times the "background" concentrations,indicating some organic carbon degradation via methanogenicmicrobial activity.
Rock-Eval Pyrolysis and Biomarker Geochemistry
The shipboard Rock-Eval data for the Lower Cretaceous sam-ples of Sites 765 and 766 are shown in Figure 6. These data clusterabout the Type III evolution line, indicating a low HI (hydrogenindex) and organic matter depleted in hydrogen. However, pyrol-ysis yields and HI values have probably been depressed becauseof the mineral matrix effect associated with high clay contents inthese samples (Espitalie et al., 1980; Orr, 1983; Katz, 1983;Peters, 1986).
More definitive information about organic matter source wasobtained from gas chromatography and GC-MS analyses of thesaturated hydrocarbon extract from selected samples (Heggie etal., this volume). The saturated hydrocarbon abundances and theratios ofn-Cnln-Cn were determined on eleven samples; eightfrom Site 766 and three from Site 765. A variety of biomarkersfrom the saturates extract were measured on seven of these sam-ples.
The presence of the C30 sterane, 24-n-propylcholestane is aspecific marine biomarker (Moldowan et al., 1985, 1990), itspresence and relative abundances in samples from both sites aresimilar to both the Mesozoic marine Toolebuc Formation in north-eastern Australia (Boreham and Powell, 1987) and the marineMonterey Formation of California (Curiale and Odermatt, 1985).Furthermore, the presence of the 4-methylsterane in all samples
600
500-
400-
300-
200-
100-
TYPE II
TYPE III
0 100 200 300
01 (mg Cθ2/g carbon)
Figure 6. Rock-Eval HI vs. OI diagram of Lower Cretaceous sediments from
Site 765 (closed squares) and Site 766 (open triangles). sample with TOC
= 5.1 wt%.
from both sites, with the 23, 24 dimethyl side chain, is specificfor and indicative of marine dinoflagellates (Summons et al.,1987). However, GC analyses of the saturated hydrocarbon ex-tracts and the ratios of n-Cnln-Cπ (Tissot and Welte, 1984),indicated contributions of terrestrial organic matter in two sam-ples from the Aptian and Barremian sections of Site 765. Twosamples of Barremian age, from Site 766, showed significantcontributions of terrestrial organic matter, while all other GCextracts indicated a predominance of marine organic matter at Site766. These limited data from both sites therefore indicate that theorganic matter extracted from the sediments was primarily of amarine source, but contained varying contributions of terrestrialorganic matter.
Shipboard palynological observations (Ludden, Gradstein, etal. 1990) indicated abundant dinoflagellate cysts in the LowerCretaceous sediments from both sites. Spores and pollen werefound scattered throughout the sediments from both sites. How-ever, at Site 765 spores and pollen appeared more abundant in theupper Aptian to lower Albian sections than elsewhere. At Site766, spores and pollen were found in the M. australis zone (mostlyof Barremian age), where they constituted 40%-80% of the paly-nomorph assemblages, although it was not possible to differ-entiate between Lower Cretaceous and reworked Jurassic grains(Ludden, Gradstein, et al., 1990).
Collectively, the organic geochemical analyses and the lowerabundances of pyrite in sediments from the Argo Abyssal Plain,as compared to those from Gascoyne Abyssal Plain, are consistentwith the burial, at Argo Abyssal Plain, of organic matter lessreactive to the sulfate-reducing bacteria, through combinations ofmixed terrestrial and oxidized marine sources.
Carbon/Phosphorus Ratios
The C/P ratios are calculated from the TOC and total phospho-rus data and summarized in Tables 3 and 4, and Figure 7. Whilethe data only approximate C/P ratios in the residual organicmatter, a comparison of the ratios between the sites provides cluesto the nature of the organic matter in the sediments. Most samplesfrom the Argo Abyssal Plain (those samples with TOC<2 wt%)have C/P ratios <IO. The one sample with TOC of 5.1 wt% has aC/P ratio of 40. These data contrast to those from the GascoyneAbyssal Plain site, where the C/P ratios are as high as 60 and morecomparable to the Redfield ratio of 105 for marine organic matter.The contrast between the sites indicates that for comparable TOC
0 1 2 3 4 5TOC (Wt%)
Figure 7. TOC (wt%) and C/P (atomic) ratios in sediments for the Argo (closed
triangles) and Gascoyne (open squares) abyssal plains.
232
ORGANIC MATTER AND PALEOCHEMISTRY OF CRETACEOUS SEDIMENTS
values, the deeper-water site of the Argo Abyssal Plain containsmore residual P. These data and the trend of Figure 7 suggestpreferential remobilization of organic carbon (over phosphorus)with increasing water depth. The result is compatible with obser-vations summarized in Ingall and Van Cappellen (1990) of lowC/P ratios in organic matter in pelagic sediments. The prolongeddegradation of organic carbon, such as occurs in pelagic sedi-ments, apparently leads to the formation of phosphorus-enrichedorganic matter (Froelich, 1982).
Ferrous and Pyrite Iron
Ferrous iron is produced as a byproduct of organic matteroxidation in sediments under sub-oxic conditions (e.g., Froelichet al., 1979) following the depletion of pore-water oxygen andnitrate, but prior to the onset of bacterial sulfate reduction. Sedi-mentary iron oxyhydroxide reduction and the release of ferrousiron to pore waters has been documented in pelagic and hemipe-lagic sediments (e.g., Klinkhammer, 1980; Bender and Heggie,1984; Heggie et al., 1987). Ferrous iron, once released to porewaters, may be redeposited in authigenic minerals or, wheresufficient reactive organic carbon is buried in the sediments todeplete the primary oxidants, reacts with hydrogen sulfide (pro-duced during bacterial sulfate reduction of sedimentary organicmatter) to form iron monosulfides and ultimately pyrite (e.g.,Froelich et al., 1979; Berner, 1981; Berner, 1982; Curtis, 1985).
Sedimentary ferrous iron abundances have been investigatedas empirical indicators of sub-oxic conditions for the followingreasons: (1) frequent appearances of red sediments, the colorbanding and gradations from red through gray, green to black, and(2) the fact that pyrite was observed scattered in only minor totrace quantities throughout these relatively deep (>2.5 km) LowerCretaceous sediments of Site 765. Ferrous iron (although in un-identified minerals) but no pyrite, suggests sub-oxic conditions(implying relatively low organic carbon burial fluxes and/or aplentiful supply of the primary oxidants, oxygen and nitrate),while pyrite occurrences are indicative of anoxic conditions,sulfate reduction, and higher organic carbon burial fluxes. Ferrousand pyrite iron abundances, when considered together, may beempirical indicators of fluctuating sub-oxic and anoxic condi-tions.
Different modes of pyrite formation, including diagenetic,syngenetic (formed before burial both in the water column and atthe sediment/seawater interface), iron-limited and carbon-limitedunder euxinic (anoxic, H2S bottom waters) and near-euxinic (verylow oxygen bottom waters) conditions, have been distinguishedby the degree of pyritization (DOP) (Raiswell and Berner, 1985).DOP, after Berner (1970), is defined as pyrite iron/total iron,although the sum of pyrite iron plus iron soluble in hot concen-trated hydrochloric acid (reactive iron) is a better indicator of theiron (rather than total iron) that is available to hydrogen sulfideto form pyrite. However, in practice both total iron and reactiveiron have been used in this application of DOP (Raiswell andBerner, 1985; Donnelly et al., 1988). DOP values calculated hereare subject to two approximations. First, pyrite iron has beencalculated stoichiometrically from the total sulfur content, assum-ing (discussed earlier) that all sulfur occurs as pyrite. Second, totalrather than reactive iron has been used in the denominator of DOP.The data of Raiswell and Biatty (in press) show, for a variety ofDevonian-Cretaceous, Cambrian-Silurian, and modern sedimentsof the Amazon shelf, that HCl-soluble iron and total iron arelinearly related, although the proportion of HCl-soluble to totaliron may vary for sediments of different ages. Donnelly et al.(1988), found that HCl-soluble iron was -30% of total iron insamples of middle Cambrian age. Because reactive iron for pyrite
formation is always less than total iron, the DOP values calculatedhere underestimate those calculated if the sum of pyrite iron andHCl-soluble iron had been used in the denominator of DOP.
Total Organic Carbon (TOC) and Degree of Pyritization (DOP)
DOP values are plotted vs. TOC in Figure 8A. The data fromSite 766 are correlated (R = 0.90), while that from Site 765 arepoorly correlated (R = 0.55). For TOC<2 wt%, the correlationsindicate iron is available for pyrite formation, pyrite is diageneticand, as the data pass through the origin, indicate deposition hasoccurred in normal marine sediments. A consideration affectingthe relation between pyrite sulfur and organic carbon is whetheror not the original deposition of organic matter was coupled to thedeposition of iron through the association of colloidal organicmatter and iron oxide particles with fine clay grains (Berner,1984; Raiswell and Berner, 1985). Total iron (used here as a proxyfor reactive iron) does not correlate with TOC in the sedimentsfrom either the Argo or Gascoyne abyssal plains (Fig. 8B). Hence,pyrite iron formation is diagenetic (Fig. 8A) and not an artifact,
TOC(wt%)
LL.
\Δ -
10 -
8 -
6 -
4 -
[
2 -
0 -
3
A
^ iD
A
D
] D fQπ
D α
4k•
B1
T0C(wt%)
Figure 8. A. TOC and DOP (degree of pyritization) for Argo (closed triangles)and Gascoyne (open squares) Abyssal Plain sediments. B. TOC and total iron,Fe(t), for Argo (closed triangles) and Gascoyne (open squares) Abyssal Plainsediments.
233
D. T. HEGGIE
because organic carbon and iron are not associated in the originaldepositing materials.
Raiswell et al. (1988) proposed that the degree of pyritization(DOP) could be used as an indicator of bottom-water oxygenconditions. These authors examined sediments of various agesclassified on paleoecological and sedimentological criteria asbeing deposited in aerobic (DOP<0.42), restricted (0.46<DOP<0.8), or inhospitable (anoxic) bottom waters (0.55<DOP<0.93).Because of the approximations noted above, the DOP values hereare not directly comparable to those of Raiswell et al. (1988).
For TOC<2 wt%, all DOP values are <0.4 (Tables 3 and 4).The TOC-rich sample from the Argo Abyssal Plain, sampled nearthe Barremian-Aptian boundary, had a DOP value of 0.9. Theapproximations, noted above, of calculated DOP values, com-bined with the considerable overlap in DOP between restrictedand inhospitable (anoxic) conditions (Raiswell et al., 1988), indi-cate the interpretation of a DOP of 0.9 (deposition from an anoxicwater column) is equivocal. However, other observations (Thu-row, pers. comm., 1991) support an interpretation of anoxic bot-tom water conditions. This sample had the following charac-teristics: (1) under a scanning electron microscope, it was almostentirely framboidal pyrite; (2) this thin interval was finely lami-nated with no evidence of bioturbation; and (3) it had clay miner-alogy similar to that from adjacent samples, suggesting it was notderived from another sediment provenance. Finally, in Figure 9,total iron vs. total AI2O3 is plotted for both Gascoyne and Argoabyssal plains samples. If all iron in the sediments were detrital,values would be expected to cluster about the average Fe(t)/Al2θ3ratio as shown by the solid line. The excess iron indicated onFigure 9, for the TOC-rich sample may be pyrite iron that isformed by precipitation in an anoxic water column, settles to thesediment surface, and is subsequently incorporated into the sedi-ments. Therefore, this TOC-rich sample may be either the localexpression of the broadly defined Barremian-Albian OceanicAnoxic Event (OAE-1) of Schlanger and Jenkyns (1976) or asub-event of OAE-1. This interpretation will require more de-tailed analyses to confirm, but there was insufficient sampleavailable from this interval for further work. Such a high abun-dance of pyrite in this sample suggests deposition of organicmatter reactive to the sulfate-reducing bacteria. However, Rock-Eval pyrolysis of this and adjacent samples (Fig. 6) did notprovide any clues to changes in the composition of the organicmatter being deposited.
0 2 4 6 8 1 0 1 2 1 4 1 6
AI2O3(wt%)
Figure 9. Total iron Fe(t) and AI2O3 (wt%) for the Argo (closed triangles) andGascoyne (open squares) abyssal plains, Lower Cretaceous sediments.
Ferrous and Pyrite Iron and Color-Banded Sediments
Plots of ferrous iron/total iron and ferrous iron + pyriteiron/total iron are summarized for both locations in Figure 10.Ferrous iron abundances increase with increasing TOC at bothsites (Figs. 10A and 10B) and indicate that ferrous iron is diage-netic. However, at low TOC (<0.2 wt%) at Site 766, FeII/Fe(t)was high in two samples and the data trend to intercept the Fespeciation axis at a non-zero value (Fig. 10A), which suggests anon-diagenetic or detrital ferrous iron component.
Ferrous iron abundances (for TOC<2 wt%) are significant inthese sediments and represent <30% and <40% of total iron at theArgo and Gascoyne abyssal plains, respectively. However, in thesample of high TOC (5 wt%), from near the Barremian/Aptianboundary, FeII/Fe(t) is <0.2, and most iron (>80%) occurs aspyrite (Fig. 10C). These data suggest sedimentary ferrous ironincreases with increases in TOC (<~2 wt%), which is ultimatelydepleted as additional TOC is buried and pyrite begins to form tobecome the ultimate sink for remobilized iron. The numericaldifferences on the Fe speciation axes between the (ferrous iron +pyrite iron) and ferrous iron represent DOP values discussed inFigure 8A. The sum of ferrous plus pyrite iron (for TOC<2 wt%)represents <~70% of total iron at Gascoyne Abyssal Plain and50% at Argo Abyssal Plain (Figs. 10A and 10B).
At Argo Abyssal Plain, the distributions of sedimentary fer-rous iron (Table 3), indicate: (1) the highest abundances of ferrousiron (<27% of total iron) in the dark-green colored Aptian clay-stones of Unit IVC (TOC<0.5 wt%); (2) that ferrous iron istypically 10%-20% of total iron in the gray and green clay stonesof Units VB and VC (TOC 0.1-1.2 wt%); and (3) the lowestabundances of ferrous iron (<10% of total iron) are found in thosered colored claystones predominant in Units VC, VI, and VII(TOC<0.2 wt%, Table 3). Pyrite iron abundances show a similartrend, being highest in samples with high TOC and the dark-grayand green Barremian and Aptian samples, and lowest in the redclaystones of Berriasian through Valanginian ages.
Selected samples of the color-banded (red, dark-green, dark-gray, black) claystones from the Lower Cretaceous sedimentUnits VA-VII, were analyzed by X-ray diffraction. The resultsare summarized in Table 6 and indicate that all red samplesconsistently contained oxidized iron minerals, i.e., hematiteand/or goethite, and these were absent in the light gray andgray-green samples. Pyrite (with a detection limit of ~5 wt%) wasnot detected in any samples. Red-colored samples always havethe lowest abundances of both pyrite and ferrous iron (Table 3).
SUMMARY
The abundances of some diagenetic byproducts that resultedfrom the burial, degradation, and preservation of organic matterwere compared in samples of Lower Cretaceous sediments fromthe Argo and Gascoyne abyssal plains.
1. Both pyrite (estimated from total sulfur) and TOC aregenerally more abundant in the Gascoyne Abyssal Plain than theArgo Abyssal Plain, although the sample of highest TOC wasmeasured in a thin (1 cm) layer from the Argo Abyssal Plain nearthe Aptian/Barremian boundary.
2. The average C/S ratio (for samples of TOC<2 wt%) in ArgoAbyssal Plain sediments is ~5. This result reflects the burial of anexcess of organic carbon that was unreactive to the sulfate-reduc-ing bacteria, over pyrite formed from metabolizable organic carb-on during sulfate reduction. The C/S ratio in the high TOC samplefrom the Aptian is 0.5 and suggests deposition of a source oforganic matter that was reactive to the sulfate-reducing bacteria.In contrast to the data from the Argo Abyssal Plain (TOC<2 wt%),the average C/S ratio from the Gascoyne Abyssal Plain sediments
234
ORGANIC MATTER AND PALEOCHEMSTRY OF CRETACEOUS SEDIMENTS
U.Ö •
0.6 -
0.4 -
Gascoyne(all data)
A
0.2 - 5 D
o.o π
AA àππ α [ g
α α π
1 - ,
A
A
D
D
0.8
1TOC(wt%)
0.6 -
0.4 -
0.2 -
Argo(TOC<2wt%)
αAD
1TOC(wt%)
1.0 -
O.8-1
0.6 -
0.4 -
Argo(all data)
D
α0.0
TOC(wt%)
Figure 10. A. TOC and ferrous iron/total iron (FeII/Fe[t], open squares) andferrous iron + pyrite iron/total iron (Fell + Fe[py)]/Fe[t], closed triangles) forthe Gascoyne Abyssal Plain sediments (TOC<2 wt%). B. TOC and ferrousiron/total iron (FeII/Fe[t], open squares) and ferrous iron + pyrite iron (Fell +Fe[py]/Fe[t], closed triangles) for Argo Abyssal Plain sediments (TOC<2wt%).C. TOC and ferrous iron/total iron (FeII/Fe[t], open squares) and ferrous iron+ pyrite iron/total iron (Fell + Fe[py]/Fe[t], closed triangles) for Argo AbyssalPlain sediments (all data).
Table 6. XRD analyses of select color-banded Aptian-Hauterivian sedi-ments from the Argo Abyssal Plain.
Core, section,interval (cm)
Depth(mbsf) Color Description
123-765C-
40R^t, 143-144
47R-3, 55-57
50R-2, 80-82
52R-1, 132-133
52R-2, 8-10
52R-2, 10-11
53R-3, 72-74
55R-2, 82-84
57R-7, 15-17
59R^t, 111-112
60R-2, 61-63
726.83 gray
790.95 It red
817.90 gray/green
835.82 It gray
836.08 It gray
836.10 gray/green
847.82 It red
865.32 It red
890.55 It red
903.50 It gray
909.61 dark red
61R-5, 105-106 923.95 red/brown
Quartz minor, feldspar trace, mont-morillonite major, mica trace
Quartz major, hematite trace, feld-spar trace, montmorillonitemajor, mica trace
Quartz major, feldspar trace, mont-morillonite major, mica trace,chlorite trace
Quartz major, feldspar trace, mont-morillonite minor mica trace
Quartz major, montmorilloniteminor, mica trace
Quartz major, feldspar trace, mont-morillonite minor, kaolinitetrace
Quartz major, feldspar trace, hema-tite trace, montmorilloniteminor, mica trace
Quartz major, hematite trace, mont-morillonite minor, mica trace
Quartz major, hematite trace, mont-morillonite trace, mica trace
Quartz major, feldspar trace, mont-morillonite trace, mica trace
Quartz major, feldspar trace, hema-tite trace, montmorilloniteminor, mica minor
Calcite major, quartz major, feld-spar trace, goethite trace,montmorillonite minor, micatrace
is 0.8 and indicates burial of organic matter from which abundantpyrite was formed.
3. The organic carbon/total phosphorus ratios in the sedimentsare low (<IO) in the Argo Abyssal Plain compared to those in theGascoyne Abyssal Plain (-60) for comparable TOC contents.These ratios are lower than the Redfield ratio of 105 for marineorganic matter and suggest preferential remobilization of organiccarbon over phosphorus with increasing water depth.
4. A consideration of the iron mass balance, including theamounts of ferrous iron and pyrite iron (expressed as a fraction oftotal iron in the sediments), indicate generally higher abundancesof both ferrous and pyrite iron at the Gascoyne Abyssal Plain site(for TOC<2 wt%). Ferrous iron and pyrite iron are related to TOC(although poorly correlated in the Argo Abyssal Plain samples)in the sediments from both sites and therefore are diageneticbyproducts. The estimates of DOP (the degree of pyritization) andevidence of bioturbation support the results of the TOC vs. Splots, that for TOC<2 wt% deposition has occurred in normalmarine sediments. One sample, near the Barremian/Aptian bound-ary, with 5.1 wt% TOC, abundant pyrite, fine laminations, and nobioturbation may represent deposition during a brief period ofanoxia in the overlying water, thus representing the local expres-sion of the Barremian-Albian Oceanic Anoxic Event.
ACKNOWLEDGMENTS
The XRF and FeO analyses were conducted at the Division ofOnshore Sedimentary and Petroleum Geology, Bureau of MineralResources, under the supervision of John Pyke and Bruce Cruik-shank. Julie Kamprad did the XRD analyses. I thank T. Donnellyfor several helpful discussions and his critical and constructivecomments on an earlier version of this manuscript. G. Hieshema,T. Powell, and C. Boreham also provided constructive comments
235
D. T. HEGGIE
on an earlier manuscript. K. H. Wolf, BMR editor, assisted in thepreparation of the final manuscript. DTH publishes with thepermission of the Executive Director of the Bureau of MineralResources, Canberra, Australia.
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Date of initial receipt: 31 August 1990Date of acceptance: 22 May 1991Ms 123B-169
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