Journal of the Geological Society, London, Vol. 151, 1994, pp. 139-152, 7 figs, 2 tables. Printed in Northern Ireland

Isotopic biogeochemistry of the Oxford Clay Formation (), UK

F. KENIG'.4, J. M. HAYES', B. N. POPP' & R. E. SUMMONS3 l Biogeochemical Laboratories, Geology Building, lndiana University, Bloomington, IN 47405, USA 'Deptartment of Geology, Geophysics and Oceanography, University of Hawaii, Honolulu, HI 96822, USA 'Australian Geological Survey Organization, PO Box 378, Canberra, 2601, Australia 4Present address: Netherland Institute of Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The Netherlands

Abstract: A total of165 samples was obtained from the Oxford Clay Formation at seven different sites. Nearly all were from the Peterborough Member (Lower Oxford Clay), but seven were from the Stewartbyand Weymouth Members (Middle and Upper Oxford Clay respectively). Five samples fromthe underlying were also examined. Stratigraphic relationships were estimated on the basis of ammonite subzones and results from all locations can be placed along a singlestratigraphic scale. The followingwere determined for allsamples: abundance and isotopic composition of organic carbon, abundances of carbonate carbon and total sulphur, andthe Rock-Eval pyrolysis parameters hydrogen index, oxygen index and T,,,. For a subset of eight samples selected to be representative ofgeochemical and apparent palaeoenvironmental variations, soluble organic compoundswere extracted and the isotopic composition of pristane,phytane, and long-chain n-alkanes determined by isotope-ratio-monitoringgas chromatograph mass spectrometry. Concentrations of organic carbon in samples from the Peterborough Member ranged from 0.5 to 16.6 % and 6 values of total organic carbon (TOC) ranged from -27.7 to -23.1% v. PDB. dominated by epifaunal bivalve assemblages have high concentrations of TOC and values of H index approaching 800, indicating preservation of hydrogen-rich organic material. Conversely, shell beds and calcareous and silty clay beds have lower abundances of TOC and values of H index dropping below 100, indicating extensive oxidation of the organic matter. Isotopiccomposition of pristaneand phytane in thePeterborough and Stewartby Members average -31.7%, those in theWeymouth Member average -29.8%. Values of 6 forlong-chain n-alkanesaverage -28%. Together these results indicate 6 valuesfor primary inputs as follows: terrestrial vascular plants, -23.5%; Peterborough Member algae, -28.2; Stewartby Member algae, -29.1%;Weymouth Member algae, -26.6%. Comparison of primary 6 values to those of TOC indicates that in some cases secondary processes enriched TOC relative to primary inputs by as much as 4%. Palaeontological evidence in these same beds indicates development of extensive food-webs and supports attribution of this isotopic enrichment to heterotrophic reworking.

TheOxford Clay is a marineargillaceous formation of Accordingly, theOxford Clay offers opportunities for Middle to Lower age which crops out geochemical studies rarely encountered in rocks of this age. in centraland southern England (Fig. 1). Two facies are Sedimentologicaland palaeontological studies of the prominent(Hudson & Martill 1991; Cox et al. 1993): an OxfordClay Formation, and more particularly of the organic-carbonrich, fissile, and richly fossiliferousshale organic-richPeterborough Member, led to palaeoenviron- foundmostly in thePeterborough Member, and a blocky mentaland palaeoecological reconstructions (Hudson & and rather calcareous mudstone with relatively low contents Martill1991 andreferences therein). Questions remain of organiccarbon, found mostly in theStewartby and aboutthe processes resulting in theaccumulation of Weymouth Members. abundantorganic matter (0.5%-16.6% TOC),and about The organic-rich Peterborough Member is not found in thedepositional conditions of the -lagerstatten for thetwo neighbouring major oil provinces,the North Sea which thesesediments are renowned(Martill & Hudson and Paris Basins. This may explain the limited number of 1991; Martill et al. this volume). organic geochemical studies on the Oxford Clay, in contrast The am of the present work is to use the content and to the many reports produced on other Jurassic source rocks isotopiccomposition of organicand inorganic phases to of proveneconomic potential: the organic-rich Toarcian reconstructprocesses controlling the formation, accumula- (e.g. Huc 1976; EspitaliC et al. 1987; Farrimond et al. 1989 tion,and preservation of organicmatter and associated andreferences therein) and Kimmeridgian shales (e.g. , primarily in the organic-rich Peterborough Member. Farrimond et a[. 1984; Huc et al. 1992 andreferences A few samples from the underlying Kellaways Formation, therein). The location of the Oxford Clay at the edge of a and the overlying Stewartby and Weymouth Members, were major sedimentary basin produced only a moderate burial, examined to provide points of comparison and to examine atits shallowest in the East Midlands where most of our theevolution of the basin in which theOxford Clay samplingwas done(Hudson & Martill 1991). A mild Formation was deposited. thermal and diagenetic history resulted in the preservation The first part of this paper summarizes results obtained of thisJurassic sediment near-pristinein condition. from the study of bulk geochemical parameters, which also 139

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eachammonite subzone at eachsite. Each subzone was divided into100 units, and each sample assigned a position within the subzone. We called the values obtained through this calculation the f 'stratigraphic age' (Table 1). Thus, 'stratigraphic ages' from 0-100 N are withinthe Enodatum-Medea Subzones, 101-200 within the JasonSubzone, 201-300 within the ObductumSubzone, 301-400 withinthe Grossouvrei Subzone, 401-500 within the Phaeinum Subzone,and501-600 within theProniae Subzone. The assumption of constant sedimentation rate is not entirely accurate, assediment accumulation was episodic (Hudson & Martill1991). Shell beds often represent diastems or have low accumulation rates (Callomon 1968); conversely, high sedimentation rates may favour the preservation of organic matter in high TOC facies (Hudson & Martill 1991). Concentrations of total organic carbon (TOC) were measured by I combustion of decalcified sediments and manometric measurement of the carbon dioxide produced. Abundances of total sulphur and total carbon (carbonate + organic) were determined using a LECO carbon-sulphur analyzer (model CS-244). To obtain the hydrogen indexand oxygen index, Rock-Eval analyses were performed on powdered whole rock samples at the Bureau of Mineral Resources (Australia)using the method described byEspitaliC et al. (1977, 1986u, b). Forisotopic analyses of totalorganic carbon (S,,,), 5-10mg of powdered,decalcified sample were combusted in a sealed quartz tube at 850°C for more than 4 hours (Knoll er al. 1986). Totalextractable material was obtained bySoxhlet extraction withdichloromethane andmethanol (3:l) hours.for48 Approximately 10g of fine-grained solvent-extracted, HCI-activated copper was added to each extraction flask to facilitate removal of Fig. 1. Schematic map showing the onshore outcrop of the Oxford elemental sulphur. All solvents were distilled in glass. Clay in England. Dots indicate locations where samples were The extractablematerials were separated using column chromatography. Silica-gel (4.2 g) was packed in a 18 mm diameter collected. D, Dogsthorpe brick pit; 0, Orton brick pit; W, column,and samples were transferred to the top of thecolumn Whittlesey core (all in the Peterborough district); S, Stewartby using a minimum quantity of hexane. Hydrocarbon fractions were brick pit; B, Bletchley brick pit; C, Calvert brick pit; SH, Stanton eluted with 20 ml of hexane. The remaining fraction was recovered Harcourt (Brown's pit.); A, Ashton Keynes (Cleveland Farm pit). with 50 ml of dichloromethane.Saturated hydrocarbons were separated from unsaturated hydrocarbons using a chromatographic column (diameter 20 mm) packed with 4 g of silica gel impregnated providedcriteria on which samplesrepresentative of the witha 10% solution of AgNO,in water:ethanol (1:3). Saturated range of bulk properties (indicated in Table 1) were selected hydrocarbons were eluted with 50ml of hexane and the unsaturated for detailed studies: specifically, analyses of biomarkers and hydrocarbons were eluted with 50 mlof methanobbenzene (1:l). The n-alkanesand branched/cyclic sub-fractions of some of the determination of I3C abundances in individualorganic saturatedhydrocarbon fractions were further separated using compounds. These latter results are discussed in the second experimental silica molecular sieve (Silicalite, PQ corporation). part of the paper. Gas chromatography (GC) of the amenable separated fractions wascarried out on a Hewlett Packard 5890 (Serie 11) instrument equippedwith a flameionization detector andan on-column Samples and methods injector. The oventemperature was programmed from 50°C to Representative samples were collected from all the biofacies defined 150 "Cat 10 "Cper minute, from 150 "C to 320 "Cat 3 "C per minute on the basis of macrofaunal assemblages (Duff 1975). 165 samples andthen maintained at 320°C for 30 minutes. The columnused were collected from four brick pits, two temporary excavations and was a Hewlett-Packard Ultra 1 (length 50m, 0.32mm i.d.) with a one core; thelocations are summarizedin Fig. 1. Specifically, 58 film thickness of 0.52 pm. sampleswere taken from acore of PeterboroughMember at Carbonisotopic compositions of individualcompounds of the Whittlesey,near Peterborough; 35 samples (P numbers)were saturated hydrocarbon fraction and of then-alkane fraction were collected from Dogsthorpe Brick Pit, Peterborough, and 5 (OCD) determined using isotope ratio monitoring-gas chromatograph mass from a temporary excavation at its base; 13 from Quest Brick Pit, spectrometry (Sano er al. 1976; Matthews & Hayes 1978; Hayes et Stewartby; 26 from Bletchley Brick Pit, and 18 from Calvert Brick al. 1990). The temperature programme and the column used were Pit:all these were collected by courtesy of the LondonBrick similar to those described above. Companyplc. In addition, eight samples came from Cleveland Farmpit, Ashton Keynes, Wilts. (ECC quarries), and twofrom Brown's pit, Stanton Harcourt, Oxon. (ARC); these are agravel pitand a waste disposal site, respectively, in which fresh Oxford Results and discussion Clay is temporarily exposed during construction work. Total organic carbon Sample sites are widely separated (Fig. l), therefore bed to bed correlations between sites on the centimetre scale of our collecting Totalorganic carbon (TOC) contents of the 165 samples are uncertain.However, eachsite is wellconstrained analysed vary between 0.5 and 16.6% by weight (Table l>. biostratigraphically (Callomon 1968; Duff 1975; Page pers. comm.), Samples were not taken at regular intervals but according to allowing approximate correlation of all samples to a common time variations in lithology.Observed concentrations of TOC scale. We assumed a duration of 430000 years for each ammonite subzone (Hallam et al. 1985). The Medea and Enodatum Subzones thereforerepresent beds ofvarying thickness, and an weregrouped together, because of the verylimited thickness of average organic-carbon content accurately representative of OxfordClay included in the EnodatumSubzone. The rate of the Peterborough Member as a whole cannot be reported. sedimentaccumulation was assumed constant for the duration of The graph of TOC content versus stratigraphic position in

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Table 1. Results of chemical and isotopic analyses of samples of the Oxford Clay

Sample Descrip-Bed" Biofacies' Relative Strat. TOC Tmaxf HIg OIh 6°C TotalTotal CO:- number tionb Elevd age" %C "C %v. c S %C PDB % %

Dogsthorpe Brick Pit OCD2 90-5 Sh DFBS 10 0 2.6 419 281 -26.4 75 2.7 1.2 0.1 OCD3 90-4 SB GSB 15 15 2.1 427 390 -27.7 79 2.4 1.1 0.3 OCD 90-3 8 Sh DFBS 48 60 8.8 415 673 -27.1 46 9.0 2.2 0.2 OCD 90-1 6/7 Sh DFBS/GSB 38 65 4.4 417 547 41 -27.7 4.8 1.5 0.4 OCD 90-2 7/8 Sh GSB/DFBS 43 70 3.7 424 577 46 -27.4 4.0 1.3 0.3 P 89-1 8 Sh DFBS 45 82 8.0 415 634 -27.0 41 8.2 2.4 0.3 P 89-2 8 Sh DFBS 50 91 6.8 417 609 -27.1 43 7.3 2.1 0.5 89-3 9 P 89-3 Sh GSB 53 96 5.6 419 567 -27.3 43 5.7 1.8 0.1 89-4 10 P 89-4 Sh GSB 54 98 4.2 422 555 -27.4 55 5.3 1.7 1.1 89-5 10 P 89-5 Sh DFBS 56 101 6.6 413 629 -26.6 45 6.8 2.0 0.2 89-6 10 P 89-6 Sh DFBS 62 110 5.1 416 610 -27.1 48 5.4 1.9 0.3 P 90-2 10 Sh DFBS 65 115 11.4 419 720 -25.9 58 12.2 3.1 0.7 88-1 10 P 88-1 Sh DFBS 66 116 13.6 407 506 37 -24.8 13.7 3.0 0.1 89-7 10 P 89-7 Con DFBS 66 116 1.4 409 486 -25.1107 11.1 5.2 9.7 89-9 10 P 89-9 Sh DFBS 66 116 12.5 419 759 -25.1 72 13.9 3.3 1.4 89-8 10 P 89-8 Sh DFBS 68 119 11.0 415 726 -24.7 87 14.0 4.0 3.0 89-12 10P 89-12 Sh DFBS 70 122 15.0 412 76 1 -24.2 64 17.0 4.0 2.0 90-1 10*P 90-1 Sh DFBS 70 122 12.2 417 710 -25.2 48 13.9 3.5 1.7 'P10 89-13 Sh DFBS 73 127 14.2 416 813 64 -23.1 15.5 4.2 1.2 89-14 10P 89-14 Sh DFBS 73 127 12.3 416 747 66 -24.7 14.2 3.5 2.0 88-2 11 P 88-2 SB GGrSB 75 130 8.3 403 564 37 -26.3 9.1 2.4 0.9 89-15 11P 89-15 Sh GSB 75 130 3.5 414 370 101 -26.6 6.2 6.4 2.7 89-16 11P 89-16 Sh GSB 75 130 2.8 414 219 -26.9115 5.5 9.1 2.7 89-20 12P 89-20 Sh DFBS 77 133 8.6 414 689 48 -26.6 9.6 2.4 0.9 89-21 12P 89-21 Sh DFBS 95 160 3.9 419 514 -27.0 65 4.2 1.3 0.2 *P12 89-22 Sh DFBS 95 160 3.8 420 222 -26.4 63 4.0 1.4 0.3 88-3 12 P 88-3 Sh DFBS 95 160 4.6 422 464 30 -26.4 4.7 1.7 0.1 89-23 12P 89-23 Sh DFBS 105 175 3.7 420 524 -26.8 58 4.1 1.4 0.4 89-24 12P 89-24 Sh DFBS 115 190 4.2 418 569 44 -26.8 4.5 1.7 0.2 89-25 13P 89-25 Sh GGrNSB 120 197 4.2 413 386 -26.4 72 6.3 3.3 2.2 P 88-4 13 SB GGrNSB 120 197 3.6 42 1 478 -26.4 61 5.1 3.5 1.5 P 88-5 14 Sh DFBS 125 20 1 5.8 417 492 -26.2 41 6.3 1.7 0.5 89-30 14P 89-30 Sh DFBS 125 201 4.9 417 518 -26.6 61 6.0 1.8 1.1 89-31 14P 89-31 Sh DFBS 175 213 7.0 416 71 1 49 -26.4 7.7 1.8 0.8 89-33 14P 89-33 Sh DFBS 225 225 5.6 418 593 58 -26.2 6.3 1.8 0.6 88-6 14 P 88-6 Sh DFBS 245 229 4.4 418 571 -26.0 61 5.5 2.1 1.1 89-34 14P 89-34 Sh DFBS 275 237 5.0 418 591 -26.3 64 5.7 1.8 0.8 89-35 14P 89-35 Sh DFBS 355 256 4.3 420 551 -26.6 65 4.9 1.6 0.6 89-36 14P 89-36 Sh DFBS 520 295 5.0 419 640 -27.2 56 5.6 1.1 0.6 89-37 15P 89-37 Sh NSB 535 299 3.3 42 1 531 -26.7 81 6.5 1.4 3.1

Stewartby Brick Pit S 90-1 6a Sh DFBS 70 54 4.6 414 539 34 -25.7 5.3 2.2 0.7 S 90-2 6a Sh DFBS 120 92 5.5 417 645 37 - 6.2 1.7 0.8 S 90-3 6b Sh DFBS 190 121 5.5 416 612 47 -25.6 6.2 1.8 0.8 S 90-4 6b Sh DFBS 310 164 5.2 417 607 -25.9 49 5.7 2.0 0.5 S 90-5 6c Sh DFBS 430 208 5.2 416 614 -26.4 46 5.9 2.0 0.7 S 90-6 6e Sh DFBS 525 246 4.1 418 552 49 -25.8 4.8 2.0 0.7 S 90-7 8 SI1 MSBICC 560 260 4.0 420 587 -26.7 56 6.8 1.3 2.8 90-8 8 S 90-8 Sh MSBICC 590 272 4.7 415 562 -26.1 55 6.6 1.7 1.9 S 90-9 8 Sh FBS 640 292 5.1 416 783 -25.8 51 6.7 1.6 1.6 90-10 10aS 90-10 Sh GrBS 720 309 5.5 417 634 43 -26.6 6.2 1.4 0.7 S 90-11 1OC Sh GrBS 950 345 6.8 413 663 -26.6 41 8.0 1.8 1.2 S 90-12 1OC Sh FBS 1010 355 9.1 412 682 -26.5 41 11.2 2.5 2.1 90-13 14S 90-13 Sh cc 1150 377 3.4 423 542 -26.4 56 5.1 1.5 1.6

Bletchley Brick Pit B 88-la 9 Sh DFBS 250 94 5.1 416 667 85 -27.0 5.9 2.2 0.7 89-6 9 B 89-6 Sh DFBS 250 96 3.6 417 385 81 -26.7 3.4 1.9 0.0 B 89-1 10 Wd GSB 270 101 2.2 416 147 124 -26.8 4.8 3.0 2.6

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Table 1. (Continued)

Sample Descrip-Bed"Biofacies' Relative Strat- TOC Tmaxf HIg OIh 6°C TotalTotal CO:- number tion' number Elevd age" %C "C f v. c S %C PDB % %

Bletchley Brick Pit (continued) B 89-3 10 Sh GSB 270 101 0.8 422 49 295 -26.6 7.2 9.9 6.4 89-4 10 B 89-4 Sh GSB 270 101 2.0 42 1 246 98 -26.5 7.5 1.7 5.5 89-5 10 B 89-5 Sh GSB 270 101 3.6 414 394 88 -26.9 3.7 1.7 0.1 89-7 12*B 89-7 Sh DFBS 300 109 5.6 718 60 1 70 -27.7 6.0 2.1 0.4 89-8 12 B 89-8 Sh DFBS 420 138 4.2 422 547 84 -26.4 4.4 1.6 0.2 88-1 12 B 88-1 Sh DFBS 500 157 3.6 419 615 78 -25.7 4.6 1.8 0.9 89-9 12 B 89-9 Sh DFBS 660 196 4.3 419 562 89 -26.1 5.0 2.2 0.7 88-2 13 B 88-2 Sh DFBS 940 242 5.9 414 73 1 66 -25.5 7.4 2.1 1.3 89-10 13?B 89-10 Con DFBS 950 243 1.1 42 1 155 239 -25.9 9.7 7.0 8.6 88-3 13 B 88-3 Sh DFBS 960 245 6.5 414 715 62 -26.0 7.2 2.3 0.6 89-13 13B 89-13 Sh DFBS 1260 292 4.7 420 610 72 -26.7 5.1 1.9 0.4 88-4 14 B 88-4 Sh NSB 1300 298 3.4 426 645 92 -26.8 4.8 1 .S 1.3 88-5 15 B 88-5 Sh GrBS 1420 323 6.9 415 690 41 -26.4 7.5 1.9 0.5 89-14 17B 89-14 Sh cc 1660 374 3.0 423 521 105 -27.3 5.8 1.8 2.8 88-6 17 B 88-6 Sh DFBS 1740 391 5.2 413 764 50 -26.6 9.2 1.6 4.0 B 89-12 18 Con FBS 1800 404 1.9 417 558 143 -27.0 10.6 0.8 8.6 88-7 20 B 88-7 Sh DFBS 1840 407 4.6 415 745 59 -26.4 6.8 9.8 3.2 89-15 20B 89-15 Sh DFBS 1840 407 4.4 417 635 82 -26.8 6.9 1.6 2.6 88-7a 24B 88-7a Sh DFBS 2400 476 5.5 405 296 66 -25.5 6.7 10.5 1.3 *B 88-8 25 Sh SM 2700 507 1.3 438 166 81 -25.5 2.7 2.5 1.4 *B25 89-17 Sh SM 3500 560 1.2 432 130 105 -26.0 3.1 l .4 1.9 B 88-9 25 Sh SM 3700 573 0.8 432 67 56 -25.4 3.0 1.1 2.2 B 89-16 25 Sh SM 3750 577 0.6 426 55 213 -25.9 3.4 0.7 2.8 Calvert Brick Pit C 89-2 2c Con GSB 300 100 0.5 429 147 200 -27.7 11.0 1.o 10.5 C 89-3 2c Sh GSB 300 100 3.7 42 1 493 92 -27.1 5.7 2.4 2.0 89-7 3b C 89-7 Sh DFBS 310 102 3.9 419 481 79 -26.1 4.9 2.3 1.o 89-4 2c C 89-4 Amm BC 440 122 0.7 415 24 251 -25.9 3.8 25.6 3.1 89-5 3b C 89-5 Sh BC 440 122 2.0 412 138 81 -26.0 1.9 2.4 0.0 89-6 3b C 89-6 Amm BC 440 122 0.5 417 200 324 -25.1 4.3 27.4 3.8 89-8 3a C 89-8 Sh DFBS 780 177 3.0 424 445 91 -26.1 3.6 1.9 0.6 89-17 4 C 89-17 Sh DFBS 1160 249 5.0 418 564 82 -26.7 5.2 1.7 0.3 89-14 5 C 89-14 Sh NSB 1370 294 7.2 413 665 57 -26.6 8.1 2.0 0.9 89-13 6 C 89-13 Sh DFBS 1375 295 8.1 414 693 54 -26.3 9.4 2.2 1.3 89-15 6 C 89-15 Sh DFBS 1380 296 5.0 414 588 86 -26.7 6.9 2.0 1.9 89-16 6 C 89-16 Con DFBS 1380 296 0.9 419 314 254 -26.5 11.3 1.9 10.4 89-12 6 C 89-12 Sh DFBS 1385 297 8.7 41 1 69 1 60 -26.2 10.4 2.3 1.7 89-10 7 C 89-10 Sh NSB 1395 299 4.2 409 353 116 -26.5 5.7 14.2 1.5 89-18 10C 89-18 Sh GrBS 1590 342 12.1 424 732 76 -26.9 13.7 3.2 l .7 89-19 11C 89-19 Sh NSB 1840 398 3.1 418 634 101 -26.6 7.1 1.8 4.0 *C12 89-20 1B Con FBS 1860 402 5.2 415 659 81 -26.6 8.2 1.6 3.0 89-21 12C 89-21 Con FBS 1860 402 1.9 42 1 567 121 -26.4 12.0 0.5 10.2

Brown's Pit BP 90-1 - Sh SM 6300 860 0.9 43 1 59 30 -25.6 4.0 1.4 3.2 BP 90-2 - Sh SM 6400 880 0.9 43 1 55 -25.4 27 4.1 0.7 3.2

Cleveland Farm Pit CF 90-1 Sh DFBS 2550 437 14.1 413 756 64 -26.2 15.5 3.5 1.4 CF 90-2 Sh DFBS 2620 44 1 5.6 413 623 -26.8 69 6.7 2.4 1.1 CF 90-3 Sh DFBS 2660 444 16.6 41 1 77 1 -25.5 59 17.8 3.9 1.2 CF 90-4 Sh DFBS 2870 458 15.2 414 731 -25.9 61 15.0 4.6 0.0 CF 90-5 Sh DFBS 3070 47 l 6.1 424 672 -26.1 70 7.4 1.9 1.4 CF 90-8 Sh SM 5500 800 0.8 424 72 21 -24.7 4.1 0.7 3.3 CF 90-7 Sh WM 6900 940 1 .o 426 78 20 -24.1 3.0 0.9 2.1 *CF 90-6 Sh WM 7000 950 0.9 425 67 11 -23.6 2.7 1.1 1.8

Whittlesey Core WKl Cb ArgLs - - 385 -77 0.5 425 51 -25.7 22 7.9 3.4 7.3 W K2 KF DkC ssc -379 - 76 1.2 429 65 -24.9 15 1.1 1.2 0.0

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Table 1. (Continued)

Sample Descrip-Biofacies'Bed" Relative Strat. TOC T,,,,,( OIhHIg 6°C TotalTotal CO:- number Elevd tionh age' %C "C %o v. c S %C PDB % %

WhifflesevCore (continued) WK3 KF DkC ssc -317 -63 1.1 427 65 -24.8 11 0.9 2.8 0.0 WK4 KF DkC ssc -275 -55 1.1 426 61 -24.5 12 1.1 1.3 0.0 W K5 KF DkC ssc -259 - 52 1.1 426 62 -24.4 12 1.o 1.6 0.0 W1 KF Silt ssc -117 - 23 1.0 406 80 -25.3 0 0.9 1.3 0.0 W2 KF CS ssc -31 -6 1.4 418 262 -26.0 7 1.5 0.9 0.1 W3 4 sc ssc 3 9 2.5 417 276 -26.6 27 2.3 1.5 0.0 W4 7/8 FSh DFBS 19 59 10.0 412 580 -27.1 40 10.1 2.4 0.1 W5 10 BkC DFBS 34 103 6.0 412 552 48 -26.8 6.1 2.0 0.1 W6 10 FSh DFBS 45 122 4.4 416 519 54 -27.2 5.0 1.9 0.6 W7 12 FSh DFBS 54 138 4.7 417 509 -27.0 51 5.1 2.0 0.4 W8 12 BkC DFBS 76 176 4.2 417 510 -27.4 51 4.7 1.4 0.5 W9 13 SB NSB 87 195 5.5 416 570 -26.4 50 6.4 1.7 0.9 W10 14 Sh DFBS 113 206 5.5 415 556 -26.7 48 6.0 1.6 0.5 W11 14 Sh DFBS 135 212 4.8 410 591 -26.8 54 5.9 1.6 1.1 W12 14 Sh DFBS 165 220 5.3 412 53 1 48 -26.6 5.8 1.6 0.5 W13 14 Sh DFBS 19.5 228 7.6 41 1 570 -26.0 43 8.2 1.8 0.6 W14 14 Sh DFBS 225 236 6.1 414 564 -25.9 46 6.9 1.6 0.7 W15 14 Sh DFBS 2.50 243 5.2 414 515 47 -26.4 5.7 2.0 0.5 W16 14 Sh DFBS 285 252 4.5 415 506 48 -26.5 5.0 1.9 0.5 W17 14 Sh DFBS 315 260 4.0 414 513 -26.6 54 4.9 1.9 0.9 W18 14 Sh DFBS 350 270 5.2 413 558 -26.2 53 6.1 2.1 0.9 W19 14 Sh DFBS 380 278 5.4 413 568 -25.9 47 6.0 1.9 0.7 W20 14 Sh DFBS 410 286 4.9 414 558 -26.1 50 5.7 1.8 0.8 W21 14 Sh DFBS 421 289 4.9 412 537 -26.1 52 5.7 1.8 0.8 W22 14 Sh DFBS 449 296 3.2 420 418 47 -26.3 4.7 1.6 1.5 W23 18 Sh DFBS 474 302 4.9 42 1 550 -26.8 46 5.6 1.1 0.7 W24 18 Sh DFBS 518 310 5.4 418 576 -26.7 47 5.8 1.3 0.4 W25 18 Sh DFBS 550 315 4.7 419 553 -26.8 46 5.1 1.2 0.4 W26 18 Sh DFBS 580 321 4.3 422 520 -27.0 48 4.8 1.1 0.5 W27 18 Sh GrSB 606 326 4.5 42 1 554 -26.9 47 5.5 1.1 1 .o W28 19 SB GrSB 614 327 2.2 420 270 -26.6 85 7.1 2.9 4.9 W29 20 Sh DFBS 629 330 3.7 419 529 -27.3 53 5.2 1.1 1.5 W30 20 ShlC MSB 644 332 3.6 42 1 491 -26.9 63 6.5 1.5 3.0 W3 1 21 Sh DFBS 680 339 6.1 413 603 -27.1 49 7.2 1.4 1.1 W32 22 Sh DFBS 715 345 5.0 423 554 -27.1 50 6.3 1.3 l .3 W33 22 Sh FBS 735 349 7.0 414 604 -26.9 43 8.1 1.6 1.1 W34 23 SB MSB 742 350 6.4 40 1 617 -26.9 43 8.5 1.9 2.0 W35 24 Sh FBS 755 352 4.7 42 1 561 52 -26.5 6.8 1.5 2.2 W36 24 Sh FBS 785 358 6.4 417 578 -27.1 40 7.3 1.3 0.9 W37 26 Sh DFBS 815 363 5.0 417 53 1 48 -27.2 6.7 2.4 1.6 W38 28 Clay cc 855 370 2.6 427 414 -27.5 56 5.5 1.4 2.9 W39 30 Clay cc 880 375 1.3 432 198 -27.2 59 5.6 1.8 4.4 W40 32 ShC DFBS 915 381 3.2 42 1 460 -26.9 51 6.2 1.2 3.0 W4 1 34 Clay cc 948 387 1.8 429 316 -27.3 54 6.1 1.5 4.3 W42 35 Clay cc 978 392 2.3 425 399 57 -27.7 5.8 1.1 3.5 W43 36 ShC DFBS 988 394 2.7 423 453 -27.5 86 6.0 1.1 3.2 W44 37 SB NSB 1015 399 3.0 425 464 -27.1 86 6.3 1.6 3.3 W45 38 ShC FBS 1023 400 3.5 420 523 -27.3 87 6.5 1.4 3.0 W46 38 ShC DFBS 1035 402 4.8 416 577 -26.7 85 7.6 1.3 2.9 W47 38 Sh DFBS 1065 405 5.6 413 573 -26.6 79 7.6 1.6 2.0 W48 39 Clay cc 1103 408 1.6 425 259 111 -27.7 3.0 1.6 1.4 W49 40 ShC DFBS 1127 41 1 4.1 417 529 94 -26.5 6.0 1.4 1.9 W50 40 ShC DFBS 1157 414 4.7 412 551 90 -26.2 7.0 1.6 2.3 W5 1 41 Clay cc 1185 417 2.1 423 340 -27.2109 4.1 1.9 2.0 W52 43 ShC DFBS 1223 420 3.2 424 362 101 -27.4 4.6 1.6 1.4 W53 44 ShC DFBS 1243 422 3.6 415 483 94 -26.4 5.6 1.3 2.0

~

a Bed designations after Duff (1978). For samples above bed 18 at Bletchley, bed numbers are defined by Hudson & Martill (this volume). For samples from Whittlesey Core, bed numbers are those defined for the King's Dyke, Whittlesey, Brick Pit (Hudson & Martill this volume). Cb, Cornbrash Formation; KF, Kellaways Formation.

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A B D E 1I K. Drontae

K. phaelnurn

grossouvre

K. obducturn

K. lawn Bed 10 af Peterboro‘ 2 K. rnedea & S encdaturn S calloviensz i 0 4 8 12 If 0 200 400 600 800 024 246

?h TOC HI Yo Crnin ?40 S tot Fig. 2. Bulk geochemical parameters obtained on whole-rocks samples (except concretions and wood) plotted versus time. Methodfor construction of the time scale is given inthe text. (A) total organic carbon (wt %TOC), (B)hydrogen index (HI, mgHC. per g C), (C) a.,., v. PDB, (D) carbonate carbon (wt %C,,,), (E)total sulphur content (weight %S).

Fig. 2A shows thatthe organic richness of Peterborough concretions. It has yielded numerous skeletons of giant Member (mostly >3% TOC) contrasts with the depletion of and reptiles and contains abundant complete ammonites. organic matter in the underlying Kellaways Formation and overlyingStewartby Member (both <1.5% TOC).The Weymouth Member is also depleted in organic carbon with Tmax all concentrationsunder 1% (Table 1). Distribution of The immaturity of the Peterborough Member is confirmed organiccarbon in thePeterborough Member is not by the low T,,, values (Tablel) which average 419 “C for all homogeneous.Bed 10 of Callomon (1968) andHudson & theshale sample; a value well belowthe threshold of oil Martill (this volume), at the base of the Jason Subzone at generation (2435°C) for organicmatter of this type Peterborough, is particularly organic rich, with TOC varying (discussed in detailbelow). High T,,, valuesfound for between 4.4 and 15%; alsocontains it septarian somesamples (carbonate concretions and beds with very

Abbreviations:Amm, ammonite; ArgLS, argillaceous limestone; BCon, bituminous concretion; BkC, blocky clay; Con, concretion; CS, clayey silt; DkC, dark clay; FSh, fissile ; SB, shell bed; SC, silty clay; Sh, shale; ShC, shaly clay; ShlC, Shelly Clay; Wd, wood. ‘Biofacies as defined by Duff (1975): Deposit feeder shale (DFBS); Grammatodon-rich shale (GrBS); Foram-rich shale (FBS); Nuculacean shell bed (NSB); shell bed (GSB); Meleagrinelfa shell bed (MSB); Gryphaea and Grammarodon shell bed (GGrSB); Mixed shell bed (GGrNSB);Silts and silty clay (SSC); Blocky claystone (BC); Calcareous clay (CC); also Stewartby Member (SM); Weymouth Member (WM). Relative elevation is the elevation in centimetres above the base of the Enodatum Subzone of each quarry or sampling site. e See ‘samples and methods’ in text. Temperature of maximum hydrocarbon yield during Rock-Eval pyrolysis. g ‘Hydrogen Index’ from Rock-Eval pyrolysis, mg hydrocarbon per gram of organic carbon. h ‘Oxygen Index’ from Rock-Eval pyrolysis, mg CO, per gram of organic carbon (technique does not release CO2 from carbonates). * Samples selected for detailed analyses: see Figs. 5,6 and 7.

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900 low TOC) cannot be representative of increased maturity. I I I Inthese instances, it is likelythat the consumption of I/ thermally labile organic matter during microbial diagenesis is responsiblefor theelevation of T,,,. Significant 750 - contributions of organic matter from non-microbial sources, such as terrestrial organic matter can also lead to increases in T,,,, butthis alternativewas excluded by petrographic 600- examination of the samples (Belin & Kenig this volume).

450 Hydrogen Index and Oxygen Index Thehydrogen index (HI), in milligrams of hydrocarbon produced during pyrolysis per gram of organic carbon, is an 300 - indicator of thematuration and/or of thepreservation of organic matter, high values (HI > 500) being representative of well-preserved,thermally immature materials (EspitaliC 150 - et al. 1977, 1986a, 6).Hydrogen indices observed for samples from the Peterborough member (Fig. 2B) are high, n averaging553 mg HCg-' C. This contrasts with values U1 I I 11 obtainedfor samples from the Kellaways Formation 150and 100 0 50 StewartbyMember (HI<250).The oxygenindex (01), 01 (mg Cq/gorg. C) alsoobtained from Rock-Eval pyrolysis, is given in milligrams of CO,produced per gram of organiccarbon. Peterborough Member: During analysis, temperatures are kept low enough to avoid 0 Deposit feedershale 0 Gryphaea shell bed pyrolytic generation of CO, from carbonates; thus the 01 is A Foram-richshale Nuculaceanshellbed anindicator of the oxygencontent of theorganic matter U Grammatodon shale Mixed shell (EspitaliC et al. 1977, 19866). Plots of HI v. 01 can be used A bed like a van Krevelen diagram, in which H/C is plotted as a + Silt and silty clay function of O/C. Thisfacilitates the identification of the M Stewartby and Weymouth Members type of organic matter present, its alteration stage and its thermalmaturity (EspitaliC et al. 1977,1986b; Tissot & Fig. 3. Plot of hydrogen index (HI) v. oxygen index (01) of all Welte 1984; Peters 1986). Results of Rock-Evalanalyses, whole-rock samples (except concretions, calcareous clays and plotted in this way (Fig.3), show samples clustered in wood). Explanations of clusters labelled A, B1, B2 and C is given severalgroups. Deposit-feeder shales arethe dominant in the text. biofacies type (Duff 1975). These form group A, plotting in the region of immature Type I1 organic matter as defined by EspitaliC et al. (1977, 1986b), and showing that the bulk of OxfordClay, these findings would indicate that deposit- accumulatedorganic matter is of marine,phytoplanktonic feeder shales were deposited under less oxic conditions than origin. The high H Index (500 to 800 mg HCg-'C) of shellbeds. Thisexplanation is consistent with the samples in this group indicates good preservation and low conclusionsreached from analyses of bio-assemblages by thermal maturity of the organic matter. Duff (1975). Groups B1 andB2 in Fig. 3 comprisesamples from Petrographicexamination of organicmatter (Belin & Gryphaea shellbeds andsamples from Nuculacean, Kenig this volume) shows that amorphous organic matter of Grammatodon and mixed shell beds, respectively. Samples phytoplanktonic origin dominates in organic-rich shales (up from the Calcareous Clay biofacies, and from the Stewartby to95%) and in asample from the Stewartby Member Member,are included in bothgroups. Trends in HIndex (75%). In spite of this dominantly marine, microbial origin, vary as a function of 0 Index for these samples and bridge organic matter from the latter (for example, sample B89 17, thegap between the lines of evolutioncharacteristic of Table 1) yields an H Index of only 130, placing it just on the Type-I1and Type-I11 organic matter. Two mechanisms type-I11 line in Fig. 3. It is not plausible that decreasing the might be invoked to explain these trends. (1) The samples marinecomponent from 90% to 75% can reduce the H fromthe shell bedbiofacies and some of thosefrom the Index from values over 650 to only 130. Accordingly,the Stewartby Member, with H and 0 Index values close to the observed trends in H Index values cannot be attributed to a line of evolution of Type-I11organic matter, might result source effect andreworking is viewed as themore likely from the mixing of Type-I1 and Type-I11 organic matter. (2) cause of thesecompositional changes. The mostobvious Thedecrease in H Index and increase in 0 Indexresult petrographic difference between the high- and low-H Index frombiological reworking of organicmatter during samples is the degree of alteration of the structured organic sedimentationand early diagenesis. Specifically, longer matter, whichchanges colour from yellow-brown to black residenceat the water-sediment interface and/or aerobic over the full range of variation (Belin & Kenig this volume). reworking of organicmatter can lead tobiologically But structured organic matter is always a minor component. mediated oxidation of hydrogen-rich organic matter (Pratt Hence, the great range of H Index, requires that significant 1984). Hollander et al. (1990)observed, in moderna chemicalchanges must also beoccurring, though not lacustrine setting, that organic matter deposited in an anoxic strongly evident petrographically, in the amorphous organic water column had higher H/C ratios and lower O/C ratios matter. thanthat deposited in anoxic water column. For the The samplesforming group C in Figure3 have low

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relatedto changes in biofacies.On average, shell beds containmore carbonate (2.1%) than do shales (1.0%). Carbonate is mostly supplied by the shells.Shells become moreconcentrated as a result of winnowing (Hudson & Martill 1991) andthus indicate slowaccumulation rates, oftenmarking diastems at subzonalboundaries (Fig. 2D; Duff 1975). These sharp variations are superimposed on a progressive increase in the abundance of carbonate carbon, from 0% to 370, from thebase to the top of the Peterborough Member. This trend is driven by progressive X increase in the concentration of coccoliths, and continued in 0 +X& the Stewartby Member (Bown in Martill et al. this volume). X *O That trend, in turn, probably reflects acontinuing marine transgression,increasing the distance tothe siliciclastic sourceand providing amore open marine environment favorable to growth of coccolithophorids.

1 10 100 Log (TOC) Total sulphur Contents of totalsulphur, graphically summarized in Fig. Peterborough Member: 2E, exhibit single point variations which can be related to 0 Depositfeeder shale 0 Gryphaea shell bed biofacieschanges and especially tothe presence of shell A Foram-richshale W Nuculaceanshell bed beds.Shell beds often mark subzone boundaries (see above) and are often preferentially pyritized (Fisher 1986). U Gratnnzatodon shale A Mlxedshell bed Four subzone boundaries are marked by sediments with a + Silt and silty clay X Calcareousclay sulphur contents of more than 7%. * Stewartby and Weymouth Members

Fig. 4. Plot of hydrogen index (HI) v. Log,,,TOC (total organic 6 13T.0(3 carbon). The "C value of organiccarbon (~5,~~)ranges between -23.1and -27.70W v. PDB (Fig.2C). Variations are correlated with unitboundaries, with particularisotopic values for H and 0 Indices. Typically, samples with these enrichment being observed in the Kellaways Formation and characteristics areconsidered thermally altered. However, inbed 10 of thePeterborough Member. Contents ofI3C given the burial history of the Oxford Clay Formation and decrease slowly butsteadily throughout theGrossouvrei the low maturity of theother samples, this cannot apply Subzone, then rise more rapidly in the Phaeinum Subzone. here. Extensive chemical alteration of the organic matter is Variations in &,,might be attributed to (i) secular changes thusindicated. This might have occurred either during in the "C content of inorganiccarbon dissolved in ocean transfer and residence at the sediment-water interface in an water; (ii) effects of thermal maturation; (iii) changes in the oxicwater column or duringresuspension and transfer of relativeabundances of marineand terrestrial organic the sediment. Suspension and transport of sediments have matter; (iv) changes in primary producer organisms, that is, beenpreviously envisaged to explain the low total succession of physiologicallydistinct organisms having accumulation rate of the Peterborough Member (Hudson & inherentlydifferent overall isotope effects associated with Martill 1991; Macquaker this volume). It is noteworthy that the fixation of carbon; (v) environmentally induced changes group C is composed of samplesfrom the Stewartby and in the isotopic fractionation imposed by primary producer Weymouth Members. organisms,for example, changes due to variations in the There is aquasi-linear relationship between hydrogen concentration of dissolved CO,; and, finally, (vi) changes in indexand Log,,,TOC for samples from the Oxford Clay the net fractionation imposed by the community, specifically (Fig. 4). A relationship of this kind would be expected for a includingeffects of heterotrophicreworking of primary process in which the rate of carbon loss hada first-order products. dependenceonhydrogen index. Considering that the metabolizability of organicmatter is atleast crudely proportional to H Index, this observation is not surprising. Effects of maturationand secular trenak. Twopotential It is consistent with the proposal that alteration of organic causes of isotopic variation can be readily excluded. First, matter during sedimentation or early diagenesis accounts for compilations of seculartrends (e.g. Holser et al. 1988) most of the observed chemical variations, without invoking indicate that the I3C content of marine inorganic carbon was theadmixture of largequantities of terrigeneousdebris. declining at a rate of less than 0.1% per Ma during deposi- Petrographic analyses, which found only minor changes in tion of these sediments. Since the time interval represented palynofacies between high- and low-H Index samples (Belin by Fig. 2 is approximately 3 Ma, only a small portion of the & Kenig this volume), support this hypothesis. observed isotopic variation can be attributed to this trend. High-frequencyglobal variations thelikethatat Cenomanian-Turonianboundary (Schlanger et al. 1987) Mineral carbon mighthave escaped notice, but none areapparent in the Twotypes of variationare evident in the abundance of carbonates analysed from these sediments (Anderson et al. carbonate carbon (Fig. 2D). Bed to bed variations can be this volume). Accordingly, changes in the I3C content of the

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inorganic carbon source cannot be responsible for the varia- isotopic compositions of pristane and phytane do not differ tions in b.,.,, observed here. Second, effects of maturation significantly and are well correlated.Accordingly, the iso- can be excluded both because indicators of significant ther- topiccompositions of pristaneand phytane are probably mal alterationare absent and because the required strat- very close tothose of primaryisoprenoid lipidsin the igraphic variations in thermal history are not plausible. We samples from which they have been isolated. As shown in are left to consider (iii) variations in the ratio of terrestrial Fig. 5, the 6 values of pristane and phytane from samples of tomarine organic matter, (iv) changesin producer organ- the Peterborough Member fall between -31 and -32.30-. isms,(v)environmental effects, and (vi)effects of Sincethese hydrocarbons can derive from any photo- reworking. synthetic organisms, either marine or terrestrial, and since they are commonly depleted in I3C by 4% relative to plant Effects of variation of primaryinputs. Estimation of the biomass, this suggests that the average 6 values of marine + isotopiccomposition of primaryorganic matter (h,,), as terrestrialbiomass primary products contributing to these distinctfrom that of thetotal organic carbon ultimately sedimentsranged between -27 and -28.30-. Anestimate preserved (6.,,,), canprovide important information. of the 6 values of terrestrial materials can be obtained by Hayes et al. (1989) demonstrated that isotopic compositions analysis of thelong-chain n-alkanes, which derivemainly of degradation products of chlorophyll could be used for this fromwaxes characteristics of higherplants (Eglinton & purpose, first employingporphyrins derived from the Hamilton1963). As shown in Table2, 6 valuesfor the hetero-aromatic nucleus of that molecule and later (Hayes et long-chain n-alkanes are near -28"A. Since n-alkyl carbon al. 1990)pristane and phytane derived from chlorophyll's skeletons are commonly a little morestrongly depleted in phytol substituent. Particular caution is required when the I3C than are polyisoprenoids like pristane and phytane, we latter alternative is chosen because acyclic isoprenoids can can estimate h,, = 6n.alby,+ 4.5 (the subscript Pt pertains to havemultiple sources (ten Haven et al. 1987;Brassell & primaryterrestrial organic matter) and conclude that a 6 Eglinton1983). In thecase of theCretaceous Greenhorn value representative of terrestrial primary biomass would be Formation, however, it was shown (Hayes et al. 1990) that near -23.5"A. Although this value may be thought surpris- isotopic compositions of pristane and phytane did not differ ingly high, it is corroborated by direct isotopic analysis of significantly, a finding in marked contrast to that in systems wood fragments found in these sediments (Table 2). in which diverse origins of phytane were apparent (Freeman No isotopicbiomarkers specific tomarine primary et al. 1990), and that the isotopic difference between por- producerscould beanalyzed in this work,but both phyrins and acyclic isoprenoids was consistently 4.5Y0, the petrographicevidence andthe isotopic compositions value expected if differences were due only to biosynthetic indicatethat the organic material in thePeterborough isotope effects. The depositional environment of the Oxford Member is dominantly of marine origin. Accepting the 95/5 Clay is much more similar to that of the Greenhorn Forma- marine/terrestrialratio indicated by petrographicobserva- tionthan tothose environments in which non-primary tion, wecan estimate that the 6 value of marineprimary phytanehas been prominent and, as shown in Fig. 5, inputs (a,,,,) was -28.20- duringthe deposition of the PeterboroughMember. Values of 6 characteristic of various marine +terrestrial mixtures can then be calculated. As shown in Table 2, not even a huge variation, to 50150 marine/terrestrial,could produce the 6.,, valuesactually observed, which range up to -23.100. Thisdemonstration -30 thatvariations inmarine vs. terrestrialinputs cannot n Q 1 account for thevariations in 6,.,, indicatesthat the observed enrichment of I3C in TOC is likely to be due to E secondaryprocesses (i.e., (vi) above, further discussed -3 1 below). 3h Theconclusion thatterrestrial inputs cannot be L: a responsiblefor the variations in 6,, is independent of 9 certain uncertainties associated with Table 2. There is, for example, evidence that enrichment of total biomass relative -32 to waxes in vascular plants can be considerably greater than 4.5'% (i.e., ?6%0, Rieley et al. 1991).Moreover, itis possible that the carbon preserved in the wood fragments is a biased sample of that initially present, cellulosic material -33 having been lost while lignin carbon survived. Since lignin is -33 -32 -3 1 -30 depletedin I3C relative tototal biomass (Benner et al. 8 1987), this effect would also lead to estimates of 6,, even pristane (%o) greaterthan those indicated in Table 2. Forthe sake of argument, one might adopt h,, = -2O"A. Even in that case, Fig. 5. Plot of 6DriaIanev. Sphytane obtained with irm-gas chromatograph mass spectrometry. Each data point is the average variations of the terrestrial contribution from 5 to 25% (the of a minimum of two values obtained for both pristane and phytane. full rangeobserved petrographically) would produce a Sample numbers are as follows: (1) P 89-13; (2) P 90-1; (3) C 89-20; change in a,.,, of less than 2%, less than half the observed (4) B 89-7; (5) P 89-22; (6) B 89-17; (7) B 88-8; (8) CF 90-6. These range of 4.60~ in thePeterborough Member alone. samples are representative of the whole range of variation in bulk Variation in marine-terrestrial organic input (alternative iii geochemical properties, as shownfor TOC and 6 "C TOC in above) can therefore be decisively rejected on the basis that Fig. 7. it providesinadequate variation in I3C, independent of

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Table 2. Isotopic compositions of primary materials, marine + terrestrial mixtures and prokaryotic biomass

Average primpry inputs, S, = Si + 4 (Si = avg. 6 of pristane + phytane) Peterborough and Stewartby Members, ai = -31.7 -27.7 WeymouthMember 6i = -29.8 -25.8

Tenigenous primary inputs, Estimated via plant wax n-alkanes, apt= 6,. + 4.5 (Sna = S of n-alkanes) n-C,,H,, (avg., n = 6) 6,, = -28.3 -23.8 n-C,,H, (avg., n = 5) 6,, = -27.6 -23.1 Estimated from analyses of wood fragments, 6,, = bwmd average for 11 samples , a,,, = -23.8 -23.8 Approximate 6, adopted here -23.5

Marineprimary input, a,, = 6,m + 4 where Si, = (6i + 27.5f)/(1 -A) (Si, = 6 for marine pristane + phytane, -27.5 = 6 for terrestrial pristane + phytane, f, = fraction of TOC of terrigenous origin). Peterborough Member ai = -31.7, f, = 0.10 -28.2 Stewartby Member 6i = -31.7, f, = 0.25 -29.1 Weymouth Member ai = -29.8, f, = 0.25 -26.6

Mixtures of marine and terrestrial primary inputs Peterborough Member, 6 = (1 -f,)(-28.2) +f,(-23.5) 95% marine/5% terrestrial -28.0 75% marine/25% terrestrial -27.0 50% marine/50% terrestrial -25.8

Stewartby Member, 6 = (1 -h)(-29.1) +f,(-23.5) 95% marine/5% terrestrial -28.8 75% marine/25% terrestrial -27.7 50% marine/50% terrestrial -26.3

Weymouth Member, 6 = (1 -f,)(-26.6) +f,(-23.5) 75% marine/25% terrestrial -25.8 50% marine/50% terrestrial -25.0

P rokaryotic inputProkaryotic 6,=6,,+4 (6, = 6 for &3-homohopane). Peterborough Member (avg. n = 5) 6, = -27.1 -23.1 Stewartby Member (avg. n = 2) 6, = -27.2 -23.2 Weymouth Member (n = 1) 6, = -26.8 -22.8

uncertainties in related biosynthetic fractionation and, thus, commonly depleted in 13Cby 4%0 relative to total biomass absolute value of 6. (Hayes et al. 1990). Movement of points parallel to the n axis indicates changes in the 6 value of primary biomass. In Effects of environmental conditions and heterotrophic theGreenhorn Formation, these were due to well docu- reworking. By default(thus far), we are led toconclude mentedchanges in the I3C content of marinedissolved thatobserved variations in are due to environmental or inorganic carbon (DIC) (i.e., the ‘carbon isotope anomaly’ biologicaleffects in themarine palaeoenvironment. These associated with theCenomanian-Turonian anoxic event). can be examined using Fig. 6, in which 6 values of isopren- The occurrence of such secular changes in marine dissolved oidhydrocarbons and TOCare compared. To showthat inorganic carbon has already been excluded for the Oxford variations in the Oxford Clay are neither extreme nor un- Clay palaeoenvironment, so the changes in Gisoprenoid must be ique, points representative of samples from the very inten- due to succession of primary producers with different overall sivelystudied Greenhorn Formation (Hayes et al. 1989, isotopeeffects (mechanism (iv)) or toenvironmental 1990) are alsoincluded. The line indicates the TOC- changes(e.g. changes in concentrations of dissolved COz, isoprenoidrelationship expected if TOC werecomprised mechanism(v)). Changes in 6is,,prenoid withinthe Peter- entirely of unalteredprimary (i.e. algal) biomass. It has borough and Stewartby Members are minimal and there is been placed by noting that isoprenoid carbon skeletons are no point in trying to decide between biological and environ-

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plausiblemechanism. First, Fischer (1991) hasshown in -22 modern settings that sinking organic particles are enriched 'I in I3C relative to plankton by 2-5%. This implies that both Bed 10 0 heterotrophsand their faecal matter are enriched in 13C 10 relative to food sources. This would seem to place a great responsibility on respiratory processes when it comes to I3C enrichment.Although itwas not mentioned by Fischer (1991), we canspeculate here that enrichment ofI3C in Bed faecalmatter might be dueto escape of "C-depleted 1 methanefrom gut communities in heterotrophs. Because 9SM CH., produced by fermentation is usually strongly depleted in I3C (Oremland 1988), this mechanism could be especially 0 0' 0y 3 efficient atproducing enrichment ofI3C infaecal matter. 0 Second, Coffin et al. (1990) haveshown that, in natural 0 Burial of settings,bacterial biomass can bemuch more strongly Primary Biomass enrichedin I3C (upto 2%) thanpreviously observed in laboratory studies. In fact, the isotopic compositions of bacterial inputs (6,) 0 Oxford Clay can be crudely monitored by analyses of &3-homohopane, a 3 Greenhorn Fm. prokaryoticproduct detected in alleight samples. As indicated in Table 2, this substance is significantly enriched in I3C relativeto pristane and phytane. The implied 6 values for prokaryotic biomass are high enough (c. -230~) 613C,,B(Avg. Isoprenoid) %, tocause significant enrichment of '.'C in TOC.However, this falls short of proving that the isotopic enrichmentis due Fig. 6. Relationships between isotopic compositionsof total organic to admixture of bacteria from zooplankton guts, from the carbon and isoprenoid hydrocarbons. TheI3C content of primary water column, or from the sediments for two reasons. First, organic content (phytoplankton) can be estimated from that of the a mass balance for bacterial v. other carbon sources cannot isoprenoids, and the line correspondsto 6 values expectedif TOC constructed.be Second, theprecise source of the represents primary materialthat had nor been reworked prior to homohopane is not known. It could derive not only from burial. For the Oxford Clay, WM, Weymouth Member; SM, heterotrophicbacteria but also from cyanobacteria (i.e., Stewartby member, and sample numberare as follows: P89-13; (1) prokaryotic photoautotrophs for which visual evidence was (2) F90-1; (3) C89-20; (4) B89-7;(5) P 89-22; (6) B89-17; (7) B88-8; found in all samples;Belin & Kenigthis volume). (8) 0-6. However, because the latter organisms are photosynthetic, they would also have contributed to the pools of pristane and phytane employed here to monitor primary inputs. The mental causes. To a first approximation, the I3C content of depletion of I3C in those substances has already been noted primaryorganic matter in these units was constant and and itfollows that "C-rich cyanobacteria(if, indeed, any variation of h,,, must beattributed to secondary effects werepresent), were not very significant producers in the (mechanism (vi)). A significant change in the 13C content of palaeoenvironment. primaryorganic matter is indicated by thevalue of Asshown in Fig. 6, the enrichments in I3C relative to observed for the samples of the Weymouth Mem- primary biomass are similar to those previously observed in ber. Specifically, as shown in Table 2, it can be estimated samplesfrom the Greenhorn Formation, aunit rich in that hp,,, changed from -28.2% in the Peterborough Mem- pelletal material composed, in that case, mainly of reworked ber to -26.6% in theWeymouth Member. Petrographic coccolithophoridalgae. Values of 6 greaterthan -24% evidence (Belin & Kenig this volume) indicates a change in were found for two samples (1 and 8 in Fig. 6). Relative to the algalpopulation and thus suggests that changes in theprimary biomass line, however, thatfrom the Gimprenoid are due to succession of primary producers with WeymouthMember is notmore strongly enriched than different isotope effects, though they could (additionally or samples from the Greenhorn Formation or from most parts alternatively)reflect changes in theconcentration of dis- of thePeterborough andStewartby Members. The solved COz. enrichmentrelative to the primary biomass in one of the Movements of points parallel to the y axis on a graph samplesfrom bed 10 (designated as point 1 inFig. 6) is like that in Fig. 6, in particular, elevation of points above indeedexceptional, butthat is theunit in which the line representing burial of primary biomass, must be due palaeontological evidence for heterotrophy is most abundant to some secondary process occurring in the palaeoenviron- andthe sediment is stronglypelleted (Martill et al. this ment. Because it is known that heterotrophs preferentially volume). On the basis of thatcorroboration and the new respire I3C depleted carbon, isotopic shifts of this kind have studiescited above, we conclude that major isotopic previouslybeen attributed to effects of heterotrophy variations in TOC fromthe Oxford Clay are due to (mechanism (vi) above; Hayes et al. 1989). The implication heterotrophic reworking of primary material. that sedimentary organic debris can be composed largely of dead respiring heterotrophs has been uncomfortable, but no Hydrogenindex v. Figure 7 showsthat the samples alternativeshave been apparent. Recently, however, two examined using compound-specificisotopic analyses are reportshave appeared allowing continued linkage of representative of theoverall sample set. Accordingly, an elevation of 6.,, toheterotrophy and providing a more overview of relationshipsbetween hydrogen richness and

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I I' I most samples from the Peterborough Member. Values of 1 H c Index indicate varying degrees of oxidation. It appears that 0 H Index is inversely related to degree of remineralization of organicmaterial, and that isotopic enrichment instead reflects the development of extensive heterotrophy. 4~ Mostpoints representative of theStewartby and WeymouthMembers have lowhydrogen indices because depositionalconditions favored oxidative alteration; varia-

v tions in hisoprenoidindicate that observed variations in 6,,, of samples from the Weymouth member are probably due to 0 *O,A changes in theisotopic composition of primaryinputs. 0 Observations in modern systems (Jasper & Hayes 1990; Rau

45+ 00 et al. 1992) indicatethat a 3%0 change in 6, (Pindicates primary product) would correspond to a 1.3-fold change in theconcentration of dissolvedCOz. Alternatively-and moreplausibly, given theabsence of otherevidence for secularchange-the shift in primary isotopic compositions -28 -27 -26 -25 -24 -23 canbe ascribed to(i) succession of primary-producer 'TOC organismswith different physiological properties and, Peterborough Member: therefore,different overall isotope effects or/and (ii) biologically mediated drop of dissolved CO, during a high 0 Depositfeeder shale 0 Grvphaea shellbed productivity event (Rau et al. 1992). A Foram-richshale m Nuculaceanshell bed 0 Gratnrnatodorl shale A Mlxedshellbed Conclusions + Sllt and silty clay The Peterborough Member of the Oxford Clay Formation is R Stewartby and Weymouth Members organic-carbon rich with contents of organic carbon mostly Fig. 7. Plot of hydrogen index (HI) v. 6.,.,, for all whole-rock above3% TOC and up to 16.6%. The hydrogen index samples (except concretions, calcareous clays and wood). For (553 mg HCg-' C, average)and values of T,,, indicate explanation of the numbered data points refer to Fig. 5. immatureorganic matter. The Stewartby and Weymouth Membershave much lower TOC and H Index,indicating variation of theenvironment to favour more complete I3C content can be offered (Fig. 7). For the deposit-feeder oxidativeremineralization of organiccarbon. The organic shales, itwas concluded above that 6,.,, variesmainly in matter in theOxford Clay sediments is mostly of marine response to variations in heterotrophy. Since decreases in H origin. Thecontribution of terrestrialorganic matter is Indexreflect oxidation of organicmatter, heterotrophy limited. might be related to loss of hydrogen. It might therefore be Variations of geochemicalparameters occur attwo expectedthat effects of heterotrophywould drive points scales.Some variationsare correlated with boundaries of downward and to the right on the axes of Fig. 7. However, ammonitesubzones and reflectdiastems and large-scale a significant trend seems instead to lead slightly upward for changes of environmentalconditions. Other variations samples of deposit-feedershale samples enriched in I3C. correspond to biofacies defined by Duff (1975) on the basis Significantly, the five deposit-feeder shale samples compris- of macrofaunalassemblages, which have characteristic ing that trend have total organic carbon contents of 12.5 to contents of organicmatter and chemical compositions. 15%. The most organic-rich deposit-feeder samples exhibit Depositionalenvironment governs thebio-assemblage a high proportion of faecal pellets containing organic matter compositionand influences the chemicalcomposition of (Belin & Kenigthis volume). Enhanced formation of sedimentary organic matter. The latter is also dependent on pelletsat times of intensetrophic activity reduces the re- thetype of organismforming the bio-assemblages. sidencetime of organicmatter in the water column sig- Deposit-feeder shales, which were deposited under dysaero- nificantly, and in consequence reduces its chances of oxida- bic conditions,are more organic-rich and more hydrogen- tion.A quick transfer from the surface waters to the rich thanshell beds and samples of Stewartbyand sediment-waterinterface allows good preservation of the WeymouthMembers, which were deposited under more organicmatter and maintenance of elevatedH Index. aerobic conditions. Within the accumulatingsediments, concentrations of or- There is little evidencefor variation in theisotopic ganic carbon would be high in proportion to available sup- composition of primaryinputs during deposition of the plies of electron acceptors. Biological activity would cease PeterboroughMember. Environmental variations were longbefore alteration had become extensive and, as ob- greatenough toproduce large changes in theextent of served, the quality of preservation would be related to the reworking and organic richness of the sediments. In spite of intensity of the input. this, there is littleevidence forproductivity-related Points representative of shell beds (filled symbols, Fig. 7) modulation of levels of availableCO, and observed all derivefrom the Peterborough Member. Variations of variations in 6.,,, aredue mainly variationsto in primary 6 valuesappear to have been minimal during heterotrophicreworking. The samples whichexhibit the deposition of thisunit. Observed values of within the highest fractionation of the I3C between primary inputs and shellbeds indicatesecondary isotopic fractionation in the totalorganic carbon were collected in abed (Bed 10, samerange observed in theGreenhorn Formation and in Peterborough;Callomon 1968; Hudson & Martillthis

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volume)that contains the greatest diversity of fossils FREEMAN,K. H.,HAYES, J. M., TRENDELJ. M. & ALBRECHT, P. 1990. representing high trophic levels in theOxford Clay Evidencefrom carbon isotopic measurements for diverse origins of Formation. sedimentary hydrocarbons. Nature, 329,48-51. HALLAM, A., HANCOCK,J. M., LA BRECOUE,J. L., LOWRIE A.& CHANNEL, J. E. T. 1985. Jurassicto Paleogene: Part 1 Jurassicand geochronology andJurassic to Paleogenemagnetostratigraphy. In: J. D.Hudson and D. M. Martill are thankedfor providing SNELLING,N. J.(ed.) The Chronology ofthe Geological Record. never-ending encouragement for the study of the Oxford Clay. J. Geological Society, London, Memoirs, 10, 118-140. D. Hudson, D. M.Martill, K.L. Duff and J.L. Reichelderfer are HAVEN,H. L. TEN, DE LEEUW,J. W., RULLKTTER,& J. SINNINGHE DAMWE, J. thanked for assistance with field collection of samples. B.N.P. and S. 1987. Restricted utility of thepristane/phytane ratio aas palaeoenvironmental indicator. 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Received 5 January 1993; revised typescript accepted 17 May 1993.

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