|
Article |
l Biogeochemical Laboratories, Geology Building, lndiana University, Bloomington, IN 47405, USA
2 Deptartment of Geology, Geophysics and Oceanography, University of Hawaii, Honolulu, HI 96822, USA
3 Australian Geological Survey Organization, PO Box 378, Canberra, 2601, Australia
A total of 165 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 Stewartby and Weymouth Members (Middle and Upper Oxford Clay respectively). Five samples from the underlying Kellaways Formation were also examined. Stratigraphic relationships were estimated on the basis of ammonite subzones and results from all locations can be placed along a single stratigraphic scale. The following were determined for all samples: abundance and isotopic composition of organic carbon, abundances of carbonate carbon and total sulphur, and the Rock-Eval pyrolysis parameters hydrogen index, oxygen index and Tmax. For a subset of eight samples selected to be representative of geochemical and apparent palaeoenvironmental variations, soluble organic compounds were extracted and the isotopic composition of pristane, phytane, and long-chain n-alkanes determined by isotope-ratio-monitoring gas chromatograph mass spectrometry.
Concentrations of organic carbon in samples from the Peterborough Member ranged from 0.5 to 16.6 % and 
values of total organic carbon (TOC) ranged from –27.7 to –23.1
v. PDB. Shales 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.
Isotopic composition of pristane and phytane in the Peterborough and Stewartby Members average –31.7, those in the Weymouth Member average –29.8. Values of for long-chain n-alkanes average –28Together these results indicate values for 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 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.
This article has been cited by other articles:
 |
|
 |
|
  K. E. Peters, C. C. Walters, and P. J. Mankiewicz Evaluation of kinetic uncertainty in numerical models of petroleum generation AAPG Bulletin, 2006; 90: 387 - 403. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 Intermittent euxinia: Reconciliation of a Jurassic black shale with its biofacies Geology, 2004; 32: 421 - 424.
|
|
|
|
 |
|
 F. Kenig, D.-J. H. Simons, D. Crich, J. P. Cowen, G. T. Ventura, T. Rehbein-Khalily, T. C. Brown, and K. B. Anderson Branched aliphatic alkanes with quaternary substituted carbon atoms in modern and ancient geologic samples PNAS, 2003; 100: 12554 - 12558. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. Dromart, J.-P. Garcia, F. Gaumet, S. Picard, M. Rousseau, F. Atrops, C. Lecuyer, and S. M. F. Sheppard Perturbation of the carbon cycle at the Middle/Late Jurassic transition: Geological and geochemical evidence Am J Sci, 2003; 303: 667 - 707. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. C. Jenkyns, H. C. Jenkyns, C. E. Jones, D. R. GrOcke, S. P. Hesselbo, and D. N. Parkinson Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography Journal of the Geological Society, 2002; 159: 351 - 378. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. S. Morgans-Bell, H. S. MORGANS-BELL, A. L. COE, S. P. HESSELBO, H. C. JENKYNS, G. P. WEEDON, J. E. A. MARSHALL, R. V. TYSON, and C. J. WILLIAMS Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset, UK Geological Magazine, 2001; 138: 511 - 539. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. D. HUDSON and D. M. MARTILL The Peterborough Member (Callovian, Middle Jurassic) of the Oxford Clay Formation at Peterborough, UK Journal of the Geological Society, 1994; 151: 113 - 124. [Abstract] [PDF]
|
|
|
|
|
|
T. F. ANDERSON, B. N. POPP, A. C. WILLIAMS, L.-Z. HO, and J. D. HUDSON The stable isotopic records of fossils from the Peterborough Member, Oxford Clay Formation (Jurassic), UK: palaeoenvironmental implications Journal of the Geological Society, 1994; 151: 125 - 138. [Abstract] [PDF]
|
|
|
|
|
|
S. BELIN and F. KENIG Petrographic analyses of organo-mineral relationships: depositional conditions of the Oxford Clay Formation (Jurassic), UK Journal of the Geological Society, 1994; 151: 153 - 160. [Abstract] [PDF]
|
|
|
|
|
|
D. M. MARTILL, M. A. TAYLOR, K. L. DUFF, J.B. RIDING, and P. R. BOWN The trophic structure of the biota of the Peterborough Member, Oxford Clay Formation (Jurassic), UK Journal of the Geological Society, 1994; 151: 173 - 194. [Abstract] [PDF]
|
|
