ISOTOPIC BIOGEOCHEMISTRY OF THE OXFORD CLAY FORMATION (JURASSIC), UK

A total of 165 samples was obtained from the,Oxford Clay Formation at seven different sites. Nearly all were from the Peterborough Member (L ower Oxford Clay), but seven were from the Stewartby and Weymouth Memb ers (Middle and Upper Oxford Clay respectively). Five samples from the underlying Kellaways Formation were also examined. Stratigraphic rela tionships 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 tota l sulphur, and the Rock-Eval pyrolysis parameters hydrogen index, oxyg en index and T-max. For a subset of eight samples selected to be repre sentative 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-r atio-monitoring gas chromatograph mass spectrometry. Concentrations of organic carbon in samples from the Peterborough Member ranged from 0. 5 to 16.6% and delta values of total organic carbon (TOC) ranged from -27.7 to -23.1 parts per thousand v. PDB. Shales dominated by epifauna l bivalve assemblages have high concentrations of TOC and values of H index approaching 800, indicating preservation of hydrogen-rich organi c 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 compos ition of pristane and phytane in the Peterborough and Stewartby Member s average -31.7 parts per thousand, those in the Weymouth Member avera ge -29.8 parts per thousand. Values of delta for long-chain n-alkanes average -28 parts per thousand. Together these results indicate delta values for primary inputs as follows: terrestrial vascular plants, -23 .5 parts per thousand; Peterborough Member algae, -28.2; Stewartby Mem ber algae, -29.1 parts per thousand; Weymouth Member algae, -26.6 part s per thousand. Comparison of primary delta values to those of TOC ind icates that in some cases secondary processes enriched TOC relative to primary inputs by as much as 4 parts per thousand. Palaeontological e vidence in these same beds indicates development of extensive food-web s and supports attribution of this isotopic enrichment to heterotrophi c reworking.