Isotopic biogeochemistry of the Oxford Clay Formation (Jurassic), UK

Abstract

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 delta 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 delta for long-chain n-alkanes average -28%. Together these results indicate delta 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 delta values to those of TOC indicates that in some cases secondary processes enriched TOC relative to primary inputs by as much as 4%. Paleontological evidence in these same beds indicates development of extensive food-webs and supports attribution of this isotopic enrichment to heterotrophic reworking.