Isotopic biogeochemistry of the Oxford Clay Formation

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5973244

Kenig, F; Hayes, J M ^[1]; Popp, B N ^[2]; Summons, R ^[3]

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1. Indiana Univ., Bloomington, IN (United States). Biogeochemical Labs.
2. Univ. of Hawaii, Honolulu, HI (United States). Dept. of Geology and Geophysics
3. Bureau of Mineral Resources, Canberra (Australia)

To better understand processes controlling production and preservation of the thermally immature organic matter in the Oxford Clay Fm. (Jurassic, U.K.), the authors measured the hydrogen index (HI), oxygen index (OI), abundance and isotopic composition of total organic carbon (TOC) and determined isotopic compositions of individual hydrocarbons (n-alkanes, pristane, and phytane). Bulk geochemical and molecular parameters vary as a function of abundance of TOC, lithology and macrofaunal biofacies. Shales dominated by epifaunal bivalve assemblages contain high abundances of TOC, high HI, low OI and high proportions of phytoplanktonic and bacterial organic matter. Conversely, shell beds and calcareous and silty clay beds have lower abundances of TOC, lower HI, and greater, but still not dominant proportions of terrestrial organic matter. [delta][sub TOC] ranges from [minus]26.5 to [minus]23.2[per thousand] vs. PDB and varies as a function of HI. In shales, abundance of C-13 increase with HI and with concentrations of TOC. This variation might be attributed to drawdown of CO[sub 2] by producers. In shell beds and in calcareous and silty clay beds, decreases in TOC and HI are correlated with enrichment of C-13, a signal that might be attributed to greater heterotrophic reworking. Isotopic compositions of pristane and phytane in each unit are equal and likely indicate that the C-13 content of algal lipids. [delta] values of TOC are as much as 8[per thousand] higher than those of pristane and phytane, indicating that the bulk of the TOC probably derives from consumer organisms. The greatest enrichment of C-13 in TOC occurs in one horizon where paleontological evidence indicates [ge]7 trophic levels in the food chain. Consideration of the overall patterns of variations allows distinction between effects of CO[sub 2] drawdown and heterotrophic reworking.

• OSTI ID: 5973244
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
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BIOGEOCHEMISTRY
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CARBON COMPOUNDS
CARBON DIOXIDE
CARBON ISOTOPES
CARBON OXIDES
CHALCOGENIDES
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ISOTOPE RATIO
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OXIDES
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