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Journal of the Geological Society

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Variation in marine total organic carbon through the type Kimmeridge Clay Formation (Late Jurassic), Dorset, UK

School of Civil Engineering and Geosciences, Drummond Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK (e-mail: r.v.tyson@ncl.ac.uk)

Bulk geochemical data from the NERC Rapid Global Geological Events project are used to appraise the pattern of variation in mean organic content through the cymodoce to rotunda zones of the type Kimmeridge Clay Formation in southern Dorset. There is a symmetrical stratigraphic pattern in mean and modal total organic carbon (TOC) values, which increase from 1–2% at the top and base to a peak of 8–9% in the middle of the formation. To remove distortion caused by the strong positive skewness of the TOC values, the mean TOC per zone was recalculated for the first mode of the TOC distribution only, and combined pragmatically with pyrolysis estimates of the reactive (marine) TOC fraction to derive marine TOC values; these also vary symmetrically from c. 0.5% to peak values of c. 5.0%. The mean TOC is negatively correlated with sedimentation rate, indicating that dilution is a significant controlling variable. Mean palaeoproductivity was assessed using modern marine sediment relationships between carbon burial efficiency, sedimentation rate (derived from RGGE cyclostratigraphic analyses), the likely dissolved oxygen range, and water depth. Best estimates of palaeoproductivity range from 40 to 150 g C m^–2 a^–1 (low to moderate by comparison with present-day shelves) and are positively but nonlinearly correlated with mean ¹³C_TOC values.

KEYWORDS: Kimmeridge Clay, 13C, total organic carbon, pyrolysis, productivity

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