Contrasting watermass conditions during deposition of the Whitby Mudstone (Lower Jurassic) and Kimmeridge Clay (Upper Jurassic) formations, UK

The Whitby Mudstone Formation (WMF) and Kimmeridge Clay Formation (KCF) are both Jurassic organic-rich ‘black shale’ units containing well-preserved marine organic matter. Despite such gross similarities they exhibit opposite trends in δ13CTOC versus wt% decarbonated total organic carbon (TOC; r=−0.76 for WMF, and 0.71 for KCF); the δ13CTOC versus TOC trend also varies geographically within both units. Only the KCF data exhibit a positive correlation between δ13CTOC and δ15Norg (r=0.76) and the presence of aryl isoprenoids (probable indicators of photic zone anoxia). Maturity differences are insufficient to explain such contrasts. Bulk and compound-specific isotopic variations in the KCF are interpreted largely as a reflection of primary productivity during deposition, rather than a dominantly diagenetic signal due to incorporation of sulphurised carbohydrate. The contrasts between the formations, and the geographic variation in coeval shelf sediments, are explained by a model stressing the relative positions and stabilities of the chemocline, pycnocline, nutricline and euphotic zone, and the effect these have on productivity, CO2 recycling, and bacterial floras.