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Organic Carbon Preservation During Oceanic Anoxic Events


Several paleoenvironmental factors influence the accumulation of organic carbon in sediments. It is widely considered that an interplay between high productivity and low bottom-water oxygen concentrations were critical factors that led to the formation of Cretaceous black shale deposits. However, little attempt has been made to actually quantifying the importance these burial drivers. We present δ13C, TOC and phosphorus (P) data for 4 sites with differing paleoenvironmental regimes. In all sites organic carbon enrichment post-dates the onset of enhanced productivity by some tens of thousands of years. This suggests that productivity is not a direct driver of organic matter burial and that preservation through anoxia is. This is consist with P-accumulation data which drops significantly during the positive δ13C excursion. Because this decrease is likely associated with falls in porewater oxygen content, this adds weight to the argument that the black shales of OAE 2 are primarily expressions of preservation. Furthermore a preliminary attempt can be made to estimate the time required for enhanced productivity to impact bottom-water oxygen levels to the extent that bacterial degradation of organic matter is slowed sufficiently to allow for its burial. In this we estimate timescales of between 50k-200k years, depending on regional paleoenvironmental conditions.