Origins and accumulation of organic matter in expanded Albian to Santonian black shale sequences on the Demerara Rise, South American margin

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• Bernasconi, Stefano M.
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• Forster, Astrid

Abstract

Ocean Drilling Program Leg 207 recovered thick sequences of Albian to Santonian organic-carbon-rich claystones at five drill-sites on the Demerara Rise in the western equatorial Atlantic Ocean. Dark-colored, finely laminated, Cenomanian-Santonian black shale sequences contain between 2% and 15% organic carbon and encompass Oceanic Anoxic Events 2 and 3. High Rock-Eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin. However, δ ¹³C _org values lie mostly between -30‰ and -27‰, and TOC/TN ratios range from 15 to 42, which both mimic the source signatures of modern C ₃ land plants. The contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations. The low δ ¹³C _org values, which are actually common in mid-Cretaceous marine organic matter, are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle. The elevated C/N ratios, which are also typical of black shales, indicate depressed organic matter degradation associated with low-oxygen conditions in the water column that favored preservation of carbon-rich forms of marine organic matter over nitrogen-rich components. Underlying the laminated Cenomanian-Santonian sequences are homogeneous, dark-colored, lower to middle Albian siltstones that contain between 0.2% and 9% organic carbon. The organic matter in these rocks is mostly marine in origin, but it occasionally includes large proportions of land-derived material.