Linking Coniacian-Santonian (OAE3) black-shale deposition to African climate variability; a reference section from the eastern tropical Atlantic at orbital time scales (ODP Site 959, off Ivory Coast and Ghana)
In: Harris, Nicholas B. (editor), The deposition of organic-carbon-rich sediments; models, mechanisms, and consequences
Society for Sedimentary Geology (SEPM), Tulsa, OK, United States
Black-shale cycles deposited in the late Cretaceous tropical Atlantic at ODP Site 959 were analyzed to reconstruct processes for organic-matter sequestration during the Coniacian-Santonian "oceanic anoxic event" (OAE3). The results from bulk organic and inorganic geochemistry suggest that black-shale accumulation was intimately linked to orbitally forced cycles in the Deep Ivorian Basin (DIB) that alternated between eutrophic conditions stimulating productivity of organic-walled plankton followed by less trophic conditions associated with carbonate production. Results from Rock-Eval Pyrolysis, bulk delta (super 13) C (sub org) analysis, and maceral analysis demonstrate a dominantly marine origin of the organic matter (OM) with only a subordinate proportion from terrestrial sources. Intervals of high organic-carbon (OC) accumulation display high hydrogen indices (HI) up to 720 mg HC/g OC, low oxygen indices (OI) of 20 mg CO (sub 2) /g OC, and bulk delta (super 13) C (sub org) varying between -28 to -26.5 per mil. The enrichment in redox-sensitive trace metals up to 2500 mu g/g for vanadium, for example, as well as carbon-sulfur relationships in black-shale intervals suggest intermittently anoxic conditions, on occasion as extreme as during the Cenomanian-Turonian OAE2. We propose that the black-shale cycles were directly linked to the climate development in equatorial Africa via the hydrological cycle. The mechanism for carbon sequestration that operated in the DIB may have worked in a similar way in other equatorial regions of Africa and South America, implying that the tropics acted as a prominent sink for OC, and consequently atmospheric CO (sub 2) , during the Coniacian-Santonian OAE3.