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Linking Coniacian-Santonian (OAE3) black shale formation to African climate variability: a reference section from the eastern tropical Atlantic at orbital time scales (ODP Site 959, off Ivory Coast/Ghana)

Lookup NU author(s): Dr Britta Beckmann, Professor Thomas Wagner

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Abstract

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 δ13Corg 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 CO2/g OC, and bulk δ13Corg varying between –28 to –26.5‰. The enrichment in redox-sensitive trace metals up to 2500 μg/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 CO2, during the Coniacian–Santonian OAE3.

Publication metadata

Author(s): Beckmann B, Hofmann P, Wagner T

Editor(s): Harris NB

Publication type: Book Chapter

Publication status: Published

Book Title: The Deposition of Organic Carbon-Rich Sediments: Models, Mechanisms and Consequences

Year: 2005

Volume: 82

Series Title: SEPM Special Publications

Library holdings: Search Newcastle University Library for this item

ISBN: 1060071X

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