| Title: | Coniacian-Santonian (OAE3) black shale formation and African climate variability: a reference section from the eastern tropical Atlantic at orbital time scales (ODP Site 959, off Ivory Coast/Ghana) |
| Author: | Thomas Wagner |
| Date Submitted: | 04/30/2002 |
| Address: | PO Box 330440
Bremen
Germany
28334
|
| Phone: | +49-421-218-7137 |
| Email: | twagner@uni-bremen.de |
| Co-Authors: | Beckmann B., University Bremen, Germany, bbeckman@uni-bremen.de; Flögel. S., Geomar Kiel, Germany, sfloegel@geomar.de; Hofmann P., University Cologne, Germany, adg03@uni-koeln.de; Sinninghe Damsté, J., NIOZ, Netherlands; damste@nioz.nl |
| Affiliation: | University Bremen, Geosciences |
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| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725538 |
| Keywords: | OAE3, Coniacian-Santonian, orbital forcing, ODP Site 959, land-ocean link, African climate variability |
| Abstract: | The Coniacian/Santonian anoxic event (OAE3) is the final of the Cretaceous OAEs and documents one important step in the transition from the Cretaceous greenhouse- to the Cenozoic icehouse-world. Compared to extensive research on the early and mid-Cretaceous OAEs, little is known about the evolution and the climate impact of the final, Coniacian/Santonian OAE3. We present detailed terrigenous and marine proxy records from ODP Site 959 (eastern tropical Atlantic off Ivory Coast/Ghana), which allow to assess controls on tropical African climate, continental run-off, and the associated formation of oil-prone, marine black shales.
At ODP Site 959, Coniacian to lowermost Campanian sediments (CC13 to CC16) are composed of alternations of finely laminated and homogenous black shale units, which we use to reconstruct the mechanisms and feed-backs of orbital forcing, land-sea interaction, and phasing of terrigenous and marine records. Orbital tuned, high resolution (down to about 300 years) organic and inorganic geochemical records from biozone CC15b (late Coniacian to early Santonian) evidence cyclic changes in depositional boundary conditions, which occur at the main frequencies of the Milankovitch bands (see Hofmann et al. for further information). Two opposing depositional modes are supported, defining boundary conditions for marine black shale accumulation and background sedimentation in relation to African climate dynamics. The transition between both modes occurred rapidly on millennial and shorter time-scales. The data propose that the latitudinal displacement of African climate belts in conjunction with fluctuations in ocean circulation were key mechanisms that forced marine organic carbon accumulation.
During the black shale mode, wind-driven upwelling in the area of ODP Site 959 was fostered leading to the enhanced accumulation of marine kerogen type I organic matter (TOC up to 16%). Preservation of specific biomarkers indicative for photic zone anoxia, i.e. isorenieratane derivates, support at least temporarely extreme (sulfidic) conditions in the upper water column, somehow comparable to the Cenomanian-Turonian OAE2. Simultanously, low Si/Al and K/Al ratios indicate enhanced continental precipitation and excess mineral run-off (e.g. Aluminium) from extensive African lateritic soil deposits to the early Ivorian Basin. In contrast, the background mode documents periods of low accumulation and preservation of kerogen type II/III organic matter. High Si/Al and K/Al ratios in these sediments support a pronounced eolian signature from southern, more arid African source areas. Apart from these differences in atmospheric and oceanic circulation, general conditions for marine organic carbon accumulation remained favorable along the southern slope of the Ivorian Basin as indicated by continously elevated TOC concentrations of about 3% (see Beckmann et al. for further information).
Global climate modelling corroborates the two endmember modes of deposition off Equatorial Africa. Accordingly, precessional forced fluctuations in precipitation and air temperature probably have triggered short-term changes in aridity and humidity over central and western Africa. |
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