HR: 17:00h
AN: PP42D-04 INVITED     [PDF]
TI: African climate variability and organic carbon accumulation in the Coniacian-Satonian eastern tropical Atlantic: Insights how insolation-cycles in the Cretaceous were transformed to marine black shales
AU: * Hofmann, P
EM: adg03@uni-koeln.de
AF: University of Cologne, Department of Geology, Zuelpicher Str. 49a, Cologne, 50674 Germany
AU: Wagner, T
EM: twagner@uni-bremen.de
AF: University Bremen, Geosciences, PO Box 330440, Bremen, 28334 Germany
AU: Beckmann, B
EM: bbeckman@uni-bremen.de
AF: University Bremen, Geosciences, PO Box 330440, Bremen, 28334 Germany
AU: Floegel, S
EM: sfloegel@geomar.de
AF: GEOMAR Research Centre, Wischofstr. 1-3, Kiel, 24148 Germany
AB: There is increasing evidence from marine proxy records that tropical regions during the late Cretaceous were hotter than previously reported and were by far exceeding modern average temperatures. Tropical sea surface temperatures in the range of 32-36$\deg$C apparently lasted from the latest Cenomanian to the early Campanian. A fundamental consequence of superheated Cretaceous tropics is a vigorous hydrological cycle operating in equatorial regions. Geological evidence supporting such an enhanced hydrological cycle and a direct link to the formation of marine black shale cycles was recently reported for ODP Site 959 from the Deep Ivorian Basin (DIB) off equatorial West-Africa. Millennial-scale marine and terrigenous proxy records from that site provide a unique opportunity to investigate short-term variability of the ocean-climate system, to discuss the role of orbital forcing and to assess the primary mechanisms how cyclic marine deposits are formed in the geological record. In the Deep Ivorian Basin formation of cyclic OAE3 black shales was favoured by the paleogeographic position of the drill site in the partly sheltered early Ivory Basin south of the paleo-equator and, most important, was directly linked to orbital-driven fluctuations in atmospheric and oceanic circulation. Dramatic changes in redox sensitive trace metal accumulation as well as the occurrence of molecular fossils of green sulfur bacteria provide evidence for extreme variations in redox conditions, with euxinic conditions occasionally even extending into the lower photic zone. The temporal establishment of a continuous euxinic water column about 200 km offshore the West-African coastline supports the conclusion that redox conditions in the Coniacian-Santonian tropical ocean at least occasionally were as extreme as during the Cenomanian-Turonian OAE-2, although much smaller in extent and restricted to short but repetitive periods. It has also been demonstrated that the terrigenous fraction represented by Si/Al and K/Al at Site 959 documents fluctuating supply of aeolian dust from a source area in southern Africa, represented by illite and quartz, and continental run-off from a tropical northern African source area, represented by smectite and kaolinite. Input from the two African source areas to the DIB is interpreted to document repetitive shifts in the position of the Intertropical Convergence Zone, which led to a succession of drier and wetter climate periods over western Africa. Recent results from climate modelling support such short-term fluctuations in African moisture balance/precipitation and allow to project the view from a single sample location off W-Africa to a regional or even global perspective.
DE: 1620 Climate dynamics (3309)
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 4267 Paleoceanography
DE: 4802 Anoxic environments
DE: 4808 Chemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting