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ABSTRACT: Oceanwide Anoxia in the Early Aptian

T. J. Bralower, M. Leckie, W. Sliter, D. Allard, M. Arthur, S. Schlanger

Normal marine deposition in the early Aptian was interrupted by an episode of oceanwide anoxia. This is recorded by the occurrence of organic carbon rich sediments in pelagic and hemipelagic land sections from Europe and DSDP/ODP (Deep Sea Drilling Project/Ocean Drill Project) sites in the North Atlantic and South Atlantic and Indian and Pacific Ocean basins. To elucidate the origin and spatial and temporal relationships of these carbonaceous sediments, we conducted an integrated biostratigraphic, lithostratigraphic, and geochemical investigation of 11 sequences from a range of geographic and oceanographic settings. Based on resulting high-resolution, integrated foraminiferal and nannofossil stratigraphy, sections can be divided into two categories: those characterized by a relatively brief interval of anoxia (about 0.5 m.y.), and those containing a longer record of dysaerobia/anoxia, which peaked in this same interval. This peak lies within the Globigerinelloides blowi foraminifera zone and the Chiastozygus litterarius nannofossil zone, and occurred shortly after magnetic Chron CMO. Foraminiferal and some nannofossil assemblages are characterized by relatively small sizes, a phenomenon whose paleoecologic significance is not fully understood. The anoxic interval is not associated with evolutionary radiations in either fossil group. A carbon isotope excursion has been measured on organic material in several sections, but it is uncertain whether this is correlatable because of the interplay of marine and terrestrial organic matter and the paucity of

carbonate in sequences analyzed. The anoxic episode affected sites in a wide range of oceanic paleoenvironments. The early Aptian bears some resemblance to the Cenomanian-Turonian anoxic interval.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990