TIMING AND PALEOCEANOGRAPHY OF OCEANIC DYSOXIA ANOXIA IN THE LATE BARREMIAN TO EARLY APTIAN (EARLY CRETACEOUS)

Normal marine deposition in the early Aptian was interrupted by an epi sode of ocean-wide dysoxia/anoxia. This event is recorded by the occur rence of organic carbon-rich sediments in land sections from Europe an d Deep Sea-Drilling Project (DSDP)/Ocean Drilling Program (ODP) sites in the North and South Atlantic, Indian and Pacific Ocean Basins. To e lucidate the origin, and spatial and temporal relationships of these c arbonaceous sediments, we have conducted an integrated biostratigraphi c, lithostratigraphic and geochemical investigation of fourteen sectio ns from a range of geographic and oceanographic settings. Based on the resulting high-resolution, integrated foraminiferal and nannofossil b iostratigraphy, it appears that most locations were characterized by a relatively brief interval of peak dysoxia/anoxia (less than a million years in duration), in the midst of a longer (approximately 2-3 milli on year) interval of intermittent oxygen deficiency which began in the late Barremian. This peak can be recognized either from organic carbo n contents or from intervals barren of calcareous plankton; in most pl aces it lies within the lower Aptian Globigerinelloides blowi foramini feral Zone and the Chiastozygus litterarius nannofossil Zone (Conuspha era rothii Subzone) and occurred shortly after magnetic Chron CMO. The dysoxic/anoxic interval affected sites in a wide range of oceanic pal eoenvironments: Nannofossil assemblage data indicate highly variable f ertility during this interval and suggest that no single model can acc ount for the origin of all organic carbon-rich horizons. These data in dicate that peak oxygen deficiency corresponded to a highly eutrophic interval but less intense dysoxic/anoxic periods were characterized by oligotrophic conditions. The late Barremian to early Aptian was a tim e of evolutionary radiation in both planktonic foraminifers and calcar eous nannofossils, no significant changes in speciation rates are asso ciated with the oxygen-deficient interval itself. The extinction of on e of the dominant Early Cretaceous nannoplankton species, Nannoconus s teinmannii may be related to events which led to dysoxia/anoxia includ ing periodically higher fertility. Tectonovolcanic events such as incr eased production of mid-plate oceanic crust may have indirectly induce d the dysoxic/anoxic episode by causing complex changes in climate, oc ean chemistry and circulation, and sea level. The lack of a clear stra tigraphic correlation between C(org)-rich horizons and the carbon isot opic record indicates that these inter-relationships are extremely com plex.