U. S. Geological Survey, United States

Normal marine deposition in the early Aptian was interrupted by an episode of ocean-wide dysoxia/anoxia. This event is recorded by the occurrence of organic carbon-rich sediments in land sections from Europe and Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) sites in the North and South Atlantic, Indian and Pacific Ocean Basins. To elucidate the origin, and spatial and temporal relationships of these carbonaceous sediments, we have conducted an integrated biostratigraphic, lithostratigraphic and geochemical investigation of fourteen sections from a range of geographic and oceanographic settings. Based on the resulting high-resolution, integrated foraminiferal and nannofossil biostratigraphy, 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 million year) interval of intermittent oxygen deficiency which began in the late Barremian. This peak can be recognized either from organic carbon contents or from intervals barren of calcareous plankton; in most places it lies within the lower Aptian Globigerinelloides blowi foraminiferal Zone and the Chiastozygus litterarius nannofossil Zone (Conusphaera rothii Subzone) and occurred shortly after magnetic Chron CMO. The dysoxic/anoxic interval affected sites in a wide range of oceanic paleoenvironments. Nannofossil assemblage data indicate highly variable fertility during this interval and suggest that no single model can account for the origin of all organic carbon-rich horizons. These data indicate 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 time of evolutionary radiation in both planktonic foraminifers and calcareous nannofossils, no significant changes in speciation rates are associated with the oxygen-deficient interval itself. The extinction of one of the dominant Early Cretaceous nannoplankton species, Nannoconus steinmannii may be related to events which led to dysoxia/anoxia including periodically higher fertility. Tectonovolcanic events such as increased production of mid-plate oceanic crust may have indirectly induced the dysoxic/anoxic episode by causing complex changes in climate, ocean chemistry and circulation, and sea level. The lack of a clear stratigraphic correlation between C (sub org) -rich horizons and the carbon isotopic record indicates that these inter-relationships are extremely complex.