Mid-Cretaceous calcareous nannoplankton at DSDP Leg 62 Holes

Moderately well-preserved calcareous-nannoplankton assemblages from the western Mid-Pacific Mountains and Hess Rise reflect paleoceanographic conditions during the middle Cretaceous. Sediments enriched in detrital organic carbon from the Barremian (NC6) at Deep Sea Drilling Project Site 463 contain impoverished assemblages indicative of strong dissolution and adverse water conditions (e.g., high turbidity, fresh-water inflow from islands). A steady increase in the nannolith genera Zygodiscus and Biscutum and in overall diversity throughout the middle Cretaceous at Site 463 is indicative of a change to more favorable surface-water conditions, but not of highly nutrient-rich surface waters. Coccolith assemblages at Site 464 are poorly preserved and thus marginal at best for quantitative investigations. Assemblages with a high relative abundance of Coccolith species indicative of high nutrient concentrations occur on southern Hess Rise (Sites 465 and 466). Organic carbon of marine origin is commonly preserved at these sites, supporting the inferences based on Coccolith assemblages. High productivity in conjunction with warm deep waters of low oxygen content seems sufficient to explain the occurrence of "black shales" in the Pacific. Injection of saline and oxygenpoor Atlantic deep water seems unlikely.