Mid-Cretaceous black shales; a result of excessive fertility or global anoxia?

In Holocene marine environments, organic carbon accumulation rates are high in areas of high productivity, rapid deposition, and in anoxic basins. Heavy metal accumulation rates are high in areas of high bulk sedimentation (resulting from the large detrital component), in areas of high fertility (heavy metal enrichment in organic tissue), and in areas of hydrothermal activity. The various environments can be characterized by the relations between their bulk sedimentation rates, amounts and types of organic carbon in the sediment, organic carbon accumulation rates, primary productivity, preservation factors (fraction of organic carbon fixed in the photic zone that becomes buried in the sediment), and accumulation rates of heavy metals (such as Cu, Ni, Zn). Differences in these relations exist, in particular, between areas where sapropel deposition is due to high supply (i.e., high fertility in localized upwelling areas) and where it is due to anoxic deep waters (such as in the Black Sea). Application of an improved calcareous nannofossil biochronology to compute accumulation rates in various DSDP sections show that the mid-Cretaceous black shales are characterized by substantially higher accumulation rates of heavy metals relative to organic carbon than is observed in Holocene upwelling settings. The implied widespread deep-water anoxia suggests high organic carbon preservation factors and correspondingly low primary productivity for large parts of the mid-Cretaceous oceans.