Organic carbon and metal accumulation rates in Holocene and mid-Cretaceous sediments: Palaeoceanographic significance

The proportion of the organic carbon produced in oceanic surface waters that is buried in the underlying sediments is highly variable. The organic-carbon preservation factor, the proportion of preserved to produced organic carbon, is much higher in anoxic than in oxic deep waters. Predictability of organic-carbon preservation factors from sedimentary evidence would enable estimation of past primary production rates from measurable accumulation rates of organic carbon in ancient sediments. Accumulation rate patterns of organic carbon, transition metals, and excess transition metals in Holocene and mid-Cretaceous sediments are evaluated as predictors of organic-carbon preservation factors. Holocene transition-metal accumulation rates are controlled dominantly by bulk accumulation rates. Euxinic deep water environments, characterized by high organic-carbon preservation factors, are more successfully identified by sedimentological than by geochemical criteria. Sedimentological evidence from several mid-Cretaceous intervals implies widespread deep-water anoxia, high organic-carbon preservation factors, and correspondingly low primary productivity for large parts of the mid-Cretaceous oceans.