Precipitation and Post-Depositional Mobility of Cadmium in Reducing Sediments: Implications for the Cd Oceanic Budget

Abstract

Reconstructions of past nutrient distribution and deep ocean circulation patterns are typically inferred from the 513C and Cd composition of benthic foraminifera from deep sea sediments; the modern oceanic distribution of 5 t3C and Cd reflect the biological cycling of nutrients (e.g. PO4) as modified by deep ocean circulation. This correlation between Cd and PO4 is the basis for using Cd, as recorded in foraminiferal shells, for reconstructing changes in deepwater circulation patterns (Boyle, 1988). Because of the relatively long residence time of Cd and PO4 (100,000-200,000 years) glacial-interglacial changes in Cd can be interpreted as reflecting variations in nutrients distribution. However, Cd and PO4 are controlled by different diagenetic processes. The strong tendency of Cd to form highly insoluble sulphide phases in the presence of HES explains its substantial enrichment in sulphide-bearing sediments as already was noted by Goldschmidt (1958). In recent studies we have investigated the possibility that temporal changes in the areal extent of reducing sediments may affect the Cd oceanic budget thus leading to de-coupling of Cd from the nutrient cycle (Rosenthal et al., 1993). The current work focuses on the precipitation of authigenic Cd in suboxic sediments and its mobility in response to post-depositional reoxidation of reducing sediments. The implications of these processes for the global Cd budget and for paleoceanographic interpretation are discussed below.