Trace Metals in Marine Organic-Rich Sediments: Distinguishing Continental Margin Upwelling Settings from Restricted Basins

Abstract:

Trace metal enrichments in sedimentary deposits are related to the processes also controlling production and preservation of organic matter (e.g. Brumsack, 2006; Tribovillard et al., 2006). Consequently, trace metals have been used in reconstructions of the depositional environment of organic-rich deposits, however most of these studies focused mainly on the redox chemistry of these trace metals. Here we present the results of a comprehensive database that has been constructed for comparing trace metal concentrations in modern organic-rich sediments from a variety of different marine settings. This database allows us to define proxies that effectively differentiate between two fundamentally different settings with enhanced organic-carbon burial: continental margin upwelling areas (e.g. Namibian Margin) and hydrographically restricted marginal marine basins (e.g. Black Sea). It is observed that relatively high Cd/Mo values are typical for samples from upwelling settings whereas Co*Mn values are relatively high in samples from restricted basins. Cd/Mo ratios are thought to reflect the relative influence of productivity versus preservation on organic carbon accumulation, in which high values in productivity driven systems (i.e. upwelling) are attributed to the enhanced transfer of Cd to the sediment by incorporation of Cd into plankton biomass. Co*Mn, on the other hand, might be used to assess circulation patterns and the relative contribution of sub-surface versus surface/river water and/or dust influx to the basin, because of the differences in the supply and reactive behaviour of Co and Mn in the water column. As both independent parameters show similar discriminating behaviour, it is proposed that cross-plotting Cd/Mo and Co*Mn provides a highly effective way to distinguish sediments deposited in upwelling versus restricted settings.