Abstract

The ‘global carbon flux’, which is dominated by large-sized organic aggregates, removes dissolved trace metals from sea water and delivers suspended particulate material to the sediment surface. Dissolved trace metals sequestered by the sinking particulates can have different speciation signatures; for example, Cu is removed principally via an organic association, whereas Mn is mainly scavenged via an inorganic surface association involving small particles which are ‘dragged down’ by the aggregates. The trace metal speciation signatures in suspended particulate material appear to be similar on a global ocean basis, but once the particulates reach the sediment surface they are subjected to a variety of processes which are constrained by the depositional environment, especially the redox conditions. In some marginal hemi-pelagic deposits the original suspended particulate trace metal speciation signatures can be retained in the buried sediment, but in open-ocean pelagic sediments deposited under oxidizing conditions the particulates suffer severe modification as the organic carbon in the sinking aggregates is destroyed during oxic diagenesis. The solid state speciation signatures of trace metals reflect these modifications, and so can be used as ‘tracers’ to monitor the chemical changes affecting particulate material incorporated into deep-sea sediments via the ‘global carbon flux’. This is illustrated with respect to the speciation signatures of Fe, Cu and Mn in surface sea water particulates and deep-sea sediments from the Atlantic Ocean.

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