TRACE-METAL SOLID-STATE SPECIATION CHANGES ASSOCIATED WITH THE DOWN-COLUMN FLUXES OF OCEANIC PARTICULATES

The 'global carbon flux', which is dominated by large-sized organic ag gregates, removes dissolved trace metals from sea water and delivers s uspended particulate material to the sediment surface. Dissolved trace metals sequestered by the sinking particulates can have different spe ciation signatures; for example, Cu is removed principally via an orga nic association, whereas Mn is mainly scavenged via an inorganic surfa ce association involving small particles which are 'dragged down' by t he aggregates. The trace metal speciation signatures in suspended part iculate material appear to be similar on a global ocean basis, but onc e the particulates reach the sediment surface they are subjected to a variety of processes which are constrained by the depositional environ ment, especially the redox conditions. In some marginal hemi-pelagic d eposits the original suspended particulate trace metal speciation sign atures can be retained in the buried sediment, but in open-ocean pelag ic sediments deposited under oxidizing conditions the particulates suf fer severe modification as the organic carbon in the sinking aggregate s is destroyed during oxic diagenesis. The solid state speciation sign atures of trace metals reflect these modifications, and so can be used as 'tracers' to monitor the chemical changes affecting particulate ma terial incorporated into deep-sea sediments via the 'global carbon flu x'. This is illustrated with respect to the speciation signatures of F e, Cu and Mn in surface sea water particulates and deep-sea sediments from the Atlantic Ocean.