Chemical Cycling at the Ocean Margin: The Sedimentary Source

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Work is in progress on the cycling of trace metals between sediment and water in the continental margin of California. Fluxes of nutrients and trace metals are being determined by using the USC free vehicle bottom lander and by calculation of gradients in pore waters of sediments on a millimeter scale. A surprising result of the initial work is that the highest rate of remineralization of metals from the sediment appears to be from well oxygenated waters, not from sediments in the depth range of the oxygen minimum zone. Moreover the high fluxes are linked with high rates of oxygen consumption, carbon oxidation, and calcium carbonate dissolution. Consequently, new work is addressing the association between metal and carbon cycling. Measurements of fluxes out of the sediments in Monterey Bay are being compared to ongoing measurements of sediment trap carbon fluxes (by Chavez) to determine whether seasonality in the carbon signal is reflected by remineralization rates of metals. A detailed study is also being made of fluxes derived from modelling pore water profiles, compared to those from the benthic chambers, which tend to give higher flux values. The discrepancy suggests rapid diagenetic reactions in the fluff layer at the sediment surface, or irrigation by infauna; the possibilities are being investigated.