Intensification of recycling of organic matter at the sea floor near ocean margins

Abstract

QUANTIFICATION of the flux of organic carbon from the ocean surface to the deep ocean and sea floor is crucial to the prediction of future levels of atmospheric carbon dioxide and the interpretation of organic carbon variations in marine sediments. We report here the results of a study of benthic exchange and metabolism based on in situ benthic flux-chamber, in situ oxygen microelectrode and shipboard pore-water measurements performed off the central California continental shelf. At the base of the continental slope, the rates of benthic carbon mineralization exceed 0.8 mol C m−2 yr−1. Seaward of the slope and rise, however, rates decrease to <0.05 mol C m−2yr−1 in the North Pacific central gyre. These results indicate that, across the northeast Pacific, about half of the input of organic carbon to the sea floor occurs within 500 km of the continental slope. Measured rates of benthic carbon oxidation near the continental margin exceed the fluxes of organic carbon determined from previous sediment-trap studies by a factor of about three. In this region, therefore, either traps significantly underestimate the mean flux of vertically sinking particulate organic carbon (perhaps by under-sampling important episodic events) or more than half of the organic-carbon input is due to processes that by-pass traps, such as near-bottom lateral or active biological transport.