In situ measurements of calcium carbonate dissolution rates in deep-sea sediments

Benthic fluxes of alkalinity, carbon dioxide, and oxygen have been measured using an in situ incubation chamber at three sites (MANOP Sites C and S and PACFLUX Site SC) in the central equatorial north Pacific. At two carbonate-rich sites (C and SC), a budget for oxygen, alkalinity, and TCO 2 fluxes indicate a net CaCO 3 dissolution rate of approximately 0.4 mmol m −2 day −1. This rate is only 20% of previous estimates but is consistent with a dissolution rate constant predicted from laboratory experiments with deep-sea sediments and derived from in situ flux measurements in sediments of the southern California borderland. Organic matter oxidation in the sediment column provides the acid for 60–100% of the calcium carbonate dissolution occurring at these sites with the remainder derived from undersaturated bottom water. At a low-carbonate site (S), no carbonate dissolution in the sediment or on the sea floor is apparent, although sediment traps indicate a rain of CaCO 3 through 3400 m. The rates of remineralization relative to resuspension or erosion at this site must differ for organic carbon and calcium carbonate, so that carbonate grains reaching the sea floor are physically removed by erosion or resuspension before they dissolve, while organic carbon is largely oxidized before it can be physically removed.