Carbon fluxes and burial rates over the continental slope and rise off central California with implications for the global carbon cycle

Abstract

In situ microelectrode, box‐core pore water gradient, and in situ benthic chamber estimates of organic carbon degradation and CaCO 3 dissolution are combined with organic‐C and carbonate‐C accumulation rates to approximate the total carbon flux to the seafloor along two transects of the continental slope and rise off central California. Microelectrode profiles of dissolved O 2 demonstrate that sediments at 13 sites, ranging in water depth from 580 to 4080 m, become anoxic below the uppermost 0.4–3 cm of the sediment column. If a current‐swept area of nondeposition on the upper slope is excluded, we find total organic‐C and carbonate‐C fluxes to the seafloor vary from 40 to 100 μmol C cm−2 yr−1 and from 32 to 91 μmol C cm−2 yr−1, respectively. From the distribution of these fluxes there is no indication that total fluxes or remineralization rates of organic or carbonate carbon are influenced markedly by conditions in the oxygen minimum zone. Instead, the upper continental rise with its system of submarine valleys and fans stands out as the most important locus for carbon deposition and remineralization. When benthic fluxes and burial rates are extrapolated over the whole slope and rise of the region, aerobic respiration is the major mechanism of organic matter oxidation, and organic‐C and carbonate‐C recycling are on average 87% and 98% efficient, respectively. These results suggest that modern sediments on the outer regions of continental margins are important sources of CO 2 that is injected directly into ocean deep water. However, if benthic carbon fluxes on the central California margin are typical of margins globally, this injection rate is less than 0.7 Gt C yr−1, which does not indicate a significant anthropogenic enhancement of carbon export to continental slopes and rises.