Regional estimates of the export flux of particulate organic carbon derived from thorium-234 during the JGOFS EqPac program

Abstract

The upper ocean 234Th activity distribution at 77 stations was measured between 12°N and 10°S, and 95°W and 170°W in the spring and autumn of 1992. A regional scavenging model was used to estimate vertical export of particulate 234Th. Given the relatively high upwelling rates in this region, particularly at equatorial latitudes near 140°W, it was necessary to include upwelling of 234Th in our model in order to quantify particulate export. Using this export flux and the measured organic C or N to 234Th ratio on particles, one can empirically determine POC and PON fluxes for this region. The estimated particulate organic C flux varies spatially and temporally within this region, ranging from 1 to 7 mmol C m −2 day −1, with enhanced export occurring over the equator. Fluxes are also enhanced along 95°W coincident with a low temperature/high nutrient peak at 4°S. Along 140°W, particulate organic C export from the upper 100 m is on the order of 2 mmol C m −2 day −1 at latitudes beyond 4°N and 4°S, with an equatorial peak of 3–5 mmol C m −2 day −1 in both spring and fall. These results suggest that a relatively small per cent of the total production is exported locally on sinking particles (particle export/primary production <5–10%). This finding of low particle export is relatively insensitive to the chosen upwelling rates or particulate organic C 234Th ratios. Given the measured C N ratio, particulate N fluxes from the upper 100 m would be 6 times lower than for POC.