Regional estimates of the export flux of particulate organic carbon derived from thorium-234 during the JGOFS EqPac program
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The upper ocean Th-234 activity distribution at 77 stations was measured between 12 degrees N and 10 degrees S, and 95 degrees W and 170 degrees W in the spring and autumn of 1992. A regional scavenging model was used to estimate vertical export of particulate Th-234. Given the relatively high upwelling rates in this region, particularly at equatorial latitudes near 140 degrees W, it was necessary to include upwelling of Th-234 in our model in order to quantify particulate export. Using this export flux and the measured organic C or N to Th-234 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 degrees W coincident with a low temperature/high nutrient peak at 4 degrees S. Along 140 degrees 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 degrees N and 4 degrees 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/Th-234 ratios. Given the measured C/N ratio, particulate N Auxes from the upper 100 m would be 6 times lower than for POC.