234Th Ratios Research Articles

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

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.

Carbon and nitrogen export during the JGOFS North Atlantic Bloom experiment estimated from 234Th: 238U disequilibria

The disequilibrium between the particle-reactive tracer 234Th ( t 1 2 = 24.1 days) and its soluble parent, 238U, was used to examine Th scavenging and export fluxes during the U.S. JGOFS North Atlantic Bloom Experiment (24 April–30 May 1989) at ∼47°N, 20°W. Four profiles of dissolved and particulate 234Th in the upper 300 m and a non-steady box model were used to quantify dissolved 234Th uptake and particle export rates. The highest export fluxes occured during the first half of May. From POC/ 234Th and PON/ 234Th ratios, particulate organic C and N fluxes were calculated. Results were 5–41 mmol C m −2 day −1 and 0.9–6.5 mmol N m −2 day −1 from the 0–35 m layer. The ratio of POC export flux to primary production ranged from 0.05 to 0.42, peaking in the first half of May. The estimated fluxes agree with the observed losses of total C and N from the upper ocean during the bloom, but yield significantly higher fluxes than were measured by floating traps at 150 and 300 m.