Particulate organic carbon flux calculated from thorium-234 measurements and sediment traps in the Eastern Tropical South Pacific

he disequilibrium between the particle-reactive 234Th (t1/2= 24.1 days) and its soluble parent 238U (t1/2= 4.5 by) is used to quantify removal rates of 234Th on sinking particles. During February-March 2010, seven 234Th profiles were measured from the surface to 300m in the Eastern Tropical South Pacific (ETSP), in a region bounded by 10°S -- 20°S and 80°W -- 100°W. Horizontal transport of 234Th in surface waters, determined using geostrophic current vectors was found to be negligible, thus only vertical transport or 1-dimensional processes should be significant. Depth-integrated Thorium deficiencies (DITD) and measured ratios of particulate organic carbon (POC) to 234Th obtained from sediment trap samples were used in a steady state, one-dimensional model to determine POC export from the upper ocean. 234Th flux from the DITD method ranged from 800 to 3120 dpm/m2-d, and the 234Th-based calculated POC export flux at 200m ranged from 2.3 to 12.9 mmolC/m2-d. At each location, drifting sediment traps were deployed for 23-65 hours at 200m to measure 234Th and POC flux directly. The flux of 234Th into the traps ranged from 48 to 747 dpm/m2-d, which was very low compared to the flux calculated using the Thorium deficiency (DITD). However, at two stations, the trap:DITD flux ratio was 0.47 (station...