POC/234Th ratios in particles collected in sediment traps in the northern South China Sea

Th-234 has been increasingly used as a tracer to estimate particulate organic carbon (POC) flux, based on calculation of the product of the POC/Th-234 ratio in sinking particles and the Th-234 flux. Large (>50 mu m) pump-collected particles are assumed to be representative of sinking particles for determining POC/Th-234 ratios, but the basis of this assumption has not been extensively investigated. Here we present POC and Th-234 data for various particle size classes (1-10, 10-50, 50-150 and >150 mu m) from trap-collected particles in the northern South China Sea (SCS, a subtropical region). Within the trap-collected POC pool the 1-10 mu m fraction contained the largest proportion of POC (26-35%), followed by the 10-50 mu m (25-29%), 50-150 mu m (19-24%), and >150 mu m (17-23%) fractions. The distribution pattern of Th-234 in the trap-collected particles was analogous to that of POC, with the smallest (1-10 mu m) particles representing the largest proportion (37-54%) of the Th-234 flux. Our preliminary results indicate that trap-collected particles <50 mu m carry most of the POC and Th-234 flux in the northern SCS, suggesting that the contribution of particles smaller than <50 mu m to the settling flux is larger than previously thought. The results indicate that it may be necessary to simultaneously measure POC/Th-234 ratios on size-fractionated trap- and pump-collected materials from different marine regions, including tropical and high latitude locations. (C) 2010 Elsevier Ltd. All rights reserved.