An assessment of particulate organic carbon to thorium - 234 ratios in the ocean and their impact on the application of 234 Th as a POC flux proxy

Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the Th activity distribution (in order to calculate Th fluxes) and an estimate of the C/ Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C/ Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C / Th either increases or is relatively invariant with increasing particle size (size classesN1 to 100s Am). Observations also suggest that C / Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2–3). We recommend sampling for C/ Th from a standard depth of 100 m, or at least one depth below 0304-4203/$ s doi:10.1016/j.m * Correspondi E-mail addr 1 Retired from 2 Present addr (2006) 213–233 ee front matter D 2006 Elsevier B.V. All rights reserved. archem.2005.10.013 ng author. Tel.: +1 508 289 2309; fax: +1 508 457 2193. ess: [email protected] (K.O. Buesseler). IAEA-MEL. Present address: 8, Allee des Orangers, 06320 Cap d’Ail, France. ess: EPOC Station Marine d’Arcachon, 33120 Arcachon, France. K.O. Buesseler et al. / Marine Chemistry 100 (2006) 213–233 214 the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple Th profiles and C/ Th samples during the course of longer observation periods to better sample temporal variations in both Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column. D 2006 Elsevier B.V. All rights reserved.