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

This review focuses on the application of Th as a proxy for the flux of particulate organic carbon (POC) from the surface ocean. The use of this Th flux approach requires determination of both the Th activity distribution (used to calculate Th fluxes), and an estimate of the C/Th ratio on sinking particles to empirically derive C fluxes. Observations have shown that in general C/Th decreases with depth and can vary between regions (highest in blooms of large diatoms and highly productive coastal settings). Ideally, a flux weighted C/Th ratio is needed, but most sampling methods separate particles based upon size (filters) or presumed sinking properties (traps). When a single sampling method is used the results 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 classes >1μm). 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 are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and make recommendations for improved application of this approach. Introduction Since the first reported measurements of thorium-234 (Th) in the ocean (Bhat et al., 1969), decreases in Th activities between open ocean and coastal waters indicated that Th activity distributions were strongly influenced by the marine particle cycle. This prompted the use of Th as a new in-situ tracer of oceanic “scavenging”, a term