The role of the particle size spectrum in estimating POC fluxes from Th234/U238 disequilibrium

Thorium-234 is increasingly being used as a tracer for particulate organic carbon (POC) export in the oceans. In particular, POC fluxes are being estimated using disequilibrium between Th234 and its parent radionuclide, U238, and estimates of the POC/Th234 ratio of particles settling out of the water column. We have investigated the effect of variations in the particle size distribution on these estimates by using model particle size spectra and size distributions of organic carbon and Th234. Simulations indicate that the POC/Th234 ratio is sensitive to differences in the distributions of organic carbon and Th234 with particle size. If these size distributions differ, then the POC/Th234 ratio is a function of particle size and estimates of the POC/Th234 ratio using size-fractioned samples are inaccurate. Consequently, size fractioning techniques, such as filtration, yield biased estimates if the quantity being measured varies with particle size. We used a model with phytoplankton, fecal pellets and aggregates to examine the assumption that the particles responsible for the Th234 flux are also responsible for the POC flux. We found that variations in the size distributions of these three populations affected POC and Th234 fluxes differently, suggesting that changes in biological interactions can lead to a preferential increase in POC or Th234 flux. We suggest that further examination of the distributions of organic carbon and Th234 with particle size and type is required to refine our understanding of the factors affecting the POC/Th234 ratio.