Th-234 in different size classes of sediment trap collected particles from the Northwestern Pacific Ocean

Abstract

234Th, a surface area specific particle-reactive radionuclide has been used to estimate particulate organic carbon (POC) export fluxes, based on the POC/234Th ratio of sinking particles and the 234Th flux, in the ocean. The POC/234Th ratios from large particles (>50 μm) are conventionally considered to represent sinking particles (collected by sediment traps), but this assumption has not been thoroughly tested. More recently, Hung and Gong (2010) separated sinking particles from the northern South China Sea (where diatoms are the most dominant group) using different Nitex screens and found that small (<50 μm) sinking particles dominate the bulk 234Th and POC flux. However, the size distribution of sinking particles has seldom been measured in picoplankton-dominated oligotrophic and upwelling conditions. Here we used Nitex screens to separate sinking particles in oligotrophic and upwelling regions. Additionally, images of selected bulk sinking particles (not subject to sequential filtration) were directly taken by scanning electron microscopy (SEM) to determine the size distribution of sinking particles. Within the trap-collected 234Th pool in the upwelling area, the 1–10 and 10–50 μm fractions had, on average, the highest amounts of 234Th, while POC did not show significant variations among the different size fractions. In the oligotrophic region, the 10–50 and 1–10 μm fractions had, on average, the highest amounts of 234Th, while the >150 and 10–50 μm had elevated shares of POC. Results thus demonstrate that 234Th was mainly carried by the smaller (<50 μm) sinking particles (∼70–80% of the total). SEM images of bulk sinking particles demonstrate that sinking particles contained abundant diatom cells, fecal pellets, detritus and small particle aggregates, mostly smaller than 50 μm. These images show that particles smaller than 50 μm indeed can account for a major fraction of the bulk sinking particles in open ocean regions. While the sequentially size-fractionated filtration might have influenced the original size distribution of sinking particles to some extent, the results provide new and consistent data which suggest that the contribution of particles smaller than 50 μm to the sinking 234Th flux can be a major fraction of the total sinking flux of this isotope, and thus, particles smaller than 50 μm cannot be ignored because the POC/234Th ratio that is often used is that of large (>50 μm) particles obtained from pump systems, not traps.