Sediment trap and in-situ pump size-fractionated POC/234Th ratios in the Mediterranean Sea and Northwest Atlantic: Implications for POC export

Measurements of particle size-fractionated POC/234Th ratios and 234Th and POC fluxes were conducted using surface-tethered, free-floating, sediment traps and large-volume in-situ pumps during four cruises in 2004 and 2005 to the oligotrophic eastern Mediterranean Sea and the seasonally productive western Mediterranean and northwest Atlantic. Analysis of POC/234Th ratios in sediment trap material and 10, 20, 53, 70, and 100 μm size-fractionated particles indicate, for most stations, decreasing ratios with depth, a weak dependence on particle size, and ratios that converge to ∼1–5 μmol dpm−1 below the euphotic zone (∼100–150 m) throughout the contrasting biogeochemical regimes. In the oligotrophic waters of the Aegean Sea, 234Th and POC fluxes estimated using sediment traps were consistently higher than respective fluxes estimated from water-column 234Th–238U disequilibrium, observations that are attributed to terrigenous particle scavenging of 234Th. In the more productive western Mediterranean and northwest Atlantic, 234Th and POC fluxes measured by sediment trap and 234Th–238U disequilibrium agreed within a factor of 2–4 throughout the water column. An implication of these results is that estimates of POC export by sediment traps and 234Th–238U disequilibrium can be biased differently because of differential settling speeds of POC and 234Th-carrying particles.