| Popis: |
Measurements of particle size-fractionated POC/ 234 Th ratios and 234 Th 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/ 234 Th 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, 234 Th and POC fluxes estimated using sediment traps were consistently higher than respective fluxes estimated from water-column 234 Th– 238 U disequilibrium, observations that are attributed to terrigenous particle scavenging of 234 Th. In the more productive western Mediterranean and northwest Atlantic, 234 Th and POC fluxes measured by sediment trap and 234 Th– 238 U 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 234 Th– 238 U disequilibrium can be biased differently because of differential settling speeds of POC and 234 Th-carrying particles. |
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