Particulate organic carbon export fluxes in the Canada Basin and Bering Sea as derived from Th-234/U-238 disequilibria

Seawater samples were collected in the water column of the Canada Basin and the Bering Sea from aboard the R/V Xue Long during August 1999. Activity concentrations of dissolved and particulate Th-234 were measured using beta counting techniques to quantify the scavenging and residence time of Th-234 and organic carbon export fluxes. Primary production (PP) and bacterial production were also determined in the study areas through in situ incubation experiments. Significant Th-234 scavenging was observed in the upper 100 m of the water column in both study areas, with up to 40% of Th-234 deficit found at Bering Sea stations and similar to15% of Th-234 deficit at the Canada Basin station. Measured PP decreased from similar to12.5 mumol C/m(3)/h in surface water to near zero at similar to100 m depth, with an integrated PP of 3.83 mmol C/m(2)/d in the Canada Basin. Bacterial production, on the other hand, was on the order of 2.0 mmol C/m(2)/d, which is up to 52% of the integrated PP. Particulate organic carbon (POC) export fluxes derived from Th-234/U-238 disequilibrium were similar to1 mmol C/m(2)/d in the Canada Basin and similar to10 mmol C/m(2)/d in the Bering Sea, with fluxes in the latter area being 5 to 10 times higher than those found in the Canada Basin. These export fluxes correspond to a ThE ratio (the ratio of Th-234-derived POC export to primary production) of 0.26 for the Canada Basin and 0.7 for the Bering Sea. The higher ThE ratios in the study areas suggest a decoupling of production and particulate export in the high-latitude ocean. Ratios of POC to particulate Th-234 (mumol C/dpm) decreased consistently with increasing depth, suggesting that organic carbon is preferentially remineralized relative to Th-234. Interestingly, the profile of particulate Th-234 in the Canada Basin showed a unique characteristic: particulate Th-234 activities increased with increasing depth, suggesting a continuous scavenging of Th-234 and a rapid settling rate of the particles