Calibration of sediment traps and particulate organic carbon export using 234Th in the Barents Sea

Profiles of particulate and dissolved Th-234 (t(1/2)=24.1 days) in seawater and particulate Th-234 collected in drifting traps were analyzed in the Barents Sea at five stations during the ALV3 cruise (from June 28 to July 12, 1999) along a transect from 78degrees15'N-34degrees09'E to 73degrees49'N-31degrees43'E. Th-234/U-238 disequilibrium was observed at all locations. Th-234 data measured in suspended and trapped particles were used to calibrate the catchment efficiency of the sediment traps. Model-derived Th-234 fluxes were similar to Th-234 fluxes measured in sediment traps based on a steady-state Th-234 model. This suggests that the sediment traps were not subject to large trapping efficiency problems (collection efficiency ranges from 70% to 100% for four traps). The export flux of particulate organic carbon (POC) can be calculated from the model-derived export flux of Th-234 and the POC/Th-234 ratio. POC/Th-234 ratios measured in suspended and trapped particles were very different (52.0 +/- 9.9 and 5.3 +/- 2.2 mumol dpm(-1), respectively). The agreement between calculated and measured POC fluxes when the POC/Th-234 ratio of trapped particles was used confirms that the POC/Th-234 ratio in trap particles is representative of sinking particles. Large discrepancies were observed between calculated and measured POC fluxes when the POC/Th-234 ratio of suspended particles was used. In the Barents Sea, vertical POC fluxes are higher than POC fluxes estimated in the central Arctic Ocean and the Beaufort Sea and lower than those calculated in the Northeast Water Polynya and the Chukchi Sea. We suggest that the latter fluxes may have been strongly overestimated, because they were based on high POC/Th-234 ratios measured on suspended particles. It seems that POC fluxes cannot be reliably derived from thorium budgets without measuring the POC/Th-234 ratio of sediment trap material or of large filtered particles. (C) 2002 Elsevier Science B.V. All rights reserved.