234Th/238U disequilibrium in the central Arctic Ocean:: implications for particulate organic carbon export

Th-234 (t(1/2) = 24.1 days) was used as a tracer to investigate scavenging and particle export in the upper ocean along a transect from the Chukchi Sea to the North Pole during the 1994 Arctic Ocean Section (August-September), Large volume water samples (201-6001) were collected using in-situ pumps for the determination of dissolved and particulate Th-234 activity and POC and PON concentrations. The lowest total Th-234/U-238 activity ratio (0.14) was observed in the Chukchi Sea, consistent with enhanced scavenging in these particle-rich shelf waters. Th-234/U-238 disequilibrium in the interior Arctic is attributed to local biological production and the associated particle export. Estimates of the export flux of POC and PON from the upper 20-30 m were made using the Th-234, POC and PON results. POC export ranged from 38 mmol C m(-2) d(-1) on the Chukchi Shelf to 0.3-7 mmol C m(-2) d(-1) (mean = 3 +/- 2 mmol C m(-2) d(-1)) within the ice-covered interior Arctic Ocean. PON export ranged from 4.8 mmol N m(-2) d(-1) on the Chukchi Shelf to 0.05-1.1 mmol N m(-2) d(-1) (mean = 0.36 +/- 0.39 mmol N m(-2) d(-1)) in the central Arctic Ocean. (C) 1998 Elsevier Science Ltd. All rights reserved.