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

234Th (t1/2 = 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 (200–6001) were collected using in situ pumps for the determination of dissolved and particulate 234Th activity and POC and PON concentrations. The lowest total 234Th/238U activity ratio (0.14) was observed in the Chukchi Sea, consistent with enhanced scavenging in these particle-rich shelf waters. 234Th/238U 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 234Th, 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.