Regional estimates of POC export flux derived from thorium-234 in the western Arctic Ocean

In order to elucidate the regional export variation of particulate organic carbon in the western Arctic Ocean, samples vertically integrated between 0 and 100 m depth or between 0and 30 m/40 m depth were collected for total ²³⁴Th measurements and those from 30 m/40 m or 100 m depth were collected for particulate ²³⁴Th measurements during the Second Chinese Arctic Expedition in July-September 2003. The removal fluxes and residence time of ²³⁴Th in the upper water column were calculated by using irreversible steady-state scavenging model. The results showed that, total ²³⁴Th was deficit relative to its parent ²³⁸U in the western Arctic Ocean except in the western Chukchi shelf and the slope regions around 160°W, indicating that scavenging and removal processes play an important role in element biogeochemical cycle in the Arctic Ocean. In the western Chukchi shelfand the slope regions around 160°W,total ²³⁴Th was excess relative to ²³⁸U, ascribing to the horizontal input of ²³⁴Th adsorbed by ice-rafted sediments. Thorinm-234 removal fluxes decreased from the shelf to the deep ocean, while the residence time of ²³⁴Th increased from shelf to offshore, demonstrating that particle scavenging and removal processes are more active in the shelfregions. The estimated POC export fluxes from 40 m in the shelf regions and from 100 m in the slope and deep ocean varied between 1.6 and 27.5 mmol/(m²·d), and between 1.8 and 14.4 mmol/(m²·d), respectively. The averaged POC export fluxes over the entire water column decreased from the shelf to the deep ocean, indicating that the Chukchi shelf is an important region for organic carbon sequestration. The high ThE ratios (ratio of POC export flux derived from ²³⁴Th/²³⁸U disequilibria to primary production) in the western Arctic Ocean suggested that the biological pump runs actively in high-latitudes.