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

Abstract

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 0 and 30 m/40 m depth were collected for total Th-234 measurements and those from 30 m/40 m or 100 m depth were collected for particulate Th-234 measurements during the Second Chinese Arctic Expedition in July-September 2003. The removal fluxes and residence time of Th-234 in the upper water column were calculated by using irreversible steady-state scavenging model. The results showed that, total Th-234 was deficit relative to its parent U-238 in the western Arctic Ocean except in the western Chukchi shelf and the slope regions around 160 degrees W, indicating that scavenging and removal processes play an important role in element biogeochemical cycle in the Arctic Ocean. In the western Chukchi shelf and the slope regions around 160 degrees W, total Th-234 was excess relative to U-238, ascribing to the horizontal input of Th-234 adsorbed by ice-rafted sediments. Thorium-234 removal fluxes decreased from the shelf to the deep ocean, while the residence time of Th-234 increased from shelf to offshore, demonstrating that particle scavenging and removal processes are more active in the shelf regions. 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/(m(2).d), and between 1.8 and 14.4 mmol/(m(2).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 (234)Tn/U-238 disequilibria to primary production) in the western Arctic Ocean suggested that the biological pump runs actively in high-latitudes.