Role of colloidal material in the removal of 234Th in the Canada basin of the Arctic Ocean

The phase partitioning of Th-234 between dissolved (< 10-kiloDalton, kD), colloidal (10 kD-0.4 mu m), and particulate (>= 0.5 mu m) matter across a horizontal transect, from a coastal station to the deep Canada Basin, and a vertical profile in the deep Canada Basin of the western Arctic Ocean was investigated. Concentrations of suspended particulate matter (SPM), dissolved, colloidal and particulate organic carbon, particulate organic nitrogen and nutrients (silicate, phosphate and nitrate) were also measured to assess transport and scavenging processes. Total Th-234 (colloidal + particulate + dissolved) indicated deficiencies relative to secular equilibrium with its parent, U-238 in the upper 100 m, which suggests active scavenging of Th-234 onto particle surfaces. In contrast, at depths > 200 m, general equilibrium existed between total Th-234 and U-238. The inventory of SPM and the specific activity of particulate Th-234 in the Canada Basin was about an order of magnitude higher than the profile reported for the Alpha Ridge ice camp station. This higher concentration of SPM in the southwestern Canada Basin is likely derived from ice-rafted sedimentary particles. Inventories of nutrients, and dissolved organic carbon and nitrogen in the upper 100 m of the Canada Basin are comparable to the other estimates for the central Arctic Ocean. Comparison of the mass concentrations of colloidal and filter-retained particulate matter as well as the activity of Th-234 in these phases indicates that only a very small component of the colloidal material is actively involved in Th scavenging. Lower values of the conditional partition coefficient between the colloidal and dissolved phase indicate that the Arctic colloids are less reactive than colloidal material from other regions. The conditional partition coefficient between the filter-retained and dissolved phases (K-f) is generally higher than that for other regions, which is attributed to the higher complexation capacity of glacio-marine sedimentary particles in these waters. The Th-234-derived export of POC for the shelf and deep Canada Basin ranges between 5.6 and 6.5 mmol m(-2) d(-1), and is in agreement with other estimates reported for the central Arctic Ocean and Beaufort Sea. (C) 2003 Elsevier Ltd. All rights reserved.