Uranium-thorium disequilibrium in north-east Atlantic waters

In this paper we report and compare the concentrations of Th-234 and U-238 measured in surface and subsurface waters collected in the course of a sampling campaign in the north east Atlantic in June-July 1998. Dissolved Th-234 concentrations in surface waters ranged from 5 to 20 Bq m(-3), showing a large deficiency relative to U-238 concentrations (typically 42 Bq m(-3)). This disequilibrium is indicative of active Th-234 scavenging from surface waters. Observed Th-234 /U-238 activity ratios, together with corresponding Th-234 particulate concentrations, were used to calculate mean residence times for Th-234 with respect to scavenging onto particles (tau(diss)) and subsequent removal from surface waters (tau(part)). Residence times in the range 5-30 days were determined for tau(diss) and 4-18 days for tau(part) (n = 14). In addition, ultrafiltration experiments at six stations in the course of the same expedition revealed that in north-east Atlantic surface waters a significant fraction (46 +/- 17%; n = 6) of the thorium in the (operationally-defined) dissolved phase (< 0.45 mum) is in colloidal form. These observations are consistent with the 'colloidal pumping' model in which it is assumed that Th-234 is rapidly absorbed by colloidal particles, which then aggregate, albeit at a slower rate, into larger filterable particles. In essence, colloids act as intermediaries in the transition from the fully dissolved to the filter-retained (> 0.45 mum) phase. Thus, the time (tau(c)) for fully dissolved Th-234 to appear in the filter-retained fraction is dependent on the rate of colloidal aggregation. Here, we determined tau(c) values in the range 3-17 days. (C) 2004 Elsevier Ltd. All rights reserved.