Uranium-thorium disequilibrium in north-east Atlantic waters
Résumé
In this paper we report and compare the concentrations of 234 Th and 238 U measured in surface and subsurface waters collected in the course of a sampling campaign in the north east Atlantic in June-July 1998. Dissolved 234 Th concentrations in surface waters ranged from 5 to 20 Bq m-3 , showing a large deficiency relative to 238 U concentrations (typically 42 Bq m-3). This disequilibrium is indicative of active 234 Th scavenging from surface waters. Observed 234 Th/ 238 U activity ratios, together with corresponding 234 Th particulate concentrations , were used to calculate mean residence times for 234 Th with respect to scavenging onto particles (s diss) and subsequent removal from surface waters (s part). Residence times in the range 5-30 days were determined for s diss and 4-18 days for s part (n ¼ 14). In addition, ultrafiltration experiments at six stations in the course of the same expedition revealed that in northeast Atlantic surface waters a significant fraction (46 AE 17%; n ¼ 6) of the thorium in the (operationally-defined) dissolved phase (<0.45 lm) is in colloidal form. These observations are consistent with the 'colloidal pumping' model in which it is assumed that 234 Th 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 lm) phase. Thus, the time (s c) for fully dissolved 234 Th to appear in the filter-retained fraction is dependent on the rate of colloidal aggregation. Here, we determined s c values in the range 3-17 days. #