Uranium–thorium disequilibrium in north-east Atlantic waters

In this paper we report and compare the concentrations of 234Th and 238U measured in surface and subsurface waters collected in the course of a sampling campaign in the north east Atlantic in June–July 1998. Dissolved 234Th concentrations in surface waters ranged from 5 to 20 Bq m –3, showing a large deficiency relative to 238U concentrations (typically 42 Bq m –3). This disequilibrium is indicative of active 234Th scavenging from surface waters. Observed 234Th/ 238U activity ratios, together with corresponding 234Th particulate concentrations, were used to calculate mean residence times for 234Th with respect to scavenging onto particles ( τ diss) and subsequent removal from surface waters ( τ part). Residence times in the range 5–30 days were determined for τ diss and 4–18 days for τ 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 μm) phase. Thus, the time ( τ c) for fully dissolved 234Th to appear in the filter-retained fraction is dependent on the rate of colloidal aggregation. Here, we determined τ c values in the range 3–17 days.