Decrease in 230Th in the Amundsen Basin since 2007: far-field effect of increased scavenging on the shelf?

This study provides dissolved and particulate Th-230 and Th-232 results as well as particulate( )(234)Th data collected during expeditions to the central Arctic Ocean (GEO-TRACES, an international project to identify processes and quantify fluxes that control the distributions of trace elements; sections GN04 and GIPY11). Constructing a time series of dissolved Th-230 from 1991 to 2015 enables the identification of processes that control the temporal development of Th-230 distributions in the Amundsen Basin. After 2007, Th-230 concentrations decreased significantly over the entire water column, particularly between 300 and 1500 m. This decrease is accompanied by a circulation change, evidenced by a concomitant increase in salinity. A potentially increased inflow of water of Atlantic origin with low dissolved Th-230 concentrations leads to the observed depletion in dissolved Th-230 in the central Arctic. Because atmospherically derived tracers (chlorofluorocarbon (CFC), sulfur hexafluoride (SF6)) do not reveal an increase in ventilation rate, it is suggested that these interior waters have undergone enhanced scavenging of Th during transit from Fram Strait and the Barents Sea to the central Amundsen Basin. The Th-230 depletion propagates downward in the water column by settling particles and reversible scavenging.