Fluxes of Th isotopes in the Santa Monica Basin, offshore California

Abstract

Fluxes of 234Th, 232Th, 230Th and 228Th increase with depth in the water column of Santa Monica Basin, following the flux of particulate inorganic matter. Temporal variability in flux is generally low for the shorter lived and predominantly radiogenic 234Th and 228Th, but high for 230Th and the nonradiogenic 232Th. The flux profiles are largely controlled by lateral advection of detrital material resuspended from the basin slope. Most of the shorter lived 234Th and 228Th scavenged from the water column are produced in situ or locally, whereas most of the authigenic 230Th is transported from elsewhere. From the concentration (in dissolved and particulate forms) and flux profiles of these isotopes, the residence time estimate of dissolved Th is 1–2 months in the upper 50 m and 1–2 years in the entire water column. The corresponding residence times for particulate Th are 4 days and ∼ 40 days, respectively. The average rate at which suspended particles work their way downward via aggregation and settling is estimated at 6–10 m/day. In the context of the documented circulation pattern in the region, these results suggest that most pollutants introduced to the Santa Monica Basin will be exported out of it.