Cochran, J.K., Armstrong, R., Miquel, J.C., Fowler, S.W., and Masque, P. NONSTEADY STATE MODELING OF THORIUM-234 IN TIMES SERIES SEDIMENT TRAP AND WATER COLUMN PROFILES IN THE MEDITERRANEAN

Abstract: The natural radionuclide ²³⁴Th is commonly used as a proxy for POC export from the euphotic zone. This application requires knowledge of the sinking flux of ²³⁴Th, often determined from the disequilibrium between ²³⁴Th and its parent 238U in the water column. Here we present time series records of ²³⁴Th in sediment traps and concomitant measurements of ²³⁴Th/238U disequilibrium in the water above the trap at two sites (coastal and deep-water) in the Mediterranean Sea. At both sites the temporal trend in ²³⁴Th flux, as recorded in the sediment traps, is not well reflected in the temporal trend of ²³⁴Th deficits in the overlying water column. The data suggest that advective transport of Th-depleted water through the area can bias the ²³⁴Th flux derived from water column profiles. This study emphasizes the importance of time series records in reconstructing fluxes from ²³⁴Th and the complementary nature of water column and sediment trap data.