Measurement of adsorbed and total 232Th/230Th ratios from marine sediments

The predictable in situ production of Th-230 from the decay of uranium in seawater, and its subsequent removal by scavenging onto falling particles, provides a valuable tool for normalizing fluxes to the seafloor. We describe a new application, determination of the Th-232 that dissolves in the water column and is removed to the seafloor. Th-232 is supplied to the ocean in continental minerals, dissolution of which leads to a measurable standing stock in the water column. Sedimentary adsorbed Th-232/Th-230 ratios have the potential to provide a proxy for estimating the amount of dissolved material that enters the ocean, both today and in the past. Ten core top samples were treated with up to eight different leaching techniques in order to determine the best method for the separating adsorbed from lattice bound thorium. In addition, separate components of the sediments were analyzed to test whether clay dissolution was an important contribution to the final measurement. There was no systematic correlation between the strength of acid used in the leach and the measured Th-232/Th-230 ratios. In all cases clean foraminifera produced the same ratio as leaches on bulk sediment. In three out of five samples leaches performed on non-carbonate detritus in the < 63 mu m size fraction were also identical. Without additional water column data it is not yet clear whether there is a simple one to one correlation between the expected deep-water Th-232/Th-230 and that produced by leaching, especially in carbonate-rich sediments. However, higher ratios, and associated high Th-232 adsorbed fluxes, were observed in areas with high expected detrital inputs. The adsorbed fraction was similar to 35-50% of the total Th-232 in seven out of ten samples. Our Th-230 normalized Th-232 fluxes are reasonable by Comparison to global estimates of detrital inputs to the ocean. In nine cases out often, the total Th-230-normalized Th-232 flux is greater than predicted from the annual dust fall at each specific location, but lower than the average global detrital input from all Sources. (c) 2008 Elsevier B.V. All rights reserved.