Fluxes of 230Th and 231Pa to the deep sea: implications for the interpretation of excess 230Th and 231Pa/ 230Th profiles in sediments

Abstract

Analysis of samples obtained with deep-sea moored sediment traps deployed at 15 sites representing a wide range of oceanic conditions confirms that the flux of 230Th scavenged to the seafloor remains close to its production rate from the decay of 234U in the overlying water column, and generally validates the use of 230Th as a normalizing tool for paleoflux reconstruction. After correction for trapping efficiency, the flux of 230Th measured in the low flux regions amounts to 90±6% of the production rate, with the notable exceptions of one site near the Arabian Sea upwelling and one site in the Weddell Sea. A 230Th flux equivalent to 120% of the production rate was found in Panama Basin. Similar or more extensive scavenging of 230Th may be occurring at a Pacific margin site off California and south of the Polar Front, but these estimates are obscured by large errors on our trapping-efficiency estimates. In contrast, the flux of 231Pa and the 231Pa/ 230Th ratio can vary strongly with particle flux, following distinct trends in different oceanic basins. In the Atlantic Ocean, 231Pa fluxes and 231Pa/ 230Th are low and not sensitive to particle flux. This is because of the short residence time of deep water in this basin resulting from thermohaline circulation, which prevents the full development of lateral concentration gradients and full expression of boundary scavenging. In the Pacific Ocean, the sensitivity of 231Pa/ 230Th to particle flux is highest, reflecting the longer residence time of deep water. In the southern ocean, 231Pa/ 230Th ratios are invariably high, even when particle fluxes are low, reflecting the predominance of opal, which fractionates minimally between the two radionuclides. Interpretation of 231Pa/ 230Th recorded in sediments is thus complex. In the Atlantic, this ratio is better suited for recording past changes in the strength of thermohaline circulation. In the Pacific, it has the best potential for providing synoptic maps of past changes in particle flux. In the southern ocean, retrieval of information from 231Pa/ 230Th is more difficult and requires a more quantitative understanding of the influence of particle composition.