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

Analysis of samples obtained with deep-sea moored sediment traps deployed a t 15 sites representing a wide range of oceanic conditions confirms that th e flux of Th-230 scavenged to the seafloor remains close to its production rate from the decay of U-234 in the overlying water column, and generally v alidates the Use Of Th-230 as a normalizing tool for paleoflux reconstructi on. After correction for trapping efficiency, the flux of Th-230 measured i n the low flux regions amounts to 90 +/- 6% of the production rate, with th e notable exceptions of one site near the Arabian Sea upwelling and one sit e in the Weddell Sea. A Th-230 flux equivalent to 120% of the production ra te was found in Panama Basin. Similar or more extensive scavenging of Th-23 0 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 trapp ing-efficiency estimates. In contrast, the flux of Pa-231 and the Pa-231/Th -230 ratio can vary strongly with particle flux, following distinct trends in different oceanic basins. In the Atlantic Ocean, Pa-231 fluxes and Pa-23 1/Th-230 are low and not sensitive to particle flux. This is because of the short residence time of deep water in this basin resulting from thermohali ne circulation, which prevents the full development of lateral concentratio n gradients and full expression of boundary scavenging. In the Pacific Ocea n, the sensitivity of Pa-231/Th-230 to particle flux is highest, reflecting the longer residence time of deep water. In the southern ocean, Pa-231/ Th -230 ratios are invariably high, even when particle fluxes are low, reflect ing the predominance of opal, which fractionates minimally between the two radionuclides. Interpretation of Pa-231/Th-230 recorded in sediments is thu s 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 Pa-231/Th-230 i s more difficult and requires a more quantitative understanding of the infl uence of particle composition. (C) 2001 Elsevier Science B.V. All rights re served.