Water column 230Th systematics in the eastern equatorial Pacific Ocean and implications for sediment focusing
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In an effort to investigate the cause of higher-than-expected sediment inventories of 230Th in the Panama Basin, thorium isotopes were measured in eight deep-water casts within the Guatemala, Panama, and Peru Basins along a meridional transect at ~86°W between 6.5°N and 8.5°S. Dissolved 230Th concentrations increase linearly from the surface to 1000m at each transect station. Below 1000m, the deep waters of the Panama Basin show the highest deficit (~50%) of 230Th assuming a reversible exchange of 230Th between dissolved and sinking particulate matter, and in comparison with the globally averaged water-column 230Th. Peru Basin waters have a larger range of dissolved 230Th concentrations (7.9-16.5fg/kg) than that within Panama Basin waters (5.7-7.1fg/kg). There is a progressive decrease in average dissolved deep-water (>1000m) 230Th concentrations from the southernmost sites in the Peru Basin toward the Panama Basin. We suggest that intense scavenging by upwelling-derived-productivity near the equator (±2°) and resuspension of ubiquitous Mn-rich particulates in Panama Basin sediments are the primary causes of the significant south-to-north lateral gradient in deep-water dissolved 230Th. Although 230Th from Peru Basin waters may be transported and then scavenged and ultimately buried in the Panama Basin, our calculations suggest that the quantity of advected 230Th is relatively small (between 15% and 30% of the total 230Th being produced within water column of the Panama Basin itself). Panama Basin sediment focusing factors greater than 1.3 cannot be explained by lateral export and excess scavenging of water column 230Th. Dissolved 232Th concentrations, in addition to being the lowest reported so far in the literature, fall within a very narrow range (6-26pg/kg), and are generally invariable with depth. This invariability suggests the dissolution of dust in surface waters as a likely sole source of dissolved 232Th for the entire water column. © 2012 Elsevier B.V.
author list (cited authors)
Singh, A. K., Marcantonio, F., & Lyle, M.