Controls on seawater 231 Pa, 230 Th and 232 Th concentrations along the flow paths of deep waters in the Southwest Atlantic

• Published in: Earth and Planetary Science Letters
• Main Authors: Deng, F.F., Thomas, A.L., Rijkenberg, M.J.A., Henderson, G.M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: North Atlantic
• Online Access: http://imis.nioz.nl/imis.php?module=ref&refid=239890

Measurements of dissolved Th-230, Pa-231 and Th-232 were made for twelve full-depth profiles along a Southwest Atlantic section during GEOTRACES cruise GA02S. Sampling captures all the main Atlantic deep water masses along their meridional flow paths and allows insight into the control on Th and Pa in a setting where waters are flowing in opposing directions, with direct relevance to understanding the use of Pa-231/Th-230 as an ocean-circulation proxy. Water-column Th-230 increases linearly with depth, in line with expected reversible scavenging models. Pa-231 increases from the surface to similar to 1200-1500 m, but is invariant or decreases with greater depth, deviating from the behavior expected for reversible scavenging. Dissolved Pa-231/Th-230 ratios display a mid-water-column maximum at similar to 1000-2000 m which is broadly coincident with Upper Circumpolar Deep Water. Below 2000 m, nuclide distributions and ratios exhibit no dependence on water mass, nor any indication of progressive change within a water mass, challenging the use of Pa-231/Th-230 as a past circulation tracer in the South Atlantic. Calculation of horizontal transport of Th-230 and Pa-231 by ocean circulation indicates a net southward export out of the Atlantic of 19% of the Pa-231 and 3% of the Th-230 produced in that ocean. This removal is all from the North Atlantic while, in the South Atlantic, removal to sediment equals production. Simple one-dimensional modeling can simulate Th-230 profiles but not the mid-water-column maximum observed in Pa-231 profiles, suggesting an additional source of Pa-231 (perhaps lateral transport from the margin) or removal at depth due to bottom scavenging. Near seafloor minima in concentrations indicates bottom scavenging of Th-230 and (231)pa, which is enhanced in the presence of nepheloid layers, particularly for 231Pa. This additional scavenging fractionates Th-230 and Pa-231 and, in the presence of nepheloid layers, may lead to an increase in sedimentary Pa-231/Th-230 ratios. Th-232 concentrations were paired with Th-230-derived residence times in the upper 250 m of the water column to test the application of Th as a tracer of dust deposition. Maxima in Th-232 indicate high dust input from the African and possibly South American continents.