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

- We present high-resolution dissolved 230Th, 231Pa and 232Th in the SW Atlantic.
- There is no clear indication of water mass control on 230Th and 231Pa in deep water.
- We estimate a net export of both nuclides out of the Atlantic by ocean circulation.
- We quantify additional bottom scavenging observed in settings of nepheloid layers.
- We apply 232Th to quantify dust input from continents to the surface ocean.

Measurements of dissolved 230Th, 231Pa and 232Th 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 231Pa/230Th as an ocean-circulation proxy. Water-column 230Th increases linearly with depth, in line with expected reversible scavenging models. 231Pa increases from the surface to ∼1200-1500 m, but is invariant or decreases with greater depth, deviating from the behavior expected for reversible scavenging. Dissolved 231Pa/230Th ratios display a mid-water-column maximum at ∼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 231Pa/230Th as a past circulation tracer in the South Atlantic. Calculation of horizontal transport of 230Th and 231Pa by ocean circulation indicates a net southward export out of the Atlantic of 19% of the 231Pa and 3% of the 230Th 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 230Th profiles but not the mid-water-column maximum observed in 231Pa profiles, suggesting an additional source of 231Pa (perhaps lateral transport from the margin) or removal at depth due to bottom scavenging. Near seafloor minima in concentrations indicates bottom scavenging of 230Th and 231Pa, which is enhanced in the presence of nepheloid layers, particularly for 231Pa. This additional scavenging fractionates 230Th and 231Pa and, in the presence of nepheloid layers, may lead to an increase in sedimentary 231Pa/230Th ratios. 232Th concentrations were paired with 230Th-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 232Th indicate high dust input from the African and possibly South American continents.