Thorium-Protactinium fate across the tropical Atlantic Ocean: what reveals the water column-sediment coupling

18 January 2021

Twenty seawater profiles and twenty core-top 231-protactinium (²³¹Pa) and 230-thorium (²³⁰Th) analyses were realised by Ng and colleagues (2020, see reference below) along five depth transects across the northern tropical Atlantic open ocean. Among them, eighteen seawater-sediment pairs from nearby sites were compared. A thorough examination of this coupling associated to published data of ²³¹Pa and ²³⁰Th in the North Atlantic Ocean, allow the authors to i) confirm the dominance of the ²³¹Pa-export signal in the deep Atlantic associated with the southward advection of North Atlantic Deep Water (NADW) but also ii) observe contrasting local increases of sedimentary ²³¹Pa/²³⁰Th above the production ratio, with higher values at intermediate depths (0.5–1.5 km) and in the eastern and mid-Atlantic sites. These last observations reflect either an additional input of ²³¹Pa to the intermediate depths, or enhanced ²³¹Pa scavenging at the eastern and mid-Atlantic sites, and/or the influence of boundary scavenging near the eastern margin.

Figure (modified from Ng, et al. 2020): Tropical Atlantic seawater ²³¹Pa/²³⁰Th data and recently-deposited seafloor sediment ²³¹Pa/²³⁰Th data. The map on the left shows the locations of seawater (square symbols) and sediment (circle symbols) data, and the red lines mark the boundary of the East-West depth section plotted on the right. The colour map in the depth section represents interpolated seawater ²³¹Pa/²³⁰Th data, while the sediment ²³¹Pa/²³⁰Th data is plotted overlain the seawater data as coloured circles. Seawater ²³¹Pa/²³⁰Th is typically higher than the sediment ²³¹Pa/²³⁰Th of a given site, reflecting the greater solubility of the ²³¹Pa isotope. Sediment ²³¹Pa/²³⁰Th values lower than the production ratio (0.093) indicate a net export of ²³¹Pa, while sediment values higher than 0.093 reflect a net input of ²³¹Pa, relative to ²³⁰Th. Sediment data may appear to be ‘below’ or ‘floating above’ the seafloor drawn in dark grey, because they are superimposed on a representative section across the Atlantic, rather than the overlying bathymetry measured at the actual sites.

Reference:

Ng, H. C., Robinson, L. F., Rowland, G. H., Chen, S. S., & McManus, J. F. (2020). Coupled analysis of seawater and sedimentary 231Pa/230Th in the tropical Atlantic. Marine Chemistry, 227, 103894. DOI: https://doi.org/10.1016/J.MARCHEM.2020.103894

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