Coupled analysis of seawater and sedimentary 231Pa/230Th in the tropical Atlantic

• Published in: Marine Chemistry
• Main Authors: Ng, Hong Chin, Robinson, Laura F, Rowland, George H, Chen, Siyuan Sean, McManus, Jerry F
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: protactinium; thorium; sediment-seawater analysis; scavenging; eddy diffusion; advection; hydrothermal scavenging; transient nepheloid layer; boundary scavengin; tropical Atlantic; North Atlantic Deep Water; North Atlantic
• Online Access: https://hdl.handle.net/1983/f4ef24a0-fc87-4153-b126-9ce32e6fae32; https://research-information.bris.ac.uk/en/publications/f4ef24a0-fc87-4153-b126-9ce32e6fae32; https://doi.org/10.1016/j.marchem.2020.103894; https://research-information.bris.ac.uk/ws/files/252912527/Ng_et_al_Tropical_Atlantic_PaTh_accepted_author_manuscript.pdf

The 231Pa/230Th ratio of seawater and in ocean sediments is a potentially powerful tool in tracing oceanic processes such as the ocean circulation and marine primary productivity. However, 231Pa/230Th reflects the combined signal of multiple controls associated with scavenging and oceanic transport, limiting its use as a paleo-proxy. Given that links between seawater and sedimentary 231Pa and 230Th distributions are crucial for understanding the cycling and removal fluxes of these nuclides, we carry out analysis of 231Pa and 230Th in both seawater (dissolved phase) and modern sediment samples acquired from five depth transects across the northern tropical Atlantic open ocean. Overall, the sediment data support the dominance of 231Pa export in the deep Atlantic Ocean (>2.5 km) associated with the southward advection of North Atlantic Deep Water. However, the seawater data provide evidence for locally enhanced scavenging of 231Pa and 230Th near the western seamounts that is likely to be associated with the influence of hydrothermal activity or episodic formation of transient nepheloid layers. At shallower depths (0.5–1.5 km), paired sediment and seawater 231Pa/230Th data reveal enhanced burial of 231Pa at the eastern and mid-Atlantic sites that is likely related to elevated marine primary productivity and/or influence of boundary scavenging at the African margin, and a net input of 231Pa potentially maintained by the northward advection of intermediate water masses and an additional mechanism of upward diffusion. Our findings provide greater insights into the range of oceanic processes that play a significant role in the cycling of 231Pa and 230Th, with implications for the controls of 231Pa/230Th distribution in the Atlantic Ocean.