Intense hydrothermal scavenging of ²³⁰Th and ²³¹Pa in the deep Southeast Pacific

Hydrothermal circulation and subsequent eruption of seawater at mid-ocean ridges and back-arc basins has great potential to modulate deep ocean biogeochemistry, acting as both a source and a sink for many trace elements and their isotopes. The influence of hydrothermal vents as a source of iron and manganese has been demonstrated in all ocean basins. However, the long-range impact of scavenging by hydrothermal particles has yet to be documented in detail. We use dissolved and particulate measurements of long-lived radiogenic (²³⁰Th, ²³¹Pa) and primordial (²³²Th) radionuclides to investigate the nature and geographic scale of scavenging processes occurring within a hydrothermal plume in the Southeast Pacific Ocean sampled during the GEOTRACES GP16 section. Due to their radioactive disequilibrium with respect to production by their parent uranium isotopes, ²³⁰Th and ²³¹Pa provide unique insights into the rates of scavenging. We find strong deficits in ²³⁰Th and ²³¹Pa coincident with elevated particulate Mn and Fe(OH)₃, indicating that trace metal scavenging is widespread and likely the result of the strong affinity of trace metals for nanoparticulate metal surface sites. The chemical composition of the particulate material is closely linked to the scavenging intensity of ²³⁰Th and ²³¹Pa, expressed as distribution coefficients between solid and solution. A comparison of ²³⁰Th and ²³¹Pa inventories with mantle-derived ³He as well as a mass balance of ²³⁰Th and ²³¹Pa suggests continuous scavenging removal over the course of the entire 4000 km transect. Unlike the two radiogenic isotopes, ²³²Th is enriched above what would be expected from assuming identical scavenging behavior to ²³⁰Th, indicating a hydrothermal source of colloidal, unreactive ²³²Th.