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

Publication Status is "Submitted" Or "In Press: 
LDEO Publication: 
Publication Type: 
Year of Publication: 
Journal Title: 
Marine Chemistry

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.