Continuous, Rapid Scavenging of Thorium and Protactinium During Westward Advection of the East Pacific Rise Hydrothermal Plume

We assess the controls on seawater distributions of ²³²Th, ²³¹Pa, and ²³⁰Th in the East Pacific Rise (EPR) Hydrothermal Plume using data from the US GEOTRACES East Pacific Zonal Transect and 1-D modeling. Approximately 60% of the ²³⁰Th has been scavenged and removed from the water column 1200 km west of the ridge axis compared to the values expected from reversible scavenging. We test three combinations of processes controlling the concentrations of Th and Pa in the EPR plume as it advects westward. Removal at the plume head and subsequent ingrowth during transit is ruled out, as is removal at the plume head and mixing with non-plume water. Instead, removal of trace elements from the water column must occur continuously as the plume-affected water mass transits westward through the basin. Our model can account for the observed longitudinal variations in ²³⁰Th, ²³¹Pa, and ²³¹Pa/²³⁰Th ratio, but requires extremely short residence times of Th and Pa of ~2 and ~20 years, respectively. Our findings have implications for particle dynamics in the EPR plume and trace metal scavenging over large areas of the tropical South Pacific.