Hydrothermal Plumes Act as a Regional Boundary Sink of ²³⁰Th in the Equatorial Pacific

Abstract An important role in the cycling of marine trace elements is scavenging, their adsorption and removal from the water column by sinking particles. Boundary scavenging occurs when areas of strong particle flux drive preferential removal of the trace metals at locations of enhanced scavenging. Due to its uniform production and quick burial via scavenging, 230 Th is used to assess sedimentary mass fluxes; however, these calculations are potentially biased near regions where net lateral transport of dissolved 230 Th violates the assumption that the flux of particulate 230 Th to the seabed equals its rate of production in the water column. Here, we present a water column transect of dissolved 230 Th along 152° W between Alaska and Tahiti (GEOTRACES GP15), where we examine 230 Th profiles across multiple biogeochemical provinces and, novelly, the lateral transport of 230 Th to distal East Pacific Rise hydrothermal plumes. We observed a strong relationship between the slope of dissolved 230 Th concentration‐depth profiles and suspended particle matter inventory in the upper‐mid water column, reinforcing the view that biogenic particle mass flux sets the background 230 Th distribution in open ocean settings. We find that, instead of the region of enhanced particle flux around the equator, hydrothermal plumes act as a regional boundary sink of 230 Th. At 152° W, we found that the flux‐to‐production ratio, and thereby error in 230 Th‐normalized sediment flux, is between 0.80 and 1.50 for hydrothermal water, but the error is likely larger approaching the East Pacific Rise.