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, ²³⁰Th is used to assess sedimentary mass fluxes; however, these calculations are potentially biased near regions where net lateral transport of dissolved ²³⁰Th violates the assumption that the flux of particulate ²³⁰Th to the seabed equals its rate of production in the water column. Here, we present a water column transect of dissolved ²³⁰Th along 152° W between Alaska and Tahiti (GEOTRACES GP15), where we examine ²³⁰Th profiles across multiple biogeochemical provinces and, novelly, the lateral transport of ²³⁰Th to distal East Pacific Rise hydrothermal plumes. We observed a strong relationship between the slope of dissolved ²³⁰Th 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 ²³⁰Th 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 ²³⁰Th. At 152° W, we found that the flux-to-production ratio, and thereby error in ²³⁰Th-normalized sediment flux, is between 0.80 and 1.50 for hydrothermal water, but the error is likely larger approaching the East Pacific Rise.