Quantifying lithogenic inputs to the North Pacific Ocean using the long-lived thorium isotopes

Abstract

Dissolved 232Th is added to the ocean though the partial dissolution of lithogenic materials such as aerosol dust in the same way as other lithogenically sourced and more biologically important trace metals such as Fe. Oceanic 230Th, on the other hand, is sourced primarily from the highly predictable decay of dissolved 234U. The rate at which dissolved 232Th is released by mineral dissolution can be constrained by a Th removal rate derived from 230Th:234U disequilibria, assuming steady-state. Calculated fluxes of dissolved 232Th can in turn be used to estimate fluxes of other lithogenically sourced dissolved metals as well as the original lithogenic supplies, such as aerosol dust deposition, given the concentration and fractional solubility of Th (or other metals) in the lithogenic material. This method is applied to 7 water column profiles from the Innovative North Pacific Experiment (INOPEX) cruise of 2009 and 2 sites from the subtropical North Pacific. The structure of shallow depth profiles suggests rapid scavenging at the surface and at least partial regeneration of dissolved 232Th at 100–200 m depth. This rapid cycling could involve colloidal Th generated during mineral dissolution, which may not be subject to the same removal rates as the more truly dissolved 230Th. An additional deep source of 232Th was revealed in deep waters, most likely dissolution of seafloor sediments, and offers a constraint on dissolved trace element supply due to boundary exchange.