Quantifying Lithogenic Inputs to the North Pacific Ocean Using the Long-lived Thorium Isotopes

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 based on ²³⁰Th:²³⁴U disequilibria. Calculated fluxes of dissolved ²³²Th can in turn be used to estimate lithogenic supplies, such as aerosol dust deposition, given the concentration and fractional solubility of Th in the lithogenic material. This method applied to water column profiles from the Innovative North Pacific Experiment (INOPEX) cruise of 2009 allows estimates of dust deposition at seven sites within the subarctic Pacific, a region in which biological productivity is debatably limited by aerosol-derived Fe. We compare our dust deposition estimates to those based on satellite optical depth and surface ocean Al concentrations. The long-lived thorium isotopes offer new insights into lithogenic material transformations in the ocean which complement more traditional tracer elements.