Aerosol-Derived Fluxes of Dissolved Trace Metals Evaluated Using Paired Thorium Isotopes

Aerosols (dust) supply dissolved ²³²Th to the ocean just as they serve as a source of micronutrients and other trace elements. Surface waters in regions downwind from major dust sources are enriched in dissolved ²³²Th, reflecting this source. The residence time of dissolved Th is evaluated by measuring the radioactive disequilibrium between dissolved ²³⁰Th and its parent, ²³⁴U. Assuming steady state, which is reasonable when dissolved ²³²Th inventories are integrated through the thermocline, the quotient of the dissolved ²³²Th inventory divided by the residence time of dissolved Th provides the mean annual flux of dissolved Th from aerosols. The supply of other dissolved trace elements (M), such as iron, can then be evaluated by multiplying experimentally-derived ratios for M/²³²Th solubility by the measured ²³²Th flux.

Fluxes of dissolved ²³²Th were measured in the subtropical North Atlantic, in a region of high Saharan aerosol flux, along the U.S. GEOTRACES section occupied in 2010 and 2011. The section ran from Portugal to Mauritania, under the plume of Saharan dust, and from there via Bermuda to Woods Hole. Fluxes of dissolved ²³²Th were evaluated by integrating inventories to a depth of 500 m, which gives a mean residence time of about three years. Integrating to this depth will average out seasonal variability of dust supply. Results will be compared to mean annual dust flux estimated using the model of Mahowald et al., 2005. Assumptions and prospects for future development will be discussed.