Potential for atmospheric acid processing of mineral dust to supply bioavailable trace metals to the oceans

Trace metal concentrations in oceans are influenced by several factors including biogeochemical cycling effects on distributions, concentrations and speciation. The major input of trace metals (and P) to the surface waters of the offshore ocean is mineral dust, predominantly from desert regions. This dust can be subject to acid processing in the atmosphere due to the presence of anthropogenic acidic gases (oxides of nitrogen and sulfur), potentially making trace metals more bioavailable when dust is deposited in the oceans. Here we present a study on the release of trace metals from a desert dust when exposed to a series of acid addition treatments. Al, Fe, Co, Cu, Zn and Pb are preferentially leached from the dust only when the calcite phase has been exhausted and the pH is no longer buffered at circumneutral values. Further acid additions quickly released the majority of leachable trace metals, although lower concentrations of most metals continue to be leached with further acid addition cycles. This contrasts with the behaviour of Ca and P, where in prior work it had been shown that dissolution mirrors closely the addition of protons to mineral surfaces demonstrating the related but contrasting processes for trace element dissolution.