Iron dissolution from volcanic ash during simulated atmospheric processing and implications for ocean fertilization

Iron (Fe) is an essential micronutrient for phytoplankton, the main primary producers of the marine environment. A low concentration of dissolved Fe limits phytoplankton growth in approximately 30 % of the ocean called High-Nutrient Low-Chlorophyll (HNLC) regions. The input of soluble, and thus bioavailable, Fe to the surface ocean has the potential to boost primary production and thereby te enhance the drawdown of atmospheric carbon dioxide (C02), with important implications for the global climate. The atmospheric deposition of mineral dust has long been considered the main source of Fe to HNLC regions, but recently the importance of volcanic ash in supplying Fe to the ocean bas begun to emerge. Iron solubility is one of the key parameters for assessing the potential for volcanic ash to fertilise the marine environment. There is a limited understanding of the factors which influence volcanic ash Fe solubility. Interaction between ash and acid during atmospheric transport may modify the distribution of Fe between dissolved and particulate phases in such a way that enhances Fe release upon wet deposition into the ocean. In this study, volcanic ash was subjected to simulated atmospheric processing in the lahoratory to assess its release of Fe under acidic conditions. Crushed basalt rock was used as a reference material to assess the effect of eruption plume conditions on Fe release from volcanic ash. The results indicate that Fe may be released from three pools in ash: i) Fe-bearing salts on the ash surface ii) Fe cations in interstices of the glass network iii) Fe in tetrahedra of the glass network. Factors such as the presence of fluoride and chloride in the plume and the partial weathering and hydrothermal alteration of ash may influence Fe release by modifying the reactivity of the ash. Comparison with previous studies suggests that atmospheric procesing may not be as an important control on volcanic ash Fe solubility as it is on mineral dust Fe solubility.