Controls on iron mobilisation from volcanic ash at low pH: insights from dissolution experiments and Mossbauer spectroscopy
Maters, E.C.; Delmelle, P.; Gunnlaugsson, H.P. (2017). Controls on iron mobilisation from volcanic ash at low pH: insights from dissolution experiments and Mossbauer spectroscopy. Chem. Geol. 449: 73-81. https://dx.doi.org/10.1016/j.chemgeo.2016.11.036
In: Chemical Geology. Elsevier: New York; London; Amsterdam. ISSN 0009-2541; e-ISSN 1872-6836, more
The deposition of volcanic ash from explosive eruptions to the surface ocean is a source of iron (Fe) for marine phytoplankton. However, the factors that determine the soluble and thus, potentially bioavailable, fraction of Fe in ash from magma source to ocean sink remain poorly understood. We investigate for the first time the release of both Fe(II) and Fe(III) from six natural ash samples in pH 1 H2SO4 representative of exposure to aqueous low pH conditions during airborne transport. Mössbauer spectroscopy analysis of the ash suggests that the overall (168 h) trends in Fetot and Fe(II)/Fetot release by individual samples are governed by dissolution of Fe-bearing aluminosilicates or oxides of the ash bulk. In contrast, the initial ( < 6 h) trends in Fe release may reflect chemical properties of the ash surface imparted by the ash's eruptive history, supported in part by X-ray photoelectron spectroscopy analysis. Preferential release of Fe(III) relative to Fe(II) immediately on ash input to acid is attributed to oxidation of ash surfaces within the eruption plume. Additionally, prior exposure of ash to a HF- or HCl-rich plume may enhance Fe release by, respectively, reducing aluminosilicate resistance to acid dissolution or lowering the pH of the liquid film on airborne ash. Altogether, our findings highlight that differing Fe speciation in the ash bulk and surface, reflecting its magmatic and eruptive history, exerts a strong influence on the Fe release behaviour of ash even from the same volcano. The relative importance of bulk versus surface controls on Fe mobilisation from ash during atmospheric transport will ultimately depend on the duration of contact with acids before deposition to the ocean.
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