Determination of Fe2+ and Fe3+ species by FIA-CRC-ICP-MS in Antarctic ice samples

Abstract

Iron is an element of great interest due to its role in primary production and in oceanic carbon cycle regulation, such that past changes in iron deposition may have influenced oceanic sequestration of atmospheric CO₂ on millennial time scales. The behavior of iron in biological and environmental contexts depends strongly on its oxidation state. Solubility in water and the capacity to form complexes are just two important characteristics that are species dependent. Distinguishing between the two iron species, Fe(II) and Fe(III), is necessary to evaluate bioavailability, as Fe(II) is more soluble and therefore more readily available for phytoplankton uptake and growth. Here, we present a novel analytical method for iron speciation analysis using Collision Reaction Cell-Inductively Coupled Plasma-Mass Spectrometry (CRC-ICP-MS) and apply it to ice core samples from Talos Dome, Antarctica. The method detection limit is 0.01 ng g⁻¹. A chelating resin, Ni-NTA Superflow, was used to separate the Fe species. At pH 2 the resin is capable of retaining Fe³⁺ with no retention of Fe²⁺. After the initial separation, we oxidized the Fe²⁺ using H₂O₂, and determined the Fe²⁺ concentration as the difference between the two measurements. Our preliminary results demonstrate higher Fe²⁺ concentrations during glacial periods than during interglacial periods. This elevated concentration of Fe²⁺ suggests that more iron was available for phytoplankton growth during the Last Glacial Maximum, than would be expected from measurements of proxies such as dust mass or total Fe.