Abstract
A more insightful view of iron in glacial systems requires consideration of iron speciation and mineralogy, the potential for iron minerals to undergo weathering in ice-water environments, the impact of freezing on concentration and speciation, and potential for glacial delivery to undergo alteration during transport into the ocean. A size fractionation approach improves recognition of iron speciation by separating dissolved Fe (0.1 wt. %) which represent samples in which the on-going transformation of ferrihydrite to goethite/hematite is incomplete. Numerical models of freezing in subglacial systems show that the nanomolar levels of soluble Fe in icebergs cannot be achieved solely by freezing, and must indicate the presence of nanoparticulate Fe and/or iron desorbed from ice or sediments during melting. Models of freezing effects in sea ice show that nanomolar levels of Fe are achievable because high concentrations of hydroxide and chloride ions maintain dissolved iron as soluble complexes. Delivery of iron through fjords is temporally and spatially variable due to circulation patterns, mixing of different sources and aggregation through salinity gradients.
Highlights
Recent studies of the iron cycle in the Southern Ocean acknowledge a range of sources, the primary focus remains on iron sourced from atmospheric dust, continental shelf sediments and hydrothermal inputs (Tagliabue et al, 2016)
Dissolved Iron Measurements of dissolved Fe are routinely made by filtration through 0.45 or 0.2 μm filters and these measurements are based on size, not speciation
Dissolved Iron Here we present speciation data for dissolved iron in icebergs and meltwaters
Summary
Recent studies of the iron cycle in the Southern Ocean acknowledge a range of sources, the primary focus remains on iron sourced from atmospheric dust, continental shelf sediments and hydrothermal inputs (Tagliabue et al, 2016). Studies of iceberg Fe delivery to the Southern Ocean by Smith et al (2007, 2013); Raiswell et al (2008, 2016); Shaw et al (2011); Death et al (2014); Duprat et al (2016) and Herraiz-Borreguero et al (2016) are supported by many others showing dissolved and particulate Fe delivery from a variety of different localized glacial sources (Lannuzel et al, 2008, 2014; Gerringa et al, 2012; De Jong et al, 2013, 2015; Annett et al, 2015, 2017; Lyons et al, 2015; Monien et al, 2017) These publications demonstrate a significant role for glacial processes in delivering potentially bioavailable Fe to the Southern Ocean. Iron measurements in biogeochemical studies should to be related to those species that are potentially bioavailable, including aqueous Fe, and fresh amorphous iron (oxyhydr)oxides
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