Iron in Glacial Systems: Speciation, Reactivity, Freezing Behavior, and Alteration During Transport

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 (<0.2 or <0.45 µm) into soluble Fe (<0.02 µm) and colloidal/nanoparticulate Fe (0.02 to 0.2 or 0.45 µm). The ranges of soluble Fe concentrations in icebergs and meltwaters are similar (tens of nanomolar). The range of colloidal/nanoparticulate Fe concentrations in icebergs are an order of magnitude higher (hundreds of nanomolar) and up to thousands of nanomolar in meltwaters. The importance of particulate iron speciation in glacial sediments is also recognized by using carefully calibrated sequential extractions with ascorbic acid (FeA comprising fresh ferrihydrite which is potentially bioavailable) and dithionite (FeD comprising all remaining (oxyhydr)oxide Fe). Iceberg and glacier sediments contain lower concentrations of FeA (0.032 ± 0.024 and 0.042 ± 0.059 wt. %) than meltwater suspended sediments (FeA 0.12 ± 0.09 wt. %). Glacier sediments also contain low concentrations of FeD (0.060 ± 0.036) but concentrations of FeD are comparable in iceberg and meltwater sediments (0.38 ± 0.24 wt. % compared to 0.31 ± 0.09 wt.%). Reactions in ice-water systems produce potentially bioavailable Fe(II) and ferrihydrite by pyrite oxidation, iron mineral dissolution (aided by low pH and organic complexes) and reduction (aided by UV radiation). Some icebergs contain high concentrations of FeA (>0.1 wt. %) which represent samples in which the ongoing 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 soluble 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.

Physical Weathering Intensity Controls Bioavailable Primary Iron(II) Silicate Content in Major Global Dust Sources

“…Bioavailable Fe contributions from terrestrial pyrite oxidation need to be considered when balancing Fe budgets and quantifying total Fe fluxes (Hawkings et al, ; Raiswell, ), but they generally correlate with primary Fe(II) silicate content in glaciogenic dust sources since they are also sourced from bedrock (Raiswell, ; Shoenfelt et al, ). Thus, loss of reactive Fe does not impact the use of Fe(II) silicates as a proxy for bioavailable physically weathered Fe reaching the ocean; Fe in pyrite and on reactive Fe(II) silicate surfaces is readily mobilized and likely becomes bioavailable to phytoplankton (Raiswell et al, ), but a primary Fe(II) silicate signature remains.…”

Dissolved Iron Supply from Asian Glaciers: Local Controls and a Regional Perspective

“…Glacially derived Fe has been distinguished in three forms: truly dissolved Fe (pore size < 0.02 μm), colloidal/nanoparticulate Fe (0.02 to 0.45 μm), and sediment-bound nanoparticulate Fe (>0.45 μm; Statham et al, 2008;Raiswell & Canfield, 2012;Hawkings et al, 2014). Recent studies in the Greenland and Antarctica showed that the largest source of Fe is sediment-bound Fe, while truly dissolved Fe is typically the least abundant phase (Hawkings et al, 2014;Hodson et al, 2017;Raiswell et al, 2018;Statham et al, 2008). Here we present a comprehensive data set of the concentrations and fluxes of filtered Fe (dFe; <0.45 μm) from three Asian glaciers over an entire melt season (Supporting Information S1).…”

“…Quantifying the transport of riverine iron (Fe) is critical because it limits the primary productivity in downstream aquatic ecosystems, some of which receive substantial glacier discharge (Martin et al, 1990;Nielsdottir et al, 2009). Ice sheets have recently been shown to deliver large amounts of Fe to coastal ecosystems through outlet glaciers (Hawkings et al, 2014;Hodson et al, 2016Hodson et al, , 2017 and icebergs (Raiswell, 2011;Raiswell et al, 2016Raiswell et al, , 2018. However, Fe release from glaciers distinct from Greenland and Antarctic ice sheets (Hood et al, 2015;Radić et al, 2014) remains poorly constrained at regional and global scales.…”

Bioavailable iron produced through benthic cycling in glaciated Arctic fjords (Svalbard)

“…Speciation, particle size, surface area, and crystallinity are physical and chemical characteristics of iron minerals that determine if it is available for biological processes. Ascorbate has been shown to selectively extract poorly crystalline, highly reactive Fe(III) 22 , which is potentially bioavailable for phytoplankton 18,23,24 and favorable for microbial reduction 25,26 . Thus, in the context of benthic cycling and early diagenesis, we define ascorbate-extractable Fe(III) as reactive and bioavailable iron (FeR).…”