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

Abstract

Ice sheets have been shown to deliver large amounts of labile iron (Fe) to aquatic ecosystems; however, the role of glaciers distinct from ice sheets in supplying labile Fe to downstream ecosystems is less well understood despite their rapid volume loss globally. Direct and continuous measurements of Fe from glaciers throughout an entire melt season are very limited to date. Here we present extensive seasonal data on 0.45‐μm‐filtered Fe (dFe) from three glaciers in Asia. Concentrations of dFe are negatively correlated with glacier discharge, and dFe yields are closely related to specific discharge. Based on our study and previously published dFe data, we estimate the release of dFe from Asian glaciers to be 23.8±14.1 Gg/a. We further compile a global data set of dFe from more than 12 glaciers, which, when combined with data on glacier discharge, suggest that the release of dFe from glaciers globally is on the order of 185±172 Gg/a. This finding suggests that glaciers may provide a substantial, but largely unrecognized source of potentially labile Fe, and may become increasingly important for the Fe biogeochemical cycle in a warming climate.

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

“…The results of the AFeR extractions are consistent with differences in Fe-mineralogical composition detected by 57 Fe Mössbauer spectroscopy, showing that iron in a Kongsfjorden plume sample had a relative abundance of 17.8±1.6% hematite, whereas material from a Kongsfjorden iceberg contained about twice as much hematite, accounting for 41.3±1.9% of the iron pool ( Figure S1, Table S2). These data corroborate results of Raiswell and coworkers 18 , who showed that FeR produced by chemical and biological weathering in subglacial systems 18,52,53 gets slowly converted into less reactive phases such as goethite or hematite in glacial ice, which may explain the higher proportion of hematite found in the icebergs. Given the low amount of FeR in the glacial sources, only a small fraction (0.6-12%) of glacially derived iron is immediately available for microbial reduction in the sediment 26 and potentially bioavailable for phytoplankton 23 .…”

“…The majority of glacially-derived iron is in the particulate form or will rapidly become particulate once in contact with oxic and saline fjord water due to oxidation and flocculation reactions 15-20 , resulting in up to 95% of glacially-sourced iron settling to fjord sediments after entering the marine environment. However, the amount and physical and chemical characteristics of glacial iron that is delivered to Arctic fjords, as well as its fate, remain poorly constrained 12,21 . Speciation, particle size, surface area, and crystallinity are physical and chemical characteristics of iron minerals that determine if it is available for biological processes.…”

The Influence of Glacial Cover on Riverine Silicon and Iron Exports in Chilean Patagonia

“…Filterable Fe concentrations are highest from glacial systems in Peru, Alaska, the Alps, and Patagonia, all of which have concentrations more than an order of magnitude higher than mean global riverine values (0.48 μM; Gaillardet et al, 2014). Such high Fe concentrations highlight that glacial systems are important components of the Fe cycle and that further research is needed to understand how Fe cycling will change in a warming world (Li et al, 2019). However, as discussed previously, details of the mineralogy, phase‐speciation and estuarine transformations are critical to determining the lability and impacts of CNFe in downstream ecosystems.…”

“…Filterable Fe concentrations have also been studied for some mountain glacial systems, with concentrations and yields exceeding those from the Greenland Ice Sheet (Li et al, 2019;Schroth et al, 2011). However, Fe exports from glacial systems are highly variable, and there are still many critical unknowns regarding the factors controlling Fe production, the bioavailability of different phases, exchange between fjord and coastal waters, and how glacial nutrient exports are likely to change in the future (Hawkings et al, 2018;Hopwood et al, 2015;Li et al, 2019;Raiswell et al, 2018;Schroth et al, 2011Schroth et al, , 2014.…”

“…Glacial weathering processes have been identified to be particularly important in the generation of potentially bioavailable iron (Fe) (Bhatia et al, 2013;Hawkings et al, 2014;Li et al, 2019;Raiswell et al, 2018; Global Biogeochemical Cycles 10.1029/2020GB006611 Schroth et al, 2011). Fe is released from the bedrock by a series of microbially mediated weathering reactions, including silicate dissolution, sulfide oxidation, and iron reduction, all of which occur in the subglacial environment (Mikucki et al, 2009;Nixon et al, 2017;Tranter et al, 2002).…”