ABSTRACT. Solute acquisition by Alpine glacial meltwaters is the result of the coupling of different pairs of reactions, one of which usually involves dissolved gases. Hence, the availability of atmospheric gases to solution is an important control on the composition of glacial meltwaters. The chemical compositions of the two main components of the bulk meltwater, quick flow and delayed flow, are dominated by different geochemical processes. Delayed flow waters are solute-rich and exhibit high p(C0 2 ) characteristics. The slow transit of these waters through a distributed drainage system and the predominance of relatively rapid reactions, such as sulphide oxidation and carbonate dissolution, in this environment maximize solute acquisition. Quick-flow waters are dilute, both because of their rapid transit through ice-walled conduits and open channels, and because the weathering reactions are fuelled by relatively slow gaseous diffusion of CO 2 into solution, despite solute acquisition being dominated by rapid surface exchange reactions. As a consequence, quick flow usually bears a low or open-system p(C0 2 ) signature. Bulk meltwaters are more likely to exhibit low p(C0 2 ) values when suspended-sediment concentrations are high, which promotes post-mixing reactions. This conceptual model suggests that the composition of both quick flow and delayed flow is likely to be temporally variable, since kinetic, rather than equilibrium, factors determine the composition.

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There is very little information about the activity of microbial communities on the surface of glaciers, though there is an increasing body of evidence to show that they strongly influence the biogeochemistry of these habitats. We measured bacterial abundance and production in cryoconite holes on Arctic, Antarctic and Alpine glaciers in order to estimate the role of heterotrophic bacteria within the carbon budget of glacial ecosystems. Our results demonstrate an active bacterial community on the surface of glaciers with doubling times that vary from a few hours to hundreds of days depending on the glacier and position (water or sediments) within the cryoconite hole. However, bacterial production is only ∼2–3% of the published literature values of community respiration from similar habitats, indicating that other types of microbes (e.g. eukaryotic organisms) may also play a role in the C cycle of glaciers. We estimate that only up to 7% of the organic C in cryoconite sediments is utilized by the heterotrophic bacterial community annually, suggesting that the surface of glaciers can accumulate organic carbon, and that this C may be important for biogeochemical activity downstream to adjacent ecosystems.

ABSTRACT. Solute acquisition by Alpine glacial meltwaters is the result of the coupling of different pairs of reactions, one of which usually involves dissolved gases. Hence, the availability of atmospheric gases to solution is an important control on the composition of glacial meltwaters. The chemical compositions of the two main components of the bulk meltwater, quick flow and delayed flow, are dominated by different geochemical processes. Delayed flow waters are solute-rich and exhibit high p(C0 2 ) characteristics. The slow transit of these waters through a distributed drainage system and the predominance of relatively rapid reactions, such as sulphide oxidation and carbonate dissolution, in this environment maximize solute acquisition. Quick-flow waters are dilute, both because of their rapid transit through ice-walled conduits and open channels, and because the weathering reactions are fuelled by relatively slow gaseous diffusion of CO 2 into solution, despite solute acquisition being dominated by rapid surface exchange reactions. As a consequence, quick flow usually bears a low or open-system p(C0 2 ) signature. Bulk meltwaters are more likely to exhibit low p(C0 2 ) values when suspended-sediment concentrations are high, which promotes post-mixing reactions. This conceptual model suggests that the composition of both quick flow and delayed flow is likely to be temporally variable, since kinetic, rather than equilibrium, factors determine the composition.

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.