Processes controlling carbon cycling in Antarctic glacier surface ecosystems

Glacier surface ecosystems, including cryoconite holes and cryolakes, are significant contributors to regional carbon cycles. Incubation experiments to determine the net production (NEP) of organic matter in cryoconite typically have durations of 6-24 hours, and produce a wide range of results, many of which indicate that the system is net heterotrophic. We employ longer term incubations to examine the temporal variation of NEP in cryoconite from the McMurdo Dry Valleys, Antarctica to examine the effect of sediment disturbance on system production, and to understand processes controlling production over the lifetimes of glacier surface ecosystems. The shorter-term incubations have durations of one week and show net heterotrophy. The longer term incubations of approximately one year show net autotrophy, but only after a period of about 40 days (~1000 hours). The control on net organic carbon production is a combination of the rate of diffusion of dissolved inorganic carbon from heterotrophic activity within cryoconite into the water, the rate of carbonate dissolution, and the saturation of carbonate in the water (which is a result of photosynthesis in a closed system). We demonstrate that sediment on glacier surfaces has the potential to accumulate carbon over timescales of months to years

Identification and analysis of low-molecular-weight dissolved organic carbon in subglacial basal ice ecosystems by ion chromatography

Determining the concentration and composition of dissolved organic carbon (DOC) in glacial ecosystems is important for assessments of in situ microbial activity and contributions to wider biogeochemical cycles. Nonetheless, there is limited knowledge of the abundance and character of DOC in basal ice and the subglacial environment and a lack of quantitative data on low-molecular-weight (LMW) DOC components, which are believed to be highly bioavailable to microorganisms. We investigated the abundance and composition of DOC in basal ice via a molecular-level DOC analysis. Spectrofluorometry and a novel ion chromatographic method, which has been little utilized in glacial science for LMW-DOC determinations, were employed to identify and quantify the major LMW fractions (free amino acids, carbohydrates, and carboxylic acids) in basal ice from four glaciers, each with a different type of overridden material (i.e. the pre-entrainment sedimentary type such as lacustrine material or palaeosols). Basal ice from Joyce Glacier (Antarctica) was unique in that 98% of the LMW-DOC was derived from the extremely diverse free amino acid (FAA) pool, comprising 14 FAAs. LMW-DOC concentrations in basal ice were dependent on the bioavailability of the overridden organic carbon (OC), which in turn was influenced by the type of overridden material. Mean LMW-DOC concentrations in basal ice from Russell Glacier (Greenland), Finsterwalderbreen (Svalbard), and Engabreen (Norway) were low (0–417nMC), attributed to the relatively refractory nature of the OC in the overridden palaeosols and bedrock. In contrast, mean LMW-DOC concentrations were an order of magnitude higher (4430nMC) in basal ice from Joyce Glacier, a reflection of the high bioavailability of the overridden lacustrine material (> 17% of the sediment OC comprised extractable carbohydrates, a proxy for bioavailable OC). We find that the overridden material may act as a direct (via abiotic leaching) and indirect (via microbial cycling) source of DOC to the subglacial environment and provides a range of LMW-DOC compounds that may stimulate microbial activity in wet subglacial sediments

Novel wireless sensors for in situ measurement of sub-ice hydrologic systems

Wireless sensors have the potential to provide significant insight into in situ physical and biogeochemical processes in sub-ice hydrologic systems. However, the nature of the glacial environment means that sensor deployment and data return is challenging. We describe two bespoke sensor platforms, electronic tracers or ‘ETracers’, and ‘cryoegg’, for untethered, wireless data collection from glacial hydrologic systems, including subglacial channels. Both employ radio frequencies for data transmission, are designed to endure harsh environmental conditions and can withstand low temperatures, high pressure, turbulence and abrasion. We discuss the design, optimization and field testing of the ETracers and cryoegg, culminating in test deployments beneath the Greenland ice sheet. The small, low-cost ETracers were able to travel through subglacial drainage channels, from where they returned water pressure measurements through 100m of ice, and could measure water depth in crevasses. The larger cryoegg was able to return multi-parameter data from moulins through 500m of wet ice to receivers up to 2km away, and from 12m depth in a proglacial lake to a receiver on the shore. The tests demonstrate that the cryoegg and ETracers are low-power, versatile, robust wireless sensor platforms suitable for glacial environments, which may be used with portable, low-cost receiving equipment

Physical weathering by glaciers enhances silicon mobilisation and isotopic fractionation

Glacial meltwaters export substantial quantities of dissolved and dissolvable amorphous silicon (DSi and ASi), providing an essential nutrient for downstream diatoms. Evidence suggests that glacially exported DSi is isotopically light compared to DSi in non-glaciated rivers. However, the isotopic fractionation mechanisms are not well constrained, indicating an important gap in our understanding of processes in the global Si cycle. We use rock crushing experiments to mimic subglacial physical erosion, to provide insight into subglacial isotope fractionation. Isotopically light DSi (δ30SiDSi) released following initial dissolution of freshly ground mineral surfaces (down to −2.12 ± 0.02 ‰) suggests mechanochemical reactions induce isotopic fractionation, explaining the low δ30SiDSi composition of subglacial runoff. ASi with a consistent isotopic composition is present in all mechanically weathered samples, but concentrations are elevated in samples that have undergone more intense physical grinding. These experiments illustrate the critical role of physical processes in driving isotopic fractionation and biogeochemical weathering in subglacial environments. Understanding perturbations in high latitude Si cycling under climatic change will likely depend on the response of mechanochemical weathering to increased glacial melt

Silicon isotopes in Arctic and sub-Arctic glacial meltwaters:The role of subglacial weathering in the silicon cycle

Glacial environments play an important role in high-latitude marine nutrient cycling, potentially contributing significant fluxes of silicon (Si) to the polar oceans, either as dissolved silicon (DSi) or as dissolvable amorphous silica (ASi). Silicon is a key nutrient in promoting marine primary productivity, contributingto atmosphericCO2 removal.We present the current understanding of Si cycling in glacial systems,focusingontheSiisotope(δ30Si)composition of glacial meltwaters. We combine existing glacial δ30Si data with new measurements from 20 subArctic glaciers, showing that glacial meltwaters consistently export isotopically light DSi compared with non-glacial rivers (+0.16‰ versus +1.38‰). Glacial δ30SiASi composition ranges from −0.05‰ to −0.86‰ but exhibits low seasonal variability. Silicon fluxes and δ30Si composition from glacial systems are not commonly included in global Si budgets and isotopic mass balance calculations at present. We discuss outstanding questions, including the formation mechanism of ASi and the export of glacial nutrients from fjords. Finally, we provide a contextual framework for the recent advances in our understanding of subglacial Si cycling and highlight critical research avenues for assessing potential future changes in these environments

The influence of glacial cover on riverine silicon and iron exports in Chilean Patagonia

Glaciated environments have been highlighted as important sources of bioavailable nutrients, with inputs of glacial meltwater potentially influencing productivity in downstream ecosystems. However, it is currently unclear how riverine nutrient concentrations vary across a spectrum of glacial cover, making it challenging to accurately predict how terrestrial fluxes will change with continued glacial retreat. Using 40 rivers in Chilean Patagonia as a unique natural laboratory, we investigate how glacial cover affects riverine Si and Fe concentrations, and infer how exports of these bioessential nutrients may change in the future. Dissolved Si (as silicic acid) and soluble Fe (0.45 mu m) phases of both Si and Fe, which are not typically accounted for in terrestrial nutrient budgets but can dominate riverine exports. Dissolved Si and soluble Fe yield estimates showed no trend with glacial cover, suggesting no significant change in total exports with continued glacial retreat. However, yields of colloidal-nanoparticulate and reactive sediment-bound Si and Fe were an order of magnitude greater in highly glaciated catchments and showed significant positive correlations with glacial cover. As such, regional-scale exports of these phases are likely to decrease as glacial cover disappears across Chilean Patagonia, with potential implications for downstream ecosystems

Investigation of subglacial weathering under the Greenland Ice Sheet using silicon isotopes

Subglacial chemical weathering plays a key role in global silicate weathering budgets, contributing to the cycling of silicon (Si) in terrestrial and marine systems and the potential drawdown of carbon dioxide from the atmosphere. Here, we use data from two Greenland Ice Sheet (GrIS) catchments to demonstrate how Si isotopes from dissolved and amorphous particulate fractions (δ30DSi and δ30ASi respectively) can be used together with major ion data to assess the degree of secondary silicate weathering product formation and redissolution in subglacial environments. We compare a time-series of summer melt seasons from the two study sites, which differ in catchment size (∼600 km2 for Leverett Glacier (LG) and ∼36 km2 for Kiattuut Sermiat (KS)). Subglacial waters from LG have elevated Na+ and K+ ions in relation to Ca2+ and Mg2+ ions, indicating a predominance of silicate weathering, whilst meltwaters from KS are characterised by carbonate weathering (hydrolysis and carbonation) throughout the melt season. Both catchments have mean δ30DSi values substantially lower than average riverine values (KS 0.41‰, LG −0.25‰, versus a global riverine mean of 1.25‰) and display a seasonal decline, which is more pronounced at LG. The δ30ASi values (discharge weighted mean values KS −0.44‰, LG −0.22‰) are lighter than the bedrock (mean values KS −0.18 ± 0.12‰, LG 0.00 ± 0.07‰) in both catchments, indicating a secondary weathering product origin or leaching of lighter isotopes during initial weathering of crushed rock. When used in combination, the major ion and silicon isotope data reveal that the extent of silicate weathering and secondary phase redissolution are more pronounced at LG compared to KS. Contrasting weathering regimes and subglacial hydrology between catchments need to be considered when estimating the δ30Si composition of silica exported into polar oceans from the GrIS, with larger catchments likely to produce fluxes of lighter δ30Si. As larger catchments dominate freshwater export to the ocean, GrIS meltwater is likely to be very light in isotopic composition, and the flux is likely to increase with ice melt as the climate warms