The Greenland Ice Sheet (GrIS) is a poorly constrained source of mercury (Hg) to Arctic ecosystems. We measured Hg concentrations and stable isotopes along an ice-to-ocean continuum to identify controls on GrIS Hg export. Early-season permafrost melt and rainfall produced high filtered total mercury (fTHg, ~17 pM) and monomethylmercury (MMHg, ~2 pM). As subglacial drainage evolved, particulate Hg doubled (from ~8 to 17 pM) and MMHg production remained elevated, indicating Hg mobilization from subglacial environments. Shifts in Hg stable isotope ratios and $$\Delta$$199Hg mass balance show supraglacial sources contribute 20–48% of exported Hg, suggesting subglacial inputs dominate the seasonal Hg flux. Fjord waters were enriched in fTHg ( ~ 10 pM) and MMHg ( ~ 2 pM) relative to rivers, consistent with particulate Hg transformations and terrestrial Hg inputs. The estimated GrIS Hg yield ( ~ 23 mmol km−2 yr−1) is similar to that of Arctic rivers and will likely increase with climate-driven mass loss. Subglacial inputs dominate the export of mercury (Hg) from the Greenland Ice Sheet, and the estimated Hg yield is similar to that of Arctic rivers, according to analysis of Hg concentrations and stable isotopes from the southwestern Greenland Ice Sheet margin during the 2023 ablation season

Recent research underscores a potential, yet overlooked, positive climate feedback mechanism: the transport of subglacially produced methane (CH4) to the atmosphere via meltwater. While the majority of research focused on release from beneath the Greenland Ice Sheet, mountain glaciers have been largely understudied, creating a gap in our understanding of the spatial distribution of subglacial CH4 emissions. Emerging research from glaciers in Iceland, Canada, Alaska, and China suggests the presence of CH4 release also from glaciers other than the Greenland Ice Sheet. Here, we explore the potential of CH4 release from outlet glaciers of the Jostedalsbreen ice cap and Midtdalsbreen in central Norway. We investigated whether meltwaters from outlet glaciers of Jostedalsbreen (Tuftebreen, Fåbergstølsbreen, Bøyabreen, Supphelebreen, Austerdalsbreen and Nigardsbreen), along with Midtdalsbreen in Finse, act as a source of CH4 to the atmosphere. We collected discrete samples for dissolved CH4 (dCH4) and CO2 concentrations at all glacier outlets multiple times throughout the melt season. Additionally, we conducted longer time-series measurements of dCH4 at Tuftebreen, Fåbergstølsbreen and Midtdalsbreen, utilizing custom-made dCH4 sensors. Accompanying these measurements were samples analysed for water chemistry and stable isotopes of dCO2 (δ13C-CO2) in samples where concentrations were elevated compared to atmospheric equilibrium. Our results indicate that dCH4 concentrations in the meltwater of all studied glaciers remained below atmospheric equilibrium concentrations throughout the melt season. In contrast, dCO2 concentrations surpassed atmospheric equilibrium levels, suggesting that the studied glacial runoffs do not act as CH4 source to the atmosphere but might contribute as a small source of CO2. This dataset of dissolved greenhouse gases enhances our understanding of the spatial distribution of subglacial CH4 emissions, fostering discussions on carbon cycling beneath glaciers and the factors influencing the presence or absence of CH4 emissions from the subglacial domain.

Far from being frozen and sterile environments, glaciers are biogeochemical reactors and regulators. In this Review, we discuss the hydrology and biogeochemistry of glacierized environments and their impact on downstream ecosystems. Supraglacial meltwaters export labile organic carbon associated with active supraglacial microbial communities, as well as carbon and nutrients delivered via atmospheric deposition. Meltwaters funnelled to the glacier bed and exiting at the glacier snout transport large quantities of rock flour as well as supraglacial and subglacial-derived organic carbon and nutrients to downstream ecosystems. Subglacial water flow paths influence rock–water contact times and vary greatly, affecting weathering reactions. For instance, the hydrology of mountain glaciers and the Greenland Ice Sheet is typically dominated by seasonal melt with short (hours) to medium (weeks) water residence times, although extended biogeochemical isolation can exist in more isolated parts of the Greenland Ice Sheet. Conversely, the Antarctic Ice Sheet is dominated by basal ice melt and residence times that can exceed years and decades. As a result, the latter supports extended biogeochemical isolation and more advanced chemical weathering. Microbial processes and physical-chemical weathering can both sequester or emit greenhouse gases, but the net effect remains unknown. Meltwaters can potentially fuel biological processes in downstream ecosystems by priming glacier-fed streams, fjords, and oceans with rock flour and nutrients. The rapid reduction in glacier area projected for the next century mandates that future research provides a critical assessment of the effects of deglaciation on watershed biogeochemistry, ecology and global biogeochemical cycles. Far from frozen and sterile environments, glaciers are biogeochemical reactors and regulators. This Review outlines key biogeochemical and associated physical processes occurring in glacierized environments and the known impacts of glaciers on elemental cycling and the Earth system.

Recent studies have shown the release of methane (CH4) from the melting Greenland Ice Sheet (GrIS) and identified it as having an additional potential positive climate feedback. This methane originates mainly from acetoclastic methanogenesis in subglacial sediments, accumulates over time, and subsequently diffuses into the subglacial hydrologic network which transports it to the ice sheet margin. The rates of methane production and emission from GrIS subglacial sediments likely depend on a number of factors, including sediment depth and distribution, organic matter content in the sediment and its reactivity, the redox conditions, and downstream methanotrophic activity; however, their relative significance remains unquantified.Here, we use a reaction-transport model that accounts for heterotrophic methane production, methane oxidation, as well as advective and diffusive methane transport to quantitatively assess the potential for biogenic methane production and emissions from subglacial sediments underneath the Greenland Ice Sheet. The model is run over a large environmental condition model ensemble (n=3840) covering the entire range of plausible subglacial sediment thickness, subglacial organic matter availability and reactivity, oxygen concentration and methanotrophic activity as constrained by available field observations from subglacial and/or similar environments and/or laboratory experiments. Model results are discussed in the context of available field observations.Results show that methanogenic activity in subglacial sediments can produce large quantities of methane (10-5 -7.9⋅101 mmol m-2 yr-1). Subglacial methane production rates compare well with observations from laboratory studies. They are strongly controlled by organic matter availability and subglacial sediment depth, but are less sensitive to the availability of oxygen in overlying waters. Only for low organic carbon contents, low methanotrophic rate constants and/or high oxygen concentrations does methane production become more sensitive to oxygen concentration in overlying waters. Simulated methane effluxes vary four orders of magnitude and again strongly depend on organic matter availability and subglacial sediment depths. However, in contrast to methane production, methane efflux is also sensitive to oxygen concentration and methanotrophic activity. Methane effluxes generally decrease with increasing oxygen concentration and their sensitivity to oxygen concentration increases with increasing methanotrophic activity. Model results show that subglacial sediments can support methane effluxes that are up to 100 times higher than the flux required to sustain observed subglacial methane fluxes at the outflow (0.653 mmol m-2 yr-1 Lamarche-Gagnon et al., 2019) for realistic organic carbon contents (0.06 - 0.5 wt%), reactivity (0.013-1.1 yr-1), subglacial sediment depths (100-500 cm) and methanotrophic rate constants (1010-1012 mol cm-3yr-1) under both anoxic and partly oxic conditions (

The basal environments of ice sheets play an important role as places of methane (CH4) production, storage, and release. Recent investigations have confirmed the release of subglacial methane of microbial origin at the western margin of the Greenland Ice Sheet (GrIS). This methane may then serve as a substrate for methane-consuming microorganisms and thus significantly shape microbial community assembly in GrIS subglacial environments. We conducted a comparative analysis of the composition of exported microbial assemblages from six regions spanning a 2,000-km transect along the western margin of the GrIS. Based on 16S rRNA gene sequences, we identified taxa predominantly affiliated with Pseudomonadota (formerly Proteobacteria), Actinomycetota (formerly Actinobacteriota), and Acidobacteriota. Within the Pseudomonadota, notable genera such as Rhodoferax, Polaromonas, and the methylotrophic Crenothrix and Methylotenera were identified as the most abundant. Importantly, we observed a pattern in community composition related to measured methane concentrations at each site, resulting in three distinct clusters: samples from sites with atmospheric methane levels (i.e., with no significant methane release), those from sites with elevated methane concentrations, and methane release hotspots. Our results align with recent findings, suggesting that microbial communities colonizing methane-emitting sites may have the potential to utilize methane as a resource, thereby reducing its release into the atmosphere and so mitigating its impact on climate change. Furthermore, our findings may facilitate the identification of potential methane release hotspots based on microbial community analysis.

Abstract. Glacial and ice sheet advances have buried large amounts of organic matter (OM), which under anoxic subglacial conditions can be microbially converted into methane (CH4). Although CH4 emissions have been observed at glacier margins, the capacity of subglacial environments to sustain such fluxes remains uncertain. To address this, we developed a reaction–transport model (RTM) to simulate CH4 production, transformation, and transport in sediments beneath warm-based regions of the Greenland Ice Sheet (GrIS) margin. The model explores a wide range of environmental conditions, including sediment thickness, OM quantity and reactivity, O2 availability, and methanotrophic activity. Model simulations show that subglacial sediments are largely anoxic. Oxygen (O2) penetration into subglacial sediments is generally restricted to the upper few tens of centimetres, with an average penetration depth of 22.8 cm. Microbial OM degradation and aerobic CH4 oxidation (AeOM) represent the main O2 sinks. Their relative contributions vary with CH4 availability. AeOM dominates in methane-rich sediments, whereas OM degradation prevails in methane-poor environments. Modeled depth-integrated methanogenesis rates range from 0.1 to 1600 mmol-CH4 m-2 yr-1 (mean 73 mmol-CH4 m-2 yr-1) and are primarily controlled by OM reactivity, with sediment depth and OM concentration exerting only a small secondary influence. This sensitivity of CH4 production rates to OM reactivity can produce sharp thresholds, where small decreases in reactivity strongly suppress CH4 fluxes. A highly variable fraction of the generated CH4 is consumed by AeOM within the shallow oxygenated zone, and is controlled by OM reactivity and the AeOM rate constant. Resulting net diffusive CH4 fluxes can range between 0–234.7 mmol m-2 yr-1. Results show that even shallow sediments (<1 m) can sustain a significant CH4 release into the subglacial environment when highly reactive OM is available, while oxidation efficiency tends to decline in thick, OM-rich deposits. Comparison with field measurements of CH4 export data from southwest GrIS catchments suggests that observed fluxes could be already sustained by subglacial sediments that contain as little as 0.6 wt% of relatively unreactive OM assuming a catchment sediment cover of 10 % with sediment depths of 9 m.

Abstract. The microbial ecosystems that lie beneath ice sheets can impact and contribute to global biogeochemical cycles, yet remain poorly understood given the logistical challenges in directly accessing the subglacial environment. Studies instead often rely on indirect sampling of subglacial systems via the collection of meltwaters emerging from ice margins. However, the origin of exported material in these waters will change over a melt season as glacier hydrology responds to changes in surface melt. Here, we reveal trends in microbial sourcing (source environment) and assemblages in a large proglacial river in southwest Greenland by investigating three microbial datasets (16S rRNA) collected during different hydrological periods over three separate summer melt seasons. By combining microbial data with high-resolution hydrological and hydrochemical measurements, we show that changes in microbial assemblages follow changes in hydrological periods, likely influenced by variations in glacial drainage expansion inland with concomittant variations in inputs of surface melt and subglacial sediment exports. We further illustrate how relative changes in microbial assemblages can inform on the state of the glacial hydrological system, and also focus on methane-cycling populations to infer their potential distribution beneath the ice. Overall, our results highlight that timing matters when sampling proglacial rivers and we caution interpretations of exported assemblages without a good understanding of the catchment and system studied; this is especially true for larger systems which undergo more complex hydrological changes over a melt season.

Glacier recession is creating new water bodies in proglacial forelands worldwide, including Antarctica. Yet, it is unknown how microbial communities of recently formed "young" waterbodies (originating decades to a few centuries ago) compare with established "old" counterparts (millennia ago). Here, we compared benthic microbial communities of different lake types on James Ross Island, Antarctic Peninsula, using 16S rDNA metabarcoding and light microscopy to explore bacterial and diatom communities, respectively. We found that the older lakes host significantly more diverse bacterial and diatom communities compared to the young ones. To identify potential mechanisms for these differences, linear models and dbRDA analyses suggested combinations of water temperature, pH, and conductivity to be the most important factors for diversity and community structuring, while differences in geomorphological and hydrological stability, though more difficult to quantify, are likely also influential. These results, along with an indicator species analysis, suggest that physical and chemical constraints associated with individual lakes histories are likely more influential to the assembly of the benthic microbial communities than lake age alone. Collectively, these results improve our understanding of microbial community drivers in Antarctic freshwaters, and help predict how the microbial landscape may shift with future habitat creation within a changing environment.

Subglacial environments provide conditions suitable for the microbial production of methane, an important greenhouse gas, which can be released from beneath the ice as a result of glacial melting. High gaseous methane emissions have recently been discovered at Russell Glacier, an outlet of the southwestern margin of the Greenland Ice Sheet, acting not only as a potential climate amplifier but also as a substrate for methane consuming microorganisms. Here, we describe the composition of the microbial assemblage exported in meltwater from the methane release hotspot at Russell Glacier and its changes over the melt season and as it travels downstream. We found that a substantial part (relative abundance 27.2% across the whole dataset) of the exported assemblage was made up of methylotrophs and that the relative abundance of methylotrophs increased as the melt season progressed, likely due to the seasonal development of the glacial drainage system. The methylotrophs were dominated by representatives of type I methanotrophs from the Gammaproteobacteria; however, their relative abundance decreased with increasing distance from the ice margin at the expense of type II methanotrophs and/or methylotrophs from the Alphaproteobacteria and Betaproteobacteria. Our results show that subglacial methane release hotspot sites can be colonized by microorganisms that can potentially reduce methane emissions.

Recent studies have shown the release of methane (CH4) through the melting Greenland Ice Sheet, and have thus identified it to have an additional potential positive climate feedback. This CH4 is thought to originate from biologically active methanogenic ecosystems in subglacial sediments, where microbes produce it by converting overridden organic carbon to CH4, which then accumulates over time. Subsequent CH4 diffusion into the subglacial hydrologic network transports it then to the ice sheet margin, where it is directly emitted to the atmosphere from supersaturated proglacial streams. Methanogenesis is highly dependent on anoxic conditions, which are in turn determined by the seasonally evolving subglacial environment subject to episodic flooding and thereby recharging oxygenated waters from surface melting. The main biogeochemical and hydrological drivers influencing the rate of CH4 production, as well as the magnitude and timing of these subglacial CH4 fluxes remain largely unknown and therefore unconstrained. Addressing these unknowns is essential because CH4 is not only a powerful greenhouse gas, but also because its unaccounted release exacerbates the ongoing climate amplification in the Arctic. The lack of observational data is primarily due to the challenging conditions for accessing the subglacial environment and the shortage of direct measurements of CH4 production, consumption, and export from the Greenland Ice Sheet and the complex nature of the subglacial system. This invites the application of reaction-transport modelling tools in combination with observational data to fill these knowledge gaps by disentangling the complex processes and drivers, and eventually quantifying CH4 cycling processes in Greenland’s subglacial sediments and their impacts on the global CH4 cycle and climate change. However, such modelling tools do not currently exist. Here, we develop a coupled subglacial sediment-cavity-stream model to explore the potential of subglacial environments to produce and accumulate methane beneath the Greenland Ice shield. The model accounts for heterotrophic methane production, methane oxidation, as well as advective and diffusive methane transport. Current field data observations are used to initialize the model, but it will also be forced over a wide range of plausible conditions (i.e. organic matter availability and reactivity, sediment thickness, terminal electron acceptor availability) that have could be found beneath the Greenland Ice shield. The results of this large model ensemble does not only help identify the most important biogeochemical and hydrological drivers on methane production and accumulation in subglacial environments, but also allows to identify areas beneath the ice sheet that could produce and accumulate important quantities of methane.These new developments present the first step in the development of a new fully coupled hydrological-biogeochemical model for subglacial environments, which will inform upscaling efforts and guide future field work.

During past periods of advance, Arctic glaciers and ice sheets overrode soil, sediments, and vegetation and buried significant stores of organic matter (OM); these glaciers are now shrinking rapidly due to climate warming. Little is known about the biogeochemical processing of the OM buried beneath glacier ice which makes the processes associated with deglaciation difficult to predict. Subglacial sediments exposed at receding glacier fronts may represent a legacy of past biogeochemical processes. Here, we analyzed sediments from retreating fronts of 19 Arctic glaciers for their mineralogical and elemental composition, contents of major nutrients, OM biomarkers (aliphatic lipids and lignin‐derived phenols), 14 C age, and microbial community structure. We show the character of the sediments is mostly determined by local glaciation history and bedrock lithology. Most subglacial sediments offer high amounts of readily bioavailable phosphorus (i.e., loose, labile, and Fe/Al P fractions) but lack readily accessible carbon substrates. The subglacial OM originated mainly from overridden terrestrial vascular plants. The results of OM biomarker analysis and 14 C dating suggest the OM substrates degrade in the subglacial environment and are reworked by the resident microbial communities. We argue the biogeochemical legacy of the perishing subglacial environments is an important determinant for the early processes of proglacial ecological succession.

&lt;p&gt;Glaciers are considered to be a biome with diverse microbial life, and their meltwaters are highly influential to downstream ecosystems by creating a unique riverine habitat template and providing resources such as nutrients and organic matter. Yet, despite unprecedented rates of glacial retreat globally, not much is known about the fate of microbial cells exported from glaciers, despite their potential to colonize and reside in downstream ecosystems. The influence of glacial meltwater on these downstream ecosystems may persist far downstream, but other sources of nutrients, organic matter, and microbial cells within the hydrological catchment likely gain influence with distance from the glacier. These include soils and thawing permafrost - partly via eroding stream banks - and benthic stream biofilms residing both within and outside the glacial environment (e.g. in tributary streams).&lt;/p&gt;&lt;p&gt;In this work, we ask how suspended microbial assemblages change with increasing distance from the source glacier, especially in terms of their composition and corresponding with abiotic environmental factors. We hypothesize that OTU richness will increase with distance from source glaciers as the importance of other catchment sources increase. Specifically, we expect &amp;#8216;cryospheric&amp;#8217; OTUs to decrease in relative abundance, and more &amp;#8216;generalist&amp;#8217; freshwater OTUs to increase. We sampled five glacier-fed streams (3 in the Austrian Alps, 1 in Iceland and 1 in Greenland) from the glacier terminus until the ocean or major riverine outlet. DNA was extracted from samples, and 16s rRNA gene amplicons were sequenced to characterize the assemblage structure. These preliminary observations improve our knowledge of the fate of glacially-exported microbial assemblages, and help us to understand the extent of their potential impact for downstream ecosystems, especially in the current age of deglaciation.&lt;/p&gt;

Cryoconite is a mixture of mineral and organic material covering glacial ice, playing important roles in biogeochemical cycles and lowering the albedo of a glacier surface. Understanding the differences in structure of cryoconite across the globe can be important in recognizing past and future changes in supraglacial environments and ice-organisms-minerals interactions. Despite the worldwide distribution and over a century of studies, the basic characteristics of cryoconite, including its forms and geochemistry, remain poorly studied. The major purpose of our study is the presentation and description of morphological diversity, chemical and photoautotrophs composition, and organic matter content of cryoconite sampled from 33 polar and mountain glaciers around the globe. Observations revealed that cryoconite is represented by various morphologies including loose and granular forms. Granular cryoconite includes smooth, rounded, or irregularly shaped forms; with some having their surfaces covered by cyanobacteria filaments. The occurrence of granules increased with the organic matter content in cryoconite. Moreover, a major driver of cryoconite colouring was the concentration of organic matter and its interplay with minerals. The structure of cyanobacteria and algae communities in cryoconite differs between glaciers, but representatives of cyanobacteria families Pseudanabaenaceae and Phormidiaceae, and algae families Mesotaeniaceae and Ulotrichaceae were the most common. The most of detected cyanobacterial taxa are known to produce polymeric substances (EPS) that may cement granules. Organic matter content in cryoconite varied between glaciers, ranging from 1% to 38%. The geochemistry of all the investigated samples reflected local sediment sources, except of highly concentrated Pb and Hg in cryoconite collected from European glaciers near industrialized regions, corroborating cryoconite as element-specific collector and potential environmental indicator of anthropogenic activity. Our work supports a notion that cryoconite may be more than just simple sediment and instead exhibits complex structure with relevance for biodiversity and the functioning of glacial ecosystems.

Glacial meltwater drains into proglacial rivers where it interacts with the surrounding landscape, collecting microbial cells as it travels downstream. Characterizing the composition of the resulting microbial assemblages in transport can inform us about intra-annual changes in meltwater flowpaths beneath the glacier as well as hydrological connectivity with proglacial areas. Here, we investigated how the structure of suspended microbial assemblages evolves over the course of a melt season for three proglacial catchments of the Greenland Ice Sheet (GrIS), reasoning that differences in glacier size and the proportion of glacierized versus non-glacierized catchment areas will influence both the identity and relative abundance of microbial taxa in transport. Streamwater samples were taken at the same time each day over a period of 3 weeks (summer 2018) to identify temporal patterns in microbial assemblages for three outlet glaciers of the GrIS, which differed in glacier size (smallest to largest; Russell, Leverett, and Isunnguata Sermia [IS]) and their glacierized: proglacial catchment area ratio (Leverett, 76; Isunnguata Sermia, 25; Russell, 2). DNA was extracted from samples, and 16S rRNA gene amplicons sequenced to characterize the structure of assemblages. We found that microbial diversity was significantly greater in Isunnguata Sermia and Russell Glacier rivers compared to Leverett Glacier, the latter of which having the smallest relative proglacial catchment area. Furthermore, the microbial diversity of the former two catchments continued to increase over monitored period, presumably due to increasing hydrologic connectivity with proglacial habitats. Meanwhile, diversity decreased over the monitored period in Leverett, which may have resulted from the evolution of an efficient subglacial drainage system. Linear discriminant analysis further revealed that bacteria characteristic to soils were disproportionately represented in the Isunnguata Sermia river, while putative methylotrophs were disproportionately abundant in Russell Glacier. Meanwhile, taxa typical for glacierized habitats (i.e., Rhodoferax and Polaromonas) dominated in the Leverett Glacier river. Our findings suggest that the proportion of deglaciated catchment area is more influential to suspended microbial assemblage structure than absolute glacier size, and improve our understanding of hydrological flowpaths, particulate entrainment, and transport.

&lt;p&gt;Glaciers and ice sheets cover around 10% of the Earth&amp;#8217;s surface and the Greenland Ice Sheet (GrIS) is the largest ice mass in the Northern hemisphere, but is melting at an increasing rate, losing ~400 km&lt;sup&gt;3&lt;/sup&gt; annually. There have been recent studies linking subglacial environments of the GrIS with methane (CH&lt;sub&gt;4&lt;/sub&gt;) production and release, presenting a possible positive climate feedback. Previous work has linked organic carbon in subglacial environments with significant CH&lt;sub&gt;4&lt;/sub&gt; export via methanogenesis. It has been hypothesised that the GrIS overlies a methanogenically active wetland environment, and thus needs to be included in the global CH&lt;sub&gt;4&lt;/sub&gt; budget.&lt;/p&gt;&lt;p&gt;However, the subglacial system of the GrIS is complex and highly heterogenous, hosting oxic and anoxic ecosystems, which have developed over a range of timescales. There are still questions outstanding surrounding the ubiquity of CH&lt;sub&gt;4&lt;/sub&gt; release from the GrIS, mainly because of the limited understanding of subglacial carbon cycling and the potential sources of CH&lt;sub&gt;4&lt;/sub&gt; in these environments. &amp;#160;&lt;/p&gt;&lt;p&gt;We present the first data from two new, complimentary projects investigating CH&lt;sub&gt;4&lt;/sub&gt; release from the GrIS margin, where we aim to quantify the production and release of CH&lt;sub&gt;4&lt;/sub&gt; into the atmosphere from the GrIS. We have developed an ambitious temporal and spatial sampling regime to evaluate the CH&lt;sub&gt;4&lt;/sub&gt; release along the western margin of the GrIS. We present the first radiocarbon (&lt;sup&gt;14&lt;/sup&gt;C) dated CH&lt;sub&gt;4&lt;/sub&gt; samples from Greenland, helping to shed light on the carbon cycling processes occurring under the ice sheet. We analyse a mixture of atmospheric CH&lt;sub&gt;4&lt;/sub&gt; exported from subglacial ice caves and dissolved CH&lt;sub&gt;4&lt;/sub&gt; from proglacial rivers draining subglacial portals to explore the age of subglacially sourced CH&lt;sub&gt;4&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;We can combine the carbon age of exported CH&lt;sub&gt;4&lt;/sub&gt; with microbial analysis and stable isotope data to improve our understanding of the environmental controls on and microbial sources of subglacial CH&lt;sub&gt;4&lt;/sub&gt; production and export. Understanding the mechanisms behind subglacial CH&lt;sub&gt;4&lt;/sub&gt; export is crucial when attempting to upscale the point source data that is available currently and we consider whether the GrIS could be a potentially important source of CH&lt;sub&gt;4&lt;/sub&gt;, leading to a substantial, yet currently understudied climatic feedback.&lt;/p&gt;

The Greenland Ice Sheet is currently not accounted for in Arctic mercury budgets, despite large and increasing annual runoff to the ocean and the socio-economic concerns of high mercury levels in Arctic organisms. Here we present concentrations of mercury in meltwaters from three glacial catchments on the southwestern margin of the Greenland Ice Sheet and evaluate the export of mercury to downstream fjords based on samples collected during summer ablation seasons. We show that concentrations of dissolved mercury are among the highest recorded in natural waters and mercury yields from these glacial catchments (521–3,300 mmol km −2 year −1 ) are two orders of magnitude higher than from Arctic rivers (4–20 mmol km −2 year −1 ). Fluxes of dissolved mercury from the southwestern region of Greenland are estimated to be globally significant (15.4–212 kmol year −1 ), accounting for about 10% of the estimated global riverine flux, and include export of bioaccumulating methylmercury (0.31–1.97 kmol year −1 ). High dissolved mercury concentrations (~20 pM inorganic mercury and ~2 pM methylmercury) were found to persist across salinity gradients of fjords. Mean particulate mercury concentrations were among the highest recorded in the literature (~51,000 pM), and dissolved mercury concentrations in runoff exceed reported surface snow and ice values. These results suggest a geological source of mercury at the ice sheet bed. The high concentrations of mercury and its large export to the downstream fjords have important implications for Arctic ecosystems, highlighting an urgent need to better understand mercury dynamics in ice sheet runoff under global warming.

The Greenland Ice Sheet harbours a wealth of microbial life, yet the total biomass stored or exported from its surface to downstream environments is unconstrained. Here, we quantify microbial abundance and cellular biomass flux within the near-surface weathering crust photic zone of the western sector of the ice sheet. Using groundwater techniques, we demonstrate that interstitial water flow is slow (~10 −2 m d −1 ), while flow cytometry enumeration reveals this pathway delivers 5 × 10 8 cells m −2 d −1 to supraglacial streams, equivalent to a carbon flux up to 250 g km −2 d −1 . We infer that cellular carbon accumulation in the weathering crust exceeds fluvial export, promoting biomass sequestration, enhanced carbon cycling, and biological albedo reduction. We estimate that up to 37 kg km −2 of cellular carbon is flushed from the weathering crust environment of the western Greenland Ice Sheet each summer, providing an appreciable flux to support heterotrophs and methanogenesis at the bed.