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The role of subglacial abrasion in sustaining subglacial microbiomes is an area of growing interest. It is likely that subglacial abrasion produces a range of bioutilisable compounds, some released from fluid inclusions in the confines of the mineral matrix and some produced via the surface free radicals that crushing produces in a variety of different minerals. As yet, the geochemical reactions which are initiated by abrasion remain poorly understood. This is largely because of the multiple potential sources and sinks of free radicals that occur in even the most simple of multi-mineralic system during crushing and subsequent wetting of the mixture. We illustrate this assertion by conducting a series of laboratory experiments where common, "simple" silicate minerals, such as quartz and feldspar, are abraded and incubated with ultra-pure water. Gaseous reaction products included CH4, H2 and CO2. The concentrations of major ions (including Na+, K+, Mg2+, and Ca2+), dissolved silicate, dissolved iron and pH of the solution during incubation are reported. These results were used to calculate HCO3- in solution and the saturation index with respect to various minerals using PHREEQCi. We found that abrasion of silicates produces bio-available gases and has the potential to increase the concentration of various weathering products in solution. Further, we found that even trace amounts of carbonates, an impurity in many natural rocks and minerals, may significantly affect the reaction products, making it challenging to concretize the reactions taking place. These results highlight the importance of conducting abrasion experiments utilising natural samples to better understand how apparently negligeable changes in mineralogy might affect the reaction products of abrasion and crushing. [ABSTRACT FROM AUTHOR]

The role of subglacial abrasion in sustaining subglacial microbiomes is an area of growing interest. It is likely that subglacial abrasion produces a range of bio-utilisable compounds, some released from fluid inclusions in the confines of the mineral matrix and some produced via the surface free radicals that crushing produces in a variety of different minerals. As yet, the geochemical reactions which are initiated by abrasion remain poorly understood. This is largely because of the multiple potential sources and sinks of free radicals that occur in even the most simple of multi-mineralic system during crushing and subsequent wetting of the mixture. We illustrate this assertion by conducting a series of laboratory experiments where common, “simple” silicate minerals, such as quartz and feldspar, are abraded and incubated with ultra-pure water. Gaseous reaction products included CH4, H2 and CO2. The concentrations of major ions (including Na+, K+, Mg2+, and Ca2+), dissolved silicate, dissolved iron and pH of the solution during incubation are reported. These results were used to calculate HCO3− in solution and the saturation index with respect to various minerals using PHREEQCi. We found that abrasion of silicates produces bio-available gases and has the potential to increase the concentration of various weathering products in solution. Further, we found that even trace amounts of carbonates, an impurity in many natural rocks and minerals, may significantly affect the reaction products, making it challenging to concretize the reactions taking place. These results highlight the importance of conducting abrasion experiments utilising natural samples to better understand how apparently negligeable changes in mineralogy might affect the reaction products of abrasion and crushing.

We present a distributed acoustic sensing (DAS) experiment at Grímsvötn, Iceland. This is intended to investigate volcano-microseismicity at Grímsvötn specifically, and to assess the suitability of DAS as a subglacial volcano monitoring tool in general. In spring 2021, we trenched a 12 km long fiber-optic cable into the ice sheet around and within the caldera, followed by nearly one month of continuous recording. An image processing algorithm that exploits spatial coherence in DAS data detects on average ~100 events per day, almost 2 orders of magnitude more than in the regional earthquake catalog. A nonlinear Bayesian inversion reveals the presence of pronounced seismicity clusters, containing events with magnitudes between −3.4 and 1.7. Their close proximity to surface volcanic features suggests a geothermal origin. In addition to painting a fine-scale picture of seismic activity at Grímsvötn, this work confirms the potential of DAS in subglacial volcano monitoring.

Little is known about Antarctic subglacial hydrology, but based on modeling, theory and indirect observations it is thought that subglacial runoff enhances submarine melt locally through buoyancy effects. However, no studies to date have examined effects of runoff on sea ice and oceanography on the continental shelf. Here we use modeled and observational estimates of runoff to force a regional model of the Amundsen Sea Embayment. We find that runoff enhances melt locally (i.e., within the ice‐shelf cavity), increasing melt at Thwaites ice shelf by up to 15 Gt/a given estimates of steady runoff, and up to 25 Gt/a if runoff is episodic as remote sensing measurements suggest. However runoff also has smaller non‐local effects on melt through freshwater influence on flow and stratification. We further find that runoff reduces summer sea‐ice volume over the continental shelf (by up to 10% with steady runoff but over 30% with episodic runoff). Furthermore runoff is much more effective at reducing sea ice than an equivalent volume of ice‐shelf meltwater—due in part to the latent heat loss associated with submarine melting. Results suggest that runoff may play an important role in continental shelf dynamics, despite runoff flux being small relative to ice‐shelf melting—and that runoff‐driven melt and circulation may be an important process missing from regional Antarctic ocean models. Plain Language Summary: A number of floating ice shelves in Antarctica are exposed to warm waters which lead to strong melting, limiting shelves' ability to buttress against ice flow from the continent and also bringing warmed and freshened waters to the ocean surface—waters which in turn influence seasonal sea ice near the continent and insulate the deep ocean from the cold atmosphere. Meanwhile, large rivers under the Antarctic ice sheet carry melt from the ice sheet base to the ocean cavities beneath ice shelves. While a few studies have looked at the role these rivers play in ice‐shelf melt, they do not examine the effects on ocean properties and sea ice in the open ocean. We find that these "rivers" not only amplify melting under ice shelves, but have far‐reaching impacts on ocean circulation. We find that while the direct contribution of water from these subglacial rivers is small compared to that of ice‐shelf melt, there is a disproportionate impact on seasonal sea ice. Key Points: Modeled and observed subglacial runoff estimates are used to force a regional model of the Amundsen Sea EmbaymentRunoff influences melt rates regionally as well as locally, and leads to reduction of summer sea ice volume on the continental shelfRunoff impacts on sea ice differ qualitatively from those of an equivalent volume of ice‐shelf melting [ABSTRACT FROM AUTHOR]

The size distributions and the shapes of detrital rock clasts can shed light on the environmental history of the clast assemblages and the processes responsible for clast comminution. For example, mechanical fracture due to the stresses imposed on a basal rock surface by a body of flowing glacial ice releases initial 'parent' shapes of large blocks of rock from outcrop, which then are modified by the mechanics of abrasion and fracture during subglacial transport. The latter processes produce subsequent generations of shapes, possibly distinct in form from the parent blocks. Lacking is a complete understanding of both the processes responsible for block shape changes and the trends in shape adjustment with time and distance away from the source outcrop. Field data on edge rounding and shape changes of Shap granite blocks (dispersed by Devensian ice eastwards from outcrop) are used herein to explore the systematic changes in block form with distance from the outcrop. The degree of edge rounding for individual blocks increases in a punctuated fashion with the distance from the outcrop as blocks fracture repeatedly to introduce new fresh unrounded edges. In contrast, block shape is conservative, with parent blocks fracturing to produce self-similar 'child' shapes with distance. Measured block shapes evolve in accord with two well-known models for block fracture mechanics - 1) stochastic and 2) silver ratio models - towards one or other of these two attractor states. Progressive reduction in block size, in accord with fracture mechanics, reflects the fact that most blocks were transported at the sole of the ice mass and were subject to the compressive and tensile forces of the ice acting on the stoss surfaces of blocks lying against a bedrock or till surface. The interpretations might apply to a range of homogeneous hard rock lithologies. [ABSTRACT FROM AUTHOR]

Initially a matter of intellectual curiosity, but now important for understanding ice‐sheet dynamics, the formation of subglacial bedforms has been a subject of scientific enquiry for over a century. Here, we use a numerical model of the coupled flow of ice, water, and subglacial sediment to explore the formation of subglacial ribs (i.e., ribbed moraine), drumlins and mega‐scale glacial lineations (MSGLs). The model produces instabilities at the ice–bed interface, which result in landforms resembling subglacial ribs and drumlins. We find that a behavioural trajectory is present. Initially subglacial ribs form, which can either develop into fields of organized drumlins, or herringbone‐type structures misaligned with ice flow. We present potential examples of these misaligned bedforms in deglaciated landscapes, the presence of which means caution should be taken when interpreting cross‐cutting bedforms to reconstruct ice flow directions. Under unvarying ice flow parameters, MSGLs failed to appear in our experiments. However, drumlin fields can elongate into MSGLs in our model if low ice–bed coupling conditions are imposed. The conditions under which drumlins elongate into MSGLs are analogous to those found beneath contemporary ice streams, providing the first mechanism, rather than just an association, for linking MSGLs with ice stream flow. We conclude that the instability theory, as realized in this numerical model, is sufficient to explain the fundamental mechanics and process‐interactions that lead to the initiation of subglacial bedforms, the development of the distinctive types of bedform patterns, and their evolutionary trajectories. We therefore suggest that the first part of the longstanding 'drumlin problem' – how and why they come into existence – is now solved. However, much remains to be discovered regarding the exact sedimentary and hydrological processes involved. [ABSTRACT FROM AUTHOR]

Monitoring the activity of subglacial volcanoes along the Aleutian Arc in Alaska is important to the safety of local populations, as well as air traffic flying through the region. However, observations of volcanic unrest are limited by accessibility and resources, particularly at glacier-covered systems, making investigations of their stability challenging. Westdahl Peak, a subglacial volcano on Unimak Island in the Aleutian Arc has experienced significant unrest and uplift since its most recent VEI three eruption in 1991-1992. Given the magnitude of observed uplift, previous investigations suggested the potential for eruption by 2010, but no such event has occurred. One hypothesis to explain this prolonged unrest is that the 1-km thick glacier may increase the stability of the magma system. However, the impact of ice caps and glaciers on the short-term stability of volcanoes is not well understood. In this study, thermomechanical finite element models are used to evaluate how the stability of a glaciated volcano is impacted by variations in ice cap thickness, magma chamber depth, geometry, magma flux rate, and seasonal changes in ice cover thickness. Our numerical experiments indicate that the presence of an ice cap (1–3 km thick) increases the average repose interval for a magma system. Among models with different magma chamber geometries, depths, and flux rates, the greatest increases in repose interval are observed in prolate systems where the increase is up to 57% for a chamber located at 5 km-depth. Spherical and oblate also experience smaller, yet significant, increases in repose interval. Additionally, the percentage increase in repose interval is not impacted by variations in magma flux rate for a given ice cap thickness and magma chamber geometry. However, flux rates do influence the timing of eruptions when the system is experiencing seasonal variations in ice thickness. Our results show that systems with low flux rates are more likely to fail when the ice thickness is at its lowest. The numerical estimates further suggest that the ice cap on Westdahl Peak, which is ∼1 km, may slightly increase the stability of the magma system. In general, given flux rates and magma chamber geometries estimated for the Westdahl system, the repose interval can increase by ∼7 years due to the Westdahl glacier. This increase is small on a geologic scale but is significant on human time scales and the impact of glaciers must be considered in future forecasting efforts.

The 10th century Eldgjá fissure eruption is the largest in Iceland in historical time. It erupted 21.0 km3 of magma, with 1.3 km3 as tephra in at least 16 explosive episodes from subaerial and subglacial vents, producing magmatic and phreatomagmatic deposits respectively. Grain-size distributions for these end-members show distinct differences at comparable distances from source: the former are coarser and unimodal; the latter are finer and bimodal. These distributions appear to record different primary fragmentation histories. In contrast, the vesicle-size distributions of pyroclasts from each type of deposit show the pyroclasts underwent similar vesicle nucleation and growth prior to fragmentation. This indicates that the role of glacial water was comparatively late-stage, re-fragmenting an already disrupting magma by quench granulation. The presence of microlite-rich domains within clasts reveals a history of complex conduit evolution, during the transition from a continuous dyke to focussed, discrete vents.

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Meltwater inputs to moulins regulate Greenland Ice Sheet sliding speeds by controlling water pressure in the most connected regions of the subglacial drainage system. While moulin storage capacities are a critical control on subglacial water pressure, few observations exist to constrain storage. Using direct observations inside moulins, we show that moulin cross‐sectional areas can be at least 500 m2, far greater than is observed at the surface or assumed in models. Moulin water level measurements and numerical modeling reveal that diurnal variability in moulin water pressure is highly attenuated in moulins with large storage volumes (∼3% ice pressure), relative to moulins with smaller storage volumes (∼25% ice pressure). Because large variability in moulin water pressure is linked to processes that ultimately reduce ice sliding speeds, ice sliding speeds in areas drained by large moulins may be more sensitive to long‐term increases in meltwater than areas drained by small moulins. Plain Language Summary: Each summer season on the Greenland Ice Sheet, meltwater forms stream networks on the ice surface that deliver water to moulins, which are holes in the ice that carry the water to the base of the ice sheet. When water backs up into moulins, and the water pressure beneath the ice increases, glacier sliding accelerates, leading to more rapid loss of ice into the ocean. We directly explored two moulins that contained immense storage volumes that are much larger than previously assumed to exist. Our observations of water levels inside moulins, and a model of water flow through the ice, indicate that storage of water within these large moulins during daily meltwater pulses has a big impact on how much the water pressure beneath the ice changes. Our work suggests that moulin sizes influence the interactions between summer melt and sliding of the Greenland Ice Sheet. Consequently, we need a more complete understanding of how moulin volumes vary in order to better predict how future increases in melt will impact the rate of ice loss from Greenland and to constrain its future contribution to sea level rise. Key Points: Moulin volumes on the Greenland Ice Sheet are much larger than previously thoughtAmplitudes of diurnal variation in moulin water level are negatively correlated with moulin volumeIce sliding speeds in areas drained by large moulins may be more sensitive to long‐term melt increases [ABSTRACT FROM AUTHOR]

Roughly 10% of the Earth's surface is permanently covered by glaciers and ice sheets and in mountain ecosystems, this proportion of ice cover is often even higher. From an ecological perspective, ice-dominated ecosystems place harsh controls on life including cold temperature, limited nutrient availability, and often prolonged darkness due to snow cover for much of the year. Despite these limitations, glaciers, and perennial snowfields support diverse, primarily microbial communities, though macroinvertebrates and vertebrates are also present. The availability and mass balance of key elements [(carbon (C), nitrogen (N), phosphorous (P)] are known to influence the population dynamics of organisms, and ultimately shape the structure and function of ecosystems worldwide. While considerable attention has been devoted to patterns of biodiversity in mountain cryosphere-influenced ecosystems, the ecological stoichiometry of these habitats has received much less attention. Understanding this emerging research arena is particularly pressing in light of the rapid recession of glaciers and perennial snowfields worldwide. In this review, we synthesize existing knowledge of ecological stoichiometry, nutrient availability, and food webs in the mountain cryosphere (specifically glaciers and perennial snowfields). We use this synthesis to develop more general understanding of nutrient origins, distributions, and trophic interactions in these imperiled ecosystems. We focus our efforts on three major habitats: glacier surfaces (supraglacial), the area beneath glaciers (subglacial), and adjacent downstream habitats (i.e., glacier-fed streams and lakes). We compare nutrient availability in these habitats to comparable habitats on continental ice sheets (e.g., Greenland and Antarctica) and show that, in general, nutrient levels are substantially different between the two. We also discuss how ongoing climate warming will alter nutrient and trophic dynamics in mountain glacier-influenced ecosystems. We conclude by highlighting the pressing need for studies to understand spatial and temporal stoichiometric variation in the mountain cryosphere, ideally with direct comparisons to continental ice sheets, before these imperiled habitats vanish completely.

Subglacial ecosystems harbor diverse chemoautotrophic microbial communities in areas with limited organic carbon, and lithological H2 produced during glacial erosion has been considered an important energy source in these ecosystems. To verify the H2-utilizing potential there and to identify the related energy-converting metabolic mechanisms of these communities, we performed metagenomic analysis on subglacial sediment samples from East Antarctica with and without H2 supplementation. Genes coding for several [NiFe]-hydrogenases were identified in raw sediment and were enriched after H2 incubation. All genes in the dissimilatory nitrate reduction and denitrification pathways were detected in the subglacial community, and the genes coding for these pathways became enriched after H2 was supplied. Similarly, genes transcribing key enzymes in the Calvin cycle were detected in raw sediment and were also enriched. Moreover, key genes involved in H2 oxidization, nitrate reduction, oxidative phosphorylation, and the Calvin cycle were identified within one metagenome-assembled genome belonging to a Polaromonas sp. As suggested by our results, the microbial community in the subglacial environment we investigated consisted of chemoautotrophic populations supported by H2 oxidation. These results further confirm the importance of H2 in the cryosphere. [ABSTRACT FROM AUTHOR]

The thermal structures of eruption-fed, subglacial volcaniclastic deposits are poorly understood because their emplacement is hazardous to observe or obscured by the glacier. Determining deposit emplacement temperature, however, supports improved understanding of the flow dynamics and emplacement processes of volcaniclastic material beneath a glacier. Understanding the emplacement temperatures of ancient volcanic deposits is also important because they can be used, in combination with field studies, to infer the eruptive environment. Here, we use paleomagnetic techniques to quantify the emplacement temperatures of two ancient, proximal, eruption-fed density current deposits at Tongariro volcano, New Zealand. Stepwise thermal demagnetisation of lithic and recycled juvenile block-sized clasts reveal randomly orientated directions of magnetisation, suggesting that the clasts were rotated within the flow but not heated. Additional data from thermomagnetic, hysteresis, and isothermal remanent magnetisation tests indicate that the principal carrier of magnetic remanence is magnetite, and that the magnetisation directions are a primary remanence rather than post-depositional chemical remanent magnetisations. Following systematic removal of any viscous remanent magnetisation, the post-emplacement equilibrium temperatures for the deposits can be estimated at <150 °C. The paleomagnetic data support field evidence for rapid cooling of clasts and waterlain deposition. The deposit-forming eruptions took place beneath a summit glacier where the freshly erupted tephra was efficiently cooled by mixing with meltwater. Lithic blocks and recycled juvenile bombs were entrained and remobilised within the cool currents that drained along meltwater channels beneath the ice. This is the first study in which paleomagnetic data have been used to determine the equilibrium temperatures of subglacial density current deposits. The data provide new insight into volcaniclastic flow dynamics beneath a glacier and current-meltwater interactions. • Eruption-fed, volcaniclastic flow deposits emplaced at <150 °C. • Magnetic stability tests rule out chemical remanence magnetisation. • Deposition from tephra-laden meltwater currents that drained beneath a glacier. • First use of paleomagnetic techniques for glaciovolcanic deposits. [ABSTRACT FROM AUTHOR]

Investigations of glacial sediment exposures in a former open pit in the Pine Point Mining District, Northwest Territories, reveal evidence of changes in stress magnitude as exhibited by the movement of deformation 'fronts' within subglacial tills, as evident through micromorphological analysis. Indications of deformation, driven by stress fluctuations, were observed throughout a set of vertical subglacial till samples associated with the Laurentide Ice Sheet. These tills exhibit multiple sub‐lithofacies variations in concert with marked changes in microshear orientation, which can be directly related to changing stress levels within a subglacial deforming bed. The evidence of changes in till stress conditions is typical of subglacial tills occurring beneath an active temperate ice mass within a soft deforming bed. As differing 'layers' of till that form a mosaic‐like pattern become immobilized and 'detached' subjacent to the deforming bed sediments, a signature of this action is recorded within the tills as sequences of specific microstructures. These microstructures are part of a spectrum of micro‐forms that develop as consequence of changes in stress, porewater content and clay content within the 'fault‐gouge'—like environment formed beneath an active temperate ice sheet. A model is developed that uses data from Pit O‐28 demonstrating where deformation 'boundary front' evidence in the form of deformation bands has developed as basal driven stress levels have penetrated active deforming sediments. The evidence from Pine Point indicates that till acts as an excellent palaeo‐strain record of past active glacial processes, subglacial basal thermal conditions and sediment mobility. [ABSTRACT FROM AUTHOR]

Abstract While the supraglacial hydrology of debris-covered glaciers is relatively well studied, almost nothing is known about how water is transported beneath the glacier surface. Here, we report the results of sixteen fluorescent dye tracing experiments conducted in April–May 2018 over the lowermost 7 km of the high-elevation, debris-covered Khumbu Glacier, Nepal, to characterise the glacier's surface and subsurface drainage system. Dye breakthroughs indicated a likely highly sinuous and channelised subsurface hydrological system draining water from the upper part of the ablation area. This flowpath was distinct from the linked chain of supraglacial ponds present along much of the glacier's lower ablation area, through which water flow was extremely slow (∼0.003 m s−1), likely reflecting the study's timing during the pre-monsoon period. Subsurface drainage pathways emerged at the glacier surface close to the terminus, and flowed into small near-surface englacial reservoirs that typically delayed meltwater transit by several hours. We observed rapid pathway changes resulting from surface collapse, indicating a further distinctive aspect of the drainage of debris-covered glaciers. We conclude that the surface and subsurface drainage of Khumbu Glacier is both distinctive and dynamic, and argue that further investigation is needed to refine the characterisation and test its regional applicability to better understand future Himalayan debris-covered glacier meltwater delivery to downstream areas. Highlights • Dye tracing used to investigate hydrology of debris-covered Khumbu Glacier, Nepal. • A subsurface drainage system exists, and is likely highly sinuous and channelised. • The subsurface drainage system does not link to the chain of supraglacial ponds. • Flow through the pond chain is slow and comprises a small part of total discharge. • Rapid pathway changes are observed, resulting from variable surface lowering. [ABSTRACT FROM AUTHOR]

Abstract Ice-contact deposits emplaced by Late Paleozoic Ice Age (LPIA) glaciers are rarely exposed due to a low degree of preservation and their capping by thick glaciomarine deglaciation sequences. In this paper, we present new data on glacial cyclicity, relative age constraints and paleoice flow interpretations for an ice-contact succession of the lowermost Itararé Group in eastern Paraná Basin (Brazil). The 80 m-thick ice-contact complex rests over the Itararé Group basal unconformity and comprises four stacked informal stratigraphic units regarded as the depositional/deformational record of three ice lobes advance/retreat cycles. Glacial cycle 1 comprises a sheet of massive diamictite resting on striated pavements carved on Devonian sandstones and topped by grooved/fluted surfaces, which indicate subglacial emplacement from a northward flowing ice lobe. The glacial cycle 2 succession was deposited in a proglacial to ice-marginal marine setting and subsequently deformed due to minor ice margin fluctuations of a westward/southwestward flowing grounded ice lobe. Glacial cycle 3 is interpreted as a thick overridden push moraine composed of grounding-line fan deposits capped by a subglacial diamictite emplaced by grounded ice advancing northwestward. A middle Visean-early Serpukhovian palynomorph assemblage was recorded in cycle two deposits, representing the oldest late Paleozoic glacial rocks in the Paraná Basin. The ice-contact complex elucidates multiple waxing and waning phases of the Windhoek Ice Sheet onto the eastern margin of the Paraná Basin during the Mississippian-Early Pennsylvanian, demonstrating that ice sheets of the early stages of the LPIA were also present in the Brazilian-African domain of southwestern Gondwana. Highlights • 3 glacial cycles with variable ice lobe kinematics are defined for an ice-contact complex in the lowermost Itararé Group. • The basal age of the Itararé Group is older than previously interpreted ranging from the middle Visean. • Ice sheets covered the west-central domain of Gondwana during the Mississippian. [ABSTRACT FROM AUTHOR]

Eskers are long sedimentary ridges that are widespread in current and previously glaciated landscapes. Eskers are believed to form when sediment carried by glacial meltwater gets deposited in subglacial tunnels, which given the importance of subglacial water for ice dynamics means that eskers can provide important information about the shape and dynamics of ice sheets and glaciers. To date, however, no physical model for their formation has been developed. Beaud et al. (2018, https://doi.org/10.1029/2018JF004779) present the first such model; they have linked an existing model for subglacial water flow with one for sediment transport, erosion, and deposition in mixed sediment/bedrock streams. The results of the model add new insights into the conditions which promote esker formation, the rate of supply of sediment to proglacial environments, and also suggest possible links between the water flow regime and the likely esker morphology. Plain Language Summary: This commentary discusses the paper by Beaud et al. (2018) which presents the first numerical, physically based model for esker formation. Eskers are long sedimentary ridges that are widespread in current and previously glaciated landscapes. They are believed to form within subglacial tunnels; this commentary discusses the importance of the configuration of the "plumbing system" of glaciers and ice sheets for their flow patterns and highlights how the model could change how esker deposits are interpreted and how the model results could be used to help infer subglacial water flow characteristics from eskers. Key Points: A new model for esker formation by Beaud et al. (2018) links sediment processes with subglacial tunnel dynamicsThe model shows sediment deposition as eskers is caused by a bottleneck in sediment transport near the snout of glaciersThe model develops realistic sedimentation patterns and will allow better understanding of the links between esker formation and ice dynamics [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract Earth has experienced periodic local to global glaciation for nearly 3 billion years, providing supra- and subglacial environments for colonization by microbial communities. A number of studies have reported on the role of microbial communities in glacial ecosystems including their influence on element cycling and weathering, but there is a paucity data on volcanic rock-hosted glacial ecosystems. Glaciers on stratovolcanoes in the Pacific Northwest override silica-rich rocks which represent analogues to an early Martian cryosphere. On these glaciers, blooms of photosynthetic snow algae support supraglacial microbial communities as has been observed on snowfields, glaciers, and ice sheets. In subglacial environments of volcanic rock-hosted glacial systems, weathering is driven, at least in part, by carbonic acid, suggesting a link between supraglacial carbon sources and subglacial heterotrophic microbial communities. Here, we report inorganic carbon assimilation and microbial community composition on glaciers across three stratovolcanoes ranging in composition from dacitic to mafic in the Pacific Northwest of the United States to begin to constrain the role of supraglacial primary productivity in subglacial weather processes. These data, coupled to contextual carbon and nitrogen isotope analyses of biomass and aqueous geochemistry, indicate snow algae drive light dependent carbon uptake across supraglacial and periglacial environments. Furthermore, snow algae microbial communities are supported by fixed nitrogen predominantly from deposition via precipitation. Our data highlight intense cycling of carbon and nitrogen driven by supraglacial microbial communities that feeds subglacial microbial communities which in turn may drive weathering processes. These results further underscore the role of glacial ecosystems in global biogeochemical cycling, especially during past global glaciations. Finally, these results lend support for glaciers as refugia for biodiversity on Earth and potentially on other bodies such as Mars where evidence exists for widespread and long-lived cryosphere including glaciers and ice sheets. [ABSTRACT FROM AUTHOR]

Radar‐sounding surveys measuring ice thickness in Greenland have enabled an increasingly "complete" knowledge of basal topography and glaciological processes. Where such observations are spatially limited, bed elevation has been interpolated through mass conservation or kriging. Ordinary kriging fails to resolve anisotropy in bed geometry, however, leaving complex topography misrepresented in elevation models of the ice sheet bed. Here, we demonstrate the potential of new high‐resolution (≤5 m) surface topography data (ArcticDEM) to provide enhanced insight into basal and englacial geometry and processes. Notable surface features, quantified via residual surface elevation, are observed coincident with documented subglacial channels, and new, smaller‐scale tributaries (<2,000 m in width) and valley‐like structures are clearly identified. Residual surface elevation also allows the extent of basal ice units to be mapped, which in conjunction with radar data indicate that they act as "false bottoms," likely due to a rheological contrast in the ice column. Plain Language Summary: Newly available high‐resolution (≤5 m) surface elevation maps of the Greenland Ice Sheet present a useful lens for examining and exploring the subglacial landscape, as well as the physical processes operating subglacially and englacially. Where radar data, which measure the subsurface geometry and properties, are sparse, we show how surface expressions can be used to enable effective interpolation between sets of bed data, allowing enhanced appreciation of ice sheet processes and boundary conditions. We also show that a subset of surface expressions originate from englacial slip surfaces or "false bottoms," allowing the extrapolation of the 2‐D areal/horizontal extent of these features, not previously possible from radar flight lines alone. Key Points: High‐resolution surface topography provides spatially coherent information on specific subglacial geometry and basal processesBasal ice units revealed by a marked expression in the ice sheet surface act as "false bottoms" and englacial slip surfacesNew clarity identifying complex, small‐scale, and anisotropic basal features where radar data are sparse [ABSTRACT FROM AUTHOR]

The temporal variations in electrical conductivity and the stable isotopes of water, δD and δ18O, were examined at Chhota Shigri Glacier, India, to understand water sources and flow paths to discharge. Discharge is highly influenced by supraglacially derived meltwater during peak ablation, and subglacial meltwaters are more prominent at the end of the melt season. The slope of the best fit linear regression line for δD versus δ18O, for both supraglacial and runoff water, is lower than that for precipitation (snow and rain) and surface ice, indicating strong isotopic fractionation associated with the melting processes. The slope of the local meteoric water line (LMWL) is close to that of the global meteoric water line (GMWL), reflecting that the moisture source is predominantly oceanic. The d-excess variation in rainwater confirms that the southwest monsoon is the main contributor during summer while the remainder including winter is mostly influenced by westerlies. [ABSTRACT FROM AUTHOR]

The geomorphology of the hidden subglacial landscape of Antarctica is relevant to our understanding of the stability of the Antarctic Ice Sheet and also to that of global interactions between plate tectonics and surface processes. We believe that geomorphology has much to contribute, but that the lack of coherent hypotheses about the origins of the subglacial landscape is holding back understanding. This paper approaches the problem by using southern hemisphere land masses in Africa and Madagascar as analogues. We find that the Antarctic landscape evolved in a similar way to passive margin evolution in southern Africa. Rifting associated with the breakup of Gondwana changed river base levels and caused rapid erosion on the flanks of rifts and was accompanied by the uplift of rift-margin mountains. Rift-margin plains, often coastal or extending inland along large rivers, are backed by an escarpment, while low-gradient continental river basins characterised the interior of Antarctica. In East Antarctica ice has removed pre-existing regolith from lowlands and excavated 2-3 km troughs below sea level along the course of major trunk rivers. The micro-continents of West Antarctica are comparable to Madagascar and apparently share a similar topography with coastal plains, backing escarpments and interior plateaux. [ABSTRACT FROM AUTHOR]

In the south-central sector of the former Fennoscandian Ice Sheet, imprints of the sub-glacial hydrological system are present as "glaciofluvial corridors," formed by glacial meltwater at the ice-bed interface during the Bølling-Allerød warm period. Many of these are interpreted as tunnel valleys and are commonly characterized by hummocks on their valley floors. Contemporary ice sheets produce increased amounts of meltwater as a consequence of global warming, and occasionally it is observed that meltwater is suddenly released from supra- and subglacial lakes, suggesting a highly dynamic subglacial hydraulic system. Studies of the imprints and deposits from such systems on formerly glaciated terrain can expand our knowledge of ice-sheet response to increased meltwater production. Here, we study sediments exposed in two hummocks within the tunnel valley at Hörda, south Sweden. One of the investigated hummocks is superposed by a small esker. This hummock consists of a diamict interpreted as a subglacial traction till, observed to be overlain by esker sediment. A second hummock displays deformed sediment at its base, which is glaciotectonically intercalated with above-lying diamict, a sub-glacial traction till. The sub-till sediments, interpreted as proglacial outwash, were deformed by overriding ice. The sediment was dated using optically stimulated luminescence (OSL), inferring a late MIS 4 or early MIS 3 age, congruent with other observations of sub-till sediments in south Sweden. The investigated hummocks on the floor of the Hörda tunnel valley are interpreted to have been formed by sub-glacial fluvial erosion simultaneous with tunnel valley formation, most probably during the latest deglaciation of the area. [ABSTRACT FROM AUTHOR]

The study of glacial landforms is important for understanding past subglacial processes and dynamics. The Nordenskiöldbreen forefield hosts numerous streamlined landforms resulting from a late Holocene glacier advance. Here, we present a geomorphological map constructed from remotely sensed imagery of both the marine and terrestrial environments. Sixteen drumlin bedforms have been identified in the mapped forefield - ten terrestrial and six submarine. A sedimentological investigation of drumlins in the Nordenskiöldbreen forefield shows that they are composed of pre-existing sediments draped by a thin layer of till which formed during the most recent advance of the glacier. Analysis of recent (2008-2012), high-resolution aerial imagery of all the glacier forelands in Svalbard suggests drumlin features are widespread. Here, we have identified 49 previously undocumented terrestrial glacier forefields, where subglacially streamlined landforms occur within the margins of the late Holocene glacier extent. Additionally, the location of 53 previously published submarine sites with streamlined landscape have been mapped. Thirty of these are sites with streamlines of late Holocene origin and 18 with Last Glacial Maximum (LGM) origin. Based on our detailed case study at the Nordenskiöldbreen forefield, and remote sensing survey, we suggest that drumlins in Svalbard form predominantly in forefields of glaciers with low-elevation termini, in areas with abundant fine-grained sediments, and where the glaciers are able to advance into an area of unconstrained topography. [ABSTRACT FROM AUTHOR]

Abstract: Drumlins are subglacial bedforms streamlined in the direction of ice flow. Common in deglaciated landscapes, they have been widely studied providing rich information on their internal geology, size, shape, and spacing. In contrast with bedform investigations elsewhere in geomorphology (aeolian and fluvial dunes and ripples for example) most drumlin studies derive observations from relict, and thus static features. This has made it difficult to gain information and insights about their evolution over time, which likely hampers our understanding of the process(es) of drumlin formation. Here we take a morphological approach, studying drumlin size and spacing metrics. Unlike previous studies which have focussed on databases derived from entire ice sheet beds, we adopt a space‐for‐time substitution approach using individual drumlin flow‐sets distributed in space as proxies for different development times/periods. Framed and assisted by insights from aeolian and fluvial geomorphology, we use our metric data to explore possible scenarios of drumlin growth, evolution and interaction. We study the metrics of the size and spacing of 36 222 drumlins, distributed amongst 71 flow‐sets, left behind by the former British‐Irish Ice Sheet, and ask whether behaviour common to other bedform phenomena can be derived through statistical analysis. Through characterizing and analysing the shape of the probability distribution functions of size and spacing metrics for each flow‐set we argue that drumlins grow, and potentially migrate, as they evolve leading to pattern coarsening. Furthermore, our findings add support to the notion that no upper limit to drumlin size exists, and to the idea that perpetual coarsening could occur if given sufficient time. We propose that the framework of process and patterning commonly applied to non‐glacial bedforms is potentially powerful for understanding drumlin formation and for deciphering glacial landscapes. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

The link between subglacial hydrology and basal sliding has prompted work on basal hydrology models with water pressure and drainage capacity as prognostic variables. We find that the Glacier Drainage System model, which belongs to a commonly used family of subglacial hydrology models that include both channelized and distributed drainage components, underpredicts winter water pressure when compared to borehole observations from western Greenland given a wide range of plausible parameter values and inputs. This problem, though previously noted by other modelers, has not been addressed. Possible causes for the discrepancy including idealized model inputs or unconstrained parameters are investigated through a series of modeling experiments on both synthetic and realistic ice sheet geometries. Numerical experiments reveal that englacial storage and hydraulic conductivity in the distributed system are the primary controls on winter water pressure in Glacier Drainage System model. Observations of temperate layer thickness and englacial water content from western Greenland imply an upper bound on englacial storage, suggesting that a reduction in hydraulic conductivity is the most plausible cause of high winter water pressure. We conclude that hydraulic conductivity acts as a proxy for the subgrid‐scale connectivity of the linked cavity system and should therefore change seasonally in correspondence with melt water availability. [ABSTRACT FROM AUTHOR]

Antarctic subice environments are diverse, underexplored microbial habitats. Here, we describe the ecophysiology and annotated genome of a Marinobacter strain isolated from a cold, saline, iron-rich subglacial outflow of the Taylor Glacier, Antarctica. This strain (BF04_ CF4) grows fastest at neutral pH (range 6-10), is psychrophilic (range: 0°C-20°C), moderately halophilic (range: 0.8%-15% NaCl) and hosts genes encoding potential low temperature and high salt adaptations. The predicted proteome suggests it utilizes fewer charged amino acids than a mesophilic Marinobacter strain. BF04_ CF4 has increased concentrations of membrane unsaturated fatty acids including palmitoleic (33%) and oleic (27.5%) acids that may help maintain cell membrane fluidity at low temperatures. The genome encodes proteins for compatible solute biosynthesis and transport, which are known to be important for growth in saline environments. Physiological verification of predicted metabolic functions demonstrate BF04 CF4 is capable of denitrification and may facilitate iron oxidation. Our data indicate that strain BF04_ CF4 represents a new Marinobacter species, Marinobacter gelidimuriae sp. nov., that appears well suited for the subglacial environment it was isolated from. Marinobacter species have been isolated from other cold, saline environments in the McMurdo Dry Valleys and permanently cold environments globally suggesting that this lineage is cosmopolitan and ecologically relevant in icy brines. [ABSTRACT FROM AUTHOR]

Geothermal areas, such as volcanoes, might have acted as glacial microrefugia for a wide range of species. The heavily glaciated but volcanically active Antarctic continent presents an ideal system for assessing this hypothesis. Ice-free terrain around volcanoes in Antarctica is, however, often restricted to small patches, whereas subglacial cave systems, formed by vented volcanic steam, can be extensive and interconnected. No observations of macrobiota have yet been made for subglacial geothermal environments in Antarctica, but these organisms are often patchily distributed and can be difficult to find. We carried out metabarcoding (eDNA) analyses of soil samples taken from exposed areas on three volcanoes in Victoria Land, and subglacial caves on Mount Erebus. We found evidence of numerous eukaryotic groups, including mosses, algae, arthropods, oligochaetes and nematodes, at both exposed and subglacial sites. Our findings support the notion that geothermal areas—including subglacial environments—can nurture biodiversity in glaciated regions. [ABSTRACT FROM AUTHOR]

Outlet glaciers of the Greenland Ice Sheet transport ice from the interior to the ocean and contribute directly to sea level rise because discharge and ablation often exceed the accumulation. To develop a better understanding of these fast‐flowing glaciers, we investigate the basal conditions of Store Glacier, a large outlet glacier flowing into Uummannaq Fjord in west Greenland. We use two crossing seismic profiles acquired near the centerline, 30 km upstream of the calving front, to interpret the physical nature of the ice and bed. We identify one notably englacial and two notably subglacial seismic reflections on both profiles. The englacial reflection represents a change in crystal orientation fabric, interpreted to be the Holocene‐Wisconsin transition. From Amplitude‐Versus‐Angle (AVA) analysis we infer that the deepest ∼80 m of ice of the parallel‐flow profile below this reflection is anisotropic with an enhancement of simple shear of ∼2. The ice is underlain by ∼45 m of unconsolidated sediments, below which there is a strong reflection caused by the transition to consolidated sediments. In the across‐flow profile subglacial properties vary over small scale and the polarity of the ice‐bed reflection switches from positive to negative. We interpret these as patches of different basal slipperiness associated with variable amounts of water. Our results illustrate variability in basal properties, and hence ice‐bed coupling, at a spatial scale of ∼100 m, highlighting the need for direct observations of the bed to improve the basal boundary conditions in ice‐dynamic models. [ABSTRACT FROM AUTHOR]

The Southern Ocean receives limited liquid surface water input from the Antarctic continent. It has been speculated, however, that significant liquid water may flow from beneath the Antarctic Ice Sheet, and that this subglacial flow carries that water along with dissolved nutrients to the coast. The delivery of solutes, particularly limiting nutrients like bioavailable iron, to the Southern Ocean may contribute to ecosystem processes including primary productivity. Using a helicopter-borne time domain electromagnetic survey along the coastal margins of the McMurdo Dry Valleys region of Southern Victoria Land, Antarctica, we detected subsurface connections between inland lakes, aquifers, and subglacial waters. These waters, which appear as electrically conductive anomalies, are saline and may contain high concentrations of biologically important ions, including iron and silica. Local hydraulic gradients may drive these waters to the coast, where we postulate they emerge as submarine groundwater discharge. This high latitude groundwater system, imaged regionally in the McMurdo Dry Valleys, may be representative of a broader system of Antarctic submarine groundwater discharge that fertilizes the Southern Ocean. In total, it has the potential to deliver tens of gigagrams of bioavailable Fe and Si to the coastal zone.

The aim of this study is to present the first evidence of triangular-shaped till landforms and related erosional features indicative of subglacial drainage within the ice stream bed of the Scandinavian ice sheet in Finland. Previously unidentified grouped patterns of Quaternary deposits with triangular landforms can be recognized from LiDAR-based DEMs. The triangular landforms occur as segments within geomorphologically distinguishable routes that are associated with eskers. The morphological and sedimentological characteristics as well as the distribution of the triangular landforms are interpreted to involve the creep of saturated deforming till, flow and pressure fluctuations of subglacial meltwater associated with meltwater erosion. There are no existing models for the formation of this kind of large-scale drainage systems, but we claim that they represent an efficient drainage system for subglacial meltwater transfer under high pressure conditions. Our hypothesis is that the routed, large-scale subglacial drainage systems described herein form a continuum between channelized (eskers) and more widely spread small-scale distributed subglacial drainage. Moreover, the transition from the conduit dominated drainage to triangular-shaped subglacial landforms takes place about 50–60 km from the ice margin. We provide an important contribution towards a more realistic representation of ice sheet hydrological drainage systems that could be used to improve paleoglaciological models and to simulate likely responses of ice sheets to increased meltwater production. [ABSTRACT FROM AUTHOR]

Since the first descriptions of Antarctic subglacial lakes, there has been a growing interest and awareness of the possibility that life will exist and potentially thrive in these unique and little known environments. The unusual combination of selection pressures, and isolation from the rest of the biosphere, might have led to novel adaptations and physiology not seen before, or indeed to the potential discovery of relic populations that may have become extinct elsewhere. Here we report the first microbiological analysis of a sample taken from a former subglacial lake sediment in Antarctica (Lake Hodgson, on the Antarctic Peninsula). This is one of a number of subglacial lakes just emerging at the margins of the Antarctic ice sheet due to the renewed onset of deglaciation. Microbial diversity was divided into 23.8% Actinobacteria, 21.6% Proteobacteria, 20.2% Planctomycetes and 11.6% Chloroflexi, characteristic of a range of habitat types ( Overall, common sequences were neither distinctly polar, low temperature, freshwater nor marine). Twenty three percent of this diversity could only be identified to “unidentified bacterium”. Clearly these are diverse ecosystems with enormous potential.

The discovery over the past two decades of viable microbial communities within glaciers has promoted interest in the role of glaciers and ice sheets (the cryosphere) as contributors to subglacial erosion, global biodiversity, and in regulating global biogeochemical cycles. In situ or in-field detection and characterisation of microbial communities is becoming recognised as an important approach to improve our understanding of such communities. Within this context we demonstrate, for the first time, the ability to detect Gram-negative bacteria in glacial field-environments (including subglacial environments) via the detection of lipopolysaccharide (LPS); an important component of Gram-negative bacterial cell walls. In-field measurements were performed using the recently commercialised PyroGene® recombinant Factor C (rFC) endotoxin detection system and used in conjunction with a handheld fluorometer to measure the fluorescent endpoint of the assay. Twenty-seven glacial samples were collected from the surface, bed and terminus of a low-biomass Arctic valley glacier (Engabreen, Northern Norway), and were analysed in a field laboratory using the rFC assay. Sixteen of these samples returned positive LPS detection. This work demonstrates that LPS detection via rFC assay is a viable in-field method and is expected to be a useful proxy for microbial cell concentrations in low biomass environments.

Direct measurement and sampling of pristine environments, such as subglacial lakes, without introducing contaminating microorganisms and biomolecules from the surface, represents a significant engineering and microbiological challenge. In this study, we compare methods for decontamination of titanium grade 5 surfaces, the material extensively used to construct a custom-made probe for reaching, measuring and sampling subglacial Lake Ellsworth in West Antarctica. Coupons of titanium were artificially contaminated with Pseudomonas fluorescens bacteria and then exposed to a number of decontamination procedures. The most effective sterilants were (i) hydrogen peroxide vapour, and (ii) Biocleanse™, a commercially available, detergent-based biocidal solution. After each decontamination procedure the bacteria were incapable of proliferation, and showed no evidence of metabolic activity based on the generation of adenosine triphosphate (ATP). The use of ultraviolet irradiation or ethyl alcohol solution was comparatively ineffective for sterilisation. Hydrogen peroxide vapour and ultraviolet irradiation, which directly damage nucleic acids, were the most effective methods for removing detectable DNA, which was measured using 16S rRNA gene copy number and fluorescence-based total DNA quantification. Our results have not only been used to tailor the Ellsworth probe decontamination process, but also hold value for subsequent engineering projects, where high standards of decontamination are required. [ABSTRACT FROM AUTHOR]

The ability to perform microbial detection and characterization in-field at extreme environments, rather than on returned samples, has the potential to improve the efficiency, relevance and quantity of data from field campaigns. To date, few examples of this approach have been reported. Therefore, we demonstrate that the approach is feasible in subglacial environments by deploying four techniques for microbial detection: real-time polymerase chain reaction; microscopic fluorescence cell counts, adenosine triphosphate bioluminescence assay and recombinant Factor C assay (to detect lipopolysaccharide). Each technique was applied to 12 subglacial ice samples, 12 meltwater samples and two snow samples from Engabreen, Northern Norway. Using this multi-technique approach, the detected biomarker levels were as expected, being highest in debris-rich subglacial ice, moderate in glacial meltwater and low in clean ice (debris-poor) and snow. Principal component analysis was applied to the resulting dataset and could be performed in-field to rapidly aid the allocation of resources for further sample analysis. We anticipate that in-field data collection will allow for multiple rounds of sampling, analysis, interpretation and refinement within a single field campaign, resulting in the collection of larger and more appropriate datasets, ultimately with more efficient science return. [ABSTRACT FROM AUTHOR]