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Subglacial hydrologic systems regulate ice sheet flow, causing acceleration or deceleration, depending on hydraulic efficiency and the rate at which surface meltwater is delivered to the bed. Because these systems are rarely observed, ice sheet basal drainage represents a poorly integrated and uncertain component of models used to predict sea level changes. Here, we report radarderived basal melt rates and unexpectedly warm subglacial conditions beneath a large Greenlandic outlet glacier. The basal melt rates averaged 14 mm · d−1 over 4 months, peaking at 57 mm · d−1 when basal water temperature reached +0.88 ◦C in a nearby borehole. We attribute both observations to the conversion of potential energy of surface water to heat in the basal drainage system, which peaked during a period of rainfall and intense surface melting. Our findings reveal limitations in the theory of channel formation, and we show that viscous dissipation far surpasses other basal heat sources, even in a distributed, highpressure system. [ABSTRACT FROM AUTHOR]

Distributed Acoustic Sensing (DAS) is a new technology in which seismic energy is detected, at high spatial and temporal resolution, using the propagation of laser pulses in a fiber‐optic cable. We show analyses from the first glaciological borehole DAS deployment to measure the englacial and subglacial seismic properties of Store Glacier, a fast‐flowing outlet of the Greenland Ice Sheet. We record compressional and shear waves in 1,043 m‐deep vertical seismic profiles, sampled at 10 m vertical resolution, and detect a transition from isotropic to anisotropic ice at 84% of ice thickness, consistent with the Holocene‐Wisconsin transition. We identify subglacial reflections originating from the base of a 20 m‐thick layer of consolidated sediment and, from attenuation measurements, interpret temperate ice in the lowermost 100 m of the glacier. Our findings highlight the promising potential of DAS technology to constrain the seismic properties of glaciers and ice sheets. Plain Language Summary: Distributed Acoustic Sensing (DAS) is a new technology for seismic surveying in which the transmission of light through fiber‐optic cables is used to record seismic energy, with unprecedented spatial resolution compared to traditional techniques. Our paper presents data from the first borehole‐glaciological deployment of DAS, in which fiber‐optic cable was installed in a 1,043 m‐deep vertical borehole on Store Glacier, a fast‐flowing outlet of the Greenland Ice Sheet. The detailed seismic anatomy of the glacier that our survey provides—an independent measurement of the seismic response every 10 m—gives new insights about its internal flow regime and temperature and even allows us to detect layers of sediment underlying it. We predict that DAS surveying will play an increasingly large role in future glaciological investigations as the recognition of its promising potential grows. Key Points: Distributed Acoustic Sensing (DAS) is a new technology for recording seismic data using laser pulses propagating in a fiber‐optic cableWe present the first borehole‐glaciological application of DAS, installing cable in a 1,043 m‐deep borehole on Greenland's Store GlacierWe evidence, at 10 m vertical resolution, anisotropic and temperate ice beyond ~880 m depth and ~20 m of consolidated subglacial sediment [ABSTRACT FROM AUTHOR]

Supraglacial lake drainage events influence Greenland Ice Sheet dynamics on hourly to interannual timescales. However, direct observations are rare, and, to date, no in situ studies exist from fast-flowing sectors of the ice sheet. Here, we present observations of a rapid lake drainage event at Store Glacier, west Greenland, in 2018. The drainage event transported 4.8 x 106 m³ of meltwater to the glacier bed in ~5 h, reducing the lake to a third of its original volume. During drainage, the local ice surface rose by 0.55 m, and surface velocity increased from 2.0 m⋅d-1 to 5.3 m⋅d-1. Dynamic responses were greatest ~4 km downstream from the lake, which we interpret as an area of transient water storage constrained by basal topography. Drainage initiated, without any precursory trigger, when the lake expanded and reactivated a preexisting fracture that had been responsible for a drainage event 1 y earlier. Since formation, this fracture had advected ~500 m from the lake's deepest point, meaning the lake did not fully drain. Partial drainage events have previously been assumed to occur slowly via lake overtopping, with a comparatively small dynamic influence. In contrast, our findings show that partial drainage events can be caused by hydrofracture, producing new hydrological connections that continue to concentrate the supply of surface meltwater to the bed of the ice sheet throughout the melt season. Our findings therefore indicate that the quantity and resultant dynamic influence of rapid lake drainages are likely being underestimated. [ABSTRACT FROM AUTHOR]

Abstract Although a substantial ice cover has been identified within the mid-latitudes of Mars, there is uncertainty regarding the formation, current and former volume, and dynamic evolution of these ice masses. Here, we present the first comprehensive ice volume estimate of martian glacier-like forms (GLFs) from systematic population scale mapping and volumetric analysis. The outlines of 1243 GLFs were manually delineated from 6 m per pixel Context Camera (CTX) images and the volume of each determined using a volume–area scaling approach. Our results show that GLFs cover a surface area of 11344 ± 393 km2 and have a total volume of 1744 ± 441 km3. Using two end-member scenarios for ice concentration by volume of 30% (pore ice) and 90% (debris-covered glacier ice), we calculate the volume of ice contained within GLFs to be between 523 ± 132 km3 (480 ± 121 Gt) and 1570 ± 397 km3 (1439 ± 364 Gt), equivalent to a mean global water layer 3 to 10 mm thick. We investigate the local topographic setting of each GLF by reference to the Mars Orbiter Laser Altimeter (MOLA) digital elevation model. Our analysis reveals that globally GLFs are on average larger in latitudes >36° and on slopes between 2 and 8°. In the northern hemisphere GLFs between 500 and 2500 m in elevation and in the southern hemisphere GLFs with a northern aspect are also larger on average. The observed spatial patterns of GLF landform and volume distribution suggests that regional to local meteorological and topographical conditions play an important role in GLF ice accumulation and/or preservation. Assuming a net accumulation rate of 10 mm a−1 typical of climatic excursions with high obliquity, we estimate a period of at least 13 ka is required to yield the average calculated GLF ice thickness of ∼130 m. Such a period is well within the timeframe of a high obliquity cycle (20–40 ka), suggesting that the current GLF volume could have formed during a single climate excursion. Highlights • We present a complete inventory of martian glacier-like form (GLFs) outlines. • Martian GLFs cover 11344 ± 393 km2 and contain between 523 ± 132 km3 and 1570 ± 397 km3 of ice. • This ice volume corresponds to a global water layer 3 to 10 mm thick. • Local microclimates, as well as solar forcing, control GLF distribution and size. [ABSTRACT FROM AUTHOR]

Intense rainfall events significantly affect Alpine and Alaskan glaciers through enhanced melting, ice-flow acceleration and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that extreme surface runoff from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runoff across an extensive bare ice surface that overwhelmed a subglacial hydrological system in transition to a less-efficient winter mode. Reanalysis data reveal that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available. Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change. [ABSTRACT FROM AUTHOR]

We report on the application of optical televiewing (OPTV) to the uppermost 630m of the North Greenland Eemian Ice Drilling (NEEM) deep ice borehole, Greenland. The resulting log reveals numerous natural and drilling-related properties, including the integrity of the borehole casing and its joints, the presence of drill-tooth scoring on the ice wall of the borehole and the presence of regularly repeated layering, interpreted to be annual, to a depth approaching 200 m. A second OPTV log was acquired from a nearby shallow borehole. With the exception of the uppermost ~10 m, this log shows a gradual decrease in luminosity with depth, interpreted as a decrease in light scattering with firnification. This shallow log also clearly images annual layers, allowing the construction of an age-depth scale. Comparing this with an independent core-based scale reveals that the OPTV record yields an age of 1724 at the deepest common point of both scales (80 m), 13 years older than the core-based record at 1737. However, all of this deviation accrues in the uppermost ~30m of the OPTV record where highly reflective snow saturates the luminosity of the borehole image, an artefact that can be reduced by further adaptation of the OPTV system. [ABSTRACT FROM AUTHOR]

Roberson, S., Hubbard, B., Coulson, H. and Boomer, I. Physical properties and formation of flutes at a polythermal valley glacier: Midre Lovénbreen, Svalbard. Geografika Annaler: Series A, Physical Geography, 93, 71–88. DOI: 10.1111/j.1468‐0459.2011.00420.x Flutes are widespread across the forefield of a polythermal valley glacier Midre Lovénbreen, Svalbard, where they consistently form on the downglacier face of cobble to boulder‐sized subglacially‐transported clasts ploughed into proglacial till. The width of flutes at Midre Lovénbreen is constant along their long‐axes and proportional to the size of their associated subglacially‐transported boulders. Frozen flutes are observed subglacially where they protrude into the debris‐rich basal ice layer of the glacier. Particle‐size analysis shows that flutes are depleted in gravel‐sized particles relative to adjacent subglacial till. Ice sampled from flutes is enriched in δ18O relative to overlying glacier ice and debris‐rich basal ice. These properties are interpreted to reflect the formation of flutes by the squeezing of till into basal cavities associated with boulders beneath warm‐based ice in the interior of the glacier. It is hypothesized that localized freeze‐on of flute sediments, within a broader zone of warm‐based ice, occurs within cavities due to either (i) a 'heat‐pump' effect operating across these boulders leading to the development of a cold patch within the cavity, and/or (ii) fluctuations in cavity length caused by slip‐stick glacial motion. Once incorporated into the basal ice layer, the frozen flute sediment would then be transported away from the boulder by ice flow, allowing the cavity to remain continuously open and the flute to propagate downflow. [ABSTRACT FROM AUTHOR]

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We outline a method of assessing the roughness of the bed beneath ice-masses, applicable to data collected in a wide variety of (glaciated) environments. Roughness at scales >102m in particular is poorly constrained and understood in glaciated systems, and regional geophysical surveys collected over the past 30 years are ideally suited to addressing this issue. We demonstrate the technique using a radio-echo sounding data set collected during the 1970s, showing that regional characterization of roughness over a wide range of wavelengths (1 to >40km) is possible using Fast Fourier Transforms, even with quite coarse horizontal sampling. Coarse sampling captures all of the variation of higher resolution analyses and the technique appears robust up to the limit imposed by the Nyquist frequency. [Copyright &y& Elsevier]

Ice loss from the Greenland Ice Sheet is currently the single largest cryospheric contributor to global sea level rise. The two most significant factors influencing ice loss are the acceleration of fast-flowing, marine-terminating glaciers, and surface run-off, which has increased steadily over the last several decades. Since almost all water from the surface is routed to the bed, hydrological systems at the base of the glacier are inherently conditioned by surface processes. Unpicking the relationships between surface (melt)water input, basal hydrology, and ice velocity is therefore key to understanding the effects of global warming on the contribution of marine-terminating glaciers to global sea level rise.Passive seismology enables the continuous observation of a range of phenomena occurring within and underneath glaciers, such as icequakes, crevassing, and tremors associated with water transport. This technique is therefore ideally suited to studying the effects of variations in surface water input over the duration of a melt season. Over the course of two field campaigns in May and June–July 2018, a network of 12 near-surface and 3 deep borehole geophones was deployed on Store Glacier, a large marine-terminating glacier in West Greenland, as part of the multidisciplinary RESPONDER project. The stations have recorded continuously since early May, capturing a wide range of phenomena including the onset and duration of the surface melting season, regional earthquakes, impulsive microseismic activity, and the drainage of a supraglacial lake. Here, we present findings in the form of spectrograms and the locations of microseismic events recorded by the network. The spectrograms show a clear and sudden transition between relative seismic quiescence and increased mid–high frequency activity on 5 June, corresponding to the onset of significant surface melting. The seismic signatures of meteorological events such as prolonged rainfall are also captured in the spectrograms. This dataset is therefore uniquely suited to investigating links between the input of surface water, microseismicity and basal hydrology. [ABSTRACT FROM AUTHOR]