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Surging and streaming of glaciers are modulated by meltwater availability and pressure which controls mechanical coupling at their beds. Using laboratory-scale experimental modelling and palaeoglaciological mapping, we explore how subglacial drainage landsystems control meltwater drainage efficiency and ice flow velocities for terrestrial-based ice lobes resting on flat horizontal and permeable beds. Two end-members regimes, surging and streaming, appear in our experiments. The surge regime is characterised by a rapid increase of drainage efficiency through development of tunnel valleys and their tributaries, thus reducing the duration of ice flow speed-up events by lowering water pressures and increasing ice-bed coupling. Tunnel valleys connected to ice lobe margins, submarginal thrust moraines, reduced ice lobe extensions and ephemeral shear margins are the most distinctive characteristics of this regime. The stream regime is characterised by disconnected channels of smaller dimensions unable to evacuate all the meltwater: this prolonged drainage inefficiency leads to sustained high ice flow velocity and steady shear margins. Small and rectilinear meltwater channels devoid of tributaries, often disconnected from ice lobe margins, and lineation swarms are diagnostic of this regime.

Surging and streaming of glaciers are modulated by meltwater availability and pressure which controls mechanical coupling at their beds. Using laboratory-scale experimental modelling and palaeoglaciological mapping, we explore how subglacial drainage landsystems control meltwater drainage efficiency and ice flow velocities for terrestrial-based ice lobes resting on flat horizontal and permeable beds. Two end-members regimes, surging and streaming, appear in our experiments. The surge regime is characterised by a rapid increase of drainage efficiency through development of tunnel valleys and their tributaries, thus reducing the duration of ice flow speed-up events by lowering water pressures and increasing ice-bed coupling. Tunnel valleys connected to ice lobe margins, submarginal thrust moraines, reduced ice lobe extensions and ephemeral shear margins are the most distinctive characteristics of this regime. The stream regime is characterised by disconnected channels of smaller dimensions unable to evacuate all the meltwater: this prolonged drainage inefficiency leads to sustained high ice flow velocity and steady shear margins. Small and rectilinear meltwater channels devoid of tributaries, often disconnected from ice lobe margins, and lineation swarms are diagnostic of this regime. [ABSTRACT FROM AUTHOR]

Understanding ice stream dynamics over decadal to millennial timescales is crucial for improving numerical model projections of ice sheet behaviour and future ice loss. In marine‐terminating settings, ice shelves play a critical role in controlling ice‐stream grounding line stability and ice flux to the ocean, but few studies have investigated the terrestrial lateral geomorphological imprint of ice shelves during deglaciation. Here, we document the terrestrial deglacial landsystem of Nioghalvfjerdsfjorden Glacier (79N) in northeast Greenland, following the Last Glacial Maximum, and the margin's lateral transition to a floating ice shelf. High‐elevation areas are influenced by local ice caps and display autochthonous to allochthonous blockfields that mark the interaction of local ice caps with the ice stream below. A thermal transition from cold‐ to warm‐based ice is denoted by the emplacement of erratics onto allochthonous blockfields. Below ~600 m above sea level (a.s.l.) glacially abraded bedrock surfaces and assemblages of lateral moraines, 'hummocky' moraine, fluted terrain, and ice‐contact deltas record the former presence of warm‐based ice and thinning of the grounded ice stream margin through time. In the outer fjord a range of landforms such as ice shelf moraines, dead‐ice topography, and ice marginal glaciofluvial outwash was produced by an ice shelf during deglaciation. Along the mid‐ and inner‐fjord areas this ice shelf signal is absent, suggesting ice shelf disintegration prior to grounding line retreat under tidewater conditions. However, below the marine limit, the geomorphological record along the fjord indicates the expansion of the 79N ice shelf during the Neoglacial, which culminated in the Little Ice Age. This was followed by 20th century recession, with the development of a suite of compressional ice shelf moraines, ice‐marginal fluvioglacial corridors, kame terraces, dead‐ice terrain, and crevasse infill ridges. These mark rapid ice shelf thinning and typify the present‐day ice shelf landsystem in a warming climate. [ABSTRACT FROM AUTHOR]

The crystal orientation fabric of glacier ice impacts its strength and flow. Crystal fabric is therefore an important consideration when modeling ice flow. Here, we show that shear‐wave splitting (SWS) measured with glacial microseismicity can be used to invert seismic anisotropy and ice fabric, if represented in a statistical sense. Rutford Ice Stream (RIS) is a fast‐flowing Antarctic ice stream, a setting crucial for informing large‐scale ice sheet models. We present >200,000 SWS measurements from glacial microseismicity, registered at a 38‐station seismic network located ∼40 km upstream of the grounding line. A representative subset of these data is inverted for ice fabric. Due to the character of SWS, which accumulates along the raypath, we include information on the depth structure from radar measurements. We find that the following three‐layer configuration fits the data best: a broad vertical cone fabric near the base of RIS (500 m thick), a thick vertical girdle fabric, orientated perpendicular to flow, in the middle (1,200 m thick), and a tilted cone fabric in the uppermost 400 m. Such a variation of fabric implies a depth‐dependent strength profile of the ice with the middle layer being ∼3.5 times harder to deform along flow than across flow. At the same time, the middle layer is a factor ∼16 softer to shear than to compression or extension along flow. If such a configuration is representative for fast‐flowing ice streams, it would call for a more complex integration of viscosity in ice sheet models. Plain Language Summary: We introduce a method to derive information on the ice viscosity of a >2 km thick Antarctic Ice Stream using very small earthquakes, which are created as the ice stream flows over its bed. When ice flows, the individual ice crystals rearrange in a specific configuration. This configuration can vary, depending on the direction of applied force, making the bulk ice harder or easier to deform. This behavior can be expressed through a variable, the ice viscosity. Here, we use the recordings of icequakes recorded at seismic stations on top of the ice to infer the configuration of ice crystals and through further mathematical calculations, the viscosity between the ice‐bed and the surface. Our results show that the crystal configuration and viscosity vary with depth, which severely affects the ice stream's behavior upon deformation. Such a behavior is so far not included in many numerical models which simulate the large‐scale behavior of ice streams. Key Points: Split shear‐waves from basal microseismicity are used to invert for ice fabricThe preferred model has three depth layers of distinct fabric including a tilted cone, thick girdle, and vertical coneIce viscosity varies with depth, direction, and component of deformation, with significant implications for ice deformation [ABSTRACT FROM AUTHOR]

• Exposure dating constrains rapid deglaciation of Antarctic ice streams ∼8 ka ago. • Paired 10Be-26Al nuclides sheds light on complex exposure history of erratics. • Ice thinning was induced by atmospheric and oceanic warming of the Early Holocene. • Eustatic and relative sea level rise paced the transition of grounded ice to ice shelf. • Buoyancy response of thinning ice stream may have accelerated grounding line retreat. The knowledge of dynamics and retreat patterns of marine-based ice streams under multiple stressors are of foremost importance for predicting Antarctic Ice Sheet response to climate changes. The Holocene palaeoglaciological record of former ice streams draining the northeast Antarctic Peninsula can elucidate the influences of changes in atmospheric and oceanic circulation and sea-level oscillations on the ice thinning and grounding line retreat. Here, terrestrial cosmogenic nuclide (TCN) dating of erratic boulders across the James Ross Island group sheds light on the pattern and timing of the ice recession along the two main arteries of the palaeo-ice drainage: Croft Trough and Prince Gustav Channel. The approach of using paired 10Be-26Al nuclides enabled an assessment of cosmogenic isotope inheritance and complex burial-exposure history, notably on the high-altitude volcanic mesas. The TCN ages suggest that the Prince Gustav Channel Ice Stream was thinning from at least ∼12 ka, with subsequent separation of the Antarctic Peninsula and James Ross Island ice masses by 10–8 ka. The transition from grounded ice to open marine conditions in the Croft Trough occurred rapidly at 8.6–7.2 ka, following the Early Holocene Warm Period, concomitant with eustatic and relative sea-level rise and incursions of warmer circumpolar waters. Grounding line retreat was possibly further accelerated by buoyancy response of thinning ice stream to low-gradient bed topography. The lessons of rapid deglaciation of James Ross Island palaeo-ice streams may provide analogues for recent or future intensification of pressures on Antarctic glaciers. [ABSTRACT FROM AUTHOR]

The near-field glacial record of the late Paleozoic Ice Age in Gondwanan basins is widely used in global paleogeographic and paleoclimatic models. Nevertheless, several late Paleozoic glacial successions still lack a detailed depositional history or reassessment of their genesis under modern glacial sedimentology concepts. One such example is the Aquidauana Formation in the western Paraná Basin, Brazil, which corresponds to about 65 % of the total outcrop area of the late Paleozoic glacial strata (Itararé Group) within the basin. Yet, the stratigraphic architecture and depositional environments are full of uncertainties as the strata are very poorly exposed. This study brings new insights on the depositional history and stratigraphic architecture for the upper half of the Aquidauana Formation by describing a 290-m-thick section cropping out along a recently developed highway in the Mato Grosso do Sul state. A subaqueous slope and glaciated outer shelf depositional setting is interpreted for the upper half of the Aquidauana Formation, which differs considerably from the glacioterrestrial and postglacial arid context traditionally proposed. More specifically, deposition chiefly took place under ice-free conditions on a subaqueous slope within an erosionally-confined channel–levee complex through sediment density flows and subaqueous landslides. Episodic glacial influence on sedimentation is recorded within the uppermost portion of the section by glacially-modified and iceberg-rafted debris as well as a deglacial trace fossil assemblage within proglacial deposits. Sediment transport toward the southeast into the basin indicates a glacial source placed westward of the Paraná Basin, reinforcing the hypothesis of ice-spreading centers located westward of the basin. The outcomes of this study also emphasize the relevance of subaqueous gravity-driven resedimentation in emplacing the late Paleozoic glacial record. [ABSTRACT FROM AUTHOR]

Basal properties beneath ice streams and glaciers are known to be a control for ice flow dynamics, hence knowledge of them is crucial for predicting sea level due to changes in glacial dynamics. Basal properties, processes and topography also drive the formation of subglacial landforms. Bed properties beneath Rutford Ice Stream (West Antarctica) have previously been described using seismic acoustic impedance measurements at a sparse spatial coverage. Here, we derive bed properties in a 15 × 17 km grid of surface radar data with coverage and sampling much higher than previous seismic studies. Bed reflection amplitudes in surface radar data were calibrated using sediment porosities (ranging from 0.4–0.5) derived from seismic acoustic impedance. We find the bed properties are spatially variable, consisting of low porosity material in some areas and soft sediment in other areas. Comparison of seismic and surface radar data imply the low porosity material to be a consolidated sediment or sedimentary rock. Mega‐scale glacial lineations (MSGLs) are ubiquitous on the bed and consist of soft, high porosity, probably deforming sediment, consistent with previous interpretations of MSGLs. We find some MSGLs have high reflectivity on their crest, interpreted as water bodies overlying high porosity sediment, whereas the trough around and the upstream end of some landforms consist of low porosity material. Integrating these different observations, we place constraints on possible explanations for the occurrence of water on the crest of landforms. Plain Language Summary: Physical properties of rocks and sediments as well as the presence of water beneath ice streams and glaciers control the flow of the ice. Understanding glacial flow is important to better constrain predictions of sea‐level changes. Within this study we present physical properties of material beneath Rutford Ice Stream in West Antarctica over a 15 × 17 km area. Our data show areas of high porosity interpreted as wet sediment and areas of low porosity beneath the ∼2 km thick ice stream. Ice movement above areas of soft sediment is assumed to be maintained by sediment deformation, whereas in areas overlain by low porosity material, sliding of the ice over this material is assumed to maintain flow. The topography beneath Rutford Ice Stream shows many elongated landforms, some over 14 km long. We show that these landforms partly consist of soft sediment. Some of these seem to have a water layer on their crest, but it is not well understood why. However, there might be variations in the sediment composition of the landforms enabling the creation of this water layer. The occurrence of materials with different properties and water show us how heterogenous the bed beneath ice streams can be, which is important for modeling ice stream motion. Key Points: Glacier bed properties (water content, porosity) are constrained using surface radar data calibrated with seismic acoustic impedanceResults show sequences of low porosity material (consolidated sediment or rock), soft sediment and free‐water beneath Rutford Ice StreamMega‐scale glacial lineations are pervasive beneath ice streams, some with a free water layer >10 km long and 100 m wide along their crest [ABSTRACT FROM AUTHOR]

Neogene volcanic centres are uncommon in the Transantarctic Mountains but at least three basaltic examples occur within 300 km of South Pole, above 2200 m asl and inland of the margin of the West Antarctic Rift System. They are the southernmost volcanoes on Earth and have yielded Early—mid Miocene isotopic ages. Two of the centres, at Mt Early and Sheridan Bluff, have been examined. The centre at Mt Early is unequivocally glaciovolcanic. It formed a tall monogenetic volcanic edifice at least 1 km high and > 1.5 km in diameter. It erupted under significantly thicker-than-modern ice, which was probably a fast-moving ice stream at the eruptive site and resulted in a distinctive constructive architecture and lithofacies. It is the first described example of a glaciovolcano erupted beneath an ice stream. The characteristics of the second centre at Sheridan Bluff indicate that it was also a monogenetic volcano but with a shield-like profile, originally c. 6 km in basal diameter but just c. 400 m high. It probably erupted in a substantial pluvial lake in an ice-poor or ice-free environment. The strongly contrasting eruptive settings now identified by the volcanic sequences at both centres examined testify to a highly dynamic Antarctic Ice Sheet during the Early—mid Miocene. [ABSTRACT FROM AUTHOR]

The BRITICE‐CHRONO Project has generated a suite of recently published radiocarbon ages from deglacial sequences offshore in the Celtic and Irish seas and terrestrial cosmogenic nuclide and optically stimulated luminescence ages from adjacent onshore sites. All published data are integrated here with new geochronological data from Wales in a revised Bayesian analysis that enables reconstruction of ice retreat dynamics across the basin. Patterns and changes in the pace of deglaciation are conditioned more by topographic constraints and internal ice dynamics than by external controls. The data indicate a major but rapid and very short‐lived extensive thin ice advance of the Irish Sea Ice Stream (ISIS) more than 300 km south of St George's Channel to a marine calving margin at the shelf break at 25.5 ka; this may have been preceded by extensive ice accumulation plugging the constriction of St George's Channel. The release event between 25 and 26 ka is interpreted to have stimulated fast ice streaming and diverted ice to the west in the northern Irish Sea into the main axis of the marine ISIS away from terrestrial ice terminating in the English Midlands, a process initiating ice stagnation and the formation of an extensive dead ice landscape in the Midlands. [ABSTRACT FROM AUTHOR]

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast‐flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice‐stream‐dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km2 of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic‐nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector‐wide ice‐sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice‐sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice‐sheet sector undergoing large size variations in response to sub‐millennial‐scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine‐terminating ice‐sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main 'Late Weichselian' ice‐sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice‐sheet build‐up in NW Scotland, with the Minch ice‐stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding‐zone wedges and moraines on the mid‐ and inner continental shelf attest to significant stabilizations of the ice‐sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present‐day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea‐level c. 26 kabp, followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice‐shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp, as the grounding‐line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding‐line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp, ice‐sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine‐instability‐triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid‐Greenland Stadial 2, with an ice‐accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice‐sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp, before significant ice‐mass losses thereafter. At the glacial termination, c. 14.5 kabp, glaciers fed outwash sediment to now‐abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice‐sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice‐sheet glaciation dominated by ice‐streaming were interspersed with periods of much more restricted ice‐cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice‐sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice‐sheet health but also to the acceleration of ice‐sheet demise via marine ice‐sheet instabilities. [ABSTRACT FROM AUTHOR]

The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m(2), significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m(2). The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region.

In order to understand ice sheet response to climate change, it is critical to examine errors associated with ice flow model boundary conditions and forcing. It is also important to understand how these errors propagate through numerical ice sheet models and contribute to uncertainty in model output. Using established uncertainty quantification methods within the Ice Sheet System Model (ISSM), we investigate the sensitivity of ice flow within the Northeast Greenland Ice Stream (NEGIS) to key fields, including ice viscosity and basal drag, and compare them with model sensitivity to climate forcing. In addition, we examine how errors in model input manifest as mass flux uncertainties during a forward simulation of the NEGIS from 1989 to 2010. Overall, we find that mass flux is most uncertain in the main outlets, Nioghalvfjerdsbræ and Zachariæ Isstrøm, and that mass flux is most sensitive to basal drag, though errors associated with basal drag are poorly constrained and difficult to quantify. Given our knowledge of errors associated with the thermal properties of ice, we estimate that in the ablation area, the effects of cryohydrologic warming contribute over 4 times more mass flux uncertainty that do errors in geothermal heat flux. We find that NEGIS total ice discharge is associated with a 0.7 Gt/yr (2.6%) uncertainty due to errors in geothermal heat flux and a 3.3 Gt/yr (11.6%) uncertainty due to the added effects of cryohydrologic warming. In comparison, errors in surface mass balance contribute 4.5 Gt/yr to NEGIS total discharge uncertainty. Key Points Mass flux is most sensitive to basal drag Mass flux is the most uncertain at the 79North outlet Geothermal heat flux uncertainty is less than that from cryohydrologic warming.

Ice at depth in ice-stream shear margins is thought to commonly be temperate, with interstitial meltwater that softens ice. Models that include this softening extrapolate results of a single experimental study in which ice effective viscosity decreased by a factor of ∼3 over water contents of ∼0.01–0.8%. Modeling indicates this softening by water localizes strain in shear margins and through shear heating increases meltwater at the bed, enhancing basal slip. To extend data to higher water contents, we shear lab-made ice in confined compression with a large ring-shear device. Ice rings with initial mean grain sizes of 2–4 mm are kept at the pressure-melting temperature and sheared at controlled rates with peak stresses of ∼0.06–0.20 MPa, spanning most of the estimated shear-stress range in West Antarctic shear margins. Final mean grain sizes are 8–13 mm. Water content is measured by inducing a freezing front at the ice-ring edges, tracking its movement inward with thermistors, and fitting the data with solutions of the relevant Stefan problem. Results indicate two creep regimes, below and above a water content of ∼0.6%. Comparison of effective viscosity values in secondary creep with those of tertiary creep from the earlier experimental study indicate that for water contents of 0.2–0.6%, viscosity in secondary creep is about twice as sensitive to water content than for ice sheared to tertiary creep. Above water contents of 0.6%, viscosity values in secondary creep are within 25% of those of tertiary creep, suggesting a stress-limiting mechanism at water contents greater than 0.6% that is insensitive to ice fabric development in tertiary creep. At water contents of ∼0.6–1.7%, effective viscosity is independent of water content, and ice is nearly linear-viscous. Minimization of intercrystalline stress heterogeneity by grain-scale melting and refreezing at rates that approach an upper bound as grain-boundary water films thicken might account for the two regimes.

Microseismicity, induced by the sliding of a glacier over its bed, can be used to characterize frictional properties of the ice‐bed interface, which are a key parameter controlling ice stream flow. We use naturally occurring seismicity to monitor spatiotemporally varying bed properties at Rutford Ice Stream, West Antarctica. We locate 230,000 micro‐earthquakes with local magnitudes from −2.0 to −0.3 using 90 days of recordings from a 35‐station seismic network located ∼40 km upstream of the grounding line. Events exclusively occur near the ice‐bed interface and indicate predominantly flow‐parallel stick‐slip. They mostly lie within a region of interpreted stiff till and along the likely stiffer part of mega‐scale glacial lineations. Within these regions, micro‐earthquakes occur in spatially (<100 m radius) and temporally (mostly 1–5 days activity) restricted event‐clusters (up to 4,000 events), which exhibit an increase, followed by a decrease, in event magnitude with time. This may indicate event triggering once activity is initiated. Although ocean tides modulate the surface ice flow velocity, we observe little periodic variation in overall event frequency over time and conclude that water content, bed topography and stiffness are the major factors controlling microseismicity. Based on variable rupture mechanisms and spatiotemporal characteristics, we suggest the event‐clusters relate to three end‐member types of bed deformation: (1) continuous creation and seismogenic destruction of small‐scale bed‐roughness, (2) ploughed clasts, and (3) flow‐oblique deformation during landform formation or along bedrock outcrops. This indicates that multiple processes, simultaneously active during glacial sliding, can accommodate stick‐slip behavior and that the bed continuously reorganizes. Key Points: We locate 230,000 micro‐earthquakes, with rupture mechanisms, at the base of a fast‐flowing West Antarctic ice stream within a 3‐month periodEvent distribution is little affected by tidal modulations and indicates basal sliding most affected by bed topography, stiffness, and fluidsEvents occur clustered, likely due to different types of bed deformation: mobile asperities, ploughed clasts and flow‐oblique bed features [ABSTRACT FROM AUTHOR]

The ice stream geometry and large ice surface velocities at the onset region of the Northeast Greenland Ice Stream (NEGIS) are not yet well reproduced by ice sheet models. The quantification of basal sliding and a parametrization of basal conditions remains a major gap. In this study, we assess the basal conditions of the onset region of the NEGIS in a systematic analysis of airborne ultra‐wideband radar data. We evaluate basal roughness and basal return echoes in the context of the current ice stream geometry and ice surface velocity. We observe a change from a smooth to a rougher bed where the ice stream widens, and a distinct roughness anisotropy, indicating a preferred orientation of subglacial structures. In the upstream region, the excess ice mass flux through the shear margins is evacuated by ice flow acceleration and along‐flow stretching of the ice. At the downstream part, the generally rougher bed topography correlates with a decrease in flow acceleration and lateral variations in ice surface velocity. Together with basal water routing pathways, this hints to two different zones in this part of the NEGIS: the upstream region collecting water, with a reduced basal traction, and downstream, where the ice stream is slowing down and is widening on a rougher bed, with a distribution of basal water toward the shear margins. Our findings support the hypothesis that the NEGIS is strongly interconnected to the subglacial water system in its onset region, but also to the subglacial substrate and morphology. Plain Language Summary: The Northeast Greenland Ice Stream (NEGIS) transports a large amount of ice mass from the interior of the Greenland Ice Sheet (GrIS) toward the ocean. The extent and geometry of the NEGIS are difficult to reproduce in current ice sheet models because many boundary conditions, such as the properties of the ice base, are not well known. In this study, we present new characteristics of the ice base from the onset region of the NEGIS derived by airborne radio‐echo sounding data. Our data yield a smooth and increasingly lubricated bed in the upstream part of our survey area, which enables the ice to accelerate. Our results confirm the hypothesis that the position of the ice stream boundaries are coupled to the subglacial hydrology system. Key Points: Basal roughness at the onset of the NEGIS hints to a geomorphic anisotropy and a change in the geomorphological regimeBasal water is funneled into the ice stream upstream and redistributed toward the shear margins further downstreamA smooth and progressively lubricated bed reduces basal traction and favors the acceleration of the NEGIS at its onset [ABSTRACT FROM AUTHOR]

The behavior of the Greenland Ice Sheet, which is considered a major contributor to sea level changes, is best understood on century and longer time scales. However, on decadal time scales, its response is less predictable due to the difficulty of modeling surface climate, as well as incomplete understanding of the dynamic processes responsible for ice flow. Therefore, it is imperative to understand how modeling advancements, such as increased spatial resolution or more comprehensive ice flow equations, might improve projections of ice sheet response to climatic trends. Here we examine how a finely resolved climate forcing influences a high-resolution ice stream model that considers longitudinal stresses. We simulate ice flow using a two-dimensional Shelfy-Stream Approximation implemented within the Ice Sheet System Model (ISSM) and use uncertainty quantification tools embedded within the model to calculate the sensitivity of ice flow within the Northeast Greenland Ice Stream to errors in surface mass balance (SMB) forcing. Our results suggest that the model tends to smooth ice velocities even when forced with extreme errors in SMB. Indeed, errors propagate linearly through the model, resulting in discharge uncertainty of 16% or 1.9 Gt/yr. We find that mass flux is most sensitive to local errors but is also affected by errors hundreds of kilometers away; thus, an accurate SMB map of the entire basin is critical for realistic simulation. Furthermore, sensitivity analyses indicate that SMB forcing needs to be provided at a resolution of at least 40 km.

Surge‐type glaciers typically undergo cyclical flow instability due to mass accumulation; however, some recent glacier surges have caused irreversible ice loss in a short period. At Vavilov Ice Cap, Russia, surge‐like behavior initiated in 2013 and by spring 2019 the ice cap had lost 9.5 Gt of ice (11% mass of the entire basin). Using time series of surface elevation and glacier velocity derived from satellite optical and synthetic‐aperture radar imagery, we identify a shift of flow pattern starting in 2017 when shear margins formed within the grounded marine piedmont fan. Multiple summer speedups correlate with warmer summers during 2015–2019 and suggest that surface melt may access the subglacial environment. Force balance analysis and examination of the Péclet number show that glacier thinning propagated upstream in 2016–2017, and diffusion became a significant dynamic response to thinning perturbations. Our results suggest that the glacier has entered a new ice stream‐like regime. Plain Language Summary: A glacier surge is a sudden speedup of glacier flow coinciding with a large advance of the ice front. Some glaciers surge periodically every 10–100 years, and so surge mechanism is thought to be independent of climate change. However, some recent surges have evacuated so much ice that another surge is unlikely to occur in the same place again. A glacier surge at the Vavilov Ice Cap, Russia, is one of these cases. Since 2013, the glacier has drained more than 11% of the ice basin (9.5 Gt) into the ocean. After the initial surge in 2013, the glacier still retains fast flow at around 1.8 km/year, an unusually high and long‐lasting speed for a glacier surge. To understand the future of the surge, we use satellite images to calculate surface elevations and ice speeds, and analyze their change over time for the glacier. The results reveal that the glacier now physically behaves more like an "ice stream," a stream of fast‐flowing ice within an ice sheet, which can probably flow at high speed for a long time and drain ice efficiently. This is the first documented case of an ice stream‐like feature ever being formed. Key Points: Outlet glacier at Vavilov Ice Cap displayed a new regime of ice dynamics in 2017, several years after an initial surge phaseThe transition indicates possible ice stream formation from the analyses of shear margin development, speeds, and Péclet numberMultiple summer speedups are identified since 2015 and their strength correlates with warmer air temperature [ABSTRACT FROM AUTHOR]

We examine the influence of different deglacial histories of the North American ice saddle, which connected the Cordilleran ice sheet and western Laurentide ice sheet, on the stability of the fast‐flowing Amundsen Gulf Ice Stream, located in the northwestern Laurentide ice sheet. We use a simplified marine‐terminating ice stream model to simulate grounding line retreat and compare our predictions to geologic evidence for the rapid collapse of the Amundsen Gulf Ice Stream at 13 ka. We show that observations of ice stream retreat can be used to distinguish between past ice unloading histories, and that the rapid retreat of the Amundsen Gulf Ice Stream near 13 ka is most dynamically consistent with substantial deglaciation of the North American ice saddle from 13 to 11.5 ka, during the Younger Dryas cooling episode. These results suggest that short‐lived changes in ice stream behavior may be used to reveal larger‐scale and longer‐duration ice sheet dynamics. Key Points: Ice streams are sensitive to history of deglaciationWe use ice stream collapse to differentiate between proposed deglacial ice histories [ABSTRACT FROM AUTHOR]

The onset of sliding in ice sheets may not take the form of a sharp boundary between regions at the melting point, in which sliding is permitted, and regions below that temperature, in which there is no slip. Such a hard switch leads to the paradox of the bed naturally wanting to refreeze as soon as sliding has commenced. A potential alternative structure is a region of subtemperate sliding. Here temperatures are marginally below the melting point and sliding velocities slower than they would if the bed was fully temperate. Rather than being controlled by a standard sliding law, sliding velocities are then constrained by the need to maintain energy balance. This thermal structure arises in temperature-dependent sliding laws in the limit of strong sensitivity to temperature. Here, we analyse the stability of such subtemperate regions, showing that they are subject to a set of instabilities that occur at all length scales between ice thickness and ice sheet length. The fate of these instabilities is to cause the formation of patches of frozen bed, raising the possibility of highly complicated cold-to-temperate transitions with spatial structures at short length scales that cannot be resolved in large-scale ice sheet simulation codes. [ABSTRACT FROM AUTHOR]

Flow organization into systems of fast-moving ice streams is a well-known feature of ice sheets. Fast motion is frequently the result of sliding at the base of the ice sheet. Here, we consider how this basal sliding is first initiated as the result of changes in bed temperature. We show that an abrupt sliding onset at the melting point, with no sliding possible below that temperature, leads to rapid drawdown of cold ice and refreezing as the result of the increased temperature gradient within the ice, and demonstrate that this result holds regardless of the mechanical model used to describe the flow of ice. Using this as a motivation, we then consider the possibility of a region of 'subtemperate sliding' in which sliding at reduced velocities occurs in a narrow range of temperatures just below the melting point. We confirm that this prevents the rapid drawdown of ice and refreezing of the bed, and construct a simple numerical method for computing steady-state ice sheet profiles that include a subtemperate region. The stability of such an ice sheet is analysed in a companion paper. [ABSTRACT FROM AUTHOR]

Reconstructions of palaeo-ice sheet retreat in response to climate warming using offshore archives can provide vital analogs for future ice-sheet behavior. At the Last Glacial Maximum, Dogger Bank, in the southern North Sea, was covered by the Eurasian Ice Sheet. However, the maximum extent and behavior of the ice sheet in the North Sea basin is poorly constrained. We reveal ice-marginal dynamics and maximum ice extent at Dogger Bank through sedimentological and stratigraphic investigation of glacial and proglacial lake sediments. We use a large, integrated subsurface dataset of shallow seismic reflection and geotechnical data collected during windfarm site investigation. For the first time, an ice stream is identified at Dogger Bank, based on preserved subglacial bedforms, eskers and meltwater channels. During ice-sheet advance, a terminal thrust-block moraine complex formed, whose crest runs approximately north-northeast to south-southwest. Subsequent ice stream shutdown caused stagnation of ice, and rapid retreat of the ice-sheet margin. The moraine complex, and outwash head from an adjacent ice-sheet lobe to the west, dammed a large (approximately 750 km2) proglacial lake. Subsequent sedimentation infilled the lake with 30 m of glacial outwash sediments. A lobate subaqueous fan formed at the ice-sheet margin, which thins toward the southeast with iceberg scours and ice-rafted debris at the base, and is onlapped by lake sediments calibrated to core as alternating clay and silt laminae, interpreted to be varves. The lake became isolated from the retreating ice-sheet margin, and ice-sheet retreat slowed. Sediment-laden meltwater was supplied to the ice-distal proglacial lake for c. 1500–2000 years. Subsequent ice-sheet retreat off Dogger Bank was more rapid due to the negative subglacial slope. The stepped retreat of rapid downwasting, slow retreat, and a final rapid phase off Dogger Bank occurred after the LGM at around 27 ka and before formation of a ribbon lake, dated previously to 23 ka and approximately 60 m lower in elevation, formed to the north of Dogger Bank. The complicated stratigraphic architecture revealed through these data improves forecasting of ground conditions for turbine footings at Dogger Bank, an important step in the provision of clean, sustainable energy.

Large-ensemble perturbed-parameter forward ice-flow modeling can provide useful insights to uncertainties in inversions for basal drag or other ice-flow parameters. Inversion and data assimilation provide estimates of poorly known parameters that are essential for accurate prognostic modeling. Because ice flow depends on many such parameters with their associated uncertainties, which may interact in nonlinear ways, full uncertainty assessment for parameter estimates is challenging. With rising computational power, it is increasingly practicable to explore co-dependencies and sensitivities. Here, we use a well-characterized higher-order flowline model configured for a well-lubricated ("shelfy") ice stream to run large ensembles, perturbing the magnitude and spatial pattern of basal drag, basal topography, and input flux. We find for steady state that ice-stream velocity and thickness along the entire domain are especially correlated to drag at the downstream end, but with greater local correlation during transients. The modeled effects of basal topographic perturbations on velocity and ice thickness are primarily local. Perturbations of input ice fluxes interact with the others in interesting ways. These insights point to the value of inversions informed by concentrated observations during forced transients such as lake-drainage events, accumulation-rate fluctuations or ice-shelf losses, and to the care needed when interpreting local results of some inversions for basal-drag parameters. • Perturbed-parameter ensembles reveal ice-flow-model uncertainties and interactions. • Basal-friction perturbations at the downstream end especially affect ice-stream flow. • During transients, basal-friction perturbations influence local ice-stream flow more. • Basal-topography perturbations mainly affect ice-stream thickness locally. • Large uncertainties in input flux can obscure other ice-flow controls. [ABSTRACT FROM AUTHOR]

The orientation of several landforms, e.g. drumlins, flutes, crag‐and‐tails, and mega‐scale glacial lineations, records the direction of the overlying ice flow that created them. Populations of such features are used routinely to infer former ice‐flow patterns, which serve as the building blocks of reconstructions of palaeo ice‐sheet evolution. Currently, the conceptualisation of flow patterns from these flow‐direction records is done manually and qualitatively, so the extractable glaciological information is limited. We describe a kriging method (with Matlab code implementation) that calculates continuous fields of ice‐flow direction, convergence, and curvature from the flow‐direction records, and which yields quantitative results with uncertainty estimates. We test the method by application to the subglacial bedforms of the Tweed Valley Basin, UK. The results quantify the convergent flow pattern of the Tweed Palaeo‐Ice Stream in detail and pinpoint its former lateral shear margins and where ice flowed around basal bumps. Ice‐flow parameters retrieved by this method can enrich ice‐sheet reconstructions and investigations of subglacial till processes and bedform genesis. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. We present a quantitative method of reconstructing palaeo ice‐flow fields from flow directions measured from subglacial bedforms (such as drumlins, crag‐and‐tails, and mega‐scale glacial lineations). Application of the method to bedforms of the Tweed Valley, UK, yields new glaciological information and insights about the Tweed Palaeo‐Ice Stream. Matlab code of our toolset and data files of the case study are given at doi: 10.15131/shef.data.6735131. [ABSTRACT FROM AUTHOR]

Firn thickness across the NE Greenland Ice Stream is a function of accumulated strain, with thinner firn in the high‐strain margins of the ice stream. We present a novel technique for extracting firn density from previously collected seismic reflection profiles and apply this technique across both shear margins of NE Greenland Ice Stream. Firn is up to 30 m thinner in the vicinity of the ice stream shear margins. Snow accumulation rates across the ice stream were calculated from airborne ice‐penetrating radar data, calibrated with ground‐based firn density measurements from a shallow core. We find that accumulation is ~20% higher in the shear margins compared to the surroundings. The higher density firn adjacent to shear margins is due to high along‐flow stresses that accelerate firn densification and develops despite the higher accumulation rate favoring lower density. These firn density variations influence subglacial hydropotential by changing the ice surface slope and overburden pressure and may influence subglacial water flow. These results demonstrate the importance of high‐resolution firn surveys in studies of shear‐margin dynamics. Plain Language Summary: Snow that falls on ice sheets is squeezed to ice under its own weight. Old snow during its transformation to ice is called firn. Ice sheets in Greenland and Antarctica are covered in 10–100 m of firn, and many studies on ice sheets require knowledge of firn properties. Observations of the firn, however, are logistically difficult and expensive to make. We present a new technique for estimating firn properties from previously collected seismic surveys. We then use this technique to show that firn is unexpectedly thin along the edges of the fast‐moving Northeast Greenland Ice Stream. Ice and firn stretch as they move into the ice stream and speed up. This causes the firn to change to ice more rapidly, contributing to formation of subtle troughs in the surface along the edge of the ice stream, which trap some extra drifting snow. Properly accounting for this extra snow and the thinner firn allows us to more accurately calculate the weight of the ice. Water under the ice moves to where there is the least weight above it. Ice moves more rapidly where more water lubricates the bed, so this new knowledge contributes to better understanding of the motion of the ice stream. Key Points: We present a novel survey of firn densification across an ice stream shear margin, with new methods for extracting firn velocity profiles from previously collected active seismic reflection profilingFirn thickness across the NE Greenland Ice Stream is a function of accumulated strain and thins by up to 30 m within high‐strain shear margins despite high accumulation thereFirn thickness variations are sufficient to impact subglacial hydropotential, affecting routing of subglacial water flow, with implications for shear margin stability [ABSTRACT FROM AUTHOR]

Abstract Rich material obtained during the glacigenic topography researches shows that the advance of the ice sheet and deglaciation of the area took place in a shape of ice streams, substreams and lobes. During the last glaciation, the south-eastern part of the Baltic Region had been covered by ice on a form of three ice streams, which, in its continental part, were dividing into smaller substreams and lobes differently named in the scientific literature. The goal of the present study is to discuss the glacial multilayerness of the Pleistocene strata in the SE Baltic Region as a phenomenon of palaeo-ice streams, substreams and lobes in the contact zone. The material of previous researches is analysed, as well as published data and new findings of the authors are used. The material available on the Middle Pleistocene Žemaitija till occurring at the depths from 10 to 175 m along the NW–SE distribution of the deposits shows differences in petrographic composition of gravel and pebble beds for the western and eastern sectors. The deposits of the Middle Pleistocene occurring at the surface the Ashmyany Upland and the Lida Plain in this region are thought to be a result of two different asynchronous ice substreams. In the area of the last (Nemunas) glaciation, the ice sheet of two stages – maximum Grūda and the younger Baltija – also formed tills differing in features and composition, thus, testifying the result of different ice streams and substreams. The statistical correlation analysis for the most widely occurring palaeosurfaces of tills in the South Lithuania revealed that the positive coefficient of adjacent surfaces was increasing with the occurrence depth. A conclusion is made that different dynamic activity of ice streams and substreams is a natural and objective phenomenon. Therefore an ice stream, substream or a lobe of the same glaciation but of different stages (phases) could move into a depression formed by another (adjacent) substream or lobe. The multilayered character of the Pleistocene strata in the South Lithuania can be explained by different manifestation of glacial till layers formed by ice streams, substreams or lobes of the same glaciation but of different stages (phases). Application of this conception makes easier the stratigraphic interpretation of the Pleistocene deposits in the region. [ABSTRACT FROM AUTHOR]

Evidence suggesting a reduction of the Laurentide Ice Sheet (LIS) during Marine Isotope Stage 3 (MIS 3) is widespread but the boundaries of the ice sheet and the timing of minimum ice extent are still poorly constrained. Radiocarbon dating of glacially transported marine mollusks in Rae Isthmus ice stream till south of Repulse Bay, Nunavut, Canada, provides new evidence for MIS 3 deglaciation in the north‐west part of Hudson Bay. Middle Wisconsinan finite 14C ages varying between 31.7 and 39.2k 14C a BP (∼35.0 and 43.7k cal a) indicate a marine event(s) for a minimum of ∼8.7k cal years. Our findings suggest ice‐free conditions in the ice stream source area in proto‐Roes Welcome Sound, and therefore significant glacial recession of the LIS margin during the Middle Wisconsinan, much farther in the interior of the ice sheet than previously known. The marine incursion possibly occurred because of a calving bay developed from the north in the Gulf of Boothia − Committee Bay area following the recession and collapse of the ice stream. After the MIS 3 marine episode(s), the topographically controlled ice stream stemming from the eastern end of the Keewatin Ice Divide was re‐activated, and maintained until deglaciation. [ABSTRACT FROM AUTHOR]

In the Polish Lowlands, three main ice lobes, referred to as Odra, Vistula and Mazury, formed during the Last Scandinavian Glaciation. These lobes protruded at least several kilometres beyond the main ice margin and it is believed that they represent terminal parts of the ice streams. Applied geostatistical analysis based on numerous geological data has allowed an approximate reconstruction of the Odra ice lobe substratum in the area of northwest Poland and northeast Germany. A spatial representation of the Odra lobe substratum clearly illustrates the adverse slope of the glacier bed, but also indicates areas morphologically prone to ice streaming. It is suggested that the ice took advantage of local topography that enabled advance out of the Baltic basin; consequently, the ice followed a topographic low which, in combination with favourable hydraulic conditions of its substratum, likely initiated ice streaming.

Recent observations indicate that many marine‐terminating glaciers in Greenland and Antarctica are currently retreating and thinning, potentially due to long‐term trends in climate forcing. In this study, we describe a simple two‐stage model that accurately emulates the response to external forcing of marine‐terminating glaciers simulated in a spatially extended model. The simplicity of the model permits derivation of analytical expressions describing the marine‐terminating glacier response to forcing. We find that there are two time scales that characterize the stable glacier response to external forcing, a fast time scale of decades to centuries, and a slow time scale of millennia. These two time scales become unstable at different thresholds of bed slope, indicating that there are distinct slow and fast forms of the marine ice sheet instability. We derive simple expressions for the approximate magnitude and transient evolution of the stable glacier response to external forcing, which depend on the equilibrium glacier state and the strength of nonlinearity in forcing processes. The slow response rate of marine‐terminating glaciers indicates that current changes at some glaciers are set to continue and accelerate in coming centuries in response to past climate forcing and that the current extent of change at these glaciers is likely a small fraction of the future committed change caused by past climate forcing. Finally, we find that changing the amplitude of natural fluctuations in some nonlinear forcing processes, such as ice shelf calving, changes the equilibrium glacier state. Plain Language Summary: We develop a very simple mathematical model to explain how change in climate causes change in marine glaciers. The model shows that this response mostly occurs in two phases, a fast phase over tens to hundreds of years, and a slow phase over thousands of years. Glaciers have a larger response when they are on flat bedrock or when changes in the length of their ice shelf occur due to iceberg detachment. Even though some glaciers have not thinned or retreated significantly in recent years because they sit on steep downward sloping bedrock, they may experience rapid thinning and retreat in the future as they continue to respond to past climate change over thousands of years. Noise in climate and other processes that cause glaciers to be noisy can potentially cause permanent changes in glacier size. These results indicate that we should include noise when making predictions of glaciers changes so that we can calculate uncertainty in future projections and the impact of noise on permanent glacier size. Key Points: The marine‐terminating glacier response to forcing occurs on two distinct time scalesThe glacier sensitivity to forcing depends on glacier state and nonlinearity in forcing processesThe level of noise in nonlinear forcing processes can change the equilibrium state of a glacier [ABSTRACT FROM AUTHOR]

This paper reconstructs the deglaciation of the Laurentide Ice Sheet (LIS; including the Innuitian Ice Sheet) from the Last Glacial Maximum (LGM), with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides. We build on a recent inventory of Laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the LIS, which we depict in a series of palaeogeographic maps. Results show that the drainage network at the LGM was similar to modern-day Antarctica. The majority of the ice streams were marine terminating and topographically-controlled and many of these continued to function late into the deglaciation, until the ice sheet lost its marine margin. Ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build-up, peak-phase and collapse of the Cordilleran-Laurentide ice saddle. The south-eastern marine margin in Atlantic Canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the LGM. In contrast, the ice streams draining towards the north-western and north-eastern marine margins in the Beaufort Sea and in Baffin Bay appear to have remained stable throughout most of the Late Glacial, and some of them continued to function until after the Younger Dryas (YD). The YD influenced the dynamics of the deglaciation, but there remains uncertainty about the response of the ice sheet in several sectors. We tentatively ascribe the switching-on of some major ice streams during this period (e.g. M'Clintock Channel Ice Stream at the north-west margin), but for other large ice streams whose timing partially overlaps with the YD, the drivers are less clear and ice-dynamical processes, rather than effects of climate and surface mass balance are viewed as more likely drivers. Retreat rates markedly increased after the YD and the ice sheet became limited to the Canadian Shield. This hard-bed substrate brought a change in the character of ice streaming, which became less frequent but generated much broader terrestrial ice streams. The final collapse of the ice sheet saw a series of small ephemeral ice streams that resulted from the rapidly changing ice sheet geometry in and around Hudson Bay. Our reconstruction indicates that the LIS underwent a transition from a topographically-controlled ice drainage network at the LGM to an ice drainage network characterised by less frequent, broad ice streams during the later stages of deglaciation. These deglacial ice streams are mostly interpreted as a reaction to localised ice-dynamical forcing (flotation and calving of the ice front in glacial lakes and transgressing sea; basal de-coupling due to large amount of meltwater reaching the bed, debuttressing due to rapid changes in ice sheet geometry) rather than as conveyors of excess mass from the accumulation area of the ice sheet. At an ice sheet scale, the ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating ice sheet, the final elimination of which was mostly driven by surface melt. [ABSTRACT FROM AUTHOR]