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Summertime solar heating of near-surface snow in central Greenland causes mass loss and grain growth. These depth hoar layers become seasonal markers which are observed in ice cores and snow pits. Mass redistribution associated with depth-hoar formation can change concentrations of immobile chemicals by as much as a factor of two in the depth hoar, altering atmospheric signals prior to archival in ice. For methanesulfonic acid (MSA) this effect is not significant because the summer maximum does not coincide with the density minimum, and the amplitude of the annual (MSA) signal is more than a factor of ten.

Bedforms of Thwaites Glacier, West Antarctica both record and affect ice flow, as shown by geophysical data and simple models. Thwaites Glacier flows across the tectonic fabric of the West Antarctic rift system with its bedrock highs and sedimentary basins. Swath radar and seismic surveys of the glacier bed have revealed soft‐sediment flutes 100 m or more high extending 15 km or more across basins downglacier from bedrock highs. Flutes end at prominent hard‐bedded moats on stoss sides of the next topographic highs. We use simple models to show that ice flow against topography increases pressure between ice and till upglacier along the bed over a distance that scales with the topography. In this basal zone of high pressure, ice‐contact water would be excluded, thus increasing basal drag by increasing ice‐till coupling and till flux, removing till to allow bedrock erosion that creates moats. Till carried across highlands would then be deposited in lee‐side positions forming bedforms that prograde downglacier over time, and that remain soft on top through feedbacks that match till‐deformational fluxes from well upglacier of the topography. The bedforms of the part of Thwaites surveyed here are prominent because ice flow has persisted over a long time on this geological setting, not because ice flow is anomalous. Bedform development likely has caused evolution of ice flow over time as till and lubricating water were redistributed, moats were eroded and bedforms grew. Plain Language Summary: Thwaites Glacier, West Antarctica, is of great interest because ongoing retreat could lead to faster future changes that could notably increase sea‐level rise. Wide‐ranging studies are thus focused on gaining broad understanding of the glacier. Geophysical surveys discussed here show that the bed of the glacier has been sculpted by the flowing ice, eroding large "moats" upglacier and to the sides of bedrock obstacles, and depositing long tails of soft sediment downglacier of those obstacles. We use simple models to show that these features formed over time because of the interactions among ice, bedrock, and subglacial water and sediment, and that the behavior of the glacier must have changed as these features developed. This knowledge does not directly affect estimates of the potential for future sea‐level rise, but does guide further studies to improve those estimates. This knowledge also may help understand the formation of glacially sculpted landscapes left in widespread regions by former ice sheets. Key Points: Extensive, well‐developed bedforms occur beneath upglacier regions of Thwaites Glacier, West Antarctica where ice flow is unremarkablePressure variations from ice flow over bedrock topography caused erosion and deposition, producing stoss‐side moats and lee‐side flutesBedforms both record and influence ice flow [ABSTRACT FROM AUTHOR]

While analysis of glacial seismicity continues to be a widely used method for interpreting glacial processes, the underlying mechanics controlling glacial stick‐slip seismicity remain speculative. Here, we report on laboratory shear experiments of debris‐laden ice slid over a bedrock asperity under carefully controlled conditions. By modifying the elastic loading stiffness, we generated the first laboratory icequakes. Our work represents the first comprehensive lab observations of unstable ice‐slip events and replicates several seismological field observations of glacier slip, such as slip velocity, stress drop, and the relationship between stress drop and recurrence interval. We also observe that stick‐slips initiate above a critical driving velocity and that stress drop magnitude decreases with further increases in velocity, consistent with friction theory and rock‐on‐rock friction laboratory experiments. Our results demonstrate that glacier slip behavior can be accurately predicted by the constitutive rate‐and‐state friction laws that were developed for rock friction. Plain Language Summary: Glacier beds and tectonic faults may at first appear to be quite different, but they share important characteristics. In both cases, motion may be smooth (aseismic creep) or earthquake‐producing "stick‐slip." A powerful physical constitutive relationship called rate‐and‐state friction has been developed to understand earthquakes and smooth slip on tectonic faults. Laboratory experiments reported here simulate glacier‐bed motion by sliding debris‐bearing ice over a rock plate under conditions that are typical for glacier beds. They produce the first laboratory icequakes. Transitions between steady and stick‐slip motions are generated by controlling shearing velocity and other conditions, as predicted by rate‐and‐state friction theory. Future studies can thus apply this physical framework to glacier slip, helping to understand ice motion and its potential to accelerate sea level rise in a warming world. Furthermore, because motion at the glacier bed is often much easier to study than tectonic faults, additional observations of glaciers may provide useful insights into earthquake behavior. Key Points: The first laboratory "icequakes" were generated and slip stability was predicted well by rate‐and‐state frictionLaboratory icequake attributes such as peak slip velocity, stress drop and healing agree well with other laboratory and field observationsThe laboratory icequake events suggest that debris‐bed contacts dominate the sudden slip mechanics [ABSTRACT FROM AUTHOR]

Glacier-erosion rates range across orders of magnitude, and much of this variation cannot be attributed to basal sliding rates. Subglacial till acts as lubricating 'fault gouge' or 'sawdust', and must be removed for rapid subglacial bedrock erosion. Such erosion occurs especially where and when moulin-fed streams access the bed and are unconstrained by supercooling or other processes. Streams also may directly erode bedrock, likely with strong time-evolution. Erosion is primarily by quarrying, aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes. Debris-bed friction significantly affects abrasion, quarrying and general glacier flow. Frost heave drives cirque headwall erosion as winter cold air enters bergschrunds, creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock, and the glacier removes the resulting blocks. Recent subglacial bedrock erosion and sediment flux are in many cases much higher than long-term averages. Over glacial cycles, evolution of glacial-valley form feeds back strongly on erosion and deposition. Most of this is poorly quantified, with parts open to argument. Glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes, motivating vigorous research. [ABSTRACT FROM AUTHOR]

Ice core data record significant and abrupt past climate changes that are associated with large and rapid changes in atmospheric greenhouse gases, such as methane. Due to the gradual close‐off of gas bubbles and the relatively fast diffusion of gases within the firn column, even a discrete or quick step increase in air composition may be smoothed or integrated in the data; current laboratory analyses of gases consider the mean gas content value across all bubbles in a sample, rather than the content of individual bubbles. The convolution of the distribution of trapping ages with the history of atmospheric composition thus smears the measured gas record in each sample. We developed a nondestructive method to determine pressure distribution in all bubbles in a sample and estimate the shape of the trapping function derived from that bubble pressure distribution and site characteristics. Our method works not only for present conditions but also through varying paleo‐atmospheric conditions, while providing accurate measurements of morphological bubble properties. The method is based on using temperature‐driven air bubble migration as a proxy for the pressure of individual bubbles, which we combine with a model for bubbly ice densification to obtain the gas trapping functions and constrain the age distribution of air bubbles for past conditions, which are preserved at different depths. The trapping functions will help us to obtain a more accurate gas signal in the future that is less attenuated through the age distribution of the gas during the close‐off process. Key Points: The gas pressure distribution in bubbly ice can be inferred by measuring velocities of bubbles migrating under a temperature gradientThe bubble close‐off history can be reconstructed from the pressure distributionGas‐trapping functions can be determined for different depths (climatic periods) [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]

The history of the Greenland Ice Sheet has been influenced by the geodynamic response to ice sheet fluctuations, and this interaction may help explain past deglaciations under modest climate forcing. We hypothesize that when the Iceland hot spot passed beneath north‐central Greenland, it thinned the lithosphere and left anomalous heat likely with partially melted rock; however, it did not break through the crust to supply voluminous flood basalts. Subsequent Plio‐Pleistocene glacial‐interglacial cycles caused large and rapidly migrating stresses, driving dike formation and other processes that shifted melted rock toward the surface. The resulting increase in surface geothermal flux favored a thinner, faster‐responding ice sheet that was more prone to deglaciation. If this hypothesis of control through changes in geothermal flux is correct, then the long‐term (105 to 106 years) trend now is toward lower geothermal flux, but with higher‐frequency (≤104 to 105 years) oscillations linked to glacial‐interglacial cycles. Whether the geothermal flux is increasing or decreasing now is not known but is of societal relevance due to its possible impact on ice flow. We infer that projections of the future of the ice sheet and its effect on sea level must integrate geologic and geophysical data as well as glaciological, atmospheric, oceanic, and paleoclimatic information. Plain Language Summary: The behavior of the Greenland Ice Sheet and its effect on future sea level depends on its geologic history as well as on greenhouse warming. The Iceland hot spot passed beneath Greenland millions of years ago, and left hot, possibly melted rock deep beneath the island. Since then, growth and shrinkage of the ice sheet have changed stresses in the rocks beneath. These stress changes may have shifted the melted rock upward, perhaps all the way to the base of the ice sheet, probably in pulses tied to times of rapid ice sheet change. This would have changed the heat flow from the Earth into the base of the ice, which affects how easily the ice sheet grows and shrinks. The future of the ice sheet depends primarily on how much the climate warms, but better understanding of the interactions between the ice and the rocks beneath will allow better predictions of ice sheet changes. Key Points: Ice sheet fluctuations affect the lithosphere and asthenosphereGreenland Ice Sheet fluctuations may have extracted melt from the track of the Iceland hot spotResulting changes in geothermal flux may help explain past and ongoing ice sheet evolution [ABSTRACT FROM AUTHOR]

Despite the importance of grounding zone sedimentation for ice sheet stability and ice sheet history, evidence for sedimentary deposits beneath West Antarctica's modern grounding zone remains sparse. Seismic surveying shows that Kamb Ice Stream has no detectable grounding zone deposit. As grounding zone deposition relies strongly on ice flow, the absence of a deposit suggests that the transition from the ice stream to the ice shelf has moved after stagnation of Kamb Ice Stream. Further support for a recent grounding zone occupation comes from satellite imagery of sub-ice shelf channel features that likely originated at previous grounding-zone locations. These features begin 25 km seaward of the current grounding zone and cut across ice flow streak lines. We estimate that retreat to the modern grounding-zone position was abrupt at rates >0.2 km a−1. [ABSTRACT FROM AUTHOR]

The Northeast Greenland Ice Stream (NEGIS) is the sole interior Greenlandic ice stream. Fast flow initiates near the summit dome, and the ice stream terminates approximately 1000 km downstream in three large outlet glaciers that calve into the Greenland Sea. To better understand this important system, in the summer of 2012 we drilled a 67m firn core and conducted ground-based radio-echo sounding (RES) and active-source seismic surveys at a site approximately 150 km downstream from the onset of streaming flow (NEGIS firn core, 75°37.61' N, 35°56.49' W). The site is representative of the upper part of the ice stream, while also being in a crevasse-free area for safe surface operations. Annual cycles were observed for insoluble dust, sodium and ammonium concentrations and for electrolytic conductivity, allowing a seasonally resolved chronology covering the past 400 yr. Annual layer thicknesses averaged 0.11m ice equivalent (i.e.) for the period 1607-2011, although accumulation varied between 0.08 and 0.14mi.e., likely due to flowrelated changes in surface topography. Tracing of RES layers from the NGRIP (North Greenland Ice Core Project) ice core site shows that the ice at NEGIS preserves a climatic record of at least the past 51 kyr. We demonstrate that deep ice core drilling in this location can provide a reliable Holocene and late-glacial climate record, as well as helping to constrain the past dynamics and ice-lithosphere interactions of the Greenland Ice Sheet. [ABSTRACT FROM AUTHOR]

Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal meltwater to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km). [ABSTRACT FROM AUTHOR]

Subglacial stick-slip motion speeds erosion by hydrofracturing and in other ways, as determined from analysis of the growing body of field data. Microearthquake monitoring commonly detects subglacial earthquakes, likely mostly from stick-slip motion of debris-laden ice over bedrock. Source parameters show that many quakes cause enough motion to greatly lower water pressure in cavities on the lee sides of bedrock steps. We calculate that the resulting expansion of higher-pressure water in nearby cracks promotes hydrofracturing, with even relatively small cracks growing unstably under thick glaciers and all cracks growing faster than for aseismic behavior. This mechanism also helps generate the step-like topography favoring block plucking. This stick-slip glacier-erosion hypothesis suggests that the erosion rate will increase with ice thickness as well as basal shear stress, ice-flow velocity, and water supply. [ABSTRACT FROM AUTHOR]

The article presents the correct information on the mistaken publisher's media release on the shrinkage of permanent ice cover in Greenland published on a map in the 2011 edition of the "Times Comprehensive Atlas of the World." It states that the 2011 ice margin encloses the ice sheet and ice bodies, and an alternative Greenland land surface classification yields represent standard deviation of 12 annual classifications. It also discusses the magnitude of sinkage rate and mass budget.

The article focuses on a study on the climate evolution at the West Antarctic ice sheet (WAIS) Divide site in West Antarctica during the late-Holocene period. It is said that instrumental data regarding climate change can be provided by ice cores as they concern constraints on the history of certain variables of climate. Under the study, bubble number-density estimates from the study area have been used in order to create a paleoclimate reconstruction for the late-Holocene period. Findings indicate a linear cooling of about ~1.7° Celsius (C) during the study period.

The article presents a study regarding the evaluation of two different cosmogenic exposure dating methods for understanding glacier size changes as affected by climate change. It mentions that by the assessment of moraine degradation and cosmogenic nuclides inheritance based on Monte Carlo models the exposure dates of moraines can be calculated. It also discusses that the representative distributions based on the youngest and oldest date mean statistics can provide correct exposure dates.

New measurements of the carbon and hydrogen isotopic ratios of methane (ö13C of CH4 and δD of CH4) over the last millennium are presented from the WAIS Divide, Antarctica, ice core (WDC05A), showing significant changes that likely were the result of human influences prior to the industrial revolution (at least as early as the 16th century of the common era (CE)). The δ13C of CH4 data corroborate the record from Law Dome, Antarctica, with high fidelity. The new SD of CH4 data set covaries with the δ13C of CH4 record. Both ö13C of CH4 and SD of CH4 were relatively stable and close to the present-day values from ∼1000 to ∼1500 CE. Both isotopic ratios decreased to minima around 1700 CE, remained low until the late 18th century, and then rose exponentially to present-day values. Our new δD of CH4 data provide an additional independent constraint for evaluating possible CH4 source histories. We searched a broad range of source scenarios using a simple box model to identify histories consistent with the constraints of the CH4 concentration and isotope data from 990-1730 CE. Results typically show a decrease over time in the biomass-burning source (found in 85% of acceptable scenarios) and an increase in the agricultural source (found in 77% of acceptable scenarios), indicating preindustrial human influence, on atmospheric methane as proposed in previous studies. [ABSTRACT FROM AUTHOR]

Determining the mass balance of the Greenland and Antarctic ice sheets (GIS and AIS) has long been a major challenge for polar science. But until recent advances in measurement technology, the uncertainty in ice sheet mass balance estimates was greater than any net contribution to sea level change. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (AR4) was able, for the first time, to conclude that, taken together, the GIS and AIS have probably been contributing to sea level rise over the period 1993-2003 at an average rate estimated at 0.4mmyr-1. Since the cut-off date for work included in AR4, a number of further studies of the mass balance of GIS and AIS have been made using satellite altimetry, satellite gravity measurements and estimates of mass influx and discharge using a variety of techniques. Overall, these studies reinforce the conclusion that the ice sheets are contributing to present sea level rise, and suggest that the rate of loss from GIS has recently increased. The largest unknown in the projections of sea level rise over the next century is the potential for rapid dynamic collapse of ice sheets. [ABSTRACT FROM AUTHOR]

Presents the electrical conductivity records for the Greenland Ice Sheet Project 2 (GISP2) and Greenland Ice-core Project (GRIP) ice-cores. Dependence of direct-current conductivity of glacial ice discrepancy between the two records.