Search

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]

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]

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]