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Johannes Weertman published his first glaciological paper in 1957 only 5 years after getting his DSc in metallurgy from the Carnegie Institute of Technology. The paper presented the very first sliding law developed quantitatively from first principles, and involved the unconventional idealization of bed roughness using cubic 'tombstones' of rock. Since 1957, there has been a great deal of progress in understanding glacier sliding, but few studies, if any, have preserved the original tombstone geometry that was a hallmark of this first theory. The current study presents a partial reanalysis of the sliding process over a bed with tombstone obstacles using modern numerical methods. The result confirms the enduring applicability of Weertman's model as a pedagogical tool and motivates new questions about (1) folding flow near bedrock obstacles that invert normal ice stratigraphy, (2) the presence and role of stress singularities on sharp edges of bedrock, and (3) the validity of a presumption that regelation flow can be plug-like. [ABSTRACT FROM AUTHOR]

Johannes Weertman published his first glaciological paper in 1957 only 5 years after getting his DSc in metallurgy from the Carnegie Institute of Technology. The paper presented the very first sliding law developed quantitatively from first principles, and involved the unconventional idealization of bed roughness using cubic 'tombstones' of rock. Since 1957, there has been a great deal of progress in understanding glacier sliding, but few studies, if any, have preserved the original tombstone geometry that was a hallmark of this first theory. The current study presents a partial reanalysis of the sliding process over a bed with tombstone obstacles using modern numerical methods. The result confirms the enduring applicability of Weertman's model as a pedagogical tool and motivates new questions about (1) folding flow near bedrock obstacles that invert normal ice stratigraphy, (2) the presence and role of stress singularities on sharp edges of bedrock, and (3) the validity of a presumption that regelation flow can be plug-like. [ABSTRACT FROM AUTHOR]

Supraglacial lakes are known to trigger Antarctic ice-shelf instability and break-up. However, to date, no study has focused on lakes on Greenland's floating termini. Here, we apply lake boundary/area and depth algorithms to Landsat 8 imagery to analyse the inter- and intraseasonal evolution of supraglacial lakes across Petermann Glacier's (81°N) floating tongue from 2014 to 2016, while also comparing these lakes to those on the grounded ice. Lakes start to fill in June and quickly peak in total number, volume and area in late June/early July in response to increases in air temperatures. However, through July and August, total lake number, volume and area all decline, despite sustained high temperatures. These observations may be explained by the transportation of meltwater into the ocean by a river, and by lake drainage events on the floating tongue. Further, as mean lake depth remains relatively constant during this time, we suggest that a large proportion of the lakes that drain, do so completely, likely by rapid hydrofracture. The mean areas of lakes on the tongue are only ~20% of those on the grounded ice and exhibit lower variability in maximum and mean depth, differences likely attributable to the contrasting formation processes of lakes in each environment. [ABSTRACT FROM AUTHOR]

On 2 March 2016, several small en échelon tabular icebergs calved from the seaward front of the McMurdo Ice Shelf, and a previously inactive rift widened and propagated by ~3 km, ~25% of its previous length, setting the stage for the future calving of a ~14 km2 iceberg. Within 24 h of these events, all remaining land-fast sea ice that had been stabilizing the ice shelf broke-up. The events were witnessed by time-lapse cameras at nearby Scott Base, and put into context using nearby seismic and automatic weather station data, satellite imagery and subsequent ground observation. Although the exact trigger of calving and rifting cannot be identified definitively, seismic records reveal superimposed sets of both long-period (>10 s) sea swell propagating into McMurdo Sound from storm sources beyond Antarctica, and high-energy, locally-sourced, short-period (<10 s) sea swell, in the 4 days before the fast ice break-up and associated ice-shelf calving and rifting. This suggests that sea swell should be studied further as a proximal cause of ice-shelf calving and rifting; if proven, it suggests that ice-shelf stability is tele-connected with far-field storm conditions at lower latitudes, adding a global dimension to the physics of ice-shelf break-up. [ABSTRACT FROM AUTHOR]

Supraglacial meltwater lakes trigger ice-shelf break-up and modulate seasonal ice-sheet flow, and are thus agents by which warming is transmitted to the Antarctic and Greenland ice sheets. To characterize supraglacial lake variability we perform a comparative analysis of lake geometry and depth in two distinct regions, one on the pre-collapse (2002) Larsen B ice shelf, Antarctica, and the other in the ablation zone of Paakitsoq, a land-terminating region of the Greenland ice sheet. Compared to Paakitsoq, lakes on the Larsen B ice shelf cover a greater proportion of surface area (5.3% cf. 1%), but are shallower and more uniform in area. Other aspects of lake geometry (e.g. eccentricity, degree of convexity (solidity) and orientation) are relatively similar between the two regions. We attribute the notable difference in lake density and depth between ice-shelf and grounded ice to the fact that ice shelves have flatter surfaces and less distinct drainage basins. Ice shelves also possess more stimuli to small-scale, localized surface elevation variability, due to the various structural features that yield small variations in thickness and which float at different levels by Archimedes' principle. [ABSTRACT FROM AUTHOR]

A conspicuous precursor of catastrophic ice-shelf break-up along the Antarctic Peninsula, reported widely in the literature, is the gradual increase in surface melting and consequent proliferation of supraglacial lakes and dolines. Here we present analytical and numerical solutions for the flexure stresses within an ice shelf covered by lakes and dolines, both isolated and arrayed. We conclude that surface water promotes ice-shelf instability in two ways: (1) by water-assisted crevasse penetration, as previously noted, and (2) by the inducement of strong tensile flexure stresses (exceeding background spreading stress by 10-100 times) in response to surface water mass loads and 'hydrostatic rebound' occurring when meltwater lakes drain. [ABSTRACT FROM AUTHOR]

The article discusses a study of the role of the Honshu earthquake and tsunami in Japan in March 2011 in an Antarctic ice-shelf calving. The calving of the rift-bounded section of ice adjacent to Guest Peninsula from the Sulzberger Ice Shelf (SIS) occurred on March 13. The event was confirmed by European Space Agency (ESA) Envisat advanced synthetic aperture radar (ASAR) images. One of the factors that enabled the tsunami to act as a trigger for ice calving was the sea-ice condition. It emphasizes the key role of environmental and tectonic conditions in the far field in ice-shelf stability in the Antarctic.

The article discusses the study which examines the vertical motions of the large tidally modulated flow that interact with the ice streams feeding the Ross Ice Shelf in Antarctica. It states that the flow of the Ross Ice Shelf front ice streams exhibits smooth and sinusoidal motions corresponding to tidal modulation. It notes that the possible linking of the ice shelf with the ice streams to form a unified system has responded to small perturbations in stresses associated with ocean tides. It confirms the presence of the sinusoidal motion and damping of very high-frequency signals despite the absence of stick-slip motions.

The article focuses on the seismic observations of glaciogenic ocean waves on icebergs and ice shelves in the Ross Sea and on the Ross Ice Shelf in Antarctica. Investigation shows that the impulsive sources of ocean surface waves are frequent in the ice-shelf and iceberg-covered environment of coastal Antarctica. Field deployment of seismometers suggests that these impulsive events are generated through glaciological mechanisms. These mechanisms include small-scale calving and edge wasting of icebergs and ice-shelf fronts, edge-on-edge closing and opening associated with iceberg collisions and the impulsive opening of void space related to ice-shelf rifting. Observations offer a background of glaciogenic ocean-wave phenomena significant to the Ross Sea.

States that model simulations of the West Antarctic ice sheet suggest that sporadic, perhaps chaotic, collapse (complete mobilization) of the ice sheet occurred throughout the past one million years. Causes of the irregular behavior; Concern that the West Antarctic ice sheet might collapse in response to future climate warming; Presentation of a whole ice-sheet model; More.

We calculate the acoustic reflection coefficient for plane waves, in a homogeneous fluid half-space, incident at arbitrary angle upon an underlying half-space of unconsolidated bottom sediments. The lower half-space, treated here as a fluid, has constant density and a sound speed that increases linearly with depth. An exact formal expression is found for the complex reflection coefficient R, in terms of a modified Bessel function. With no absorption, | R | is unity for all angles except at normal incidence, as would also be expected from ray theory. Assuming bottom absorption, we derive an analytical approximation for R, valid for incident angles [variant_greek_theta] (measured from the interface normal) in the range 10°<=[variant_greek_theta]<=70°, for acoustic frequencies f>=10 Hz, and to about 80° for f>=70 Hz. A high-frequency limit and a small-angle approximation are found. Use of a pseudolinear gradient, suitable at large angles, could extend [variant_greek_theta] to 90°. A more realistic model having layers in the upper part of the bottom is discussed briefly. [ABSTRACT FROM AUTHOR]

Global sea-level rise is caused, in part, by more rapid ice discharge from Antarctica, following the removal of the restraining forces of floating ice-shelves after their break-up. A trigger of ice-shelf break-up is thought to be stress variations associated with surface meltwater ponding and drainage, causing flexure and fracture. But until now, there have been no direct measurements of these processes. Here, we present field data from the McMurdo Ice Shelf, Antarctica, showing that the filling, to ~2 m depth, and subsequent draining, by overflow and channel incision, of four surface lakes causes pronounced and immediate ice-shelf flexure over multiple-week timescales. The magnitude of the vertical ice-shelf deflection reaches maxima of ~1 m at the lake centres, declining to zero at distances of <500 m. Our results should be used to guide development of continent-wide ice-sheet models, which currently do not simulate ice-shelf break-up due to meltwater loading and unloading. Meltwater ponding on top of ice shelves is thought to play a role in ice-shelf flexure and fracture, however in-situ evidence of these mechanisms is lacking. Here, the authors provide field-based evidence showing the impact of the filling and draining of four surface lakes on ice-shelf flexure in Antarctica. [ABSTRACT FROM AUTHOR]

Climate varied enormously over the most recent ice age-for example, large pulses of ice-rafted debris, originating mainly from the Labrador Sea, were deposited into the North Atlantic at roughly 7,000-year intervals, with global climatic implications. Here we show that ocean tides within the Labrador Sea were exceptionally large over the period spanning these huge, abrupt ice movements, which are known as Heinrich events. We propose that tides played a catalytic role in liberating iceberg armadas during that time. [ABSTRACT FROM AUTHOR]

We calculate the acoustic reflection coefficient for plane waves, in a homogeneous fluid half space, incident at arbitrary angle upon an underlying half space of bottom sediments. The latter, treated here as a fluid of constant density, has a sound speed c increasing with depth z, dc/dz being constant (a behavior frequently encountered). An exact formal expression is found for the complex reflection coefficient R, in terms of a modified Bessel function. With no absorption, |R| is unity for all angles, as expected from ray theory. Assuming bottom absorption, we derive an analytic approximation for R, valid at low acoustic frequencies (up to a few kilohertz, at least). Numerical results are described. Incident angles too near the normal must be excluded, but a different approximation serves for small angles, including normal incidence. A more realistic model having layers in the upper part of the bottom is examined. In effect, it now reduces to problems already solved. [Supported by Naval Electronics Systems Command, Department of the Navy, Washington, DC, under Contract N00039-76-C-0081.] [ABSTRACT FROM AUTHOR]