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We use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5 13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5 10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

Interferometric synthetic aperture radar (InSAR) observations of ice-shelf tidal deformation reveal the wide transition between grounded and floating ice as well as local areas where the ice shelf is only grounded at low tide, a condition that we call ephemeral grounding. Ephemeral grounding creates a subtle, local disturbance on the vertical motion field of the ice-shelf surface in response to changes in occanic tide which is detected with millimetric precision using InSAR. These ice-shelf features are, however, not expected to produce a noticeable disturbance on the ice-shelf velocity field. To illustrate the influence of ephemeral grounding on ice-shelf creep flow, we use a finite-element model in which ephemeral grounding is incorporated through a variable basal friction coefficient. The results show that while ice rises (permanently grounded areas) have a pronounced influence on the ice-shelf velocity field, areas of ephemeral grounding have a vanishingly small influence. What is thus of most interest is the capacity for observations of ephemeral grounding to reveal subtle changes in ice-shelf thickness over time. We discuss an example in the Thwaites Glacier area, West Antarctica, where multi-year data show how ice rises become ephemeral grounding and subsequently disappear. This result is consistent with the grounding-line retreat and ice thinning of Thwaites Glacier.

Fifteen synthetic aperture radar (SAR) images of the Ronne Ice Shelf (also referred to as the Filchner-Ronne Ice Shelf), Antarctica, obtained by the European remote-sensing satellites ERS-1 and -2, are used to study ice-shelf dynamics near two ends of the iceberg-calving front. Interferograms constructed from these SAR images are used to resolve the ice-shelf displacement along several directions in response to both ocean tide and long-term creep flow. Tidal motion is separated from creep flow using differential interferometry, i.e. two or more interferograms in which fringe patterns common to all are predominantly associated with creep flow. Creep-flow velocities thus determined compare well with prior ice-shelf velocity surveys. Using these data, we studied the influence of large-scale rifts, ice rises and coastal separation on the ice-shelf flow. Many of the large rifts that appear to form the boundaries where tabular icebergs may eventually detach from the ice shelf are filled with a melange of sea ice, ice-shelf debris and wind-blown snow. The interferograms show that this melange tends to deform coherently in response to the ice-shelf flow and has sufficient strength to trap large tabular ice-shelf fragments for several decades before the fragments eventually become icebergs. In many instances, the motion of the tabular fragments is a rigid-body rotation about a vertical axis that is driven by velocity shear within the melange. The mechanical role of the rift-filling melange may be to bind tabular ice-shelf fragments to the main ice shelf before they calve. This suggests two possible mechanisms by which climate could influence tabular iceberg calving. First, spatial gradients in oceanic and atmospheric temperature may determine where the melange melts and, thus, the location of the iceberg-calving margin. Second, melting or weakening of ice melange as a consequence of climate change could trigger a sudden or widespread release of tabular icebergs and lead to rapid ice-shelf disintegration.

AbstractWe compare European remote-sensing satellite (ERS) synthetic aperture radar interferograms with artificial interferograms constructed using output of a finite-element ice-shelf flow model to study the dynamics of Filchner-Ronne Ice Shelf (FRIS), Antaretica, near Hemmen Ice Rise (HIR) where the iceberg-calving front intersects Berkner Island. We find that the model must account for rifts, mechanically competent sea ice which fills rifts, and ice softening in coastal boundary layers in order to agree with the ice-deformation pattern implied by observed interferograms. Analysis of the stress field in the model experiment that best matches the observed interferograms suggests that: (1) HIR introduces weakness into the ice shelf through the generation of large-scale rifts, and (2) the melange of sea ice and ice-shelf fragments that fills the rifts stabilizes the shelf front by providing mechanical coupling between the fractured shelf front and the adjacent coast. The rift-filling melange could melt more easily than the surrounding ice shelf and thus could represent a vulnerability of the FRIS to climate warming.

A time-dependent, flow-line ice-sheet model is used to explore interactions between last glacial climate, the Laurentide ice sheet, and the Great Lakes. We seek to understand perturbations in the ice-flow field that caused three neighboring lobes to leave different geomorphic and sedimentary records. Driven by reconstructed air-temperature variations from 65 to 18 ka, the simulated lobe dynamics are consistent with constraints from loess and till chronologies, till stratigraphy, and ice-surface profiles. Contrasts in lobe dynamics are best explained by the geometry of lakes along each flow line. As ice entered these lakes, calving influenced glacier mass balance. In particular, deep water in Lake Superior likely delayed ice advance into northern Wisconsin by promoting large calving losses, whereas lobes to the east that encountered shallower water were less affected by calving. The Driftless Area of Wisconsin may thus owe its existence, at least in part, to the presence of Lake Superior. Our results suggest that morainal-bank evolution should be treated in ice-sheet models in lacustrine and shallow-marine settings. Determining the sediment flux to the morainal bank remains a difficult task. For example, ice advance across central Lake Superior was probably sediment-flux limited. Otherwise, the Driftless Area would have been glaciated. Keywords: Laurentide ice sheet, numerical model, Great Lakes, glacial geology, mass balance.

We used a numerical ice-sheet model to reconstruct the North American Laurentide Ice Sheet during the last glacial maximum. Our model simulates ice-sheet conditions that can be specified experimentally as either a rigid substrate (hard bed) or a wet, deformable till (soft bed); basal sliding is excluded. We use geologic records of former basal ice-sheet processes to prescribe the distribution of hard and soft beds. Our reconstruction of the Laurentide Ice Sheet is significantly lower in ice-surface height and contains less ice volume than the CLIMAP (maximum) reconstruction. In contrast, our reconstruction agrees well with the ICE-4G reconstruction, both in height and volume. Because the ICE-4G reconstruction is based on the inversion of relative sea-level data, whereas our reconstruction is based on glacial geology and ice mechanics, this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the Laurentide Ice Sheet that is most consistent with observations of relative sea-level change and other geodynamic considerations.

The collapse of the West Antarctic ice sheet (WAIS) may have occurred sporadically and chaotically during the last 1,000,000 years. An ice sheet collapse occurs when ice flow turns a partly grounded, partly floating ice sheet into an entirely floating ice sheet. The random nature of these collapses results from the slow equilibration time of the basal till's distribution. Attempts at forecasting new collapses due to global warming must incorporate the irregularity that has marked the collapse of WAIS in the past.

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]

D.H. Mooers claims that the ice margin used for Minnesota which was taken from studies done by Dyke and Prest in 1987 was incorrect. He believes that the ice dome southwest of Hudson Bay may have been smaller. A reconstruction of the Laurentide ice sheet was done again using 21,000-22,000 14C year B.P. as suggested by Mooers. The results obtained agrees with Mooers' contention that an ice dome southwest of the Hudson Bay is 600 meters shorter than what it would have been had it been reconstructed using Dyke and Prest's ice margin.

Ice sheet modelling experiments on two large, but temporary, glacial meltwater inflows show that the breaking up of the Eurasian ice sheet (EIS) led to the first of the two meltwater inflows. The increase in the rise of the sea-level was caused when the ice sheet moved quickly toward iceberg-calving margins, and not when the ice-sheet melted.

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]