Charles R. Bentley

Sedimentation beneath ice shelves — the view from ice stream B

Abstract Ice-shelf development is favored by rapid flow of cold ice from outlet glaciers or ice streams into protected embayments with localized high spots. Basal melting of ice shelves is rapid near the ice front and may occur near the grounding line. Ice from outlet glaciers may contain significant englacial debris that is deposited as a dropstone diamicton in regions of basal melting. Englacial debris is sparse or absent in ice streams. Evidence from ice stream B, draining into the Ross Ice Shelf of West Antarctica, suggests that the rapid ice velocity arises from deformation of a several-meter-thick, water-saturated basal till layer that is eroding an unconformity on sediments beneath and that has deposited a “till delta” tens of meters thick and tens of kilometers long at the grounding line. Sea-level fall would cause “conveyor belt” recycling of this till delta and grounding-line advance across the Ross Sea to the edge of the continental shelf, forming an ice sheet with a low, ice-stream profile resting on a several meter-thick deforming till layer eroding an unconformity. The modern Ross Sea is characterized by a regional unconformity overlain by a diamicton of probable latest Pliocene-Pleistocene age measuring several meters to tens of meters thick. We hypothesize that this diamicton is a deformed glacial till and that the Ross Sea sediments record one or more expansions of the till-lubricated West Antarctic ice sheet to the edge of the continental shelf.

A model for Holocene retreat of the West Antarctic Ice Sheet

Abstract Marine ice sheets are grounded on land which was below sea level before it became depressed under the ice-sheet load. They are inherently unstable and, because of bedrock topography after depression, the collapse of a marine ice sheet may be very rapid. In this paper equations are derived that can be used to make a quantitative estimate of the maximum size of a marine ice sheet and of when and how rapidly retreat would take place under prescribed conditions. Ice-sheet growth is favored by falling sea level and uplift of the seabed. In most cases the buttressing effect of a partially grounded ice shelf is a prerequisite for maximum growth out to the edge of the continental shelf. Collapse is triggered most easily by eustatic rise in sea level, but it is possible that the ice sheet may self-destruct by depressing the edge of the continental shelf so that sea depth is increased at the equilibrium grounding line. Application of the equations to a hypothetical “Ross Ice Sheet” that 18,000 yr ago may have covered the present-day Ross Ice Shelf indicates that, if the ice sheet existed, it probably extended to a line of sills parallel to the edge of the Ross Sea continental shelf. By allowing world sea level to rise from its late-Wisconsin minimum it was possible to calculate retreat rates for individual ice streams that drained the “Ross Ice Sheet.” For all the models tested, retreat began soon after sea level began to rise (∼15,000 yr B.P.). The first 100 km of retreat took between 1500 and 2500 yr but then retreat rates rapidly accelerated to between 0.5 and 25 km yr −1 , depending on whether an ice shelf was present or not, with corresponding ice velocities across the grounding line of 4 to 70 km yr −1 . All models indicate that most of the present-day Ross Ice Shelf was free of grounded ice by about 7000 yr B.P. As the ice streams retreated floating ice shelves may have formed between promontories of slowly collapsing stagnant ice left behind by the rapidly retreating ice streams. If ice shelves did not form during retreat then the analysis indicates that most of the West Antarctic Ice Sheet would have collapsed by 9000 yr B.P. Thus, the present-day Ross Ice Shelf (and probably the Ronne Ice Shelf) serves to stabilize the West Antarctic Ice Sheet, which would collapse very rapidly if the ice shelves were removed. This provides support for the suggestion that the 6-m sea-level high during the Sangamon Interglacial was caused by collapse of the West Antarctic Ice Sheet after climatic warming had sufficiently weakened the ice shelves. Since the West Antarctic Ice Sheet still exists it seems likely that ice shelves did form during Holocene retreat. Their effect was to slow and, finally, to halt retreat. The models that best fit available data require a rather low shear stress between the ice shelf and its sides, and this implies that rapid shear in this region encouraged the formation of a band of ice with a preferred crystal fabric, as appears to be happening today in the floating portions of fast bounded glaciers. Rebound of the seabed after the ice sheet had retreated to an equilibrium position would allow the ice sheet to advance once more. This may be taking place today since analysis of data from the Ross Ice Shelf indicates that the southeast corner is probably growing thicker with time, and if this persists then large areas of ice shelf must become grounded. This would restrict drainage from West Antarctic ice streams which would tend to thicken and advance their grounding lines into the ice shelf.

Water-pressure coupling of sliding and bed deformation. I: Water system. II: Velocity-depth profiles. III: Application to ice stream B, Antarctica

Analysis of the likely behavior of a water system developed between ice and an unconsolidated glacier bed suggests that, in the absence of channelized sources of melt water, the system will approximate a film of varying thickness. The effective pressure in such a film will be proportional to the basal shear stress but inversely proportional to the fraction of the bed occupied by the film. These hypotheses allow calculation of the sliding and bed-deformation velocities of a glacier from the water supply and basal shear stress, as discussed in the second and third papers in this series.

A method of combining ICESat and GRACE satellite data to constrain Antarctic mass balance

Measurements from the Geoscience Laser Altimeter System (GLAS) aboard NASAs ICESat satellite (2001 launch) will be used to estimate the secular change in Antarctic ice mass. We have simulated 5 years of GLAS data to infer the likely accuracy of these GLAS mass balance estimates. We conclude that ICESat will be able to determine the linear rate of change in Antarctic ice mass occurring during those 5 years to an accuracy of similar to 7 mm/yr equivalent water thickness when averaged over the entire ice sheet. By further including the difference between the typical 5-year trend and the long-term (i.e., century-scale) trend, we estimate that GLAS should be able to provide the long-term trend in mass to an accuracy of about +/-9 mm/yr of equivalent water thickness, corresponding to an accuracy for the Antarctic contribution to the century-scale global sea level rise of about +/-0.3 mm/yr. For both cases the principal error sources are inadequate knowledge of postglacial rebound and of complications caused by interannual and decadal variations in the accumulation rate. We also simulate 5 years of gravity measurements from the NASA and Deutsches Zentrum fur Luft-und Raumfahrt (DLR) satellite mission Gravity Recovery and Climate Experiment (GRACE)(2001 launch). We find that by combining GLAS and GRACE measurements, it should be possible to slightly reduce the postglacial rebound error in the GLAS mass balance estimates. The improvement obtained by adding the gravity data would be substantially greater for multiple, successive altimeter and gravity missions.

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.

Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC, West Antarctica

Digital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6 km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing remanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method. Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.

Ice-Core Analysis on the Siple Coast of West Antarctica

Pit and core studies at the Upstream B camp (UpB) and ridge BC (BC) on the Siple Coast, West Antarctica, have revealed a number of interesting results. Both sites have 10 m temperatures near -26.5 ° C and accumulation rates near 0.09 m a1 of ice. At both sites, the low densities of annual depth-hoar layers arise in the first year following snow deposition. Densification rates at UpB are accelerated compared to BC and to other sites, probably owing to stress enhancement of power-law-creep densification caused by the large longitudinal deviatoric stresses at UpB. The connectivity of firn, measured by the number of bonds per grain and by the fraction of total surface area per grain involved in bonds, correlates well with density and shows no significant dependence on grain-size. The longitudinal stresses at UpB cause the ice to develop a fabric of horizontally elongated bubbles and grains, interpenetrating grains, and strain shadows between about 30-70 m. Ice at BC below about 95 m shows rapid grain growth and development of a bimodal grain-size distribution, but no preferred orientation of grains or pores. The temperaturedepth profile to lOO m at BC suggests a basal heat flux near 1.9 heat-flow units (HFU), typical of a region of active rifting. Surface-melt events occur about once every 50 years, and correlate well between UpB and BC.

Grain growth in polar ice. II: Application

Grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials (microparticles, bubbles, and dissolved impurities) are characterized fully. Dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice. The high concentration of soluble impurities in Wisconsinan ice from the Dome C (Antarctica) ice core (and perhaps other ice cores) probably causes the small grain-sizes observed in that ice. Microparticles have little effect on grain growth in ordinary ice. In ice layers that appear dirty owing to concentrations of volcanic tephra (such as in the Byrd Station (Antarctica) ice core) or of morainal material, micro particles reduce grain-growth rates significantly. The relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks, so grain growth in firn is limited by boundary migration rather than by neck growth. Bubbles formed by pore close-off at the firn-ice transition are less mobile than grain boundaries, causing bubble-boundary separation whenever geometric constraints are satisfied; however, such separation reduces grain-growth rates by only about 10%. The observed linear increase of grain area with time is thus predicted by theory, but the growth rate depends on soluble-impurity concentrations as well as on temperature.

Field Studies of Bottom Crevasses in the Ross Ice Shelf, Antarctica

Analyse des crevasses observees par sondages radar aeriens et terrestres, localisation de leurs points de formation

Antarctic crustal modeling from the spectral correlation of free‐air gravity anomalies with the terrain

We investigated the use of enhanced spectral correlation theory for modeling the crustal features of the Antarctic from regional observations of gravity and terrain. The analysis considered 1°-gridded free-air gravity anomalies and topographic rock, ice, and water components for the region south of 60°S. We modeled terrain gravity effects at 150-km altitude by Gauss-Legendre quadrature (GLQ) integration assuming densities of 2800 kg/m3 for rock, 900 kg/m3 for ice, and 1030 kg/m3 for seawater. These effects are substantial relative to the free-air anomalies and must be compensated by the effects of subsurface density variations. Significant terrain-correlated free-air anomalies were revealed by the wavenumber correlation spectrum between the free-air anomalies and the modeled terrain gravity effects, which we interpreted mostly to reflect possible isostatic imbalances of the crust. Subtracting the terrain-correlated free-air anomalies from the total free-air anomalies and topographic gravity effects yielded terrain-decorrelated free-air anomalies and the gravity effects of isostatically compensated terrain features, respectively, which are uncorrelated with each other. The compensating effects that annihilate the latter were attributed to undulations of the Moho, which we estimated by inversion using GLQ integration and a mantle-to-crust density contrast of 400 kg/m3. The inversion produced a Moho map with nearly 40 km of total relief that agrees very well with deep seismic refraction soundings. For East Antarctica, a bimodal variation in crustal thickness was found: the crustal wedge between the eastern Weddell Sea (≃330°E) and the eastern flank of the Gamburtsev Subglacial Mountains (≃90°E) has a mean thickness of about 37 km, whereas the mean crustal thickness is near 32 km for the northern half of the rest of East Antarctica up to the western flank of the Transantarctic Mountains (≃150°E). For West Antarctica and the oceanic regions, mean crustal thicknesses of about 30 km and 14 km, respectively, are inferred. The terrain-decorrelated free-air anomalies may be related to long-wavelength, large-amplitude subcrustal density variations and to much shorter-wavelength, smaller-amplitude intracrustal density variations.