Hugh F. J. Corr

New Boundary Conditions for the West Antarctic Ice Sheet: Subglacial Topography of the Thwaites and Smith Glacier Catchments

Airborne radar sounding over the Thwaites Glacier (TG) catchment and its surroundings provides the first comprehensive view of subglacial topography in this dynamic part of the West Antarctic Ice Sheet (WAIS) and reveals that TG is underlain by a single, broad basin fed by a dendritic pattern of valleys, while Smith Glacier lies within an extremely deep, narrow trench. Subglacial topography in the TG catchment slopes inland from a broad, low-relief coastal sill to the thickest ice of the WAIS and makes deep connections to both Pine Island Glacier and the Ross Sea Embayment enabling dynamic interactions across the WAIS during deglaciation. Simple isostatic rebound modeling shows that most of this landscape would be submarine after deglaciation, aside from an island chain near the present-day Ross-Amundsen ice divide. The lack of topographic confinement along TGs eastern margin implies that it may continue to widen in response to grounding line retreat.

Distortion of isochronous layers in ice revealed by ground-penetrating radar

In addition to measuring ice-sheet thickness, ground-penetrating radar can be used to delineate reflections in ice sheets. These reflections are generally accepted to result from layers of isochronous deposition of snow and can reveal much about the dynamics of the ice flow. Here we present ground-penetrating radar data from Fletcher Promontory, Antarctica, which show arches and troughs in isochronous ice layers to a depth of 100 m. We demonstrate that the origin of these features can be determined by their growth with depth, and many features result from local anomalies in accumulation rate which can be correlated with the ice surface slope. One arch appears to be the result of a local anomaly in vertical strain-rate. Its proximity to the ice divide, width, and growth with depth, indicate that this arch is one of a class of features postulated by Raymond but not previously shown to exist in the field. Such a feature is an indication of the nonlinear rheology of ice and requires that palaeoclimate records from ice cores extracted from the vicinity of ice divides underlain by similar features should be specifically corrected for such effects on ice-flow.

Breakup and conditions for stability of the northern Larsen Ice Shelf, Antarctica

The breakup of ice shelves has been widely regarded as an indicator of climate change, with observations around the Antarctic Peninsula having shown a pattern of gradual retreat, associated with regional atmospheric warming and increased summer melt and fracturing processes. The rapid collapse of the northernmost section of the Larsen Ice Shelf (Larsen A), over a few days in January 1995, indicated that, after retreat beyond a critical limit, ice shelves may disintegrate rapidly. Here we use a finite-element numerical model that treats ice as a continuum without fracture to examine the breakup history between 1986 and 1997 of the two northern sections of Larsen Ice Shelf (Larsen A and Larsen B), from which we establish stability criteria for ice shelves. Analysis of various ice-shelf configurations reveals characteristic patterns in the strain rates near the ice front which we use to describe the stability of the ice shelf. On Larsen A, only the initial and final ice-front configurations show a stable pattern. Larsen B at present exhibits a stable pattern, but if the ice front were to retreat by a further few kilometres, it too is likely to enter an irreversible retreat phase.

Widespread Persistent Thickening of the East Antarctic Ice Sheet by Freezing from the Base

A large fraction of the ice at Dome A, Antarctica, did not form by the usual process of snowfall compaction. An International Polar Year aerogeophysical investigation of the high interior of East Antarctica reveals widespread freeze-on that drives substantial mass redistribution at the bottom of the ice sheet. Although the surface accumulation of snow remains the primary mechanism for ice sheet growth, beneath Dome A, 24% of the base by area is frozen-on ice. In some places, up to half of the ice thickness has been added from below. These ice packages result from the conductive cooling of water ponded near the Gamburtsev Subglacial Mountain ridges and the supercooling of water forced up steep valley walls. Persistent freeze-on thickens the ice column, alters basal ice rheology and fabric, and upwarps the overlying ice sheet, including the oldest atmospheric climate archive, and drives flow behavior not captured in present models.

Precise measurement of changes in ice-shelf thickness by phase-sensitive radar to determine basal melt rates

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Observation and Parameterization of Ablation at the Base of Ronne Ice Shelf, Antarctica

Parameterizations of turbulent transfer through the oceanic boundary layer beneath an ice shelf are tested using direct measurements of basal ablation. Observations were made in the southwestern part of Ronne Ice Shelf, about 500 km from open water. The mean basal ablation rate was measured over a month-long and a year-long period using phase-sensitive radar to record the thinning of the ice shelf. Ocean temperatures were observed within about 25 m of the ice shelf base over the period of the radar observations, while the tidally dominated ocean currents were estimated from tidal analysis of collocated current observations from an earlier period. Ablation rates derived using these ocean data and a number of bulk parameterizations of turbulent transfer within the boundary layer are compared with the direct measurements. The ablation rates derived using a parameterization that explicitly includes the impact of ocean currents on the turbulent transfer of heat and salt match the observations to within 40%; with suitable tuning of the drag coefficient,the mismatch can be reduced below the level of the observational errors. Equally good agreement can be obtained with two slightly simpler, current-dependent parameterizations that use constant turbulent transfer coefficients,andtheoptimalvaluesforthecoefficientsatthis particularlocationon RonneIce Shelfaregiven.

Aerogravity evidence for major crustal thinning under the Pine Island Glacier region (West Antarctica)

The West Antarctic Rift System provides critical geological boundary conditions for the overlying West Antarctic Ice Sheet. Previous geophysical surveys have traced the West Antarctic Rift System and addressed the controls that it exerts on the West Antarctic Ice Sheet in the Ross Sea Embayment. However, much less is known about the rift system under the Amundsen Sea Embayment, a key sector of the West Antarctic Ice Sheet, which is thinning significantly today. New aerogravity data over the Pine Island Glacier region, one of the fastest flowing glaciers within the Amundsen Sea Embayment, sheds new light into the crustal structure under this dynamic part of the West Antarctic Ice Sheet. Three-dimensional (3-D) inversion of terrain-decorrelated free-air and Bouguer gravity anomaly data reveal significant crustal thinning beneath the catchment of Pine Island Glacier. Under the Byrd Subglacial Basin and the newly identified Pine Island Rift, Moho depth is estimated to be 19 ± 1 km. This is the thinnest crust observed beneath the West Antarctic Ice Sheet. Estimates of lithosphere rigidity ( T e ), based on isostatic models, yield a T e of 5 ± 5 km, which is comparable to values from modern rift systems such as the Basin and Range Province. Major crustal thinning, coupled with low lithosphere rigidity, attest to the considerable impact of continental rifting beneath this part of the West Antarctic Ice Sheet. In analogy with the better known Ross Sea segment of the West Antarctic Rift System we suggest that the Amundsen Sea Embayment was affected by distributed Cretaceous rifting, followed by Cenozoic narrow-mode rifting. Narrow-mode rifting within the Pine Island Rift is particularly important as it may serve as a geological template for enhanced glacial flow associated with Pine Island Glacier.

Location for direct access to subglacial Lake Ellsworth: An assessment of geophysical data and modeling

Ellsworth is 14.7 km ×3 .1 km with an area of 28.9 km 2 . Lake depth increases downlake from 52 m to 156 m, with a water body volume of 1.37 km 3 . The ice thickness suggests an unusual thermodynamic characteristic, with the critical pressure boundary intersecting the lake. Numerical modeling of water circulation has allowed accretion of basal ice to be estimated. We collate this physiographic and modeling information to confirm that Lake Ellsworth is ideal for direct access and propose an optimal drill site. The likelihood of dissolved gas exchange between the lake and the borehole is also assessed. Citation: Woodward, J., A. M. Smith, N. Ross, M. Thoma, H. F. J. Corr, E. C. King, M. A. King, K. Grosfeld, M. Tranter, and M. J. Siegert (2010), Location for direct access to subglacial Lake Ellsworth: An assessment of geophysical data and modeling, Geophys. Res. Lett., 37, L11501, doi:10.1029/ 2010GL042884.

Channelized Ice Melting in the Ocean Boundary Layer Beneath Pine Island Glacier, Antarctica

Active Ice How, exactly, does warm ocean water erode an ice shelf? In a field study of an ice shelf at Pine Island, Antarctica, Stanton et al. (p. 1236) collected data from radar, seismic surveys, and oceanographic sensors inserted in holes bored through the ice shelf. The results show that localized, intensive melting occurs in a complex network of discreet channels that are formed on the underside of the shelf. This pattern of melting may limit the absolute rate of ice-shelf mass loss. A complex pattern of channelized melting exists on the underside of the ice shelf of Pine Island Glacier in Antarctica. Ice shelves play a key role in the mass balance of the Antarctic ice sheets by buttressing their seaward-flowing outlet glaciers; however, they are exposed to the underlying ocean and may weaken if ocean thermal forcing increases. An expedition to the ice shelf of the remote Pine Island Glacier, a major outlet of the West Antarctic Ice Sheet that has rapidly thinned and accelerated in recent decades, has been completed. Observations from geophysical surveys and long-term oceanographic instruments deployed down bore holes into the ocean cavity reveal a buoyancy-driven boundary layer within a basal channel that melts the channel apex by 0.06 meter per day, with near-zero melt rates along the flanks of the channel. A complex pattern of such channels is visible throughout the Pine Island Glacier shelf.

Subglacial Lake Ellsworth: A candidate for in situ exploration in West Antarctica

Radio-echo sounding reveals a 10 km-long lake beneath ∼3.4 km of ice near the Ellsworth Mountains in West Antarctica, 20 km from the ice divide. Subglacial Lake Ellsworth is located within a distinct topographic hollow, which is ∼1.5 km deeper than the surrounding bed. Judging by bed slopes flanking the lake, the water depth is at least 10s of metres. Calculations of basal temperature reveal the ice base to be warm both now and during full glacial periods. As the environments of subglacial lakes are broadly similar, life may be expected in Lake Ellsworth as in any other. Given this, its physical characteristics, and the fact that the West Antarctic Ice Sheet has been accessed on several occasions, Lake Ellsworth is an excellent candidate for in situ examination.