Robert W. Jacobel

Fractures as the main pathways of water flow in temperate glaciers.

Understanding the flow of water through the body of a glacier is important, because the spatial distribution of water and the rate of infiltration to the glacier bottom is one control on water storage and pressure, glacier sliding and surging, and the release of glacial outburst floods. According to the prevailing hypothesis, this water flow takes place in a network of tubular conduits. Here we analyse video images from 48 boreholes drilled into the small Swedish glacier Storglaciären, showing that the glaciers hydrological system is instead dominated by fractures that convey water at slow speeds. We detected hydraulically connected fractures at all depths, including near the glacier bottom. Our observations indicate that fractures provide the main pathways for surface water to reach deep within the glacier, whereas tubular conduits probably form only in special circumstances. A network of hydraulically linked fractures offers a simple explanation for the origin and evolution of the englacial water flow system and its seasonal regeneration. Such a fracture network also explains radar observations that reveal a complex pattern of echoes rather than a system of conduits. Our findings may be important in understanding the catastrophic collapse of ice shelves and rapid hydraulic connection between the surface and bed of an ice sheet.

Surface velocity and mass balance of Ice Streams D and E, West Antarctica

Over 75 000 surface-velocity measurements are extracted from sequential satellite imagery of Ice Streams D and E to reveal a complex pattern of flow not apparent from previous measurements. Horizontal and vertical strain rates, calculated from surface velocity, indicate that the bed experiences larger basal shear where the surface of these ice streams is rougher. Ten airborne-radar profiles and one surface-based radar profile of ice thickness make possible the calculation of mass balance for longitudinal sections of each ice stream. Improved data-collection methods increase data density, substantially reducing random errors in velocity. However, systematic errors continue to limit the ability of the flux-differencing technique used here to resolve local variations in mass balance. Nevertheless, significant local variations in mass balance are revealed, while, overall, Ice Streams D and E are in approximate equilibrium. An earlier estimate of the net mass balance for Ice Stream D is improved.

The accumulation pattern across Siple Dome, West Antarctica, inferred from radar-detected internal layers

The spatial distribution of accumulation across Siple Dome,West Antarc-tica, is determined from analysis of the shapes of internal layers detected by radio-echo sounding (RES) measurements. A range of assumed accumulation patterns is used in an ice-flow model to calculate a set of internal layer patterns. Inverse techniques are used to determine which assumed accumulation pattern produces a calculated internal layer pattern that best matches the shape of internal layers from RES measurements. All of the observed internal layer shapes at Siple Dome can be matched using a spatially asymmetric accumulation pattern which has been steady over time. Relative to the divide, the best-fitting accumulation pattern predicts 40% less accumulation 30 km from the divide on the south flank of Siple Dome and 15^40% more accumulation 30 km from the divide on the north flank. The data also allow the possibility for a small time variation of the pattern north of the divide. The mismatch between the calculated and the observed layer shapes is slightly reduced when the accumulation rate north of the divide is higher in the past (>5 kyr BP) than at present. Sensitivity tests show that the predicted change in the spatial accumulation pattern required to cause the slight Siple Dome divide migration (inferred from previous studies) would be detectable in the internal layer pattern if it persisted for 42 kyr. Our analysis reveals no evidence that such a change has occurred, and the possible change in accumulation distribution allowed by the data is in the opposite sense. Therefore, it is unlikely that the Siple Dome divide migration has been caused by a temporal change in the spatial pattern of accumulation. This conclusion suggests the migration may be caused by elevation changes in Ice Streams C and D at the boundaries of Siple Dome.

Changes in the configuration of ice stream flow from the West Antarctic Ice Sheet

Surface-based ice-penetrating radar profiles on the northeast flank of Siple Dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream. The scar defines the southwestern boundary of an ice stream flowing from ice stream C to ice stream D, close to where it enters the Ross Ice Shelf. Our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across, underlain by a zone of disturbed internal stratigraphy revealed by the radar. Burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1.3 ka. The configuration of the ice streams draining the West Antarctic Ice Sheet into the Ross Ice Shelf evidently changes with time, and attempts to predict the evolution of the ice sheet must incorporate this observation.

Migration of the Siple Dome ice divide, West Antarctica

The non-linearity of the ice-flow law or a local accumulation low over an ice divide can cause isochrones (internal layers) to be shallower under the divide relative to the flanks, forming a divide bump in the internal layer pattern. This divide signature is analyzed using ice-flow models and inverse techniques to detect and quantify motion of the Siple Dome ice divide, West Antarctica. The principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide. The calculated migration rate is 0.05-0.50 m a -1 toward Ice Stream D and depends slightly on whether the divide bump is caused by the non-linearity of ice flow or by a local accumulation low. Our calculations also suggest a strong south-north accumulation gradient of 5-10 x 10 -6 a -1 in a narrow zone north of the divide. A consequence of divide migration is that pre-Holocene ice is thickest about 0.5 km south of the present divide position. Divide motion indicates that non-steady processes, possibly associated with activity of the bounding ice streams, are affecting the geometry of Siple Dome. The migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths. This suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 -10 4 years.

Spatial variation of radar-derived basal conditions on Kamb Ice Stream, West Antarctica

Abstract Radar profiles of bed echo intensity can survey conditions at the ice–bed interface and test for the presence or absence of water. However, extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice. We used the relationship between bed echo intensities from constant-offset radar data and ice thickness to estimate depth-averaged attenuation rates at several locations on and near Kamb Ice Stream (KIS), West Antarctica. We found values varying from 29 dBkm–1 at Siple Dome to 15 dBkm–1 in the main trunk region of KIS, in agreement with a previous measurement and models. Using these attenuation-rate values, we calculated the relative bed reflectivity throughout our KIS surveys and found that most of the bed in the trunk has high basal reflectivities, similar to those obtained in the location of boreholes that found water at the bed. Areas of lower bed reflectivity are limited to the sticky spot, where a borehole found a dry bed, and along the margins of KIS. These results support previous hypotheses that the basal conditions at locations like the sticky spot on KIS control its stagnation and possible reactivation.

Changes in the margin of Ice Stream C, Antarctica

We present results from satellite imagery, ice motion surveys, and ice-penetrating radar studies of a portion of the north margin of Ice Stream C, one of the ice streams draining the West Antarctic Ice Sheet to the Ross Embayment. Our studies suggest that the shutdown of Ice Stream C about 150 years ago was not a single event but a sequence involving stagnation of ice and migration of the ice stream boundary. Ground-based studies confirm the inference from imagery that a series of former shear zones exist decreasing in age towards the ice stream center. A region of ice stream trunk, including a former margin, lies sheared and folded between the (recent) inner and (older) outer margins of the area. Ice motion and topographic surveys give some constraint on the time of shutdown of the outer margin. The results provide a forum for discussing shutdown mechanisms. Possible causes for the stepwise migration of the north margin of Ice Stream C include a gradual decrease in ice flux, a reduction in the available water or hydrostatic pressure in the basal till, or a freezing of the till layers on the northern side. Introduction Drainage of the West Antarctic Ice Sheet (WAIS) is controlled by ice streams which deliver the majority of the mass flux to the bounding ice shelves. In the Ross Embayment, Ice Streams A through E drain the interior in a dynamic system that has changed in both flux and configuration over a range of temporal and spatial scales. Four ice streams are currently active, while Ice Stream C stagnated approximately 150 years ago (Retzlaf and Bentley, 1993; Shabtaie and Bentley, 1987), leading to questions about the conditions necessary to sustain rapid ice motion and the relationship of this event to other changes in the drainage configuration from the ice sheet. At the millennial scale, the system has lost approximately two-thirds of its mass since the end of the last glacial epoch (Bindschadler, 1998). In the nearer term, both ice stream velocities and ice stream widths are observed to fluctuate on a scale of years to decades at rates of 1-2 % per year for ice speed, and 0.1% per year for width (e. This suggests that ice streams may be constantly adjusting to local changes in basal lubrication and evolving at their margins. Additional evidence for change at an intermediate timescale has come from the morphology preserved in …

Radar internal layers from the Greenland Summit

Ice penetrating radar measurements made over the summit region of Greenland show returns from internal layers which can be used to augment the interpretation of climate information from the two deep cores recently recovered from this area. These reflecting surfaces, believed to represent isochrones, give information about the stress regime near the summit, and may aid in a better calibration of the age depth scale between the two cores — particularly in the lowest 10% of ice thickness where there is currently disagreement. The approximate depth at which internal echoes become discontinuous corresponds with the observations of steep inclinations and overturned folds on the scale of centimeters in the core samples. However the deepest internal layers which can be distinguished in the profiles place constraints on the scale and location of high angle or overturned folds.

Estuaries beneath ice sheets

Interactions between subglacial hydrology and the ocean make the existence of estuaries at the grounding zones of ice sheets likely. Here we present geophysical observations of an estuary at the downstream end of the hydrologic system that links the active subglacial lakes beneath Whillans Ice Stream to the ocean beneath the Ross Ice Shelf, Antarctica. This subglacial estuary consists of a hydropotential low upstream of the grounding zone, which is linked to the ocean by a hydropotential trough and a large subglacial channel. This subglacial channel, which is imaged using active source seismic methods, has an apparent width of 1 km and a maximum depth of 7 m. The hydropotential trough continues upstream of the grounding zone and results from an along-flow depression in surface elevations. Pressure differences along the trough axis are within a range that can be overcome by tidally induced processes, making the interaction of subglacial and ocean water likely.

First results from radar profiles collected along the US-ITASE traverse from Taylor Dome to South Pole (2006-2008)

Abstract The 2006/07 and 2007/08 US-ITASE traverses from Taylor Dome to South Pole in East Antarctica provided opportunities to survey the subglacial and englacial environments using 3 MHz and 200MHz radar. We present first results of these new ground-based radar data. A prominent basal deformation layer indicates different ice-flow regimes for the northern and southern halves of the Byrd Glacier drainage. Buried dune stratigraphy that appears to be related to the megadunes towards the west occurs at depths of up to 1500 m. At least two new water-filled subglacial lakes were discovered, while two recently drained lakes identified from repeat ICESat surface elevation surveys appear to be devoid of water.