Abstract:
This award supports a project to evaluate radio-echo intensities in the available SOAR ice-penetrating radar data along grids covering Lake Vostok, and along four regional tracks from Ridge B toward the lake. The project has two objectives; first, it will examine the upper surface of the lake and reflectors hypothesized to be a boundary between the meteoric and accreted ice. They will provide ... crucial knowledge on the dynamic evolution of the lake. Second, this project will examine a poorly understood echo-free zone within the deep ice in central East Antarctica. This zone may consist of distorted stagnant ice, while its upper boundary may be a shear zone. The SOAR radar data provide a unique resource to examine spatiotemporal water circulation patterns that should be understood in order to select the best direct-sampling strategy to the lake. The Vostok ice core provides a unique opportunity to do this work. First, the path effects, i.e. propagation loss and birefringence, will be derived at the ice-core site using ice temperature, chemistry, and fabric data. Second, lateral variations of the propagation loss will be estimated by tracking chemistry associated with radar-detected isochronous layers, and by inferring temperatures from an ice-flow model that can replicate those layers. Ice-fabric patterns will be inferred from anisotropy in the reflectivity at about 100 radar-track cross-over sites. In terms of broader impacts, a graduate student will be trained to interpret the radar data in the light of radar theory and glaciological context of Lake Vostok and summer workshops for K-12 teachers will be provided in Seattle and New York. This project will contribute to ongoing efforts to study Lake Vostok and will complement the site selection for a North Vostok ice core, which has been proposed by Russia and France as an IPY program.
Both a change in personnel as well as an achievement of the initial purpose of the project allowed us to move the direction toward dynamic drainage and water flow beneath glaciers and ice sheets with particular emphasis on the interior of Antarctica and overdeepenings.
Purpose:
Findings:
We have investigated the hypothesis that the meteoric–accreted ice boundary is visible as a weak reflector in the radar data. Using ice-core and borehole-temperature data, we seek possible reflection causes that explain the radar-derived reflectivity (~48 dB). Ice-core data show large variations in the number density of inclusions, soluble chloride-ion concentration ([Cl]), ... charge-balance-derived acidity ([H+]) and fabric near the reflector. We estimate that the reflectivity due to insoluble mineral inclusions is 128 dB, based on a likely range of inclusion permittivities diameters. An impurity- and temperature-dependent ice-conductivity model predicts a reflectivity of 50 dB near the reflector due to a large [H+] and [Cl] contrast if both the soluble H+ and Cl ions have entered into and created defects in the ice lattice. The reflectivity due to the observed fabric contrast is 61 dB. These results suggest that neither mineral inclusions nor fabric contrasts cause the observed reflection, and that the impurity-concentration contrast (principally [H+]) is likely the primary cause. Combined with the spatial pattern of MAIB radar detection, this reflection cause suggests that a necessary condition for radar detection of a meteoric accreted ice boundary is a large temperature gradient between the basal ice and the icelake interface as the basal ice rafts onto the lake. This temperature gradient in turn causes faster accretion rates, greater incorporation of impurities in the accreted ice, a larger conductivity contrast and, finally, a larger reflectivity.
Results of other objectives:
Major research findings of the subglacial sediment transport paper include that overdeepenings, closed depressions under glaciers and ice sheets, do not actively trap sediment as previously anticipated. Where the bed slopes steeply upward, the water system can supercool, and freeze-on ice causes the water depth to decrease. This decrease means that water velocity increases, and sediment erodes along the base of the glacier.
Further investigation will determine how these and similar processes act beneath Antarctica, particularly, with respect to the active subglacial lakes in West Antarctica. Most of the subglacial systems in Antarctica drain through overdeepenings, so particular effects on hydrology could be quite large.
Joseph A. MacGregor, Kenichi Matsuoka, Michelle R. Koutnik, Edwin D. Waddington, Michael Studinger and Dale P. Winebrenner, "Millennially averaged accumulation rates for the Lake Vostok region inferred from deep internal layers", Annals of Glaciology, vol. 51, (2009), p. ., " " Accepted
J.A. MacGregor, K. Matsuoka and M. Studinger, "Radar detection of accreted ice over Lake 1 Vostok, Antarctica", ... EPSL, vol. , (2009), p. ., " " Accepted
Wolovick, M.J. and R.E. Bell, and T.T. Creyts, and N. Frearson, "Identification and routing of subglacial water networks under Dome A", The Journal of Geophysical Research --- Earth Surface Processes, vol. , (2012), p. ., " " Submitted
Creyts, T.T. and G.K.C. Clarke and M. Church, "Glaciofluvial sediment redistribution across basal overdeepenings and the relation to glaciohydraulic supercooling", The Journal of Geophysical Research --- Earth Surface Processes, vol. , (2012), p. ., " " Near submission (final review before submission)
