C13B-1071
Subglacial lakes in Iceland. A review.
Subglacial lakes in Iceland are expressions of glacier-volcanic interaction. At several of the central volcanoes under the ice caps Vatnajokull and Myrdalsjokull hydrothermal activity results from the interaction of water with magmatic intrusions at shallow depths in the crust. Ice is continuously melted at the glacier bed creating permanent depressions in the glacier surface that reveal this hydrothermal activity. The meltwater may be trapped in a lake that rises as a dome above the bed due to relatively low basal fluid potential under the depression. High ice overburden pressure at the rim around the depression seals the lake that expands as water flows toward the depression, until the hydraulic seal is broken and water drains out in a jokulhlaup. The best-known examples of such subglacial lakes are Grimsvotn in the interior of Vatnajokull and the Skafta cauldrons. We describe the geometry, refill rate, mass and energy balance of these subglacial lakes.
C13B-1072
A Dynamic View of Subglacial Lakes in a Glaciation/Deglaciation Cycle
Topographic features likely to correspond to subglacial lakes can be identified as pits in the water pressure potential surface. Given an ice sheet, the primary source of uncertainty in this potential surface is due to the effective pressure. To a first approximation effective pressure may be assumed to be zero, allowing the water pressure potential to be determined from ice thickness and bed elevation. Pits are defined as points on a surface lower or equal in height to all neighbors. Well developed and robust algorithms for identification of pits in surfaces can be found in the hydrological literature. Such an algorithm has been adapted to run after each time step of a thermo-mechanically coupled ice sheet model. To investigate the dynamic nature of subglacial lakes a complete glaciation/deglaciation cycle is run. Sub-glacial lakes are identified as regions where there is both basal melting and a pit. The area and depth of the pit in the hydraulic potential field gives an indication of the volume of the lake. In these cycles, a dynamic picture of the emergence, expansion and subsequent loss of subglacial lakes appears. Areas that were likely to have been persistent subglacial lakes during the last ice age are identified, quantified, and mapped. This map will be of use to investigators wishing to explore the geological and biological records of paleo subglacial lakes.
C13B-1073
Are Antarctic Subglacial Lakes Markers of Geologic Boundaries?
Airborne geophysics has revealed the locations of more than 150 subglacial lakes in the Antarctic interior. The spatial distribution of lakes beneath an ice sheet depends on geothermal flux at the ice-bed interface, ice thickness and flow, surface temperature and accumulation rate, as well as the occurrence of basins in the bed topography. The dynamics of the lake environment, as well as the diversity of controls acting on it, account for the difficulty in reproducing or predicting the distribution of known and probable lakes from models. To date, most efforts to predict the distribution of subglacial lakes have focused on the glaciological facets of the problem. Here we examine the role tectonics may play in the distribution of subglacial lakes. Lake Vostok lies along a tectonic boundary between two distinct geologic provinces-relatively flat-lying continental margin sediments to the west of the lake and folded and compressed crust material to the east. This suture is possibly the result of Precambrian tectonic activity which produced the Vostok basin and thus is the fundamental tectonic control on the location of Lake Vostok. This work examines the relative contribution of glaciological and tectonic controls to determine if other subglacial lakes may also reside on geologic or tectonic boundaries.
C13B-1074
Great lakes, cold lakes, and fuzzy lakes: An algorithm for characterizing diverse water environments beneath the East Antarctic ice sheet.
Understanding subglacial lakes and their interaction with the underlying geology and overlying ice sheet requires the ability to rapidly locate such bodies in the large data volumes of airborne radar sounding data. Historically, subglacial lakes in airborne ice penetrating radar have been detected by identifying portions of the base of the ice interface which appear nearly horizontal. Horizontality is defined as a slope nearly 11 times the surface slope but in the opposite direction of surface slope, which is equivalent to a flat hydropotential surface. The character of the reflection should be bright on an absolute scale as well as bright relative to reflections in the surrounding region. The reflection's amplitude should be exceptionally consistent across the entirety of the reflection. It is also appropriate to use proxies for the ice sheet's basal temperature to identify subglacial lakes. In fact, an ideal subglacial lake will meet all criteria, including hydraulic flatness, brightness (absolute and relative), specularity, and high basal temperature. We have designed an algorithm that automatically identifies portions of the flight lines that match all or at least some of these criteria. This algorithm has now been applied to over 50,000 line kilometers of data from four regions totaling over 250,000 square kilometers. This data was acquired in East Antarctica with the UTIG airborne geophysical platform which includes an ice penetrating radar. A review of the results suggests that many of the lakes found in previous studies fail one or more of the subglacial lake identification tests. It is known that many "lakes" in East Antarctica are in regions where the estimated basal temperature is far below the pressure melting point of ice. We classify these as "cold lakes". Some areas are identified as lakes yet show non-specular reflections and are referred to as "fuzzy lakes". More interestingly is that a number of these sub-par lakes intersect more perfect lakes. The distribution and nature of subglacial lakes, their classifications, and understanding gained from their intersections is the focus of this presentation.
C13B-1075
Modeling englacial attenuation using ice-core data for radar sounding of basal conditions
Observation of the radar reflectivity of ice-sheet beds is a primary tool for discriminating wet from frozen beds, and for finding subglacial lakes. However, uncertainty in englacial radar attenuation and its spatial variation introduces corresponding uncertainty in estimates of basal reflectivity. Radio-frequency ice-sheet attenuation is dependent on the impurity (acidity and salinity) and temperature profiles in the ice column that is being probed. Modeling englacial radar attenuation using modeled temperature profiles and assumed, depth-averaged impurity concentrations reduces such uncertainties only modestly. Here we develop a physical attenuation model based on ice-core chemistry and temperature data that can be used, in conjunction with ice flow modeling, to estimate englacial attenuation over wide areas around an ice core (e.g., Vostok). We test the model initially at Siple Dome, West Antarctica, where both ice-core and independent radar attenuation data are available for comparison. The modeled depth-averaged attenuation rate there is 32 dB km$^{-1}$, which lies between values of 26 dB km$^{-1}$ and 35 dB km$^{-1}$ measured at Siple Dome [Winebrenner et al., {\it Ann. Glaciol.} {\bf 37}, 226-232, 2003]. A synthesis of experimental conductivity data shows that uncertainty in computed attenuation rates depends on uncertainties in conductivity parameterizations, which increase with increasing impurity concentrations and decreasing temperature. When the ice temperature is below about -15°C, fine-depth-scale impurity variations (mostly acid) control the attenuation rate; above -15°C, the higher temperature controls the attenuation rate more strongly, as the attenuation increases with temperature following an Arrhenius relationship.
C13B-1076
Time variability of radar reflections from englacial and subglacial waters
We have conducted radar imaging at Bench Glacier, Alaska aimed at investigating englacial and subglacial waters. Repeat measurement intervals span years, seasons, days, and hours. Data are included from a logging system that continuously collects radar traces. We used four radar systems, a range of frequencies from 5-100 Mhz, and a variety of data collection techniques. The radar data were collected in conjunction with observations made in over 50 boreholes drilled through the ice. We found variations in englacial scattering events dependent on season and time of day. In radar images collected in June of different years, with two different radar systems and two different frequencies, layering based on scattering events was evident. Two distinct layers were present within the glacier: an upper layer defined by a notable lack of scatterers and high radar velocity, and a lower layer defined by numerous scatterers and relatively low radar velocity. The boundary between the two layers is abrupt, and ranges in depth from 20 to 50 m below the glacier surface. Many of the scatterers are large (i.e. meters in diameter). This is indicated by imaging of the top and bottom of some of these events in 25 and 100 MHz data, and by the fact that the long wavelength of the 5 MHz systems detected the features. Our repeated measurements elucidate changes in englacial and subglacial water processes over time scales of hours to seasons.