C54A-01
Identification and Characterization of Dynamic Alpine Subglacial Lakes Using InSAR, Radio- Echo Sounding, and Crevasse Interpretation
We use interferometric synthetic aperture radar (InSAR), radio-echo sounding (RES), and crevasse interpretation to identify and characterize two dynamic alpine subglacial lakes in Glacier Bay National Park, Alaska. Although significant literature exists on large subglacial lakes in Antarctica, little research has been done on alpine subglacial lakes. Subglacial and subaerial glacier-dammed lakes and the catastrophic floods (jokulhlaups) that release are a hazard in glacierized mountain regions around the world. Many glacier-dammed lakes form subglacially during periods of glacier retreat and downwasting, but are not identified until they become subaerially exposed or release a jokulhlaup. The two lakes discussed here are dammed by Brady Glacier in southeast Alaska, 120 km west of Juneau. Initially, a conspicuous, 3-km-long crevasse in the glacier drew our attention to Hinge Lake, so named because of its hinge-like appearance. For the InSAR analysis, we utilized 20 ascending and descending ERS-1 and -2 tandem radar images provided by the European Space Agency. We obtained a DEM from Glacier Bay National Park that was based on data from the SRTM mission, with gaps filled using photogrammetry data. We co-registered and processed raw SAR signal data into complex, single-look images, created interferograms, and unwrapped the phase. To simplify the analysis, we assumed zero horizontal glacier movement. This assumption is valid because ice is flowing into a closed depression and all interferograms analyzed in this study show very little or zero horizontal motion. To further characterize the lakes, we conducted a RES survey to determine ice depths and substrate. We deduced principle stresses by interpreting crevasses patterns in combination with vertical displacement data derived from interferograms. A time series of interferograms shows vertical motion over large areas of the two lakes. A hydraulic connection between the two lakes is inferred from contemporaneous vertical displacement of the overlying ice. The RES survey provided minimum depths of floating ice. We explain crevasse patterns using a two-fold stress field caused by simple downslope ice movement and vertical displacement of the overlying ice due to filling and draining of the lakes. This study demonstrates that a combination of InSAR, RES, and glaciological interpretation can effectively identify and characterize alpine subglacial lakes. Knowledge of these lakes is important for understanding glacier motion, outburst flood potential and routing, and glacier mass balance. This research is a component of a dissertation that seeks to identify and characterize the glacier-dammed lakes of Glacier Bay, Denali, and Wrangell - St. Elias National Parks using ground-truthed radar and optical remote sensing techniques. It is our aim to eventually apply these techniques to glacier-dammed lakes worldwide.
C54A-02
Geophysical observations from the South Pole region of the Antarctic Ice Sheet: Evidence for a subglacial lake?
Radio-echo sounding and satellite altimetry observations have been used to identify a catalog of well over one hundred subglacial lakes beneath the Antarctic Ice Sheet. These lakes provide a unique laboratory for studying life in extreme environments, and may also contain paleoclimate records and ecological records that may date back as far as ~35 million years, when the Antarctic continent was ice-free. One of these lakes, which is near the South Pole (and is hereafter referred to as Lake Amundsen-Scott), is typical of many subglacial lakes in its radar signature and subglacial morphology. However, both temperature modeling and radar reflection strength modeling have cast doubts on the presence of free water at the base of the ice sheet near the South Pole. We set out to reconcile these contradictions and establish the truth behind Lake Amundsen-Scott through a series of geophysical techniques. Here we present the results of several geophysical experiments performed in the proposed Lake Amundsen- Scott region, along with temperature modeling of the local ice column. In the 2006-2007 Antarctic field season, we collected ~9 km of seismic reflection and refraction data, as well as ~15 km of ice-penetrating radar profiles, in order to characterize the firn, the local ice column, and the subglacial environment. Kinematic GPS measurements were also made to determine if basal conditions are reflected in the surface expression of the region. Our temperature modeling supports the potential for liquid water at the bed, suggesting that the base of the ice sheet is at the pressure melting point here. The amalgamation of these results will reveal whether or not Lake Amundsen-Scott truly exists, as well as determine the validity of similar lakes in the current subglacial lake inventory.
C54A-03
Spatial Variation of Basal Conditions on Kamb Ice Stream
Radar profiles of bed echo intensity provide a way to survey conditions at the ice-bed interface and test for the presence or absence of water. However, extracting information about bed properties from bed echo intensities requires an estimate of the dielectric attenuation loss through the ice. A recent survey [MacGregor et al., 2007] found that the few reported values of depth-averaged attenuation rates in West Antarctica vary by a factor of 3, presumably due to spatial variations in the chemistry and temperature profiles of the ice. Thus, a single value for depth-averaged ice-sheet attenuation cannot be assumed, even over a relatively small region. We measured attenuation rates at several locations on and near Kamb Ice Stream (KIS) by examining basal echo intensity values as a function of ice thickness from constant-offset radar data acquired in 2004-2006. Our values obtained for Siple Dome of 29 dB/km agree with previous measurements [Gades et al., 2000] and the recent calculations and model results of MacGregor et al. [2007]. On KIS, we measured attenuation values of 20 dB/km over the "sticky spot", where ice has become stagnant. This value is consistent with an attenuation model over the sticky spot calculated using borehole temperature data [Engelhardt, 2005] and chemistry data from the Siple Dome ice core. Our radar profiles in the main trunk region of KIS yield a slightly lower value of 15 dB/km, presumably because colder ice is still being advected from inland West Antarctica. Using these values of attenuation, we calculated the basal radar reflectivity at the ice-bed interface in the regions of all our surveys. We found that most regions of the bed in the trunk of KIS have high basal reflectivities and that these values are similar to those obtained in locations where water was found in the Caltech boreholes [Engelhardt, 2005]. Areas of lower bed reflectivity are limited to the sticky spot, where a borehole found a dry bed, and along the margins of KIS. We use these results to hypothesize about the stagnation of KIS and the possibility of its reactivation. http://www.stolaf.edu/other/cegsic
C54A-04
Chemistry of an Antarctic Subglacial Environment – The role of subglacial geochemical processes in global biogeochemical cycles and quantifying subglacial hydrological processes
It is well known that subglacial environments especially the availability of basal water plays an important role for the dynamic of ice sheets. Hydrological processes however are far from being understood as direct observations are hampered through kilometer thick ice. Over the past years, it also has become more and more evident that despite the cold and isolation wet subglacial environments provide a viable habitat for life. Geochemical and biogeochemical processes in the sub ice environment not only can create and release chemical compounds, which can be used as natural tracers helpful in elucidating and quantifying subglacial hydrological processes, subglacial bio/geochemical processes may also play an important role in global geochemical cycles, like the global carbon cycle, or the cycling of iron and fertilization of the oceans. Here we present the first geochemical measurements of the geochemical composition of basal water and pore water from beneath the West Antarctic Ice Sheet. Our results point towards an oxygen depleted environment in the Upstream C area (Kamb Ice Stream). Geochemical water and sediment also indicate the removal of inorganic carbon potentially constituting a significant flux and release of inorganic carbon across the ice sheet grounding zone into the sub ice shelf cavity. Our results also point out that subglacial environments are far from being understood and that sample recovery and insitu observations will be crucial for understanding subglacial environments and their role for ice sheet dynamic, the interaction between ice sheets, the underlying lithosphere and the oceans as well as the impact of subglacial processes on global geochemical cycles.
C54A-05 INVITED
Microbial Energetics Beneath the Taylor Glacier, Antarctica
Subglacial microbiology is controlled by glacier hydrology, bedrock lithology, and the preglacial ecosystem. These factors can all affect metabolic function by influencing electron acceptor and donor availability in the subglacial setting leaving biogeochemical signatures that can be used to determine ecosystem processes. Blood Falls, an iron-rich, episodic subglacial outflow from the Taylor Glacier in the McMurdo Dry Valleys Antarctica provides an example of how microbial community structure and function can provide insight into subglacial hydrology. This subglacial outflow contains cryoconcentrated, Pliocene-age seawater salts that pooled in the upper Taylor Valley and was subsequently covered by the advance of the Taylor Glacier. Biogeochemical measurements, culture-based techniques, and genomic analysis were used to characterize microbes and chemistry associated with the subglacial outflow. The isotopic composition of important geochemical substrates (i.e., δ34Ssulfate, Δ33Ssulfate, δ18Osulfate, δ18Owater, Δ14SDIC) were also measured to provide more detail on subglacial microbial energetics. Typically, subglacial systems, when driven to anoxia by the hydrolysis of organic matter, will follow a continuum of redox chemistries utilizing electron acceptors with decreasing reduction potential (e.g., Fe (III), sulfate, CO2). Our data provide no evidence for sulfate reduction below the Taylor Glacier despite high dissolved organic carbon (450 μM C) and measurable metabolic activity. We contend that, in the case of the Taylor Glacier, the in situ bioenergetic reduction potential has been ‘short-circuited' at Fe(III)-reduction and excludes sulfate reduction and methanogenesis. Given the length of time that this marine system has been isolated from phototrophic production (~2 Mya) the ability to degrade and consume increasingly recalcitrant organic carbon is likely an important component to the observed redox chemistry. Our work indicates that glacier hydrology imparts strong feedbacks on the availability of oxygen as an electron acceptor and may be a robust regulator of the in situ metabolism. This biogeochemical regulation in turn affects the chemical nature of subglacial efflux. Blood Falls demonstrates that measurements of geochemistry and microbial diversity can support models of subglacial hydrology.
C54A-06
Glaciohydraulic Supercooling and Basal Ice Formation at Outlet Glaciers of Vatnajökull and Oraefajokull, Iceland
The hydrology of glaciers is affected by the subglacial environment and in particular overdeepenings that may result in supercooling of subglacial waters and the entrainment of debris into basal ice. Warm-based outlet glaciers of Vatnajökull and Oraefajökull, Iceland, are often characterized by overdeepenings in their terminal regions and up to 100-m long exposures of meters-thick, debris-laden (4 to 35% by vol.) basal zones. During the period 2002 to 2005, we collected bulk samples (300g) of the basal ice, englacial ice, melt water and frazil ice from a number of these outlet glaciers to evaluate probable origins of the basal zone and thus the role of basal topography and hydrology in its generation. Analyses of the 18O and D content show delta 18O and D values for the basal ice generally plot below the meteoric water line and are less negative then values for englacial ice. The difference between the mean delta 18O values of the basal and englacial ice is approximately 2.41 per mil, while the mean delta D values differ by about 11.12 per mil. The delta 18O and delta D values of frazil ice also generally plot below the meteoric water line and overlap those of the basal ice. Mean delta 18O values of frazil ice and meltwater differ by approximately 1.93 per mil, while the mean deuterium values differ by about 13.16 per mil. Little difference exists between the mean delta 18O and D values of meltwater and englacial ice. Tritium measured in basal ice ranges from 1.9 to 6.1 TU and averages 2.9 TU, whereas in englacial ice it is essentially 0. Both the physical properties and isotopic composition of the basal zone ice and debris in the outlet glaciers sampled is consistent with a glaciohydraulic supercooling origin resulting from subglacial discharge out of overdeepenings.
C54A-07
Subglacial Depositional Processes from Wireless Subglacial Probe Experiments.
An understanding of the subglacial environment is vital to our understanding of the relationship between glacier dynamics and climate change. In recent years, great advances have been made in our understanding of the subglacial environment, by direct in situ experiments. We show the results associated with subglacial depositional processes from a new wireless subglacial probe installed beneath Briksdalsbreen, Norway that behaved like a clast within the subglacial environment. The results include: a) an annual pattern of basal sliding and subglacial deformation, indicating early summer lodgement and late summer deformation; b) The amount and nature of clast rotation within the deforming layer, with implications for till sedimentation and till fabric production; c) Evidence for subglacial deformation over a wide range of basal shear stresses, which indicates the style of subglacial behaviour. http://www.glacsweb.org
C54A-08
Stabilizing feedbacks in-glacier bed erosion: Constraints from numerical simulations
Recently, the idea of a stabilizing feedback in subglacial erosion was introduced using a simple thermodynamic balance (Alley and others, 2003). The hypothesis states that glaciers with an active basal hydrology will modify the bed slope so that it is adverse to and approximately 20--70% steeper than the glacier surface slope based on a thermodynamically-controlled supercooling threshold. When water flow or sediment supply changes, bed slopes will be driven back to this threshold slope. In the ablation zone where sufficient surface meltwater reaches the ice--bed interface, water flowing up a bed slope adverse to ice flow can supercool and freeze to the glacier sole. Because the density of ice is greater than water, the remaining water pressurizes and sediments will not be mobilized effectively. Glacier beds will thus tend to be at the slope where supercooling occurs subglacially. In order to test this hypothesis, we construct a numerical model of one dimensional water flow based on the balances of mass, momentum, and enthalpy. These balances quantify water, ice, and sediment movement at the base of the glacier. Upstream conditions are simplified to recreate a melt season with diurnal meltwater fluctuations. The model is fully transient and does not rely on steady state or other similar assumptions. Simulations reveal behavior that cannot be inferred from simplified models. For example, while total simulated ice accretion is comparable to field estimates, locations of simulated ice accretion along the ice--bed interface conflict with steady-state models, which tend to overpredict accretion amounts. Simulations also indicate that much sediment deposition occurs prior to water being supercooled. As a result, sediment deposition and supercooling are largely decoupled in simulations and may be either partially or largely decoupled in the field. In addition, sediment deposition tends to smooth subglacial topography rather than enhance it. This result stems from the rate of change of sediment transport being determined by the curvature of the water flowpath. A prominent result is that glaciers will tend to smooth out their beds rather than enhance them. This result runs contrary to the hypothesis proposed for stabilizing feedbacks in glacier-bed erosion. We discuss additional results and implications of these results and encourage dialogue on stabilizing feedbacks.