C33D-01 13:40h
Use of a characteristic time scale of microwave emission to determine accumulation variability in Antarctica
Relationship of the Passive Microwave Characteristic Time Scale of Emission to Accumulation Rate in Antarctica Authors: Lora S. Koenig1, Eric J. Steig1, Dale P. Winebrenner2 1) Department of Earth and Space Sciences, University of Washington 2) Applied Physics Laboratory, University of Washington Passive microwave sensors offer a potential tool for retrieving accumulation rates over the Greenland and Antarctic ice sheets. However, no retrieval method developed to date is reliable in both temporal and spatial domains. A new retrieval method is presented that shows considerable promise. The characteristic timescale of emission ($\tau_{0}$) is the ratio of the microwave extinction length in the firn, squared, to the firn thermal diffusivity. This characteristic time scale arises in a convolution expression that relates physical temperature to microwave brightness temperature, replacing the "emissivity" term in the traditional Rayleigh-Jeans approximation. $\tau_{0}$ can be estimated for the entire Antarctic continent by comparing thermal infrared observations of physical surface temperature from the AVHRR satellite with passive microwave brightness temperatures at the 37 GHz vertically polarized channel measured by the Scanning Multichannel Microwave Radiometer (SMMR) and Special Senor Microwave Imager (SSM/I). Comparison between $\tau_{0}$ and independent estimates of accumulation rate from radar-echo-sounding observations near Byrd Station Antarctica shows a strong linear relationship for accumulation rates over a broad range -- from 10 to 50 cm/year ice equivalent. Averaged over the 18 years of available data, $\tau_{0}$ varies over this area from a few days to more than three months. Estimates of $\tau_{0}$ over short time intervals of three years show patterns reminiscent of expected accumulation rate variability, and are of the correct magnitude to plausibly relate to temporal accumulation rate changes. Additional radar accumulation measurements from West Antarctica, which provide temporal as well as spatial estimates of accumulation over broad areas, are currently being compared with calculations of $\tau_{0}$ to further examine the extent to which the observed spatial relationship holds in the temporal domain.
C33D-02 13:55h
Subglacial Volcanism in West-Antarctica - A Geologic and Ice Dynamical Perspective
Subglacial volcanic eruptions may increase the contribution of the West-Antarctic Ice-Sheet (WAIS) to global sea-level rise in the near-future by enhancing basal melt water production and ice flow lubrication. Geophysical data have led scientists to believe that the ice sheet may be located over an extensive, young volcanic province containing ~1 million cubic kilometers of basalts (Behrendt, 1964; Behrendt et. al., 1991; 1995; 1998). While not all scientists may recognize this theory of widespread subglacial volcanism, so far no scientific paper has challenged its existence. Here we present the first geologic constraints on the presence/absence of widespread Late Cenozoic subglacial volcanism beneath the WAIS and investigate the potential influence of an individual subglacial volcano (Blankenship et. al., 1993) on the flow dynamic of WAIS. Properties of subglacial sediments indicate limited presence of subglacial volcanic rocks. Moreover, the only two basaltic pebbles, recovered from the region, are of Mesozoic-Paleozoic age (~100 to ~500 million years). While these findings reduce the potential for widespread near-future increases in ice discharge from WAIS due to eruptions of subglacial volcanoes, they do not rule out the presence of individual hot spots associated with volcanic centers beneath the WAIS. Fuel for the existence of a proposed volcano (Mt. Casertz) on the Whitmore Mountain Ross Sea Transitional Crust (WRT; Blankenship et. al., 1993), in the southern part of the WAIS, comes from thermo-dynamical modeling in comparison with observed ice velocities. Ice velocities (Joughin et. al., 1999; 2002) downstream of Mt. Casertz indicate significant basal sliding, where thermo-dynamical models suggest that the ice sheet is frozen to its base. Routing of basal melt water, produced in the vicinity of Mt. Casertz, may lubricate the ice base in parts of the WRT, thus enabling basal sliding and enhancing the discharge of ice in this sector of the WAIS. The only means to resolve any further questions on the existence of subglacial volcanism in West-Antarctica and its potential impact on the dynamic of the ice sheet, requires drilling into potential volcanic centers and the recovery of volcanic rocks for dating and geochemical analysis.
C33D-03 14:10h
Mechanisms for the Survival of Subglacial Lake Vostok During the Buildup of the Antarctic Ice Sheet
There is some debate on the origin of subglacial Lake Vostok and whether the water within the present subglacial lake system contains biota that survived the buildup of the Antarctic ice sheet. One theory suggests that Lake Vostok existed as a preglacial lake before glaciation of the continent at around 15 Ma ago, survived the subsequent period of ice sheet growth, and remained stable beneath the thick ice cover to the present day (Duxbury et al., 2001, JGR 106(E1)). Another hypothesis challenges this view by stating that the early phase of buildup would have resulted in ice grounding throughout the trough which the lake now occupies (Siegert, 2004, JGR 109(E02007)). Here, I present a physical mechanism that allows for subglacial Lake Vostok to survive the buildup of the ice sheet, by taking into account the interaction of the ice sheet with the preglacial / subglacial lake. This hypothesis is supported by the fact that the surface of a preglacial lake as well as the interface between a subglacial lake and the overriding ice sheet can be regarded as a slippery spot. Model simulations -- with the 3D higher-order ice-sheet model of Pattyn (2003, JGR 108 (B8)) -- demonstrate that due to the interaction of the ice sheet with the lake surface, ice-sheet surface slopes near the edge of the ice sheet remain low, so that subglacial water is not driven out of the subglacial trough due to enhanced hydraulic potential gradients. Survival of the lake after initiation of the Antarctic ice sheet implies that possible microorganisms and their remnants within the water can be older than 5-30 Ma. It is further discussed why subglacial troughs near the edge of the Antarctic continent (such as Astrolabe subglacial basin) are at present devoid of water.
C33D-04 14:25h
The Influence of a Subglacial Dilatant Till Beneath the Upstream Reaches of Bindschadler Ice Stream: Implications for Streaming Ice Flow
Subglacial sediments play an important role in the streaming ice flow along the Siple Coast of West Antarctica. It has been hypothesized that a soft and deformable bed acts as a lubricant in enhancing ice flow over an otherwise rugged bedrock floor. The only prior observation of a subglacial till along the Siple Coast was beneath Whillans Ice Streams. This single measurement leads to uncertainties about the regional extent and overall importance of tills in ice drainage. We present the results of a reflection seismic experiments performed just upstream of the inferred onset region of Bindschadler Ice Stream. Changes in the reflectivity of a seismic reflector along the ice-bed interface have proven to be useful in determining the subglacial properties along a portion of Bindschadler Ice Stream. The amplitude versus offset (AVO) technique measures how the amplitude of a seismic reflector varies with increasing source to receiver offset (or angle of incidence of the seismic wave relative to a given interface). Using this method, we suggest that the bed of the ice stream is composed of an unconsolidated and saturated sediment layer. The source-receiver geometry of the seismic reflection experiments allowed us to observe arrivals up to an angle of incidence of 30 degrees. The most notable change observed is a 180 degree phase shift of the basal reflection at incidence angles ranging from 12-25 degrees. Analysis of these amplitude variations reveals P-wave velocities of $<$1700m/s and S-wave velocities of $<$200m/s, indicative of a dilatant and possibly deforming till layer. We suggest that this till layer may be closely linked to the channelization and onset of streaming ice flow.
C33D-05 14:40h
Determining Sliding Velocity and Shear-Strain Magnitude From Basal Sediments of Past Ice Sheets
Ice sheets slide over their basal sediments and commonly deform them. Despite meticulous description of such sediments, they have not been used to estimate rates or magnitudes of basal motion. Thus, although a common assertion is that modeling of past ice sheets can benefit from studies of basal sediments, their actual utility in modeling studies has been minimal. We have developed methods for estimating sliding velocity and till shear-strain magnitude from basal sediments of past ice sheets. The first method involves balancing shear traction on clasts that have plowed through the bed surface with resistance to plowing provided by the bed. The shear traction on clasts is provided by the sliding theory of Lliboutry, and plowing resistance is estimated using a geotechnical theory of cone penetration. The result is an expression for sliding speed of a past ice sheet that depends only on the size distribution of clasts that plowed and the thermomechanical properties of ice and clasts. This method was applied to sizes of clasts that plowed through outwash near Peoria, Illinois, to estimate the sliding speed of the Illinoian ice sheet in that area: 60-170 m/a. The second method involves shearing till in laboratory experiments to study the evolution of till microstructural properties as a function of shear-strain magnitude. Microstructural anisotropy is quantified by collecting multiple intact samples (20 mm cubes) and determining the strength of fabric defined by principal directions of magnetic susceptibility. These directions depend on alignment of needle-shaped magnetite grains. Fabrics formed by directions of maximum susceptibility do not become steady until shear strains of 20-50. Therefore, laboratory calibrations of fabric strength to shear-strain magnitude allow the extent of bed deformation to be determined from susceptibility fabrics of basal till. These studies can provide quantitative inputs to ice-sheet models that have been unavailable previously.
C33D-06 14:55h
The Role of Plastic Necking in Ice Shelf Rifting: A Comparison with Lithospheric Rifts
Ice shelf rifts are through-cutting fractures that penetrate the entireice depth. Most treatments of rift propagation assume that the fractureprocess can be treated as brittle (i.e. ice is treated as an elasticmaterial and viscous effects are neglected). In this approach the riftis treated as a thin planar crack in the ice. However, over the timescales of rift propagation (years to decades), viscous strains exceedelastic strains by several orders of magnitude, making this aninappropriate assumption. We suggest that a more realistic formulationis to consider the rifting process as the result of strain localizationdue to plastic flow - sometimes called the "necking instability". Inthis theory deformation occurs in a zone with a characteristic lengthscale of several ice thicknesses rather than in an small zone around therift tip. Here we present the linear stability analysis which leads toplastic necking under plane strain conditions and determine theconditions where small perturbations in either the bottom profile orsurface profile of the ice shelf become amplified. The predictedsurface topography expected from a small initial thickness perturbationis compared with GLAS elevation profiles across ice shelf rifts, as wellas SRTM profiles across emerged rifts along tectonic plate boundaries.
C33D-07 15:10h
Biweekly variation in the flow speeds of Rutford Ice Stream, West Antarctica.
GPS measurements conducted in Dec 2003 to February 2004 on Rutford Ice Stream revealed fluctuations in surface velocities with a periodicity of two weeks. Tidal-analysis of the data, and comparison with GPS measurements made in the same time period on Ronnie Ice Shelf, show these temporal variations in flow to be tidally controlled and related to spring-neap tidal cycles. In total, five GPS stations were operated on the ice stream over a period of about 50 days. A further reference station was situated on Fletcher Promontory. Distance from the reference station to all other GPS units was less than 60 km. Positions were calculated using the local reference station, and by the use of precise point processing. In contrast to what similar type of measurements have reviled on the ice plain of Whillans Ice Stream, there is no indication of any stick-slip motion on the Rutford Ice Stream. Tidal effects on flow were observed at the upper-most station some 50 km upstream from the grounding line. Analysis of the spatial variation in tidal amplitudes constituents suggests that tides affect the flow some tens of km further upstream. Preliminary model calculations suggest that the lack of any diurnal variability in surface velocities may be caused by the particular bed-to-surface transfer characterisitcs of the ice stream.
C33D-08 15:25h
SOUTHBERG: An in-situ investigation of the seismic symphony of Iceberg C16, Ross Sea, Antarctica
We present preliminary results of the operation, from November 2003 to January 2004, of a PASSCAL array of 4 short-period seismometers on Iceberg C16, presently grounded North of Ross Island. A broadband STS-2 instrument was also run at the central station. Results from an uninterrupted 33-day window during which all 4 stations were operational are: (i) GPS tracking indicates that the 4-station polygon remained rigid but drifted (about 20 m) and rotated (0.1 degree) abruptly on Christmas Day; (ii) most of the background seismic noise is concentrated in the 20-50 mHz range, which includes the natural bobbing and rolling frequencies of the ice sheet; (iii) a large variety of seismic signals originating from the ice were detected, including: (Type I) short events featuring a white spectrum, probably simple episodes of cracking in the ice; (Type II) long episodes of relatively broad-band noise, of variable amplitude, lasting hundreds to thousands of seconds, occasionally superimposed over somewhat preferential frequencies; and (Type III) resonant signals featuring extremely narrow spectral lines (with overtones), similar to those recorded in Polynesia in 2000 from drifting icebergs. Type III signals are correlated among the four stations of the network, but do not seem to be interpretable as the mere passage of a seismic wavefront, thereby suggesting the resonance of a normal mode of the whole structure. A Type III signal on 15 January 2004 coincides with an episode of collision with and friction against B-15A, as documented from GPS data obtained on the latter. We may present additional data to be retrieved from Station C16-A, which wintered over, and will be revisited in October 2004.