C41C-01 INVITED
The Role of Subglacial Water and Tectonics in the Onset of Fast Ice Flow
Dramatic change in the polar regions is focused along the ice sheet margins and will be a direct result of ice dynamics and fast ice flow. As ice streams and outlet glaciers transport ice into the global oceans, water and sediments are key subglacial lubricants. In West Antarctic, the distribution of sediments is controlled both by the location of fault bounded rift basins and marine sediments. The production of water will be influenced by local variations in geothermal flux and the presence of active subglacial volcanism. In East Antarctica, large volumes of subglacial water was first identified in relatively stable lakes. The largest of these lakes appear to be rest along well defined tectonic boundaries. Recently water has been imaged actively flowing between lakes in the interior. Large tectonically controlled East Antarctic lakes have been linked to the onset of fast flow of the Recovery Ice Stream.
C41C-02
Coupled Ice, Water and Crustal Systems Beneath the West Antarctic Ice Sheet from Aerogeophysical Observations with Implications for Fast Glacier Flow and Habitats.
Abundant subglacial water has been hypothesized as a key control on the existence and evolution of fast glacier flow within the West Antarctic Ice Sheet (WAIS). In addition, the origin of this water will dictate the diversity of life that can exist beneath the ice sheet. Large scale basal melting of the ice is controlled by glaciological stresses and the regional distribution of geothermal flux whereas subglacial hydrothermal systems will be controlled by local geology and a local concentration of high geothermal flux.In this talk, airborne measurement of ice thickness and surface slope coupled with radar reflection coefficients are used to characterize the water systems associated with both local and regional geology beneath the Ross and Amundsen Sea Embayments of the WAIS inferred from airborne gravity and magnetics observations. In addition, we utilize the internal layering of the ice sheet to evaluate candidates for subglacial hydrothermal systems. We conclude that 1) the upper reaches of Kamb ice stream in the Ross Sea Embayment of the WAIS are underlain by a pervasive water sheet that is "energetic"; 2) the regional geologic framework for this water sheet is an adjacent intrusive/extrusive complex that lies less than 1000 meters below the ice surface and is characterized by significant mantle upwarping; 3) there is direct evidence from layer downdraw within the ice that this complex is both a source of water for Kamb ice stream's pervasive water sheet and that this water may arise from a subglacial hydrothermal system; and, 4) similar features, often associated with subglacial lakes, exist at depths up to several thousand meters beneath the Amundsen Sea Embayment of the WAIS.
C41C-03
Neogene Tectonic Events in the Marie Byrd Land Sector of the West Antarctic Rift System: Their Potential Impact on Ice Sheet Evolution and Stability.
The West Antarctic rift system is buried beneath 1-4 km of ice over much of its extent, obscuring vast areas that could provide clues about the potential for active volcanism beneath the ice sheet, and whether significant tectonic movement has taken place in Cenozoic time. This study explores the consequences of viewing the ice as basin fill, and of approximating the mass equivalent of ice as unconsolidated sediment. It then compares the results with active rift systems elsewhere in the world. The results suggest (1) that the interior rift trough is relatively cool, and the potential there for destabilizing subglacial eruptions is low, (2) that extension and over- deepening of interior basins, like the Bentley Subglacial Trench, have taken place beneath the ice sheet in Neogene time, and (3) that dome uplift and the growth of large volcanoes along the Marie Byrd Land coast, together with the subsidence of interior basins, have greatly increased the topographic relief of the rift system in Neogene time. Recent studies suggest that West Antarctic glaciation first appeared during the Oligocene. The implication of this study is that the Oligocene ice sheet originated on a low relief landscape near sea level, adjacent to a much shallower inland sea, and has since evolved in an environment of progressive basin deepening, dome uplift, and volcanism, unlike that of any other ice sheet in the recent past.
C41C-04
Crustal and Lithospheric Structural Controls on Thwaites Glacier, West Antarctica
Thwaites Glacier in the Amundsen Sea Embayment of West Antarctica is changing rapidly. Satellite observations show that the glacier is accelerating, its grounding line is retreating, and its floating portion is thinning. These changes are dynamic and could be related to the nature of the sub-ice geology, though the geology is not well- understood. We know from the Ross Sea Embayment ice streams of West Antarctica that the locations of subglacial water (and sediment) are critical to initiating fast flow. Subglacial water can be created in areas where the base of the ice is at the pressure melting point, particularly in areas that may have elevated geothermal heat flux. To fully understanding the thermal state of the base of the ice, we must determine the deeper geology of the continent below. We hypothesize that the crustal and lithospheric structure of the Thwaites Glacier catchment (TGC) impacts the behavior of Thwaites Glacier through spatially heterogeneous geothermal heat flux. Here we use airborne gravity results collected primarily by the University of Texas at Austin in 2004-2005 to test this hypothesis. The airborne gravity free-air and Bouguer anomalies can be used in two ways to estimate sub-ice earth structure. First, the long-wavelength Bouguer gravity anomalies reflect changes in Moho depth. Based on spectral estimates of the gravity anomalies for gross crustal structure, there are two crustal provinces in the TGC: the West Antarctic Rift System in the eastern side of the catchment with a Moho at approx. 27 km b.s.l., and the Marie Byrd Land crustal block in the western side of the catchment with a Moho at approx. 20 km b.s.l. To determine the spatial variability of crustal thickness across these crustal provinces, we invert the long-wavelength Bouguer anomalies for Moho depth. This method will also more precisely locate the boundary between the two provinces. Second, we use admittance/coherence techniques on the gravity and topography to estimate the elastic thickness (Te) of the lithosphere containing the crustal blocks. The elastic thickness of the lithosphere is related to the lithosphere's actual thickness and thus the temperature at the base of the lithosphere. Based on our crustal and lithospheric structure results, we will comment on both the potential heterogeneity of subglacial heat flux and the creation of deep sedimentary basins.
C41C-05
New Type of Basaltic Subglacial Sheet-Like Sequence and Implications for the Inferred Thickness of Associated ice
Basaltic volcanic sequences erupted subglacially have been classified empirically into two major types, thought to correspond to eruptions under thick and thin ice, respectively. The latter are called subglacial sheet-like sequences from the appearance of the outcrops. Only one type of sheet-like sequence has been described so far. However, there is now evidence that there are at least two types of subglacial sheet-like sequence, with significantly different implications for interpretations of associated palaeo-ice sheet thicknesses. The first type, which is relatively well known, is regarded as a diagnostic product of eruptions associated with relatively thin glaciers (< c. 150 m) of any thermal regime. It is called the Mount Pinafore type after sequence holotypes described in Antarctica. A second type of sheet-like sequence is now proposed based on significant differences in the lithofacies and lithofacies associations. In particular, the eruptions are typically not explosive except sometimes in the latest stages. They are probably linked genetically to subglacial pillow volcanoes. The individual sequences only provide an indication of minimum thicknesses of the associated overlying ice, although theoretical considerations suggest the coeval overlying ice mass must have been substantial, probably in excess of 1000 m. The sequences are currently only known to crop out in Iceland but similar volcanic sequences probably exist beneath the West Antarctic Ice Sheet (WAIS). As the latter are presently covered by about 3 km of ice, they are inaccessible and cannot be scrutinised directly. The impact of eruptions of this type is still to be assessed but they may have an important influence on future WAIS stability.
C41C-06
Crustal geothermal controls on ice sheet dynamics of Greenland
Rapid ice flow in the northeast quadrant of the Greenland Ice Sheet is associated with unusually high heat flow. Heat flux can be greatly increased in deep valleys to promote basal melting with additional feedback due to locally increased friction. However, crustal thinning can also enhance heat flow because the relatively thermally conductive mantle is closer to the surface. In addition to incised topography, relatively shallow Moho also occurs beneath the northeast quadrant of the Greenland Ice Sheet. We made regional three-dimensional thermal models that include the effects of topographic and mantle relief. These effects can strongly enhance the heat flux at the base of the ice sheet.
C41C-07
Evidence for an Extensive Tributary-Type Ice Flow System in the Interior of the Laurentide Ice Sheet: Implications for Ice Sheet Stability
It has been demonstrated that many modern ice streams are fed by complex systems of tributaries extending far into the interior of ice sheets, thus raising questions about their potential implications in past and modern ice sheet response to climate forcing. However, little is known about the timing of initiation of these systems and the mechanisms involved in their development. Here we present evidence for a major glacial dynamics shifts in the southwestern sector of the Laurentide Ice Sheet (LIS) that lead to the development of a tributary-type flow system prior to the final retreat of the LIS in that sector. The system apparently extended more than 1500 km into the interior of the ice sheet, a distance similar to what has been modeled for east and west Antarctica (~ 1000 km). This interpretation is based on the analysis of SRTM-derived digital elevation models over the North American Interior Plains, satellite imagery, available digital geologic maps, 14C databases and associated published ice margin retreat sequences as well as limited new field observations. The development of this tributary-type flow system may be coeval with readvance of the James Lobe (~ 14.6 cal ka BP) in South Dakota. This system also clearly crosscuts a large but less complex set of paleo-ice stream tracks. This older (~ 16.0 cal ka BP) ice flow configuration is associated with prominent ice marginal glaciotectonic features in southern Alberta and southwestern Saskatchewan. As the ice sheet thinned and the ice margin retreated, the tributary system evolved into a set of smaller outlet lobes just prior to the retreat of the ice margin onto the Canadian Shield (~ 12.5 cal ka BP). These smaller lobes left several belts of ice-thrust features. Based on the proposed reconstruction, we suggest that the development of the tributary-type ice flow system in the interior of the LIS marked the onset of its collapse. If such internal reorganization of ice-flow systems was indeed characteristic of the collapsing LIS, concerns over the stability of modern ice sheets become even more justified as flow systems of this type are known to exist in Antarctica and Greenland.
C41C-08
Thinning and Acceleration of East Antarctic Outlet Glaciers
The East Antarctic Ice Sheet (EAIS) is Earth's largest freshwater reservoir and has the potential to raise global sea level by ~55 meters. Because of its large size and the logistical challenge of conducting measurements, as well as limits on latitudinal coverage or spatial resolution of some existing remote sensing instruments, potential regional-scale mass imbalances of the EAIS are difficult to detect with the datasets currently available. ASTER satellite imagery provides high-resolution coverage farther south than many existing datasets, opening up new possibilities for investigation. Here, we describe ice dynamic characteristics and mass balance estimates for four large outlet glaciers draining the EAIS through the Transantarctic Mountains -- David, Mulock, Byrd and Nimrod -- based on an analysis of new ASTER-derived ice velocity and surface elevation maps. The conventional view of largely-stable outlet glaciers in this sector of East Antarctica may be open for reassessment. Results indicate that Byrd Glacier, one of the largest glaciers in Antarctica, entered a phase of modest acceleration and thinning beginning in 2006-2007. The current flow speeds are anomalous in the context of an historical record of velocity measurements dating back to the 1960s. Similar changes also appear to be occurring on Mulock and Nimrod glaciers, although their historical records are of much shorter duration. These results are interesting because they demonstrate the dynamic nature of outlet glaciers draining Earth's largest ice sheet, and point to a possible mechanism for a significant EAIS contribution to near-term sea level rise.