C51B-0381
Upper Pliocene Diatom Record From Northern Basin, Ross Sea; Potential Correlation to Other Antarctic Continental Shelf Records
The Northern Basin seismic record suggests a dynamic East Antarctic Ice Sheet with at least eight Late Neogene glacial expansions. The seismic-stratigraphic record indicates relatively thick late Neogene sequences on the outer continental shelf and upper slope and possibly undisturbed interglacial sequences. In 2003 Nathaniel B. Palmer (NBP03-01) collected 10 piston and jumbo piston cores in a depth transect from Mawson Bank to Drygalski Basin. The sediment cores range from 46 cm to over 4 meters in length, only recovering the surface of upper seismic unit. The lithologies of the cores include diatom mud, bioclastic sediment, silt, sand, clay and diamicton. Diatom analyses reveals that two of these cores, NBP03-01 PC2 and NBP03-01 PC8, have penetrated into undisturbed Upper Pliocene sequences. Upper Pliocene diatomaceous units are also recovered in Prydz Bay ODP sites 742 age ca. 1.8-2.2 Ma, and 1166 ages ca 1.8-2.2 and 2.5-2.8, sandwiched between diamictons. Recovery of Pliocene and Pleistocene sediment records of interglacial events is rare on the Antarctic continental shelf and the obtained records have been difficult to correlate with any certainty. With the recent recovery of the ANDRILL MIS (AND-1 core) the potential for correlation of these records has improved significantly. The sediment record from ANDRILL MIS comprises 13 diatomaceous units, all with their own specific diatom assemblage and key species. Comparison of diatom assemblages using key taxa and PCA show a strong correlation between Northern Basin PC2 and PC8, ODP Site 742 and MIS diatom unit IV placed at the Gauss-Matuyama boundary. Continued multiproxy analyses and independent dating will be performed to further constrain the correlation. The initial results have implications for regional correlation of interglacial events, and suggest that other Pliocene sediment records may be preserved in the Northern Basin region.
C51B-0382
Do laminated sediment-gravity-flow deposits on the Antarctic Peninsula continental shelf record ice shelf grounding events?
Drifts on the continental rise of the Antarctic Peninsula may contain a high-resolution record of ice-sheet expansion on the adjacent continental shelf (Baker and Carmerlenghi, 1999). During major ice-sheet grounding events, sediment is delivered to the outer continental shelf and upper slope. Associated slope instabilities trigger additional mass transport to the continental rise and abyssal plain. Therefore, major glacial periods should be recorded on the continental rise by laminated sediment-gravity-flow deposits within the drifts. When grounded ice retreats during interglacials, sedimentation rates would decline and the tops of laminated sections should be bioturbated. In this ongoing study, we compile the laminated-bioturbated couplets from ODP Leg 178 Sites 1095, 1096, and 1101. We are in the process of evaluating whether the timing and frequency of the sediment cycles on the drift are possibly related to orbital forcing as opposed to an uneven distribution of sediment input from the outer continental shelf and/or bioturbation on the drift.
C51B-0383
Numerical Modeling of West Antarctic Ice Sheet grounding-line stability under the influence of changing continental shelf physiography
Bart and Iwai (in prep.) utilize morphologic and biologic evidence to hypothesize that the Antarctic Peninsula's outer continental shelf overdeepened during a transitory period of increased glacial erosion in the early Pliocene. They attribute the enhanced erosion of the continental shelf to a period of regional warming on the peninsula's pacific margin that produced an associated increased flux of snow and ice. On the over-deepened outer continental shelf, the deeper-water sills presumably permitted a larger influx of relatively warm circumpolar deep water. If correct and if early Pliocene overdeepening of the continental shelf was a continent-wide phenomenon, then this relatively recent modification of the Antarctic continental shelf may have greatly altered the way in which the marine terminus of the Antarctic Ice Sheet interacts with global thermohaline circulation. Our ongoing numerical modeling experiments of the Eastern Basin Ross Sea continental shelf are designed to evaluate the influence of changing shelf morphology (primarily water depth) and water-mass properties (primarily, water temperature) on West Antarctic Ice Sheet grounding-line stability.
C51B-0384 INVITED
Sedimentary characteristics of Late Pleistocene till from the Transantarctic Mountains, Antarctica
Particle size distributions were measured from till samples collected at seventeen moraines along the Byrd and Nimrod Glaciers. Approximately 2 g of material was separated from till samples, treated with 35% H2O2, and measured 3 to 5 times on a Malvern Mastersizer 2000. All sites are dominated by sand- and gravel-sized material with distinct spatial patterns in the silt and clay content. Till from the north side of Byrd Glacier and along the trunk of Nimrod glacier have variable modes in the sand fraction and <5% clay. The tills are primarily derived from locally eroded bedrock. Moraines found at the head of both glaciers contain 15% - 70% fines (silt plus clay) and are interpreted to represent the presence of subglacially-derived sediment. Till deposited in the Ross Sea from outlet glaciers likely contains a mixture basal debris derived from the East Antarctic craton and material eroded as glaciers cross the Transantarctic Mountains.
C51B-0385
ESTIMATION OF ACCUMULATION RATES ON THE CATCHMENT OF THWAITES GLACIER, WEST ANTARCTICA
West Antarctica has been found to be experiencing rapid changes of ice dynamics as a response of rapid climate changes in the past century, therefore the assessment of mass balance is a priority for different scientific communities. To assess the problem of mass influx, we construct a map of snow accumulation rates over the catchment of Thwaties Glacier, West Antarctica, based on a least square inversion of mass influx along flow lines. The data parameters are surface velocities and shallow internal isochron layers traced from radio echo soundings (UTIG-AGASEA survey 2004-2005). Our calculations account for firn compaction, convergence and divergence of ice flow as well high velocities over the ice trunk. As an assessment of mass balance in Thwaites Glacier, we compare our estimates of ice influx, which represents thousand years of accumulation, against published data of mass outflux.
C51B-0386
Enhancing a RADARSAT/ICESat Digital Elevation Model of West Antarctica Using MODIS Imagery
An image enhancement approach is used to develop a new digital elevation map of West Antarctica, combining multiple MODIS images and both radar altimetry and ICESat laser altimetry Digital Elevation Model (DEM) data. The method combines the wide image coverage of MODIS, and its high radiometric sensitivity (which equates to high sunward slope sensitivity), with the high precision and accuracy of ICESat and combined ICESat and radar altimetry DEMs. We calibrate brightness-to-slope relationships for several MODIS images of the central West Antarctic using smoothed DEMs derived from both sources. Using the calibrations, we then created, first, a slope map of the ice sheet surface from the image data (regressing slope information from many images), and then integrated this absolute slope map to yield complete DEMs for the region. ICESat (as of September 2007) has acquired a series of eleven near-repeat tracks over the Antarctic during the period September 2003 to April 2007, covering the continent to 86 deg S. ICESat data are acquired as a series of spot elevations, averaging a ~60m diameter surface region every ~172m. However, ICESat track paths have spacings wide enough (2 km at 85 deg; 20 - 50 km at 75 deg) that some surface ice dynamical features (e.g. flowlines, undulations, ice rises) are missed by the track data used to construct the ICESat DEM. Radar altimetry can provide some of the missing data north of 81.5 deg, but only to a maximum resolution of about 5 km. A set of cloud-cleared MODIS band 1 data from both the Aqua and Terra platforms acquired during the 2003-2004 austral summer, used in generating the Mosaic of Antarctica, MOA, surface morphology image map, were used for the image enhancement. Past analyses of the slope-brightness relationship for MODIS have shown ice surface slope precisions of +/- 0.00015. ICESat spot elevations have nominal precisions of ~5 cm under ideal conditions, although thin-cloud effects and mislocation errors can magnify these. Only cloud-free areas of MODIS scenes that also meet specific slope, grain-size, solar zenith, brightness, latitude, and elevation criteria are used for image enhancement. A suite of applications for an enhanced DEM are identified and explored. A full representation of the WAIS undulation field permits a better investigation of the relationship between accumulation and topography, and surface temperature and topography. Further, addressing the shape modifications introduced by the variations in accumulation across undulations is a necessary prerequisite before inverting surface topography for bed elevation. Lastly, surface topography and detailed bed topography are both required for inferring sub-ice-sheet hydrostatic pressure. We will discuss these potential applications.
C51B-0387
Analysis of Changes in Ice Elevation, Ice Mass and Ice Deformation --- Methods and Case Studies from Greenland and Antarctic Glaciers
We introduce and utilize a combination of old and new geo-spatial analysis methods to derive and synthesize ice- elevation changes, ice-mass changes and changes in the dynamics of a glacier, with the objective of exploiting remote-sensing data as indicators of geophysical processes of cryospheric change. Data types include ICESat, Cryosat, radar altimetry, multispectral image data and videographic data. The geographic objectives of our paper are Pine Island Glacier in the Amundsen Sea Embayment (West Antarctica) and Jakobshavns Isbrae (West Greenland), both rapidly changing outlet glaciers, which follow geological troughs. Glaciological conclusions derived in ongoing projects and outlooks for future studies will be presented.
C51B-0388 INVITED
Exploring under the weak underbelly of the West Antarctic Ice Sheet with recent aerogeophysical data
There are growing concerns over how the West Antarctic Ice Sheet (WAIS) will respond to global warming and the major societal implications that its potential collapse would have. An area of particular concern is the Amundsen Sea Embayment (ASE), where glaciers such as Pine Island Glacier (PIG) and Thwaites Glacier (THW) have been shown to be melting, thinning, accelerating and retreating rapidly. This part of the WAIS has also been referred to as the "weak underbelly of the WAIS" where rapid deglaciation could occur. Previous aerogeophysical surveys flown over the Siple Coast (SC) ice streams have imaged subglacial geology, and have highlighted the importance of considering geology when assessing the dynamics and long-term stability of the WAIS. The SC ice streams overlie broad and narrow sediment-infilled rift basins of the West Antarctic Rift System (WARS). Subglacial sediments and Cenozoic to Recent magmatic features associated with the WARS may represent geological templates for enhanced glacial flow there. However, whether the WARS extended beneath the catchments of the dynamic ASE glaciers remained poorly constrained due to the paucity of geophysical data. Approximately 100,000 line km of new aerogeophysical data were collected over this region as part of a collaborative effort between the University and Texas and the British Antarctic Survey, comprising airborne radar, aeromagnetic and airborne gravity. The new airborne radar datasets yielded an improved picture of the deep narrow subglacial trough beneath the trunk of PIG, and the broader basin where THW flows, as well as the narrow basins where tributary flow occurs. A bedrock high was imaged on the flank of PIG, which would rise above sea-level after isostatic compensation following deglaciation, and could perhaps impede ice-sheet collapse initiated near the grounding line by preventing its progress into the deep Byrd Sublglacial Basin. The new potential field data uncovers the WARS in ASE region. Aeromagnetic patterns suggest that several rift basins were highly magmatic in the Cenozoic west of 102W, in contrast to a much more weakly magmatic area to the east. Remarkable boundaries between the Marie Byrd Land block, the WARS and the Ellsworth Mountains are imaged. Modelling of Bouguer gravity data indicates a generally higher degree of crustal thinning compared to the SC region, and low lithospheric rigidities, both of which may be indicative of a modern rift system. Rifting and magmatic processes may have induced elevated heat-flow at both local and regional scale, thereby increasing the availability of water at the base of the ice sheet. Subglacial sedimentary basins, which may further enhance fast glacial flow in the ASE are revealed by our aeromagnetic and isostatic residual gravity images.
C51B-0389
A structural glaciological analysis of the 2002 Larsen B Ice Shelf collapse
Recent ice-shelf collapse events have been linked to rapid climatic warming of the Antarctic Peninsula. Here we provide a detailed structural glaciological analysis of the changes in surface structures on the Larsen B Ice Shelf ice shelf from a series of visible-channel satellite images acquired between January 2000 and April 2002 and the RAMP mosaic of 1997. Mapped features include the ice-shelf edge, rifts (fractures that penetrate the entire ice shelf), crevasses, longitudinal linear surface structures (also called flow stripes or streaklines), medial moraines, and meltwater features (streams, ponds and dolines). Longitudinal surface structures are particularly important because they can be used to define individual tributary-glacier flow units and their contribution to the ice shelf. Using this approach, we define domains on the ice shelf related to glacier source areas and demonstrate that, prior to collapse, the central Larsen B Ice Shelf consisted of four sutured flow units fed by Crane Glacier, Jorum Glacier, Punchbowl Glacier and Hektoria/Green/Evans Glacier. Between these active glacier-fed flow units were less-active flow units. Prior to collapse, large open rift systems (with floating brash ice) were present ~40 km from the ice-shelf edge at Foyn Point and Cape Disappointment because of the strong lateral shear in the zone separating active and less-active flow units. We suggest that the ice shelf was pre-conditioned to collapse by partial rupturing of the sutures between flow units. Rupturing may have been caused by changes in velocity and shear stresses within the tributary glacier flow units, leading to weaker coupling with islands and peninsulas bounding the shelf.
C51B-0390
210-Pb Study of Fjord Sedimentation Rates in the South Shetland Islands and Antarctic Peninsula
As part of a study concerning the factors controlling rates of glacial erosion and sedimentation across climatic regimes, sixteen kasten cores were collected from ten fjords in the South Shetland Islands and Antarctic Peninsula in April and May, 2007 from the R/VIB Nathaniel B. Palmer. They were sampled for 210Pb measurements of accumulation rates and associated sedimentological observations (x-radiography, sedimentology). The cores were collected from basins within a few kilometers of the calving front at the respective glacier, where sediment accumulation was observed in 3.5 kHz sub-bottom profiles. Core lengths ranged from 34-289 cm; in 7 of the 16 core locations, multiple deployments were required to retrieve even a short sediment core due to the hardness of the bed. For most of the cores (10 of 16) sandy mud has accumulated steadily at rates ranging from 1 to 10 mm/y over the past century, as tightly defined by clear linear trends of 10 to 15 log 210Pb values versus depth, with r2 ranging from 0.75-0.98. The rates of accumulation appear to be independent of distance from the calving front, and most likely reflect variability in glacial-marine sediment delivery to, and basin morphology of, individual fjords. Several cores show evidence of variable sedimentation with pulses of sand emplacement. Three cores contained a surficial layer >50 cm thick of uniform 210Pb activity, indicating recent, rapid deposition of a large turbidity current or mass flow. The consistency of many of the 210Pb profiles implies that, over the past century, the glacial-marine processes that supplied, transported and deposited sediment in these sub-polar and polar fjords have not varied markedly. This contrasts with other glacial-marine systems in warmer regions such SE Alaska, where sedimentation in the fjords from calving temperate glaciers tends to vary significantly, and where the accumulation tends to slow as the calving front recedes rapidly from the core location. The rates of sedimentation reported here for the last 10 to 100 years off the Antarctic Peninsula will be compared to those on a millennial time scale derived from 14C data, and will be interpreted in the context of corresponding bathymetric and glaciological data as they become available.
C51B-0391
Recent Measurements of Ice Flux From Outlet Glaciers of the South Shetlands and Antarctic Peninsula
One of the most significant events in the evolution of the Antarctic climate and cryosphere was the pronounced glacial erosion in the late Cenozoic that led to the considerable depth and landward sloping profile of the continental shelf, affecting both ice sheet dynamics and the oceanographic processes that drive circulation on the shelf. Understanding how changes in glacial conditions during the LGM may have led to enhanced glacial erosion, however, requires us to first identify the factors that control the amount and rate of glacier erosion in Antarctica today. The bays and fjords of the Antarctic Peninsula contain a rich history of climate change recorded both in proxy climate data (e.g., forams, oxygen isotopes) and in sediment accumulation rates that reflect changes in glacial erosion and sediment transfer. Prior studies revealed large variations in the rate of sediment accumulation across the Peninsula, with a general trend of decreasing sedimentation from north to south and west to east, attributed to climate-driven differences in glacier dynamics. Little is known to date, however, about the individual dynamics of the glaciers in these fjords, and the variability in their sediment delivery, particularly as many of them start to accelerate and retreat (Cook et al., 2005; Rignot, 1998; Angelis and Skvarca, 2003). As part of a study concerning the factors controlling rates of glacial erosion and sedimentation across climatic regimes, the cross-sectional area at the ELA of 16 tidewater glaciers in the South Shetland Islands and the western coast of the Antarctic Peninsula were measured in April 2007. The cross-section area of each glacier is then combined with the mean surface velocity, measured remotely using SAR interferometry, to reconstruct the contemporary ice flux through these glacier systems. The study area spans almost 4° of latitude and 8° of mean annual temperature, encompassing both sub-polar and polar regimes, from Maxwell Bay, South Shetland Islands (62°10' S) to Beascochea Bay, Graham Coast (65°31' S). For the sub-polar glaciers (2 of 16), where the ELA was significantly above sea level, the cross-sectional area at the ELA was measured using ice-penetrating radar. Velocity stakes were also measured at the ELA of these glaciers to ground-truth the satellite measurements of surface velocity. For the remaining polar glaciers, whose ELAs are at sea-level, multi-beam swath bathymetry was used from aboard the RV/IB Nathaniel B. Palmer together with estimates of ice cliff heights to determine the cross-sectional area of the calving front. The variability in ice flux across the sub-polar and polar climatic regimes of the western Antarctic Peninsula reported here will be compared to bathymetric data and sediment accumulation rates derived from 210-Pb and 14-C chronology in the adjacent fjords, as they become available, to examine the influence of both climate and ice flux on the rate of glacial erosion and sedimentation, and to infer potential changes in sediment delivery to the shelf as the region warms.
C51B-0392
New Results and Future IPY Investigations in Hektoria Trough, Inner Larsen-B Embayment, Antarctic Peninsula
We present new results from a sediment core collected in the inner Larsen-B embayment, Antarctic Peninsula. The 2002 break-up of the Larsen-B Ice Shelf enabled access to the termini of glaciers that formerly fed into the ice shelf. United States Antarctic Program cruise LMG05-02 discovered and mapped an 800-m deep glacial trough off Hektoria Glacier at the northern end of the Larsen-B embayment. Sub-bottom profiling reveals the presence of at least 20-m of unconsolidated sediment in the trough, which varies in character between acoustically laminated, and packages with weak, discontinuous reflectors. A 2.5-m kasten core was recovered from the Hektoria trough during cruise LMG05-02. The sediment consists of silty clay with a minor component of very fine sand, and is laminated in the uppermost meter. Downcore magnetic and geochemical parameters were used to investigate the terrigenous sediment supplied to Hektoria Trough. Bulk sediment geochemistry suggests a constant sediment source that is chemically consistent with upper continental crust. Magnetic parameters reveal several intervals in which the magnetic material supplied to the trough varies in abundance and particle size. We observe shifts at approximately 850, 5900, and 7900 14C yr B.P., based on a preliminary age model. Superimposed on these shifts are periodic pulses of coarser material into the trough, which may be related to fluctuations in the location of the grounding line with respect to the core site. Our preliminary age model indicates an average sedimentation rate of 21 cm/kyr, which suggests that the basin fill extends back to perhaps 100,000 yr B.P., a rare archive of the late Pleistocene in an ice proximal site.
C51B-0393
Large Contribution of Antarctic 'Small' Glaciers to Sea-level Rise
Glaciers have generally experienced mass loss in the last couple of decades with strongly accelerated ice wastage during the last decade. We present a methodology for grid-based global assessment of mass loss of "small" glaciers defined as all glaciers outside the ice sheets in Greenland and Antarctica, and their sea-level equivalent during the period 1961-2004. Regional and global estimates of glacier mass loss are computed from glacier area, mass balance sensitivities, and ERA-40 temperature and precipitation trends for each glacierized grid cell. Annual and seasonal mass balance sensitivities to temperature and precipitation changes are computed for almost 100 glaciers based on calibration of a simple temperature-index regression model to observations of seasonal mass balances using ERA-40 re-analysis data as climate input. The mass balance sensitivities are then extrapolated to each glacierized grid cell by means of a continentality index and precipitation as derived for each grid cell of the ERA-40 grid. Results indicate that previous global assessments based on extrapolation of measured mass balances may have underestimated glacier mass loss during the last 40 years due to underestimation of high mass balance sensitivities in areas were measurements are scarce. We obtain a global sea-level rise equivalent estimate of 0.62 mm/yr, of which one third originates from the glaciers and ice caps around Antarctica, and hence their contribution is considerably higher than previously assumed.
C51B-0394
Influence of oceanic boundary conditions in simulations of Antarctic climate and surface mass balance change during the coming century
This work reports on high-resolution (60 km) atmospheric general circulation model simulations of the Antarctic climate for the periods 1981-2000 and 2081-2100. Our analysis focuses on the surface mass balance change, one of the components of the total ice sheet mass balance, and its impact on global eustatic sea level. Contrary to previous simulations, in which we directly used sea surface boundary conditions produced by a coupled ocean-atmosphere model for the last decades of both centuries, we applied an anomaly method here in which the present-day simulations use observed sea surface conditions, while the simulations for the end of the 21st century use the change in sea surface conditions taken from the coupled simulations superimposed on the present-day observations. We show that the use of observed oceanic boundary conditions clearly improves the simulation of the present-day Antarctic climate, compared to model runs using boundary conditions from a coupled climate model. Moreover, although the spatial patterns of the simulated climate change are similar, the two methods yield significantly different estimates of the amplitude of the future climate and surface mass balance change over the Antarctic continent. These differences are of similar magnitude as the inter-model dispersion in the current IPCC exercise: Selecting a method for generating boundary conditions for a high- resolution model model may be just as important as selecting the climate model itself. Using the anomaly method, the simulated mean surface mass balance change over the grounded ice sheet from 1981-2000 to 2081-2100 is 43 mm water equivalent per year, corresponding to an eustatic sea-level decrease of 1.5 mm/year. A further result of this work is that future continental-mean surface mass balance changes are dominated by the coastal regions, and that high-resolution models, which better resolve coastal processes, tend to predict stronger precipitation changes than models with lower spatial resolution. http://www-lgge.obs.ujf-grenoble.fr/~gerhard/resultats/KrinnerEA.jc2008.pdf
C51B-0395
Effect of firn depth and density variations on elevation changes in Antarctica reveals opposing ice sheet mass balance pattern
Satellite radar altimetry is a valuable tool to monitor ice sheet elevation change (dH/dt). Before dH/dt observations can be translated into long-term mass changes, corrections must be made for short-term variability in firn densification rate and accumulation. We calculated this contribution for the observational period of ERS-2 (1995- 2003), using 6-hourly output of a regional atmospheric model (1980-2004) to drive a time-dependent firn densification model. The results show that interannual anomalies in snow accumulation can cause ice sheet elevation changes comparable to those observed by satellite radar altimetry. Taking these firn depth fluctuations into account, the satellite-derived elevation change reveals a pattern of persistent large mass losses in the Amundsen Sea sector in West Antarctica, but also shows that the East Antarctic ice sheet grows, resulting in a net mass increase of 19 Gt/yr for the grounded ice sheet, equivalent to a sea level lowering of 0.05 mm/yr.
C51B-0396
Flow pattern and rheology of marine ice from Nansen Ice Shelf, Antarctica: constraints for modeling
Antarctic ice sheet stability is largely controlled by dynamics of transition zones such as ice shelves. These zones are e.g. acting as confining walls for grounded ice flowing towards the ocean. Marine ice, which is an important by-product of ice-shelf/ocean interactions, is a poorly-known component of ice shelves, although it is supposed to play a major role on their dynamics. In order to integrate more realistic mechanical data of marine ice into ice flow models, rheological, crystallographic and geochemical studies have been carried out on marine ice samples from ice cores extracted near the grounding line of Nansen Ice Shelf (NIS), Terra Nova Bay, Antarctica. Preliminary results indicate that NIS marine ice 1) is deforming slower than isotropic ice under similar pure shear conditions, and 2) that Nansen Ice Shelf dynamics are not in agreement with commonly considered stress fields in ice shelves, as revealed by highly contrasted fabric patterns observed in the cores. It is shown that the topographical settings (margins and pinning points) might play an as important role as the grounding line activity on the dynamics of the ice shelf. The NIS flow pattern and rheological characteristics are discussed here with a view to provide physical constraints for modeling small to medium-scale ice shelves in the context of global change.
C51B-0397
Debris-Rich Basal Ice Layers Effects on Polar Glacier Dynamics
Deformation in a glacier is concentrated near the bed where shear stresses are highest. Deformation may be enhanced significantly when the basal ice of a glacier consists of debris-rich ice, which is typically weaker than clean ice. We present results from a study exploring (1) the importance of these basal layers to the large-scale flow dynamics in the terminus region of Taylor Glacier, a cold-based outlet glacier of the East Antarctic Ice Sheet and (2) the effects of deformation on the longitudinal variation in the thickness of these layers. We use a two-layer two-dimensional flow-band model to study the relative contributions from the debris-rich basal ice and the overlying clean ice to the deformation rates in the terminus region of Taylor Glacier. Field data, including ground penetrating radar, ablation rate, ice temperature, and surface velocity measurements, allow us to validate model results and determine both the best combination of clean and debris-rich basal ice softness parameters, and the spatial variability of the basal ice thickness. We present theoretical evidence supported by three years of observations that the debris-rich basal ice structure (1) controls the large-scale deformation field in the terminus region of Taylor Glacier and (2) is affected by the deformation field such that the layer varies in thickness along the flow-band. As many polar glaciers and ice sheets have prominent debris-rich basal ice layers that crop out at their margins, and interactions between these weak layers and the overall glacier dynamics are significant, it would be worthwhile to incorporate rheologically distinct layers in models of polar glacier dynamics.
C51B-0398
Modelling the Circulation and Melt/Freeze Beneath the Amery Ice Shelf, East Antarctica
Most of the mass lost from the Antarctic Ice Sheet is via basal melting and iceberg calving from its fringing ice shelves. The overall melting rate has important implications for the stability of ice shelves, water mass formation and the flow of glacial ice from the continent. The Amery Ice Shelf is the third largest ice shelf in Antarctica and receives glacial ice from a drainage basin that covers an area of ~16% of East Antarctica. Studies of the ice/ocean interaction for the Amery Ice Shelf region are presented that use numerical modeling of the circulation and basal melting and freezing. Recently, it has been found that large freshwater subglacial floods, at the base of the ice sheet, can drain directly into the sub-ice-shelf ocean cavity. These events occur on short time scales (months) and may also influence the sub-ice-shelf thermohaline circulation. The numerical model used is the Regional Ocean Modeling System (ROMS) which is a three dimensional high resolution hydrodynamic ocean model with ice/ocean thermodynamics. Simulations with ROMS are done to investigate the sensitivity of the circulation and melt/freeze beneath the Amery Ice Shelf to sub glacial flooding at the deep grounding line, and seasonal cycles, by the formation and melting of sea ice in Prydz Bay.
C51B-0399
The Physico-chemical Effects of Sea Ice on Ocean Surface pCO2 and Atmospheric Flux in Antarctica
Sea ice impedes gas exchange between atmosphere and ocean, however sea ice formation itself concentrates all solutes, including gases, in the brine pockets that form within the ice. As well as solute rejection, several physicochemical factors conspire to elevate the CO2 concentration in the liquid brine: (1) as salinity increases the gas solubility decreases, (2) bicarbonate dissociates in favor of CO2 concentration and (3) calcium carbonate can precipitate, decreasing alkalinity. Brine with a high concentration of CO2 may vent gas to the atmosphere or convey that excess gas (relative to atmospheric equilibrium) to the water column through gravity-driven brine drainage. In Antarctica, winter mixing transports Circumpolar Deep Water into the mixed layer, bringing heat and excess CO2 into contact with the base of the sea ice. We explore these effects on CO2 concentration using a one-dimensional model of sea ice formation, brine drainage and the equilibrium kinetics of the seawater carbonate system as an approximation for CO2 in brine. The eventual fate of excess CO2 in sea ice depends on the ice permeability, but may also be regulated by light and salt adapted algal communities that are most dense in the lower 20 cm of the ice.