C51A-0068 

Question of Ages of Cenozoic Volcanic Centers Inferred Beneath the West Antarctic Ice Sheet (WAIS) in the West Antarctic Rift System (WR) from Coincident Aeromagnetic and Radar Ice Sounding Surveys

* Behrendt, J C (behrendj@colorado.edu), INSTAAR, University of Colorado, Boulder, CO 80309-0450, United States * Behrendt, J C (behrendj@colorado.edu), U.S. Geological Survey, Federal Center, Denver, CO 80225, United States Finn, C A (cfinn@usgs.gov), U.S. Geological Survey, Federal Center, Denver, CO 80225, United States Blankenship, D D (blank@utig.ig.utexas.edu), Institute of Geophysics, University of Texas, Austin, TX 78759-8345, United States

The recently acquired radar ice sounding surveys (Holt, et al., 2006) extending the 1990s Central West Antarctica (CWA) aerogeophysical survey to the Amundsen and Bellingshausen sea coasts allows us to revise a thought experiment reported by Behrendt et al., 1991 from very limited bed elevation data. Were the ice of the WAIS flowing through the WR to be compressed to the density of crustal rock, almost all of the area beneath the WAIS would be at or above sea level, much >1 km elevation. There are only about 10-20% of the very deep areas (such as the Bentley subglacial trench and the Byrd Subglacial Basin) filled with 3-4-km thick ice that would be well below sea level. The age of the 5-7-km high rift shoulder bounding the asymmetric WR from northern Victoria Land through the Horlick Mountains (where it diverges from the Transantarctic Mountains) to the Ellsworth Mountains has been reported as old as Cretaceous. Volcanic exposures associated with the West Antarctic rift system in the present WAIS area extend at least to 34 Ma and the West Antarctic ice sheet has flowed through the rift possibly as far back in time as 25 Ma. Active volcanism has been reported for the WR at only a few widely scattered locations, so speculations about present volcanic activity beneath the WAIS are quite uncertain, and it is probably quite rare. The Central West Antarctic aeromagnetic and radar ice sounding survey carried out in the 1990s revealed about 1000 "volcanic centers" characterized by 100-1000 nT shallow source magnetic anomalies, at least 400 of which have associated bed topography. About 80% of these show relief <200 m and have been interpreted as smoothed off as they were erupted (injected) into the moving WAIS. Several kilometer-thick highly magnetic sources are required to fit these anomalies requiring high remanent magnetizations in the present field direction. We interpreted these sources as subvolcanic intrusions which must be younger than about 100 Ma because the Antarctic plate has been in its approximately present position since that time. Eighteen anomalies have >600 bed relief and were interpreted as erupted subaerially at a time when the WAIS was absent. At least one of these subaerially erupted peaks (Mt. Resnik, having 2 km bed relief) was erupted through a magnetic reversal. About 100 "volcanic" anomalies show reversed magnetic polarization indicating these must be at least as old as the Brunes-Matayama reversal at about 780 Ka. Essentially no volcanic rocks or detritus has been reported from the few drill holes that have penetrated the WAIS, although some have speculated, from the presence of smectite recovered from rock cores into the Ross Sea continental shelf, that this mineral has resulted from alteration of volcanic rock erupted beneath the WAIS. We consider the absence of volcanic samples from beneath the WAIS is not evidence of their absence. This seems particularly true considering the long time of the apparently coincident volcanism beneath the WAIS, possibly as great as 25 Ma, and the relatively brief age of the ice presently comprising the WAIS, about 200 Ka at most (e.g. perhaps the bulk of the volcanic centers are >10 Ma). Because none of the volcanic rocks or subvolcanic intrusions inferred to underlie the "volcanic centers" marked by high amplitude anomalies and low relief bed topography has been directly sampled, the question of their age cannot be answered.

C51A-0069 

Polar Seismic TETwalker: Integrating Engineering Teaching and Research

* Gifford, C M (cgifford@cresis.ku.edu), Center for Remote Sensing of Ice Sheets, 316 Nichols Hall Irving Hill Road, Lawrence, KS 66045, United States Ruiz, I), University of Puerto Rico, PO Box 9021, Mayaguez, 00681, Puerto Rico Carmichael, B L), Elizabeth City State University, 1704 Weeksville Rd, Elizabeth City, NC 27909, United States Wade, U B), Elizabeth City State University, 1704 Weeksville Rd, Elizabeth City, NC 27909, United States Agah, A (agah@ku.edu), Center for Remote Sensing of Ice Sheets, 316 Nichols Hall Irving Hill Road, Lawrence, KS 66045, United States

Based on the TETwalker robot platform at NASA/Goddard Space Flight Center, the Center for Remote Sensing of Ice Sheets (CReSIS) has begun work on designing and modeling the integration of seismic surveying equipment into the TETwalker robot architecture for use in polar environments. Employing multiple Seismic TETwalker robots will allow gathering of polar seismic data in previously inaccessible or unexplored terrains, as well as help significantly reduce human involvement in such harsh environments. NASA's TETwalker mobile robot uses a unique form of mobility to topple across the surface and over obstacles. This robot therefore does not suffer the fate of other wheeled and tracked robots if tipped over. It is composed of extending struts and nodes, forming a tetrahedral shape which can be strategically adjusted to change the robot's center of gravity for toppling. Of the many platforms the TETwalker architecture can form, the 4-TETwalker robot (consisting of four ground nodes, a center payload node, and interconnecting struts) has been the focus of current research. The center node has been chosen as the geophone deployment medium, designed in such a way to allow geophone insertion using any face of the robot's structure. As the robot comes to rest at the deployment location, one of its faces will rest on the surface. No matter which side it is resting on, a geophone spike will be perpendicular to its face and an extending strut will be vertical for pushing the geophone into the ground. Lengthening and shortening struts allow the deployment node to precisely place the geophone into the ground, as well as vertically orient the geophones for proper data acquisition on non-flat surfaces. Power source integration has been investigated, incorporating possible combinations of solar, wind, and vibration power devices onboard the robot models for long-term survival in a polar environment. Designs have also been modeled for an alternate center node sensor package (e.g., broadband seismometer) and other structures of the node-and-strut TETwalker robot architecture. It is planned to take the design models and construct a physical prototype for future testing in Greenland and Antarctica. This work involved three undergraduate students from underrepresented groups as part of the CReSIS Summer REU program, aimed at involving these groups in science and engineering research.

C51A-0070 

Ground surface temperature history of the past 50000 years inferred from deep borehole temperature profiles in Canada: implications for conditions at the base of the Laurentide ice sheet.

* Chouinard, C (cchouin@olympus.geotop.uqam.ca), GEOTOP-UQAM-McGill, University of Quebec at Montreal, POB 8888, sta. "downtown", Montreal, QC H3C3P8, Canada Mareschal, J (mareschal.jean-claude@uqam.ca), GEOTOP-UQAM-McGill, University of Quebec at Montreal, POB 8888, sta. "downtown", Montreal, QC H3C3P8, Canada

Several deep boreholes (>2000m) have been logged for temperature near Sudbury and Manitouwadge in Ontario, Canada. Thermal conductivity and radiogenic heat production measurements were made on core samples from all boreholes. From these temperature depth profiles, the heat production, and the thermal conductivity data, we calculated the perturbations to the steady-state temperature regime. These perturbations are interpreted as caused by past temperature changes at the ground surface. The depth of these holes is sufficient to infer variations in ground surface temperature over the past 20,000-50,000 years (i.e. including the last glacial maximum (LGM)). We have used two different inversion techniques, one based on the singular value decomposition algorithm and the other based on a Monte Carlo search of parameter space to obtain the ground surface temperature history (GSTH) of the past 50,000 years for each site. Both sites show that temperatures at the base of the glacier were only marginally colder (<5K) during the LGM that at present. We compare the ground surface temperature history at these two sites with GSTHs previously inferred from other deep boreholes located in central and eastern Canada. The new results confirm that over most of southern Canada the temperatures at the base of the Laurentide ice sheet during the LGM were everywhere near the melting point of ice, with only weak regional differences.

C51A-0071 

Moho topography of the West Antarctic Rift System from inversion of aerogravity data: ramifications for geothermal heat flux and ice streaming

* Studinger, M (mstuding@ldeo.columbia.edu), Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States

The West Antarctic rift system, a region of thinned continental crust, dominates the lithospheric structure of the Ross Embayment in West Antarctica. Parts of the rift are host to the West Antarctic Ice Sheet, a marine-based ice sheet prone to instability. It has long been hypothesized that the lithospheric structure beneath the West Antarctic Ice Sheet is a major influence on the formation, nature and dynamics of the ice sheet. The structure of the crust- mantle boundary is a fundamental geophysical parameter for understanding lithospheric processes and for geodynamic interpretation. I use aerogravity data to derive a map of the crust/mantle boundary beneath the West Antarctic Ice Sheet and to reveal the impact of relative changes in thickness of the crust and lithosphere on surface heat flow and ice streaming. Before inverting the observed Bouguer gravity field for Moho topography the gravity effect of the crust/mantle boundary has to be isolated from shallower sources. The observed radial power spectrum is matched with the model spectra of a number of equivalent source layers. First, the short wavelengths of the observed spectrum are matched with a shallow source equivalent layer model spectrum. The model spectrum is then removed from the observed power spectrum and the remaining residual spectrum is used to match the next deeper equivalent source layer. The Moho topography derived from filtered gravity coincides well with two independent seismically determined Moho estimates. In general, the geologic contributions to continental surface heat flow can be separated into mantle heat flow and radiogenic heat flow from heat producing elements within the crust. Assuming an exponentially decreasing distribution of radioactive elements, the continental surface heat flow can be expressed as a function of crustal and lithospheric thicknesses. Within the survey area there is a region of thinner crust that corresponds to elevated heat flow of several tens of mW/m2 compared to the background level. This region coincides with the confluence and onset of several ice stream tributaries, namely Kamb Ice Stream. The region with elevated heat flow appears to pin point the location of ice stream onsets and tributaries. http://pubs.usgs.gov/of/2007/1047/ea/of2007-1047ea031.pdf

C51A-0072 

The Present ice age: A 5 Million Year Record of Glacial-Interglacial Oscillations Based on Volcanic Strata in Skaftafell, SE-Iceland

* Helgason, J (jhelgason@internet.is), National Land Survey of Iceland, Stillholt 14, Akranes, 300, Iceland Duncan, R (rduncan@coas.oregonstate.edu), College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, United States

Stratigraphic, paleomagnetic and Ar-Ar age dating of 5 km thick overlapping volcanic sections in Skaftafell, SE- Iceland, has revealed close to 16 glacial-interglacial transitions during the last 5 m. y. Extensive mapping has provided a detailed window into the onset and evolution of the present ice age that intensified at roughly 3 Ma. Lithology of volcanic strata provided means to distinguish glacial and interglacial conditions. Volcanic strata that accumulated under ice free conditions is mapped as widespread lava formations whereas subglacially formed volcanics piled up on top of volcanic fissures. Through magnetostratigraphic work and Ar-Ar age dating correlations were made with the geomagnetic time scale to further constrain timing of events. The lowest part of the section consists of lavas that formed when erosion was minimal and the area had remained virtually ice free over an extended period. The earliest tillites in the area are dated at 4.0 Ma and a thick hyaloclastite horizon is dated at 3.2 Ma. Roughly at 2.6 Ma the record shows evidence of frequent glaciations. The predominantly normal Gauss magnetic polarity chron (2.33-3.35 Ma) plays a major role in mapping the ice age onset as units below consist of reversely magnetized lavas deposited during the late Gilbert magnetic polarity chron (3.35-3.82 Ma). Through field mapping erosional horizons have been traced that show deepening of valleys from roughly 100 m during early Gauss to at least 350 m at 1.7 Ma, and to over 2000 m at present. http://www.skaftafell.org

C51A-0073 

Megadunes and Geologic Maps of Snow/Firn of East Antarctica: Implications for Major Climatic Change, Accumulation Rates, Ice Flowage, and Bedrock Structures

* Wise, D U (dwise@geo.umass.edu), Dept. of Geosciences, Univ. of Massachusetts, Amherst, MA 01003, United States Cianfarra, P (cianfarr@uniroma3.it), Dept. of Geosciences, Universta Roma Tre, Rome, N/A, Italy Salvini, F (salvini@uniroma3.it), Dept. of Geosciences, Universta Roma Tre, Rome, N/A, Italy

Recent satellite mosaics of East Antarctica (EA) contradict current megadune origin models based on modern wind patterns, sastrugi, and slow, snow/firn accumulation rates. The images invariably show older megadune fields buried by younger snow/firn or cut by younger structures with sastrugi as surface decorations. An alternative model proposes these enigmatic, 2-4 km spaced, zebra-striped, snow ripple marks are relics of a past climate wherein summer glaze preserved winter antidunes that grew at rates 10 to 50 times present values. This requires an earlier Holocene, warm climatic excursion with extremely rapid snow accumulation from moisture- rich air off more open winter seas. Following this, present-day snow/firn has slowly accumulated over about 2/3 of EA. This, plus limited apparent erosion / ablation of megadune areas, implies relatively minor, net accumulation over the post-megadune surface, now collapsing from deeper, bedrock-controlled outflow. A new type of snow/firn geologic map supports these interpretations using the Megadune Formation (MF) as a basal unit. A new class of transitional high relief dunes helps define the MF. These "Duke of York" (DOY) dunes (after that nursery rhyme of uphill-downhill marches) marks the top of the MF, transitional from uphill-migrating, upper flow regime antidunes into overlying, downhill-migrating, modern- style, lower flow regime dune fields and smooth snow/firn deposits. The MF crops out flowing over bedrock highs and disappears into snow basins above bedrock lows. One map suggests flowage over a 900 km-long, bedrock, fault-line scarp passing nearly under the South Pole. An opposite facing partner forms a bedrock horst, blocking drainage of a stagnant basin now filled as the large, flat area E of the pole. A map of Argus Dome, highest divide of the plateau, suggests high relief DOY dunes formed a walled, shallow basin around a former divide. That basin, now completely filled with firn, forms the present crest, with trend changed by about 45 degrees. These models, geologic maps, and their interpretations may be "outrageous hypotheses" but at least they are testable by correlation with shallow ice cores and radar traverses.

C51A-0074 

Cenozoic East Antarctic Ice Sheet Evolution From Wilkes Land Continental Margin Sediments and IODP Drilling in 2009

* Escutia, C (cescutia@ugr.es), Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad d Granada, Fuentenueva, s/n, Granada, 18002, Spain Cooper, A (akcooper@pacbell.net), Department of Geological and Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, United States Eittreim, S (akcooper@pacbell.net), Unites States Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States Tanahashi, M (tanahashi-m@aist.go.jp), Geological Survey of Japan Geological Survey of Japan, Site C-7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan Ishihara, T (ishihara-t@aist.go.jp), Geological Survey of Japan Geological Survey of Japan, Site C-7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan De Santis, L (ldesantis@ogs.trieste.it), Geofisica Sperimentale, Borgo Grotta Gigante 42/c, Sgonico, 34010, Italy O'Brien, P (phil.obrien@ga.gov.au AF:

The long-term history of glaciation along the East Antarctic Wilkes Land margin is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice to the Wilkes Land continental shelf resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low- angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across regional unconformity WL-U8 is interpreted to represent the transition, during the late Miocene-Pliocene, from a glacial regime characterized by a warm dynamic ice sheet (i.e., ice sheets come and go) to a regime dominated by a cold-based and persistent ice sheet. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, inferred to be of middle Miocene age. During deposition of the two uppermost sequences (WL-S8 and WL-S9), there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin during the late Miocene. Sequence WL-S9 forms under polar regime during the Pliocene–Pleistocene, when most sediment delivered to the margin is trapped in the outer shelf and slope forming steep prograding wedges. During the warmer but still polar, Holocene, biogenic sediment accumulates quickly in deep inner-shelf basins during the high-stand intervals. These sediments contain an ultrahigh resolution (annual to millennial) record of climate variability. Drilling of the Wilkes Land margin by IODP scheduled for 2009 is designed to test the above inferred ages and history of glaciation. Drilling of the Wilkes Land margin will be a contribution by the ACE (Antarctic Climate Evolution) Program of SCAR (Scientific Committee on Antarctic Research) to the forthcoming International Polar Year.

C51A-0075 

Reconstruction of the Last Outburst Flood of Glacial Lake Agassiz-Ojibway in Hudson Bay and Hudson Strait

* Lajeunesse, P (patrick.lajeunesse@ggr.ulaval.ca), Departement de Geographie & Centre d'Etudes Nordiques, Universite Laval, Quebec, QC G1K 7P4, Canada St-Onge, G (guillaume_st-onge@uqar.qc.ca), Institut des sciences de la mer (ISMER), Universite du Quebec a Rimouski, Rimouski, QC G5L 3A1, Canada

Hudson Bay and Hudson Strait were the sites of a rapid deglaciation that culminated in the catastrophic drainage of proglacial Lake Agassiz-Ojibway into the North Atlantic at ~8.47 cal kyr BP. It has previously been suggested that this sudden outburst of freshwater may have weakened the thermohaline circulation and triggered the 8200 cal BP cold event recorded in Greenland ice cores. Evidence for the outburst flood included geomorphic features observed on the seafloor of southern Hudson Bay and the identification of a centimeter to decimeter- thick hematite-rich red layer present in Hudson Strait sediment cores. However, unequivocal evidence is still lacking in order to define the way the lake drained (i.e., either by a breach through the ice-dam, a supraglacial spillover or a subglacial flood), whether it drained by one or more pulses and the location of the northward flood routes toward Hudson Bay. In this paper, we present new seafloor images and sediment cores collected onboard the CCGS Amundsen in Hudson Bay and Hudson Strait in 2004 and 2005 that shed light on the dynamics of the final drainage of the ice-dammed lakes. We found that this sudden outburst flood combined with subsequent currents displaced icebergs back-and-forth in a former calving bay to produce preferentially oriented arc-shaped scours on the seafloor. In addition, fields of giant sandwaves were identified in southern Hudson Bay in areas unaffected by arcuate iceberg scours, suggesting that they were protected from iceberg scouring by overlying glacier ice during the lake drainage event. The subglacial origin of the widely distributed sandwaves and the occurrence of many submarine channels lead us to propose that the drainage of Lake Agassiz-Ojibway took place by a buoyant lifting of the rapidly thinning LIS along many subglacial routes that spread over the entire southern Hudson Bay region. We also reveal that the red bed contains two layers deposited by hyperpycnal flows (hyperpycnites). These two red layers can be correlated to a red bed previously identified as a regional isochron in Hudson Strait and associated with the final drainage of Lake Agassiz-Ojibway around 8500 cal BP. Regardless of the exact timing of the catastrophic drainage, these hyperpycnites suggest that they were deposited following two pulses, which is in agreement with one of the scenarios proposed in previous studies for lake final drainage.

C51A-0076 

A New 3035.22m Deep Ice Core At Dome Fuji, Antarctica And Reconstrction Of Global Climate And Environmental Change Over Past 720kyr

* Motoyama, H (motoyama@pmg.nipr.ac.jp), National Institute of Polar Research, Kaga 1-9-10, Itabashi-ku, Tokyo, 173-8515, Japan ice core project members, D (motoyama@pmg.nipr.ac.jp), Ice Core Consortium, National Institute of Polar Research, Kaga 1-9-10, Itabashi-ku, Tokyo, 173-8515, Japan

The Japanese Antarctic Research Expedition succeeded in a deep ice core down to 3,035.22m in depth at Dome Fuji station (77 19'S, 39 42'S, 3,810m a.s.l.) in January 26, 2007. A previous deep ice core of 2503m length was drilled at Dome Fuji in December 1996, going back to past 340kyr. Due to the lack of hole liquid, the drill got stuck in the borehole and we could not recover it. On the assumption that the snow depositional condition from surface to 2,503m deep continues to the bottom of the ice sheet, one million-year-old ice can be expected to lie near the bedrock. For this reason, the second deep ice core drilling at Dome Fuji was started with the aim of drilling down to the bedrock. The new drilling site was constructed and ice core drilling was started in December 2003 using a new excellent deep ice core drill developed in Japan. Average coring length was 3.7m/run and the ice core drilling speed was 178m/week until 3000m deep. Under 3,000m deep, it is difficult to drill the ice sheet because of warm ice near the bedrock. Around 3,030m, the small inclusion was collected in ice. It seems to be the rock or organic matter. The water in the basal ice (or the subglacial water) cozed out to the borehole and was frozen again. The freezing water was also corrected. The ice core was transported to Japan and many kinds of analysis were started at various ice core laboratories. The oxygen isotope profile of Dome Fuji ice core suggests that the deeper part of ice goes back to 720kyr, which corresponds to MIS (Marine Isotope Stage) 17. We think that the age of bottom of ice core, which is younger than we estimated, is based on discontinuous melting of ice sheet near bedrock. Analysis carried out so far confirms that the Dome Fuji ice core have the reliable environmental and climatic record at least back to 720kyr. Overwintering glaciological and meteorological observations were performed during the past four years at Dome Fuji station. Moreover, these scientific observations were carried out between traverse routes from Syowa station to Dome Fuji station These scientific research turns into the important information when we clarify the environment information from ice core. http://polaris.nipr.ac.jp/~domef/home/eng/index-e.html

C51A-0077 

Basal heterogeneity of Thwaites Glacier, Antarctica: Results from advanced ice penetrating radar data

* Young, D A (duncan@ig.utexas.edu), University of Texas at Austin, J.J. Pickle Research Campus University of Texas Institute for Geophysics Mail Code R2200 10100 Burnet Road, Austin, TX 78758, United States Blankenship, D D (blank@ig.utexas.edu), University of Texas at Austin, J.J. Pickle Research Campus University of Texas Institute for Geophysics Mail Code R2200 10100 Burnet Road, Austin, TX 78758, United States Holt, J W (jack@ig.utexas.edu), University of Texas at Austin, J.J. Pickle Research Campus University of Texas Institute for Geophysics Mail Code R2200 10100 Burnet Road, Austin, TX 78758, United States Kempf, S E (scottk@ig.utexas.edu), University of Texas at Austin, J.J. Pickle Research Campus University of Texas Institute for Geophysics Mail Code R2200 10100 Burnet Road, Austin, TX 78758, United States

Thwaites Glacier dominates the mass balance of the northern West Antarctic Ice Sheet. Subglacial topography and crustal variability strongly control the organization of flow in this catchment, and will likely dictate the response of this glacier to climate change. Basal roughness is symptomatic of the coupling between the ice sheet and the underlying crust, although causality can be unclear. Using airborne ice penetrating radar data collected In the 2004-05 austral summer, a comprehensive aerogeophysical survey was flown across the northern WAIS. The AGASEA/BBAS survey, (a collaborative effort between the University of Texas at Austin and the British Antarctic Survey) collected 50, 000 line kilometers of coherent radar data. We use this data to analyze the basal roughness of Thwaites Glacier using both along track slope statistics and lengthening of the bed echo in individual records. The first method applies to hectometer to decimeter length scales. We improve on initial efforts using low resolution unfocused radar data by using a new high resolution focused SAR algorithm, decreasing length scales of analysis from hundreds of meters to tens of meters. The second method, examining the length of the received bed echo, is instructive as to the distribution of slopes on the the scale of the radar carrier wave, integrated over the footprint of the radar. Overall, we find a self-affine landscape with a similar topological structure to that found in many erosional terrains. We find that large portions of Thwaites Glacier are underlain by very smooth material, which also corresponds to regions of low bed strength. Unlikely the Siple Coast ice streams to the south, however, we find little evidence of the downstream advection of this material. We suggest that the nature of ice sheet basal coupling in the Thwaites Catchment is different to that found on the Siple Coast. http://www.ig.utexas.edu/research/projects/agasea/

C51A-0078 

Crustal thinning and low Lithospheric Rigidity Revealed Beneath the Catchment of Pine Island Glacier: Implications for the West Antarctic Ice Sheet

Jordan, T A (tomj@bas.ac.uk), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, United Kingdom * Ferraccioli, F (ffe@bas.ac.uk), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, United Kingdom Holt, J W (jack@ig.utexas.edu), The University of Texas at Austin, Institute of Geophysics, J.J. Pickle Research Campus, Bldg. 196; 10100 Burnet Road, Austin, TX 78758-4, United States Diehl, T M (theresa@ig.utexas.edu), The University of Texas at Austin, Institute of Geophysics, J.J. Pickle Research Campus, Bldg. 196; 10100 Burnet Road, Austin, TX 78758-4, United States Corr, H F (hfjc@bas.ac.uk), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, United Kingdom Blankenship, D D (blank@ig.utexas.edu), The University of Texas at Austin, Institute of Geophysics, J.J. Pickle Research Campus, Bldg. 196; 10100 Burnet Road, Austin, TX 78758-4, United States Vaughan, D G), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, United Kingdom

Within the West Antarctic Ice Sheet (WAIS) glaciers flowing into the Amundsen Sea Embayment (ASE) are known to be presently thinning and retreating fast, leading to accelerated global sea level rise. Crustal structures may provide critical, but largely unconstrained, geological boundary conditions for enhanced ice flow over this highly dynamic "collapse prone" sector of the WAIS. During the 2004-05 field season an integrated aerogeophysical survey was conducted over the catchment of Pine Island Glacier, as part of a joint US-UK exploration effort over the ASE. Here we examine 30,000- line km of airborne gravity data, which provide new insights on crustal properties and tectonic structure of a segment of the underlying West Antarctic Rift System (WARS). Modern continental rifts are often associated with thinned crust, high heat flow, and low lithospheric rigidity. Knowledge of the lithospheric rigidity is important when estimating the amount of long-term sea-level rise associated with deglaciation processes. Comparison between the observed gravity data and isostatic compensation models suggests that the lithospheric rigidity is regionally low beneath the catchment of Pine Island Glacier. Our estimated value of equivalent elastic thickness (T_e) 0-10 km is significantly lower compared to previous estimates beneath the better studied Ross Sea segment of the WARS (~30km). Modelling of the Bouguer and terrain de-correlated gravity anomalies reveals several segments of highly thinned continental crust beneath Pine Island Glacier, the Byrd Subglacial Basin and the Bentley Subglacial Trench. Crustal thinning may increase regional heat-flow, and hence increase the availability of water at the base of the ice sheet, which has implications for the long-term stability of the WAIS. Additionally we image thick subglacial sedimentary basins, which may further enhance fast glacial flow in the ASE region.

C51A-0079 

Shackleton ice Shelf (Antarctica): Preliminary Results

* Urbini, S (stefano.urbini@ingv.it), Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Roma, 00143, Italy Bianchi, C), Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Roma, 00143, Italy Cafarella, L), Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Roma, 00143, Italy Forieri, A), Università di Milano, Dipartiimento Scienze della Terra, Sezione Geofisica, Via Cicognara 7, Milano, 20100, Italy Tabacco, I), Università di Milano, Dipartiimento Scienze della Terra, Sezione Geofisica, Via Cicognara 7, Milano, 20100, Italy Zirizzotti, A), Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Roma, 00143, Italy Baskaradas, J), Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Roma, 00143, Italy Young, N), Antarctic Cooperative Research Centre and Australian Antarctic Division, Private Bag 80, Hobart, 7001, Australia

In this poster we present the interpretation of radio echo sounding (RES) measurements collected during 2003 Antarctic expedition on the region between Mirny (66° 33' S, 93° 01' E) and Casey (66° 17' S, 110° 32' E) stations. The expedition has been made by Italy and Australia and the radar survey provided data on ice thickness and bed morphology of the outlet glaciers and their floating portions. Data were acquired by means of an airborne radio echo sounding system (named INGV-IT) for remote-sensing studies of the polar ice in Antarctica. The aim of our work was to define the morphological characteristics of the Shackleton ice shelf. In fact several analysis were made on the RES data set: maps of bedrock morphologies, ice thickness, ice surface and its gradient. The different information coming from the maps were combined to define the main characteristics of the area. In particular it was well defined the morphological structure of the main ice drainage of the region corresponding to Northcliffe, Denman and Adams outlet glaciers. The analysis of the ice thickness of the ice shelf also gave important information about actual anchorage (floating) line.

C51A-0080 

Contemporary ice-mass changes and glacial-isostatic adjustment in the polar regions from GRACE

Sasgen, I (sasgen@gfz-potsdam.de), GeoForschungsZentrum Potsdam, Telegrafenberg A17, Potsdam, 14467, Germany Martinec, Z (zdenek@gfz-potsdam.de), GeoForschungsZentrum Potsdam, Telegrafenberg A17, Potsdam, 14467, Germany * Fleming, K (kevin@gfz-potsdam.de), GeoForschungsZentrum Potsdam, Telegrafenberg A17, Potsdam, 14467, Germany

Mass changes in the polar regions are inferred by making use of about 4 years of Gravity Recovery and Climate Experiment (GRACE) gravity fields. The time series of each of the Stokes coefficients are decomposed into their linear, annual and semi-annual components, and we apply the Student's t-test to assess the statistical significance of the linear temporal trends in the Stokes potential coefficients. We solve the inverse-gravimetric problem by adjusting forward models describing the prominent geoid-height changes arising from ice-mass changes in Antarctica, Greenland, Alaska and Patagonia, and of the glacial-isostatic adjustment (GIA) due to the last glacial-interglacial transition. We show that, although all used data sets (GFZ, CSR; JPL and CNES) consistently reflect the prominent mass changes, differences in the mass-change estimates are considerably larger than the uncertainties estimated by the propagation of the GRACE errors. We then use the bootstrapping method based on the four releases and six time intervals, each with 3.5 years of data, to quantify the variability in the mean mass-change estimates. We find that for most regions the variability of the mass changes estimates lies below +/-50% demonstrating the reliability of results obtained using GRACE observations.

C51A-0081 

Integrating drilling results adjacent to the Transantarctic Mountains front with high-resolution aeromagnetic data: an example from Cape Roberts

Armadillo, E (egidio@dipteris.unige.it), Dipartimento per lo Studio del Territorio e delle sue Risorse, Universita' di Genova, V.le Benedetto XV, 5, Genova, 16132, Italy * Ferraccioli, F (ffe@bas.ac.uk), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 OET, United Kingdom Gambetta, M (marco.gambetta@gmail.com), Istituto Nazionale di Geofisica e Vulcanologia, Sez. di Geofisica Marina, Via Pezzino Basso, Fezzano, 19020, Italy Talarico, F (talarico@unisi.it), Dipartimento di Scienze della Terra, Università di Siena, Via del Laterino, 8, Siena, 53100, Italy Zunino, A (andrea.zunino@dipteris.unige.it), Dipartimento per lo Studio del Territorio e delle sue Risorse, Universita' di Genova, V.le Benedetto XV, 5, Genova, 16132, Italy Bozzo, E (bozzo@dipteris.unige.it), Dipartimento per lo Studio del Territorio e delle sue Risorse, Universita' di Genova, V.le Benedetto XV, 5, Genova, 16132, Italy

We present new magnetic models and enhanced maps derived from a high resolution aeromagnetic (HRAM) survey that was performed over a rift basin at the edge of the East Antarctic Ice Sheet (EAIS) and adjacent to the Transantarctic Mountains front of southern Victoria Land as part of the site survey for the Cape Roberts Drilling Project (CRP). The aim of the CRP was to investigate the early history of the EAIS and the tectonics of the West Antarctic Rift System and Transantarctic Mountains by drilling a continuous core through Cenozoic strata. In summary, the completed CRP cores provided 1500 metre of shallow marine Cenozoic strata that recorded climate and mountain/basin history for the period from 17 to 34 Ma ago. Magnetic susceptibility, P-wave velocity and density/porosity log data can now be analysed together with reprocessed and enhanced HRAM data. The magnetic signatures of Cenozoic sedimentary strata have been modeled and several magnetic trends and anomalies are re-discussed in conjunction with drilling results, bathymetric data and published structural interpretations derived from multichannel seismic reflection data. The results of our study suggest that correlation between stratigraphic sequences and magnetic anomaly patterns could potentially be useful also for interpretation of more recent aeromagnetic surveys in southern McMurdo Sound performed in the frame of the Antarctic Drilling Program (ANDRILL), which is targeting Middle Miocene to Quaternary strata, and hence providing new insights into Antarctic climate evolution and ice sheet history.

C51A-0082 

Using new airborne gravity data to revisit uplift of the Transantarctic Mountains and to investigate the adjacent Wilkes Subglacial Basin beneath the East Antarctic Ice Sheet

Moss, C (ffe@bas.ac.uk), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 OET, United Kingdom Jordan, T (tomj@bas.ac.uk), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 OET, United Kingdom * Ferraccioli, F (ffe@bas.ac.uk), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 OET, United Kingdom Watts, T (Tony.Watts@earth.ox.ac.uk), Department of Earth Sciences, University of Oxford, Park Road, Oxford, OX1 3PR, United Kingdom Armadillo, E (egidio@dipteris.unige.it), Dipartimento per lo Studio del Territorio e delle Sue Risorse, Viale Benedetto XV, 5, Genova, 16132, Italy Bozzo, E (bozzo@dipteris.unige.it), Dipartimento per lo Studio del Territorio e delle Sue Risorse, Viale Benedetto XV, 5, Genova, 16132, Italy Corr, H (hfjc@bas.ac.uk), British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 OET, United Kingdom Caneva, G (caneva@dipteris.unige.it), Dipartimento per lo Studio del Territorio e delle Sue Risorse, Viale Benedetto XV, 5, Genova, 16132, Italy

The Transantarctic Mountains (TAM) are the highest and longest rift-related Cenozoic mountain range of our planet. Models for the evolution of the East Antarctic Ice Sheet (EAIS) state that, during post-late Eocene cooling local glaciers and ice caps nucleated on the highlands of the East Antarctic craton, including the proto-TAM. These ice caps expanded and then merged during major climatic Neogene cooling events. Differential Neogene uplift of TAM blocks has also been advocated as a possible way of reconciling opposing views of stabilists and dynamists for the EAIS during warm periods. The TAM are therefore a natural laboratory to study interplays between rifting, mountain uplift, ice sheets, landscape evolution and climate change. The rift basins adjacent to TAM front are relatively well understood, but less is known about the deep crustal structure and uplift mechanisms of the TAM rift flank, and even less is known about the enigmatic Wilkes Subglacial Basin (WSB) in the hinterland of the TAM. During the 2005/06 Antarctic field season a major collaborative UK-Italian aerogeophysical survey was flown linking the Ross Sea Rift basins offshore with the TAM and the WSB. 60,000 line km of new airborne radar, aerogravity and aeromagnetic data were collected. New Free-Air gravity, Bouguer gravity and isostatic residual maps will be presented. The Free-Air gravity map reveals several linear lows and highs within this part of the WSB adjacent to northern Victoria Land, consistent with airborne radar evidence for previously unrecognised deep trenches and highland blocks in the hinterland of the TAM. To model TAM uplift we will initially utilise gravity and topography data over a relatively low elevation block further south along the TAM, the Prince Albert Mountains. Here independent wide-angle seismic data are available onshore, and relative proximity to the Cape Roberts drilling site offshore provides some information on timing of rifting, magnitude of TAM uplift, and climate evolution.

C51A-0083 

Calcareous Nannofossil Evidence for Marine Isotopic Stage 31 (1 Ma) in ANDRILL MIS Core (Western Ross Sea, Antarctica)

* Villa, G (giuliana.villa@unipr.it), Dipartimento Scienze della Terra, Universit¡§¡è di Parma, Viale Usberti, 157, Parma, 43100, Italy Persico, D (davide.persico@unipr.it), Dipartimento Scienze della Terra, Universit¡§¡è di Parma, Viale Usberti, 157, Parma, 43100, Italy Wise, S W (wise@gly.fsu.edu), Department of Geological Sciences, Florida State University, Department of Geological Sciences, 4100; Florida State University, Tallahassee, FL 32306-4100, United States

Quaternary sediments south of the Antarctic Divergence (AD) at about 63-65° S were long considered to be barren of calcareous nannofossils until their discovery there within the past decade. The study of such fossils in Quaternary sediments of periantarctic basins and their fluctuations and relationships with glacial-interglacial cycles, therefore, is of fundamental importance in reconstructing palaeoclimatic conditions for this time interval. To further verify the presence of calcareous nannofossils within Antarctic Quaternary and older Neogene sediments, we have undertaken a micropalaeontological study of the recently cored ANDRILL McMurdo Ice Shelf (MIS) sediments. During drilling, calcareous nannofossils in the form of calcareous dinoflagellates were detected in 31 samples of some 530 examined. These occurred along with calcareous spicules (calciospongia) and occasional fragments of ostracods and foraminifers within several biogenic carbonate intervals at about 27, 31, and 91-98 mbsf, but only in scattered samples between 98 and 426 and 554 to 556 mbsf. Subsequent shore-based analysis of 15 toothpick samples from 93-95 mbsf revealed for the first time the presence of Pleistocene coccolithophorids at these high southern latitudes (77° S), including Coccolithus pelagicus, Coccolithus crassipons, Pseudoemiliania lacunosa, and Reticulofenestra sp. in sediments of Marine Isotopic Stage 31. As the lower temperature limit for living calcareous nannoplankton is 2.5°C, the presence of these nannofossils, though rare, is an indication of ice-free sea surface temperatures warmer than today in the Ross Sea at 1 Ma.

C51A-0084 

New Antarctic Bedrock and Moho Topography Models from GRACE: Implications for the Cryosphere

* Smith, A E (adrienne@ldeo.columbia.edu), Department of Earth and Environmental Sciences Columbia University, 106 Geoscience Lamont-Doherty Earth Observaory 61 Route 9W, Palisades, NY 10964, United States Bell, R E (robinb@ldeo.columbia.edu), Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States Studinger, M (mstuding@ldeo.columbia.edu), Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States Velicogna, I (isabella@colorado.edu), Jet Propulsion Laboratory (NASA), 4800 Oak Grove Drive, Pasadena, CA 91109, United States Velicogna, I (isabella@colorado.edu), CIRES and Dept. of Physics, University of Colorado, Campus Box 390, Boulder, CO 80309, United States

Topography and crustal structure of glaciated continents are critical to the stability and dynamics of the overlying ice sheet but remain difficult to recover on a continental scale. Specifically, the slope and undulation of the underlying bedrock acts as a fundamental control on the flow and stability of the ice sheet. Until recently, large regions of East Antarctica lacked a reasonable topographic model because only scattered observations of ice thickness had been available to constrain the bedrock elevation. The satellite gravity from NASA's Gravity Recovery and Climate Experiment (GRACE) has created a new opportunity to model the sub-ice topography by providing the first geophysical dataset that covers the whole of Antarctica. We have used gravity inversion, a classic geophysical technique, to predict the topographic variation of the bedrock surface. GRACE's free–air anomalies are also sensitive to long wavelength structures including the undulation of the ice sheet surface and the topography on the Moho. We are able to predict Moho topography by removing the gravity effect of BEDMAP as an a priori model, and gain insights into the crustal structure and lithospheric strength. This approach also enables us to map changes in the lithosphere's elastic thickness using a flexural model. Here we present both results from GRACE: new bedrock and mantle topography models for the Antarctic continent. These new continental scale models will enable us to produce new estimates of ice volume and investigate the geodynamic control on ice sheet processes.

C51A-0085 

Evidence for Northern Hemisphere Glaciation Back to 44 Ma From Ice-Rafted Debris in the Greenland Sea

Shorttle, O (os258@cam.ac.uk), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom * Tripati, A (atri02@esc.cam.ac.uk), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom Eagle, R A (rae28@cam.ac.uk), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom Dawber, C (cfd25@esc.cam.ac.uk), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom Morton, A (a.c.morton@heavyminerals.fsnet.co.uk), CASP, 181A Huntingdon Road, Cambridge, Cam CB3 ODH, United Kingdom Dowdeswell, J (jd16@cam.ac.uk), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom Atkinson, K (ka298@cam.ac.uk), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom Bahe, Y (yannick.bahe@gmx.de), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom Shaw, R (kincsem@ntlworld.com), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom Thanabalasundaram, L (Lavaniya_89@yahoo.de), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom Khadun, E (nutemma@hotmail.com), University of Cambridge, Downing Street, Cambridge, Cam CB2 3EQ, United Kingdom

The widely accepted age estimate for the onset of glaciation in the Northern Hemisphere ranges between 2 and 15 million years ago (Ma). However, recent studies indicate the date for N. Hemisphere glacial onset may be significantly older [1,2,3]. We report the presence of ice-rafted debris (IRD) in ~44 to 39 Ma sediments from the Greenland Sea, evidence for glaciation in the North Atlantic during the Middle Eocene to Late Eocene. We also have developed a high-resolution record of ice-rafting for the late Eocene through early Oligocene (39-30 Ma). Detailed sedimentological evidence indicates that glaciers extended to sea level in the region during part of the study interval, allowing icebergs to be produced. Peaks in IRD accumulation are observed at ~40-42 Ma, IRD may have been sourced from tidewater glaciers, small ice caps, and/or a continental ice sheet. Foraminiferal records from the deep Pacific show that several shifts in seawater δ¹⁸O of greater than 0.6‰ occurred during these intervals, consistent with the build-up of ice in both hemispheres [1,4]. [1] Tripati, A., Backman, J., Elderfield, H. and Ferretti, P. Eocene bipolar glaciation associated with global carbon cycle changes. Nature 436, 341–346 (2005). [2] Moran, K., Backman, J., Brinkhuis, H., Clemens, S.C., Cronin, T., Dickens, G.R., Eynaud, F., Gattacceca, J., Jakobsson, M., Jordan, R.W., Makinski, M., King, J., Koc, N., Krylov, A., Martinez, N., Matthiessen, J., McInroy, D., Moore, T.C., Onodera, J., O'Regan, M., Plike, H., Rea, B., Rio, D., Sakamoto, T., Smith, D.C., Stein, R., St. John, K., Suto, I., Suzuki, N., Takahashi, K., Watanabe, M., Yamamoto, M., Farrell, J., Frank, M., Kubik, P., Jokat, W., and Kristoffersen, Y., 2006, The Cenozoic palaeoenvironment of the Arctic Ocean, Nature, 441, 601–605. [3] Eldrett, J., Harding, I., Wilson, P., Butler, E. and Roberts, A., 2007, Continental ice in Greenland during the Eocene and Oligocene, Nature, 446, 176-179. [4] Dawber, C. and Tripati, A., 2007, Evidence for Early Cenozoic glaciation from a record of seawater δ¹⁸O at ODP Site 1209: Exploring the paradigm of an ‘ice-free' Middle Eocene, Fall AGU abstract.

C51A-0086 

A Local Provenance for Glacial Tills at the Margin of the East Antarctic Ice Sheet, Transantarctic Mountains, Antarctica

* Farmer, G L (farmer@colorado.edu), Dept. of Geological Sciences and CIRES, Dept. of Geological Sciences and CIRES, University of Colorado, Campus Box 399, Boulder, CO 80309, United States Licht, K (klicht@iupui.edu), Department of Earth Sciences, Indiana University-Purdue University Indianapolis, 723 W. Michigan St., Indianapolis, IN 46202, United States Palmer, E (emersonpalmer@mac.com), Department of Earth Sciences, Indiana University-Purdue University Indianapolis, 723 W. Michigan St., Indianapolis, IN 46202, United States

The Nd and Sr isotopic compositions of fine-grained (<63 micron) fractions from tills transported by the Byrd Glacier and its tributaries in the Transantarctic Mountains were determined to asses the nature of crustal rocks underlying the East Antarctic ice sheet (EAIS) and to investigate changes in the sources of glacial detritus as a function of position along an active glacial flow path. Our data reveal that the tills have isotopic compositions that typically reflect variations in the composition of local bedrocks. The furthest upstream samples, from the Lonewolf Nunataks, have the lowest measured ε_Nd values (~-20) and highest ⁸⁷Sr/⁸⁶Sr (0.725 to 0.736) of any of the sampled tills. Although the local bedrock is mapped as Late Paleozoic to Mesozoic Beacon Group sedimentary rocks, the measured values likely reflect erosion of Precambrian crust similar to that exposed in nearby areas (Miller Range). Northern tributaries to the Byrd Glacier (along Brittania Ridge and the Bates Nunatak) have significantly higher measured ε_Nd (-8 to -15) and lower ⁸⁷Sr/⁸⁶Sr (0.712 to 0.723) that correspond to erosion of the Beacon Group rocks exposed in these regions. These results indicate that the tills deposited along nunataks do not necessarily represent far-traveled glacial detritus transported from the interior of the EAIS but are often products of more local bedrock erosion. Fine grained material in lateral moraines along the Byrd Glacier have ε_Nd values from -14 to -17 and ⁸⁷Sr/⁸⁶Sr from 0.729 to 0.739 that are consistent with their derivation from material eroded along the main glacier, combined with variable amounts of detritus provided by the tributary glaciers. Overall, our data support the assertion that outlet glaciers draining major continental ice sheets deliver detritus to continental margins that represents a complex mixture of the rocks glacially eroded within its entire catchment, and may be dominated by material eroded at or near the continental margin, itself.

C51A-0087 

Solving for ice-surface elevation history in the southern Ross Sea using inverse methods and surface-exposure ages

* Todd, C E (toddce@plu.edu), Department of Geosciences Pacific Lutheran University, 158 Rieke Science Center, Tacoma, WA 98447, United States Waddington, E D (edw@ess.washington.edu), Department of Earth and Space Sciences University of Washington, Box 351310, Seattle, WA 98195, United States Koutnik, M R (mkoutnik@u.washington.edu), Department of Earth and Space Sciences University of Washington, Box 351310, Seattle, WA 98195, United States Conway, H (conway@ess.washington.edu), Department of Earth and Space Sciences University of Washington, Box 351310, Seattle, WA 98195, United States Stone, J (stone@ess.washington.edu), Department of Earth and Space Sciences University of Washington, Box 351310, Seattle, WA 98195, United States

Surface-exposure ages calculated from Be-10 concentrations in glacial erratics provide a history of ice-surface elevations along Reedy Glacier from approximately 17 kyr before present (B.P.) through the Holocene. These surface-exposure ages show that the most recent maximum ice thickness at Reedy Glacier occurred at different times at different locations along the glacier. For example, deposits at the Quartz Hills, located 70 m from the head of the glacier, have ages of 17.3 +/- 1.1 kyr – 14.3 +/- 0.9 kyr B.P., and indicate that the glacier surface was 250 m higher than present during this time. In contrast, deposits at the margin of McCarthy Glacier, a tributary of Reedy Glacier, show that ice thicknesses exceeded modern ice thicknesses by ~ 200 m from 9.4 +/- 0.7 kyr to 8.0 +/- 0.5 kyr B.P. Surface-exposure ages from nunataks near the mouth of Reedy Glacier provide a history of ice- surface elevations that begins approximately 7 kyr B.P., but these deposits do not capture the most recent maximum ice-surface elevation as these peaks were overrun by ice during the most recent maximum ice thickness in this area. The history of ice-surface elevation at the mouth of Reedy Glacier provides a useful constraint on past ice- surface elevations in the southern Ross Sea Sector of the West Antarctic Ice Sheet (WAIS). Improved constraints on ice-surface elevations in this region are needed to improve ice-sheet reconstructions of the WAIS during the last glacial maximum (LGM). Thus, we develop an inverse procedure to solve for the history of ice-surface elevations at the mouth of Reedy Glacier from surface-exposure-age data. We use an ice-flow model for our forward algorithm. This inverse procedure finds the history of ice-surface elevations that (1) yields glacier surfaces that best match our surface-exposure-age data at a defined tolerance, and (2) is within a range of ice- surface elevations that is physically reasonable. Results suggest that LGM ice thickness at the mouth of Reedy Glacier, in the southern Ross Sea Embayment, was approximately 1000 to 1200 m above modern sea level.

C51A-0088 

Ice Stream Shear Margin Migration

* Schoof, C G (cschoof@eos.ubc.ca), Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada Rempel, A W (rempel@uoregon.edu), Department of Geology, University of Oregon, 1272 University of Oregon, Eugene, OR 97403, United States

Ice streams are the major conduits through which ice drains from the interior areas of an ice sheet to its margins. Their discharge can be highly sensitive to their geometry: simple models of ice streams suggest that their discharge scales as their width to the fifth power. The migration of ice stream margins is therefore an important component of ice stream dynamics, but the processes involved are poorly understood. The flow of ice streams is caused by the presence of lubricating subglacial meltwater, and the margins of an ice stream correspond to a transition from high basal water pressure under the ice stream to low pressure or a frozen bed outside. In order for an ice stream to widen, meltwater must be supplied to its margins. Fast sliding in the centre of the ice stream leads to the frictional dissipation of heat, which produces meltwater, but this meltwater is not immediately available to weaken the margins. Here, we investigate how subglacial drainage from the centre of the ice stream and the disspiation of heat due to shearing of ice in the margins leads to widening of an ice stream (or to shrinking, if the sum of meltwater sources is too small). We find that the transition from temperate to frozen bed plays a crucial role in this process, and that considerations of frost heave processes are necessary for a complete description of shear margin migration. Surprisingly, the strength of hydrological and thermal controls in the migration process seems to overwhelm the effect of different sliding parameterizations, and subglacial drainage parameters may play a bigger role in controlling ice stream dynamics than the details of the basal friction law.

C51A-0089 

High Resolution Geothermal Heat Flux Data--Implications for Ice Sheet Dynamics and Model Uncertainty

* Johnson, J V (johnson@cs.umt.edu), Department of Computer Science The University of Montana, 417 Social Science Building, Missoula, MT 59812, United States Naslund, J A (Jens-Ove.Naslund@skb.se), Swedish Nuclear Fuel and Waste Management Co (SKB), Box 5864, Stockholm, 102 40, Sweden Pattyn, F (fpattyn@ulb.ac.be), Laboratoire de Glaciologie Département des Sciences de la Terre et de l'Environnement (DSTE), CP 160/03 Université Libre de Bruxelles Av. F. Roosevelt 50, Bruxelles, B-1050, Belgium Jansson, P (Peter.Jansson@natgeo.su.se), Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, SE-106 91, Sweden

Airborne surveys of radiogenic activity in Sweden and Finland have been utilized to create a 5 km resolution map of the geothermal heat flux. Statistical analysis of these data reveal contiguous areas where heat flux is more or less than two standard deviations from the mean. These anomalous areas have a length scale of approximately 22 km. In order to investigate the significance of these findings, thermo-mechanically coupled ice sheet models having both nearly complete and simplified stress treatments are used. Experiments which are formulated for finer scales (~180 km, 10,000 years) utilize the higher order stress treatments, and experiments that treat larger scales (~ 2000 km, 200,000 years) use the reduced stress models. Experiments involve both the measured data, as well as proxies for it, as is appropriate. Experiments also treat a range of sliding relations, and basal water treatments. A novel method of producing synthetic data from the original dataset is formulated, and used to assign confidence intervals to uncertainty estimates. In most all cases, it is found that the ice sheet model is effectively averaging the high resolution geothermal heat flux data, and the results are only marginally different than what would be found using a uniform heat flux boundary condition near the mean of the data set. In cases where it is significantly different, it is the interaction with basal water that provides the mechanism to amplify the impact of a heat flux anomaly.

C51A-0090 

Is a 3-Dimensional Stress Balance Ice-Stream Model Really Better Than a 2-Dimensional "Reduced Order" Ice-Stream Model?

* Sergienko, O (olga@neptune.gsfc.nasa.gov), ORAU/NASA Goddard Space Flight Center, Hydrospheric and Biospheric Sciences Laboratory Code 614, bldg. 33, Rm. A109 NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States MacAyeal, D R (drm7@midway.uchicago.edu), Department of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave, Chicago., IL 60637, United States

With growing observational awareness of numerous ice-stream processes occurring on short time and spatial scales, e.g., sub-ice-stream lake volume changes and grounding-line sediment wedge build-up, the question of how well models based on "reduced-order" dynamics can simulate ice-stream behavior becomes paramount. Reduced-order models of ice-streams are typically 2-dimensional, and capture only the largest-magnitude terms in the stress tensor (with other terms being constrained by various assumptions). In predicting the overall magnitude and large-scale pattern of ice-stream flow, the reduced-order models appear to be adequate. Efforts underway in the Glaciological Community to create 3-dimensional models of the "full" ice-stream stress balance, which relax the assumptions associated with reduced-order models, suggest that a cost/benefit analysis should be done to determine how likely these efforts will be fruitful. To assess the overall benefits of full 3-dimensional models in relation to the simpler 2-dimensional counterparts, we present model solutions of the full Stokes equations for ice-stream flow over a variety of basal perturbations (e.g., a sticky spot, a subglacial lake, a grounding line). We also present the solutions derived from reduced 2-dimensional models, and compare the two solutions to estimate effects of simplifications and neglected terms, as well as to advise on what circumstances 3-dimensional models are preferable to 2-dimensional models.

C51A-0091 

Spontaneous generation of pure ice-streams via flow instability: Role of longitudinal shear stresses and subglacial till

* Sayag, R (sayag@fas.harvard.edu), Harvard University, Department of Earth and Planetary Sciences and school of Engineering, Cambridge, MA 02138, United States Tziperman, E (eli@eps.harvard.edu), Harvard University, Department of Earth and Planetary Sciences and school of Engineering, Cambridge, MA 02138, United States

A significant portions of the ice discharge in ice sheets is drained through ice streams, with subglacial sediment (till) acting as a lubricant. The known importance of horizontal friction in shear margins to ice stream dynamics suggests a critical role of longitudinal stresses. The effects of subglacial till and longitudinal stresses on the stability of an ice sheet flow are studied by linear stability analysis of an idealized ice-till model in two horizontal dimensions. A power law-viscous constitutive relation is used, explicitly including longitudinal shear stresses. The till, which has compressible-viscous rheology, affects the ice flow through bottom friction. We examine the possibility that pure ice streams develop via a spontaneous instability of ice flow. We demonstrate that this model can be made intrinsically unstable for a seemingly relevant range of parameters, and that the wavelengths and growth rates that correspond to the most unstable modes are in rough agreement with observed pure ice streams. Instabilities occur due to basal friction and melt water production at the ice-till interface. The most unstable wavelength arise due to selective dissipation of both short and long perturbation scales. Longitudinal stresses stabilize modes with short transverse wavelengths while a physical process that can be described as a gravitationally driven flow divergence feedback stabilizes long transverse wavelengths. These results do not change qualitatively for a Newtonian ice flow law, indicating no significant role to shear thinning, although this may very well be due to the restrictive assumptions of the model and analysis.

C51A-0092 

Tropical Mountain Glaciers on Mars: Altitude-Dependence of Ice Accumulation, Accumulation Conditions, Formation Times, Glacier Dynamics, and Implications for Planetary Spin- Axis/Orbital History

* Fastook, J L (fastook@maine.edu), University of Maine, Climate Change Institute, Orono, ME 04469, United States Head, J W (James_Head@brown.edu), Brown University, Geological Sciences, Providence, RI 02912, United States Marchant, D R (marchant@bu.edu), Boston University, Earth Sciences, Boston, MA 02215, United States

Lobate deposits up to ~166,000 km2 in area are found on the northwest flanks of the huge Tharsis Montes volcanos in the tropics of Mars. Recent spacecraft data have confirmed earlier hypotheses that these lobate deposits are glacial in origin. Increased knowledge of polar-latitude terrestrial glacial analogs in the Antarctic Dry Valleys has been used to show that the lobate deposits are the remnants of cold-based glaciers that formed in the extremely cold, hyper-arid climate of Mars. Mars atmospheric general circulation models (GCM) show that these glaciers form during periods of high obliquity when upwelling and adiabatic cooling of moist polar air favor deposition of snow on the northwest flanks of the Tharsis Montes. We present a simulation of the Tharsis Montes ice sheets produced by a static accumulation pattern based on the GCM results and compare this with the nature and extent of the geologic deposits. We use the fundamental differences between the atmospheric snow accumulation environments (mass balance) on Earth and Mars, geological observations and ice sheet models to show that two equilibrium lines should characterize ice sheet mass balance on Mars, and that glacial accumulation should be favored on the flanks of large volcanos, not on their summits as seen on Earth. Predicted accumulation rates from such a parameterization, together with sample spin-axis obliquity histories, are used to show that mean obliquity in excess of 45 degrees and multiple 120,000 year obliquity cycles are necessary to produce the observed deposits. Our results indicate that the formation of these deposits required multiple successive stages of advance and retreat before their full extent could be reached, and thus imply that spin-axis obliquity remained at these high values for millions of years during the Late Amazonian period of Mars history. Spin-axis obliquity is one of the main factors in the distribution and intensity of solar insolation, and thus in determining the climate history of Mars. Unfortunately, reconstruction of past climate history is inhibited by the fact that calculations of spin-axis geometry histories prior to about 20 Ma ago are not possible due to the chaotic nature of the solutions. We show, however, that the geological record, combined with glacial modeling, can be used to provide insight into the nature of the spin-axis/orbital history of Mars in the Late Amazonian, and to begin to establish data points for the geologically-based reconstruction of the climate and orbital history of Mars.

C51A-0093 

Numerical improvement on simulation over the margin area of the Greenland ice sheet

* SAITO, F (saitofuyuki@jamstec.go.jp), Japan Agency for Marine-Earth Science and Technology, Frontier Research Center for Global Change, 3173-25 Showamachi, Kanazawa-ku, Yokohama, 236-0001, Japan Abe-Ouchi, A (abeouchi@ccsr.u-tokyo.ac.jp), Japan Agency for Marine-Earth Science and Technology, Frontier Research Center for Global Change, 3173-25 Showamachi, Kanazawa-ku, Yokohama, 236-0001, Japan Abe-Ouchi, A (abeouchi@ccsr.u-tokyo.ac.jp), Center for Climate System Research, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8568, Japan Blatter, H (heinz.blatter@env.ethz.ch), Institute for Atmospheric and Climate Science, ETH Zurich, CHN L 11 Universitaetstrasse 16, Zurich, CH-8092, Switzerland

Generally, simulated topography by an ice-sheet model has large error near the margin compared to the real (or analytical) ones. There are several reasons for the error near the margin, such as uncertain processes and parameters, ignored processes, and also the error due to numerical properties. Saito and Abe-Ouchi (2005) demonstrate that difference near the margin in the simulation may significantly affect the sensitivity experiment such as global warming experiments. Thus improvement on the simulation especially near the margin is a highly important subject. Saito et al. (2007) present an improved numerical scheme to reduce the error at the ice-sheet margin. In the present paper we applied the scheme to Greenland ice sheet to investigate its influence on simulated thickness and temperature. In addition, ignoring higher-order stress terms may have some effect on the error. We have been developing a model including higher-order stress components and had applied it to ideal configuration (e.g., Saito et al., 2006). In the present work, preliminary result will be also shown by the model applied to the Greenland ice sheet.

C51A-0094 

The North Polar Ice Cap of Mars at Varying Obliquities, Simulated With a Coupled Atmosphere/Ice-Sheet Model

Stenzel, O J (stenzel@mps.mpg.de), Max-Planck-Institute for Solar System Research, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany * Greve, R (greve@lowtem.hokudai.ac.jp), Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo, 060-0819, Japan Grieger, B (Bjoern.Grieger@sciops.esa.int), European Space Astronomy Centre (ESAC), P.O. Box 50727, Madrid, 28080, Spain Fraedrich, K (klaus.fraedrich@zmaw.de), Meteorological Institute, University of Hamburg, Bundesstr. 55, Hamburg, 20146, Germany Kirk, E (e.kirk@gmx.de), Meteorological Institute, University of Hamburg, Bundesstr. 55, Hamburg, 20146, Germany Lunkeit, F (Frank.Lunkeit@zmaw.de), Meteorological Institute, University of Hamburg, Bundesstr. 55, Hamburg, 20146, Germany Keller, H U (keller@mps.mpg.de), Max-Planck-Institute for Solar System Research, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany

Two numerical models, a general circulation model of the atmosphere (Planet Simulator Mars) and a dynamic/thermodynamic ice sheet model (SICOPOLIS) for the perennial north polar H₂O ice cap and the underlying layered deposits (considered as a morphological unit), have been coupled in order to simulate the climate system of Mars. The experimental set-up includes three runs of the atmospheric part for two Martian years each for obliquities of 15°, 25.2° and 35°, respectively, which covers the variability of the last 5~Ma. The climatic fields of the second model year are then used to force the ice component over a period of 125~Ma, with the present-day ice cap used as initial condition. At 15° obliquity, glaciation southwards to 70°N takes place within the first 10~Ma. Given an unlimited amount of available water, the ice cap grows even further through extensive glacial flow until equilibrium is reached. In case of the high obliquity of 35°, the difference between precipitation and evaporation is negative southward of about 85°N, and the ice cap shrinks until glacial flow compensates for evaporation in the vicinity of the ice sheet. The ice cap subsequently grows again and reaches 82°N at the end of the simulation after 125~Ma. At an obliquity of 25.2° the ice cap reaches equilibrium with its margin at 78°N. In all cases, it takes tens of millions of years to reach the equilibrium state, and we conclude that the present-day north polar ice cap is most likely not in or close to equilibrium with the present-day climate. Re-running the simulations with ice-free initial conditions shows that an ice cap forms only for obliquities less than a critical value of approximately 22°.

C51A-0095 

Calving Dynamics in an Ice-Stream/Ice-Shelf Model: Are Basal and Lateral Drag Stabilizing?

* Dupont, T K (tdupont@uci.edu), Dept. of Earth System Science, University of California 3226 Croul Hall, MC 3100, Irvine, CA 92697, United States Alley, R B (ralley@geosc.psu.edu), Dept. of Geosciences and EESI, Pennsylvania State University 517 Deike Building, University Park, PA 16802, United States Horgan, H (hhorgan@geosc.psu.edu), Dept. of Geosciences and EESI, Pennsylvania State University 517 Deike Building, University Park, PA 16802, United States Parizek, B R (hhorgan@geosc.psu.edu), The Pennsylvania State University, 181 Smeal Building College Place, DuBois, PA 15801, United States Joughin, I (ian@apl.washington.edu), Polar Science Center, APL, University of Washington 1013 NE 40th Street, Seattle, WA 98105, United States

Predicting the contributions of ice sheet dynamics to sea-level variations requires an understanding of the dynamics of ice-stream/ice-shelf systems. The behavior if ice shelves is particularly important given their ability to buttress the outflow of ice streams. This buttressing derives from frictional drag on the sides and areas of local grounding. Ice-shelf buttressing can be modified by changing the extent of the shelf, as shown most dramatically by the accelerations of glaciers feeding the collapsed portion of the Larsen B ice shelf (Rignot and others, 2004; Scambos and others, 2004). This leads directly to the consideration of iceberg calving, as the kinematic balance of calving and outward velocity at the ice front govern the migration of the ice front through time. In this work we apply an empirically derived calving rule to ice-shelf/ice-stream dynamics. The calving rule relates the rate of calving to the square root of the longitudinal strain rate. The shelf/stream model couples dynamic mass balance and stress equilibrium component models for a depth and width-integrated stream/shelf system. The ice front dynamically adjusts according to the difference between the ice-front velocity and the calving rate. Preliminary results indicate that an ice shelf with no lateral or basal drag is unstable with respect to the ice-front position. Here use the model to examine the ability of drag to stabilize the ice-front position, as well as the response of a stream/shelf system to perturbations of this drag.

C51A-0096 

A Least Squares Collocation Method for Modeling Ice, Water and Sediment Transport

* Egholm, D L (david@geo.au.dk), Department of Earth Sciences University of Aarhus, Høegh-Guldbergs Gade 2, Aarhus, DK8000, Denmark Nielsen, S B (sbn@geo.au.dk), Department of Earth Sciences University of Aarhus, Høegh-Guldbergs Gade 2, Aarhus, DK8000, Denmark Pedersen, V K (vivi.pedersen@geo.au.dk), Department of Earth Sciences University of Aarhus, Høegh-Guldbergs Gade 2, Aarhus, DK8000, Denmark

We present a new least squares collocation method for computational simulation of ice related flow processes on a three dimensional topographic surface. The collocation approach is predicated on 1) irregular, cell-based discretization of the topographic surface, 2) local second-order polynomial approximation of the bed topography and ice surface, and 3) Runge-Kutta time marching. The ice flow is simulated in 2D planform using a second- order shallow-ice approximation (SOSIA). Hence, higher order asymptotics, including contributions from longitudinal stress gradients and ice surface curvatures, are addressed. The collocation approach leads to a general and highly parallel algorithm based on discrete cell interactions, which is well suited for simulating also other types of surface flow processes, such as fluvial and hillslope related sediment transport. The inherent benefit of the method thus relates primarily to the ease with which several types of earth surface processes may be simulated simultaneously. Using this new integrated modeling platform, it is our objective to study landform generation by simulating glacial and fluvio-glacial erosion, sediment transport, and deposition. The dynamics of the ice and of the processes acting under glaciers must to some degree be manifested by the landforms produced by glaciers, and we wish to test quantitatively the potential of established sub-glacial dynamical relations for explaining well known large scale glacial landforms such as U-shaped valleys and cirques.

C51A-0097 

The Role of Glacial Erosion in Limiting Ice Sheet Extents

* Jamieson, S (Stewart.Jamieson@ed.ac.uk), University Of Edinburgh, School of GeoSciences, Institute of Geography, Drummond St., Edinburgh, EH8 9XP, United Kingdom Hulton, N (Nick.Hulton@ed.ac.uk), University Of Edinburgh, School of GeoSciences, Institute of Geography, Drummond St., Edinburgh, EH8 9XP, United Kingdom

We aim to identify and quantify feedbacks between ice dynamics and glacial erosion. Whilst geological and geomorphological evidence indicates that ice sheets generally oscillate in time with orbital forcing, their extents are not necessarily a direct function of the amplitude of this forcing. Benthic δ¹⁸O records document glacial-interglacial fluctuations and indicate that maximum Pleistocene global ice volume occurs around 400 ka. However, geomorphological evidence in a number of regions is contradictory, with the most extensive ice masses often occurring 100's of kyrs prior to peaks in the δ¹⁸O record. For example, the glacial landforms of Patagonia preserve a record of just such behaviour with each successive glacial advance since 1.15 Ma covering an area less extensive than the previous expansion. This implies that other processes are modifying the linkages between ice sheets and climate. We ask: Could glacial erosion of bedrock have caused ice sheets to self-regulate their extents? Ground-breaking experiments by Oerlemans (1984) demonstrated that erosion induced margin retreat was indeed possible. He showed that retreat could be achieved but only where eroding ice streams were smaller in width than the wavelength of lithospheric response. In Patagonia however, the scales of retreat are much larger than this lithospheric wavelength – but could erosion still be an important factor? We use the GLIMMER 3-D thermomechanical ice sheet model (Payne, 1999) with an added erosion component to simulate long-term landscape evolution under theoretical ice sheets (Jamieson et al., 2007). We show that models of glacial erosion can generate feedbacks on a significant scale such that ice sheets can self-limit their extents over periods of 10⁵ - 10⁶ years regardless of the flexural response of the land surface. Erosion around the ELA enables increasingly efficient ice drainage, and the mass balance of the ice sheet thus shifts towards a more negative state. At the same time, the thermal regime of the ice alters and the drawdown and capture of warm ice into ‘streams' causes more focussed selective erosion. The main control on the pattern of retreat is the pre-existing topography, which strongly controls erosion patterns. Ice streams retreat progressively in response to lowering valley floors and the impact of erosion induced self-limiting remains over successive glacial cycles. References: Jamieson, S.S.R., Hulton, N.R.J. and Hagdorn, M., 2007. Modelling landscape evolution under ice sheets. Geomorphology, doi:10.1016/j.geomorph.2007.02.047. Oerlemans, J., 1984. Numerical experiments on large-scale glacial erosion. Zeitschrift fur Gletscherkunde und Glazialgeologie, 20: 107-126. Payne, A.J., 1999. A thermomechanical model of ice flow in West Antarctica. Climate Dynamics, 15(2): 115-125.

C51A-0098 

Implications of Increasing Vertical Resolution in an Isopycnal Model of an Ice Shelf Cavity

* Little, C M (cmlittle@princeton.edu), Department of Geosciences Princeton University, Guyot Hall Princeton NJ 08544, Princeton, NJ 08544, United States

Boundary layer dynamics and thermodynamic feedback processes govern the efficiency of oceanic heat delivery to ice shelves. Isopycnal models offer an opportunity to resolve thin meltwater-freshened layers, improving the representation of the oceanic boundary layer and water masses modified in the cavity. However, heat, freshwater, and momentum fluxes between ice and ocean pass through a variable density bulk mixed layer (BML). Increased vertical resolution impacts the properties of the BML, and may modify the rate and spatial distribution of basal melting. To investigate these impacts, a series of simulations using the Hallberg Isopycnal Model, modified to represent sub-ice shelf processes, is conducted in an idealized, strongly forced, east-west aligned ice shelf cavity. Meltwater mixtures remain in the BML or fill intermediate density isopycnal layers (from 1-22 layers, depending on resolution) above a uniform (1.4°C) source water mass. Since mixing may mute the benefits of increased resolution, these simulations incorporate differing parameterizations, including a minimum BML thickness, background (tidal) velocities, and Richardson number-dependent entrainment. Water in the BML and, if present, intermediate layers, is advected weakly (O(10^-2) ms^-1) towards a southern boundary current. Meltwater flux near the ice shelf front is dominated by O(10^-1) ms^-1 flow in this boundary layer. The thickness of the meltwater-enriched outflow (20-160 m) increases in step with vertical resolution; gradients in tracers are apparent in all but the lowest resolution cases. Area-averaged melt rates of 16-35 myr^-1 indicate a strong sensitivity to near-boundary mixing; the spatial distribution of melting reveals the influence of vertical resolution under different regimes. Under uniformly high mixing, melting rates are dominated by "upstream" regions and are insensitive to resolution. Weaker imposed mixing induces a shift to a "shear-driven" regime, with melting intensified in the fast-flowing boundary current. Stronger shear-driven entrainment (which may be supported by observations of relatively dilute outflow) reveals greater resolution- dependence, as entrainment into the BML and isopycnal layers are controlled by different parameterizations. These results provide guidance for the appropriate level of vertical resolution, evidence for the importance of consistency in the BML and isopycnal interior, and suggest observational evidence that might constrain these results.

C51A-0099 

Inferring Histories of Accumulation, Ice Flow, and Ice Thickness from Internal Layers in Ice Sheets

* Koutnik, M R (mkoutnik@ess.washington.edu), University of Washington, Department of Earth and Space Sciences Box 351310, Seattle, WA 98195, United States Waddington, E D (edw@ess.washington.edu), University of Washington, Department of Earth and Space Sciences Box 351310, Seattle, WA 98195, United States Conway, H (conway@ess.washington.edu), University of Washington, Department of Earth and Space Sciences Box 351310, Seattle, WA 98195, United States Winebrenner, D P (dpw@apl.washington.edu), University of Washington, Applied Physics Laboratory Box 355640, Seattle, WA 98195, United States

Isochronous internal layers in ice sheets can be mapped with ice-penetrating radar. The shapes of these internal layers retain information about spatial and temporal patterns of accumulation and the history of ice flow. Deeper layers retain information from further in the past, but are more difficult to interpret because particles forming a deep layer have traveled through spatial and temporal gradients in accumulation and strain rate. To properly recover valuable information about the histories of ice dynamics and accumulation from deep internal layers, we solve an inverse problem. Solving this inverse problem uses an ice-sheet model in a new formulation, and recovers additional information from existing radar data. An inverse problem consists of a forward algorithm and an inverse algorithm. The forward algorithm makes a prediction of the data using a given set of model parameters, and the inverse algorithm adjusts the values of the unknown model parameters that control this prediction. Our forward algorithm calculates spatial and temporal gradients in ice temperature, ice thickness, and accumulation. Our inverse algorithm finds a spatially smooth accumulation history that also produces a prediction of the internal-layer architecture that fits the actual layer data within the uncertainties in the data. In addition to knowing the accumulation pattern, understanding past changes in ice thickness and the position of the ice divide are necessary to properly interpret an ice core. We are using radar data from central West Antarctica, near the upcoming West Antarctic Ice Sheet (WAIS) Divide ice core site, to infer the histories of accumulation and ice dynamics, and to support interpretation of this ice core. Radar data are also being collected over the Martian Polar Layered Deposits (PLD). Using synthetic layers, we have explored the feasibility of recovering accumulation patterns, and possibly accumulation rates on Mars. We present preliminary results for central West Antarctica, and for portions of the Martian North PLD.

C51A-0100 

Increased dynamic thinning due to the enhanced basal flow induced by surface meltwater in the Greenland ice sheet

* Wang, W (weili.wang@nasa.gov), SGT, NASA/GSFC, Code 614.1, Greenbelt, MD 20771, United States Li, J (jun.li@nasa.gov), SGT, NASA/GSFC, Code 614.1, Greenbelt, MD 20771, United States Zwally, J (Jay.zwally@nasa.gov), Cryospheric Sciences Branch, NASA/GSFC, Code 614.1, Greenbelt, MD 20771, United States

The satellite altimetry measurements during the time periods of ERS (1992 -2002) and ICESat (since 2003) have shown that the ice surface averaged thinning-rate at Swiss Camp Site, near the equilibrium line in the coastal region of West Greenland, increases from 0.14 m yr-1 to 0.68 m yr-1. Recent flowline modeling study indicates that the thinning during ICESat period is due to about 35% ice dynamic response to the increased seasonal basal sliding induced by surface meltwater and this dynamic thinning-rate increases about an order of magnitude from ERS period to ICESat period. A 3-dimensional ice flow model is developed with incorporation of the seasonal basal sliding induced by the penetration of surface meltwater through ice-sheet moulins and crevasses. The surface meltwater is determined based on the satellite observed surface temperature during the time period of 1982 -2006. The model is applied to the Greenland ice sheet to estimate the dynamic thinning caused by surface meltwater enhancing basal sliding, which provides a mechanism for rapid dynamic response of the ice sheet to climate change.

C51A-0101 

A Comparison of Two Models That Predict the Tidal Influence on the Flow of the Ross Ice Shelf, Antarctica

* Brunt, K M (kbrunt@uchicago.edu), University of Chicago, 5734 S. Ellis Ave, Department of Geophysics, Chicago, IL 60615, United States MacAyeal, D R (drm7@midway.uchicago.edu), University of Chicago, 5734 S. Ellis Ave, Department of Geophysics, Chicago, IL 60615, United States

Three stations near the calving front of the Ross Ice Shelf, Antarctica, recorded GPS data through a full spring-to- neap tidal cycle in 2005 and yielded a tantalizing diurnal signal, similar to those observed in other ice shelves [Doake et al., 2002] and ice streams [Anandakrishnan et al., 2003 and Bindschadler et al., 2003 a and b]. Based on the steady timing of the signal, it was hypothesized that the signal was related to the tides, which are strongly diurnal in the Ross Sea. To assess the influence of the tide on the ice-shelf motion, two model experiments using ocean tidal fields provided by Padman et al. [2002] were conducted and the results were compared. The first, more general model approach satisfies the full non-linearity of the governing stress-balance equations using numerical techniques. The solution obtained suggests that the tidal influence is small (less than 10 percent of the mean) compared to the time-averaged flow of the ice shelf. The small size of this response suggests that a linearization of the problem may be a fruitful and a much simpler approach to the problem. Thus, the second model approach uses a linearization of the governing stress balance equations and provides a check on the initial approach. The main non-linearity in the governing equations is associated with the vertically averaged effective viscosity. To linearize the equations, viscosity is associated with that computed for an ice shelf in the absence of tidal forcing.

C51A-0102 

Modelling the behaviour of Greenland outlet glaciers

* M. Nick, F (faezeh.nick@durham.ac.uk), University of Durham, Department of Geography, South Road, Durham, DH1 3LS, United Kingdom Vieli, A (andreas.vieli@durham.ac.uk), University of Durham, Department of Geography, South Road, Durham, DH1 3LS, United Kingdom

Rapid ice-sheet changes such as thinning and flow acceleration originate in, and spread from restricted regions of fast flow such as ice streams and outlet glaciers. Such rapid changes as observed for several Greenland outlet glaciers cannot be explained by the existing large-scale ice-sheet models that are based on the Shallow Ice Approximation, as they lack the necessary spatial resolution, an adequate treatment of the grounding line motion and do not consider the longitudinal transfer of stresses. The aim of this study is, firstly, to develop an adequate numerical model that can simulate this behaviour of outlet glaciers, in particular, to include longitudinal stresses and grounding line migration into a flowline model. Secondly, such a model is then used to investigate the interaction between ice-marginal processes and discharge from the interior through upstream transmission of perturbations. We use remotely sensed data (e.g. changes of surface elevation, velocity and front position) from Helheim Glacier, East Coast Greenland to refine and validate the model. We compare model response to perturbations in the frontal region and evaluate the role of longitudinal stresses in the retreat-thinning feedback. Such a comparison gives us important insights on the requirements for numerical models to improve our ability to predict future ice-sheet change.

C51A-0103 

New present day temperature parameterization and Degree Day model for Greenland

* Fausto, R S (rfo@gfy.ku.dk), University of Copenhagen, Juliane Maries Vej 26-32, Copenhagen, 2100, Denmark * Fausto, R S (rfo@gfy.ku.dk), Geological Survey of Denmark and Greenland, Oester voldgade 10, Copenhagen, 1350, Denmark Ahlstrom, A P (apa@geus.dk), Geological Survey of Denmark and Greenland, Oester voldgade 10, Copenhagen, 1350, Denmark Boggild, C E (carl.egede.boggild@unis.no), University centre in Svalbard, PB 156, Longyearbyen, 9171, Norway Johnsen, S J (sigfus@gfy.ku.dk), University of Copenhagen, Juliane Maries Vej 26-32, Copenhagen, 2100, Denmark

Surface temperature, melt rate and melt area for the Greenland Ice Sheet is parameterized using data from American and Danish Automatic Weather Stations (AWS) located near and on the ice sheet. The surface temperature is parameterized in terms of mean annual temperature, mean July temperature and mean annual precipitation. Melt rate is calculated using a positive degree day model, that accounts for firn warming, rainfall, refreezing of melt water and different degree day factors for ice and snow under warm and cold conditions. The temperature parameterization and degree day model has a high integration performance in numerical schemes and it is optimized for the use in large scale ice sheet models with spatial resolution of roughly 10-20 km. The positive degree-day factors for melt and refreezing of melt are adjusted in order to have the modeled melt area to fit as closely as possible with the observed values based on satellite algorithms. Methods and results are presented together with a discussion of the performance of these simple observation based models versus more sophisticated models.

C51A-0104 

Rheology of the Brunt Ice Shelf Inferred by Data Assimilation The Role of Marine Ice

* Khazendar, A (ala.khazendar@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States Rignot, E (eric.rignot@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States Larour, E (eric.larour@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States

The disintegration of ice shelves in Antarctic Peninsula over the past two decades clearly demonstrated the connection between the removal of ice shelves and the acceleration of their tributary glaciers. This enhanced flow of continental ice to the ocean contributes directly to global sea level rise and emphasizes the need for improved understanding of ice shelf evolution in a warming climate. An important, but not yet thoroughly examined, factor in ice shelf stability is the extent of marine ice presence, and its effect on shelf flow and mechanical integrity. The Brunt Ice Shelf on the east coast of the Weddell Sea presents a rare setting of rafts of meteoric ice being embedded in large, visible expanses of an ice mélange largely composed of marine ice. Different crystallographic structure and salinity and impurity contents should give marine ice a distinct rheology. In this work, we use satellite radar interferometric observations to infer the spatial distribution of the Brunt Ice Shelf ice rheology (flow law parameter B) by an inverse control method. We test the hypothesis of meteoric and marine ice bodies composing the ice shelf having different rheologies, and examine how this distribution affects ice shelf flow and its mechanical competence. We further use the inferred rheology distribution to locate the zones of weakness in the ice shelf, many of which are cut by large rifts filled with a mélange, and their influence on ice shelf stability. The outcome of this study makes it possible to better constrain the rheology of ice shelves known to have significant marine ice sections when velocity measurements are not available, while the methods applied demonstrate the importance (and difficulty) of deriving realistic rheologies to simulate ice shelf flow by numerical methods reliably. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, Cryospheric Sciences Program.

C51A-0105 

Greenland: Beyond "ice-sheet" dynamics

* Parizek, B R (parizek@geosc.psu.edu), The Pennsylvania State University, DuBois Campus 181 Smeal Building College Place, DuBois, PA 15801, United States Alley, R B (ralley@geosc.psu.edu), The Pennsylvania State University, 517 Deike Building, University Park, PA 16802, United States Dupont, T K (tdupont@uci.edu), University of California, Irvine, Department of Earth System Science 3226 Croul Hall, Irvine, CA 92697, United States du Bois, I J (dubois4@tcnj.edu), The College of New Jersey, Department of Physics P.O. Box 7718, Ewing, NJ 08628-0718, United States

Subglacial dynamics likely play a critical roll in coupling shelf, outlet-glacier, and inland-ice flow regimes. Furthermore, strong indirect evidence indicates that surface meltwater can penetrate through roughly 1 km of ice in Greenland and lubricate basal motion (Zwally et al., 2002), thereby linking surface and basal processes. Historically, whole ice-sheet models simulate regions of "streaming" flow based on basal temperatures and an a priori knowledge of subglacial sediment distribution. In order to capture observed changes in ice flow, both temporal and spatial evolution of controlling basal properties are required. Coupling surface melt and basal processes to a flowline model that incorporates longitudinal and vertical-shear stresses, we assess the role of variable lubrication and the potential feedbacks between lubrication and moulin initiation on the dynamics of the Greenland ice sheet.

C51A-0106 

Lagrangian Transient Firn Compaction Model With Heat Transfer

* Lundin, J (jdrees@u.washington.edu), University of Washington, Johnson Hall 070, Box 351310, 4000 15th Avenue NE, Seattle, WA 98195-1310, United States Waddington, E (edw@ess.washington.edu), University of Washington, Johnson Hall 070, Box 351310, 4000 15th Avenue NE, Seattle, WA 98195-1310, United States

Paleoclimate applications require knowledge of the ice-age gas-age offset (delta age) to correlate proxies in the ice (e.g. the temperature proxy δ18O in H2O) with trapped atmospheric gas. We develop a modular one-dimensional Lagrangian transient firn compaction model to find delta age. Finite volumes proportional to surface accumulation are advected downward, as they thin vertically primarily due to increases in density. The volumes can also extend horizontally due to strain in the underlying ice sheet. The evolution of each finite volume is governed by mass conservation and a constitutive relation for firn. Firn-compaction rate depends on temperature, so the compaction model is coupled to a Lagrangian heat-diffusion model, in which the thermal properties can depend on density. Upper boundary conditions for the coupled model are the histories of accumulation rate, surface density, and surface temperature. The lower boundary conditions must be provided from a larger ice-sheet model in which the firn model can be embedded. These conditions are the the thermal boundary condition, horizontal extension with depth, and vertical velocity at the bottom of the firn. Delta age is defined as the age of the ice where the density of bubble close of is met. The firn model can be incorporated into a larger inverse problem, creating a self-consistent system among three lines of questioning, that have until now been treated independently: (1) delta-age (2) inferred spatial and temporal accumulation histories from dated radar layers, and (3) physically meaningful interpolations of depth-age between sparse dated gas data points. This firn compaction model with a modular design, can be applied to any ice sheet given the requisite boundary conditions, further contributing to frontier work in ice sheet modeling.

C51A-0107 

Large scale modeling of Antarctica ice flow using inverse control methods on ice rheology and basal drag.

* larour, e (eric.larour@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States rignot, e (eric.rignot@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States

Large scale modeling of Antarctica and Greenland ice flow is a difficult problem that needs to be addressed if we want to be able to understand the present and future evolution of these continent's mass balance. A good first approximation for modeling ice flow is to work within the shallow ice approximation, using for example the forward model developped by MacAyeal (1989). Implementing a finite element solution to this model, and working with parallel cluster technologies, we have been able to develop a model capable of modeling the entire Antarctica ice flow within a reasonable degree of accuracy. Every ice sheet/ice shelf subsystem that constitue Antarctica are taken into account Realistic modeling can only be carried out if the rheology of ice and the basal drag at the bedrock interface are known. These parameters cannot be measured, and we have to resort to inverse control methods using satellite measurements of velocities, to infer those parameters. We have developped a control method that simultaneously inverts for ice rheology and basal drag. This is important as those two parameters often play similar roles in constraining ice flow. Using this dual control method, we can target subsystems of Antarctica and gradually improve the quality of our forward model. We have so far implemented this method on the entire Pine Island and Thwaites basin, where new data for surface and thickness elevation are avaialble. Results show a realistic pattern of distribution for ice rheology and basal friction, and improvements in the forward model for the entire Antarctica continent are significant. From those results, patterns can be extrapolated for areas of Antarctica where ice rheology and basal friction are totally unknown.

C51A-0108 

Measuring U-Series Isotopes in Polar Ice: Toward an Absolute Ice Chronometer

* Aciego, S M (aciego@erdw.ethz.ch), Institute for Isotope Geochemistry and Mineral Resources, ETH-Zurich Clausiusstrasse 25, NW C83.1, Zurich, 8092, Switzerland Bourdon, B (bourdon@erdw.ethz.ch), Institute for Isotope Geochemistry and Mineral Resources, ETH-Zurich Clausiusstrasse 25, NW C83.1, Zurich, 8092, Switzerland Schwander, J (schwander@climate.unibe.ch), Climate and Environmental Physics, University of Bern Sidlerstrasse 5, Bern, 3012, Switzerland Stocker, T (stocker@climate.unibe.ch), Climate and Environmental Physics, University of Bern Sidlerstrasse 5, Bern, 3012, Switzerland

Comparison of ice records between ice sheets, alpine glaciers, and marine records currently rely on a combination of ice layer counting, matching relative time scales, and interpolation. U-series recoil from mineral aerosols (dust) into the ice matrix is one possible technique for determining the absolute age of ice, independent of any other parameters. However, the low concentrations of the U-series parents and daughters have made previous measurements difficult and the results ambiguous. We present here the first results of work we have undertaken for determining U-series recoil ages in ice cores. The primary difficulty of this technique is the extremely low concentrations of dust in polar ice samples, and therefore, of the recoil daughter products in the ice. Previous work on dust provenance indicates 0.01 to 1 mg of dust concentration per kilogram of ice from the ice cores of Greenland and Antarctica. Given these conditions, U and Th dissolved in the water fraction of the aerosol-ice system may overwhelm the total U-series budget. Constraining the possible "initial" U and Th is the first step in determining the feasibility of this dating method for ice cores. We have implemented new geochemical techniques: ultra-clean ice processing, multiple ion counter ICP-MS measurements of U and Th, and quantification of total recoveries of the aerosol and water fractions using both established USGS standards, an internal lab loess standard that best approximates the dust fraction found in ice cores, and [U]-[Th] standards SRM960 and Th105. Dissolution experiments using U and Th spikes with these standards indicate recovery of the dust and dissolved fractions are better than 99%. We present here the first concentration measurements of U from the water fraction (<0.2 microns) of freshly deposited South Pole snow (20pg/kg), as well as a series of measurements from the upper section (~128m) of the Dye 3 ice core in Greenland which thus far range from 410pg/kg to 520fg/kg U dissolved in the water fraction. The high 234U/238U activity ratio (>1) in the Greenland ice core indicates the source of the uranium within the dissolved fraction is not inconsistent with seaspray. Given these results we model, using a simple Monte Carlo simulation, the expected errors in measuring absolute ages of polar ice. Using an estimate of surface roughness factor of 160 (based on recent BET measurements of dust), a lognormal distribution of grain sizes around 2 microns, and concentrations of dust over glacial periods on the order of 0.5mg/kg, we find that our current blanks, detection limits and internal precision should allow us to measure the ages of polar ice with an accuracy of 1% for Antarctic Ice cores and 5-10% for Greenland Ice cores (2 sigma).

C51A-0109 

Ice Cores Dating With a New Inverse Method Taking Account of the Flow Modeling Errors

* Lemieux-Dudon, B (lemieux@lgge.obs.ujf-grenoble.fr), Laboratoire de Glaciologie et de Geophysique de l'Environnement, CNRS, 54 rue Moliere, St Martin d'Heres, 38402, France Parrenin, F (parrenin@lgge.obs.ujf-grenoble.fr), Laboratoire de Glaciologie et de Geophysique de l'Environnement, CNRS, 54 rue Moliere, St Martin d'Heres, 38402, France Blayo, E (Eric.Blayo@imag.fr), Laboratoire Jean Kuntzmann, 51 rue des Mathematiques, Grenoble, 38041, France

Deep ice cores extracted from Antarctica or Greenland recorded a wide range of past climatic events. In order to contribute to the Quaternary climate system understanding, the calculation of an accurate depth-age relationship is a crucial point. Up to now ice chronologies for deep ice cores estimated with inverse approaches are based on quite simplified ice-flow models that fail to reproduce flow irregularities and consequently to respect all available set of age markers. We describe in this paper, a new inverse method that takes into account the model uncertainty in order to circumvent the restrictions linked to the use of simplified flow models. This method uses first guesses on two flow physical entities, the ice thinning function and the accumulation rate and then identifies correction functions on both flow entities. We highlight two major benefits brought by this new method: first of all the ability to respect large set of observations and as a consequence, the feasibility to estimate a synchronized common ice chronology for several cores at the same time. This inverse approach relies on a bayesian framework. To respect the positive constraint on the searched correction functions, we assume lognormal probability distribution on one hand for the background errors, but also for one particular set of the observation errors. We test this new inversion method on three cores simultaneously (the two EPICA cores : DC and DML and the Vostok core) and we assimilate more than 150 observations (e.g.: age markers, stratigraphic links,...). We analyze the sensitivity of the solution with respect to the background information, especially the prior error covariance matrix. The confidence intervals based on the posterior covariance matrix calculation, are estimated on the correction functions and for the first time on the overall output chronologies.

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.