Day 2 - The Antarctic ice sheet
Title: ICECAP – revealing the hidden landscape of East Antarctica
Presenter: Roland Warner
Australian Antarctic Division
Antarctic Climate & Ecosystems Cooperative Research Centre
Authors: Jason L Roberts1,2, Roland C Warner1,2, Duncan A Young3, Jamin S Greenbaum3, Martin J Siegert4, Donald D Blankenship3, Tas D van Ommen1,2, Andrew P Wright5, Anne M Le Brocq5, Dustin M Schroeder3, Gregory Ng3, Cyril Grima3, Neal W Young2, Stefan Vogel1,2
1 - Australian Antarctic Division, Channel Highway, Kingston, Tasmania, 7050, Australia, Roberts@aad.gov.au, Roland.Warner@aad.gov.au, Tas.Van.ommen@aad.gov.au, Stefan.Vogel@aad.gov.au
2 - Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart, Tasmania 7001, Australia, Neal.Young@utas.edu.au
3 - Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78758, USA, duncan@utig.ig.utexas.edu, jamin.greenbaum@gmail.com, blank@utig.ig.utexas.edu, dschroed@gmail.com, ngg@utig.ig.utexas.edu, cyril.grima@gmail.com
4 - School of Geographical Sciences, University of Bristol, University Road, Clifton, BS8 1SS, UK, M.J.Siegert@bristol.ac.uk
5 - School of Geography, University of Exeter, Rennes Drive, Exeter, EX4 4RJ, UK, A.P.Wright@exeter.ac.uk, A.LeBrocq@exeter.ac.uk
Abstract
ICECAP is a major international airborne geophysics collaboration exploring the hidden aspects of the East Antarctic ice sheet. Measurements of bedrock topography, ice sheet basal conditions, ice thickness and internal layering, will underpin more realistic computer simulations of the future contribution of the ice sheet to sea level change. Tracing internal layer structures will connect major ice core sites and inform ice flow dynamics studies. As part of NASA’s Operation IceBridge our LiDAR surface profiling along ICESAT tracks contributes to understanding the current rate of ice sheet change.
Five seasons operating from Casey Station have yielded the first realistic bedrock map for a vast region of East Antarctica, including the Aurora Subglacial Basin. The landscapes revealed beneath the ice have provided insights into the early history of the Antarctic ice sheet, and into subglacial hydrology. Magnetic measurements have highlighted significant geological structures. Airborne gravimetry is revealing the bathymetry of ocean cavities beneath the floating ice shelves. Detailed coverage of major outlet glacier systems will enable realistic simulations of the critical connections between ocean, ice shelves and outlet glacier flows. ICECAP has collected over three hundred thousand kilometres of measurements - a major contribution to the latest compilation of ice sheet measurements.
In coming seasons ICECAP plans to operate from Davis Station, exploring one of the largest remaining regions of unmapped bedrock in East Antarctica, and to profile the Amery Ice Shelf, where gravimetric measurements of the sub ice shelf ocean bathymetry will improve modelling of ice shelf-ocean interactions.
Title: On the benefit of airborne snow radar and altimetry data for the assessment of ice sheet change: A case study over Law Dome and the Totten Glacier
Presenter: Christopher Watson
University of Tasmania
Authors: Christopher Watson1, Veit Helm2 and the TOT-Cal/CryoVExANT 2011 team
1 - University of Tasmania
2 - Alfred Wegener Institute
Abstract
Variation in the surface elevation of the Antarctic ice sheet remains an important indicator of change, particularly when considering marine grounded outlet glaciers where the change is anticipated to be the most rapid and significant. In addition to changes in ice volume (as derived from altimetry), the surface mass balance (SMB) must be properly understood to reliably estimate and interpret changes in ice sheet mass balance as derived from the altimetry and mass flux techniques. SMB remains poorly constrained, limited by sparse sampling and variable quality in situ data. Airborne snow radar and altimeter data provides a strategic and cost effective dataset to aid in the understanding at the regional scale - filling the spatial and temporal gap often found with satellite altimetry, and providing unparalleled information on spatial-temporal variability in SMB.
In this contribution, we review the joint TOT-Cal/CryoVExANT 2011 project which has obtained a unique airborne dataset over Law Dome and the Totten Glacier in East Antarctica. The study contributes directly to the validation of the CryoSat-2 altimetry mission, as well as assisting in observing and understanding regions where surface lowering is known to be approaching ~2 m/yr. In 2011/12, the AWI Polar-6 aircraft completed aerial surveys across the region equipped with scanning LiDAR and the ESA Airborne SAR/Interferometric Altimeter System (ASIRAS). Together, these data provide a window into recent changes in surface elevation and spatio-temporal variability in accumulation rate. We review these findings in the context of providing support for ongoing airborne geophysical capability across the region.
Title: Ice surface elevation change on the George V Coast of East Antarctica: a new hybrid high resolution record
Presenter: Donald Blankenship
University of Texas at Austin
Authors: Duncan A. Young1, Laura E. Lindzey1, Donald D. Blankenship1, Jamin S. Greenbaum1, Emmanuel Le Meur2
1 - University of Texas at Austin, Austin, Texas, USA, duncan@ig.utexas.edu
2 - LGGE, Grenoble, France: elemeur@lgge.obs.ujf-grenoble.fr
Abstract
The ice streams of the George V Coast drain from the large Wilkes Subglacial Basin behind East Antarctica's Transantarctic Mountains, into fragmentary ice shelves exposed to the Southern Ocean. As part of NASA's Operation Ice Bridge, the ICECAP program has been conducting integrated aerogeophysical observations over these glaciers, targeting Mertz Glacier, Ninnis Glacier and the glaciers feeding Cook Ice Shelf, using the resources of the French Polar Institute. Our instrument suite included multi-frequency radar, gravimetry, magnetics and the ALAMO (Airborne Laser Altimetry with Mapping Optics) hybrid laser/lidar altimeter system. A primary objective of these observations is to extend and improved the 2003-2009 surface elevation record of GLAS (Geoscience Laser Altimeter System) instrument onboard NASA's ICESat satellite. We can improve on the GLAS only record as small pointing errors in GLAS lead to measurements that deviate from the nominal track by several hundred meters. In areas of high slope, this confounds elevation change due to surface slope with the dH/dt signal. With accurate surface slopes, the impacts of these two effects can be separated. This is particularly important in the coastal ice streams, which have both rapidly changing elevations and high surface slopes. Here we report on a combined ALAMO-GLAS slope corrected surface elevation record for Cook Ice Shelf, and find clear evidence for ice surface lowering over the deep trench that defines fast ice flow in this ice catchment.
Title: Constraints on ice volume changes of the East and West Antarctic Ice Sheets based on cosmogenic exposure ages from the Darwin-Hatherton outlet glaciers of the Transantarctic Mountains
Presenter: Bryan Storey
Gateway Antarctica, University of Canterbury
Authors: Bryan Storey1, Kurt Joy1 and David Fink2
1 - Gateway Antarctica, University of Canterbury, New Zealand
2 - Australian Nuclear Science and Technology Organization, Sydney, Australia
Abstract
About 80 cosmogenic 10Be and 26Al exposure ages of erratics from 3 locations flanking the Darwin-Hatherton outlet glaciers documents 2.5 Ma of ice volume evolution allowing a reconstruction of its quaternary paleo-ice surface. Pleistocene ice thickness is some 800 to 400 meters thicker between 2.5 to 0.5 Ma years ago than today. However at all 3 locations, exposure ages of mapped glacial drifts younger than 0.5 Ma did not show any evidence for a distinct LGM advance. At Lake Wellman a cluster of mid-elevation moraine boulders from the Britannia Drift, previously taken to demarcate the LGM advance, have exposure ages ranging from 30 to 40 ka. At Dubris Valley, the same drift returned ages of 120-125 ka. At Diamond Hill, the confluence of the Darwin Glacier and Ross Ice Shelf, cosmogenic dates show a similar trend to that seen further upvalley – the WAIS was approximately 900 meters thicker than the current Rose Ice Shelf configuration at about1.5 Ma and with only minor advances in the last 10 ka and an absence of LGM ages. The absence of a LGM signal is perplexing. We suggest the idea that while WAIS expansion during the early Pleistocene was large, LGM ice volume in the Darwin-Hatherton glaciers was not as large as previously estimated and perhaps little different from what is observed today (at most 50 m above current ice surface). These results raise interesting questions about how the Antarctic ice sheets respond to global change.
Title: Basal conditions of two Transantarctic Mountain outlet glaciers from observation-constrained diagnostic modeling
Presenter: Nicholas Golledge
Antarctic Research Centre, Victoria University of Wellington
GNS Science
Authors: N.R. Golledge1,2, O.J. Marsh3, W. Rack3, R.S. Jones1
1 - Antarctic Research Centre, Victoria University of Wellington, Wellington, NZ, nick.golledge@vuw.ac.nz, Richard.S.Jones@vuw.ac.nz
2 - GNS Science, Avalon, Lower Hutt, NZ
3 - Gateway Antarctica, University of Canterbury, Christchurch, NZ, oliver.marsh@pg.canterbury.ac.nz, wolfgang.rack@canterbury.ac.nz
Abstract
Response of Antarctic ice-sheets to environmental change is determined largely by conditions at their inaccessible and unobservable beds. In an attempt to better understand two outlet glaciers of the East Antarctic Ice Sheet in the Transantarctic Mountains, we present new velocity data from ice surface GPS installations, which we use with newly-acquired airborne radar data as well as with continental gridded datasets to constrain a diagnostic glacier flow model. The observational data are primary inputs to a modeling procedure that first calculates the basal thermal regime of each glacier, then iterates the basal sliding parameter and deformation rate parameter until the fit of simulated to observed surface velocities is optimised. We find that the two glaciers have both frozen and thawed areas at their beds, facilitating partial sliding. Glacier flow arises from a balance between sliding and deformation that fluctuates along the length of each glacier, with the amount of sliding typically varying by up to two orders of magnitude but with deformation rates far more constant. Whilst the two glaciers differ significantly in mean flow speed and discharge, sensitivity analysis illustrates that ocean-driven thinning at glacier grounding lines would likely result in acceleration and retreat of both glaciers, primarily as a consequence of enhanced discharge arising from the combined effects of longitudinal coupling due by reduced basal traction and increased floatation arising from up-glacier deepening beds.
Title: A comparison of anisotropic ice flow relations for use in ice sheet modelling
Presenter: Adam Treverrow
Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania
Authors: Adam Treverrow1, Roland C. Warner1,2, William F. Budd1,3, Tim H. Jacka1
1 - Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Australia, 7004, adamt0@utas.edu.au, w.f.budd@utas.edu.au, jglac@bigpond.com
2 - Australian Antarctic Division, Channel Highway, Kingston, Australia, 7050, Roland.Warner@aad.gov.au
3 - Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia, 7004
Abstract
Incorporating a physically accurate description of ice deformation processes into ice sheet models is a key component to reducing the uncertainty in predictions of ice sheet contributions to changes in global mean sea level. Recent developments in some ice sheet models, in particular the capability to determine the three dimensional distribution of stresses throughout an ice sheet makes it possible to incorporate a description of anisotropic ice flow properties into large-scale dynamic simulations. Despite these developments, the selection of an appropriate numerical relationship between the ice strain rates and the stresses driving the flow remains an issue in improving the accuracy of model simulations. In this study we use observations from Law Dome, East Antarctica - including crystallographic data from the Dome Summit South (DSS) ice core - to model the vertical distribution of deviatoric stress components at the DSS borehole site. We compare predictions of four anisotropic flow relations that have been proposed in the literature. These range from grain-scale flow relations incorporating a description of ice microdynamic processes, including nearest-neighbour grain interactions, through to empirical flow relations derived from the results of laboratory ice deformation experiments. For grain-scale relations the effect of polycrystalline anisotropy on enhancing flow rates is based primarily on consideration of crystal orientations. This leads to lower estimates of flow enhancement when compared to scalar flow relations in combined stress situations. An empirical relationship, where the effects of anisotropy are parameterised by a scalar function of the deviatoric stresses, provides the most realistic and computationally efficient simulations.
