Ice Flow of the Antarctic Ice SheetE. Rignot,1,2* J. Mouginot,1 B. Scheuchl11 Dept. Earth System Science, University of Ca...
corresponding to slow motion in East Antarctica, and a              suggests uncertainties in DEM-derived ice divide or re...
Wilkes Land, Mertz Glacier splits around a high-ground                 This organization of ice sheet flow into a complex ...
8. J. L. Bamber, J. Gomez-Dans, The accuracy of digital          25. E. Rignot, K. Echelmeyer, W. Krabill, Penetration dep...
lines outline subglacial lakes discussed in the text. Thickblack lines along the coast are interferometrically-derived ice...
Science 2011-rignot-science.1208336
Science 2011-rignot-science.1208336
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Science 2011-rignot-science.1208336

  1. 1. Ice Flow of the Antarctic Ice SheetE. Rignot,1,2* J. Mouginot,1 B. Scheuchl11 Dept. Earth System Science, University of California Irvine, Irvine, CA, 92697 USA. 2Jet Propulsion Laboratory, CaliforniaInstitute of Technology, Pasadena, CA 91109, USA.*To whom correspondence should be addressed. E-mail: erignot@uci.eduWe present a reference, comprehensive, high-resolution, from topographic divides to a set of narrow, fast-movingdigital mosaic of ice motion in Antarctica assembled from glaciers that control most of the ice sheet discharge into themultiple satellite interferometric synthetic-aperture radar ocean. This lack of broad-scale and detailed observation ofdata acquired during the International Polar Year 2007- ice motion has placed a fundamental limit on the capability2009. The data reveal widespread, patterned, enhanced and reliability of numerical models of ice sheet evolution (6). Downloaded from www.sciencemag.org on August 25, 2011flow with tributary glaciers reaching hundreds to On the eve of the international polar year, internationalthousands of kilometers inland, over the entire continent. space agencies worked together to enable a complete InSARThis view of ice sheet motion emphasizes the importance survey of Antarctica. Here, we employ spring 2009 data fromof basal-slip dominated tributary flow over deformation the Canadian Space Agency (CSA)s and MacDonald,dominated ice sheet flow, redefines our understanding of Dettwiler and Associates Ltd. (MDA)s RADARSAT-2,ice sheet dynamics, and has far-reaching implications for spring 2007-2008-2009 data from European Space Agencythe reconstruction and prediction of ice sheet evolution. (ESA)s Envisat ASAR, and fall 2007-2008 data from the Japan Aerospace Exploration Agency (JAXA)s AdvancedIce velocity is a fundamental characteristic of glaciers and ice Land Observing Satellite (ALOS) PALSAR, complementedsheets that measures the rate at which ice is transported from by patches of CSAs RADARSAT-1 data from fall 2000 (7)the interior regions toward the ocean, the location of preferred and ESAs ERS-1/2 data from spring 1996 (2). Each radarchannels of ice transport, and how ice mass evolves with instrument contributes its unique coverage and performancetime. Traditional measurements from ground-based stations level (see Fig. S1 in supporting online material (SOM)).are limited relative to the size of the continent, leading to an The final mosaic assembles 900 satellite tracks and moreincomplete picture of Antarctica. Satellite radar than 3,000 orbits of radar data (Fig. 1). The data are geo-interferometry or InSAR has been successfully employed to referenced with a precision better than one pixel, here 300 m,map glacier flow independent of cloud cover, solar to an Earth-fixed grid using a digital elevation model (DEM)illumination or the presence of surface features (1). Until (8). Absolute calibration of the surface velocity data relies onnow, however, the vast extent of East Antarctica, which control points of zero motion distributed along the coastcomprises 77% of the continent, has been devoid of quality (stagnant areas near ice domes or emergent mountains) anddata; only a few floating ice shelves have been mapped, and along major ice divides (areas of zero surface slope in thecomprehensive velocity mapping has been limited to the DEM) in a set of coast-to-coast ASAR tracks (fig. S1). Thelower reaches of key outlet glaciers (2). mapping precision varies with instrument, location, technique Balance velocity calculated from ice thickness, surface of analysis, repeat cycle, time period and data stacking.slope and snow accumulation data provides insights about the Nominal errors range from 1 m/yr along major ice dividespotential flow pattern of the ice sheet (3), but the technique with high data stacking to about 17 m/yr in areas affected byassumes an ice sheet in mass equilibrium, which is not correct ionospheric perturbations (fig. S2). In terms of strain rate, oreverywhere, and that ice flows perpendicular to surface changes in velocity per unit length, data noise is at the 3 xcontours, which is a simplification of the gravitational driving 10−4 /yr level, which is sufficient to reveal effective strainstress equation of motion (4). The precision of balance rates along tributary shear margins over the vast majority ofvelocity is affected by uncertainties in snow accumulation, the continent (Fig. 2A).major gaps in ice thickness data, and poor topographic Ice velocity ranges from a few cm/yr near divides to a fewcoverage near the South Pole. Nor does the technique apply km/yr on fast moving glaciers and floating ice shelves, or 5to floating ice shelves or replicate the correct width and orders of magnitude. The histogram in surface velocity has aextent of ice streams and tributaries (5). As a result, there is bi-modal distribution with a main peak at 4-5 m/yrno clear picture of ice sheet motion at the continental scale, / www.sciencexpress.org / 18 August 2011 / Page 1 / 10.1126/science.1208336
  2. 2. corresponding to slow motion in East Antarctica, and a suggests uncertainties in DEM-derived ice divide or recentsecond peak at 250 m/yr driven by the fast flow of glaciers changes in ice dynamics and enhanced flow to the West. Theand ice shelves. The fastest glaciers, Pine Island and western tributary of Foundation Ice Stream offers a naturalThwaites, are several times faster than any other glacier, and division between east and west Antarctica flow that was notunique in terms of how far inland fast flow prevails. This is known before. Academy Glacier, a major eastern tributary ofindeed the sector of most rapid change at present, over the Foundation, extends to the West over the South Pole and towidest area, and with the greatest impact on total ice sheet the East in parallel to Support-Force Ice Stream for more thanmass balance (2). Other fastest-moving glaciers include the 400 km, which is counter to the traditional view of well-Larsen B glaciers in the Peninsula, which accelerated in defined drainage basins.response to ice shelf collapse (9); the Ferrigno and Land Our map reveals to its full extent the tributaries ofglaciers in the West; and the Ninnis, Frost, Totten, Denman Recovery, Slessor and Bailey ice streams, which areand Shirase glaciers in the East. grounded below sea level and are likely underlaid by thick There is reasonably good agreement between observed marine sediments that favor rapid basal motion (16–17). Thesurface velocity and balance velocity (Fig. 2B) near ice tributaries share common sources that extend 1,000 km inlanddivides, however, large differences exist within each basin of the grounding line through a meander of slow movingand especially near the coast, demonstrating that the direct areas. Recovery Ice Streams two main tributaries broadenmeasurement of ice velocity is crucial to capture continental- inland and reach beyond the four subglacial lakes thought to Downloaded from www.sciencemag.org on August 25, 2011wide ice motion accurately. Figure 1 reveals a wealth of new be a possible source of fast motion (18). The presence of 2information. For instance, the exact pathway of ice along the subglacial lakes may accelerate the flow of the tributariescoastline is not without surprise. In Queen Maud Land, the coming from the East, but much less clearly for the tributariesmain trunk of Jutulstraumen is not to the South through Penck coming from the West. The impact of the abrupt reduction intrough but to the east of Neumayer Cliffs (10). The Sor basal friction associated with subglacial lakes is not apparentRondane Mountains were known to deflect ice flow to the in the broad pattern of ice motion. Similarly, at Lake Vostok,East and to the West through Hansenbreen and Brydbreen the largest subglacial lake in Antarctica, flow disturbancesglaciers, but the main ice sheet discharge is from two large remain at the sub-meter scale per year (19) and are not(80-km), un-named ice streams that flow at 100-200 m/yr expressed beyond the lake.around the Belgica Mountains for more than 500 km inland Bedrock topography has a clear impact on the flow pattern(11). Farther east, the fast-flowing core of Shirase Glacier of ice (Fig. 2C). Restricted ice motion is observed over majordoes not extend far inland, but little-studied Rayner Glacier to subglacial (Gamburtsev) and around emergent mountainsthe east flows above 100 m/yr for more than 200 km inland, (Newfoundland, Sor Rondane, Victoria Land, TransAntarctic,presumably along a deep subglacial trough (12). In the Executive Committee Range, Ellsworth, Graham Land),Antarctic Peninsula, the tributaries of Wilkins Ice Shelf and which deflect flow around them or limit flow through narrowof the northern sector of George VI Ice Shelf abruptly glaciers. The tributaries of Lambert Glacier extend inlandtransition to zero velocity when they mix with the floating ice among slow-moving areas of higher ground, hence highershelves. We attribute this spectacular termination of the basal friction, with some tributaries emerging as narrow, fastglaciers to massive rates of basal ablation of the ice shelves streams from the flanks of the Gamburtsev Mountains, whichby the underlying warm ocean (13). is surprising this far inland from the coast. The ice flowing A most interesting aspect is the spatial pattern of tributary down Byrd Glacier, the most extensive basin drained by aflow. Each major glacier is the merger of several tributaries single glacier, originates from 4 tributaries that spread inlandthat extend hundreds of km inland. While this was observed over more than 1,000 km. The tributaries are wider and morein the partial mapping of Siple Coast (14) and Pine Island diffuse than in the balance velocity map, which reveals a(15), this is now observed over the entire ice sheet. The map form of ice motion that is neither ice stream flow nor icereveals to its full extent the dendritic nature of drainage sheet flow but an intermediate regime which we denotesystems, the anastomosing distribution of tributaries, the patterned enhanced flow.narrowing (acceleration) and widening (deceleration) of Conversely, major flow pathways do not follow thetributaries inland, and their extension well beyond grounding deepest subglacial basins, hence the divide between Pinelines toward topographic divides. On Pine Island, Thwaites Island, Rutford and Evans runs across Byrd and Bentleyand Siple Coast, the tributaries extend to ice divides trenches, and the divide between Cook Ice Shelf and Davideverywhere we have data. Pine Island and Rutford share a Glacier runs across the Wilkes subglacial basin. Tottencommon source in the South that connects two different sides Glacier extends 1,000 km inland through two majorof West Antarctica. Furthermore, the velocity divide is offset tributaries that reach the Aurora subglacial basin, alongsideto the East of the topographic divide by 10-15 km, which Denman, another marine-based fast-flowing glacier. In / www.sciencexpress.org / 18 August 2011 / Page 2 / 10.1126/science.1208336
  3. 3. Wilkes Land, Mertz Glacier splits around a high-ground This organization of ice sheet flow into a complex set ofregion to reach Astrolabe trench to the West and Wilkes meandering, size-varying, speed-varying, anastomosingsubglacial basin to the East. But Ninnis Glacier extends tributaries most certainly dominated by basal-slip motionstraight, deep south into Wilkes subglacial basin. These two challenges the view of ice sheet flow constrained by internalglaciers exhibit an acceleration in flow at a bedrock step (20). deformation, disconnected from the coastal regions, that wasA similar initiation is visible on one western tributary of adopted as the background model for continental scale iceRecovery Ice Stream and the western tributary of Totten, but sheet modeling (6, 21). Actual observations of continental-over the vast majority of Antarctica, we find no area where scale ice motion reveal a new flow regime that initiates nearfast flow initiates at a bedrock step; instead, we observe topographic divides and involves a significant amount oftributary flow extending slowly, diffusively and gradually basal-slip motion. Much remains to be understood about theinland. mechanisms of basal motion and patterned enhanced flow, Tributary shear margins, detected as zones of effective but our observations already imply a tighter connectionstrain rate orders of magnitude higher than the background between coastal sectors and interior regions than in the(Fig. 2A), appear at velocities of about 30 m/yr, i.e., at hypothetical case of a uniform ice sheet flow because thesubstantially smaller values than the 100 m/yr quoted from concentration of ice fluxes along preferred channels enhancesthe partial mapping of Siple Coast (14). This extensive the diffusivity of perturbations. It is likely that this patternednetwork of shear margins reaches a few hundred km inland enhanced flow is not unique to Antarctica but a common Downloaded from www.sciencemag.org on August 25, 2011and expresses a strong coupling between coastal and inland feature of ice sheets. The mapping of Antarctic ice motionflow of ice. Yet patterned enhanced flow initiates at even therefore redefines our view of ice sheet flow dynamics andlower speed, in both west and east Antarctica. Patterned the way ice sheets have been modeled in the past, withenhanced flow here means flow that is not uniform, implications for improving reconstructions of past anddominated by deformational velocity, but includes a major ongoing changes, and especially for modeling the evolutionbasal slip component which varies spatially. Basal slip may of ice sheet dynamics in a warming climate.be attributed the presence of sub-glacial valleys thatchannelize thicker ice that generates more friction, more heat References and Notesand more melt water for lubrication and may also accumulate 1. R. M. Goldstein, H. Engelhardt, B. Kamb, R. Frolich,wet sediments that facilitate sliding; or the presence of Satellite radar interferometry for monitoring ice sheetdeformable or erodible, well lubricated beds not tied in with motion: application to an Antarctic ice stream. Sciencebed topography (21). Bed conditions are poorly constrained 262, 1525 (1993).by observations in Antarctica. 2. E. Rignot et al., Recent Antarctic ice mass loss from radar In Figure 2D, we calculate the ice sheet deformational interferometry and regional climate modelling. Naturevelocity by selecting an effective creep parameter A that best Geosci. 1, 106 (2008).fits the data in a 200-km wide band around the ice divides in 3. J. L. Bamber, D. G. Vaughan, I. Joughin, WidepreadQueen Maud Land where internal deformation is expected to complex flow in the interior of the Antarctic Ice Sheet.dominate ice motion (see SOM). Our calculation reproduces Science 287, 1248 (2000).the ice motion pattern reasonably well over a larger domain, 4. A. M. LeBrocq, A. J. Payne, M. J. Siegert, West Antarcticwhich provides confidence that this simple approach captures balance calculations: impact of flux-routing algorithm,the general pattern of deformational velocity. Deformational smoothing algorithm and topography. Computers andand measured surface velocity start to differ within a few Geosci. 32, 1780 (2006).hundred km of ice divides, typically at velocities above about 5. J. L. Bamber, E. Rignot, Unsteady flow inferred for15 m/yr (fig. S3). The difference expresses uncertainties in A Thwaites Glacier, and comparison with Pine Islandand ice thickness, but also for the most part indicates the Glacier, West Antarctica. J. Glaciol. 48, 237 (2002).presence of basal slip because the pattern of enhanced flow is 6. P. Lemke et al. Observations: Changes in Snow, Ice andquite different from the pattern of large deformational Frozen Ground. In Climate Change 2007: The Physicalvelocity. A doubling of A does not improve the model fit (fig. Science Basis. Contribution of Working Group I to theS4). In a second simulation, we calculate the deformational Fourth Assessment Report of the Intergovernmental Panelvelocity obtained for a rigid bed (21), i.e. where the speed on Climate Change Eds. Solomon, S. et al., Cambridgeonly depends on the power of the driving stress, but the University Press, Cambridge, United Kingdom and Newagreement between measured and deformational velocity is York, NY, USA (2007).not improved (fig. S5). We conclude that basal slip is a 7. K. C. Jezek, K. Farness, R. Carande, X. Wu, N. Labelle-significant component of ice motion in Antarctica, which Hammer, RADARSAT 1 synthetic aperture radardevelops close to ice divides. observations of Antarctica: Modified Antarctic Mapping Mission, 2000. Radio Sci. 38, 1 (2003). / www.sciencexpress.org / 18 August 2011 / Page 3 / 10.1126/science.1208336
  4. 4. 8. J. L. Bamber, J. Gomez-Dans, The accuracy of digital 25. E. Rignot, K. Echelmeyer, W. Krabill, Penetration depth elevation models of the Antarctic continent. Earth Plan. of interferometric syntheticaperture radar signals in snow Sci. Lett. 237, 516 (2005). and ice. Geophys. Res. Lett. 28(18) 3501-3504 (2001).9. E. Rignot et al., Accelerated ice discharge from the 26. A. L. Gray, K. E. Mattar, Influence of Ionospheric Antarctic Peninsula following the collapse of Larsen B ice Electron Density Fluctuations on Satellite Radar shelf. Geophys. Res. Lett. 31, L18401 (2004) Interferometry. Geophys. Res. Lett. 27(10), 1451-145410. C. W. Swithinbank, in Satellite Image Atlas of Glaciers of (2000). the World, USGS Prof. Paper 1386-B (1988). 27. R. Michel, E. Rignot, Flow of Glaciar Moreno, Argentina,11. F. Pattyn, S. De Brabander, A. Huyghe, Basal and thermal from repeat?pass Shuttle Imaging Radar images: control mechanisms of the Ragnhild glaciers, East Comparison of the phase correlation method with radar Antarctica. Ann. Glaciol. 40, 17 (2005). interferometry, J. Glaciol. 45, 93100 (1999).12. I. Allison, R. Frew, I. Knight, Bedrock and ice surface 28. I. Joughin, R. , Kwok, M. Fahnestock, Interferometric topography of the coastal regions of Antarctica between Estimation of Three-Dimensional Ice-Flow Using 48oE and 64oE. Polar Record 21, 241 (1982). Ascending and Descending Passes, IEEE Trans. Geosc.13. A. Jenkins, S. S. Jacobs, Circulation and melting beneath Rem. Sens. 36, 25-37 (1998). George VI Ice Shelf, Antarctica. J. Geophys. Res. 113, 29. D. T. Sandwell et al. Accuracy and resolution of ALOS C04013 (2008). Interferometry: Vector deformation maps of the Fathers Downloaded from www.sciencemag.org on August 25, 201114. I. Joughin et al., Tributaries of West Antarctic Ice Day intrusion at Kilauea, IEEE Trans. Geos. Rem. Sci. 46, Streams revealed by RADARSAT Interferometry. Science 3524-3533 (2008) 286, 283 (1999). 30. F. Pattyn, Antarctic subglacial conditions inferred from a15. M. Stenoien, C. R. Bentley, Pine Island Glacier, hybrid ice sheet/ice stream model, Earth Planet. Sci. Lett. Antarctica: A study of the catchment using interferometric 295 451-461 (2010). synthetic aperture radar measurements and radar altimetry. 31. A. M. Le Brocq, A. J., Payne, M. J . Siegert, R. B. Alley, J. Geophys. Res. 105, 21761 (2000) A subglacial water-flow model for West Antarctica, J.16. D. M. Rippin, J. L. Bamber, M. J. Siegert, D. G. Glaciol. 55, 879-888 (2009). Vaughan, H. F. J. Corr, Basal topography and ice flow in Acknowledgments: This work was performed at the the Bailey/ Slessor region of East Antarctica. J. Geophys. University of California Irvine and at Caltechs Jet Res. 108, 6008 (2003). Propulsion Laboratory under a contract with the National17. J. L. Bamber et al., East Antarctic ice stream tributary Aeronautics and Space Administrations MEaSUREs and underlain by major sedimentary basin. Geology 1, 33 Cryospheric Science Programs. Data acquisitions are (2006). courtesy of the IPY Space Task Group. The digital ice18. R. Bell, M. Studinger, C. A. Shuman, M. Fahnestock, I. motion map will be available as a MEaSUREs Earth Joughin, Large sub-glacial lakes in East Antarctica at the Science Data Record (ESDR) at the National Snow and Ice onset of fast-flowing ice streams. Nature 445, 904 (2007). Data Center, Boulder, CO. We thank two anonymous19. R. Kwok, M. J. Siegert, F. D. Carsey. Ice motion over reviewers for their comments. Lake Vostok, Antarctica: constraints on inferences regarding the accreted ice. J. Glaciol. 46, 689 (2000). Supporting Online Material20. N. F. Mc Intyre, The dynamics of ice-sheet outlets. J. www.sciencemag.org/cgi/content/full/science.1208336/DC1 Glaciol. 31, 99 (1985). Materials and Methods21. K. Cuffey and W. S. B. Paterson, The physics of glaciers Figs. S1 to S5 (Academic Press, Amsterdam, 4th ed., 2010). Table S122. T. Haran, J. Bohlander, T. Scambos, T. Painter, M. References (26–31) Fahnestock. 2006. MODIS mosaic of Antarctica (MOA) image map. Boulder, Colorado USA: National Snow and 13 May 2011; accepted 27 July 2011 Ice Data Center. Digital media. Published online 18 August 2011; 10.1126/science.120833623. E. Rignot, J. Mouginot, B. Scheuchl, Antarctic grounding line mapping from differential satellite radar Fig. 1. Antarctic ice velocity derived from ALOS PALSAR, interferometry. Geophys. Res. Lett. 38, L10504 (2011). Envisat ASAR, RADARSAT-2 and ERS-1/2 satellite radar24. M. B. Lythe, D.G. Vaughan and the BEDMAP interferometry color coded on a logarithmic scale and Consortium, BEDMAP: a new ice thickness and subglacial overlaid on a MODIS mosaic of Antarctica (22) with topographic model of Antarctica. J. Geophys. Res. 106, geographic names discussed in the text. Pixel spacing is 300 11335 (2001). m. Projection is polar stereographic at 71oS secant plane. Thick black lines delineate major ice divides (2). Thin black / www.sciencexpress.org / 18 August 2011 / Page 4 / 10.1126/science.1208336
  5. 5. lines outline subglacial lakes discussed in the text. Thickblack lines along the coast are interferometrically-derived icesheet grounding lines (23).Fig. 2. (A) Effective strain rate, · , color coded from 3 x 10−4yr−1 to greater than 15 x 10−3 yr−1; (B) balance velocity (3)color coded on a logarithmic scale as in Fig. 1; (C) bedelevation (24) with geographic names discussed in the text;(D) surface velocity calculated from internal deformationwith a creep parameter, A = 9 x 10−25 s−1 Pa−3. Thick blacklines in (B-D) and white lines in (A) delineate major icedivides (2) and grounding lines (23). Downloaded from www.sciencemag.org on August 25, 2011 / www.sciencexpress.org / 18 August 2011 / Page 5 / 10.1126/science.1208336

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