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Ice Flow of the Antarctic Ice Sheet
E. Rignot,1,2* J. Mouginot,1 B. Scheuchl1
1
 Dept. Earth System Science, University of California Irvine, Irvine, CA, 92697 USA. 2Jet Propulsion Laboratory, California
Institute of Technology, Pasadena, CA 91109, USA.

*To whom correspondence should be addressed. E-mail: erignot@uci.edu

We present a reference, comprehensive, high-resolution,            from topographic divides to a set of narrow, fast-moving
digital mosaic of ice motion in Antarctica assembled from          glaciers that control most of the ice sheet discharge into the
multiple satellite interferometric synthetic-aperture radar        ocean. This lack of broad-scale and detailed observation of
data acquired during the International Polar Year 2007-            ice motion has placed a fundamental limit on the capability
2009. The data reveal widespread, patterned, enhanced              and reliability of numerical models of ice sheet evolution (6).




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flow with tributary glaciers reaching hundreds to                     On the eve of the international polar year, international
thousands of kilometers inland, over the entire continent.         space agencies worked together to enable a complete InSAR
This view of ice sheet motion emphasizes the importance            survey of Antarctica. Here, we employ spring 2009 data from
of 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 Agency
the reconstruction and prediction of ice sheet evolution.          (ESA)'s Envisat ASAR, and fall 2007-2008 data from the
                                                                   Japan Aerospace Exploration Agency (JAXA)'s Advanced
Ice velocity is a fundamental characteristic of glaciers and ice
                                                                   Land Observing Satellite (ALOS) PALSAR, complemented
sheets that measures the rate at which ice is transported from
                                                                   by patches of CSA's RADARSAT-1 data from fall 2000 (7)
the interior regions toward the ocean, the location of preferred
                                                                   and ESA's ERS-1/2 data from spring 1996 (2). Each radar
channels of ice transport, and how ice mass evolves with
                                                                   instrument contributes its unique coverage and performance
time. 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 more
incomplete 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 on
now, however, the vast extent of East Antarctica, which
                                                                   control points of zero motion distributed along the coast
comprises 77% of the continent, has been devoid of quality
                                                                   (stagnant areas near ice domes or emergent mountains) and
data; only a few floating ice shelves have been mapped, and
                                                                   along major ice divides (areas of zero surface slope in the
comprehensive velocity mapping has been limited to the
                                                                   DEM) in a set of coast-to-coast ASAR tracks (fig. S1). The
lower 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 divides
potential flow pattern of the ice sheet (3), but the technique
                                                                   with high data stacking to about 17 m/yr in areas affected by
assumes an ice sheet in mass equilibrium, which is not correct
                                                                   ionospheric perturbations (fig. S2). In terms of strain rate, or
everywhere, and that ice flows perpendicular to surface
                                                                   changes in velocity per unit length, data noise is at the 3 x
contours, which is a simplification of the gravitational driving
                                                                   10−4 /yr level, which is sufficient to reveal effective strain
stress equation of motion (4). The precision of balance
                                                                   rates along tributary shear margins over the vast majority of
velocity 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 few
coverage near the South Pole. Nor does the technique apply
                                                                   km/yr on fast moving glaciers and floating ice shelves, or 5
to floating ice shelves or replicate the correct width and
                                                                   orders of magnitude. The histogram in surface velocity has a
extent of ice streams and tributaries (5). As a result, there is
                                                                   bi-modal distribution with a main peak at 4-5 m/yr
no clear picture of ice sheet motion at the continental scale,

                                   / www.sciencexpress.org / 18 August 2011 / Page 1 / 10.1126/science.1208336
corresponding to slow motion in East Antarctica, and a              suggests uncertainties in DEM-derived ice divide or recent
second peak at 250 m/yr driven by the fast flow of glaciers         changes in ice dynamics and enhanced flow to the West. The
and ice shelves. The fastest glaciers, Pine Island and              western tributary of Foundation Ice Stream offers a natural
Thwaites, are several times faster than any other glacier, and      division between east and west Antarctica flow that was not
unique in terms of how far inland fast flow prevails. This is       known before. Academy Glacier, a major eastern tributary of
indeed the sector of most rapid change at present, over the         Foundation, extends to the West over the South Pole and to
widest area, and with the greatest impact on total ice sheet        the East in parallel to Support-Force Ice Stream for more than
mass 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 of
glaciers in the West; and the Ninnis, Frost, Totten, Denman         Recovery, Slessor and Bailey ice streams, which are
and 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). The
surface velocity and balance velocity (Fig. 2B) near ice            tributaries share common sources that extend 1,000 km inland
divides, however, large differences exist within each basin         of the grounding line through a meander of slow moving
and especially near the coast, demonstrating that the direct        areas. Recovery Ice Stream's two main tributaries broaden
measurement of ice velocity is crucial to capture continental-      inland and reach beyond the four subglacial lakes thought to




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wide ice motion accurately. Figure 1 reveals a wealth of new        be a possible source of fast motion (18). The presence of 2
information. For instance, the exact pathway of ice along the       subglacial lakes may accelerate the flow of the tributaries
coastline is not without surprise. In Queen Maud Land, the          coming from the East, but much less clearly for the tributaries
main trunk of Jutulstraumen is not to the South through Penck       coming from the West. The impact of the abrupt reduction in
trough but to the east of Neumayer Cliffs (10). The Sor             basal friction associated with subglacial lakes is not apparent
Rondane 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 disturbances
glaciers, 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 major
does not extend far inland, but little-studied Rayner Glacier to    subglacial (Gamburtsev) and around emergent mountains
the 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 narrow
of the northern sector of George VI Ice Shelf abruptly              glaciers. The tributaries of Lambert Glacier extend inland
transition to zero velocity when they mix with the floating ice     among slow-moving areas of higher ground, hence higher
shelves. We attribute this spectacular termination of the           basal friction, with some tributaries emerging as narrow, fast
glaciers to massive rates of basal ablation of the ice shelves      streams from the flanks of the Gamburtsev Mountains, which
by 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 a
flow. Each major glacier is the merger of several tributaries       single glacier, originates from 4 tributaries that spread inland
that extend hundreds of km inland. While this was observed          over more than 1,000 km. The tributaries are wider and more
in 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 ice
reveals to its full extent the dendritic nature of drainage         sheet flow but an intermediate regime which we denote
systems, the anastomosing distribution of tributaries, the          patterned enhanced flow.
narrowing (acceleration) and widening (deceleration) of                 Conversely, major flow pathways do not follow the
tributaries inland, and their extension well beyond grounding       deepest subglacial basins, hence the divide between Pine
lines toward topographic divides. On Pine Island, Thwaites          Island, Rutford and Evans runs across Byrd and Bentley
and Siple Coast, the tributaries extend to ice divides              trenches, and the divide between Cook Ice Shelf and David
everywhere we have data. Pine Island and Rutford share a            Glacier runs across the Wilkes subglacial basin. Totten
common source in the South that connects two different sides        Glacier extends 1,000 km inland through two major
of West Antarctica. Furthermore, the velocity divide is offset      tributaries that reach the Aurora subglacial basin, alongside
to 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
Wilkes Land, Mertz Glacier splits around a high-ground                 This organization of ice sheet flow into a complex set of
region to reach Astrolabe trench to the West and Wilkes            meandering, size-varying, speed-varying, anastomosing
subglacial basin to the East. But Ninnis Glacier extends           tributaries most certainly dominated by basal-slip motion
straight, deep south into Wilkes subglacial basin. These two       challenges the view of ice sheet flow constrained by internal
glaciers exhibit an acceleration in flow at a bedrock step (20).   deformation, disconnected from the coastal regions, that was
A similar initiation is visible on one western tributary of        adopted as the background model for continental scale ice
Recovery 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 near
fast flow initiates at a bedrock step; instead, we observe         topographic divides and involves a significant amount of
tributary flow extending slowly, diffusively and gradually         basal-slip motion. Much remains to be understood about the
inland.                                                            mechanisms of basal motion and patterned enhanced flow,
    Tributary shear margins, detected as zones of effective        but our observations already imply a tighter connection
strain 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 the
substantially smaller values than the 100 m/yr quoted from         concentration of ice fluxes along preferred channels enhances
the partial mapping of Siple Coast (14). This extensive            the diffusivity of perturbations. It is likely that this patterned
network of shear margins reaches a few hundred km inland           enhanced flow is not unique to Antarctica but a common




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and expresses a strong coupling between coastal and inland         feature of ice sheets. The mapping of Antarctic ice motion
flow of ice. Yet patterned enhanced flow initiates at even         therefore redefines our view of ice sheet flow dynamics and
lower speed, in both west and east Antarctica. Patterned           the way ice sheets have been modeled in the past, with
enhanced flow here means flow that is not uniform,                 implications for improving reconstructions of past and
dominated by deformational velocity, but includes a major          ongoing changes, and especially for modeling the evolution
basal slip component which varies spatially. Basal slip may        of ice sheet dynamics in a warming climate.
be attributed the presence of sub-glacial valleys that
channelize thicker ice that generates more friction, more heat     References and Notes
and 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 sheet
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the ice motion pattern reasonably well over a larger domain,       4. A. M. LeBrocq, A. J. Payne, M. J. Siegert, West Antarctic
which 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 and
and 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 for
15 m/yr (fig. S3). The difference expresses uncertainties in A         Thwaites Glacier, and comparison with Pine Island
and ice thickness, but also for the most part indicates the            Glacier, West Antarctica. J. Glaciol. 48, 237 (2002).
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quite different from the pattern of large deformational                Frozen Ground. In Climate Change 2007: The Physical
velocity. A doubling of A does not improve the model fit (fig.         Science Basis. Contribution of Working Group I to the
S4). In a second simulation, we calculate the deformational            Fourth Assessment Report of the Intergovernmental Panel
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only depends on the power of the driving stress, but the               University Press, Cambridge, United Kingdom and New
agreement 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 radar
develops close to ice divides.                                         observations of Antarctica: Modified Antarctic Mapping
                                                                       Mission, 2000. Radio Sci. 38, 1 (2003).

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8. J. L. Bamber, J. Gomez-Dans, The accuracy of digital          25. E. Rignot, K. Echelmeyer, W. Krabill, Penetration depth
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14. I. Joughin et al., Tributaries of West Antarctic Ice            Day intrusion at Kilauea, IEEE Trans. Geos. Rem. Sci. 46,
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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 Caltech's Jet
    Res. 108, 6008 (2003).                                          Propulsion Laboratory under a contract with the National
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19. R. Kwok, M. J. Siegert, F. D. Carsey. Ice motion over           reviewers for their comments.
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20. 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 Methods
21. K. Cuffey and W. S. B. Paterson, The physics of glaciers     Figs. S1 to S5
    (Academic Press, Amsterdam, 4th ed., 2010).                  Table S1
22. 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.1208336
23. 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 radar
24. 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
lines outline subglacial lakes discussed in the text. Thick
black lines along the coast are interferometrically-derived ice
sheet grounding lines (23).
Fig. 2. (A) Effective strain rate, · , color coded from 3 x 10−4
yr−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) bed
elevation (24) with geographic names discussed in the text;
(D) surface velocity calculated from internal deformation
with a creep parameter, A = 9 x 10−25 s−1 Pa−3. Thick black
lines in (B-D) and white lines in (A) delineate major ice
divides (2) and grounding lines (23).




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Science 2011-rignot-science.1208336

  • 1. Ice Flow of the Antarctic Ice Sheet E. Rignot,1,2* J. Mouginot,1 B. Scheuchl1 1 Dept. Earth System Science, University of California Irvine, Irvine, CA, 92697 USA. 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA. *To whom correspondence should be addressed. E-mail: erignot@uci.edu We present a reference, comprehensive, high-resolution, from topographic divides to a set of narrow, fast-moving digital mosaic of ice motion in Antarctica assembled from glaciers that control most of the ice sheet discharge into the multiple satellite interferometric synthetic-aperture radar ocean. This lack of broad-scale and detailed observation of data acquired during the International Polar Year 2007- ice motion has placed a fundamental limit on the capability 2009. The data reveal widespread, patterned, enhanced and reliability of numerical models of ice sheet evolution (6). Downloaded from www.sciencemag.org on August 25, 2011 flow with tributary glaciers reaching hundreds to On the eve of the international polar year, international thousands of kilometers inland, over the entire continent. space agencies worked together to enable a complete InSAR This view of ice sheet motion emphasizes the importance survey of Antarctica. Here, we employ spring 2009 data from of 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 Agency the reconstruction and prediction of ice sheet evolution. (ESA)'s Envisat ASAR, and fall 2007-2008 data from the Japan Aerospace Exploration Agency (JAXA)'s Advanced Ice velocity is a fundamental characteristic of glaciers and ice Land Observing Satellite (ALOS) PALSAR, complemented sheets that measures the rate at which ice is transported from by patches of CSA's RADARSAT-1 data from fall 2000 (7) the interior regions toward the ocean, the location of preferred and ESA's ERS-1/2 data from spring 1996 (2). Each radar channels of ice transport, and how ice mass evolves with instrument contributes its unique coverage and performance time. 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 more incomplete 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 on now, however, the vast extent of East Antarctica, which control points of zero motion distributed along the coast comprises 77% of the continent, has been devoid of quality (stagnant areas near ice domes or emergent mountains) and data; only a few floating ice shelves have been mapped, and along major ice divides (areas of zero surface slope in the comprehensive velocity mapping has been limited to the DEM) in a set of coast-to-coast ASAR tracks (fig. S1). The lower 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 divides potential flow pattern of the ice sheet (3), but the technique with high data stacking to about 17 m/yr in areas affected by assumes an ice sheet in mass equilibrium, which is not correct ionospheric perturbations (fig. S2). In terms of strain rate, or everywhere, and that ice flows perpendicular to surface changes in velocity per unit length, data noise is at the 3 x contours, which is a simplification of the gravitational driving 10−4 /yr level, which is sufficient to reveal effective strain stress equation of motion (4). The precision of balance rates along tributary shear margins over the vast majority of velocity 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 few coverage near the South Pole. Nor does the technique apply km/yr on fast moving glaciers and floating ice shelves, or 5 to floating ice shelves or replicate the correct width and orders of magnitude. The histogram in surface velocity has a extent of ice streams and tributaries (5). As a result, there is bi-modal distribution with a main peak at 4-5 m/yr no clear picture of ice sheet motion at the continental scale, / www.sciencexpress.org / 18 August 2011 / Page 1 / 10.1126/science.1208336
  • 2. corresponding to slow motion in East Antarctica, and a suggests uncertainties in DEM-derived ice divide or recent second peak at 250 m/yr driven by the fast flow of glaciers changes in ice dynamics and enhanced flow to the West. The and ice shelves. The fastest glaciers, Pine Island and western tributary of Foundation Ice Stream offers a natural Thwaites, are several times faster than any other glacier, and division between east and west Antarctica flow that was not unique in terms of how far inland fast flow prevails. This is known before. Academy Glacier, a major eastern tributary of indeed the sector of most rapid change at present, over the Foundation, extends to the West over the South Pole and to widest area, and with the greatest impact on total ice sheet the East in parallel to Support-Force Ice Stream for more than mass 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 of glaciers in the West; and the Ninnis, Frost, Totten, Denman Recovery, Slessor and Bailey ice streams, which are and 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). The surface velocity and balance velocity (Fig. 2B) near ice tributaries share common sources that extend 1,000 km inland divides, however, large differences exist within each basin of the grounding line through a meander of slow moving and especially near the coast, demonstrating that the direct areas. Recovery Ice Stream's two main tributaries broaden measurement 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, 2011 wide ice motion accurately. Figure 1 reveals a wealth of new be a possible source of fast motion (18). The presence of 2 information. For instance, the exact pathway of ice along the subglacial lakes may accelerate the flow of the tributaries coastline is not without surprise. In Queen Maud Land, the coming from the East, but much less clearly for the tributaries main trunk of Jutulstraumen is not to the South through Penck coming from the West. The impact of the abrupt reduction in trough but to the east of Neumayer Cliffs (10). The Sor basal friction associated with subglacial lakes is not apparent Rondane 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 disturbances glaciers, 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 major does not extend far inland, but little-studied Rayner Glacier to subglacial (Gamburtsev) and around emergent mountains the 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 narrow of the northern sector of George VI Ice Shelf abruptly glaciers. The tributaries of Lambert Glacier extend inland transition to zero velocity when they mix with the floating ice among slow-moving areas of higher ground, hence higher shelves. We attribute this spectacular termination of the basal friction, with some tributaries emerging as narrow, fast glaciers to massive rates of basal ablation of the ice shelves streams from the flanks of the Gamburtsev Mountains, which by 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 a flow. Each major glacier is the merger of several tributaries single glacier, originates from 4 tributaries that spread inland that extend hundreds of km inland. While this was observed over more than 1,000 km. The tributaries are wider and more in 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 ice reveals to its full extent the dendritic nature of drainage sheet flow but an intermediate regime which we denote systems, the anastomosing distribution of tributaries, the patterned enhanced flow. narrowing (acceleration) and widening (deceleration) of Conversely, major flow pathways do not follow the tributaries inland, and their extension well beyond grounding deepest subglacial basins, hence the divide between Pine lines toward topographic divides. On Pine Island, Thwaites Island, Rutford and Evans runs across Byrd and Bentley and Siple Coast, the tributaries extend to ice divides trenches, and the divide between Cook Ice Shelf and David everywhere we have data. Pine Island and Rutford share a Glacier runs across the Wilkes subglacial basin. Totten common source in the South that connects two different sides Glacier extends 1,000 km inland through two major of West Antarctica. Furthermore, the velocity divide is offset tributaries that reach the Aurora subglacial basin, alongside to 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. Wilkes Land, Mertz Glacier splits around a high-ground This organization of ice sheet flow into a complex set of region to reach Astrolabe trench to the West and Wilkes meandering, size-varying, speed-varying, anastomosing subglacial basin to the East. But Ninnis Glacier extends tributaries most certainly dominated by basal-slip motion straight, deep south into Wilkes subglacial basin. These two challenges the view of ice sheet flow constrained by internal glaciers exhibit an acceleration in flow at a bedrock step (20). deformation, disconnected from the coastal regions, that was A similar initiation is visible on one western tributary of adopted as the background model for continental scale ice Recovery 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 near fast flow initiates at a bedrock step; instead, we observe topographic divides and involves a significant amount of tributary flow extending slowly, diffusively and gradually basal-slip motion. Much remains to be understood about the inland. mechanisms of basal motion and patterned enhanced flow, Tributary shear margins, detected as zones of effective but our observations already imply a tighter connection strain 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 the substantially smaller values than the 100 m/yr quoted from concentration of ice fluxes along preferred channels enhances the partial mapping of Siple Coast (14). This extensive the diffusivity of perturbations. It is likely that this patterned network 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, 2011 and expresses a strong coupling between coastal and inland feature of ice sheets. The mapping of Antarctic ice motion flow of ice. Yet patterned enhanced flow initiates at even therefore redefines our view of ice sheet flow dynamics and lower speed, in both west and east Antarctica. Patterned the way ice sheets have been modeled in the past, with enhanced flow here means flow that is not uniform, implications for improving reconstructions of past and dominated by deformational velocity, but includes a major ongoing changes, and especially for modeling the evolution basal slip component which varies spatially. Basal slip may of ice sheet dynamics in a warming climate. be attributed the presence of sub-glacial valleys that channelize thicker ice that generates more friction, more heat References and Notes and 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 sheet deformable or erodible, well lubricated beds not tied in with motion: application to an Antarctic ice stream. Science bed 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. Nature velocity 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, Widepread Queen 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 Antarctic which 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 and and 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 for 15 m/yr (fig. S3). The difference expresses uncertainties in A Thwaites Glacier, and comparison with Pine Island and 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 and quite different from the pattern of large deformational Frozen Ground. In Climate Change 2007: The Physical velocity. A doubling of A does not improve the model fit (fig. Science Basis. Contribution of Working Group I to the S4). In a second simulation, we calculate the deformational Fourth Assessment Report of the Intergovernmental Panel velocity obtained for a rigid bed (21), i.e. where the speed on Climate Change Eds. Solomon, S. et al., Cambridge only depends on the power of the driving stress, but the University Press, Cambridge, United Kingdom and New agreement 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 radar develops 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. 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), 1451-1454 10. 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, 2011 14. 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 a 15. 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 Caltech's Jet Res. 108, 6008 (2003). Propulsion Laboratory under a contract with the National 17. J. L. Bamber et al., East Antarctic ice stream tributary Aeronautics and Space Administration's 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 ice 18. 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 anonymous 19. 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 Material 20. 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 Methods 21. K. Cuffey and W. S. B. Paterson, The physics of glaciers Figs. S1 to S5 (Academic Press, Amsterdam, 4th ed., 2010). Table S1 22. 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.1208336 23. 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 radar 24. 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. lines outline subglacial lakes discussed in the text. Thick black lines along the coast are interferometrically-derived ice sheet grounding lines (23). Fig. 2. (A) Effective strain rate, · , color coded from 3 x 10−4 yr−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) bed elevation (24) with geographic names discussed in the text; (D) surface velocity calculated from internal deformation with a creep parameter, A = 9 x 10−25 s−1 Pa−3. Thick black lines in (B-D) and white lines in (A) delineate major ice divides (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