Sensitivity of the divide position at Siple Dome, West Antarctica, to boundary forcing

A linearized perturbation about a two-dimensional Vialov-Nyc ice-shect profile is used to investigate the sensitivity of the divide position at Siple Dome, West Antarctica, to small changes in the accumulation pattern and in the elevation of its lateral boundaries at the margins of Ice Streams C and D. Relaxation time-scales for the ice-sheet surface and divide position are derived from the perturbation theory. For Siple Dome, these time-scales are short: 450 800 years for surface adjustment, and 200-350 years for divide position adjustment. These short time-scales indicate that Siple Dome responds quickly to forcing at its boundaries. Therefore, the recent migration of the Siple Dome divide (determined from previous work) is probably a response to an ongoing, sustained forcing rather than a response to a long-past climate event such as the transition from the Last Glacial Maximum to the Holocene. Based on our analysis, the inferred rate of migration of the Siple Dome divide could be attained by: (1) a steady increase in the south north spatial accumulation gradient of 0.1-1.5 × 10−9 a −2, or (2) a steady increase (decrease) in elevation of the Siple Dome lateral boundary adjacent to a relict margin of Ice Stream D (Ice Stream C) of 0.005-0.040 m a−1 over the past several thousand years. The required forcing is quite small, and suggests that major changes in the configuration of Ice Streams C and D associated with major changes in the elevation at boundaries of Siple Dome have not occurred over the past several thousand years.

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The elevation history of ice streams and the spatial accumulation pattern along the Siple Coast of West Antarctica inferred from ground-based radar data from three inter-ice-stream ridges

Journal of Glaciology ◽

10.3189/172756501781832197 ◽

2001 ◽

Vol 47 (157) ◽

pp. 303-313 ◽

Cited By ~ 40

Author(s):

N. A. Nereson ◽

C. F. Raymond

Keyword(s):

Spatial Pattern ◽

Internal Layer ◽

Windward Side ◽

West Antarctica ◽

Ice Streams ◽

Amundsen Sea ◽

The Past ◽

Ice Stream ◽

Siple Dome ◽

History Of

AbstractMeasurements of the surface and internal layer geometry from ice-penetrating radar and global positioning system surveys on three inter-ice-stream ridges in West Antarctica (Siple Dome, ridge DE and ridge BC) are examined with ice-flow models to infer (1) the history of the divide position at each site and (2) the spatial pattern of accumulation across the ridges. We find that the divide position is most steady at Siple Dome, somewhat steady at ridge DE and highly variable at ridge BC. Data from Siple Dome and ridge DE show evidence for steady northward motion of the ice divide for the past few thousand years. The layers beneath ridge BC suggest a 5 km northward shift of the divide position within the past several hundred years. Assuming the divide shifts are all due to changing elevation of the bounding ice streams, we infer the relative elevation history for segments of Ice Streams B–E. The northward displacement of the divide for all ridges implies a progressive relative thinning of the ice streams from E to B, with most dramatic recent thinning (100 m in <103 years) of Ice Stream B relative to Ice Stream C. Analysis of the internal layer pattern across the ridges indicates a south–north accumulation gradient with higher accumulation rates on the northern flanks of the ridges in all three cases. The inferred accumulation distribution is nearly uniform on the northern flanks, decreases sharply within a few ice thicknesses across the divides, and then decreases gradually farther to the south. The north/south decrease is strongest for ridge DE and weakest for ridge BC. This spatial pattern and the reduction in gradient strength with distance from the Amundsen Sea is consistent with the hypothesis that storms from the Amundsen Sea carry moisture first south then west over West Antarctica and deposit more snow on the windward side of ridges due to orographic lifting. This pattern has been stable for at least the past several thousand years.

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Migration of the Siple Dome ice divide, West Antarctica

Journal of Glaciology ◽

10.3189/s0022143000002148 ◽

1998 ◽

Vol 44 (148) ◽

pp. 643-652 ◽

Cited By ~ 7

Author(s):

N. A. Nereson ◽

C. F. Raymond ◽

E. D. Waddington ◽

R. W. Jacobel

Keyword(s):

Migration Rate ◽

Internal Layer ◽

Internal Layers ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Flow Models ◽

The Past ◽

Siple Dome ◽

Local Accumulation

AbstractThe non-linearity of the ice-flow law or a local accumulation low over an ice divide can cause isochrones (internal layers) to be shallower under the divide relative to the flanks, forming a “divide bump” in the internal layer pattern. This divide signature is analyzed using ice-flow models and inverse techniques to detect and quantify motion of the Siple Dome ice divide, West Antarctica. The principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide. The calculated migration rate is 0.05-0.50 m a−1 toward Ice Stream D and depends slightly on whether the divide bump is caused by the non-linearity of ice flow or by a local accumulation low. Our calculations also suggest a strong south-north accumulation gradient of 5-10 x 10−6 a−1 in a narrow zone north of the divide. A consequence of divide migration is that pre-Holocene ice is thickest about 0.5 km south of the present divide position. Divide motion indicates that non-steady processes, possibly associated with activity of the bounding ice streams, are affecting the geometry of Siple Dome. The migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths. This suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 103-104 years.

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Elevation of ice-stream margin scars after stagnation

Journal of Glaciology ◽

10.3189/172756500781833241 ◽

2000 ◽

Vol 46 (152) ◽

pp. 111-118 ◽

Cited By ~ 13

Author(s):

N. A. Nereson

Keyword(s):

Finite Difference ◽

Flow Model ◽

Small Scale ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Ice Surface ◽

Ice Stream ◽

Adjustment Time ◽

Siple Dome

AbstractThe evolution of an inter-ice-stream ridge flanked by stagnated ice streams is simulated using a finite-difference, continuity ice-flow model. The model tracks the elevation of small-scale topographic undulations on the ice surface (“scars”) which form at ice-stream margins, and shows that after ice-stream stagnation these surface features are lifted onto the flanks of the evolving ridge before they are carried downslope by ice flow. The model is applied to the stagnant ice streams bounding Siple Dome, West Antarctica: “Siple Ice Stream” (SIS) on the northeast flank near Ice Stream D, and the “Duckfoot” area (DF) on the south flank near Ice Stream C. The volume-adjustment time-scale corresponding to the evolution of Siple Dome and these stagnant ice-stream areas is 1500–2000 years. The present geometry and elevation of the scar features, in addition to measurements of the present mass flux across the ridge, are used to estimate stagnation ages for SIS and DF. These measurements suggest that both SIS and DF stagnated 200–500 years ago.

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Migration of the Siple Dome ice divide, West Antarctica

Journal of Glaciology ◽

10.1017/s0022143000002148 ◽

1998 ◽

Vol 44 (148) ◽

pp. 643-652 ◽

Cited By ~ 53

Author(s):

N. A. Nereson ◽

C. F. Raymond ◽

E. D. Waddington ◽

R. W. Jacobel

Keyword(s):

Migration Rate ◽

Internal Layer ◽

Internal Layers ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Flow Models ◽

The Past ◽

Siple Dome ◽

Local Accumulation

AbstractThe non-linearity of the ice-flow law or a local accumulation low over an ice divide can cause isochrones (internal layers) to be shallower under the divide relative to the flanks, forming a “divide bump” in the internal layer pattern. This divide signature is analyzed using ice-flow models and inverse techniques to detect and quantify motion of the Siple Dome ice divide, West Antarctica. The principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide. The calculated migration rate is 0.05-0.50 m a−1toward Ice Stream D and depends slightly on whether the divide bump is caused by the non-linearity of ice flow or by a local accumulation low. Our calculations also suggest a strong south-north accumulation gradient of 5-10 x 10−6a−1in a narrow zone north of the divide. A consequence of divide migration is that pre-Holocene ice is thickest about 0.5 km south of the present divide position. Divide motion indicates that non-steady processes, possibly associated with activity of the bounding ice streams, are affecting the geometry of Siple Dome. The migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths. This suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 103-104years.

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The accumulation pattern across Siple Dome, West Antarctica, inferred from radar-detected internal layers

Journal of Glaciology ◽

10.3189/172756500781833449 ◽

2000 ◽

Vol 46 (152) ◽

pp. 75-87 ◽

Cited By ~ 63

Author(s):

N. A. Nereson ◽

C. F. Raymond ◽

R.W. Jacobel ◽

E. D. Waddington

Keyword(s):

Accumulation Rate ◽

Small Time ◽

Internal Layer ◽

Internal Layers ◽

West Antarctica ◽

Ice Streams ◽

Accumulation Pattern ◽

The North ◽

Siple Dome ◽

Radio Echo Sounding

AbstractThe spatial distribution of accumulation across Siple Dome, West Antarctica, is determined from analysis of the shapes of internal layers detected by radio-echo sounding (RES) measurements. A range of assumed accumulation patterns is used in an ice-flow model to calculate a set of internal layer patterns. Inverse techniques are used to determine which assumed accumulation pattern produces a calculated internal layer pattern that best matches the shape of internal layers from RES measurements. All of the observed internal layer shapes at Siple Dome can be matched using a spatially asymmetric accumulation pattern which has been steady over time. Relative to the divide, the best-fitting accumulation pattern predicts 40% less accumulation 30 km from the divide on the south flank of Siple Dome and 15–40% more accumulation 30 km from the divide on the north flank. The data also allow the possibility for a small time variation of the pattern north of the divide. The mismatch between the calculated and the observed layer shapes is slightly reduced when the accumulation rate north of the divide is higher in the past (> 5kyr BP) than at present. Sensitivity tests show that the predicted change in the spatial accumulation pattern required to cause the slight Siple Dome divide migration (inferred from previous studies) would be detectable in the internal layer pattern if it persisted for > 2 kyr. Our analysis reveals no evidence that such a change has occurred, and the possible change in accumulation distribution allowed by the data is in the opposite sense. Therefore, it is unlikely that the Siple Dome divide migration has been caused by a temporal change in the spatial pattern of accumulation. This conclusion suggests the migration may be caused by elevation changes in Ice Streams C and D at the boundaries of Siple Dome.

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Bed properties of Siple Dome and adjacent ice streams, West Antarctica, inferred from radio-echo sounding measurements

Journal of Glaciology ◽

10.3189/172756500781833467 ◽

2000 ◽

Vol 46 (152) ◽

pp. 88-94 ◽

Cited By ~ 96

Author(s):

A. M. Gades ◽

C. F. Raymond ◽

H. Conway ◽

R. W. Jagobel

Keyword(s):

Basal Layer ◽

Water Layer ◽

Thickness Variation ◽

West Antarctica ◽

Ice Streams ◽

A Value ◽

Ice Stream ◽

Siple Dome ◽

Radio Echo Sounding ◽

Echo Sounding

AbstractWe have used ground-based radio-echo sounding (RES) profiles to reveal the spatial distribution of basal and internal ice properties across Siple Dome, West Antarctica, and under the dormant ice streams on its flanks. The RES-detected bed-reflection power, corrected for the effects of instrumentation and ice-thickness variation, is nearly constant across Siple Dome at a value suggesting spatially homogeneous basal properties of ice frozen to bedrock. Till, if present under the dome, must be thin (<0.1 m). The high basal reflectivity measured under now dormant “Siple Ice Stream” (SIS) and Ice Stream C suggests that they are underlain by either a thin (<0.05 m) water layer or a thick (>1 m) thawed or frozen till layer. The evidence that the dormant SIS is not frozen directly to underlying bedrock (but is separated by a water or till layer) is a further indication that it was once an active ice stream, and suggests that streaming motion may have ceased before the basal layer was frozen. The absence of a thick till layer beneath Siple Dome is consistent with its apparent stability as an inter-ice-stream ridge in the past and may suggest that it will remain as a stable limitation of ice-stream width in the future.

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Of isbræ and ice streams

Annals of Glaciology ◽

10.3189/172756403781816347 ◽

2003 ◽

Vol 36 ◽

pp. 66-72 ◽

Cited By ~ 88

Author(s):

Martin Truffer ◽

Keith A. Echelmeyer

Keyword(s):

Ice Sheet ◽

Shear Zones ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Bed Topography ◽

Ice Stream ◽

Fast Ice ◽

Fast Flow ◽

End Members

AbstractFast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.

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Deformation in Rutford Ice Stream, West Antarctica: measuring shear-wave anisotropy from icequakes

Annals of Glaciology ◽

10.3189/2013aog64a033 ◽

2013 ◽

Vol 54 (64) ◽

pp. 105-114 ◽

Cited By ~ 14

Author(s):

S.R. Harland ◽

J.-M. Kendall ◽

G.W. Stuart ◽

G.E. Lloyd ◽

A.F. Baird ◽

...

Keyword(s):

Shear Wave ◽

Flow Direction ◽

Transversely Isotropic ◽

Shear Wave Splitting ◽

Ice Crystal ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Ice Stream ◽

Wave Splitting

Abstract Ice streams provide major drainage pathways for the Antarctic ice sheet. The stress distribution and style of flow in such ice streams produce elastic and rheological anisotropy, which informs ice-flow modelling as to how ice masses respond to external changes such as global warming. Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shear-wave splitting from three-component icequake seismograms to characterize ice deformation via crystal-preferred orientation. Over 110 high-quality measurements are made on 41 events recorded at five stations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this is the first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitude of the splitting ranges from 2 to 80 ms and suggests a maximum of 6% shear-wave splitting. The fast shear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider three mechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdle model (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model). Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy – an HTI component is needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment of cracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination of mechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropy in an ice stream, as predicted by ice fabric models.

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Surface Features of Ice Stream B, Marie Byrd Land, West Antarctica

Annals of Glaciology ◽

10.3189/s0260305500001385 ◽

1986 ◽

Vol 8 ◽

pp. 168-170 ◽

Cited By ~ 14

Author(s):

P.L. Vornberger ◽

I.M. Whillans

Keyword(s):

Aerial Photographs ◽

Relative Importance ◽

West Antarctica ◽

Ice Streams ◽

Longitudinal Compression ◽

West Antarctic Ice Sheet ◽

Surface Features ◽

West Antarctic ◽

Ice Stream ◽

Ice Stream B

Aerial photographs have been obtained of Ice Stream B, one of the active ice streams draining the West Antarctic Ice Sheet. A sketch map made from these photographs shows two tributaries. The margin of the active ice is marked by curved crevasses and intense crevassing occurs just inward of them. Transverse crevasses dominate the center of the ice streams and diagonal types appear at the lower end. A “suture zone” originates at the tributary convergence and longitudinal surface ridges occur at the downglacier end. The causes of these surface features are discussed and the relative importance of four stresses in resisting the driving stress is assessed. We conclude that basal drag may be important, longitudinal compression is probably important at the lower end, and longitudinal tension is probably most important near the head of the ice stream. Side drag leads to shearing at the margins, but does not restrain much of the ice stream.

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Is Ice-Stream Evolution Revealed By Satellite Imagery?

Annals of Glaciology ◽

10.3189/s0260305500008740 ◽

1990 ◽

Vol 14 ◽

pp. 273-277 ◽

Cited By ~ 5

Author(s):

S.N. Stephenson ◽

R.A. Bindschadler

Keyword(s):

Stream Flow ◽

Temporal Evolution ◽

Thematic Mapper ◽

Landsat Thematic Mapper ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Ice Shelves ◽

Ice Stream ◽

Grounding Line

Ten Landsat Thematic Mapper images together show Ice Streams E, D and most of Ice Stream C on Siple Coast, West Antarctica. The images are interpreted to reveal aspects of both spatial and temporal evolution of the ice streams. Onset of ice-stream flow appears to occur at distributed sites within the ice-stream catchment, and the apparent enhanced flow continues in channels until they join, forming the main ice stream. Most crevassing on these ice streams is associated with features of horizontal dimensions between 5 and 20 km. We suggest these features are caused by bed structures which may be an important source of restraint to ice flow, similar to ice rumples on ice shelves. A pattern of features near the grounding line of the now-stagnant Ice Stream C are interpreted as having formed because there was a period of reduced flux before the ice stream stopped.

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