North American Ice Sheet build-up during the last glacial cycle, 115–21 kyr
Johan KlemånKrister N. JanssonHernán De AngelisArjen P. StroevenClas HättestrandGöran AlmNeil F. Glasser
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Last Glacial Maximum
Wisconsin glaciation
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Abstract Small perturbations in Antarctic environ-mental conditions can culminate in the demise of the Antarctic ice sheet’s western sector. This may have happened during the last interglacial period, and could recur within the next millennium due to atmospheric warming from trace gas and CO 2 increases. In this study, we investigate the importance of sea-level, accumulation rate, and ice influx from the East Antarctic ice sheet in the re-establishment of the West Antarctic ice sheet from a thin cover using a time-dependent numerical ice-shelf model. Our results show that a precursor to the West Antarctic ice sheet can form within 3000 years. Sea-level lowering caused by ice-sheet development in the Northern Hemisphere has the greatest environmental influence. Under favorable conditions, ice grounding occurs over all parts of the West Antarctic ice sheet except up-stream of Thwaites Glacier and in the Ross Sea region.
Antarctic ice sheet
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Ice divide
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Last Glacial Maximum
Chronology
Surface exposure dating
Deglaciation
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Since publication of the paper by Bard et al. (1990) it has been known that GCM studies of the climate of the last glacial maximum (LGM), employed lower boundary conditions appropriate to this time but astronomical parameters of an era 3000 years later. The LGM boundary conditions from CLIMAP were for 18 kyr BP on the 14 C timescale. The GCM simulations employed the insolation regime appropriate to 18 kyr BP on the sidereal timescale whereas the appropriate LGM insolation regime is that of 21 kyr BP. These studies also used the CLIMAP ice sheet reconstruction. However, on the basis of most recent analyses the reconstruction by Tushingham and Peltier (1991) is to be preferred. Hyde et al. (1989) showed that a simple EBM compared favourably with the NCAR CCM, when both were used to simulate the temperature distribution of the LGM. Here we shall employ the same EBM to study the effect on LGM climate of the timing mismatch, and of the different horizontal extents of the different ice sheet reconstructions. In each case the climatic effect is found to be significant. Thus we cannot claim an accurate LGM simulation unless the orbital and terrestrial inputs match to within 1,000 years and unless we employ the best possible ice sheet reconstruction in the analysis.
Last Glacial Maximum
Paleoclimatology
Ice-sheet model
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An ice age model is proposed in which glacial‐interglacial global climatic cycles are controlled by interactions between the cryosphere, hydrosphere, and atmosphere in the Atlantic environment. In the model, climatic change results from instabilities which develop in the snowfields or ice sheets of North America, Europe, and Antarctica. Disintegration of the West Antarctic ice sheet (that portion of the Antarctic ice sheet lying in the western hemisphere) initiates a chain of events which culminates in a global ice age. Ten independent bodies of data can be interpreted as evidence that the West Antarctic ice sheet has been and is disintegrating. The dynamics of the Ross Sea ice drainage system of Antarctica is examined to determine what controls disintegration and recovery of the West Antarctic ice sheet. It is concluded that disintegration is controlled by ice streams which drain the inherently unstable West Antarctic ice sheet and recovery is controlled by outlet glaciers which drain the inherently stable East Antarctic ice sheet. Glacial stability in both cases is determined by the degree of coupling between the ice sheet and its bed. Ice drainage channels develop when this coupling is weakened normal to the margin of an ice sheet and can lead to surges in ice streams or outlet glaciers. Ice shelves develop when this coupling is weakened parallel to the margin of an ice sheet and can lead to a rapid grounding line retreat of floating ice tongues or ice shelves. An inflection maximum on the ice sheet surface migrates inland during a surge and migrates seaward after the surge is spent. A transition zone between the ice sheet and the ice shelf widens during a grounding line retreat inland and narrows during a grounding line advance seaward. Inflection line and grounding line migrations combine to give the ice sheet a concave surface during retreat and a convex surface during advance. A train of surging segments in an ice stream lowers the ice sheet in stages, creating a terraced ice stream surface which causes rapid discontinuous retreats of the grounding line. Rapid glacial recovery following a surge can truncate the advancing ice sheet‐ice shelf boundary. Today at least one West Antarctic ice stream is terraced and at least one East Antarctic outlet glacier is truncated in the Ross Sea ice drainage system. If this condition is general, the West Antarctic ice sheet is disintegrating along the Siple Coast as a result of surging ice streams and is recovering along the Transantarctic Mountains as a result of thickening outlet glaciers. The competition between these processes will provide a critical test of the ice age model, which predicts that progressive disintegration of the West Antarctic ice sheet results in progressive growth of adjacent parts of the East Antarctic ice sheet.
Ice divide
Antarctic ice sheet
Ice-sheet model
Ice core
Fast ice
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Last Glacial Maximum
Ice-sheet model
Proxy (statistics)
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Abstract A model of a non-linearly viscous ice sheet is used to investigate the influence of net mass-balance pattern, basal boundary condition, and subglacial topography on the size and shape of ice sheets. The aim is to enable geological evidence of the extent of former ice sheets to be used as indicators of palaeoclimate. A series of curves are presented showing the relationships between ice-sheet span, net mass balance, and equilibrium-line altitude (ELA) for zero and complete isostatic compensation. These are applicable to a very wide range of basal boundary conditions. The way in which they can be used to reconstruct net mass-balance gradients for former ice sheets is demonstrated. Changes in the basal boundary condition only have a strong influence on glacier span when they occur in the terminal zone. Ice-sheet expansion and contraction is not merely accompanied by changes in snow-line elevation, but also by changes in the net mass-balance gradient. The combinations of these required to cause ice-sheet expansion and contraction are analysed. A non-linearly viscous model for ice suggests that ice-sheet volume changes may not be a simple function of their change in areal extent.
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Greenland ice sheet
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The increased temperatures in the Arctic accelerate the loss of land based ice stored in glaciers. The Greenland Ice Sheet is the largest ice mass in the Northern Hemisphere and holds ~10% of all the freshwater on Earth, equivalent to ~7 metres of global sea level rise. A few decades ago, the mass balance of the Greenland Ice Sheet was poorly known and assumed to have little impact on global sea level rise. The development of regional climate models and remote sensing of the ice sheet during the past decade have revealed a significant mass loss. To monitor how the Greenland Ice Sheet will affect sea levels in the future requires understanding the physical processes that govern its mass balance and movement. In the southeastern and central western regions, mass loss is dominated by the dynamic behaviour of ice streams calving into the ocean. Changes in surface mass balance dominate mass loss from the Greenland Ice Sheet in the central northern, southwestern and northeastern regions. Little is known about what the hydrological system looks like beneath the ice sheet; how well the hydrological system is developed decides the water’s impact on ice movement. In this thesis, I have focused on radar sounding measurements to map the subglacial topography in detail for a land-terminating section of the western Greenland Ice Sheet. This knowledge is a critical prerequisite for any subglacial hydrological modelling. Using the high-resolution ice thickness and bed topography data, I have made the following specific studies: First, I have analysed the geological setting and glaciological history of the region by comparing proglacial and subglacial spectral roughness. Second, I have analysed the subglacial water drainage routing and revealed a potential for subglacial water piracy between adjacent subglacial water catchments with changes in the subglacial water pressure regime. Finally, I have looked in more detail into englacial features that are commonly observed in radar sounding data from western Greenland. In all, the thesis highlights the need not only for accurate high-resolution subglacial digital elevation models, but also for regionally optimised interpolation when conducting detailed hydrological studies of the Greenland Ice Sheet.
Greenland ice sheet
Future sea level
Ice-sheet model
Ice divide
Glacier morphology
Antarctic ice sheet
Glacier mass balance
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Analysis of newly available, high resolution DEM and LiDAR imagery permits detailed geomorphological mapping of glacially streamlined subglacial bedforms (drumlins and megascale glacial lineations; MSGLs) in unprecedented detail over a very large (170,500 km2) area of eastern North America extending from the western end of Lake Erie east through the Lake Ontario basin to the Hudson Valley east of the Adirondack Mountains, and south to the Finger Lakes. Digital imagery and field work identify the beds of eight paleo-ice streams that record a major region-wide ice streaming event within the Laurentide Ice Sheet (LIS) which appears to have commenced after c. 14,400 ybp. A large southwestward-flowing ice stream (the Ontario-Erie Ice Stream; OEIS) sourced from the southern Quebec sector of LIS was initiated along the axis of the Ontario basin coeval with its counterpart to the east, the long-recognized St. Lawrence Ice Stream flowing from the same Quebec source to Gulf of St. Lawrence. Deep ice-frontal lakes at the margin of the retreating ice sheet may have played a role in the onset of fast flow similar to its easternmost marine-terminating margin. It is hypothesized that drumlin bedforms formed under steady state flow regimes are reshaped and dissected into MSGLs by deforming subglacial debris as fast flow propagates upglacier from the ice margin or grounding line. Ice is pulled out of ice sheets by headward-propagating ice streams; the formation of MSGLs from drumlins reflects accompanying subglacial erosion by deforming subglacial sediment to reduce bed relief and basal drag.
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Antarctic ice sheet
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ABSTRACT Study of satellite images from most of the area of the Canadian mainland once covered by the Laurentide ice sheet reveals a complex pattern of superimposed drift lineations. They are believed to have formed subglacially and parallel to ice flow. Aerial photographs reveal patterns of superimposition which permit the sequence of lineation patterns to be identified. The sequential lineation patterns are interpreted as evidence of shifting patterns of flow in an evolving ice sheet. Flow stages are recognised which reflect roughly synchronous integrated patterns of ice sheet flow. Comparison with stratigraphic sections in the Hudson Bay Lowlands suggests that all the principal stages may have formed during the last, Wisconsinan, glacial cycle. Analogy between Flow stage lineation patterns and the form and flow patterns of modern ice sheets permits reconstruction of patterns of ice divides and centres of mass which moved by 1000–2000 km during the glacial period. There is evidence that during the early Wisconsinan, ice sheet formation in Keewatin may have been independent of that in Labrador–Quebec, and that these two ice masses joined to form a major early Wisconsinan ice sheet. Subsequently the western dome decayed whilst the eastern dome remained relatively stable. A western dome then re-formed, and fused with the eastern dome to form the late Wisconsinan ice sheet before final decay. Because of strong coupling between three-dimensional ice sheet geometry and atmospheric circulation, it is suggested that the major changes of geometry must have been associated with large scale atmospheric circulation changes. Lineation patterns suggest very little erosional/depositional activity in ice divide regions, and can be used to reconstruct large scale patterns of erosion/deposition. The sequence of flow stages through time provides an integrative framework allowing sparse stratigraphic data to be used most efficiently in reconstructing ice sheet history in time and space.
Lineation
Wisconsin glaciation
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