Eleven paleogeographic maps and a summary ice retreat map outline the history of advance, retreat, and readvances of the Laurentide Ice Sheet along with associated changes in proglacial drainage and relative sea level oscillations for Late Wisconsinan and Holocene times. The text outlines pertinent chronological control and discusses the paleoglaciology of the ice sheet, with attention to location and migration of ice divides, their attendant domes and saddles, and to ice streams, ice shelves, and mechanisms of deglaciation. At 18 ka the ice sheet consisted of 3 sectors with an interlocked system of ice divides joined at intersector saddles. A throughgoing superdivide is recognized and named the Trans Laurentide Ice Divide. The ice sheet retreated slowly from 18 to 13 ka, mainly along the west and south margins, but still held a near maximum configuration at 13 ka. A regional change in flow pattern over the Prairies just before 14 ka is thought to represent a large reduction in ice volume, but not in extent, and likely was triggered by a switch from nondeforming to deforming bed conditions. Retreat between 13 and 8 ka was vastly more rapid in the west than in the east, which resulted in eastward migration of the divide system of Keewatin Ice but relatively static divides of Labrador and Foxe Ice. By 10 ka the Trans Laurentide Ice Divide had been fragmented as Hudson Ice became increasingly autonomous. By 8 ka Hudson Ice had disappeared, little ice was left in Keewatin, but Foxe Ice still held its near maximum configuration and Labrador Ice was still larger than Foxe Ice. Repeated surging along aquatic margins and calving back of margins thinned by surging probably was the most important mechanism of deglaciation of Keewatin and Hudson Ice. The core of Foxe Ice disintegrated at 7 ka but retreat and readvance of Foxe Ice remnants continued throughout the Holocene.

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Late Wisconsinan and Holocene History of the Laurentide Ice Sheet

Experiments beneath Breidamerkurjokull in Iceland have led to development of flow laws for the subglacial till, relating strain rate to shear stress and effective pressure and assuming either Bingham fluid or nonlinearly viscous fluid behavior. Water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment, which inhibits sliding at the ice/sediment interface. Where water pressures are equal or near to ice pressures, infiltration does not occur and sliding may result. A one-dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation. This predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice/bed interface or unstable states where zero or negative effective pressures are predicted. Time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds. It is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed. It is suggested that unstable deformation at zero or negative effective stress leads to “piping” in subglacial sediments at the glacier terminus and the growth of sediment-floored, subglacial tunnels. Their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation. Where they discharge large water volumes, subglacial sediments flow laterally toward them producing “tunnel valleys.” This sediment is then removed by water flowing along the axial tunnel. Tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas.

Sediment deformation beneath glaciers: Rheology and geological consequences

Ice-rafted debris in sediment cores from the North Atlantic suggests that the Laurentide ice sheet (LIS) periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7,000 years. Here, I propose that Heinrich events (i.e., what these episodes are called) were caused by free oscillations in the flow of the Laurentide ice sheet which arose because the floor of Hudson Bay and Hudson Strait is covered with soft, unconsolidated sediment that forms a slippery lubricant when thawed. The proposed Heinrich event cycle has two phases. The growth phase occurs when the sediment is frozen and the LIS is stranded (immobile) on a rigid bed. The volume of the LIS slowly grows during this phase at a rate dictated by snow accumulation. The purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway (i.e., an ice stream such as those which occur in West Antarctica today) developes through Hudson Strait. The volume of the LIS rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the Labrador Sea. The periodicity T=π/κ(−kθsl2G∼)2≈7000 years of the proposed Heinrich event cycle is a function of the thermal conductivity and diffusivity of ice, k and κ, respectively, the atmospheric sea level temperature θsl (in degrees Celsius), and the excess geothermal heat flux defined by G∼=G−kΓ where Γ is the atmospheric lapse rate, and G is the geothermal heat flux. Agreement between the predicted T and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism I propose. An alternative mechanism in which Heinrich events are forced by periodic variations in external climate is implausible, because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet.

Binge/purge oscillations of the Laurentide Ice Sheet as a cause of the North Atlantic's Heinrich events

We present a new time-slice reconstruction of the Eurasian ice sheets (British–Irish, Svalbard–Barents–Kara Seas and Scandinavian) documenting the spatial evolution of these interconnected ice sheets every 1000 years from 25 to 10 ka, and at four selected time periods back to 40 ka. The time-slice maps of ice-sheet extent are based on a new Geographical Information System (GIS) database, where we have collected published numerical dates constraining the timing of ice-sheet advance and retreat, and additionally geomorphological and geological evidence contained within the existing literature. We integrate all uncertainty estimates into three ice-margin lines for each time-slice; a most-credible line, derived from our assessment of all available evidence, with bounding maximum and minimum limits allowed by existing data. This approach was motivated by the demands of glaciological, isostatic and climate modelling and to clearly display limitations in knowledge. The timing of advance and retreat were both remarkably spatially variable across the ice-sheet area. According to our compilation the westernmost limit along the British–Irish and Norwegian continental shelf was reached up to 7000 years earlier (at c. 27–26 ka) than the eastern limit on the Russian Plain (at c. 20–19 ka). The Eurasian ice sheet complex as a whole attained its maximum extent (5.5 Mkm2) and volume (~24 m Sea Level Equivalent) at c. 21 ka. Our continental-scale approach highlights instances of conflicting evidence and gaps in the ice-sheet chronology where uncertainties remain large and should be a focus for future research. Largest uncertainties coincide with locations presently below sea level and where contradicting evidence exists. This first version of the database and time-slices (DATED-1) has a census date of 1 January 2013 and both are available to download via the Bjerknes Climate Data Centre and PANGAEA (www.bcdc.no; http://doi.pangaea.de/10.1594/PANGAEA.848117).

The last Eurasian ice sheets - a chronological database and time-slice reconstruction, DATED-1

The North Atlantic Ocean during the last deglaciation

Although theories of glacier movement generally assume that glaciers flow over rigid rock beds, there are many places where glaciers rest on beds of deformable sediment, and the great Pleistocene ice sheets which extended from time to time over much of Northern Europe and North America were largely underlain by such beds. Observations show that a large proportion of the forward move-ment of a glacier lying on such a bed may be contributed by deformation of the bed rather than the glacier. A theory is developed in which the glacier surface profile is related to the hydraulic and strength properties of potentially deformable bed materials. If these have a high hydraulic transmissibility, melt water is readily discharged sub-glacially, the bed is stable, and the profile is a normal parabolic one, governed by the rheological properties of ice. If bed transmiss-ibility is low, water pressures build up, the bed begins to deform, and a lower equilibrium profile will develop, so that in an extreme case the glacier approximates to a thin flat sheet, similar to an ice shelf.-from Authors

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Stability of Temperate Ice Caps and Ice Sheets Resting on Beds of Deformable Sediment

Satellite imagery and data from ground surveys are used to reconstruct the integrated pattern of the principal longitudinal and transverse features produced on a continent-wide scale by the last ice sheets in Europe and North America. From modern analogues, it is argued that most longitudinal features reflect flow in the outer zone of the ice sheet, and that most major transverse features reflect relatively stable ice-sheet margins. These principles are tested and, using them alone, detailed patterns for the decay of the last ice sheets in North America, Europe and the British Isles are produced, and periods during which they attained near steady-states identified. These patterns can be calibrated by dated sequences to yield deglaciation isochrons. Application of glaciological models to these geological reconstructions generates detailed prediction of net ablation for the period of ice-sheet decay and, by using evidence of last glaciation stratigraphy, models of the dynamic behaviour of the ice sheets throughout the last glacial period are constructed. These enable volumetric changes, oceanic isotopic changes and erratic dispersal pathways to be reconstructed. Erratic dispersal patterns give a good indication of the long-term distribution of centres of ice sheet mass. Discrepancies between predicted and empirical oceanic isotopic records indicate ways in which the conventional continental timescale of glacial change must be altered to fit the better-dated deep ocean record. In addition discrepancies between predicted and empirical erratic dispersal patterns suggest that conventional views of ice-sheet behaviour based on high latitude models may be inappropriate to the dynamically more active mid-latitude ice sheets based in large part on deformable sediment beds.

A. S. Jones

Papers

Stability of Temperate Ice Caps and Ice Sheets Resting on Beds of Deformable Sediment

Glacial geology and glaciology of the last mid-latitude ice sheets