Mid-latitude Ice Sheets Research Articles

Tunnel valley formation beneath deglaciating mid-latitude ice sheets: Observations and modelling

The geological record of landforms and sediments produced beneath deglaciating ice sheets offers insights into inaccessible glacial processes. Large subglacial valleys formed by meltwater erosion of sediments (tunnel valleys) are widespread in formerly glaciated regions such as the North Sea. Obtaining a better understanding of these features may help with the parameterisation of basal melt rates and the interplay between basal hydrology and ice dynamics in numerical models of past, present, and future ice-sheet configurations. However, the mechanisms and timescales over which tunnel valleys form remain poorly constrained. Here, we present a series of numerical modelling experiments, informed by new observations from high-resolution 3D seismic data (6.25 m bin size, ∼4 m vertical resolution), which test different hypotheses of tunnel valley formation and calculate subglacial water routing, seasonal water discharges, and the rates at which tunnel valleys are eroded beneath deglaciating ice sheets. Networks of smaller or abandoned channels, pervasive slump deposits, and subglacial landforms are imaged inside and at the base of larger tunnel valleys, indicating that these tunnel valleys were carved through the action of migrating smaller channels within tens of kilometres of the ice margin and were later widened by ice-contact erosion. Our model results imply that the drainage of extensive surface meltwater to the ice-sheet bed is the dominant mechanism responsible for tunnel valley formation; this process can drive rapid incision of networks of regularly spaced subglacial tunnel valleys beneath the fringes of retreating ice sheets within hundreds to thousands of years during deglaciation. Combined, our observations and modelling results identify how tunnel valleys form beneath deglaciating mid-latitude ice sheets and have implications for how the subglacial hydrological systems of contemporary ice sheets may respond to sustained climate warming.

Tunnel valley infill and genesis revealed by high-resolution 3-D seismic data

AbstractLandforms produced beneath former ice sheets offer insights into inaccessible subglacial processes and present analogues for how current ice masses may evolve in a warming climate. Large subglacial channels cut by meltwater erosion (tunnel valleys [TVs]) have the potential to provide valuable empirical constraints for numerical ice-sheet models concerning realistic melt rates, water routing, and the interplay between basal hydrology and ice dynamics. However, the information gleaned from these features has thus far been limited by an inability to adequately resolve their internal structures. We use high-resolution three-dimensional (HR3-D) seismic data (6.25 m bin size, ∼4 m vertical resolution) to analyze the infill of buried TVs in the North Sea. The HR3-D seismic data represent a step-change in our ability to investigate the mechanisms and rates at which TVs are formed and filled. Over 40% of the TVs examined contain buried glacial landforms including eskers, crevasse-squeeze ridges, glacitectonic structures, and kettle holes. As most of these landforms had not previously been detected using conventional 3-D seismic reflection methods, the mechanisms that formed them are currently absent from models of TV genesis. The ability to observe such intricate internal structures opens the possibility of using TVs to reconstruct the hydrological regimes of former mid-latitude ice sheets as analogues for contemporary ones.

Prolonged Monsoonal Moisture Availability Preconditioned Glaciation of the Tibetan Plateau During the Mid‐Pleistocene Transition

AbstractPaleohydrological data comprising pollen assemblages and leaf‐wax hydrogen isotopes (δDwax) from paleolake sediments in the Qaidam Basin (China) provide evidence for a link between increased moisture availability on the Tibetan Plateau and global cooling during the Mid‐Pleistocene Transition. Notably, they document the persistence of humid and cold conditions during Marine Isotope Stages 24–22 (936–866 ka) suggesting that boundary conditions favorable for extended glaciation on the Tibetan Plateau first developed at ~900 ka. Our δDwax results indicate a strong influence of proximal (monsoonal) moisture sources during that glacial, in agreement with the intensification of the interhemispheric moisture transport resulting from Antarctic ice volume increase at ~900 ka. The consistency of our results with other marine and terrestrial climate data sets suggests that extended glaciation on the Tibetan Plateau may have initiated ~500 ka earlier than previously assumed, implying that midlatitude ice sheets actively contributed to global cooling during the Mid‐Pleistocene Transition.

Plucking enhanced beneath ice sheet margins: evidence from the Grampian Mountains, Scotland

ABSTRACTConcentrations of boulders are a common feature of landscapes modified by former mid-latitude ice sheets. In many cases, the origin of the boulders can be traced in the up-ice direction to a cliff only tens to hundreds of metres distant. The implication is that a pulse of plucking and short boulder transport occurred beneath thin ice at the end of the last glacial cycle. Here we use a case study in granite bedrock in the Dee Valley, Scotland, to constrain theory and explore the factors involved in such a late phase of plucking. Plucking is influenced by ice velocity, hydrology, effective ice pressure, the extent of subglacial cavities and bedrock characteristics. The balance between these factors favours block removal beneath thin ice near a glacier margin. At Ripe Hill in the Dee Valley, a mean exposure age of 14.2 ka on blocks supports the view that the boulder train formed at the end of ice sheet glaciation. The late pulse of plucking was further enhanced by ice flowing obliquely across vertical joints and by fluctuations in sub-marginal meltwater conditions. An implication of the study is that there is the potential for a wave of ice-marginal plucking to sweep across a landscape as an ice sheet retreats.

Preservation of Midlatitude Ice Sheets on Mars

AbstractExcess ice with a minimum age of tens of millions of years is widespread in Arcadia Planitia on Mars, and a similar deposit has been found in Utopia Planitia. The conditions that led to the formation and preservation of these midlatitude ice sheets hold clues to past climate and subsurface structure on Mars. We simulate the thermal stability and retreat of buried excess ice sheets over 21 Myr of Martian orbital solutions and find that the ice sheets can be orders of magnitude older than the obliquity cycles that are typically thought to drive midlatitude ice deposition and sublimation. Retreat of this ice in the last 4 Myr could have contributed ~6% of the volume of the north polar layered deposits (NPLD) and more than 10% if the NPLD are older than 4 Myr. Matching the measured dielectric constants of the Arcadia and Utopia Planitia deposits requires ice porosities of ~25–35%. We model geothermally driven vapor migration through porous ice under Martian temperatures and find that Martian firn may be able to maintain porosity for timescales longer than we predict for retreat of the ice.

Theoretical framework and diagnostic criteria for the identification of palaeo-subglacial lakes

The Antarctic Ice Sheet is underlain by numerous subglacial lakes, which comprise a significant and active component of its hydrological network. These lakes are widespread and occur at a range of scales under a variety of conditions. At present much glaciological research is concerned with the role of modern subglacial lake systems in Antarctica. Another approach to the exploration of subglacial lakes involves identification of the geological record of subglacial lakes that once existed beneath former ice sheets. This is challenging, both conceptually, in identifying whether and where subglacial lakes may have formed, and also distinguishing the signature of former subglacial lakes in the geological record. In this work we provide a synthesis of subglacial lake types that have been identified or may theoretically exist beneath contemporary or palaeo-ice sheets. This includes a discussion of the formative mechanisms that could trigger onset of (or drain) subglacial lakes. These concepts provide a framework for discussing the probability that subglacial lakes exist(ed) beneath other (palaeo-)ice sheets. Indeed we conclude that the former mid-latitude ice sheets are likely to have hosted subglacial lakes, although the spatial distribution, frequency and type of lakes may have differed from today's ice sheets and between palaeo-ice sheets. Given this possibility, we propose diagnostic criteria for identifying palaeo-subglacial lakes in the geological record. These criteria are derived from contemporary observations, hydrological theory and process-analogues and provide an observational template for detailed field investigations.

Ice dynamics and sediment movement: last glacial cycle, Clyde basin, Scotland

AbstractThe nature and behaviour of sediment beneath glaciers influences how they flow and respond to changing environmental conditions. The difficulty of accessing the bed of current glaciers is a key constraint to studying the processes involved. This paper explores an alternative approach by relating sediments under the beds of former mid-latitude ice sheets to changing ice behaviour during a glacial cycle. The paper focuses on the partly marine-based Pleistocene British-Irish ice sheet in the Clyde basin, Scotland. A three-dimensional computation of subsurface glacial sediment distribution is derived from 1260 borehole logs. Sediment distribution is linked to an empirically based reconstruction of ice-sheet evolution, permitting identification of distinctive phases of sedimentation. Maximum sediment mobilization and till deposition (~0.04ma_1) occurred during ice advance into the basin from adjacent uplands. Transport distances were generally short. Subglacial processes were influenced locally by the relative stiffness of pre-existing sediments, the permeability of the sub-till lithology, and topography; the resulting mean till thickness is 7.7 m with a high standard deviation of 7.0 m. In places, focused till deposition sealed pre-existing permeable substrates, promoting lower effective pressures. Sediment remobilization by meltwater was a key process as ice margins retreated through the, basin.

Importance of sampling across an assemblage of glacial landforms for interpreting cosmogenic ages of deglaciation

Deglaciation chronologies for some sectors of former ice sheets are relatively poorly constrained because of the paucity of features or materials traditionally used to constrain the timing of deglaciation. In areas without good deglaciation varve chronologies and/or without widespread occurrence of material that indicates the start of earliest organic radiocarbon accumulations suitable for radiocarbon dating, typically only general patterns and chronologies of deglaciation have been deduced. However, mid-latitude ice sheets that had warm-based conditions close to their margins often produced distinctive deglaciation landform assemblages, including eskers, deltas, meltwater channels and aligned lineation systems. Because these features were formed or significantly altered during the last glaciation, boulder or bedrock samples from them have the potential to yield reliable deglaciation ages using terrestrial cosmogenic nuclides (TCN) for exposure age dating. Here we present the results of a methodological study designed to examine the consistency of TCN-based deglaciation ages from a range of deglaciation landforms at a site in northern Norway. The strong coherence between exposure ages across several landforms indicates great potential for using TCN techniques on features such as eskers, deltas and meltwater channels to enhance the temporal resolution of ice-sheet deglaciation chronologies over a range of spatial scales.

The subglacial thermal organisation (STO) of ice sheets

This paper examines ice-sheet wide variations in subglacial thermal regime and ice dynamics using the landform record exposed on the beds of former mid-latitude ice sheets (the Laurentide, Cordilleran, Fennoscandian and British-Irish Ice Sheets). We compare the landform patterns beneath these former ice sheets to the flow organisation beneath parts of the contemporary Antarctic Ice Sheet inferred from RADARSAT-1 Antarctic Mapping Project (RAMP) data. The evidence preserved in the landform record and observed on contemporary ice masses can be grouped into four major ice-dynamical components that collectively define the subglacial thermal organisation (STO) of ice sheets. These ice-dynamical components are frozen-bed patches, ice streams, ice-stream tributaries and lateral shear zones. Frozen-bed patches appear at a wide range of spatial scales, spanning four orders of magnitude. In some areas, frozen-bed zones comprise large proportions of the bed (e.g. near the ice divide in continental areas), whilst in other areas they constitute isolated “islands” in areas dominated by thawed-bed conditions. Ice streams, narrow zones of fast flow in ice sheets that are otherwise dominated by slow sheet flow, are also common features of Quaternary ice sheets. Tributaries to ice streams flow at velocities intermediate between full ice-stream and sheet flow, and may divert ice drainage from one primary ice-stream corridor to an adjacent one. Sharp lateral boundaries between landforms indicate sliding and non-sliding conditions, respectively. These lateral boundaries represent important discontinuities in the glacial landscape and mark the location of shear zones between thawed-bed ice streams and intervening frozen-bed areas. We use the landform evidence in the area around Great Bear Lake, Canada to trace the evolution of an ice-stream web through time, demonstrating that frozen-bed patches are integral components of this complex system. We conclude that frozen-bed patches are important for the stability of ice sheets because they laterally constrain and isolate peripheral drainage basins and their ice streams.

Glacial sedimentary evidence supporting stick-slip basal ice flow

Last Glacial (Late Devensian) subglacial sediments in the southern Sperrin Mountains, north central Ireland, comprise tabular bedrock rafts which are interbedded with diamicton (till) and brecciated bedrock and separated by glaciotectonic shears. Bedrock rafts and diamicton beds alternate laterally and vertically in the profile, suggesting the ice-bed interface was chequered with both high-strength (rock rafts) and low-strength (diamicton) patches during accumulation of the sediment pile. The overall facies arrangement is interpreted qualitatively as reflecting stick-slip basal ice flow and is related to the redistribution of free subglacial meltwater at the ice-bed interface. It is inferred that stick phases occur over bedrock rafts as meltwater migrates to lower-pressure regions, infilling shallow cavities on the surrounding diamicton surface. Slip occurs when porewater pressure increases and the meltwater layer backfills across bedrock raft surfaces, causing ice-bed uncoupling. The presented field evidence suggests a stick slip rather than pervasive subglacial deformation model may be a better approximation for the flow of parts of the Late Devensian mid-latitude ice sheets.

The dimensions and topographic setting of Antarctic subglacial lakes and implications for large-scale water storage beneath continental ice sheets

Research Article| February 01, 1999 The dimensions and topographic setting of Antarctic subglacial lakes and implications for large-scale water storage beneath continental ice sheets Julian A. Dowdeswell; Julian A. Dowdeswell 1Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom Search for other works by this author on: GSW Google Scholar Martin J. Siegert Martin J. Siegert 1Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom Search for other works by this author on: GSW Google Scholar GSA Bulletin (1999) 111 (2): 254–263. https://doi.org/10.1130/0016-7606(1999)111<0254:TDATSO>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Julian A. Dowdeswell, Martin J. Siegert; The dimensions and topographic setting of Antarctic subglacial lakes and implications for large-scale water storage beneath continental ice sheets. GSA Bulletin 1999;; 111 (2): 254–263. doi: https://doi.org/10.1130/0016-7606(1999)111<0254:TDATSO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Radio-echo sounding (RES) at 60 MHz has penetrated to the ice-sheet bed on about 400 000 km of flight track covering more than 50% of the 13.5 x 106 km2 Antarctic ice sheet. About 70 subglacial lakes have been identified by characteristic strong, mirror-like, and very flat RES reflectors. Most are located in the ice-sheet interior, where numerical modeling predicts basal melting, and 33% are within 100 km of the ice crest. Mean ice thickness above the lakes is about 3000 m. About 75% of lakes have radio-echo lengths of <10 km, and only 5% are >30 km. The largest is Vostok Lake, which is 230 km long, 14 000 km2 in area, and has a water volume of about 2000 km3. More than 60% of lakes have maximum local bedrock elevations of <400 m and bed gradients of <0.1 adjacent to their margins. Extrapolating the observed side-slope topography implies water depths from tens to several hundred meters in a few cases. About 20 000 km2 of subglacial lakes are inferred from RES, which can be doubled for the entire ice sheet to yield an estimated total area of about 40 000 km2, excluding Vostok Lake. Assuming average lake depths between 50 and 250 m, and including the 2000 km3 of water stored within Vostok Lake, we calculate an envelope of about 4000 to 12 000 km3 for the likely volume of water stored in modern Antarctic subglacial lakes, equivalent to 10–35 mm of global sea-level rise. In these volume calculations it is assumed that no subglacial lakes the size of Lake Vostok remain to be discovered and that RES length measurements along single transects are representative of overall lake dimensions. Our estimates of the total water volume of Antarctic subglacial lakes, which may be a suitable analogue for full glacial Quaternary ice sheets, do not necessarily conflict with the possible significance of large meltwater floods during the deglaciation of former midlatitude ice sheets. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Large-scale Bedrock Displacement by Cirque Glaciers

Three moraine complexes containing large rafts of fractured-bedrock formed by small cirque glaciers occur in Reindalen, Svalbard. Glaciotectonic landforms cored by rock rafts have been described from mid-latitude ice sheets of Pleistocene age in the past, but we believe this to be the first time large-scale bedrock rafts have been reported from moraines formed by small cirque glaciers. These moraine complexes consist of an imbricate stack of thrust slabs composed of fractured-bedrock and diamicton. Although the exact mechanism are unclear, we suggest that these moraines form by the displacement of subhorizontal strata within the cirque floor to form an initial stack of rock-cored thrust slabs, which subsequently confine the cirque glacier. Subsequent glacier advances, against this “barrier,” establishes a strong compressive regime within the glacier snout, which leads to erosion and moraine development. Estimates of the thickness of the rock rafts are used to derive erosion rates, which are an order of magnitude greater than those previously reported for arctic regions.

The impact of glaciation on the groundwater regime of Northwest Europe

Growth of mid-latitude ice sheets during the glacial cycles of the Quaternary repeatedly reorganises the pattern of groundwater flow on a continent-wide scale. Relatively small scale non-glacial catchments are replaced by catchments which are integrated on the scale of continental ice sheets. Simulations are presented of the response to glaciation of a large part of the western European groundwater system during the last two (Saalian, Weichselian) glacial cycles. A two-dimensional model along an ice sheet flowline from western Sweden to The Netherlands illustrates the impact of glaciation on flow in the vertical plane, and a vertically integrated model illustrates its impact on areal patterns of flow. Hydraulics heads, hydraulic gradients and flow velocities are increased far above their modern values, and relatively shallow aquifers are completely flushed out during glacial periods. There are significant implications for groundwater chemistry and geological structures. Large seepage pressures generated near to ice sheet margins and major impacts on the distribution of effective pressures will produce structures such as hydrofractures, sediment dykes, sediment volcanoes, loading structures etc. The model can be readily applied to hydrocarbon resorvoirs.

The origin of till sequences by subglacial sediment deformation beneath mid-latitude ice sheets

A theory of erosion and deposition as a consequence of subglacial sediment deformation over beds of unlithified sediment is reviewed and applied to large-scale till sequences formed on the southern flanks of the North American and British and European ice sheets during the last glacial cycle. The distribution of till thickness, till lithology in relation to source materials and intra-till erosion surfaces along a flowline in the Michigan lobe of the North American ice sheet are shown to be compatible with the deformational theory but not with other modes of till genesis. It is then demonstrated, in the case of the British ice sheet, how the assumption of a deformational origin for tills can be used to infer time-dependent patterns of ice-sheet dynamic behaviour. By reference to an example from the Netherlands, it is argued that many till sequences interpreted as melt-out tills are more likely to have formed by subglacial sediment deformation.

Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation

AbstractA theory of erosion, transport and deposition of unlithified sediments by glaciers is presented. It predicts the large-scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle, together with the resultant intensity of large-scale lineations (drumlins) which will be incised in the landscape. The theory also predicts the dispersal patterns of subglacial lithologies, together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till. It predicts major erosional discontinuities within tills and the formation of boulder pavements. It suggests that the dominant proportion of the lowland tills produced by Pleistocene mid-latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine-grained.The theory is based on an analysis of glacier-dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition.

The origin of till sequences by subglacial sediment deformation beneath mid-latitude ice sheets

A theory of erosion and deposition as a consequence of subglacial sediment deformation over beds of unlithified sediment is reviewed and applied to large-scale till sequences formed on the southern flanks of the North American and British and European ice sheets during the last glacial cycle. The distribution of till thickness, till lithology in relation to source materials and intra-till erosion surfaces along a flowline in the Michigan lobe of the North American ice sheet are shown to be compatible with the deformational theory but not with other modes of till genesis. It is then demonstrated, in the case of the British ice sheet, how the assumption of a deformational origin for tills can be used to infer time-dependent patterns of ice-sheet dynamic behaviour. By reference to an example from the Netherlands, it is argued that many till sequences interpreted as melt-out tills are more likely to have formed by subglacial sediment deformation.

Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation

AbstractA theory of erosion, transport and deposition of unlithified sediments by glaciers is presented. It predicts the large-scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle, together with the resultant intensity of large-scale lineations (drumlins) which will be incised in the landscape. The theory also predicts the dispersal patterns of subglacial lithologies, together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till. It predicts major erosional discontinuities within tills and the formation of boulder pavements. It suggests that the dominant proportion of the lowland tills produced by Pleistocene mid-latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine-grained.The theory is based on an analysis of glacier-dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition.

Plate motions, crustal and lithospheric mobility, and paleomagnetism: Prospective viewpoint

The shift in viewpoint from a static to a mobilistic solid Earth, as brought about by paleomagnetism, opened many new directions of research that continue to be exciting today and hold prospects for an exciting future. Here five topics of current research on Earth's mobilistic surface are reviewed and their future directions discussed. First, dividing Earth's solid surface into two domains, (1) nearly rigid plate interiors and (2) plate boundaries, some of them very wide, leads to a review and discussion of the kinematics of wide plate boundaries. Many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries, which cover ∼15% of Earth surface, than the few dozen parameters needed to describe the kinematics of plate interiors, which cover ∼85% of Earth's surface. Second, the question of the degree of rigidity of plate interiors is discussed. The integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well‐determined cases less than 2 or 3 mm/yr. Future investigations will no doubt be aimed at narrowing these limits and to expanding the area of Earth's surface over which limits are known. Third, space geodetic data, mainly from very long baseline inter‐ferometry, satellite laser ranging, and the Global Positioning System, have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years. Already, however, significant differences of a few millimeters per year are emerging, and the tectonic and dynamic significance of these differences need to be evaluated. Fourth, despite more than two decades of investigation, the question of how fast hotspots move relative to one another is still a contentious issue. One group of researchers maintains that maximum speeds are 3 mm/yr, whereas another maintains that speeds are 10–20 mm/yr or more. The differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion. Fifth, paleomagnetists have estimated true polar wander over the past 200 Myr by equating it with the apparent polar wander of hotspots. However, the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing. Further progress requires the incorporation of these uncertainties. A possible exception is the apparent polar wander of the hotspots over the past ∼10–20 Myr or less, which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions. The cause of this apparently significant, geologically recent shift of the pole is poorly understood. Compared with the rate and direction of the secular shift of the pole observed this century from the International Latitude Service and for 1976 to 1994 from space geodetic data, the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets, which is the commonly accepted explanation for the shift observed over the past century. I speculate instead that the apparently rapid uplift of the Tibetan Plateau sometime during the past ∼10–20 Myr is at least partly responsible for the geologically recent shift of the pole. Finally, I conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of Earth's solid surface chiefly because of the key assumption of the rigidity of plate interiors, which permits specific predictions to be made. This revolution in quantification still has far to go and holds exciting prospects for future tectonic studies.

Ice Core Facility

To support its program of studying ice cores taken from the Greenland, Antarctic and midlatitude ice sheets, the Division of Polar Programs of the National Science Foundation is soliciting proposals for operating its ice core curatorial facility.NSF's current ice core repository is at the State University of New York at Buffalo, where it has been operated under annual contract since 1975. Beginning in Fiscal Year 1991, whoever manages the NSF facility, now to be called the U.S. Ice Core Repository, will do it under a 5‐year cooperative agreement.

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

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.