Style Of Glaciation Research Articles

Major tunnel valleys and sedimentation changes document extensive Early Pleistocene glaciations of the Barents Sea

Sedimentary records of Early Pleistocene (~2.6–0.8 Ma) glaciations are sparse on shelves, yet trough mouth fans on adjacent continental slopes provide a continuous record of ice-sheet and climate development throughout the Quaternary. Here, we interpret high-quality 3D seismic reflection data combined with borehole and chronostratigraphic information from a shelf-slope setting in the southwestern Barents Sea to study meltwater and sediment inputs to the deep ocean, focusing on the onset of Pleistocene glaciations. Sandy deposits were brought to the slopes of the high-latitude Bear Island Fan by a preglacial contourite-turbidite system from ⁓2.6–2.4 Ma. Muddy glacigenic debris flows document the first shelf-edge glaciation at ⁓2.4 Ma. From 1.78–0.78 Ma, muddy turbidity-current- and debris-flow-derived sediments were delivered from shelf to slope via six tunnel valleys measuring up to 12 km in width and 200 m in depth. These tunnel valleys and associated downslope deposits formed during the 41-kyr climate cycles of the Early Pleistocene, and evidence abundant channelized, meltwater discharges from these glaciations. Following the mid-Pleistocene transition to 100-kyr cycles, a change in the style of glaciation is suggested by a change in landform and facies associations consistent with a reduced meltwater contribution. This study shows that the Norwegian-Barents shelf was extensively glaciated in the Early Pleistocene, with a first shelf-edge glaciation from ~2.4 Ma.

Past glaciation of temperate‐continental mountains: A model for a debris‐charged plateau icefield/cirque glacier landsystem in the Southern Carpathians, Romania

AbstractReconstructing the extent, style, timing and drivers of past mountain glaciation is crucial in both understanding past atmospheric circulation and predicting future climate change. Unlike in high‐elevation mountains situated in maritime and continental climates, less is known of past glaciation in mid‐altitude mountains, located in transitional climates, such as the Southern Carpathians of Romania. Despite these mountains harbouring a rich glacial geomorphology, this has never been systematically mapped according to well‐established morphological criteria, nor confidently related to former styles of glaciation. Therefore, filling this gap is important for not only accurately identifying glacial extents but also for establishing past glaciation styles and relating them to past ice dynamics and climate. Here, we devise the first glacial landsystem model for the Southern Carpathians, to enhance our understanding of the glaciation style and dynamics of past mountain glaciers in temperate‐continental climates. We present a geomorphological map, from which we infer the spatial and temporal evolution of glacial, periglacial and paraglacial landforms. We then assess the links between the mountain geomorphology and glaciation style and dynamics to construct a glacial landsystem model for debris‐charged plateau icefields and cirque glaciers in temperate‐continental mountain settings.

Possible new evidence for Mid-Pleistocene glaciation in the Vale of Pickering, North Yorkshire, UK

Whilst the Late Devensian glaciation (MIS2) of the Vale of Pickering is well-documented, earlier glaciations within it are not. A proposed limited glaciation in the Mid-Pleistocene, thought to be of Marine Isotope Stage 8 (MIS) age is not well constrained. This paper aims to obtain preliminary ages for two of the most prominent geomorphic features in the Vale of Pickering to see if they related to pre-Devensian glaciations. New luminescence dating by infra-red stimulation of feldspars from sand accumulations near the summit of Gallows Hill, part of the Wykeham Moraine, and from a section through poorly sorted fluvial sand and gravel on the flanks of the Hutton Buscel Terrace in Yedman Dale gave ages of 176 ± 14 ka and 156 ± 12 ka, respectively. Evidence suggests they represent a glacial incursion (MIS 6) into the Vale of Pickering blocking its eastern end and forming a pre-Devensian Glacial Lake Pickering. Whilst they could be older, this style of glaciation is very different to the limited plateau ice-field proposed for MIS 8 at the western end of the Vale of Pickering. Taken at face value, these preliminary ages suggest that the Vale of Pickering was partially glaciated in MIS 6 as part of a wider ice sheet and contemporary with the Saalian glaciation in Europe.

Pattern, style and timing of British–Irish Ice Sheet retreat: Shetland and northern North Sea sector

ABSTRACTThe offshore sector around Shetland remains one of the least well‐studied parts of the former British–Irish Ice Sheet with several long‐standing scientific issues unresolved. These key issues include (i) the dominance of a locally sourced ‘Shetland ice cap’ vs an invasive Fennoscandian Ice Sheet; (ii) the flow configuration and style of glaciation at the Last Glacial Maximum (i.e. terrestrial vs marine glaciation); (iii) the nature of confluence between the British–Irish and Fennoscandian Ice Sheets; (iv) the cause, style and rate of ice sheet separation; and (v) the wider implications of ice sheet uncoupling on the tempo of subsequent deglaciation. As part of the Britice‐Chrono project, we present new geological (seabed cores), geomorphological, marine geophysical and geochronological data from the northernmost sector of the last British–Irish Ice Sheet (north of 59.5°N) to address these questions. The study area covers ca. 95 000 km2, an area approximately the size of Ireland, and includes the islands of Shetland and the surrounding continental shelf, some of the continental slope, and the western margin of the Norwegian Channel. We collect and analyse data from onshore in Shetland and along key transects offshore, to establish the most coherent picture, so far, of former ice‐sheet deglaciation in this important sector. Alongside new seabed mapping and Quaternary sediment analysis, we use a multi‐proxy suite of new isotopic age assessments, including 32 cosmogenic‐nuclide exposure ages from glacially transported boulders and 35 radiocarbon dates from deglacial marine sediments, to develop a synoptic sector‐wide reconstruction combining strong onshore and offshore geological evidence with Bayesian chronosequence modelling. The results show widespread and significant spatial fluctuations in size, shape and flow configuration of an ice sheet/ice cap centred on, or to the east of, the Orkney–Shetland Platform, between ~30 and ~15 ka BP. At its maximum extent ca. 26–25 ka BP, this ice sheet was coalescent with the Fennoscandian Ice Sheet to the east. Between ~25 and 23 ka BP the ice sheet in this sector underwent a significant size reduction from ca. 85 000 to <50 000 km2, accompanied by several ice‐margin oscillations. Soon after, connection was lost with the Fennoscandian Ice Sheet and a marine corridor opened to the east of Shetland. This triggered initial (and unstable) re‐growth of a glaciologically independent Shetland Ice Cap ca. 21–20 ka BP with a strong east–west asymmetry with respect to topography. Ice mass growth was followed by rapid collapse, from an area of ca. 45 000 km2 to ca. 15 000 km2 between 19 and 18 ka BP, stabilizing at ca. 2000 km2 by ~17 ka BP. Final deglaciation of Shetland occurred ca. 17–15 ka BP, and may have involved one or more subsidiary ice centres on now‐submerged parts of the continental shelf. We suggest that the unusually dynamic behaviour of the northernmost sector of the British–Irish Ice Sheet between 21 and 18 ka BP – characterized by numerous extensive ice sheet/ice mass readvances, rapid loss and flow redistributions – was driven by significant changes in ice mass geometry, ice divide location and calving flux as the glaciologically independent ice cap adjusted to new boundary conditions. We propose that this dynamism was forced to a large degree by internal (glaciological) factors specific to the strongly marine‐influenced Shetland Ice Cap.

Sedimentary processes and seabed morphology of the Southwest Greenland margin

Bathymetric and seismic data from the Southwest Greenland margin have been used to provide an integrated shelf-to-basin overview of the margin architecture between 57° N and 64° N. A variety of glacially formed morphologic features, contour current-related erosion and sedimentary deposits, and evidence of downslope sediment transport via canyons, channels and gullies is found here. The study area is characterized by two major canyons bordering Fylla Bank in the northern part and a narrow shelf and steep slope to the south, where erosion due to strong boundary currents occur down to c. 3000 m water depth. The narrow shelf area appears to be an intra-ice stream area, and numerous channels and gullies on the upper slope point to hyperpycnal melt water release from a stable or retreating wide ice front. Further southward, the shelf is widening and the morphology indicates dominance of former ice stream activity. This difference in glaciation style may reflect the different bedrock types. Sedimentary and morphologic characteristics of the Fylla Bank canyons and some of the slope gullies and channels point to actual cascading of dense winter water or hyperpycnal melt water flow from the shelf. Deep-water channels at the base of the slope evidence transport of Greenland-derived sediment to the central Labrador Sea basin and buried channels further west indicate a former contribution to the Northwest Atlantic Mid-Ocean Channel (NAMOC) system. However, in contrast to widespread turbidite channels on the present seabed bordering NAMOC to the west, east of NAMOC contourite deposits have largely covered the deep-water tributary turbidite channels originating from the Greenland margin.

Topographic, lithologic and glaciation style influences on paraglacial processes in the upper Sil and Luna catchments, Cantabrian Mountains, NW Spain

Deglaciated mountainous areas are usually affected by a great range of paraglacial processes that involve progressively denudation of glacial imprints. This study focuses on paraglacial processes and glacial landform preservation in six zones located in two catchments of the Cantabrian Mountains (NW Spain). These zones are adjacent but display important topographic, lithologic and glaciation style differences. An analysis of how these variables conditioned the intensity and diversity of paraglacial processes in these zones was investigated through a detailed study of their relief, lithology and paleo glacier surface using GIS. A comprehensive cartography of paraglacial landforms was also accomplished, considering landslides, alluvial fans and rock glaciers that show signs of having been conditioned by glaciation. Moraines and debris-mantled slopes were also included in the analysis to examine their grade of preservation depending on the studied variables. Data comparison show that differences in glaciation style influenced paraglacial processes. In the ancient icefield area, relief modification by glacial action was limited and scarce paraglacial processes occur. By contrast, in the upper areas located in the southern ranges where an alpine-style of glaciation was installed, paraglacial processes were very active, generating many rock glaciers. Lithology explains some paraglacial landforms distribution: 1) Landslides are associated with incohesive (sandy and shale) rocks; 2) Rock glaciers mainly occur in quartzite areas; 3) In limestone areas subterranean drainage contributes to glacial landscape preservation, but suffosion dolines affect some glacial deposits. Topography is also a key factor in paraglacial processes: 1) Moraines are well-preserved in gently sloping valleys, but rare in the steeper where they are usually transformed into moraine (or debris)-mantled slopes; 2) Alluvial fans are more frequent in the steeper valleys, but more dissected by postglacial river action. 3) Elevation and orientation show little influence on paraglacial landforms, except in the generation of some periglacial features such as rock glaciers.

Chapter 4 Conceptual glacial ground models: British and Irish case studies

Abstract Former glaciation style is dictated by physiography and ice dynamics and is encoded in glacial landsystem imprints. As a holistic evaluation of sediment–landform associations and their genetic relationships to the processes involved in terrain development, glacial landsystems can facilitate a preliminary prediction of expected subsurface conditions using depositional surface morphology and wider physiographic setting. This chapter provides exemplars representative of the widely variable glacial depositional environments of the British Isles. The glacial deposits of the British Isles are viewed in terms of the dominant landsystems in the Quaternary sediment–landform record and can be grouped under four categories: (1) ice-sheet-related deposits and (2) upland (hard bedrock) glacial deposits, organized according to subglacial footprints, ice-marginal complexes and supraglacial assemblages; (3) glaciofluvial sediment–landforms, organized according to whether they are ice-contact or proglacial in nature; and (4) subaqueous depositional sequences, related to ice-proximal and ice-distal environments. These glacial landsystems are related to the concept of Quaternary domains in an attempt to translate sediment–landform assemblages into a format that has practicability in engineering geology. In this respect the regional distribution of landsystems resonates to some degree with the classification schemes of ‘glaciogenic subgroups’ and ‘till formation domains’. Beyond the glaciogenic subgroup and domain classifications, landsystems further identify localized complexities and ensure a higher level of detail for site investigations where intensive Quaternary geological assessments have yielded a range of data including geomorphological mapping and outcrop investigations with three-dimensional analyses of borehole archives.

Evidence for restricted Loch Lomond Stadial plateau ice in Glen Turret and implications for the age of the Turret Fan

Despite a wealth of research on the patterns and timing of glaciation in Glen Roy over the last 150 years, glacial events within Glen Turret remain heavily debated. These debates centre on the extent and source of Loch Lomond Stadial (Younger Dryas) ice in Glen Turret, and the implications for the age and genesis of the Turret Fan. Here we present details of recent systematic geomorphological mapping of Glen Turret and the neighbouring valleys to the north and east. The geomorphological evidence recorded indicates a plateau icefield style of glaciation centred on the Carn Dearg plateau, of which the Turret Glacier was an outlet. A morphostratigraphical approach is used to identify a relative chronology of glacial events, and suggests that the Turret Fan may have formed prior to the Loch Lomond Stadial. A reconstruction of the Carn Dearg plateau icefield is presented, which was connected to the larger Monadhliath Icefield to the east. Equilibrium line altitudes for the outlet glaciers range from 560±20m to 646±20m and are comparable with those calculated for surrounding regions. This research suggests that the Turret Fan is predominantly an older feature that was deposited by a more extensive plateau ice-sourced Turret Glacier prior to the Loch Lomond Stadial, most likely during or immediately after deglaciation of the last ice sheet.

Heinabergsjökull and Skalafellsjökull, Iceland: active temperate piedmont lobe and outwash head glacial landsystem

A 1:15,000 scale map of the glacial geomorphology and surficial geology of the Heinabergsjökull and Skalafellsjökull glacier forelands in southeast Iceland depicts a landsystem imprint of actively receding temperate glaciers in a mountain terrain with a high glacifluvial sediment yield. The landsystem is characterised by the three diagnostic depositional domains for active temperate glacier systems (marginal morainic; subglacial; glacifluvial/glacilacustrine) together with site-specific landform-sediment assemblages indicative of jökulhlaup drainage from ice-dammed lakes. Other features are overridden moraines and fluted kame terraces, indicative of ice-marginal and glacifluvial palimpsests preserved beneath temperate glacier ice. A significant outwash head in front of Heinabergsjökull records the long-term accumulation of proglacial outwash and was responsible for a radical change in proglacial drainage patterns (topographically unrestricted to restricted) once the glacier snout had receded from the ice-contact face of the landform. The Heinabergsjökull/ Skalafellsjökull foreland constitutes a modern analogue for active temperate piedmont lobes associated with the construction of large outwash heads fed by high glacifluvial sediment yields. This is one of the most common glacial depositional scenarios associated with the more restricted, mountain-based, average glaciation style during a typical cold stage.

Glaciation style and the geomorphological record: evidence for Younger Dryas glaciers in the eastern Lake District, northwest England

The Younger Dryas (c. 12,900–11,700 years ago) in Britain witnessed renewed glaciation, with the readvance of ice masses that had survived the preceding Lateglacial Interstadial as well as the formation of new glaciers. The extents of these former glaciers have been mapped by many workers over the past fifty years, usually as a basis for palaeoclimatic investigations. It has frequently been asserted that the landform record is sufficiently clear to allow accurate ice mass reconstructions at or near maximum extents. Detailed geomorphological mapping in the eastern Lake District in NW England, however, demonstrates that this confidence may not always be warranted. Whereas previous workers have interpreted the well-developed moraines that exist in some locations as evidence for an alpine-style of glaciation, with ice restricted to a small number of valleys, this study shows that the most recent glaciation to affect the area was characterised by: (i) extensive summit icefields, which supplied ice to the surrounding valleys; and (ii) a much greater volume of ice in the valleys than previously thought. The discovery that summit icefields were relatively common at this time is consistent with recent studies elsewhere in the Lake District and beyond. More significant, however, is the recognition that changing glacier–topographic interactions over both space and time appears to have had a profound impact on valley-floor glacial landform development, with the absence of clear moraines not necessarily indicating ice-free conditions at this time. This complicates glacier reconstructions based solely on the geomorphological record. Similar geomorphological complexity may be present in other areas that previously supported summit icefields, and this needs to be taken into account in glacier reconstructions.

Late pleistocene ice fluctuations and glacial geomorphology of the archipiélago de chiloé, southern chile

Most of the last glacial maximum (LGM) glacier record west of the southern Andes (40–55° S) is today submerged under the Pacific Ocean and therefore the Archipiélago de Chiloé (42–43° S) provides an unusual opportunity to study local sediment and landform associations to help understand paleoglacial features of the former Patagonian ice sheet (PIS). In this context, this work presents the first comprehensive glacial geomorphologic mapping of the central region of the Archipiélago de Chiloé, which is located in a transitional geomorphic region between the Chilean Lake District (CLD, 39–41° S, 73° W) and northwest Patagonia (∼43–48° S, 74° W).The Chilotan glacial geomorphology and sediment associations resulted from a warm‐based glacier that characterizes a typical active glacial temperate landsystem, which in central Chiloé combines deposits and landform units originated in subglacial and subaerial environments. Paleoglacial features that occur in central Chiloé are characteristic of an ice‐sheet style of glaciation, which differentiates it from a typical Alpine glacial style defined previously for the CLD. Therefore, the Archipiélago de Chiloé represents a geographical break point where the PIS became the large ice mass that occupied the Patagonian Andes during the last glacial period (Llanquihue Glaciation).A double ice‐contact slope on the east face of the Cordillera de La Costa provides evidence for the most extensive Early Llanquihue glacial advance on Isla Grande de Chiloé. The most prominent LGM advance in the area occurred at 26 000 cal yr BP, coincident with regional stadial conditions, and is defined by a big moraine along the east coast of the island.

Sedimentary evidence against a local ice-cap on the Shetland Isles at the Last Glacial Maximum

Abstract The Shetland Isles occupy a critical position equidistant to major ice-dispersal centres of the British-Irish and Fennoscandian ice sheets during the last glaciation, and have long been the subject of controversy regarding the timing, geometry and style of glaciation on the Shetland Isles. Previous reconstructions of the last glaciation of Shetland, favour development of a local ice-cap, possibly subsequent to early inundation by Fennoscandian ice. However, this model requires complex ice shed migrations and dynamic changes to explain the evidence. Recent studies identify an extensive landform record of NW-flowing ice extending across the region from the North Sea Basin, incompatible with evidence from striae and erratic boulder transport studies. This paper presents glacigenic lithofacies descriptions and structural (clast-fabric, shear-planes) analysis of seventeen sections from around the Shetland archipelago. The sedimentary record of glaciation supports a model of inundation by ice from the North Sea Basin, and includes no sediments representing multiple phases or dynamic local ice-cap glaciation. A preliminary framework which integrates all previous datasets with the results of this study proposes: (1) an early phase of local glaciation that was subsequently overwhelmed by extension of a major ice stream flowing from the North Sea Basin at the LGM; and (2) absence of an extensive local ice-cap formed over Shetland during deglaciation. This study concludes by identifying key areas that require further investigation, most notably more detailed analysis of ice-flow indicators and sediments in the eastern islands of Whalsay, Fetlar and Bressay, and further work to establish the temporal relationships between glacially eroded bedrock and associated subglacial till emplacement.

Glacial geomorphology and paleoglaciation patterns in Shaluli Shan, the southeastern Tibetan Plateau — Evidence for polythermal ice cap glaciation

Glacial geomorphological mapping from satellite imagery and field investigations provide the basis for a reconstruction of the extent and style of glaciation of the Shaluli Shan, a mountainous area on the southeastern Tibetan Plateau. Our studies provide evidence for multiple glaciations, including the formation of regional ice caps and valley glaciers. The low-relief topography within the Shaluli Shan, the Haizishan Plateau, and Xinlong Plateau display zonal distributions of glacial landforms that is similar to those imprinted by Northern Hemisphere ice sheets during the last glacial cycle, indicating the presence of regional, polythermal ice caps. Abundant alpine glacial landforms occur on high mountain ranges. The pattern of glaciated valleys centered on high mountain ranges and ice-scoured low relief granite plateaus with distinctive patterns of glacial lineations indicate a strong topographic control on erosional and depositional patterns by glaciers and ice caps. In contrast to the Shaluli Shan, areas farther north and west on the Tibetan Plateau have not yielded similar landform evidence for regional ice caps with complex thermal basal conditions. Such spatial differences across the Tibetan Plateau are the result of variations in climate and topography that control the extent and style of glaciations and that reinforce the importance of detailed geomorphological mapping for understanding paleoclimate variations and characteristics of former glaciations.

Modeling the flow of glaciers in steep terrains: The integrated second‐order shallow ice approximation (iSOSIA)

The rugged topography of mountain ranges represents a special challenge to computational ice sheet models simulating past or present glaciations. In mountainous regions, the topography steers glaciers through relatively narrow and steep valleys, and as a consequence hereof, the flow rate of alpine‐style glaciers varies significantly at length scales comparable to that of the topography. Localized flow rate undulations generate longitudinal and transverse stress gradients within the ice, which, in turn, are of known importance to the flow itself, whether by internal deformation of the ice or by basal sliding, and to the interaction with topography through glacial erosion. Standard models capable of simulating mountain range–scale glaciations build on the so‐called shallow ice approximation, which, among other parameters, neglects the longitudinal and transverse stress gradients, and therefore fails to capture the full effects of the rugged topography and related feedbacks between erosion by glacial sliding and the extent and style of glaciation. Here we present and test a new depth‐integrated model framework which, on the one hand, takes into account the “higher‐order” effects related to steep and rugged bed topography while it still, on the other hand, provides the computational efficiency needed for three‐dimensional simulations of glaciation and landscape evolution in response to, for example, long‐term climate variations. The model framework (integrated second‐order shallow ice approximation (iSOSIA)) is based on depth integration of the second‐order shallow ice approximation. Although depth integration is not guaranteed to maintain so‐called second‐order accuracy, the results of computational benchmark experiments show that iSOSIA, in spite of its efficient depth‐integrated structure, performs well in situations with steep and rapidly undulating bed topography.

Towards defining the transition in style and timing of Quaternary glaciation between the monsoon-influenced Greater Himalaya and the semi-arid Transhimalaya of Northern India

To help delineate the transition in pattern and timing of glaciation between two contrasting regions, Lahul to the south and Ladakh to the north, moraines in the Puga and Karzok valleys of Sanskrit in the Transhimalaya of northern India were mapped and dated using cosmogenic 10Be. In Lahul, Late Quaternary glaciation was extensive with total valley glacial systems being >100 km in extent, whereas glaciation in Ladakh has been comparatively restricted, with glaciers advancing only ∼15 km from the contemporary glaciers during the last 200 ka. In the Puga valley, glaciers advanced >15 km at ∼129 ka and ∼10 km at ∼46 ka, ∼4.2 ka, and ∼0.6 ka. In the Karzok valley, glaciers advanced ∼1 km at ∼3.6 ka. Boulder exposure ages from a large moraine complex in Karzok indicate a glacial advance at ∼80 ka of ∼4 km from the present ice margin. The oldest moraine in Karzok is ∼311 ka, indicating that glaciers advanced >10 km from the present ice margin during or before marine isotope stage 9. The glacial chronology of the two valleys shows a lack of early Holocene glaciation and generally asynchronous glaciation between them. Moraines in the Puga and Karzok valleys broadly correlate with previous studies in the Zanskar Range but the paucity of data for many of the glacial stages across the Zanskar region makes the correlations tentative. The lack of early Holocene glaciation in the Puga and Karzok valleys is in stark contrast to many regions of the Himalaya, including Lahul, and the restricted glacial extent in Zanskar is more similar to the style of glaciation in Ladakh. The similarity between the glacial records in the Puga and Karzok study areas suggests that the transition to Lahul style glaciation is to the south of the Karzok valley, showing that this geographical transition is abrupt.

Quaternary glaciation of Gurla Mandhata (Naimon’anyi)

The Quaternary glaciation of Gurla Mandhata (Naimona’nyi), an impressive, isolated dome-shaped massif situated in a remote region of southern Tibet, was examined using glacial geomorphic methods and dated using 10Be terrestrial cosmogenic nuclides. The oldest moraines, representing an expanded ice cap that stretched ∼ 5 km into the foreland of the Gurla Mandhata massif, likely formed during marine isotope stage (MIS) 10 or an earlier glacial cycle. Another glacial expansion of coalescing piedmont type occurred during the early part of the last glacial cycle or the penultimate glacial cycle, after which glaciers became restricted to entrenched valley types. Impressive latero-frontal moraines at the mouths of most valleys date to the early part of the last glacial cycle and MIS 3. A succession of six sets of moraines within the valleys and up to the contemporary glaciers show that glaciers advanced during the Lateglacial, Early Holocene, Neoglacial and possibly Little Ice Age. The change of style of glaciation throughout the latter part of the Quaternary, from expanded ice caps to deeply entrenched valley glaciers, might reflect: (1) climatic controls with reduced moisture supply to the region over time; and/or (2) tectonic controls reflecting increase basin subsidence due to detachment faulting and enhanced valley incision.

The stratigraphic signature of the late Cenozoic Antarctic Ice Sheets in the Ross Embayment

A 1284.87-m-long sediment core (AND-1B) from beneath the McMurdo sector of the Ross Ice Shelf provides the most complete single section record to date of fluctuations of the Antarctic Ice Sheets over the last 13 Ma. The core contains a succession of subglacial, glacimarine, and marine sediments that comprise ∼58 depositional sequences of possible orbital-scale duration. These cycles are constrained by a chronology based on biostratigraphic, magnetostratigraphic, and 40 Ar/ 39 Ar isotopic ages. Each sequence represents a record of a grounded ice-sheet advance and retreat cycle over the AND-1B drill site, and all sediments represent subglacial or marine deposystems with no subaerial exposure surfaces or terrestrial deposits. On the basis of characteristic facies within these sequences, and through comparison with sedimentation in modern glacial environments from various climatic and glacial settings, we identify three facies associations or sequence “motifs” that are linked to major changes in ice-sheet volume, glacial thermal regime, and climate. Sequence motif 1 is documented in the late Pleistocene and in the early Late Miocene intervals of AND-1B, and it is dominated by diamictite of subglacial origin overlain by thin mudstones interpreted as ice-shelf deposits. Motif 1 sequences lack evidence of subglacial meltwater and represent glaciation under cold, “polar”-type conditions. Motif 2 sequences were deposited during the Pliocene and early Pleistocene section of AND-1B and are characterized by subglacial diamictite overlain by a relatively thin proglacial-marine succession of mudstone-rich facies deposited during glacial retreat. Glacial minima are represented by diatom-bearing mudstone, and diatomite. Motif 2 represents glacial retreat and advance under a “subpolar” to “polar” style of glaciation that was warmer than present, but that had limited amounts of subglacial meltwater. Sequence motif 3 consists of subglacial diamictite that grades upward into a 5- to 10-m-thick proglacial retreat succession of stratified diamictite, graded conglomerate and sandstone, graded sandstone, and/or rhythmically stratified mudstone. Thick mudstone intervals, rather than diatomite-dominated deposition during glacial minima, suggest increased input of meltwater from nearby terrestrial sources during glacial minima. Motif 3 represents Late Miocene “subpolar”-style glaciation with significant volumes of glacially derived meltwater.

Quaternary glaciation of Muztag Ata and Kongur Shan: Evidence for glacier response to rapid climate changes throughout the Late Glacial and Holocene in westernmost Tibet

The glacial geology of two massifs, Muztag Ata and Kongur Shan, in western Tibet was examined to help define the timing and style of glaciation in the semiarid regions of western Tibet. Remote sensing, geomorphic mapping, and 10Be terrestrial cosmogenic nuclide (TCN) surface-exposure dating of boulders on the moraines and sediment in depth profiles show that glaciers advanced at least 12 times during at least the last two glacial cycles. Over this time, the style of glaciation changed progressively from one that produced ice caps to one that produced less extensive and more deeply entrenched valley glaciers. The timing of the two earliest glaciations is poorly defined, but they likely occurred prior to the penultimate glacial cycle (the Karasu glacial stage) and the early part of the last glacial cycle or during the penultimate glacial cycle (the Subaxh glacial stage). In contrast, the timing of later glacial advances (the Olimde glacial stage) is relatively well defined showing quasiperiodical oscillations on millennial time scales (17.1 ± 0.3 ka, 13.7 ± 0.5 ka, 11.2 ± 0.1 ka, 10.2 ± 0.3 ka, 8.4 ± 0.4 ka, 6.7 ± 0.2 ka, 4.2 ± 0.3 ka, 3.3 ± 0.6 ka, 1.4 ± 0.1 ka, and a few hundred years before the present). These data suggest that since the global Last Glacial Maximum (LGM), the glaciers in western Tibet likely responded to Northern Hemisphere climate oscillations (rapid climate changes), with minor influences from the south Asian monsoon. This study provides the first well-defined glacial geologic evidence to suggest that glaciers in western Tibet respond to rapid climate changes on millennial time scales throughout the Late Glacial and Holocene.

Geomorphology of anomalously high glaciated mountains at the northwestern end of Tibet: Muztag Ata and Kongur Shan

Muztag Ata and Kongur Shan massifs represent a significant area of anomalously high topography at the northwestern end of the Tibetan Plateau, rising to > 7500 m above sea-level (asl) from the plateau that has an average elevation of ~ 3500 m asl. These massifs provide an excellent opportunity to test geomorphic concepts, such as the glacial buzz-saw model. Using remote sensing, digital elevation modeling, field mapping and terrestrial cosmogenic nuclide (TCN) methods, the massifs were examined to determine the relative importance of tectonics and geomorphic processes in shaping the regional landscape and to provide a framework for testing geomorphic models. The gneiss domes that underlie the peaks are the result of exhumation along the Kongur detachment fault that has unroofed the massifs at a rate of between 4–6 km/Ma over the last few million years. This has resulted in rapid uplift and active seismicity, which is exemplified by the numerous fresh fault scarps throughout the region and large historic earthquakes. The geomorphic system is dominated by glaciation and the region contains extensive successions of moraines and paraglacial landforms, including fans, terraces and landslides. Glaciers have oscillated considerably throughout the latter part of the Quaternary, and three major glacier stages are recognized (Karasu [oldest], Olimde and Subaxh [youngest] glacial stage) that include at least 10 smaller glacial advance. The style of glaciation has changed over time from expanded ice caps to piedmont glaciers to valley and cirque glaciers. This possibly reflects a change in climate and/or topographic constraints as the massifs grew and became incised. The topography and glaciers in the region vary across the massifs divided by a broadly N–S trending high ridge and watershed. The western portion, situated upwind (the stoss slopes) of the mid-latitude westerlies, that bring moisture to the region, has gentle high topography and small valley glaciers. In contrast, on the eastern leeward slopes, gradients are higher and long debris-covered valley glaciers are present. The hypsometry of the region indicate two peaks in the distribution frequency of elevation (3600–4100 m and 4400–4800 m asl). These two elevation zones are consistent in space with the former equilibrium-line altitudes during the Olimde and Subaxh glacial stages and suggest that glacial erosion (most effective at the ELA) has helped control topography. This observation supports the glacial buzz-saw hypothesis, which argues that glaciers determine hypsometry by means of rapid surface erosion. Based on TCN methods, basin-wide rates of erosion range from ~ 0.1 to 1.4 km/Ma and are five to ten times lower than the unroofing rate of both massifs. The discrepancy over different time scales suggests that initial unroofing was produced by abrupt tectonic uplift and that the unroofing of the massifs has continued at a slower pace during the Late Quaternary.

Configuration and Dynamics of the Laurentide Ice Sheet During the Late Wisconsin Maximum

Prior to 1943 the Laurentide Ice Sheet was considered to have three major domes centered in Keewatin, Labrador, and Patricia (TYRRELL, 1898 a, b; 1913). FLINT (1943) argued that these centres were of only local and temporary importance and favoured a single-domed ice sheet. Despite the lack of supporting geological evidence, and despite the proposition of a Foxe Dome in the interim (IVES and ANDREWS, 1963), the single-dome concept was not seriously challenged until the late 1970's and, in fact, is still strenuously supported (HUGHES era/., 1977 ; DENTON and HUGHES, 1981). This paper extends and modifies recent conclusions that the Laurentide Ice Sheet had more than one dome at the Late Wisconsin maximum. We propose a model incorporating five domes (M'Clintock, Foxe, Labrador, Hudson, and (?) Caribou) based on the position of ice divides, ice flow patterns, drift composition, late-glacial features, postglacial isostatic recovery and free-air gravity anomalies. Our Labrador and Hudson domes closely correspond to Tyrrell's Labradorean and Patrician ice sheets; our Caribou and M'Clintock domes together with the Franklin Ice Complex over the Queen Elizabeth Islands north of the Laurentide Ice Sheet, correspond to Tyrrell's original Keewatin Ice Sheet. The style of glaciation of the Foxe Basin region was not known to Tyrrell, but our reconstruction of the Foxe Dome is in close agreement with the original proposal of Ives and Andrews. Like Tyrrell, our reconstruction is based on field evidence obtained through extensive mapping; the single dome model continues to be unsupported by geological data.