Extensive Glaciation Research Articles

The Thurwieser rock avalanche (Italian Alps): Description and dynamic analysis

A rock avalanche of about 3 · 10 6 m 3 detached from the South-East flank of the Punta Thurwieser ridge (Italian Central Alps) on the 18th of September, 2004. Due to the characteristics of the moving material (i.e. rock, debris, ice, snow) and the path of propagation (e.g. presence of a glacier), the mass was able to rapidly cover long distances (about 3000 m). Among the possible causes of the event, attention is mainly focused on permafrost degradation and changes in glacier extension due to global warming. In the present work, the dynamics of the event and the role played by characteristics of the propagation path are numerically investigated with the RASH3D code, based on a continuum mechanics approach. Results obtained with two rheological hypotheses (Frictional and Voellmy) are compared and discussed. In particular, it emerges that 1) the use of a Voellmy rheology with a reduced friction angle on the glacier is necessary to reproduce the dynamics of the Thurwieser event, and 2) a lack in knowledge of geometrical and geomechanical information may lead to wrong interpretations of the event and inaccurate calibration of numerical models.

Late Pleistocene mountain glaciation in Alaska: key chronologies

AbstractMoraine sequences of mountain glaciers can be used to infer spatial and temporal patterns of climate change across the globe. Alaska is an accessible high‐latitude location in the Northern Hemisphere and contains a rich record of alpine glaciation. Here, we highlight the key chronologies from three mountain ranges in Alaska that reveal the timing and spatial extent of Late Pleistocene glaciation, and pay particular attention to age of the penultimate glaciation. The most extensive glacier advance of the last glaciation occurred prior to the last global glacial maximum. Cosmogenic exposure ages from moraine boulders in three sites spanning 800 km indicate that this penultimate advance most likely culminated during marine isotope stage (MIS) 4 or early MIS 3. During MIS 2, more limited glacier expansion generated multiple moraines that span from prior to the global Last Glacial Maximum (LGM) through the Lateglacial period. Glaciers retreated from their terminal positions ca. 27–25 ka in arctic Alaska and ca. 22–19 ka in southern Alaska. Moraines in at least two ranges date to 12–11 ka, indicating a glacial advance during the Younger Dryas period. Reconstructed equilibrium‐line altitudes of both penultimate and MIS 2 glaciers were lowered only 300–600 m – much less than elsewhere in the Americas. Alaska is documented to have been more arid during MIS 2, perhaps due in large part to the exposure of the Bering–Chukchi platform during eustatic sea‐level lowering. The restricted ice extent is also consistent with the output of climate models that simulate a lack of significant summer cooling. Copyright © 2008 John Wiley & Sons, Ltd.

Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO2 levels

It is thought that the Northern Hemisphere experienced only ephemeral glaciations from the Late Eocene to the Early Pliocene epochs (about 38 to 4 million years ago), and that the onset of extensive glaciations did not occur until about 3 million years ago. Several hypotheses have been proposed to explain this increase in Northern Hemisphere glaciation during the Late Pliocene. Here we use a fully coupled atmosphere-ocean general circulation model and an ice-sheet model to assess the impact of the proposed driving mechanisms for glaciation and the influence of orbital variations on the development of the Greenland ice sheet in particular. We find that Greenland glaciation is mainly controlled by a decrease in atmospheric carbon dioxide during the Late Pliocene. By contrast, our model results suggest that climatic shifts associated with the tectonically driven closure of the Panama seaway, with the termination of a permanent El Niño state or with tectonic uplift are not large enough to contribute significantly to the growth of the Greenland ice sheet; moreover, we find that none of these processes acted as a priming mechanism for glacial inception triggered by variations in the Earth's orbit.

A new bathymetric compilation highlighting extensive paleo–ice sheet drainage on the continental shelf, South Georgia, sub‐Antarctica

A grid derived from a new compilation of marine echo‐sounding data sets has allowed us to visualize and map the geomorphology of the entire continental shelf around South Georgia at an unprecedented level of detail. The grid is the first continuous bathymetric data set covering South Georgia to include multibeam swath bathymetry and represent them at a subkilometer resolution. Large and previously undescribed glacially eroded troughs, linked to South Georgia's modern‐day fjords, radiate from the island, marking the former pathways of large outlet glaciers and ice streams. A tectonic or geological influence is apparent for the major troughs, where glaciers have exploited structural weaknesses on the continental block. Bed forms lining the troughs give some first insights into glacial dynamics within the troughs, suggesting arteries of fast flowing ice occupied these topographic depressions in the past and operated over both bedrock and sedimentary substrates. On the outer shelf and within the troughs, large ridges and banks are also common, interpreted as terminal, lateral, and recessional moraines marking former positions of ice sheets on the shelf and their subsequent reorganization during deglaciation. A small trough mouth fan has developed at the mouth of at least one of the cross‐shelf troughs, demonstrating a focused sediment delivery to the margin. Slides and slide scars are also present on parts of the margin, showing that margin stability, perhaps also related to glaciation, has been an important factor in depositional processes on the continental slope. Implications of the new observations are that ice sheets have been more extensive on South Georgia than any previous studies have reported. Their age may date back to late Miocene times, and evolution of the shelf system has probably involved numerous late Cenozoic glacial episodes. However, relatively fresh seafloor geomorphology coupled with evidence from other maritime‐Antarctic islands (Heard Island and Kerguelen Island) indicating extensive glaciation at the Last Glacial Maximum raises the possibility that the extent of sub‐Antarctic glaciation for the Last Glacial period has, until now, been underestimated.

Catching a Cold

CLIMATE SCIENCE Earth has wandered in and out of periods of extensive glaciation for hundreds of millions of years. The oxygen isotopic record of seawater indicates that the current Antarctic ice sheet began to form about 34 million years ago, across the Eocene-Oligocene transition. In contrast,

Three-dimensional radiocarbon modeling: A tool to assess the last glacial ocean circulation and radiocarbon chronologies

PAGES News • Vol.16 • No 2 • April 2008 Sp ec ia l S ec tio n: D at aM od el C om pa ri so n during the LIA. Again, these changes can be attributed to shifts of the SHWs. Additionally, lacustrine investigations (Haberzettl, 2006; Stine and Stine, 1990) indicate lower temperatures and wetter conditions in Patagonia during the LIA. Pollen-based results point to vegetation-type changes from dry to wet species. The latter can also be attributed to shifts in the SHWs (Mayr et al., 2007). Further, the analysis of glaciers and moraines in Patagonia show extensive glaciation during the LIA (Harrison et al., 2006, Koch and Kilian, 2005, Thompson et al., 1986). Therefore, the comparison between proxy-based reconstructions and results from the GCM simulation show that both, model and proxy data indicate a climatically anomalous period between the mid16th and 19th century over southern South America. This suppors the hypothesis that the LIA, as indicated in proxy based and modeled northern hemispheric temperatures, is also reflected in hydrological variables over parts of the southern hemisphere. References Harrison, S., Glasser, N., Winchester, V., Haresign, E., Warren, C. and Jansson, K., 2006: A glacial lake outburst flood associated with recent mountain glacier retreat, Patagonian Andes, The Holocene, 16(4): 611-620. Lara, A. and Villalba, R., 1993: A 3620-Year Temperature Record From Fitzroya cupressoides Tree Ring in Southern South America, Science, 260: 104-1106. Mayr, C. et al., 2007: Holocene variability of the Southern Hemisphere westerlies in Argentiniean Patagonia (52°S), Quaternary Science Reviews, 26: 579-584. von Storch, H., Zorita, E., Jones, J., Dimitriev, Y., Gonzalez-Rouco, F. and Tett, S., 2004: Reconstructing past climate from noisy data, Science, 306: 679-682. Wagner, S., Widmann, M., Jones, J., Haberzettl, T., Lucke, A., Mayr, C., Ohlendorf, C., Schabitz, F. and Zolitschka, B., 2007: Transient simulations, empirical reconstructions and forcing mechanisms for the Mid-Holocene hydrological climate in Southern Patagonia, Climate Dynamics, 29: 333-355.

10.1007/s11720-008-1008-6

The morphological structure of mountains forming under conditions of extensive continental and mountain glaciation is described. They are characterized by a general hummocky profile with alternating dome-shaped summits, steps, valley-shaped depressions, and microrelief represented by roche moutonees, greywethers, and nunataks with isolated peaks in the form of high cones and prisms. The under-ice mountains are represented by a step morphological landscape with glacial steps and leveling surfaces. The step pattern of their morphological landscape is assumed to be related to successive layering of ice according to its movement velocity and the formation of a jet current against the background of general dominant displacements of ice masses. This makes under-ice mountains morphologically similar to medium-height and low mountainous relief of the temperate humid belt in the Northern Hemisphere with dominant landslides on their slopes.

Early and Middle Wisconsinan Environments of Eastern Beringia: Stratigraphic and Paleoecological Implications of the Old Crow Tephra

The widespread Beringian Old Crow tephra occurs in Imuruk Lake (Alaska) core V, above the Blake paleomagnetic event and below Radiocarbon dates, which provide an extrapolated tephra age between 87 000 - 105 000 BP. Exposure KY-11 (Alaska), where the tephra occurs in a dated lacustrine sequence, provides corroboration. Fossil pollen records show that O.C.T. was deposited across northern Beringia on birch-shrub tundra vegetation during an interval of colder climate. A series of climatic oscillations followed tephra deposition. A prolonged period of cold-arid climate ( = marine isotope Stage 4) preceded an interval of warmer than present climate starting ca. 60 000 BP (beginning Stage 3). During this interval, designated the Koy-Yukon thermal event, an exposed Bering land bridge promoted an interglacial type climate that led to significant biotic changes and permafrost degradation. O.C.T. occurs on drift of the Mirror Creek Glaciation which is equivalent to other presumed Early Wisconsinan glaciations in Alaska and Yukon. These glaciations could not have occurred later than marine Stage 5. Stage 4 was fully as cold as Stage 2 (Late Wisconsinan), yet seems not to have been a time of extensive glaciation. The Middle Wisconsinan, 30 000 to more than 80 000 BP, was a nonglacial interval with several climate fluctuations, one of which, the Koy-Yukon thermal event, was warmer than at present.

The peatland/ice age hypothesis revised, adding a possible glacial pulse trigger

Carbon sequestering in peatlands is believed to be a major climate‐regulating mechanism throughout the late Phanerozoic. Since plant life first evolved on land, peatlands have been significant carbon sinks, which could explain significant parts of the large variations in atmospheric carbon dioxide observed in various records. The result is peat in different degrees of metamorphosis, i.e. lignite, hard coal and graphite. During phases of extensive glaciations such as the 330–240 Ma Pangea Ice Age, atmospheric carbon dioxide was critically low. This pattern repeats itself during the Pleistocene when carbon dioxide oscillates with an amplitude of c. 200–300 ppmv. This paper suggests that the ice age cycles during the Pleistocene are generated by the interglacial growth of peatlands and the subsequent sequestering of carbon into this terrestrial pool. The final initiation of ice age pulses towards the end of inter‐glacials, on the other hand, is attributed to the cyclic influx of cosmic dust to the Earth surface, which in turn regulates cloud formation and the incoming shortwave radiation. These shorter cycles have a frequency of c. 1000‐1250 years and might be connected to sunspot or other low frequency solar variations. In a wider context the ice age cycling could be regarded as an interplay between terrestrial life on the high latitudes of the northern hemisphere and the marine subsurface life in the southeast.If the results presented here are correct, the present global warming might just be the early part of a new warm period such as the Bronze Age and the Roman and Medieval Warm periods. This could be caused by entry into another phase of decreasing influx rates of cosmic dust. The increasing concentrations of atmospheric carbon dioxide might have contributed to this warming but, most important of all, it might temporarily have saved us from a new ice age pulse.

Bedrock landsliding, river incision, and transience of geomorphic hillslope‐channel coupling: Evidence from inner gorges in the Swiss Alps

The formation of inner gorges cut into bedrock has been explained as relief rejuvenation by fluvial incision in response to rapid base level drop, repeated glaciations, frequent pore pressure–driven landsliding focused at hillslope toes, or catastrophic outburst flows from natural dam failures. Prominent inner gorges occur in soft Mesozoic Bündner schist and lower Tertiary flysch units of the formerly glaciated Alpenrhein catchment, eastern Swiss Alps. Their channel and hillslope morphologies differ from basins characterized by strong glacial or landslide imprints, while formally resembling the theoretically predicted transient response of detachment‐limited bedrock rivers to rapid base level fall by headward knickpoint migration. Assuming a postglacial onset of fluvial bedrock incision into a Last Glacial Maximum surface in response to base level drop induced by downwasting of trunk valley glaciers requires downcutting rates E > 20 mm yr−1 and requires hillslopes to adjust by frequent landsliding toward development of a threshold state. We test this scenario using data on surface uplift, geomorphometry, geomorphic hillslope coupling, and probabilistic slope stability models. We find that adjustment of inner gorge walls through landsliding is mainly strength limited and structurally controlled, and threshold conditions are restricted to the lower 25% of local hillslope relief. Mass movement processes on upper hillslopes remain largely decoupled from channel incision despite inferred postglacial specific sediment yields of 104 m3 km−2 yr−1 from the studied basins. Conversely, several constraints imposed by fluvial bedrock detachment, postglacial sediment yields, and bedrock landsliding argue for a pre‐Holocene origin for at least some of the inner gorges in the area. This implies partial protection of fluvial gorge topography by subglacial sediment fill during the last (Würm) extensive glaciation and implies that glaciers were insufficient to fully eradicate fluvially sculpted bedrock topography. This leads us to conclude that lithology and major climate oscillations should be considered as further alternative controls on inner gorge formation.

Modeled patterns of Late Pleistocene glacier inception and growth in the Southern and Central Rocky Mountains, USA: sensitivity to climate change and paleoclimatic implications

High-resolution modern climate data for the Southern and Central Rocky Mountains provide a starting point for analysis of climate changes necessary to produce regional Late Pleistocene glaciation. A GIS-based model was used to assess the sensitivity of glaciation in different ranges to climatic change and the combinations of temperature and precipitation change necessary to sustain the last glacial maximum (LGM) extent of glaciation in each range. Extensive glaciation initiates first in the Wind River Range of Wyoming under every climate change scenario tested. In absence of precipitation change a summer temperature depression of 6–8 °C would be necessary to maintain LGM ice extents in Colorado and Wyoming. If precipitation was halved, necessary summer temperature depression would be 8–10 °C; if precipitation was doubled, a depression of 3.5–5.5 °C would suffice. Given model uncertainties, these values may underestimate necessary temperature depression by as much as 2 °C. Under all scenarios tested, LGM glaciation in Utah, particularly the Wasatch Range, requires either more temperature depression for given precipitation change, or more precipitation for a given temperature depression than is required in the Colorado/Wyoming Rockies. A summer temperature depression of 7 °C, which would suffice to sustain LGM ice extent with little change from modern precipitation in the Colorado/Wyoming Rockies, would need to be coupled with a near doubling of precipitation to maintain LGM ice extent in the Wasatch Range. This difference appears to reflect LGM precipitation enhancement in the Wasatch Range, and to a lesser degree the Uinta Mountains, resulting from the presence of Lake Bonneville immediately upwind of these ranges.

A quantification of the glacial imprint on relief development in the French western Alps

The morphology of the western Alps has been strongly influenced by Quaternary glaciations. On the basis of observations of glacial morphology in the Belledonne, Grandes Rousses, Taillefer and Pelvoux-Ecrins Massifs (south-eastern France), we reconstitute the glacial trimline and Equilibrium Line Altitude (ELA) during the most extensive glaciation (MEG). Our best estimate of the MEG ELA is 1800 ± 100m. Using digital elevation models, we compare our glacial reconstruction with the relief structure of nine major catchments draining the massifs. Modal elevations of the largest catchments occur at 2000–2500m and coincide with minima in plots of mean slope angles as a function of elevation. Modal elevations and slope minima occur between the modern and MEG ELAs, confirming a strong glacial imprint on relief. In order to quantify glacial valley carving in the massifs, we isolated high-elevation, low-relief surfaces that form rock shoulders adjacent to the glacial valleys from a Digital Elevation Model and constructed an interpolated surface passing through these. Subtracting the present-day topography from this surface allows us to quantify the maximum glacial valley depths. Maximum valley depths determined in this manner are typically > 1000m, with spatial maxima occurring around the location of the MEG ELA in most valleys. These numbers do not take into account glacial valley widening and local glacial overdeepenings. The also neglect, however, potential pre-glacial fluvial valley incision, which could account for 20–50% of the measured valley depths. In spite of these problems, inferred valley depths are reasonably well correlated with the mean reconstructed ice thickness, and constitute about half of the sub-ridgeline relief of the studied catchments. These results lead us to propose a significant Quaternary increase in the relief of the French western Alps, controlled by climate and associated with the initiation of alpine glaciations. For reasonable values of the effective elastic thickness of the lithosphere, the isostatic response to glacial valley carving reaches values of ∼ 300m across the massifs. This number is insufficient to substantially offset topographic lowering due to regional denudation, and we conclude that the isostatic response to glacial valley carving has not increased peak elevations significantly.

Spatial and temporal patterns of grain size and chemical weathering of the Chinese Red Clay Formation and implications for East Asian monsoon evolution

Palaeoclimatic interpretation of the Red Clay in the Chinese Loess Plateau is particularly important for understanding of the East Asian monsoon evolution from Pliocene to the Quaternary. In this study, a N–S transect involving five loess-Red Clay sections across the Loess Plateau was selected for detailed investigation with analyses of grain size and major element geochemistry, in order to address the spatial pattern. Results indicate that the Red Clay contains large amount of coarse (<6ϕ, i.e. >16 μm) fraction and shows a prominent N–S decreasing gradient much similar to that of the overlying loess, suggesting that the dust transport was dominantly by low-level northerly/northwesterly winds. Compared with overlying loess, the Red Clay is markedly fine and rather constant through all the sections, implying a distant and steady dust-source area about 200 km further north during most of the Pliocene time. Pedogenic leaching intensities inferred from geochemical data display a clear decreasing trend from south to north both for the Red Clay and the loess, suggesting that the modern spatial pattern of the East Asian monsoon regime has been maintained at least for the past 5 Ma. The N–S spatial gradient, however, might be smaller during the Pliocene than the Quaternary period. Temporally, two short periods of climatic deterioration implied by marked grain-size coarsening and low weathering intensity occurred at 5.3 and 3.5 Ma, respectively, which divide the Pliocene into two major stages. The 5.3–3.5 Ma stage is characterized by a steady state climate through the entire region, consistent with other records. In the 3.5–2.7 Ma stage, the Red Clay in the Jingbian section shows a noticeable coarsening in grain size than the previous stage, indicating some southward extension of the deserts, in agreement with mass accumulation rate record in the north Pacific. But the eolian dust deposition remained relatively stable in major part of the Loess Plateau until 2.7 Ma. The Red Clay deposition was then terminated by massive and coarse loess deposition in the entire region, suggesting dramatic intensification of the winter monsoon in response to the onset of extensive glaciation of Northern Hemisphere.

Critical approach to methods of glacier reconstruction in high Asia and discussion of the probability of a Qinghai-Xizang (Tibetan) inland ice

This overview discusses old and new results as to the controversy on the past glacier extension in High Asia, which has been debated for 35 years now. This paper makes an attempt to come closer to a solution. H.v. Wissmann’s interpretation (1959) of a small-scale glaciation contrasts with M. Kuhle’s reconstruction (1974) of a large-scale glaciation with a 2.4 million km2 extended Qinghai-Xizang (Tibetan) inland glaciation and a Himalaya-Karakorum icestream network. Both opinions find support but also contradiction in the International and Chinese literature (Academia Sinica). The solution of this question is of supraregional importance because of the subtropical position of the concerned areas. In case of large albedo-intensive ice surfaces, a global cooling would be the energetical consequence and, furthermore, a breakdown of the summer monsoon. The current and interglacial heat-low above the very effective heating panel of the Qinghai-Xizang (Tibetan) Plateau exceeding 4000 m, which gives rise to this monsoon circulation, would be replaced by the cold-high of an inland ice. In addition, the plate-tectonically created Pleistocene history of the uplift of High Asia — should the occasion arise up to beyond the snowline (ELA) — would attain a paleoclimatically great, perhaps global importance. In case of a heavy superimposed ice load, the question would come up as to the glacio-isostatic interruption of this primary uplift. The production of the loesses sedimentated in NE-China and their very probable glacial genesis as well as an eustatic lowering of the sea-level by 5 to 7 m in the maximum case of glaciation are immediately tied up with the question of glaciation we want to discuss. Not the least, the problems of biotopes of the sanctuary-centres of flora and fauna, i.e., interglacial re-settlement, are also dependent on it. On the basis of this Quaternary-geomorphological-glaciological connection, future contributions are requested on the past glaciation, the current and glacial permafrost table and periglacial development, the history of uplift, and the development of Ice Age lakes and loess, but also on the development of vegetation and fauna in High Asia.

Coupling glacial erosion and tectonics at active orogens: A numerical modeling study

Climate change indirectly alters the distribution of tectonic uplift at active orogens by modifying the action of surface processes, which in turn alters mountain topography. The impact of alpine glaciation on tectonic activity is explored here. The predictions of previous analytical, critical wedge models are compared with the output of a numerical model that explicitly couples rock uplift produced by convergence with surface erosion. Glacial and fluvial and erosive processes are calculated over a two‐dimensional grid, which uses an ice evolution model to calculate the rate of glacial erosion due to ice sliding. Tectonic uplift is determined by a two‐dimensional finite element convergence model. The simulations performed support the predictions of previous one‐dimensional analytical modeling of fluvially dominated orogens: Increasing precipitation decreases orogen width by the one quarter power. The simulations show that glacially dominated landscapes have similar dependencies. The total glacial erosion yield is determined to be proportional to the glacial coverage, sublinearly dependent on the precipitation rate, and linearly dependent on the glacial erosion constant that relates erosion with sliding speed. Climate also determines the distribution of tectonic uplift in the model. If the equilibrium line altitude (ELA) is high and glacial processes are limited to peak regions, the center of the mountain range experiences the highest rate of rock uplift. Extensive glaciation increases rock uplift rates on the flanks of the range. The style of glacial erosion is also important: A simple glacial buzzsaw does not create the same tectonic response as models that physically simulate glacial erosion. The simulations support the idea that a climate cooling of the magnitude recorded in the late Cenozoic has the potential to more than double the rate of rock uplift in appropriate orogens. The implications of the model are discussed for observations from the Olympic Mountains of Washington State, the Southern Alps of New Zealand, the Alaskan coastal range, and the southern Andes.

Palaeolatitude of glacial deposits and palaeogeography of Neoproterozoic ice ages

At the end of the Neoproterozoic, the Earth may have experienced important environmental changes, with a transition between two supercontinents (from Rodinia to Gondwana), extensive glaciations with ice caps reaching the Equator and the beginning of metazoan diversification. In such a context, the palaeomagnetic record can be used to constrain both the palaeogeography and the palaeoclimate (palaeolatitudinal distribution of glacial deposits). Here we present an up-to-date geochronological and palaeomagnetic database for the Neoproterozoic glacial deposits, including poles recently obtained on ‘cap carbonates’ from China, Oman, and Amazonia. The database comprises ten poles (from eight different cratons), obtained directly on the glacial deposits or on the overlying ‘cap carbonate’, and two other palaeolatitudes derived from reference poles coeval to well-dated glacial units in the same craton. The occurrence of glacial deposition at low latitudes (<30 °) is attested by some good-quality poles, two of them well dated at ∼740 and ∼635 Ma. Based on these poles and on reference poles obtained on igneous rocks, tentative palaeogeographic reconstructions for ∼750, ∼620, and ∼580 Ma (ages for which the database has limited but still sufficient entries) were performed in order to investigate the tectonic context within which the glacial events were produced.

Quaternary sedimentation, margin architecture and ocean circulation variability around the Faroe Islands, North Atlantic

The Quaternary development offshore the Faroe Islands has been studied using high-resolution seismic and core data from the R/V DANA 2000 cruise and previous cruises. Several glacial-related features and deposits are observed, all bearing witness to former extensive glaciations of the Faroe area. On the shelves, overlaying a mid-Pleistocene glacial erosional surface, glacial and glacimarine deposits form a sheet geometry interrupted by ridges of sediment that are likely to represent ice-front deposits. An iceberg turbate north of the Faroe Islands provides evidence of large-scale drift of ultra-deep draft (>600 m) icebergs in the Nordic Seas at pre-Weichselian glacial stage(s). Marginal and transverse troughs found on the eastern and western shelf are suggested to have formed during the same glacial period(s) as the iceberg turbate. Iceberg plough-marks and abundant ice rafted material of non-Faroese origin, together with the relict moraine ridges encircling the Faroe Islands at around the 100 and 200 m water depth contours, indicate that the outer shelf was probably ice free during the Weichselian ice age. On the slopes and basinal parts, the formation of fine-grained contourites was favoured during (Weichselian) glacial stages when bottom currents were reduced. Sediment overloading during these glacial stages resulted in repeated slope instability, causing mass failures of the contourite deposits.

First results of cosmogenic dated pre-Last Glaciation erratics from the Montoz area, Jura Mountains, Switzerland

The Jura Mountains are a perialpine chain bending from the SW to the NW of Switzerland. Alpine ice reached only marginal areas of this chain during the Last Glacial Maximum (LGM) but remaining erratic boulders of alpine origin indicate older glaciations of unknown age, tentatively correlated either with the classical Rissian or with the Most Extensive Glaciation. Presented here are the first results of dating these pre-LGM boulders by using cosmogenic radionuclides ( 10Be) and noble gases ( 21Ne). Both data sets are in good agreement within 1 σ error, with radionuclide apparent exposure ages ranging from 60 to 107 ka, and noble gas apparent ages from 73 to 123 ka. Considering the importance of erosion on older boulders, the age of the erratics corresponds most probably to Marine Isotope Stage 6. These results are encouraging for further studies on pre-LGM erratic boulders in the mid-latitudes.

Exposure dating of Late Glacial and pre-LGM moraines in the Cordon de Doña Rosa, Northern/Central Chile (~31° S)

Abstract. Despite the important role of the Central Andes (15–30° S) for climate reconstruction, knowledge about the Quaternary glaciation is very limited due to the scarcity of organic material for radiocarbon dating. We applied 10Be surface exposure dating (SED) on 22 boulders from moraines in the Cordon de Doña Rosa, Northern/Central Chile (~31° S). The results show that several glacial advances in the southern Central Andes occurred during the Late Glacial between ~14.7±1.5 and 11.6±1.2 ka. A much more extensive glaciation is dated to ~32±3 ka, predating the temperature minimum of the global LGM (Last Glacial Maximum: ~20 ka). Reviewing these results in the paleoclimatic context, we conclude that the Late Glacial advances were most likely caused by an intensification of the tropical circulation and a corresponding increase in summer precipitation. High-latitude temperatures minima, e.g. the Younger Dryas (YD) and the Antarctic Cold Reversal (ACR) may have triggered individual advances, but current systematic exposure age uncertainties limit precise correlations. The absence of LGM moraines indicates that moisture advection was too limited to allow significant glacial advances at ~20 ka. The tropical circulation was less intensive despite the maximum in austral summer insolation. Winter precipitation was apparently also insufficient, although pollen and marine studies indicate a northward shift of the westerlies at that time. The dominant pre-LGM glacial advances in Northern/Central Chile at ~32 ka required lower temperatures and increased precipitation than today. We conclude that the westerlies were more intense and/or shifted equatorward, possibly due to increased snow and ice cover at higher southern latitudes coinciding with a minimum of insolation.

The extent and chronology of Cenozoic Global Glaciation

The Quaternary is synonymous with extensive glaciation of Earth's mid- and high-latitudes. Although there were local precursors, significant glaciation began in the latest Eocene (ca 35 Ma) in eastern Antarctica. It was followed by glaciation in mountain areas through the Miocene (in Alaska, Greenland, Iceland and Patagonia), later in the Pliocene (e.g. in the Bolivian Andes and possibly in Tasmania) and in the earliest Pleistocene (e.g. in the Alps, New Zealand, Iceland and Greenland). Today, evidence from both the land and the ocean floors demonstrates that the major continental glaciations, outside the polar regions, rather than occurring throughout the 2.6 Ma of the Quaternary, were markedly restricted to the last 1 Ma—800 ka or less. Marine Isotope Stage (MIS) 22 (ca 870–880 ka) included the first of the ‘major’ worldwide events with substantial ice volumes that typify the Later Pleistocene glaciations (i.e. MIS 16, 12, 10, 6, 4-2).