Article

Evidence for rapid ice flow and proglacial lake evolution around the central Strait of Magellan region, southernmost Patagonia

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Abstract

This paper presents a detailed palaeoglaciological reconstruction of ice sheet dynamics in the Seno Skyring, Seno Otway and Strait of Magellan region of the former Patagonian Ice Sheet, with a particular focus on previously hypothesised zones of rapid ice flow and the evolution of proglacial lakes. Geomorphological mapping from a combination of satellite imagery and oblique and vertical aerial photographs reveals a variety of glacial landforms that are grouped into several discrete flow-sets and associated ice margin positions. The most distinct features are represented by flow-sets of highly elongate streamlined glacial lineations on both sides of the Strait of Magellan. Based on the shape and dimensions of the flow-sets and their abrupt lateral margins, a transverse and longitudinal variation in glacial lineation length and elongation ratio, and the reported presence of a potentially deformable bed and thrust moraines, the flow-sets are interpreted as zones of rapid ice flow within the Otway and Magellan lobes. We hypothesise that this provides evidence for contemporaneous surge-like advances within the lobes, which may explain the asymmetry in the lobate margin positions on either side of the strait. The mechanisms that initiated rapid flow are unclear, but are likely to have been influenced by internal factors such as a change in thermal/hydrological conditions at the bed. The topography of the region suggests ice-dammed lakes would have formed as the ice lobes retreated. The westernmost of the former lakes, Lake Skyring, is delimited by a series of palaeo-shorelines surrounding the present-day lake Laguna Blanca and we reconstruct lake evolution based on manipulation of a digital elevation model. The size and orientation of meltwater channels and a large outwash plain indicate that Lake Skyring drained eastwards towards the Strait of Magellan, probably quite rapidly. We conclude that the potential for quasi-independent surge-like behaviour within adjacent lobes raises the possibility that, during climate-driven ice expansion, some advances in this region may have been partly controlled by secondary internal feedback mechanisms. Copyright © 2012 John Wiley & Sons, Ltd.

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... Well-preserved glacial geomorphology relating to the former Patagonian Ice Sheet provides a record of the fluctuations of its margins throughout the Quaternary (Clapperton, 1993;. This record can also be used to reconstruct ice-sheet dynamics Lovell et al., 2012) and may be supplemented with chronological information to constrain how the ice sheet changed over time because of climatic forcing (e.g. McCulloch et al., 2005b;Douglass et al., 2006;Kaplan et al., 2008a,b;Hein et al., 2010). ...
... The locations of the former ice lobes are marked by prominent straits and sounds, which were established by one or more major glacial events during the Quaternary (Rabassa, 2008;Kaplan et al., 2009). Glacial geomorphology relating to the former ice lobes was first described in detail by Caldenius (1932) and has since been updated, and sometimes reinterpreted, by numerous workers (Meglioli, 1992;Clapperton et al., 1995;Benn and Clapperton, 2000a,b;Rabassa et al., 2000;Bentley et al., 2005;McCulloch et al., 2005b;Lovell et al., 2011Lovell et al., , 2012Darvill et al., 2014Darvill et al., , 2015a. Darvill et al. (2014) showing the locations of figures and locations mentioned in the text. ...
... Relatively little attention has been given to the nature of ice dynamics recorded by the glacial geomorphology, with the exception of localized studies on the gLGM or post-gLGM limits. Previous work has highlighted factors affecting ice-lobe dynamics and rates of advance and retreat, such as the influence of pro-glacial lakes (Porter et al., 1992;Lovell et al., 2012), subglacial thermal regime (Benn and Clapperton, 2000a,b;Bentley et al., 2005) and evidence for rapid ice flow over a soft-sediment bed (Clapperton et al., 1995;Lovell et al., 2012). As such, there is a need for a regional glacial history that incorporates geomorphological evidence for ice dynamics and reassesses previously published chronological data. ...
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Reconstructions of former ice masses from glacial geomorphology help to constrain the nature and timing of glaciation in relation to climatic forcing. This paper presents a new reconstruction of the glacial history of five ice lobes in southernmost South America: the Bahía Inútil − San Sebastián, Magellan, Otway, Skyring and Río Gallegos ice lobes. We use previous geomorphological mapping of glacial landforms to reconstruct former glacial limits and proglacial lakes, demarcate flow-sets from the distribution of glacial lineations, and evaluate glacial landsystem signatures and their palaeoglaciological implications. Evidence suggests that the ice lobes predominantly reflect active temperate glacial landsystems, which may have switched to polythermal systems when periods of cold-based ice developed ephemerally. This complex landsystem signature implies that the ice lobes were sensitive to regional climate variability, with active re-advances during overall retreat of the ice margins. There is also evidence for periods of fast ice flow and possible surge-like activity in the region, followed by the rapid retreat or even collapse of some of the ice lobes in association with proglacial lakes. Constraining our new reconstruction with published chronological information suggests that at least some of the ice lobes advanced before the global Last Glacial Maximum (gLGM: ca. 26.5–19 ka) during the last glacial cycle. Our new reconstruction demonstrates a more complex picture of ice dynamics than has previously been portrayed, and one in which the advance and retreat of the ice lobes was likely to have been primarily driven by changes in climate. As such, ice advances before the gLGM in the southernmost part of the Patagonian Ice Sheet are likely to indicate a wider climatic forcing at this time.
... Studies in Patagonia on glacier dynamics inferred from the glacial geomorphological record, have been limited to a few selected sites (e.g. Lovell et al., 2012). Such studies across the Southern Alps of New Zealand include Porter (1975), Suggate (1990), and Bacon et al. (2001). ...
... We suggest that the swath of attenuated bedforms that occur in isolation northwest of Lake Alexandrina (Fig. 3C, D), as well as their parallel concordance and the abrupt lateral margins of this zone, are similar to those in areas of former rapidly flowing ice (cf. Clark, 1999, 2001;Evans et al., 2008Evans et al., , 2014; Lovell et al., 2012), in this case within an active temperate piedmont glacier lobe (cf. Boulton, 1987;Hart, 1999;Evans and Twigg, 2002;Kovanen and Slaymaker, 2004;Evans et al., 2018a). ...
... There is spatial variation in lineation elongation (cf. Lovell et al., 2012) with lineations clearly showing a lateral transition in both length and elongation ratio, with the longest and most elongate lineations focused within a narrow zone (Fig. 3C, D). The orientation of lineations exhibits a high degree of parallel concordance and there are no examples of crosscutting lineations, suggesting they formed relatively rapidly (isochronously), with minimal modification or re-moulding during subsequent deglaciation (cf. ...
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Quaternary glaciations have created impressive landform assemblages that can be used to understand palaeo-glacier extent, character and behaviour, and hence past global and local glacier forcings. However, in the southern hemisphere and especially in New Zealand, the Quaternary glacial landform record is relatively poorly investigated with regard to glaciological properties. In this study, a 1 m digital elevation model (DEM) was generated from airborne LiDAR data and supplemented with aerial imagery and field observations to analyse the exceptionally well-preserved glacial geomorphology surrounding Lake Tekapo, New Zealand. We describe a rich suite of Last Glacial Maximum (LGM) and recessional ice-marginal, subglacial, supraglacial, glaciofluvial and glaciolacustrine landform assemblages. These represent two landsystems comprising i) fluted till surfaces with low-relief push moraine ridges; and ii) crevasse-squeeze ridges, ‘zig-zag’ eskers and attenuated lineations. The former landsystem records the behaviour of an active temperate glacier and the latter landsystem, which is superimposed upon and inset within the former, strongly suggests intermittent surge phases. The two landsystem signatures indicate a sequential change in ice-marginal dynamics during recession that was likely to have been partially non-climatically driven. Overall, we present the first evidence of surge-type glacier behaviour in New Zealand.
... The second glacial landform-sediment association, related to the outer moraine complexes, is interpreted to have been formed during surge-like events of the Chagan Uzun glacier (terminology after Lovell et al. (2012)). The identified combination of wide hummocky belts of controlled-moraine exhibiting double lobate/ interdigitating terminus outlines (CUMCs 1 to 3), and hillside-scale folding accompanied by extensive faulting (observed within CUMC 3), strongly indicates a highly dynamic glacier behaviour (e.g., Evans and Rea, 2003;Schomacker and Kjaer, 2007;Grant et al., 2009;Lovell, 2014). ...
... Furthermore, the presence of soft sediments, such as those encountered by the former Chagan Uzun Glacier in its foreland, has been identified as a factor favouring ice flow acceleration, especially when saturated, due to enhanced bed deformation and basal sliding processes (e.g. Anandakrishnan et al., 1998;Jiskoot et al., 2000;Lovell et al., 2012). Similar highly deformed sediments with folds several tens of meters high, and with entire moraine belts formed from the pre-existing soft sediments, thus have been observed for surging glaciers in Iceland (Benediktsson et al., 2008) and Svalbard Lovell, 2014) and for fast flowing outlet glaciers in Patagonia (Benn and Clapperton, 2000;Lovell et al., 2012). ...
... Anandakrishnan et al., 1998;Jiskoot et al., 2000;Lovell et al., 2012). Similar highly deformed sediments with folds several tens of meters high, and with entire moraine belts formed from the pre-existing soft sediments, thus have been observed for surging glaciers in Iceland (Benediktsson et al., 2008) and Svalbard Lovell, 2014) and for fast flowing outlet glaciers in Patagonia (Benn and Clapperton, 2000;Lovell et al., 2012). ...
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The Southern part of the Russian Altai Mountains is recognized for its evidence of catastrophic glacial lake outbursts. However, little is known about the late Pleistocene paleoglacial history, despite the interest in such reconstructions for constraining paleoclimate. In this study, we present a detailed paleoglaciological reconstruction of the Chagan Uzun Valley, in the Russian Altai Mountains, combining for the first time detailed geomorphological mapping, sedimentological logging, and in situ cosmogenic 10Be and 26Al surface exposure dating of glacially-transported boulders. The Chagan Uzun Valley exhibits the most impressive glacial landforms of this sector of the Altai, with extensive lobate moraine belts deposited in the intramontane Chuja Basin, reflecting a series of pronounced former glacial advances. Observations of “hillside-scale” folding and extensive faulting of pre-existing soft sediments within the outer moraine belts, together with the geomorphology, strongly indicate that these moraine belts were formed during surge-like events. Identification of surge-related features is essential for paleoclimate inference because these features correspond to a glacier system that is not in equilibrium with the contemporary climate, but instead largely influenced by various internal and external factors. Therefore, no strict relationship can be established between climatic variables and the pronounced distal glacial extent observed in the Chagan Uzun Valley/Chuja basin. In contrast, the inner (up-valley) glacial landforms of the Chagan Uzun valley were likely deposited during retreat of temperate valley glaciers, close to equilibrium with climate, and so most probably triggered by a general warming. Cosmogenic ages associated with the outermost, innermost, and intermediate moraines all indicate deposition times clustered around 19 ka. However, the actual deposition time of the outermost moraine may slightly predate the 10Be ages due to shielding caused by subsequent lake water coverage. This chronology indicates a Marine Isotope Stage (MIS) 2 last maximum extent of the Chagan Uzun Glacier, and an onset of the deglaciation around 19 ka. This is consistent with other regional paleoclimate proxy records and with the Northern Hemisphere glaciation chronology. Finally, this study also highlights the highly dynamic environment in this area, with complex interactions between glacial events and the formation and drainage of lakes.
... During glacial maxima, the icefields coalesced to form a single large ice mass extending westwards to the continental shelf edge and eastwards into steppe-like plains (Caldenius, 1932;Clapperton and Clapperton, 1993;Coronato and Rabassa, 2011;Glasser et al., 2011b;Hein et al., 2010;McCulloch et al., 2000;Mercer, 1968Mercer, , 1976. During ice recession, a series of large proglacial lakes formed along the eastern ice margin, dammed between the ice sheet and higher ground or moraines (Bourgois et al., 2016;Caldenius, 1932;Glasser et al., 2016b;García et al., 2014;Lovell et al., 2012;Martinod et al., 2016;McCulloch and Bentley, 1998;Sagredo et al., 2011;Thorndycraft et al., 2019a;Turner et al., 2005). On the eastern side of the Andes, terminal moraines from these maxima often form the present-day continental watershed divide, following lake drainage events causing Atlantic to Pacific drainage reversals and underfit valleys in the sectors 42 -49°S and 51 -53°S (Thorndycraft et al., 2019a). ...
... Our compilation builds on the "Glacial Map of South America" with updates from around the former PIS (e.g., Ackert et al., 2008;Bendle et al., 2017b;Coronato et al., 2009;Darvill et al., 2014Darvill et al., , 2015aDe Muro et al., 2018; Input data Fig. 5. Davies, et al. Earth-Science Reviews 204 (2020) 103152 García, 2012;García et al., 2014;Izagirre et al., 2018;Lovell et al., 2011Lovell et al., , 2012Waldmann et al., 2010;Davies et al., 2018;Thorndycraft et al., 2019b) and original mapping by the authors of this study of key landforms in data-sparse regions. At the scale of the PIS, glacial landforms were mapped from remotely sensed images, particularly satellite imagery (for details on overall methods, see: Boulton and Clark, 1990;Chandler et al., 2018;Clark, 1997;Clark et al., 2012Clark et al., , 2018Jansson and Glasser, 2005). ...
... In the lowlands, a land-terminating glacial landsystem, with an assemblage of nested lateral-frontal moraine arcs, outwash plains, meltwater channels, sedimentary glacial lineations (including drumlins) and inset hummocky terrain (e.g. Bendle et al., 2017b;Coronato et al., 2009;Darvill et al., 2014;Ercolano et al., 2004;Lovell et al., 2012;Martin et al., 2019;Ponce et al., 2013); iii. A lowlands glaciolacustrine landsystem, with landforms such as deltas and shorelines, and localised ice-contact glaciofluvial features (e.g. ...
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We present PATICE, a GIS database of Patagonian glacial geomorphology and recalibrated chronological data. PATICE includes 58,823 landforms and 1,669 geochronological ages, and extends from 38°S to 55°S in southern South America. We use these data to generate new empirical reconstructions of the Patagonian Ice Sheet (PIS) and subsequent ice masses and ice-dammed palaeolakes at 35 ka, 30 ka, 25 ka, 20 ka, 15 ka, 13 ka (synchronous with the Antarctic Cold Reversal), 10 ka, 5 ka, 0.2 ka and 2011 AD. At 35 ka, the PIS covered of 492.6 x10³ km², had a sea level equivalent of ~1,496 mm, was 350 km wide and 2090 km long, and was grounded on the Pacific continental shelf edge. Outlet glacier lobes remained topographically confined and the largest generated the suites of subglacial streamlined bedforms characteristic of ice streams. The PIS reached its maximum extent by 33 – 28 ka from 38°S to 48°S, and earlier, around 47 ka from 48°S southwards. Net retreat from maximum positions began by 25 ka, with ice-marginal stabilisation then at 21 – 18 ka, which was then followed by rapid irreversible deglaciation. By 15 ka, the PIS had separated into disparate ice masses, draining into large ice-dammed lakes along the eastern margin, which strongly influenced rates of recession. Glacial readvances or stabilisations occurred at least at 14 – 13 ka, 11 ka, 6 – 5 ka, 2 – 1 ka, and 0.5 – 0.2 ka. We suggest that 20th century glacial recession (% a⁻¹) is occurring faster than at any time documented during the Holocene.
... In the Patagonia Fuegian region, marine terrace deposits have historically attracted the attention of many researchers (Aguirre, Richiano, & Sirch, 2006;Andersson, 1906;Auer, 1974;Bujalesky, & Gonzales Bonorino,1990;Codignotto, 1984;Feruglio, 1933;Gordillo, Coronato, & Rabassa, 1990;Halle, 1910;Markgraf, 1980;Mörner, 1987;Porter, Clapperton, & Sudgen, 1992;Porter, Stuiver, & Heusser, 1984;Rabassa et al.1992;Rabassa, Serrat, Marti, & Coronato, 1988;Urien, 1966), however, the geomorphology of the area described in this paper is still largely unknown, particularly when discussing the contiguous spatial distribution of four orders of Holocene terraces of marine and transitional origin. Most of the previous studies described, sometimes with great accuracy, the glacial evolution of this region and associated geomorphological features (Benn & Clapperton, 2000;Bentley, Sugden, Hulton, & McCulloch, 2005;Darvill, Stokes, Bentley, & Lovell, 2014;Darvill, Stokes, Bentley, Evans, & Lovell, 2016;Glasser & Jansson, 2008;Lovell, Stokes, & Bentley, 2011;Lovell, Stokes, Bentley, & Benn, 2012;McCulloch & Davies, 2001) with some reference to Holocene paleo shoreline and other coastal landforms (Isla & Bujalesky, 2008;Rostami, Peltier, & Mangini, 2000). However, the previous literature did not complete coastal geomorphological mapping of the area discussed herein but focused on single outcrops relevant to the interpretation of regional glacial evolution (Benn & Clapperton, 2000;Schellmann & Radtke, 2003). ...
... Coastal terrace sequences are mostly developed above glacial features showing clear angular discordance and sharp erosive truncation. These features were produced by processes ranging from glacial and lacustrine to littoral (marine), typically related to glacial-interglacial phases (Benn & Clapperton, 2000;Darvill et al., 2014Darvill et al., , 2016Lovell et al., 2011Lovell et al., , 2012McCulloch & Davies, 2001). ...
Article
This paper presents a coastal geomorphology map of some of the Chilean region of the Straits of Magellan, and is based on a combined geomorphological and sedimentological approach applicable to a larger section of the coast. The mapped area is located in Tierra del Fuego between the Segunda Angostura of the Straits of Magellan and Bahía Inútil. A detailed geomorphological map was compiled at a scale of 1:50,000, describing a sequence of 4 marine and transitional (glacio-lacustrine to marine) terrace orders and raised marine shorelines found between 0 and + 20 m. These features are mainly linked to glacio-eustasy and secondarily to Holocene neo-tectonics. This research derives from the interpretation of aerial photographs and remote sensing imagery along with geomorphological-stratigraphic field surveys and geochemical data. The Main Map illustrates an area with dominant palaeo-glacial and glacio-lacustrine morphology with extensive lacustrine deposits and well-developed lake systems characterising the Porvenir region.
... Here, the ice lobes have been forced to flow through depressions and up reverse slopes, depositing latero-frontal moraines on the edge of the higher ground on several occasions (Fig. 12B). The depressions were subsequently occupied by ice-marginal lakes as ice retreated from the higher ground (Lovell et al., 2012) and are likely to have been infilled during and following deglaciation (see Section 5.2.), which may explain why they are not particularly noticeable in the modern landscape. ...
... As noted in Section 2.2, effort is often made to link past glacier dimensions to climate (e.g., Dyke and Savelle, 2000;Kerschner and Ivy-Ochs, 2008;Barr and Clark, 2012b). However, the moraine record upon which many reconstructions are based is not a reflection of palaeoclimate alone (Spedding and Evans, 2002;Swift et al., 2002;Kirkbride and Winkler, 2012;Lovell et al., 2012). In particular, topography exerts a significant control on moraine formation, preservation, and ease of identification (Warren and Hulton, 1990;Warren, 1991;Kaplan et al., 2009;Anderson et al., 2012). ...
Article
Ice-marginal moraines are often used to reconstruct the dimensions of former ice masses,which are then used as proxies for palaeoclimate. This approach relies on the assumption that the distribution ofmoraines in the modern landscape is an accurate reflection of former ice margin positions during climatically controlled periods of ice margin stability. However, the validity of this assumption is open to question, as a number of additional, nonclimatic factors are known to influence moraine distribution. This review considers the role played by topography in this process, with specific focus on moraine formation, preservation, and ease of identification (topoclimatic controls are not considered). Published literature indicates that the importance of topography in regulating moraine distribution varies spatially, temporally, and as a function of the ice mass type responsible for moraine deposition. In particular, in the case of ice sheets and ice caps (>1000 km2), one potentially important topographic control on where in a landscape moraines are deposited is erosional feedback, whereby subglacial erosion causes ice masses to become less extensive over successive glacial cycles. For the marine-terminating outlets of such ice masses, fjord geometry also exerts a strong control on where moraines are deposited, promoting their deposition in proximity to valley narrowings, bends, bifurcations, where basins are shallow, and/or in the vicinity of topographic bumps. Moraines formed at the margins of ice sheets and ice caps are likely to be large and readily identifiable in the modern landscape. In the case of icefields and valley glaciers (10–1000 km2), erosional feedback may well play some role in regulating where moraines are deposited, but other factors, including variations in accumulation area topography and the propensity for moraines to form at topographic pinning points, are also likely to be important. This is particularly relevant where land-terminating glaciers extend into piedmont zones (unconfined plains, adjacent to mountain ranges) where large and readily identifiable moraines can be deposited. In the case of cirque glaciers (b10 km2), erosional feedback is less important, but factors such as topographic controls on the accumulation of redistributed snow and ice and the availability of surface debris, regulate glacier dimensions and thereby determine where moraines are deposited. In such cases, moraines are likely to be small and particularly susceptible to post-depositional modification, sometimes making them difficult to identify in the modern landscape. Based on this review,we suggest that, despite often being difficult to identify, quantify, and mitigate, topographic controls on moraine distribution should be explicitly considered when reconstructing the dimensions of palaeoglaciers and that moraines should be judiciously chosen before being used as indirect proxies for palaeoclimate (i.e., palaeoclimatic inferences should only be drawn from moraines when topographic controls on moraine distribution are considered insignificant).
... Lovell, Stokes, and Bentley (2011) produced a map of the glacial geomorphology within the Otway and Skyring lobes region (area shown in Figure 3). It was the most detailed map produced of the area and was subsequently used to interpret the glacial dynamics of the ice lobes, including surge-like advances and the development of pro-glacial lakes during retreat (Lovell, Stokes, Bentley, & Benn, 2012). ...
... (1) The glacial geomorphology is dominated by landforms associated with meltwater (channels and outwash plains) and possibly indicative of deposition in a slow-moving or stagnant outlet (kettle-kame topography and hummocky terrain). This contrasts with discrete 516 C.M. Darvill et al. areas of large, well-defined lineations within the inner reaches of the Magellan and Otway lobes, hypothesised to result from more active ice-streaming (Lovell et al., 2012). (2) Moraines to the north differ markedly from those to the south. ...
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This paper presents a glacial geomorphological map of the landforms created by five large ice lobes that extended eastwards from the southernmost reaches of the Patagonian Ice Sheet during the Quaternary period. The study is focussed on Tierra del Fuego, but also updates previous mapping of the Skyring and Otway lobes, and the resulting level of detail and extent is a significant advance on previous work in the region. The map has been created as the necessary precursor for an improved understanding of the glacial history of the region, and to underpin a programme of dating glacial limits in the region. It was produced using Landsat ETM+ and ASTER satellite imagery and vertical aerial photography, supplemented by Google Earth (TM) imagery and field-checking. Eleven landform types were mapped: moraine ridges, subdued moraine topography, kettle-kame topography, glacial lineations, irregular and regular hummocky terrain, irregular dissected ridges, eskers, meltwater channels, former shorelines and outwash plains. The map reveals three important characteristics of the glacial geomorphology. First, the geomorphic systems are largely dominated by landforms associated with meltwater (channels, outwash plains and kettle-kame topography). Second, there is a difference in the nature of landforms associated with the northern three ice lobes, where limits are generally marked by numerous clear moraine ridges, compared to those to the south, where hummocky terrain and drift limits prevail. Finally, cross-cutting landforms offer evidence of multiple advances, in places, which has implications for the timing of limit deposition, and thus for the design and interpretation of a dating programme.
... Several glacial geomorphological maps have been published to investigate the nature and chronology of past glacier fluctuations e.g. Benn and Clapperton (2000); ; Clapperton et al. (1995); Darvill, Stokes, Bentley, Evans, and Lovell (2017); Darvill, Stokes, Bentley, and Lovell (2014); Glasser and Jansson (2008) ;Lovell, Stokes, and Bentley (2011);Lovell, Stokes, Bentley, and Benn (2012); McCulloch and Bentley (1998); Rabassa, Coronato, and Martinez (2011). ...
... During Glacial stages B and C, the glacial lobes retreated reaching the western shore of Seno Otway and Seno Skiring after 19 ka (Darvill et al., 2017;Kilian et al., 2013;Lovell et al., 2011;. During the ice margin retreat from the distinct lobate positions, ice and topography would have blocked the lower ground contained within the former Skyring and Otway lobes, impeding drainage and damming proglacial lakes (Lovell et al., 2012). Consequently, proglacial lakes would have formed in front of both Skyring and Otway lobes likely draining initially through northern spillways towards the Atlantic. ...
Article
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This paper illustrates a detailed geomorphological map (scale 1:50,000) of the marine and transitional terraces (glacio-lacustrine to marine) and raised shorelines linked to Holocene glacio-eustasy and neo-tectonics in the northern area of the Brunswick Penìnsula (Chilean region of the Strait of Magellan). The mapped area is located in Tierra del Fuego between the Segunda Angostura and Seno Otway. This map is the result of geomorphological field survey data integrated with the interpretation of aerial photographs and remote sensing imagery. The survey has allowed the mapping of a sequence of terraces and raised shorelines to be completed. The sequence mainly consists of four orders of marine and glacio-lacustrine terraced deposits, with elevations ranging from 25 to 1 m above mean sea level. The map also presents other landforms and deposits, with their formation linked to littoral, fluvial, glacial and aeolian processes.
... Better understanding of the links between surging, debris entrainment and geomorphology is important as it can help to provide an insight into processes acting at the bed [Christoffersen et al., 2005; Larsen et al., 2006]. It also refines the application of surge landsystem models for the interpretation of proglacial geomorphology of glaciers with no known surge history [e.g., Croot, 1988; Ottesen and Dowdeswell, 2006; Grant et al., 2009] or in palaeo-environments [e.g., Evans et al., 1999; Lovell et al., 2012] and it ensures that glacier-climate inferences are not drawn erroneously from surge moraine systems. ...
Article
The englacial entrainment of basal debris during surges presents an opportunity to investigate processes acting at the glacier bed. The subsequent melt-out of debris-rich englacial structures during the quiescent phase produces geometrical ridge networks on glacier forelands that are diagnostic of surge activity. We investigate the link between debris entrainment and proglacial geomorphology by analyzing basal ice, englacial structures, and ridge networks exposed at the margins of Tunabreen, a tidewater surge-type glacier in Svalbard. The basal ice facies display clear evidence for brittle and ductile tectonic deformation, resulting in overall thickening of the basal ice sequence. The formation of debris-poor dispersed facies ice is the result of strain-induced metamorphism of meteoric ice near the bed. Debris-rich englacial structures display a variety of characteristics and morphologies and are interpreted to represent the incorporation and elevation of subglacial till via the squeezing of till into basal crevasses and hydrofracture exploitation of thrust faults, reoriented crevasse squeezes, and preexisting fractures. These structures are observed to melt-out and form embryonic geometrical ridge networks at the base of a terrestrially grounded ice cliff. Ridge networks are also located at the terrestrial margins of Tunabreen, neighboring Von Postbreen, and in a submarine position within Tempelfjorden. Analysis of network characteristics allows these ridges to be linked to different formational mechanisms of their parent debris-rich englacial structures. This in turn provides an insight into variations in the dominant tectonic stress regimes acting across the glacier during surges.
... Surging ice lobes/ice streams during the deglaciation of the Laurentide Ice-Sheet (LIS) have been identified by e.g. Clayton et al. (1985); Clark (1994); Colgan et al. (2003) and Evans et al. (2014), and Lovell et al. (2012) presented a palaeoglaciological reconstruction of Pleistocene ice sheet dynamics in the Strait of Magellan region, incorporating evidence for rapid surgelike ice flow and proglacial lake evolution. Although suggested it was possible to use modern surge-type glaciers in Iceland as analogues for fast flowing ice/ice streams during the deglaciation of the LIS, recent studies by e.g. ...
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Surging glaciers are potential analogues for land-terminating palaeo-ice streams and surging ice sheet lobes, and research on surge-type glaciers is important for understanding the causal mechanisms of modern and past ice sheet instabilities. The geomorphic signatures left by the Icelandic surge-type glaciers vary and range from glaciotectonic end moraines formed by folding and thrusting, crevasse-squeeze ridges, concertina eskers, drumlins and fluted forefields, to extensive dead-ice fields and even drift sheets where fast ice-flow indicators are largely missing. We outline some outstanding research questions and review case studies from the surge-type outlets of Brúarjökull, Eyjabakkajökull and Tungnaárjökull (Vatnajökull ice cap), Múlajökull and Sátujökull (Hofsjökull ice cap), Hagafellsjökull and Sudurjökull (Langjökull ice cap), Kaldalónsjökull, Leirufjardarjökull and Reykjarfjardarjökull (Drangajökull ice cap), as well as the surge-type cirque glaciers in northern Iceland. We review the current understanding of how rapid ice flow is sustained throughout the surge, the processes that control the development of the surge-type glacier landsystem and the geological evidence of surges found in sediments and landforms. We also examine if it is possible to reconstruct past surge flow rates from glacial landforms and sediments and scale-up present-day surge processes, landforms and landsystems as modern analogues to past ice streams. Finally, we also examine if there is a climate/mass-balance control on surge initiation, duration and frequency.
... Se aprecia que los sitios se localizan en una zona de planicies y lomajes altos con algunos sectores ligeramente ondulados; este relieve obedece a una extensa red de drenaje de deshielos, que muestra una tendencia clara de dirección Oeste-Este, donde un complejo sistema de canales (sinuosos y anastomosados) submarginales y marginales se orientan al Este, cortando una gran llanura de depositación con alineaciones glaciales (ejemplo, drumlins). Esta descripción concuerda con las realizadas inicialmente por Clapperton (1989) y recientemente por Lovell et al. (2012). En la Figura 1 se incluyen los perfiles de suelo y su posición en el paisaje, asociados a cada comunidad vegetal estudiada. ...
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South Patagonia (53° S) has long been famous for its rugged terrain, opceanic climate and often hydromorphic vegetation and soil. Volcanism, tectonic and glacial process are imprinting their actions along a territory, with vegetation-soil patterns that has been known since Darwin’s time. Soil fertility and moisture seem to control the overall floristic gradient in a simple structure, often changed by anthropic disturbations. The goal of this study was to establish general relationships between three major plant communities (Empetrum rubrum or “murtilla”, Festuca gracillima or “coirón”, and hydromorphic Poas or “vegas”) and the properties of fluvio-glacial mineral soils that support them. Soil under “murtilla” shows more constraints to plant growth, determined by a low pH, high aluminum activity and low amounts of non acid cations.
... Glacial lineations consist of linear landforms with a strong common orientation parallel to inferred iceflow (Figure 3; Spagnolo et al., 2014). Many of these landforms were previously mapped in the Strait of Magellan area by Darvill et al. (2014) and Lovell et al. (2011), who used them to reconstruct former ice lobe dynamics (Darvill et al., 2016;Lovell, Stokes, Bentley, & Benn, 2012). We found glacial lineations mostly around Gente Grande Peninsula and north of the Strait of Magellan. ...
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We present a geomorphic map of the landforms created by the Patagonian Ice Sheet during the local Last Glacial Maximum and perhaps prior glaciations in southernmost Patagonia. Building on prior work, the new mapping focuses in unprecedented detail on the right lateral and frontal landforms formed by the Strait of Magellan ice lobe. We produced the map using aerial orthophotography, Sentinel-2 and SPOT satellite imagery, ALOS PALSAR digital elevation model and fieldwork to ground-truth preliminary interpretations. We delineate at least five glacial events defined by a sequence of moraine drifts and associated glaciofluvial features. In contrast to previous studies, we propose the Magellan ice lobe extended ∼65 km farther, to the Primera Angostura peninsula, during the local Last Glacial Maximum. Our study provides a new context to establish a precise glacial chronology of the Magellan ice lobe during the last glacial cycle in the middle-high latitudes of the Southern Hemisphere.
... Interest in these glacial landform assemblages has increased in recent years as information on the timing of glacier fluctuations may yield insight into past variations in Southern Westerly Wind changes (Boex et al., 2013;García et al., 2012;Moreno et al., 2009Moreno et al., , 2015 and interhemispheric glacial and climate synchroneity (Denton et al., 1999;Murray et al., 2012;Sugden et al., 2005). Moreover, geomorphological studies, including glacial land-system approaches, have enabled detailed reconstructions of former ice dynamics Darvill, Stokes, Bentley, Evans, & Lovell, 2016;Lovell, Stokes, Bentley, & Benn, 2012). Whilst such methods have been applied in southernmost Patagonia, around central Patagonia and the North Patagonian Icefield (NPI) (∼46-48°S) previous work has focused on constraining the timing of glacial fluctuations, with less attention given to the detailed nature of landform-sediment assemblages (Glasser, Harrison, & Jansson, 2009). ...
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This paper presents a glacial geomorphological map of landforms produced by the Lago General Carrera–Buenos Aires and Lago Cochrane–Pueyrredón ice lobes of the former Patagonian Ice Sheet. Over 35,000 landforms were digitized into a Geographical Information System from high-resolution (<15 m) satellite imagery, supported by field mapping. The map illustrates a rich suite of ice-marginal glacigenic, subglacial, glaciofluvial and glaciolacustrine landforms, many of which have not been mapped previously (e.g. hummocky terrain, till eskers, eskers). The map reveals two principal landform assemblages in the central Patagonian landscape: (i) an assemblage of nested latero-frontal moraine arcs, outwash plains or corridors, and inset hummocky terrain, till eskers and eskers, which formed when major ice lobes occupied positions on the Argentine steppe; and (ii) a lake-terminating system, dominated by the formation of glaciolacustrine landforms (deltas, shorelines) and localized ice-contact glaciofluvial features (e.g. outwash fans), which prevailed during deglaciation.
... This is supported by observations from the margins of surgetype glaciers in other regions, such as Iceland (e.g., Sharp, 1985;Croot, 1987Croot, , 1988bAndrzejewski, 2002;Bennett et al., 2004;Benediktsson et al., 2008Benediktsson et al., , 2009Benediktsson et al., , 2010Benediktsson et al., , 2015Schomacker et al., 2014;Ingólfsson et al., 2016) and West Greenland (e.g., Yde et al., 2005;Roberts et al., 2009;Larsen et al., 2010). Such observations also lend support to interpretations that glacitectonised moraine sequences in palaeo settings are related to ice streaming and surgelike activity at the margins of former ice sheets (e.g., Clayton et al., 1985;Evans et al., , 2008Hart, 1999;Benn and Clapperton, 2000;Kehew et al., 2005;Lovell et al., 2012;Darvill et al., 2017;Gribenski et al., 2016). ...
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Some surge-type glaciers on the High-Arctic archipelago of Svalbard have large glacitectonic composite ridge systems at their terrestrial margins. These have formed by rapid glacier advance into proglacial sediments during the active surge phase, creating multicrested moraine complexes. Such complexes can be formed during single surge advances or multiple surges to successively less-extensive positions. The few existing studies of composite ridge systems have relied on detailed information on internal structure and sedimentology to reconstruct their formation and links to surge processes. However, natural exposures of internal structure are commonly unavailable, and the creation of artificial exposures is often problematic in fragile Arctic environments. To compensate for these issues, we investigate the potential for reconstructing composite ridge system formation based on geomorphological evidence alone, focusing on clear morphostratigraphic relationships between ridges within the moraine complex and relict meltwater channels/outwash fans. Based on mapping at the margins of Finsterwalderbreen (in Van Keulenfjorden) and Grønfjordbreen (in Grønfjorden), we show that relict meltwater channels that breach outer parts of the composite ridge systems are in most cases truncated upstream within the ridge complex by an inner pushed ridge or ridges at their ice-proximal extents. Our interpretation of this relationship is that the entire composite ridge system is unlikely to have formed during the same glacier advance but is instead the product of multiple advances to successively less-extensive positions, whereby younger ridges are emplaced on the ice-proximal side of older ridges. This indicates that the Finsterwalderbreen composite ridge system has been formed by multiple separate advances, consistent with the cyclicity of surges. Being able to identify the frequency and magnitude of former surges is important as it provides insight into the past behaviour of surge-type glaciers and, if absolute dating is possible, allows for the assessment of surge-type glacier response to climate change on decadal to centennial timescales. Although further investigations into the internal structure of these deposits should be sought where possible, our study demonstrates that geomorphology could be an invaluable tool for reconstructing the formation of composite ridge systems.
... For example, preliminary remote mapping may reveal features that suggest former glacier lobes may have surged (e.g. Lovell et al., 2012). Systematic study of sediment-landform assemblages, sediment exposures and other evidence, with reference to modern analogues (e.g. ...
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Geomorphological mapping is a well-established method for examining earth surface processes and landscape evolution in a range of environmental contexts. In glacial research, it provides crucial data for a wide range of process-oriented studies and palaeoglaciological reconstructions; in the latter case providing an essential geomorphological framework for establishing glacial chronologies. In recent decades, there have been significant developments in remote sensing and Geographical Information Systems (GIS), with a plethora of high-quality remotely-sensed datasets now (often freely) available. Most recently, the emergence of unmanned aerial vehicle (UAV) technology has allowed sub-decimetre scale aerial images and Digital Elevation Models (DEMs) to be obtained. Traditional field mapping methods still have an important role in glacial geomorphology, particularly in cirque glacier, valley glacier and icefield/ice-cap outlet settings. Field mapping is also used in ice sheet settings, but often takes the form of necessarily highly-selective ground-truthing of remote mapping. Given the increasing abundance of datasets and methods available for mapping, effective approaches are necessary to enable assimilation of data and ensure robustness. This paper provides a review and assessment of the various glacial geomorphological methods and datasets currently available, with a focus on their applicability in particular glacial settings. We distinguish two overarching ‘work streams’ that recognise the different approaches typically used in mapping landforms produced by ice masses of different sizes: (i) mapping of ice sheet geomorphological imprints using a combined remote sensing approach, with some field checking (where feasible); and (ii) mapping of alpine and plateau-style ice mass (cirque glacier, valley glacier, icefield and ice-cap) geomorphological imprints using remote sensing and considerable field mapping. Key challenges to accurate and robust geomorphological mapping are highlighted, often necessitating compromises and pragmatic solutions. The importance of combining multiple datasets and/or mapping approaches is emphasised, akin to multi-proxy approaches used in many Earth Science disciplines. Based on our review, we provide idealised frameworks and general recommendations to ensure best practice in future studies and aid in accuracy assessment, comparison, and integration of geomorphological data. These will be of particular value where geomorphological data are incorporated in large compilations and subsequently used for palaeoglaciological reconstructions. Finally, we stress that robust interpretations of glacial landforms and landscapes invariably requires additional chronological and/or sedimentological evidence, and that such data should ideally be collected as part of a holistic assessment of the overall glacier system.
... In Patagonia, the PíoXI Glacier, located at the Southern Patagonian Ice field, is the only active surge-type glacier in the region (Wilson et al., 2016). In the past, nearby ice lobes surrounding the Magellan Strait showed fast-flow ice conditions related to surge-like advances (Benn and Clapperton, 2000;Lovell et al., 2012;Darvill et al., 2017). ...
Article
The valley of Lake Viedma represents one of the main ice flow axes during the Quaternary glaciations of Patagonia. Geomorphological mapping of the eastern sector of the lake, between the shoreline and the end moraine systems attributed to the Last Glaciation, allowed us to recognize a large number of ice-marginal and subglacial landforms. The landform assemblage of this area matches with previous models of surging glacier landsystems around the world. We identified the three geomorphological zones that characterise surging glacier landsystems, with some mixing of the zones, possibly related to multiple surges. The outer zone includes push moraines, thrust-block moraines, overridden moraines and hummocky moraines. In the middle zone, overridden moraines and hummocky moraines were developed. Finally, the inner zone is composed of megaflutes, crevasse-squeeze ridges, eskers and till eskers in a till plain. We interpreted these geomorphological features as evidence of surge activity of the Lake Viedma Lobe. A combination of topographic setting, soft-bedrock and the presence of a proglacial lake in contact with ice could have contributed to the development of surge activity in the Viedma paleoglacier. It could have been triggered by a change in the glacial thermal regime of the Viedma paleoglacier from polar during the Last Glacial Maximum to polythermal or temperate during the beginning of the Late Glacial.
... Proglacial lake evolution during the last deglaciation played a significant role in ice sheet dynamics (e.g. Jansson, 2003;Stokes and Clark, 2004;Lovell et al., 2012;Carrivick and Tweed, 2013), climate (Teller, 2004), hydrology and marine circulation (Soulet et al., 2013), sediment flux and landscape evolution (Larsen et al., 2011;Evans et al., 2012;Winsemann et al., 2016). A major improvement in understanding the pattern and behaviour of the numerous proglacial lakes along the southern Canadian Interior Plains has been to consider the evolution of the whole proglacial system at the sub-ice sheet scale (Teller, 2004). ...
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The extent of proglacial lakes following the initial separation of the southwest Laurentide Ice Sheet from the Cordilleran Ice Sheet and its eastward retreat from the Canadian Rocky Mountains has been reconstructed across regions of Alberta at a range of scales. However, to date, no studies have integrated all available geological information to produce a province-wide deglacial reconstruction that considers the evolution of proglacial lakes as components of the ice-marginal system. In this paper, we utilize a geologically constrained shoreline projection method with a high resolution digital elevation model to reconstruct the evolution of the ice-marginal system along the southwest LIS during the last deglaciation. This method provides new details on the configuration, volume, drainage history and routing of ∼240 proglacial lakes as they migrated across Alberta and establishes a succession of paleogeographic reconstructions that can place geological evidence of regional and local ice-flow reorganizations into a spatiotemporal context. These reconstructions demonstrate that although the evolution of proglacial lakes was largely driven by the topography of the emerging landscape and the configuration of the ice margin, positive feedbacks in the ice-marginal system, particularly where margins transitioned from terrestrial to subaqueous settings played a major role in deglacial ice dynamics. Narrow, ribbon-shaped lakes that paralleled the ice margin induced relatively minor changes in style and rate of deglaciation, whereas the evolution of progressively larger, longer-lived lakes extending obliquely to the ice margin promoted surging and subsequent rapid retreat.
... Anderson et al., 2005;Huss et al., 2007;Sugiyama et al., 2007). The Quaternary record suggests that glacier velocity can accelerate due to the drainage of ice-dammed lakes (Meinsen et al., 2011;Lovell et al., 2012) and thus for glacier dynamics to be decoupled, at least temporarily, from climatic perturbations. ...
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Proglacial lakes are ubiquitous within the Quaternary record and can provide exceptional breadth and depth of palaeoenvironmental information. Present deglaciation is increasing the number and size of proglacial lakes around the world. This study provides a synthesis of knowledge on proglacial lake character and behaviour and critically evaluates the importance of proglacial lakes from a geological perspective. We show how ‘ice-marginal’ or ‘ice-contact’ lakes and other distal proglacial lakes can be distinguished from each other by geomorphological, sedimentological, chemical and biological characteristics. The key controls on proglacial lake geomorphology and sedimentology are outlined and discussed. Proglacial lakes can exacerbate mountain glacier and ice sheet margin ablation via mechanical and thermal stresses, but very large lakes can moderate summer air temperatures and relatively retard summer ice ablation. Proglacial lakes interrupt meltwater flux and are very efficient sediment traps. Hydrological routing and consequent geomorphological activity can be radically modified by sudden drainage of proglacial lakes and resultant glacial lake outburst floods; exceptionally large proglacial lake drainages affected global ocean circulation and global climate during the Quaternary. Overall, analyses of proglacial lakes can provide a valuable insight into (i) patterns, character and behaviour of mountain glaciers, ice sheets and glaciations, and (ii) the impacts of past, present and future deglaciation.
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The history of glaciations on Southern Hemisphere sub-polar islands is unclear. Debate surrounds the extent and timing of the last glacial advance and termination on sub-Antarctic South Georgia in particular. Here, using sea-floor geophysical data and marine sediment cores, we resolve the record of glaciation offshore of South Georgia through the transition from the Last Glacial Maximum to Holocene. We show a sea-bed landform imprint of a shelf-wide last glacial advance and progressive deglaciation. Renewed glacier resurgence in the fjords between c. 15,170 and 13,340 yr ago coincided with a period of cooler, wetter climate known as the Antarctic Cold Reversal, revealing a cryospheric response to an Antarctic climate pattern extending into the Atlantic sector of the Southern Ocean. We conclude that the last glaciation of South Georgia was extensive, and the sensitivity of its glaciers to climate variability during the last termination more significant than implied by previous studies.
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Glacitectonic composite ridge systems are found at the margins of a number of surge-type glaciers globally. On the High-Arctic archipelago of Svalbard, the pioneering work of Croot (Glaciotectonics: forms and processes. Balkema, Amsterdam, 1988) highlighted the coincidence between composite ridge systems and surge-type glaciers on the island of Spitsbergen. These observations have contributed significantly to our understanding of the links between glacier surges and the landforms they produce. We update this work and expand it to the whole archipelago using the Norwegian Polar Institute’s TopoSvalbard aerial photograph archive to identify 50 composite ridge systems. These are found on all four of the largest islands: Spitsbergen, Nordaustlandet, Edgeøya and Barentsøya, and at the margins of both tidewater and land-terminating glaciers. Of the 50 composite ridge systems, 49 are associated with glaciers that have either been documented as surge-type or contain indicative geomorphological evidence of surging in the form of crevasse-squeeze ridge (CSR) networks. This provides further support for the established link between composite ridge systems and surging. Based on the proportion of glaciers that are documented as being of surge-type and those that display indicative evidence of surging (but have not been observed to surge), we conclude that at least 32.6% of all glaciers in Svalbard surge or are likely to have surged. This study contributes to the understanding of the links between glacier surging and specific landforms/landform assemblages (composite ridge systems and CSR networks), which has applications in other modern glacial environments and at the margins of former ice masses in palaeoglaciological settings.
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The Quaternary of Tierra del Fuego is represented by glacial, glacio#uvial, glaciolacustrine, marine and aeolian deposits. Six drift units have been described, the oldest dating from the Late Pliocene: these are, starting from the oldest, the RmHo Grande, Sierra de los Frailes, Cabo VmHrgenes, Punta Delgada, Primera Angostura and Segunda Angostura Drifts. Neoglacial and `Little Ice Agea events are represented in cirques and higher mountain valleys. Marine deposits and raised beaches were formed during Middle Pleistocene, Late Pleistocene and Holocene interglacial stages. After the de"nitive ice-retreat (10 ka ago) vegetation changed from tundra and cold steppe to subantarctic forest environments. Marine deposits re#ect also colder conditions than the present ones. Aeolian processes prevailed in northern Tierra del Fuego, where semiarid conditions and the frequent westerlies favoured the retransportation of "ner materials coming from pre-existing deposits. Before the opening of the Magellan Straits, earliest human colonization occurred in northern Tierra del Fuego ca. 11 ka BP, in tundra-like environmental conditions. Pedestrian hunters of camelids and foxes co-existed with Pleistocene fauna that became extinct during Late Glacial}Earliest Holocene times. The steppe area (inland and Atlantic coast) was successively occupied since then until recent times. On the other hand, the Beagle Channel coasts were occupied since 6 ka BP by hunter}gatherer groups adapted to maritime littoral conditions. Finally, the easternmost area of the island was inhabited at least since 1.5 ka BP. The human settlements in these latter areas occurred under environmental conditions similar to the present ones, when the Fuegian forest was de"nitively established
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Please click here to download the map associated with this article.This paper describes The Glacial Map of southern South America. This is a new map of the glacial geomorphology of southern South America between latitudes 38°S and 56°S, approximately the area covered by the former Patagonian Ice Sheets. The map was compiled from interpretation of remotely sensed images (Landsat 7 ETM+, pan-sharpened Landsat 7 and ASTER scenes). The mapped geomorphological features include terminal moraines, glacial lineations, ribbed moraine, glacier debris stripes, trimlines, empty cirques, plateau edge, volcanoes, meltwater channels, deltas, glacial lake shorelines, alluvial fans, sandar, and glacial lake outburst ood tracks. The map also indicates the current extent of major icefields and glaciers in the area, as well as other topographic features such as lakes, rivers, shorelines, deltas, plateau surfaces and volcanoes. We briey describe the most commonly occurring glacial landforms and provide an overview of their distribution within the mapped area.
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This chapter reviews that the landscape morphology of the southernmost end of South America shows many erosional and depositional features of glacial origin. Many of these features are related to ancient glaciations, although most of them represent the more recent Quaternary glacial events. In all cases, the glaciers that formed this landscape were derived from the mountain ice sheet of the Darwin Cordillera, from which ice tongues emerged in all directions. The genesis of the various ice lobes depended upon the ice thickness in each glacial episode and the underlying superficial relief. According to the regional morphology and stratigraphy, the formation of the different ice lobes would have been physically defined from the older to the younger glaciations in a progressive succession. As indicated in the map, the Sierra de los Frailes glaciation would have expanded over a vast extension, between the Rio Gallegos valley and Central Tierra del Fuego. The existence of an ancient glaciation, named the Rio Grande Glaciation, has been interpreted from the observation of scattered erratic boulders along the Rio Grande valley and neighbouring areas and more, a few tens of kilometres further north in the Rio Chico valley. This area belongs to a large basin that drains the western and central portions of Tierra del Fuego toward the Atlantic ocean. Moreover, this area received the outwash discharge of the glaciers occupying the Carmen Sylva and Beauvoir ranges during the Pleistocene. The region adjacent to the Atlantic coast in southeastern Tierra del Fuego is shown in the map as covered by “undifferentiated drifts” because the till deposits observed in various positions along the mountain valleys have not been studied in detail.
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The terrestrial glacial record reflects past snowline variability and atmospheric temperature changes. When combined with secure chronologies, these data can be used to test models of ice-age climate. We present new in situ cosmogenic 10Be, 26Al, and 3He exposure ages, supported by limiting 40Ar/39Ar and 14C ages, for seven of the youngest moraines east of Lago Buenos Aires, Argentina, 46.5°S, that were deposited by a large outlet glacier of the Patagonian Ice Cap. Following a major glaciation that deposited extensive moraines prior to 109 ka, paired 10 Be-26 Al ages indicate that the next youngest complex of moraines was deposited from 23.0 ± 1.2 to 15.6 ± 1.1 ka (1σ). During the last glaciation, ice was at its maximum extent prior to 22 ka and at least five moraines were deposited in less than 10 k.y. These data are in good agreement with three 14C ages of ca. 16 ka from varved sediment banked on top of the youngest of these five moraines and limiting 3He ages, which range from ca. 33 to 19 ka. The most extensive ice marginal deposits preserved within the last 109 k.y. were formed during marine oxygen isotope stage 2; no moraines dating to stage 4 were found. For stage 2, the distribution of ages at Lago Buenos Aires is similar to cosmogenic nuclide-based glacial chronologies from western North America. In fact, the structure of the last mid-latitude South American ice age-specifically, the overall timing, a maximum ice extent prior to 22 ka, and deglaciation after 16 ka-is indistinguishable from that of the last major glaciation in the Northern Hemisphere, despite a maximum in Southern Hemisphere insolation during this period. The similar midlatitude glacial history in both hemispheres implies that a global climate forcing mechanism, such as atmospheric cooling, as opposed to oceanic redistribution of heat, synchronizes the ice age climate on orbital time scales.
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Ross ice streams supply over 90% of the ice volume flowing out of the Ross sector of the West Antarctic ice sheet (WAIS). Stoppage of Ice Stream C (ISC) ca. 150 years ago appears to have pushed this sector of WAIS from negative into positive mass balance [Joughin and Tulaczyk, 2002]. We propose an explanation for the unsteady behavior of ISC using a new numerical ice-stream model, which includes an explicit treatment of a subglacial till layer. When constrained by initial conditions emulating prestoppage geometry, dynamics, and mass balance of ISC, the model yields a rapid (~100 years) stoppage of the main ice-stream trunk. The stoppage is triggered by basal freeze-on, which consolidates and strengthens the subglacial till. Our numerical simulations produce results consistent with a number of existing observations, for example, continuing activity of the two tributaries of ISC. The model always yields rapid stoppage unless we specify ice-stream width that is smaller than its prestoppage values (maximum of ~80 km). We conjecture that if ISC was active for at least a few thousand years before slowdown, its width was significantly smaller than today to sustain the long active phase. Ice-stream width is a key control that helps determine whether ice-stream flow is sustainable over a long term. Our work indicates that the recent stoppage of Ice Stream C could have been part of inherent ice-stream cyclicity, and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns.
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Fast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the "ice-stream" type (represented by the Siple Coast ice streams in West Antarctica) and the "isbrae" type (represented by Jakobshavn Isbrae in Greenland). The typical ice stream is wide, relatively shallow (~1000 m), has a low surface slope and driving stress (~10kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbrae type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zones tend to be wide relative to the isbrae width, and the location of isbrae and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbraes occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.
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Subglacially-produced drift lineations provide spatially extensive evidence of ice flow that can be used to aid reconstructions of the evolution of former ice sheets. Such reconstructions, however, are highly sensitive to assumptions made about the glaciodynamic context of lineament generation; when during the glacial cycle and where within the ice sheet were they produced. A range of glaciodynamic contexts are explored which include: sheet-flow submarginally restricted; sheet-flow pervasive; sheet-flow patch; ice stream; and surge or re-advance. Examples of each are provided. The crux of deciphering the appropriate context is whether lineations were laid down time-transgressively or isochronously. It is proposed that spatial and morphometric characteristics of lineations, and their association with other landforms, can be used as objective criteria to help distinguish between these cases. A logically complete ice-sheet reconstruction must also account for the observed patches of older lineations and other relict surfaces and deposits that have survived erasure by subsequent ice flow. A range of potential preservation mechanisms are explored, including: cold-based ice; low basal-shear stresses; shallowing of the deforming layer; and basal uncoupling.
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We analyzed the possible controls on the distribution of surge-type glaciers in Svalbard using multivariate logit models including 504 glaciers and a large number of glacial and geological attributes. Specifically we examined the potential effect of geological boundaries, mass-balance conditions and thermal regime on surging. It was found that long glaciers with relatively steep slopes overlying young fine-grained sedimentary lithologies with orientations in a broad arc clockwise from northwest to southeast are most likely to be of surge type. No relation between lithological boundaries and surge potential could be established. Possible explanations for length being conducive to surging are transport-distance-related substrate properties, distance-related attenuation of longitudinal stresses and the possible relation between thermal regime and glacier size. Analysis of glaciers with recorded radio-echo sounding reveals that a polythermal regime, accumulation-area ratios close to balance and a large elevation span increase the surge potential. The logit models also enabled us to detect 19 new surge-type glaciers, to reclassify six glaciers as normal and to identify unusual surge-type glaciers. Our model results suggest that a polythermal regime and fine-grained potentially deformable beds are conducive to the surge potential of Svalbard glaciers.
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Ice cores provide a record of changes in dust flux to Antarctica, which is thought to reflect changes in atmospheric circulation and environmental conditions in dust source areas. Isotopic tracers suggest that South America is the dominant source of the dust, but it is unclear what led to the variable deposition of dust at concentrations 20-50 times higher than present in glacial-aged ice. Here we characterize the age and composition of Patagonian glacial outwash sediments, to assess the relationship between the Antarctic dust record from DomeC (refs9,13) and Patagonian glacial fluctuations for the past 80,000 years. We show that dust peaks in Antarctica coincide with periods in Patagonia when rivers of glacial meltwater deposited sediment directly onto easily mobilized outwash plains. No dust peaks were noted when the glaciers instead terminated directly into pro-glacial lakes. We thus propose that the variable sediment supply resulting from Patagonian glacial fluctuations may have acted as an on/off switch for Antarctic dust deposition. At the last glacial termination, Patagonian glaciers quickly retreated into lakes, which may help explain why the deglacial decline in Antarctic dust concentrations preceded the main phase of warming, sea-level rise and reduction in Southern Hemisphere sea-ice extent.
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We examine the deglaciation of the eastern flank of the North Patagonian Icefield between latitudes 46° and 48°S in an attempt to link the chronology of the Last Glacial Maximum moraines and those close to present-day outlet glaciers. The main features of the area are three shorelines created by ice-dammed lakes that drained eastwards to the Atlantic. On the basis of 16 14C and exposure age dates we conclude that there was rapid glacier retreat at 15–16 ka (calendar ages) that saw glaciers retreat 90–125 km to within 20 km of their present margins. There followed a phase of glacier and lake stability at 13.6–12.8 ka. The final stage of deglaciation occurred at c. 12.8 ka, a time when the lake suddenly drained, discharging nearly 2000 km3 to the Pacific Ocean. This latter event marks the final separation of the North and South Patagonian Icefields. The timing of the onset of deglaciation and its stepped nature are similar to elsewhere in Patagonia and the northern hemisphere. However, the phase of lake stability, coinciding with the Antarctic Cold Reversal and ending during the Younger Dryas interval, mirrors climatic trends as recorded in Antarctic ice cores. The implication is that late-glacial changes in southern Patagonia were under the influence of the Antarctic realm and out of phase with those of the northern hemisphere.
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AbStRACt. Newly described outcrops, excavations and sediment cores from the region of Última Esperanza, Magalla-nes, contain tephra derived from the large late-glacial explosive R1 eruption of the Reclús volcano in the Andean Austral Volcanic Zone. New radiocarbon dates associated to these deposits refine previous estimates of the age, to 14.9 cal kyrs BP (12,670±240 14 C yrs BP), and volume, to >5 km 3 , of this tephra. The geographic and stratigraphic distribution of R1 also place constraints on the evolution of the ice-dammed proglacial lake that existed east of the cordillera in this area between the termination of the Last Glacial Maximum (LGM) and the Holocene. This proglacial lake generated wave-cut terraces, and also caves, such as the Cueva de Milodón, along the highest prominent terrace. The current elevation of these terraces depends on the total amount of post-glacial isostatic rebound, which is unknown. Due to differential rebound, the highest prominent lake terraces decrease in height from west-to-east, from ~170 m a.s.l. on Península An-tonio Varas west of Seno Última Esperanza, to ~150 m a.s.l. around Lago Sofía, and down to ~125 m a.s.l. along their easternmost margin. The presence of thick deposits of R1 tephra in some of the caves around Lago Sofía implies that the proglacial lake had already dropped below its highest level prior to the time of this eruption, and, in fact, even earlier, prior to 16.1 cal kyrs BP (13,560±180 14 C yrs BP), when land mammals first occupied these caves. The depositional environment of R1 in a core from Dumestre bog suggests that the lake level was in fact <80 m a.s.l. at the time of this eruption. The original lake may have drained to this level across the low elevation pass between Fiordo Obstrucción and Seno Skyring, and subsequently into Seno Otway and the Pacific Ocean, when Canal Jerónimo opened up prior to the R1 eruption. Another suite of cores, from the Eberhard site, indicate that the lake persisted at >70 m a.s.l. until 12.8 cal kyrs BP (10,695±40 14 C yrs BP). However, a 14.2 cal kyrs BP (12,125±85 14 C yrs BP) Mylodon pelvis from a nearby site, located at only ~7 m a.s.l., suggests that the lake could have emptied, for at least a brief period, to this low level at this time. This latter datum, combined with the lack of any prominent terraces between the highest ones (170-125 m a.s.l.) and much lower ones (at only 30 m a.s.l. on Península Antonio Varas and 20 m a.s.l. along the coast north and south of Puerto Natales), suggests abrupt changes in the lake level after the R1 eruption. The likely mechanism for producing these changes in Última Esperanza was the catastrophic failure and subsequent re-sealing of an ice dam in Paso Kirke, the only below sea-level pathway west to the Pacific north of Fjordo Obstrucción. The final stage of lake drainage, from the lower terrace level (20-30 m a.s.l.) occurred at 10.3 cal kyrs BP. RESuMEn. Evolución de lagos proglaciales embalsados por hielo en Última Esperanza, Chile: Implicancias de la explosión volcánica tardiglacial R1 del volcán Reclús, Zona Volcánica Austral Andina. En este trabajo reportamos hallazgos de tefras derivadas de la gran explosión volcánica tardiglacial R1 del volcán Reclús situado en la Zona Volcánica Austral Andina, a partir de nuevos afloramientos, excavaciones y testigos sedimentarios de lagos y pantanos, obtenidos en la región de Última Esperanza, Magallanes. Nuevas fechas asociadas a estos depósitos permiten refinar su edad a 14,9 ka cal AP (12.670±240 14 C años AP) y su volumen a >5 km 3 . Además, la ubicación geográfica y estratigráfica de R1 permite acotar la evolución del lago proglacial represado por hielo que se desarrolló al este de la cordillera al intervalo temporal entre el término del Último Máximo Glacial y el Holoceno. Este lago proglacial generó terrazas y cuevas, producto de la acción del oleaje, como la Cueva de Milodón, a lo largo de la terraza más alta y conspicua. La altitud actual de estas terrazas depende de la cantidad total de rebote isostático posglacial, el cual se desconoce. Debido a las variaciones en el rebote isostático posglacial, las terrazas lacustres más altas y prominentes disminuyen en altitud de oeste a este, desde ~170 m s.n.m. en la Península Antonio Varas, al oeste del Seno Última Esperanza, a 150 m s.n.m. alrededor del lago Sofía y descienden hasta ~125 m s.n.m. a lo largo de su margen más oriental. La presencia de grandes depósitos de la tefra R1 en algunas de las cuevas alrededor del lago Sofía indican que el lago proglacial ya había descendido, con respecto a su nivel más alto, antes de la erupción de R1 y de hecho incluso antes de 16,1 ka cal AP (13.560±180 14 C años AP) que es cuando los mamíferos terrestres ocuparon por primera vez estas cuevas. El ambiente deposicional de R1, en el registro sedimentario del pantano Dumestre, sugiere incluso que el nivel del lago era inferior a 80 m s.n.m. durante el momento de esta erupción. Probablemente el lago proglacial original habría drenado a este nivel a través de sectores de baja altitud ubicados entre fiordo Obstrucción y seno Skyring, siguiendo hacia el seno Otway para desembocar en el Océano Pacífico, una vez que el canal Jerónimo ya estaba libre de hielo antes de la erupción de R1. Otro grupo de testigos, del sitio Eberhard, indican que el lago persistió a >70 m s.n.m. hasta 12,8 ka cal AP (10.695± 14 C años AP). Sin embargo, a 14,2 ka cal AP (12.125± 14 C años AP), la pelvis de Mylodon de un sitio cercano, ubicado a 7 m s.n.m., sugiere que el lago podría haberse vaciado temporalmente durante este período. Este último dato, combinado con la ausencia de terrazas prominentes entre las más altas (170-125 m s.n.m.) y las más bajas (a solo 30 m s.n.m. en la Península Antonio Varas y 20 m s.n.m. a lo largo de la costa norte y sur de Puerto Natales), sugiere cambios abruptos en el nivel del lago proglacial después de la erupción de R1. Probablemente el mecanismo que ocasionó estos cambios en Última Esperanza fue la ruptura catastrófica y el subsecuente resellamiento del dique de hielo que bloqueaba el Paso Kirke, el único paso al Océano Pacífico bajo el nivel del mar al norte del fiordo Obstrucción. El drenaje final del lago, desde la terraza inferior (20-30 m s.n.m.), ocurrió a los 10,3 ka cal AP.
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We report the results of glacial geomorphological mapping of the Strait of Magellan and Bahía Inútil, southernmost South America. Our aims are to determine the pattern and process of deglaciation during the last glacial–interglacial transition, and to provide a firm geomorphological basis for the interpretation of radiocarbon, cosmogenic isotope and amino acid dates for the timing of deglaciation. The area is important because it lies in a southerly location, providing a link between Antarctica and southern mid-latitudes, and also lies in the zone of the southern westerlies which are a key element in regional climate change.
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Glacier fluctuations in the Strait of Magellan tell of the climatic changes that affected southern latitudes at c. 53–55°S during the Last Glacial Maximum (LGM) and Late-glacial/Holocene transition. Here we present a revised chronology based on cosmogenic isotope analysis, 14C assays, amino acid racemisation and tephrochronology. We unpick the effect of bedrock-derived lignite which has affected many 14C dates in the past and synthesise new and revised dates that constrain five glacier advances (A to E). Advance A is prior to the LGM. LGM is represented by Advance B that reached and largely formed the arcuate peninsula Juan Mazia. Carbon-14and 10Be dating show it occurred after 31 250 cal yrs BP and culminated at 25 200–23 100 cal yrs BP and was then followed by the slightly less extensive advance C sometime before 22 400–20 300 cal yrs BP. This pattern of an early maximum is found elsewhere in South America and more widely. Stage D, considerably less extensive, culminated sometime before 17 700–17 600 cal yrs BP and was followed by rapid and widespread glacier retreat. Advance E, which dammed a lake, spanned 15 500–11770 cal yrs BP. This latter advance overlaps the Bølling-Allerød interstadials and the glacier retreat occurs during the peak of the Younger Dryas stadial in the northern hemisphere. However, the stage E advance coincides with the Antarctic Cold Reversal (c. 14800–12700 cal yrs BP) and may indicate that some millennial-scale climatic fluctuations in the Late-glacial period are out of phase between the northern and southern hemispheres.
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In the summers of 1993, 1994 and 1995, video and Global Positioning System location data and 35 mm photographs were collected in a series of systematic survey flights undertaken over the Bering Glacier and Bagley Ice Field system (Alaska) in an effort to characterize surge-crevasse patterns and surge propagation. During survey flights in late August 1995, we observed that the 1993–94. Bering Glacier surge was continuing and still expanding affecting new areas farther up in Bagley Ice Field. New crevasse fields, similar in pattern to the first surge crevasses we had observed in June 1993 below Khitrov Hills and in other isolated areas of central Bering Glacier and in July 1994 near the head of Bering Glacier (near the junction of Bering Glacier and Bagley Ice Field, in both upper Bering Glacier and Bagley Ice Field), were opening in eastern Bagley Ice Field and in the “Stellet” side of Bagley Ice Field. The type of crevasses seen in the new fields suggested that the surge was propagating into these areas. By analysis and interpretation of the brittle-deformation patterns apparent in the crevasse patterns, some aspects of the past kinematic framework of the surge can be deduced. This approach may lead to a more general classification of ice-surface structures and to their linkage to ongoing processes.
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It has been suggested that extremely long subglacial bedforms (e.g. attenuated drumlins and mega-scale glacial lineations) record former areas of fast-flowing ice and that bedform elongation ratio is a useful proxy for ice velocity. Despite the availability of much data pertaining to the measurement and analysis of subglacial bedforms, these assumptions have rarely been explicitly addressed in detail. In this paper, we demonstrate that long subglacial bedforms (length:width ratios S10:1) are indicative of fast ice flow. Using satellite imagery, we mapped over 8000 lineaments associated with a highly convergent flow pattern near Dubawnt Lake, District of Keewatin, Canada. This flow pattern is unusual in that it displays a large zone of convergence feeding into a main 'trunk' and then diverging towards the inferred ice margin. The 'bottleneck' pattern is taken to record an increase and subsequent decrease in ice velocity and we analysed transverse and longitudinal variations in bedform morphometry. The main trunk of the flow pattern (down-ice of the convergent zone) is characterized by mega-scale glacial lineations of great length (up to 13 km) and high elongation ratios (up to 43:1). The down-ice variations in elongation ratio reflect exactly what we would expect from a terrestrial ice stream whose velocity increases in the onset zone passes through a maximum in the main trunk and slows down as the ice diverges at the terminus. It is suggested that any unifying theory of drumlin formation must be able to account for the association between long subglacial bedforms and fast ice flow, although it is not assumed that fast ice flow always produces attenuated bedforms. A further implication of this work is that many more ice streams may be identified on the basis of attenuated subglacial bedforms, radically altering our views on the flow dynamics of former ice sheets.
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Glaciers and Glaciation is the classic textbook for all students of glaciation. Stimulating and accessible, it has established a reputation as a comprehensive and essential resource. In this new edition, the text, references, and illustrations have been thoroughly updated to give today's reader an up-to-the minute overview of the nature, origin, and behavior of glaciers and the geological and geomorphological evidence for their past history on earth. The first part of the book investigates the processes involved in forming glacier ice, the nature of glacier/climate relationships, the mechanisms of glacier flow, and the interactions of glaciers with other natural systems such as rivers, lakes, and oceans. In the second part, the emphasis moves to landforms and sediment, the interpretation of the earth's glacial legacy, and the reconstruction of glacial depositional environments and palaeoglaciology.
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The location and behaviour of ice streams is one of the most important controls on ice sheet configuration and stability. In order to reconstruct former ice sheets we need to know ice stream location and timing. Once identified, the beds of palaeo-ice streams provide an unprecedented opportunity to glean information about their basal environment, something that remains very difficult under contemporary ice streams. This paper represents the first synthesis and discussion of palaeo-ice stream research from a variety of former ice sheets and includes new insights relating their configuration and activity to the evidence they leave behind. The ambiguous use of the term 'ice stream' is addressed and four possible configurations for ice streams are presented. Diagnostic landforms and landform assemblages that we would expect an ice stream to produce are discussed and the temporal context of bedform generation is described as either a 'rubber stamped' or 'smudged' bedform imprint, resulting from isochronous or time-transgressive landform generation. In particular, we focus on the configuration of terrestrially terminating ice streams, for which there are no modern analogues. Technological advances in marine geophysics are helping to generate a convergence of interest between Antarctic glaciology and palaeo glaciology. It is suggested that investigations around the fringes of contemporary ice sheets, (i.e. West Antarctica) can provide evidence that directly links the geomorphological record of palaeo-ice streams with their contemporary counterparts. In addition, computational advances and modelling adaptations have permitted the incorporation of ice streams within ice sheet models. We argue that data from palaeo-ice stream beds are invaluable to ice sheet/stream modelling experiments and will help us understand ice stream operation and the linkages between climate perturbations and both palaeo and contemporary ice sheets.
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The existence of glacial lakes Naskaupi, McLean, Minto, Mélèzes, and Wapussakatoo during the deglaciation of Labrador/Ungava, Canada, was recognized at the end of 1950s. However, glacial lakes have seldom been included in regional reconstructions to constrain the outline of successive ice margins during the glacial retreat in Labrador/Ungava. Reconstruction of the ice flow succession in Labrador/Ungava has often primarily focused on till lineation systems. These reconstructions often depict a late glacial ice dispersal center situated over central Labrador/Ungava. More recent studies on glacial geomorphology including meltwater features, however, suggest that north-central Labrador/Ungava exhibited cold-based conditions at least during the latest deglaciation. Cold-based conditions inhibit basal sliding and formation of landforms, except for meltwater traces such as meltwater channels, glacial lake shorelines and deltas. This situation implies that meltwater traces are the main source of information when reconstructing the spatial retreat pattern during a cold-based deglaciation. Evidence presented in this study, such as glacial lake shorelines, fossil deltas, and spillway and drainage channels in north-central Labrador/Ungava, indicates the existence of numerous previously unmapped glacial lakes. The mapped glacial lake features are synthesized to a reconstruction of several glacial lake stages which, in turn, are used to constrain the late glacial ice margin retreat pattern over the inferred cold-based areas of north-central Labrador/Ungava. A total of 26 glacial lakes (65 sub-stages) existed during the deglaciation of Labrador/Ungava. These lakes were impounded along the southern margin of the shrinking ice sheet. The required damming ice margins indicate that the last ice remnant of the Laurentide Ice Sheet in Labrador/Ungava was situated over the southern Ungava Bay and the adjacent southern shore.
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The glacier Holmstrømbreen, in Spitsbergen, surged into the ice contact scarp of a proglacial outwash sequence at some time during its Neoglacial maximum. The outwash sediments were pushed along a decollement to produce a moraine in which deformation extended for 1.5 km beyond the furthest extent of the glacier front. The style of folding and faulting and the nature of the pre-, syn- and post-tectonic sedimentary sequence across the whole push moraine is described from a continuously exposed section of the push moraine which extends from its proximal to distal extremities. The precise extent of incremental compressive shortening of the pushed sediments, of some 900 m, is established. The depth to the underlying decollement is inferred to be an average of about 30 m, indicating that stresses and movement were transmitted through a thin nappe with an aspect ratio of about 1 in 30. It is suggested that this nappe was frozen and that an artesian water pressure head of 60 m immediately beneath it reduced friction along its base to a very low value. It is calculated that a glacially generated force of about 1.5×107 kN was responsible for pushing the sediment nappe. The nature of the glacially controlled groundwater flow system rather than the magnitude of longitudinal forces generated by the glacier is the principle determinant of large-scale push moraine characteristics.
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In the summers of 1993, 1994 and 1995, video and Global Positioning System location data and 35 mm photographs were collected in a series of systematic survey flights undertaken over the Bering Glacier and Bagley Ice Field system (Alaska) in an effort to characterize surge-crevasse patterns and surge propagation. During survey flights in late August 1995, we observed that the 1993-94 Bering Glacier surge was continuing and still expanding, affecting new areas farther up in Bagley Ice Field. New crevasse fields, similar in pattern to the first surge crevasses we had observed in June 1993 below Khitrov Hills and in on other isolated areas of central Bering Glacier and in July 1994 near the head of Bering Glacier (near the junction of Bering Glacier and Bagley Ice Field, in both upper Bering Glacier and Bagley Ice Field), were opening in eastern Bagley Ice Field and in the "Steller" side of Bagley Ice Field. The type of crevasses seen in the new fields suggested that the surge was propagating into these areas. By analysis and interpretation of the brittle-deformation patterns apparent in the crevasse patterns, some aspects of the past kinematic framework of the surge can be deduced. This approach may lead to a more general classification of ice-surface structures and to their linkage to ongoing processes.
Article
Geomorphological evidence of Quaternary glaciations that developed along the larger glacial valleys of the Eastern side of the Andes in Southern Patagonia and the Fuegian Archipelago, between 47°08′S and 54°55′S, are herein presented. The boundaries of the glacial advances that took place during the Early, Middle and Late Pleistocene, including the Last Glacial Maximum and Late Glacial moraines, have been indicated.
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During the Pleistocene, east of Lago Buenos Aires, Argentina, at 46.5degreesS, at least 19 terminal moraines were deposited as piedmont glaciers from the Patagonian ice cap advanced onto the semi-arid high plains adjacent to the southern Andes. Exceptional preservation of these deposits offers a rare opportunity to document ice-cap fluctuations during the last 1.2 m.y. Ar-40/ Ar-39 incremental-heating and unspiked K-Ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence. The Ar-40/Ar-39 isochron ages of three lavas that overlie till 90 km east of the Cordillera at Lago Buenos Aires, and another 120 km from the Andes along Rio Gallegos at 51.8degreesS that underlies till, strongly suggest that the ice cap reached its greatest eastward extent ca. 1100 ka. At least six moraines were deposited within the 256 k.y. period bracketed by basaltic eruptions at 1016 +/- 10 ka and 760 +/- 14 ka. Similarly, six younger, more proximal moraines were deposited during an similar to651 k.y. period bracketed by an underlying 760 14 ka basalt and the 109 +/- 3 ka Cerro Volcan basalt flow that buried all six moraines. Coupled with in situ cosmogenic surface exposure ages of moraine boulders, the 109 ka age of Cerro Volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6. Further westward toward Lago Buenos Aires, six additional moraines younger than the Cerro Volcan basalt flow occur. Surface exposure dating of boulders on these moraines, combined with the C-14 age of overlying varved lacustrine sediment, indicates deposition during the Last Glacial Maximum (LGM, 23-16 ka). Although Antarctic dust records signal an important Patagonian glaciation at 60-40 ka, moraines corresponding to marine oxygen isotope stage 4 are not preserved at Lago Buenos Aires; apparently, these were overrun and obliterated by the younger ice advance at 23 ka. Notwithstanding, the overall pattern of glaciation in Patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m.y. We hypothesize that tectonically driven uplift of the Patagonian Andes, which began in the Pliocene, yet continued into the Quaternary, in part due to subduction of the Chile rise spreading center during the past 2 m.y., maximized the ice accumulation area and ice extent by 1.1 Ma. Subsequent deep glacial erosion has reduced the accumulation area, resulting in less extensive glaciers over time.
Article
This chapter overviews the Patagonian and Fuegian glaciations—starting in the Late Miocene, when the junction of global, cooler climatic conditions, and the final rise of the southern Andes enabled the formation of mountain glaciers in the area. The chapter presents the absolute chronology of the Patagonian terrestrial glacial sequences, basically dated by means of 40Ar/39Ar dating techniques on volcanic rocks associated with glacial landforms and deposits, and cosmogenic isotope dating techniques on erratic boulders and glacial erosional surfaces. In some cases, the magnetostratigraphy of glacial deposits is available, thus allowing the correlation with the Pampean continental sequences and with the global ocean record. Patagonia and Tierra del Fuego are some of the regions of the world still largely covered by ice and snow. Three major mountain ice sheets can be observed along the Patagonian and Fuegian Andes. These three ice sheets are the Northern Patagonian Ice Field, the Southern Patagonian Ice Field, and the Darwin Cordillera Ice Field. The chapter includes also the most significant information available on the glaciations of the Chilean side of the Andes, along the same latitudinal belt.
Article
High-resolution palaeoenvironmental data for the Wisconsin Late-glacial and early Holocene is provided from two sites, Puerto del Hambre and Estancia Esmeralda II, in the central section of the Strait of Magellan, southern Chile. Palaeoecological and lithostratigraphical evidence is correlated with other palaeoenvironmental data from the Magellan region using 14C dating and tephrochronology. The oldest record, from Puerto del Hambre, commences at 14,47014Cyrbp (17,330cal. yrbp), following retreat of the Magellan glacier. Climatic warming after that time is indicated by ice retreat from the central section of the Strait of Magellan and palynological evidence. However, mean annual temperature continued to be cooler than present until c. 10,30014C yrbp (12,270–12,110cal. yrbp). The Magellan environment was relatively humid between c. 14,100 and c. 12,55014Cyrbp (16,910 and 15,170–14,410cal. yrbp). At c. 12,55014Cyrbp, there was a sharp decrease in effective moisture. Levels of humidity fluctuated, but remained relatively low until c. 10,300 14Cyrbp. This period of cool and drier climate is contemporary with a significant glacier advance dated to between c. 12,700 and 10,30014Cyrbp (15,330 and 12,270–12,110cal. yrbp), which would have produced an enhanced rainshadow effect. Around 10,30014Cyrbp a shift to warmer conditions occurred. Between c. 10,300 and 855014Cyrbp (12,270–12,110 and 9530cal. yrbp) an extreme arid phase is recorded, which led to an increase in fires. At c. 855014Cyrbp an increase in available moisture allowed the eastward spread of Nothofagus forest. At 826514Cyrbp (9270–9160cal. yrbp) there was a relative rise in sea level in the Strait of Magellan. This marine incursion lasted until 397014Cyrbp (4420cal. yrbp), reaching its maximum around c. 645014Cyrbp (7420–7340cal. yrbp).
Article
Regional mapping in southern Minnesota has illuminated a suite of landforms developed by the Des Moines Lobe that delimit the position of the lobe at its maximum and at lesser readvances. The ice lobe repeatedly advanced, discharged its subglacial water, and subsequently stagnated. Recent glaciological research on Antarctic ice streams has led some glacial geologists to postulate that ice streams drained parts of the marine-based areas of the Laurentide Ice Sheet. I postulate that such ice streams may develop in land-based areas of an ice sheet as well, and that the Des Moines Lobe, 200 km wide and 900 km long, was an outlet glacier of an ice stream. It appears to have been able to advance beyond the Laurentide Ice Sheet as long as adequate water pressure was maintained. However, the outer part of the lobe stagnated because subglacial water that facilitated the flow was able to drain away through tunnel valleys. Stagnation of the lobe is not equivalent to stoppage of the ice stream, because ice repeatedly advanced into and onto the stagnant margins, stacking ice and debris. Similar landforms are also seen in other lobes of the upper midwestern United States.
Article
Drumlins in South America are described and discussed in some detail for the first time. They occur in the Chilean part of southern Patagonia close to the Magellan Straits. Here, the majority of drumlins are spindle-shaped and have asymmetric cross-profiles, with the steeper slopes consistently on the same side (southeast); elongated enclosed hollows are commonly associated with the steeper slopes. Till, with blade-shaped clasts showing preferred alignment roughly parallel with the drumlin long axes, composes at least the outer 2 m, and up to 6 m of the stoss end, of some drumlins. Thick sequences of glaciofluvial and glaciolacustrine sediments may underlie much of the main drumlin field. The subglacial deformation of earlier Quaternary sediments may explain the spatial pattern and morphological characteristics of drumlins in Patagonia.
Article
Please click here to download the map associated with this article.This paper presents a detailed glacial geomorphological map covering over 16,000 km of the Seno Skyring-Seno Otway-Strait of Magellan region in southernmost Patagonia. It builds on previously published maps produced at a variety of scales and is re-mapped in detail for the purposes of reconstructing the pre-Last Glacial Maximum (LGM) glacial dynamics of the region, with particular focus on deciphering the glacial landsystem north east of Seno Otway, which has been postulated as a zone of ice streaming. Additional areas of interest include the reconstruction of proglacial lakes dammed by the Skyring and Otway lobes; their drainage during various stages of retreat; and a landsystems approach to the overall reconstruction of the combined Skyring-Otway-Magellan ice lobes. Mapping was conducted using a combination of Landsat ETM+ and ASTER satellite imagery and oblique and vertical aerial photographs, and is centred on approximately 53_S, 71_W. Seven main landform types have been mapped: glacial lineations, moraines, meltwater channels, irregular dissected ridges, eskers, outwash plains and former shorelines. The map records several episodes of ice flow, as revealed by glacial lineations, with the area around Laguna Cabeza del Mar exhibiting spectacular elongate drumlins. Drumlin fields are associated with three major ice lobes whose extent is marked by a series of moraine ridges and lateral meltwater channels. Large parts of the area were also subjected to proglacial meltwater erosion and deposition, as recorded by large tracts of outwash and associated channels.
Article
The distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice. The recent discovery of a small effective pressure beneath one ice stream consequently had led to some gross errors in the velocities predicted by the models. The difficulty may be resolved if it is true, as recent experiments suggest, that ice stream B, and by extrapolation other Ross ice streams as well, slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock. -from Author
Article
Satellite images of the North American continent are discussed which reveal previously unrecognized patterns of crossing drift lineations which aerial photography and ground surveys reveal to be sediment ridges oriented parallel to former glacial flow. The lineations reflect unexpected shifts of centers of mass of the Laurentide ice sheet throughout the last glacial cycle. The relative chronology of the lineations is examined and correlated with stratigraphy and dating, and the development of the ice sheet is followed through the glacial cycle.
Article
A total of 204 surging glaciers has been identified in western North America. These glaciers surge repeatedly and probably with uniform periods (from about 15 to greater than 100 years). Ice flow rates during the active phase may range from about 150 m/year to > 6 km/year, and horizontal displacements may range from < 1 to > 11 km. Ice reservoir and ice receiving areas can be defined for surging glaciers, and the reservoir area does not necessarily coincide with the accumulation area. Glaciers of all shapes, sizes, and longitudinal profiles can surge, and no unusual "ice dams" or bedrock constrictions are evident. Surges occur in many different climatic, tectonic, and geologic environments, but only in certain limited areas (mainly in the Alaska, eastern Wrangell, and St. Elias mountains). Three types of surging glaciers are defined: (I) large to moderate-sized glaciers with large displacements and very fast flow, (II) large to moderate glaciers with moderate displacements and flow rates, and (III) small glaciers with small displacements and moderate to fast flow rates. All three types involve an inherent instability which is self-triggered at regular intervals, but with Type I surges an additional (unknown) mechanism produces the very high flow rates.
Article
Flow features on the surface of the Ross Ice Shelf, West Antarctica, record two episodes of ice stream stagnation and reactivation within the last 1000 years. We document these events using maps of streaklines emerging from individual ice streams made using visible band imagery, together with numerical models of ice shelf flow. Forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration. According to our analysis, Whillans Ice Stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and MacAyeal Ice Stream either stopped or slowed significantly between 800 and 700 years ago, restarting about 150 years later. Until now, ice-stream scenarios emphasized runaway retreat or stagnation on millennial timescales. Here we identify a new scenario: century-scale stagnation and reactivation cycles, as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains. This introduces uncertainty into predictions for future sea-level withdrawals by the West Antarctic Ice Sheet, which are based in part on recent slowing of Whillans Ice Stream and the stagnant condition of Kamb Ice Stream.
Article
A major surge of the glacier complex of Sortebrae, East Greenland, occurred between 1992 and 1995.The impact of this surge on the topography of Sortebrae was examined through the production of a pre-surge (1981) digital elevation model (DEM) from interpolated digital map data, and a post-surge DEM from differential synthetic aperture radar interferometry using European Remote-sensing Satellite (ERS) imagery.The combined vertical error is typically 30-40 m; however, downdraw of up to 270 m in the reservoir zone, and uplift of up to 145 m in the receiving zone were measured. The upper glacier reservoir area discharged in excess of 24.3 ± 9.5 km3 volume over the surge, of which ~12.5 km3 was stored in the advanced lower glacier, the balance being lost, predominantly to calving.
Article
Bakaninbreen in Svalbard and Trapridge Glacier in Yukon Territory, Canada, are two prominent examples of surging glaciers which are thought to be controlled by their thermal regime. Both glaciers have developed large bulges which have propagated forward as travelling wave fronts, and which are thought to divide relatively stagnant down- stream cold-based ice from faster-moving warm-based upstream ice. Additionally, both glaciers are underlain by a wet, metres thick layer of deforming till. We develop a simple model for the cyclic surging behaviour of these glaciers, which interrelates the motion of the ice and till through a description of the subglacial hydrology. We find that oscillations (surges) can occur if the subglacial hydrological transmissivity is sufficiently low and the till layer is sufficiently thin, and we suggest that these oscillations are associated with the development and propagation of a travelling wave front down the glacier. We therefore interpret the travelling wave fronts on bothTrapridge Glacier and Bakaninbreen as mani- festations of surges. In addition, we find that the violence of the surge in the model is associ- ated with the resistance to ice flow offered by undulations in the bed, and the efficiency with which occasional hydrological events can release water accumulated at the glacier sole.
Article
Ice streams are critical regulatory mechanisms in contemporary ice sheets. It has been inferred that they also had a significant effect on the dynamics of former ice sheets. Subsequently, many people have invoked their widespread occurrence from a variety of formerly glaciated areas. Hypothesised locations, however, have often out-weighed meaningful evidence. This paper addresses the problem, using the characteristics of contemporary ice streams as a basis for their identification from former ice-sheet beds. A convergence of knowledge gained from contemporary ice-stream research, coupled with theories of glacial geomorphology, allows several geomorphological criteria to be identified as suggestive signatures of ice-stream activity. It is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams. The criteria are used to construct conceptual land-system models of the beds of former ice streams, and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based.
Article
Glacier surges tend to be initiated in relatively small regions, then propagate down-glacier, up-glacier and/or across-glacier. The processes controlling patterns and rates of surge propagation, however, are incompletely understood. In this paper, we focus on patterns of surge propagation in two confluent glaciers in Svalbard, and examine possible causes. One of these glaciers, Bakaninbreen, surged in 1985-95. The surge propagated ∼7 km down-glacier, but did not cross the medial moraine onto the other glacier, Paulabreen. When Paulabreen surged between 2003 and 2005, the surge wave travelled several km down-glacier, but its lateral boundary stayed very close to the medial moraine. The confluent glaciers formerly extended into a fjord, and bathymetric mapping and historical observations show that an active subglacial conduit has existed between Bakaninbreen and Paulabreen since at least the early 20th century. The existence of a persistent subglacial conduit below the medial moraine was confirmed when we entered and mapped a Nye channel at the confluence of Bakaninbreen and Paulabreen. We argue that the conduit acts as a barrier to surge propagation. If pressurized water below one branch of the glacier system reaches the conduit, water can be readily evacuated, preventing its propagation into the other branch.
Article
The identification of surging glaciers and ice streams in glaciated landscapes is of major importance to the understanding of ice-sheet dynamics and for reconstructing ice sheets and climate. No single landform or diagnostic criterion has yet been found with which to identify surging glaciers. A surging-glacier land-system model is constructed using observations and measurements from contemporary surging-glacier snouts in Iceland, Svalbard, U.S.A. and Canada for differentiating ancient surging margins from other non-surging palaeoglaciers. This integrates the suite of landforms, sediments and stratigraphy produced at surging-glacier margins. Landforms produced during surging include thrust moraines, concertina eskers and subglacial crevasse-squeeze ridges. Sedimentary sequences are usually characterized by multiple stacked diamictons and stratified interbeds, which display severe glaciotectonic contortion and faulting. Hummocky moraine, comprising interbedded stratified sediments and mass-flow diamictons, has also been associated with surge margins where large quantities of supraglacial and englacial debris entrained during the surge event have melted out in situ. An example of the application of the land-system model is presented for east-central Alberta, Canada. A surging palaeo-ice stream is identified within this part of the southwestern Laurentide ice sheet, where thrust-block moraines, crevasse-squeeze ridges, flutings, hummocky moraine and glaciotectonized sediments are juxtaposed.
Article
Different types of fast ice flow (both spatial and temporal) in valley glaciers (surging glaciers, tidewater glaciers and deforming-bed glaciers) and ice sheets (ice streams and deforming-bed ice-sheet flow) are discussed briefly. Although there are unlikely to be any specific individual landforms associated with fast ice flow, there may be landform assemblages. At valley glacier scale, it is suggested that there are two landform assemblages: (1) an ice-thrust type, dominated by bulldozed push moraines and hummocky moraines (associated with glaciers with a high supraglacial sediment supply, a coarse-grained substrate and a coarse-grained proglacial sediment wedge); and (2) a bed-flow type dominated by "squeeze" push moraines, flutes and drumlins (associated with glaciers with a low supraglacial sediment supply and fine-grained substrate). The ice-thrust type alone is only associated with discontinuous fast flow (on both rigid and deforming beds); whilst the bed-flow type is associated with both continuous and discontinuous fast flow. It is suggested that these two landform assemblages may also be indicative of fast ice flow at ice-sheet scale, in particular the bed-flow style. If that is the case, then discontinuous fast ice flow may be indicated by the ice-thrust landform assemblage and the bed-flow style where drumlins are present. It is also suggested that specific evidence for ice streams includes the distinctive land-form assemblages within valley or fan-like locations, and a predictable pattern of velocity reflected by drumlin elongation ratios.
Article
Ice streams drain much of the interior West Antarctic Ice Sheet and buffer the main ice reservoir from oceanic influences. The slow-flowing interior feeds the floating Ross Ice Shelf with ice via fast-flowing ice streams that are believed to modulate sea-level change through their control of inland ice storage. Understanding ice-stream behaviour, and predicting the response to climate change, requires a better knowledge of the subglacial geology. It is known that a thawed ice-bed and high-pressure basal water are necessary, but not sufficient, conditions to cause ice streaming. Moreover, it has been hypothesized that a soft sedimentary bed is also required, because of its intrinsic low frictional resistance to flow, and owing to its high erodibility so as to generate till that can deform and lubricate ice motion, or to bury rough features and smooth the bed for sliding. Here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill. The ice stream is within the basin and the ice outside the basin is slow-flowing. The basin fill presents an order-of-magnitude lower frictional resistance to ice flow than the subglacial material outside the basin. We conclude that the ice stream position is dependent on subglacial geology.
Article
The southwestern part of the Laurentidc Ice Sheet, in central North America, repeatedly surged during the last part of the Wisconsin Glaciation. Evidence includes the extreme lobation of the ice margin, the gentle slopes of lateral moraines and other marginal features, a radiocarbon chronology indicating extremely rapid marginal advance and retreat, and the abundance of supraglacial flow till. Rapid ice movement was caused by subglacial water and was probably limited to areas of slowly permeable substrate, which slowed the escape of the water.
Chapter
IntroductionKey considerations in designing an inversion modelIce streamsAn inversion modelExamples of inversion model applicationConclusion
Article
We measured in situ 10Be, 26Al and 36Cl on glacial deposits as old as 1.1 Myr in the southernmost part of Patagonia and on northern Tierra del Fuego to understand boulder and moraine and, by inference, landscape changes. Nuclide concentrations indicate that surface boulders have been exposed for far less time than the ages of moraines they sit upon. The moraine ages are themselves constrained by previously obtained 40Ar/39Ar ages on interbedded lava flows or U-series and amino acid measurements on related (non-glacial) marine deposits. We suggest that a combination of boulder erosion and their exhumation from the moraine matrix could cause the erratics to have a large age variance and often short exposure histories, despite the fact that some moraine landforms are demonstrably 1 Myr old. We hypothesize that fast or episodic rates of landscape change occurred during glacial times or near the sea during interglacials. Comparison with boulder erosion rates and exhumation histories derived for the middle latitudes of semi-arid Patagonia imply different geomorphic processes operating in southernmost South America. We infer a faster rate of landscape degradation towards the higher latitudes where conditions have been colder and wetter.
Article
In southern South America, the maximum areal extent of ice during the Quaternary Period, the Greatest Patagonian Glaciation (GPG, [Mercer, J.H., 1983. Cenozoic glaciation in the southern hemisphere. Annual Reviews of Earth and Planetary Science 11, 99–132.]), occurred at 1.1 Ma and subsequent glaciations were overall less extensive. The GPG preceded global minimum temperatures and maximal volume of ice, which occurred in the last ~ 800 kyr, as recorded in the marine δ 18 O record. Significant modification of the drainage morphology of the southern Andes from a non-glaciated to glaciated landscape occurred throughout the Quaternary Period. We infer a non-climatic relationship between glacial modification of the mountains and the decreasing extent of ice and we discuss processes of landscape development that could have caused the trend. Specifically, these include modification of valleys, such as development from a V-to a U-shape, and lowering of mass-accumulation areas. Such changes would strongly affect glacial dynamics, the mass balance profile and mass-flux during succeeding glaciations, especially for low-gradient outlet glaciers occupying low areas. Other areas around Earth (at least where ice has been warm-based) also may exhibit a non-random trend of decreasing extent of ice with time, ultimately because of glacial erosion in the Quaternary Period.
Article
This paper critically appraises the evidence for a succession of ice-dammed lakes in the central Strait of Magellan (c. 53°S) c. 17 000–12 250 cal. yr BP. The topographic configuration of islands and channels in the southern Strait of Magellan means that the presence of lakes provides compelling constraints on the position of former ice margins. Lake shorelines and glacio-lacustrine sediments have been dated by their association with a key tephra layer from Volcan Reclús (c. 15 510–14 350 cal. years bp) and by 14C-dated peats. The timing of glacial lake formation and associated glacier readvances is at odds with the rapid and widespread glacier retreat of the Patagonian ice fields further north after c. 17 000 cal. yr bp, suggesting rather that the lakes were coeval with the Antarctic Cold Reversal and persisted to the Late-glacial/Holocene transition. This apparent asymmetrical latitudinal response in glacier behaviour may reflect overlapping spheres of northern hemisphere and Antarctic climatic influence in the Magellan region.
Article
Evidence for former fast glacier flow (ice streaming) in the southwest Laurentide Ice Sheet is identified on the basis of regional glacial geomorphology and sedimentology, highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream. Preliminary mapping from a digital elevation model of Alberta identifies corridors of smoothed topography and corridor-parallel streamlined landforms (megaflutes to mega-lineations) that display high levels of spatial coherency. Ridges that lie transverse to the dominant streamlining patterns are interpreted as: (a) series of minor recessional push moraines; (b) thrust block moraines or composite ridges/hill–hole pairs constructed during readvances/surges; and (c) overridden moraines (cupola hills), apparently of thrust origin. Together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms. Localised cross-cutting and/or misalignment of flow sets indicates temporal separation and the overprinting of ice streams/lobes. The fast-flow tracks are separated by areas of interlobate or inter-stream terrain in which moraines have been constructed at the margins of neighbouring (competing) ice streams/outlet glaciers; this inter-stream terrain was covered by more sluggish, non-streaming ice during full glacial conditions. Thin tills at the centres of the fast-flow corridors, in many places unconformably overlying stratified sediments, suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams/outlet glaciers is consistent with subglacial deformation theory. In this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory, sometimes with fast-flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally. The occurrence of minor push moraines and flutings and associated landforms, because of their similarity to modern active temperate glacial landsystems, are interpreted as indicative of ice lobe marginal oscillations, possibly in response to seasonal climatic forcing, in locations where meltwater was more effectively drained from the glacier bed. Further north, the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging, possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice-bed decoupling and drawdown initiated by the development of ice-dammed lakes. Copyright © 2008 John Wiley & Sons, Ltd.