Browsing by Author "Glasser, NF"

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    10Be and 26Al exposure history of the highest mountains in Wales: evidence from Snowdon and the Glyders
    (International Union for Quaternary Research (INQUA), 2019-07-28) Hughes, PD; Glasser, NF; Fink, D
    Cosmogenic 10Be and 26Al exposure ages from samples collected from the mountains of Glyders (1001 m) and from Snowdon (1085 m) in Wales provide new insights into the glacial history of the highest mountains in the British Isles outside of Scotland. Sample elevations range from 598 to 1013 m and taken from centimetre-thick quartz veins on bedrock and also one boulder. The summits of the Glyders are characterised by intensely modified frost-shattered surfaces and have long been recognised as exemplars of mountain summit periglacial activity in the British Isles. However, numerous glacially-transported boulders on the highest ground indicate that ice overran the summits. Bedrock samples at c. 960 m yielded 10Be exposure ages of 72 ka and 153 ka. In addition, a glacially-transported boulder at 985 m gave a 10Be age of 57 ka. This boulder sample is important because it negates issues of inheritance that are possible with bedrock samples and it provides the closest estimate of the timing of ice thinning and the true exposure age of the Glyders summits. All 26Al ages were consistent indicating non-complex histories. These results clearly confirm the Glyder summits were overtopped by the Welsh Ice Cap during marine isotope stage (MIS) 4, when ice in this area was thicker than at the global last glacial maximum (LGM) in MIS 2. The summits were revealed as ice thinned during the transition from MIS 4 to 3. Both the geomorphological evidence and our new cosmogenic ages support recent ice cap modelling predictions that the summits then stood as nunataks above the LGM ice sheet surface in MIS 2. The oldest 10Be (and 26Al) age of ~150 ka from the frost-shattered summit tor displays significant nuclide inheritance and previous work has demonstrated evidence of gibbsite in the summit soils. The wide range in 10Be apparent exposure ages and the evidence of glacially-transported boulders on intensely frost-shattered bedrock suggests erosion of the Glyder summit surfaces largely proceeded by plucking/quarrying rather than abrasion. This would have occurred under cold-based conditions with ice flow dominated by internal deformation rather than sliding. In contrast, at altitudes below 900 m ice-scoured rock surfaces on both the Glyders and neighbouring Snowdon yield exposure ages consistent with deglaciation after the global LGM in MIS 2. Based on these ages and similar results from other summits in North Wales the Welsh Ice Cap rapidly thinned after c. 20 ka leading to a phase of alpine-style glaciation. However, on Snowdon, arête crests yielded very young apparent ages of ~ 5 ka. These young ages reflect continual stripping of the arête rock surfaces through the current interglacial.
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    Cenozoic landscape evolution of an East Antarctic oasis (Radok Lake area, northern Prince Charles Mountains), and its implications for the glacial and climatic history of Antarctica
    (Elsevier, 2007-03) Hambrey, MJ; Glasser, NF; McKelvey, BC; Sugden, DE; Fink, D
    Ice-free areas Antarctica reveal a multi-million year history of landscape evolution, but most attention up to now has focused on the Transantarctic Mountains. The Amery Oasis in the northern Prince Charles Mountains borders the Lambert Glacier-Amery Ice Shelf System that drains 1 million km(2) of the East Antarctic Ice Sheet, and therefore provides a record of fluctuations of both local and regional ice since the ice sheet first formed in early Oligocene time. This glacial record has been deciphered by (i) geomorphological mapping from aerial photographs and on the ground, (ii) documenting the relationship between thick well-dated, uplifted glaciomarine strata and the underlying palaeolandscape, (iii) examining surficial sediment facies, and (iv) surface-exposure dating using Be-10 and Al-26. The SE Amery Oasis records at least 10 million years of landscape evolution beginning with a pre-late Miocene phase of glacial erosion, followed by deposition of glaciomarine strata of the Battye Glacier Formation (Pagodroma Group) in late Miocene time. A wet-based ice sheet next expanded over the SE Amery Oasis, following which deposition of the glaciomarine Pliocene Bardin Bluffs Formation (Pagodroma Group) took place. Both formations were uplifted;, by at least 500 and 200m, respectively. Their tops are characterised by geomorphological surfaces upon which intensive periglacial activity took place. Higher-level bedrock areas were subjected to deep weathering and torformation. Early Pleistocene time was characterised by expansion of a cold-based ice sheet across the whole area, but it left little more than patches of sandy gravel and erratic blocks. Late Pleistocene expansion of local ice (the Battye Glacier) saw deposition of moraine-mound complexes on low ground around Radok Lake and ice-dammed lake phenomena. Subglacial drainage of the lake escaped to the east exhuming the sediment-filled gorges. Holocene landscape modification has been relatively superficial. Overall, the landscape of the Amery Oasis evolved primarily under the influence of wet-based (probably polythermal) glaciers in Miocene and Pliocene times, whereas the Quaternary Period was characterised mainly by cold-based glaciers that had comparatively little impact on the landscape. © 2007, Elsevier Ltd.
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    A community-based geological reconstruction of Antarctic ice sheet deglaciation since the last glacial maximum
    (Elsevier, 2014-09-15) Bentley, MJ; O'Cofaigh, C; Anderson, JB; Conway, H; Davies, B; Graham, AGC; Hillenbrand, CD; Hodgson, DA; Jamieson, SSR; Larter, RD; Mackintosh, AN; Smith, JA; Verleyen, E; Ackert, RP; Bart, PJ; Berg, S; Brunstein, D; Canals, M; Colhoun, EA; Crosta, X; Dickens, WA; Domack, E; Dowdeswell, JA; Dunbar, R; Ehrmann, W; Evans, J; Favier, V; Fink, D; Fogwill, CJ; Glasser, NF; Gohl, K; Golledge, NR; Goodwin, I; Gore, DB; Greenwood, SL; Hall, BL; Hall, K; Hedding, DW; Hein, AS; Hocking, EP; Jakobsson, M; Johnson, JS; Jomelli, V; Jones, RS; Klages, JP; Kristoffersen, Y; Kuhn, G; Leventer, A; Licht, K; Lilly, K; Lindow, J; Livingstone, SJ; Massé, G; McGlone, MS; McKay, RM; Melles, M; Miura, H; Mulvaney, R; Nel, W; Nitsche, FO; O'Brien, PE; Post, AL; Roberts, SJ; Saunders, KM; Selkirk, PM; Simms, AR; Spiegel, C; Stolldorf, TD; Sugden, DE; van der Putten, N; van Ommen, TD; Verfaillie, D; Vyverman, W; Wagner, B; White, DA; Witus, AE; Zwartz, D
    A robust understanding of Antarctic Ice Sheet deglacial history since the Last Glacial Maximum is important in order to constrain ice sheet and glacial-isostatic adjustment models, and to explore the forcing mechanisms responsible for ice sheet retreat. Such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and Sub-Antarctic islands. Here, we report a new synthesis of those datasets, based on an accompanying series of reviews of the geological data, organised by sector. We present a series of timeslice maps for 20 ka, 15 ka, 10 ka and 5 ka, including grounding line position and ice sheet thickness changes, along with a clear assessment of levels of confidence. The reconstruction shows that the Antarctic Ice sheet did not everywhere reach the continental shelf edge at its maximum, that initial retreat was asynchronous, and that the spatial pattern of deglaciation was highly variable, particularly on the inner shelf. The deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small Antarctic contribution to meltwater pulse 1a. We discuss key areas of uncertainty both around the continent and by time interval, and we highlight potential priorities for future work. The synthesis is intended to be a resource for the modelling and glacial geological community. © 2014 The Authors. CC BY license
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    Rapid thinning of the Welsh Ice Cap at 20–19 ka Based on 10Be Ages
    (Cambridge University Press, 2017-01-20) Hughes, PD; Glasser, NF; Fink, D
    New 10Be ages from the summits of three mountain areas of North Wales reveal a very similar exposure timing as the Welsh Ice Cap thinned after the global Last Glacial Maximum. Eight bedrock and one boulder sample gave a combined arithmetic mean exposure age of 19.08 ± 0.80 ka (4.2%, 1σ). Similar exposure ages over a 320 m vertical range (824 to 581 m altitude) show that ice cap thinning was very rapid and spatially uniform. Using the same production rate and scaling scheme, we recalculated six published 10Be exposure ages from the nearby Arans, which also covered a similar elevation range from 608 to 901 m and obtained an arithmetic mean of 19.41 ± 1.45 ka (7.5%, 1σ). The average exposure age of all 15 accepted deglaciation ages is 19.21 ± 1.07 (5.6%, 1σ). The complete dataset from North Wales provides very strong evidence indicating that these summits became exposed as nunataks at 20–19 ka. This result provides important insight to the magnitude of ice surface lowering and behavior of the Welsh Ice Cap during the last deglaciation that can be compared to other ice masses that made up the British-Irish Ice Sheet. © University of Washington
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    The timing of deglaciation from mountain summits to cirques in Wales: 10Be and 26Al exposure dates from Cadair Idris
    (Copernicus GmbH, 2021-04-19) Hughes, PD; Glasser, NF; Fink, D; Dortch, J; Fülöp, RH; Wilcken, KM; Fujioka, T
    Cosmogenic 10Be and 26Al exposure ages from 20 erratic samples collected from Cadair Idris (893 m), a mountain in southern Snowdonia, Wales, provide evidence for the timing of deglaciation from summits to cirques at the end of the Late Pleistocene. The summit of the mountain is characterised by intensely modified frost-shattered surfaces that have long been identified as a representing a former nunatak. Numerous glacially-transported quartz boulders on the highest ground indicate that ice overran the summit at some point in the Pleistocene. Two quartz boulders, one with preserved striations, sampled at c. 856 m near the summit of Cadair Idris yielded consistent 10Be and 26Al paired exposure ages of 75 ka to 60 ka (using anuhigh-latitude sea level 10Be spallation production rate of 4.20 at/g/y, scaled by the Lal/Stone scheme). A glacially polished bedrock quartzite outcrop at 735 m gave an age of 17.5 ka. Immediately below this, cirque and down-valley recessional moraine ages, covering an elevation of 480 m to 350 m ranged from 10 to 15 ka respectively. These results confirm that Cadair Idris was overridden by the Welsh Ice Cap during marine isotope stage (MIS) 4, when ice was thicker than at the global last glacial maximum (LGM) in MIS 2. This is consistent with findings from northern Snowdonia. The highest Welsh summits, including Cadair Idris, emerged above a thinning Welsh Ice Cap (British Irish Ice Sheet) during the transition from MIS 4 to 3. The summit area above ~800 m then stood as nunataks above the LGM ice sheet surface in MIS 2. The Welsh Ice Cap then rapidly thinned over Cadair Idris at ~20-17 ka based on ages from high-level ice-moulded bedrockThis is supported by more new ages from high-level paired erratics and bedrock samples on several other mountains throughout Snowdonia, leading to a phase of alpine-style deglaciation. Valley glaciers initiated their retreat up-valley from ~17 to 14 ka after Heinrich Event 1. A later phase of glacier stabilisation or still stand formation produced classic cirque moraines near the rim of a present cirque lake basin (480 m elevation) yielding 10Be ages of 13-10 ka during the Younger Dryas.