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History of Ice Sheet Elevation in East Antarctica: Paleoclimatic Implications

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History of Ice Sheet Elevation in East Antarctica: Paleoclimatic Implications Earth and Planetary Science Letters 290 (2010) 281–288 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl History of ice sheet elevation in East Antarctica: Paleoclimatic implications Xiaohan Liu a,b,⁎, Feixin Huang a,c, Ping Kong b, Aimin Fang b, Xiaoli Li b, Yitai Ju c a Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100085, China b Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China c Mineral Resource Institute of China Metallurgical Geology Bureau, Beijing, 100025, China article info abstract Article history: The multi-disciplinary study of past ice surface elevations in the Grove Mountains of interior East Antarctica Received 17 July 2007 provides direct land-based data on the behavior of the East Antarctic Ice Sheet since the Pliocene. The glacial Received in revised form 3 December 2009 geology, the ages of cold desert soils, the depositional environment of younger moraine sedimentary Accepted 6 December 2009 boulders and their spore–pollen assemblages combine to imply a possible significant shrinkage of the Ice Available online 13 January 2010 Sheet before the Middle Pliocene Epoch, with the Ice Sheet margin retreating south of the Grove Mountains Editor: Y. Ricard (∼450 km south from its present coastal position). Exposure age measurements of bedrock indicate that the elevation of the ice surface in the Grove Mountains region subsequently rose at about mid-Pliocene to at Keywords: least 200 m higher than today's levels. The ice surface then progressively lowered, with some minor Grove Mountains fluctuations. Middle to Late Pleistocene exposure ages found on the lowest samples, at the ice/bedrock fluctuation of ice surface contact line, indicate a long period with ice surface elevations kept at the current level or complex – spore pollen assemblage fluctuation history during the Quaternary Epoch. cosmogenic nuclide exposure age © 2009 Elsevier B.V. All rights reserved. Pliocene 1. Introduction constrain ice sheet models during known glacial maxima and minima in the post-14 Ma time-frame, when the earth's geography and its climate Interest in the past behavior of the East Antarctic Ice Sheet (EAIS) has system have roughly resembled their present configurations. been piqued by the growing awareness of an active subglacial Land-based data concerning the past ice surface elevation in the hydrological system (Wingham et al., 2006). This and other findings interior of the EAIS are scarce because of extremely difficult access. question whether the EAIS is as stable as had hitherto been assumed. As Outside the Transantarctic Mountains region, only a few paleo ice levels the world's largest glacial system, the EAIS first formed around 34 Ma ago, have been reported (Ingolfsson et al., 1998). In the Lambert Graben, associated in part with the thermal isolation of Antarctica by the opening northern Prince Charles Mountains, glacial sedimentary strata have of the Southern Ocean in conjunction with declining global CO2 levels recorded fluctuations in Neogene glacial ice extent (Mabin, 1992; (DeConto and Pollard, 2003). The ice sheet there oscillated on Adamson et al., 1997; Hambrey and McKelvey, 2000a,b), and observa- Milankovitch frequencies (Naish et al., 2001).TheEAISextendedbeyond tions of glacial topography reveal expansion and retreat of the EAIS its present continental margin during glacial times and retreated during margin in the region of Dronning Maud Land (Bunger Hills) (Jonsson, interglacial times to expose a coastal terrain that supported a low 1988; Holmlund and Näslund, 1994; Lintinen and Nenonen, 1997). This woodland forest (Barrett, 2007). Around 14 Ma ago the EAIS entered a paper provides new data on past ice sheet elevations from the Grove more stable state, during which many believe ice persisted in central East Mountains, 400 km inland from the coast at Prydz Bay, where the Antarctica with the ice volume and ice extent varying modestly, perhaps nunataks protrude hundreds of meters above the contemporary ice by less than 1/3 of today's 66 m of sea level equivalent (Kennett and surface (e.g., Mt. Harding, at 2338 m). Field studies were undertaken in Hodell, 1993; Barrett, 1996). A proposal in the 1980's that the EAIS might this region by the Chinese National Antarctic Research Expedition have shrunk by as much as 2/3 to allow the ocean to flood central East (CHINARE) from 1998 to 2005. The results of this endeavor include Antarctica (Burckle and Potter, 1996)hasbeencontested(Hicock et al., interpretations of glacial geology, recognition and analyses of cold desert 1996; Stroeven and Prentice, 1997; Harwood and Webb, 1998), but soils, lithologic analyses of sedimentary boulders of moraines, along with continues to have some credence (Haywood et al., 2002). Regardless of analyses of spore–pollen assemblages. Samples of bedrock from two that issue, a key constraint on our understanding of the EAIS behavior nunataks provide in situ cosmogenic nuclide 10Be and 26Al exposure ages. remains—the lack of direct evidence of ice sheet surface levels to 2. Geographic setting ⁎ Corresponding author. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China. Fax: +86 10 62375165. The Grove Mountains emerge as a group of 64 isolated nunataks 2 E-mail address: [email protected] (X. Liu). scattered over an area of ∼3200 km within the EAIS (72°20′Sto73°10′ 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.12.008 282 X. Liu et al. / Earth and Planetary Science Letters 290 (2010) 281–288 S, and 73°50′Eto75°40′E). The nunataks can be divided into 5 ranges are usually jagged, with summit areas containing well-developed forming a ridge–valley topography with an NNE trend. The regional ice ventifacts facing the dominant wind from the SE (Fig. A1). The wind has flows north-westward, perpendicular to the ridges and away from the cut meters into hard rock, suggesting that these higher areas have been ice central part of the EAIS (i.e., Dome Argus), where the ice surface free for at least several hundred thousand years. However the lower elevation exceeds 4 050 m above sea level (Fig. 1). slopes, within ∼100 m from recent ice surface, contain more recent glacial The Grove Mountains nunataks extend along the local seasonal features, such as fresh trimlines and erratics. Lower nunataks have been equilibrium line separating the more coastal zone of net ablation from eroded into “roches moutonnée” forms with oriented surface striae, the inland zone of net accumulation. Because they form obstacles to presumably from the flow of overriding ice (Fig. A2). We consider this ice flow, these nunataks are responsible for the existence of boundary between the effects of wind and glacial erosion to indicate the neighboring blue ice regions, where wind-induced ablation exceeds level of a former persistent ice surface, perhaps during the late Pliocene or the local snow accumulation. The ice surface elevation in this area early Quaternary, that has not been subsequently overtopped. averages ∼2000 m above sea level. Mount Harding in the central part of the Grove Mountains appears These nunataks consist mainly of upper amphibolite to granulite crescent shaped, open to the northwest. Both the northern and southern facies metamorphic rocks, syn-orogenic to late orogenic granite, and ends form steep crests, protruding ∼200 m above the current ice post tectonic granodioritic aplite and pegmatite. The time of surface. The central segment of the ridge-line between two summits deformation (∼550 Ma) indicates that the Grove Mountains, like descends progressively until it reaches the ice surface at a central col, similar rocks in Prydz Bay, form part of the Pan-African orogenic belt with a relic ice tongue hanging on the lee side. A stagnant field of blue (Liu et al., 2003). The absence of active structures and earthquakes, ice, tens of square kilometers wide, lies inside the crescent. An arc- and the lack of Cenozoic volcanism suggest that this region, along shaped ice-cored moraine ∼100 m wide and over 5 km long extends with Prydz Bay, has been geologically stable since at least the Late along the western edge of this blue ice field (Fig. A3). An old ice tongue Mesozoic Era (Tingey, 1991). once overrode the central col of Mount Harding, leaving this terminal moraine as the ice surface lowered. The gravel cover of this moraine has 3. Glacial geology protected the blue ice beneath from ablation since the last ice retreat, leaving it ∼25 m above the surface of the surrounding ice. The slopes of nunataks facing due the upstream of ice flow show smoothly abraded and striated bedrock, with occasional patches of 4. Sedimentary boulders and spore–pollen assemblages diamicton. The slopes facing the downstream of ice flow typically form bluffs that have been steepened by glacial abrasion. The nunataks leave The ice-cored moraine consists mainly of clasts of local metamorphic trails of superglacial debris tens of km in length on the blue ice surface, rock derived from Mount Harding, sub-angular to angular in shape and marking the present ice flow trace. The upper parts of the higher nunataks ranging in size from centimeters to meters. It also contains exotic clasts Fig. 1. The sketch map showing locality, landscape and ice flow lines in central part of Grove Mountains. X. Liu et al. / Earth and Planetary Science Letters 290 (2010) 281–288 283 of similar size range taken by the ice flow from some distance to the assemblages representing a continental flora. The compositions of south-east, beneath the interior of the EAIS. The exotic clasts include these assemblages resemble the Neogene Weddellian biogeocenose, metamorphic rock such as high-pressure granulite, ophiolitic rocks, including the major pollen types such as Penaceae, Podocarpus, meteorites, and some sedimentary rocks with subrounded shapes and Araucardiaceae, Chenopodiaceae and Artemisia (Fang et al., 2005b).
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