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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, Chinab Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, Chinac Mineral Resource Institute of China Metallurgical Geology Bureau, Beijing, 100025, China

a b s t r a c ta r t i c l e i n f o

Article history:Received 17 July 2007Received in revised form 3 December 2009Accepted 6 December 2009Available online 13 January 2010

Editor: Y. Ricard

Keywords:Grove Mountainsfluctuation of ice surfacespore–pollen assemblagecosmogenic nuclide exposure agePliocene

The multi-disciplinary study of past ice surface elevations in the Grove Mountains of interior East Antarcticaprovides direct land-based data on the behavior of the East Antarctic Ice Sheet since the Pliocene. The glacialgeology, the ages of cold desert soils, the depositional environment of younger moraine sedimentaryboulders and their spore–pollen assemblages combine to imply a possible significant shrinkage of the IceSheet before the Middle Pliocene Epoch, with the Ice Sheet margin retreating south of the Grove Mountains(∼450 km south from its present coastal position). Exposure age measurements of bedrock indicate that theelevation of the ice surface in the Grove Mountains region subsequently rose at about mid-Pliocene to atleast 200 m higher than today's levels. The ice surface then progressively lowered, with some minorfluctuations. Middle to Late Pleistocene exposure ages found on the lowest samples, at the ice/bedrockcontact line, indicate a long period with ice surface elevations kept at the current level or complexfluctuation history during the Quaternary Epoch.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Interest in the past behavior of the East Antarctic Ice Sheet (EAIS) hasbeen piqued by the growing awareness of an active subglacialhydrological system (Wingham et al., 2006). This and other findingsquestion whether the EAIS is as stable as had hitherto been assumed. Astheworld's largest glacial system, theEAISfirst formedaround34 Maago,associated in part with the thermal isolation of Antarctica by the openingof the Southern Ocean in conjunction with declining global CO2 levels(DeConto and Pollard, 2003). The ice sheet there oscillated onMilankovitch frequencies (Naish et al., 2001). The EAIS extended beyondits present continental margin during glacial times and retreated duringinterglacial times to expose a coastal terrain that supported a lowwoodland forest (Barrett, 2007). Around 14 Ma ago the EAIS entered amore stable state, duringwhichmany believe ice persisted in central EastAntarctica with the ice volume and ice extent varyingmodestly, perhapsby less than 1/3 of today's 66 m of sea level equivalent (Kennett andHodell, 1993; Barrett, 1996). A proposal in the 1980's that the EAISmighthave shrunk by as much as 2/3 to allow the ocean to flood central EastAntarctica (Burckle and Potter, 1996) has been contested (Hicock et al.,1996; Stroeven and Prentice, 1997; Harwood and Webb, 1998), butcontinues to have some credence (Haywood et al., 2002). Regardless ofthat issue, a key constraint on our understanding of the EAIS behaviorremains—the lack of direct evidence of ice sheet surface levels to

constrain ice sheet models during known glacial maxima andminima inthe post-14 Ma time-frame, when the earth's geography and its climatesystem have roughly resembled their present configurations.

Land-based data concerning the past ice surface elevation in theinterior of the EAIS are scarce because of extremely difficult access.Outside the Transantarctic Mountains region, only a few paleo ice levelshave been reported (Ingolfsson et al., 1998). In the Lambert Graben,northern Prince Charles Mountains, glacial sedimentary strata haverecorded fluctuations in Neogene glacial ice extent (Mabin, 1992;Adamson et al., 1997; Hambrey and McKelvey, 2000a,b), and observa-tions of glacial topography reveal expansion and retreat of the EAISmargin in the region of Dronning Maud Land (Bunger Hills) (Jonsson,1988; Holmlund and Näslund, 1994; Lintinen and Nenonen, 1997). Thispaper provides new data on past ice sheet elevations from the GroveMountains, 400 km inland from the coast at Prydz Bay, where thenunataks protrude hundreds of meters above the contemporary icesurface (e.g., Mt. Harding, at 2338 m). Field studies were undertaken inthis region by the Chinese National Antarctic Research Expedition(CHINARE) from 1998 to 2005. The results of this endeavor includeinterpretations of glacial geology, recognition and analyses of cold desertsoils, lithologic analyses of sedimentary boulders ofmoraines, alongwithanalyses of spore–pollen assemblages. Samples of bedrock from twonunataks provide in situ cosmogenic nuclide 10Be and 26Al exposure ages.

2. Geographic setting

The Grove Mountains emerge as a group of 64 isolated nunataksscattered over an area of∼3200 km2within the EAIS (72°20′S to 73°10′

Earth and Planetary Science Letters 290 (2010) 281–288

⁎ Corresponding author. Institute of Geology and Geophysics, Chinese Academy ofSciences, Beijing, 100029, China. Fax: +86 10 62375165.

E-mail address: [email protected] (X. Liu).

0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2009.12.008

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S, and 73°50′E to 75°40′E). The nunataks can be divided into 5 rangesforming a ridge–valley topography with an NNE trend. The regional iceflows north-westward, perpendicular to the ridges and away from thecentral part of the EAIS (i.e., Dome Argus), where the ice surfaceelevation exceeds 4 050 m above sea level (Fig. 1).

The Grove Mountains nunataks extend along the local seasonalequilibrium line separating the more coastal zone of net ablation fromthe inland zone of net accumulation. Because they form obstacles toice flow, these nunataks are responsible for the existence ofneighboring blue ice regions, where wind-induced ablation exceedsthe local snow accumulation. The ice surface elevation in this areaaverages ∼2000 m above sea level.

These nunataks consist mainly of upper amphibolite to granulitefacies metamorphic rocks, syn-orogenic to late orogenic granite, andpost tectonic granodioritic aplite and pegmatite. The time ofdeformation (∼550 Ma) indicates that the Grove Mountains, likesimilar rocks in Prydz Bay, form part of the Pan-African orogenic belt(Liu et al., 2003). The absence of active structures and earthquakes,and the lack of Cenozoic volcanism suggest that this region, alongwith Prydz Bay, has been geologically stable since at least the LateMesozoic Era (Tingey, 1991).

3. Glacial geology

The slopes of nunataks facing due the upstream of ice flow showsmoothly abraded and striated bedrock, with occasional patches ofdiamicton. The slopes facing the downstream of ice flow typically formbluffs that have been steepened by glacial abrasion. The nunataks leavetrails of superglacial debris tens of km in length on the blue ice surface,marking thepresent iceflow trace. The upper parts of thehigher nunataks

are usually jagged, with summit areas containing well-developedventifacts facing the dominant wind from the SE (Fig. A1). The wind hascutmeters intohard rock, suggesting that thesehigher areashavebeen icefree for at least several hundred thousand years. However the lowerslopes,within∼100 m from recent ice surface, containmore recent glacialfeatures, such as fresh trimlines and erratics. Lower nunataks have beeneroded into “roches moutonnée” forms with oriented surface striae,presumably from the flow of overriding ice (Fig. A2). We consider thisboundary between the effects of wind and glacial erosion to indicate thelevel of a former persistent ice surface, perhaps during the late Pliocene orearly Quaternary, that has not been subsequently overtopped.

Mount Harding in the central part of the Grove Mountains appearscrescent shaped, open to the northwest. Both thenorthern and southernends form steep crests, protruding ∼200 m above the current icesurface. The central segment of the ridge-line between two summitsdescends progressively until it reaches the ice surface at a central col,with a relic ice tongue hanging on the lee side. A stagnant field of blueice, tens of square kilometers wide, lies inside the crescent. An arc-shaped ice-cored moraine ∼100 m wide and over 5 km long extendsalong the western edge of this blue ice field (Fig. A3). An old ice tongueonce overrode the central col of Mount Harding, leaving this terminalmoraine as the ice surface lowered. The gravel cover of thismoraine hasprotected the blue ice beneath from ablation since the last ice retreat,leaving it ∼25 m above the surface of the surrounding ice.

4. Sedimentary boulders and spore–pollen assemblages

The ice-coredmoraine consistsmainly of clasts of localmetamorphicrock derived fromMount Harding, sub-angular to angular in shape andranging 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.

282 X. Liu et al. / Earth and Planetary Science Letters 290 (2010) 281–288


of similar size range taken by the ice flow from some distance to thesouth-east, beneath the interior of the EAIS. The exotic clasts includemetamorphic rock such as high-pressure granulite, ophiolitic rocks,meteorites, and some sedimentary rocks with subrounded shapes andvarnished, striated, and pitted surfaces. Lithologies of the sedimentaryboulders range fromweakly consolidated diamicton to well consolidat-ed sandstone and gravel-bearing argillaceous sandstone with clay orcalcareous cement (Fig. A4).Most of these sedimentary boulders shouldhave ages older than the formation of the continental scale stable EAIS(e.g. 14 Ma), but some of them, the glaciofluvial genetic, weaklyconsolidated diamicton, exhibit strictly younger features than those ofwell consolidated sedimentary boulders.

35 blocks of sedimentary rocks were collected from the ice-coredmoraines for the analyses in order to determine the hydrodynamicenvironment during the formation of these boulders, especially theyoungweakly consolidated diamicton. All these samples consist of non-sorted clasts of various sizes and sandy mud matrix and showing amassive texture. We chose the matrix of 11 consolidated to half-consolidated samplesmaking thin sections and statistically counted themineral components of their sandy particles. According to the EMSanalysis upon the compositions of the fine cementing minerals in thematrix, there are two kinds of cementation in these debris. One iscemented mainly by calcite, the other is cemented by clay. A total of 7samples were selected for geochemical analysis. There are two groupsthat can be subdivided according to the major element contents. Onehas a high content in CaO, low in SiO2, Al2O3 and FeO, cemented bycalcite. The other has contrary compositions, cemented by clay. Suchdifferences show it attributed not only to the different source rocks, itmay also result from the differences in their sedimentary environmentsand weathering conditions, i.e. they may come from differentlithological units. Quartz grains of similar sizes (0.25 to 0.5 mm indiameter) were picked out from two samples and observed under ascanning electron microscope (SEM). Quartz grain shapes from thediamicton matrix are angular (70–80%), subangular (10–20%), sub-rounded (5–10%) and rounded (∼5%). More than 50% of the grainsdisplay surface textures (such as oriented striae, V-shaped pits, orconchoidal fractures) that suggestmechanical abrasion typical of glacialtransport. 4 sandstones were chosen for the grain sizes characteristicsanalysis under an IBAS-2000 image analysis instrument. The values ofmean size (Mz) are similar, and their skewness (SK1) all belongs tominus, showing the feature of subglacial tillites. As for the standarddeviation (σ1) and kurtosis (Kg), there are differences between thesamples, which show different degrees of sorting, and may representthe post-depositional modifications of the grain size distribution byflowing water or changes of their medium dynamics during theprocesses of transportation and sedimentation. Such deduction issupported by the frequency curve and log-probability plot of samples(Fang et al., 2004; Fang et al., 2005a).

From the analysis upon their textural characteristics, mineralcomponents of clasts as well as matrix, geochemical compositions,and surface textures of quarts grains, coupled with the grain sizecharacteristics, indicate that these sedimentary rocks have very badtextural and compositional maturity, they suffered post-depositionalmodifications by subglacial streams or melt-water action. Suchcomplex multi-media dynamic conditions exist only on the marginof warm-based ice sheets. We interpret these diamicton clasts asproducts of a marginal regime of wet-based ice sheet, rather thanfrom the cold interior of the EAIS. These clasts could have originated i)prior to the formation of a persistent EAIS in themiddleMiocene, or ii)during a subsequent period of large scale collapse and retreat of theEAIS margin.

Our study has identified more than 25 species of Neogene plantmicrofossil from the diamicton clasts. These spore and pollenassemblages derive from a suite of glaciogenic strata hidden beneaththe EAIS, and therefore provide useful information on the evolution ofthe EAIS. Most of the pollen and spores appear to be in situ

assemblages representing a continental flora. The compositions ofthese assemblages resemble the Neogene Weddellian biogeocenose,including the major pollen types such as Penaceae, Podocarpus,Araucardiaceae, Chenopodiaceae and Artemisia (Fang et al., 2005b).The herbaceous angiosperms are important for distinguishing andsubdividing the Neogene stratum. The first herbage occurs in the earlyMiocene, with increasing species and abundance into the middleMiocene. These reached their peak in the Pliocene, when majorassemblages occurred in some special environments, such as theassemblage of Chenopodiaceae, Atemisia and Polygonaceae. Althoughthe abundance of herbaceous pollen is not high in our samples, someAtemisiaepollenites, Chenopodiaceae and Gramineae are common.We infer therefore a Neogene, possibly Pliocene, age for thesesedimentary clasts (Fang et al., 2007).

These young exotic sedimentary clasts in the Grove Mountainshave some common lithologic features that compare with otherPliocene strata in East Antarctica. Comparable strata include thePagodroma Group outcrops in the Prince Charles Mountains (White-head and McKelvey, 2001; McKelvey et al., 2001), and the SфrsdalFormation outcrops in the Vestford Hills (Quilty, 1992; Quilty et al.,2000). They are all composed of a series of glaciogenic andnonglaciogenic units of compact sedimentary rocks overlapping theTertiary strata with unconformity surface, and possess an amount ofreworked micro-fossils. Preliminary results of our pollen assemblageanalyses of these young clasts resemble those from the Meyer DesertFormation, non-marine glaciogenic deposits forming the upper part ofthe Sirius Group in the Transantarctic Mountains, with a biostrati-graphic age of less than 3.8 Ma based on reworked marine diatomdata (Ashworth and Cabtrill, 2004).

Further evidence of a Pliocene retreat of the ice sheet margin inthis sector of Antarctica comes from the Vestford Hills and LarsemannHills (mainly marine deposits with deep water foraminifera (Quilty,1992; Quilty et al., 2000; Harwood et al., 2000)), and fiord deposits inthe northern Prince Charles Mountains (Mabin, 1992; Adamson et al.,1997; Hambrey and McKelvey, 2000a,b). Whitehead and others(Barker et al., 1998;Whitehead, 2000) have recently reported that thesedimentary facies of Pliocene glacial deposits in the southern PrinceCharles Mountains is continental, and Baker et al. (Whitehead andMcKelvey, 2002) indicate that the corresponding strata in the PrydzBay derive from deep water ooze. The gradual transition from deepwater to continental facies, from Prydz Bay, via the Vestford Hills andLarsemann Hills, to the Prince Charles Mountains shows significantpaleo-topographic features, and supports the idea that the margin ofthe EAIS was once located some distance upstream of the GroveMountains, at more than ∼450 km south of the margin's presentposition.

5. Desert soils

The southern slope of Mount Harding exposes several cold desertsoil patches, distributed along a sub-horizontal zone higher than theregional wind–ice erosion limit of ∼100 m above the recent icesurface (Fig. 1) (Fig. A5). The preservation of such soils indicates thatthe regional ice sheet's surface has not overtopped this limit elevationsince the soil formed. These soil layers are characteristics of thewidespread occurrence of surface desert pavement, abundant water-soluble salts, strongly stained upper portion of soil profile, slightlyacid and negligible organic matter content. A 1:5 soil–water extractsanalysis indicates that the dominant cations are Mg2+ and Na+,followed by Ca2+ and K+, and the main anion is SO2−

4, then Cl− andNO3

−. The water laid structures and water-soluble salts indicate iontransportation in the frigid and arid environment. The distribution ofthe salts is related to the maximum content of moisture and claymineral. Clay fraction migration occurs in the soils, which is differentfrom that of other cold desert soils in Antarctica. The upper horizons ofsome soil profiles are generally stained, a process called rubification,

283X. Liu et al. / Earth and Planetary Science Letters 290 (2010) 281–288


primarily because of the weathering of iron-bearing minerals. Thereddish hues of cold desert soils have been attributed to relativelyhigh concentrations of dithionite-extractable Fe (Fed) (Li et al., 2003).According to the available data observed at present time, the dailytemperature in the Grove Mountains ranges from −13.1 to −22.6 °Cin January, i.e. the highest temperature in summer is far below zero.As the formation of soils requires melt-water, their presence indicatesa warmer climate event, with at least seasonal ice-thawing, onceexisted in the area.

Because of the lack of fossils and organic carbon, the weatheringstage analysis of cold desert soils has become a common chronologicaltool in Antarctica. The weathering processes can be indicated in thesoil profiles by a number of distinctive morphological properties,related to a number of factors, including topography, climate, andtime. With the characteristics of surface rocks, soil colors, horizondifferentiation, salt in soils and soil depth, it is possible to recognize upto six weathering stages in soils formed onmoraine and regolith alongthe Transantarctic Mountains (Campbell and Claridge, 1975; Camp-bell and Claridge, 1987). Different weathering stages correspond todifferent soil ages. With the increase of weathering, the configurationof surface rocks varies from fresh, unstained, coarse and angular ofstage 1 to very strongly stained, rounded, crumbled bedrock of stage6; the soil color varies from pale olive to light grey of stage 1 to strongbrown to yellowish red and dark red of stage 6; the soil horizondevelopment varies from nil of stage 1 to very distinct of stage 6; thesoil salts absent from stage 1, present in a few flecks in stage 2, form acontinuous horizon 20–30 cm below the surface in stage 6. Based onstandard soil weathering characteristics from stage 1 to stage 6, theweathering feature of the soils in the Mount Harding is considered to

belong to the third–fourth weathering stages, mainly the fourth stage.Surface boulders of the soils are rare and show very strong cavernousweathering or rounding, but most boulders are small, polished, andwell stained. Some desert varnish and ventifacts are also found in thiszone. Within the profile, soil colors are 5YR5/4 in the upper horizons,paler in lower horizons. In the profile of this area, salts are oftenabundant beneath surface boulders and also through the profiles.They are most commonly found in a continuous or discontinuous

Table 1Occurrence of spore–pollen from the glacial erratics in the Grove Mountains.

Spore and pollen types S1501 S1507 S1509 S1514 S1604 S1605 S1606 Nj02 Nj04 Nj05 Nj07 Nj08 Total

Cryptogam spores 28Toroisporis (Lygodiaceae) 4 1 5Granulatisporites (Pteridaceae?) 1 1 2Osmunda (Osmundaceae) 5 1 4 1 11Polypodiaceae 1 1 2Magnastriatites (Parkeriaceae) 1 4Deltoidospora 3 1Trilete spore 3 1 3

Gymnospermous pollen 99Araucardiaceae 3 1 2 6Podocarpus (Podocarpaceae) 9 3 1 6 1 1Dacrydium (Podocarpaceae) 1 2 1 1 20Taxodiaceae 1 5Pinus (Pinaceae) 24 3 7 2 1 3 40Keteleeria (Pinaceae) 1 1Picea (Pinaceae) 22 1 2 25Tsuga (Pinaceae) 1 1

Angiospermous pollen 64Chenopodiaceae 2 2 5 2 2 1 2 16Artemisia (Asteraceae) 2 2 2 1 7Asteraceae 1 1 2Gramineae 2 2Fraxinoipollenites (Oleaceae) 2 2Oleoidearumpollenites (Oleaceae) 1 1Oleaceae 1 1Operculumpollis 1 1Nothofagidites (Nothofagus) 2 1 1 1 5Rhus 2 1 3Quercus (Fagaceae) 1 1 3 1 3 3 12Juglans (Juglandaceae) 1 1Pterocarya (Juglandaceae) 1 1Liquidambar (Hamamelidaceae) 2 2Ulmus (Ulmaceae) 1 1 1 3Ulmoidepites (Ulmaceae) 1 1Tilia 1 1Proteacidites(Proteaceae) 1 1 2Tricolpopollenites 1 1Total grains 33 6 17 4 81 9 16 2 1 5 8 9 191

Fig. 2. Plot of 26Al/10Be vs. 10Be concentrations for R9204, R9207, R9213 and R9216. 10Beconcentrations have been normalized to sea level and high latitude according to thescaling method of Lal (Lal, 1991), modified by Stone (Stone, 2000).



horizon 5–10 cm below the surface of the soil. All these weatheringfeatures suggest that they formed between 0.5 and 3.5 Ma beforepresent (Li et al., 2003). Despite such large range of ages, this providessome solid evidence constraining the timing of local ice surfacefluctuations.

6. Exposure ages

To provide a chronology, we sampled bedrock profiles on Zakhar-off Ridge (Fig. A6) and Mount Harding (two typical nunataks in theGrove Mountains), and calculated surface exposure dates using in situcosmogenic nuclides 10Be and 26Al (Huang et al., 2008). The twobedrock slopes sampled are topographically smooth with stable lowangles of about ∼5°. They consist of granitic gneiss, with thin foliation(exfoliation) layers parallel to the slope surfaces.

The cosmogenic nuclide lab at the Institute of Geology andGeophysics, Chinese Academy of Sciences, carried out chemicalpreparations. Samples were first crushed to 0.1–1.0 mm size, afterwhich, each sample underwent magnetic separation. Quartz sampleswere purified by leaching 4 or 5 times in a hot ultrasonic bath with amixed solution of HF and HNO3 (Kohl and Nishiizumi, 1992), and werecompletely dissolved together with ∼0.5 mg 9Be carrier. Beryllium andaluminum were separated by ion chromatography, their hydroxidesprecipitated, and then baked to oxides at 850 °C. Procedure blankswereused to correct the measured values. Total Al concentrations werequantified in aliquots of the dissolved quartz by ICP-OES, and 10Be and26Al concentrations were measured by the accelerator mass spectrom-etry (AMS) at the Australian Nuclear Science and TechnologyOrganisation (ANSTO). The minimum exposure ages shown in Table 1were calculated using the scalingmethod of Lal (Lal, 1991), modified byStone (Stone, 2000) for Antarctica. This study used production rates of

5.1 and 31.1 atoms g−1 yr−1 for 10Be and 26Al, respectively, for sea leveland high latitude in the calculation.

Preliminary exposure ages for twelve bedrock samples indicate 10Beminimum exposure ages ranging from 3.29 to 0.57 Ma, and the 26Alminimum exposure ages ranging from 2.13 to 0.18 Ma (Huang et al.,2008). Samples from the crests of Zakharoff Ridge and Mount Hardinghave minimum 10Be ages of 2.00±0.22 and 2.30±0.26 Ma, respec-tively. These dates suggest that the crests have been above the ice sheetsurface since at least the Plio-Pleistocene boundary. Adopting a‘reasonable’ erosion rate of 5–10 cm/Ma increases the exposure agesof these two samples into themid-Pliocene. Thus the bedrock exposureages indicate an ∼200 m decrease of the ice sheet surface elevation inthe Grove Mountains since mid-Pliocene time. Analyses of sevensamples from the two profiles resulted in inconsistent 10Be and 26Alminimum exposure ages, and the five lower elevation samplessuggested complex exposure histories. The higher two samples(R9204 and R9207), which have old 10Be minimum exposure ages(N2.9 Ma), but much younger 26Al minimum exposure ages (b1.8 Ma)(Huang et al., 2008), presented a dilemma for interpretation.

To determine the cause of the inconsistent 10Be and 26Al minimumexposure ages for R9204 and R9207, we re-treated the four samplesfrom the south Mount Harding profile that had appeared inconsistent.This time we modified our procedures to avoid errors in the Alconcentration measurement. In the new procedure, we isolatedaliquots for Al measurement by ICP-OES before fume off HF in theprocess of quartz dissolution. We checked the new procedure bycomparing multi aliquot Al concentrations with those for identical 1 gquartz fractions that showed consistency within 10%. Table 1 lists themeasured values and calculated ages for the four samples. Fig. 2 showsthe 26Al/10Be ratios vs. 10Be concentrations. With this new procedure,the four samples in the south Mount Harding profile show consistent

Fig. 3. Minimum 10Be exposure ages vs. altitudes. The ages of samples 9204, 9207, 9213 and 9216 have been corrected, compared to the previous minimum exposure ages (Huanget al., 2008) (Table 2 reflects the new values).

Table 2Elevations and minimum exposure ages of bedrock samples from south Mount Harding, Grove Mountains.

Sample Elev(m)

Quartz(g)

10Be(106 atoms/g)

26Al(106 atoms/g)

26Al/10Be Minimum 10Be age(Ma)

Minimum 26Al age(Ma)

R9204 2275 17.10 74.44±2.00 249.63±8.42 3.35±0.23 3.29±0.51 2.51±0.77R9207 2250 12.71 74.83±2.24 254.34±8.16 3.40±0.22 3.47±0.58 3.02±1.29R9213 2200 16.05 34.09±0.96 130.20±17.99 3.82±0.27 1.03±0.09 0.71±0.16R9216 2175 25.36 32.96±1.01 157.03±7.21 4.76±0.30 1.01±0.09 0.98±0.12

Minimum 10Be and 26Al exposure ages calculated using the scaling method for Antarctica from Lal (Lal, 1991), modified by Stone (Stone, 2000). Errors of minimum 10Be and 26Alexposure ages include: 2% from AMS, 6% from production rate, 1% from Be carrier, and 4% from ICP-OES for Al.



10Be and 26Al minimum exposure ages, and their 10Be minimumexposure ages remain the same as the previous results. This change inprocedure indicates that the apparently inconsistent 10Be and 26Alages of the four samples (Huang et al., 2008) resulted from Alconcentration errors. Our data now show that all samples in thisprofile have simple exposure histories. The following discussion isbased on the minimum 10Be exposure ages because we believe themto be fairly accurate and closer to the actual exposure ages thanminimum 26Al ages, which may have been influenced by erosion.

Overall, the re-calculated bedrock exposure ages tend to be olderfor samples at higher elevations and younger at lower elevations, andthe minimum exposure ages of R9204 and R9207 are ∼3.29 and3.47 Ma. These ages suggest that deglaciation in the Grove Mountainsbegan no later thanmid-Pliocene and lasted no longer thanmid to latePleistocene.

Fig. 3 shows the minimum 10Be exposure ages vs. elevation on thetwo profiles. The oldest minimum 10Be exposure age on the MountHarding profile is N3.0 Ma. But the age on the pinnacle (∼200 m abovecurrent ice surface) is 2.30 Ma. On the top of the Zakharoff Ridge profile(∼150 m above current ice surface), the oldest minimum 10Beexposures age is 2.00 Ma. All of our samples exhibit minimum 10Beexposure ages that decrease downwards to the Pleistocene. Therelationship between the 10Be minimum exposure ages and theelevations of samples does not display a linear trend, but does showthe general tendency of a steadily decreasing ice sheet. The cause of thisnon-linear appearance could be due to the nature of the rock samples(different erosion rates) and/or errors in the cosmogenic nuclide geo-chronology.

The relationship between the cosmogenic nuclide concentrationsand the elevations of the samples shows that the 10Be and 26Alconcentrations have not yet reached secular equilibrium, indicating acontinuously decrease of the ice surface elevation from the mid-Pliocene. Furthermore, the oldest ages on the profiles would implythat ice covered the summits before the mid-Pliocene Epoch. Theboundary observed in the field between wind and glacial erosionmentioned above (about 100 m higher than today's ice level) exhibitsages near the Pliocene/Pleistocene boundary, and provides noevidence of a higher subsequent ice surface (Fig. 3).

7. Conclusion and discussion

Based on geomorphic evidence, inferred ages of soil formation,lithologic analyses of sedimentary boulders, and spore–pollen assem-blages, coupled with in situ cosmogenic nuclide exposure ages, wesuggest that:

1. The margin of the EAIS may have been south of the GroveMountains region, some 450 km south of its present coastalposition, before the mid-Pliocene Epoch.

2. The elevation of the ice surface in the Grove Mountains region roseto more than 200 m higher than today at about the mid-Pliocene,and remained high even during theMiddle Pliocene ClimaticWarmEvent (3.29–2.97 Ma), based on the sea surface temperature anddeep-sea bottom-water temperatures from ostracode Mg/Ca ratios(Dowsett et al., 1996; Cronin et al., 2005).

3. The ice surface then progressively lowered with some minorfluctuations during the general global cooling trend based onreconstructions from deep-marine oxygen isotope records,resulted probably from less precipitation inland of the EAIS.

4. Exposure ages of the lowest samples, from the ice/bedrock contactline, are Middle to Late Pleistocene. This indicates two possibilities:one is that the EAIS surface have remained at the current level for along period, the other is that the ice surface has re-raised to thecurrent limit of today from a lower level, with a complex history offluctuation during the Quaternary Epoch (Fig. 4).

Our results, obtained with a combination of methods, support adynamic model of the interior EAIS since the Pliocene. Because the10Be and 26Al concentrations have not yet reached secular equilibriumat the tops of the two profiles we sampled, the highest ice surface levelin the Grove Mountains may have been still much higher. The onset oflowering of the ice surface might have occurred earlier than we inferhere.

There have been similar efforts at looking at exposure age dating invarious age ranges in the EAIS. D. Fink et al. have reported the historyof ice surface fluctuation from the Pliocene–Pleistocene boundary upto the beginning of the Holocene in the Lambert Glacier Region. Theages cluster into three groups that decrease in age with bothdecreasing altitude (from 1260 to 70 m above sea level) andincreasing proximity to the Lambert Glacier-Amery Ice Shelf drainagesystem (Fink et al., 2006). A. Mackintosh et al. presented that thecoastal area thinned by at most 350 m in this region during the past13 ka (Mackintosh et al., 2007). S. Strasky et al. have concluded that amajor ice advance reaching elevations of about 500 m above presentice levels occurred between 1.125 and 1.375 Ma before the present inVictoria Land with erosion-corrected 21Ne and 10Be exposure ages.Subsequent ice fluctuations were of lesser extent with complexexposure histories (Strasky et al., 2009). L.D. Nicola et al. reportedrelict landscape features eroded by extensive ice overriding the wholecoastal area before at least 6 Ma in the Terra Nova Bay region. Sincethen, summit surfaces were continuously exposed and well preservedunder polar conditions with negligible erosion rates. The combinationof stable and radionuclide isotopes of drifts documents complexmultiple Pleistocene glacial cycles (Nicola et al., 2009). P. Oberholzeret al. have yielded minimum ages of 11 to 34 ka, 309 ka, and 2.6 Ma.Taking erosion into account, the oldest surface is 5.3 Ma old. The icesurface lowered before the Mid-Pliocene. Subsequent glacial eventsreached an elevation of hundreds of meters, but were restricted to the

Fig. 4.History of ice surface fluctuations in the Grove Mountains region since the Pliocene. Bold line represents the ice surface fluctuation through time, the solid line is supported bybedrock exposure ages, and the dashed lines on both sides of the solid line are inferred based on erosion, topographic features, and soils. The ice margin retreat, probably before theMiddle Pliocene Warmth (3.29–2.97 Ma), is based on the sedimentary boulders and their spore–pollen assemblages.



valley. Such advances took place at least twice throughout thePleistocene (Oberholzer et al., 2003) (Fig. 5).

Our evidence from the sedimentary boulders supporting a Plioceneice sheet collapse remains weak. The age of the comparable MeyerDesert Formation remains unresolved. Data from reworked marinediatoms and age estimations for paleosols on glacial deposits suggesta Pliocene age (3 Ma). However, new cosmogenic dating and dating ofvolcanic ash deposits in the Dry Valleys (Transantarctic Mountains)indicate that the Sirius Group deposits and correlated deposits on theBeardmore Glacier are Middle Miocene, appropriate for the transitionfrom wet-to-cold based glaciation (Marchant et al., 2002; Franciset al., 2007; Lewis et al., 2007, 2008). Herbaceous pollen and theposition distribution of Pliocene strata outcropping around theLambert Glacier region reveal a rapid ice sheet collapse at a youngage as Pliocene. The chronology of sedimentary boulders needs to becorroborated by further pollen assemblage studies and the offshoresediment record in Prydz Bay. Our scenario corresponds roughly withclimatic events on a global scale. For example, an extensive ice sheetstarted to form in the northern hemisphere during the Pliocene. Morefrequent, but lower amplitude, fluctuations of the ice surface duringthe Quaternary correspond with the glacial cycles recorded in thenorthern hemisphere. Overall, our new land-based data imply that theEAIS ice surface fluctuation depended not simply on the global surfacetemperature, but possible other factors such as large scale tectonicevents have to be considered.

Despite the report of Cenozoic up-lifting (presumed to beisostatic) of bedrock along the west coast of the Lambert Graben(Hambrey and McKelvey, 2000a,b), we did not detect evidence ofregional tectonic activity in the Grove Mountains region from ourgeological investigations. The bedrock shows a stable high meta-morphic crust basement without active structures such as faults,thrusting, young sediments or Cenozoic volcanism and earth-quakes, etc. Consequently, our conclusions do not consider regionalgravitational isostatic correction during the fluctuation of the icesheet.

Acknowledgements

This work was supported by grants from National Natural ScienceFoundation of China (40631004 and 40506003), and field logisticswas supported by the Chinese Polar Research Administration grantsduring 1998–2005. We thank Professor Paul Tapponnier and KennethJ. Hsü for their constructive comments. We thank Professor PeterBarrett and Tim Naish, together with Editor Claude Jaupart for theirinsightful and patient comments that helped to improve this study.We thank Dr. Bill Isherwood for assistance with the wording of thismanuscript.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.epsl.2009.12.008.

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