Exploring former subglacial Hodgson Lake, Antarctica Paper I: site description, geomorphology and limnology

Quaternary Science Reviews 28 (2009) 2295–2309

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Exploring former subglacial Hodgson Lake, Antarctica Paper I: site description, geomorphology and limnology Dominic A. Hodgson a, *,1, Stephen J. Roberts a,1, Michael J. Bentley b, a, James A. Smith a, b, Joanne S. Johnson a, Elie Verleyen c, Wim Vyverman c, Andy J. Hodson d, Melanie J. Leng e, Andreas Cziferszky a, Adrian J. Fox a, David C.W. Sanderson f a

British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK Department of Geography, University of Durham, South Road, Durham DH1 3LE, UK Department of Biology, Section of Protistology and Aquatic Ecology, University of Ghent, Krijgslaan 281 S8, 9000 Gent, Belgium d Department of Geography, University of Sheffield, Sheffield S10 2TN, UK e Natural Environment Research Council, Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK f Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride, Glasgow G75 OQF, UK b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 May 2008 Received in revised form 18 March 2009 Accepted 14 April 2009

At retreating margins of the Antarctic Ice Sheet, there are a number of locations where former subglacial lakes are emerging from under the ice but remain perennially ice-covered. This paper presents a site description of one of these lakes, Hodgson Lake, situated on southern Alexander Island, west of the Antarctic Peninsula (72 00.5490 S, 68 27.7080 W). First, we describe the physical setting of the lake using topographic and geomorphological maps. Second, we determine local ice sheet deglaciation history and the emergence of the lake using cosmogenic isotope dating of glacial erratics cross-referenced to optically stimulated luminescence dating of raised lake shoreline deltas formed during ice recession. Third we describe the physical and chemical limnology including the biological and biogeochemical evidence for life. Results show that the ice mass over Hodgson Lake was at least 295 m thick at 13.5 ka and has progressively thinned through the Holocene with the lake ice cover reaching an altitude of c. 6.5 m above the present lake ice sometime after 4.6 ka. Thick perennial ice cover persists over the lake today and the waters have remained isolated from the atmosphere with a chemical composition consistent with subglacial melting of catchment ice. The lake is ultra-oligotrophic with nutrient concentrations within the ranges of those found in the accreted lake ice of subglacial Lake Vostok. Total organic carbon and dissolved organic carbon are present, but at lower concentrations than typically recorded in continental rain. No organisms and no pigments associated with photosynthetic or bacterial activity were detected in the water column using light microscopy and high performance liquid chromatography. Increases in SO4 and cation concentrations at depth and declines in O2 provide some evidence for sulphide oxidation and very minor bacterial demand upon O2 that result in small, perhaps undetectable changes in the carbon biogeochemistry. However, in general the chemical markers of life are inconclusive and abiotic processes such as the diffusion of pore waters into the lake from its benthic sediments are far more likely to be responsible for the increased concentrations of ions at depth. The next phases of this research will be to carry out a palaeolimnological study of the lake sediments to see what they can reveal about the history of the lake in its subglacial state, and a detailed molecular analysis of the lake water and benthos to determine what forms of life are present. Combined, these studies will test some of the methodologies that will be used to explore deep continental subglacial lakes. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction

* Corresponding author. Tel.: þ44 1223 221 635; fax: þ44 1223 221 259. E-mail address: [email protected] (D.A. Hodgson). 1 These authors contributed equally to this work. 0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.04.011

Subglacial lakes in Antarctica have remained largely unexplored except using remote geophysical technologies, for example airborne radio echo-sounding (cf. Siegert et al., 2005), satellitebased altimeters (Wingham et al., 2006) and surface geophysics.

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Two lakes, Vostok and Ellsworth, have received most attention (Ellis-Evans and Wynn-Williams, 1996; Siegert et al., 2003, 2006). Lake Vostok is the largest and deepest of the more than 145 subglacial Antarctic lakes and an access hole has been drilled through w3600 m of ice to within w130 m of the lake ice-water interface. However, there are still considerable logistic and methodological challenges to overcome if such lakes are to be physically sampled without introducing drilling fluid from the access hole and other contaminants that might affect subsequent research on the water chemistry and the biological communities living there. Direct exploration of subglacial lakes under the main bodies of the East and West Antarctic Ice Sheets has therefore yet to be achieved. In contrast, at the margins of the Antarctic Ice Sheet, there are a number of locations where former subglacial lakes are emerging from under the ice following deglaciation from Last Glacial Maximum (LGM) limits (Hodgson et al., 2006). Some, such as Progress Lake in the Larsemann Hills, East Antarctica, are known to have been overridden by relatively thin ice masses at the LGM due to the proximity of ice-free habitats (Hodgson et al., 2001a; Hodgson et al., 2005), yet have remained ice-covered until relatively late in the Holocene when increased light penetration and the onset of seasonal ice-free conditions have reinstated phototrophic microbial activity (Hodgson et al., 2006). Others, such as the lake described in this paper, were, from geomorphological evidence, overridden by much larger ice masses and today remain sealed beneath thick perennial lake ice. The great value of these latter lakes is that they provide an opportunity to study the physical, chemical and biological features of subglacial lakes that have remained locked under the ice during most of the mid to late Pleistocene, providing a relatively low-cost test bed for some of the technologies and hypotheses that will be employed in the exploration of deep continental subglacial lakes (Siegert et al., 2006). In this paper, we present a multidisciplinary site description of emerging subglacial Hodgson Lake on southern Alexander Island, west of the Antarctic Peninsula. First, we describe the physical setting of the lake using topographic and geomorphological maps. Second, we determine local ice sheet deglaciation history and the emergence of the lake using cosmogenic isotope dating of glacial erratics cross-referenced to optically stimulated luminescence dating of raised lake shoreline deltas formed during the process of ice recession. Third, we describe the physical and chemical limnology including the biological and biogeochemical evidence for life. Future papers will describe the palaeolimnology of the lake (Hodgson et al., 2009) the regional dating of shoreline deltas (Roberts et al., in press) and the molecular search for life. 2. Site description & geological context Alexander Island is a large highly glaciated island (400  80 km) situated to the south west of the Antarctic Peninsula between 68 700 S and 72 600 S (Fig. 1). It is predominantly ice-covered, with some exposed nunataks, and a few notable ice-free areas such as the Ablation Point Massif. Numerous glaciers flow off Alexander Island; west into the Bach and Wilkins Ice Shelves and Bellingshausen Sea, and east into the George VI Ice Shelf. The latter is fed both by outlet glaciers from Alexander Island and the ice cap from Palmer Land on the Antarctic Peninsula (Smith et al., 2007a) and occupies most of George VI Sound. Geologically, Alexander Island consists of thick fluviatile, deltaic and submarine fan sedimentary rocks forming the Fossil Bluff Group. This was deposited unconformably during the Late Jurassic– Late Albian (c. 100 Ma) on the older deformed metasediments of the LeMay Group, the oldest rocks to outcrop on Alexander Island, formed during the middle Jurassic, prior to the break-up of

Gondwana. The Fossil Bluff Group is a fore-arc sedimentary sequence characterised by interbedded black mudstones/siltstones, conglomerates, sedimentary breccias and arkosic sandstone strata (Horne, 1968; Elliott, 1975; Taylor et al., 1979). Citadel Bastion is located in the south-eastern part of Alexander Island (Fig. 1). It is one of a number of nunataks at the southern end of a mountain ridge, dissected by glaciers, oriented north–south along the east coast. Citadel Bastion is a member of the Triton Point formation. Its upper 900 m is composed of very fine to very coarse sandstones and mudstones with interbedded palaeosols and occasional beds of vitric tuff representing fluvial channel floodplain deposits with meandering facies including notable exposures of fossil forests (Nichols and Cantrill, 2002; Howe, 2003; Howe and Francis, 2005). Alexander Island is geologically distinct from Palmer Land (Antarctic Peninsula) which is composed of Late Jurassic to Early Cretaceous calc-alkaline igneous rocks and fore-arc and backarc volcaniclastic sedimentary sequences (Saunders et al., 1980). Climate data at Citadel Bastion are limited to basic monitoring undertaken during the summer field campaign. However, annual temperatures at the British Antarctic Survey automatic weather station at nearby Mars Oasis (15 km distant at 71540 S, 68 130 W), on Southern Alexander Island range between 29  C and þ4.27  C (mean 5.15  C) and relative humidity between 41.63% and 106.53% (mean 80%). In 2000, aerial reconnaissance of southern Alexander Island (Fig. 2a) identified an extensive area of flat ice and snow within the cirque bounded by Citadel Bastion and Corner Cliffs (72 00.5490 S, 68 27.7080 W). Further aerial and ground reconnaissance in 2001 established that this was the site of a perennially ice-covered freshwater lake, with dimensions of c. 2 x 1.5 km, and a 93.4 m deep water column under 3.6–4.0 m of lake ice (Fig. 1). On its northern side the lake is bounded by the Saturn Glacier, which flows east towards George VI Sound where it discharges into George VI Ice Shelf, 1–2 km downstream, but does not enter the cirque (Figs 1a and 2a). BEDMAP geophysical survey data (Lythe et al., 2000) suggest that the Saturn Glacier occupies a c. 300–400 m deep trough adjacent to Citadel Bastion. Originally proposed as ‘Citadel Lake’, in 2007 the UK Antarctic Place-names Committee named the lake Hodgson Lake. 3. Methods 3.1. Topographic and geomorphological mapping Topographic and geomorphological maps were compiled by the Mapping and Geographical Information Centre, (BAS) at 1:25,000 scale from aerial photography (BAS/4/03: 012-025, January 2003) and GPS-surveyed ground control. Altitudes were referenced to vertical datum WGS84, and included accurate GPS determinations of the height of the lake ice surface and photogrammetric height measurements of key landforms. Lake bathymetry was measured through holes drilled in the lake ice, using acoustic echo sounders calibrated against a plumb line (precision <0.1 m). 3.2. Exposure history The exposure history of the Citadel Bastion-Corner Cliffs cirque was determined using cosmogenic isotope analysis of four erratic boulders (two pairs) sampled in the vicinity of glacially-striated bedrock on the summit of Citadel Bastion (465.4 m) and on an adjacent col (c. 297 m) (Fig. 1c). This was cross-referenced to the regional deglaciation history based on similar analyses of glacial erratic boulders either side of George VI Sound, for example at Two Step Cliffs, Moutonne´e Valley and in the Batterbee Mountains (Bentley et al., 2006). Pure quartz was extracted from crushed rock

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samples following the procedures of Kohl and Nishiizumi (1992). Samples were measured by accelerator mass spectrometry (AMS) at the Scottish Universities Environmental Research Centre (SUERC), East Kilbride, UK. Methods are described in detail in Bentley et al. (2006). To constrain the later phases of catchment deglaciation luminescence age determinations were carried out on two raised lake shoreline deltas on the southern shore of the lake situated at 8.4 and 6.5 m above the present lake level. The equivalent dose (ED) was determined on the 90–150 mm quartz fraction using a modified SAR-OSL protocol (Murray and Wintle, 2000). Samples were prepared following standard methods (Sanderson et al., 2001) and analysed using an automated Riso TL/OSL system (TL-DA-15) using blue LEDs with a wavelength of 470 nm for stimulation. High resolution, low-level gamma spectrometry was undertaken on finely powdered bulk sediment to determine the concentration of isotopes in the U- and Th- decay chains and assess the degree of secular disequilibrium. Further details of the experimental methods are presented in Roberts et al. (in press).

a

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3.3. Physical, chemical and biological limnology Vertical water column profiles of conductivity, temperature, pH and dissolved oxygen (DO) were measured at 4 m depth intervals to 90 m using a YSI 650 water quality meter, calibrated for DO within 24 h of measurements following the manufacturer’s instructions. Water samples for chemical analyses were collected in the field and froze within a few hours of collection. Thawed sub-samples were handled in glass apparatus. Analyses of cations, anions, total nitrogen (N), total organic carbon (TOC), dissolved organic carbon (DOC) and nutrients were measured in samples from fixed depths in the water column by the Analytical Laboratory, Centre for Ecology and Hydrology, Natural Environment Research Council, UK. Sodium, potassium, calcium, magnesium, iron, aluminium, manganese and silicon were determined by ICP-OES (United Kingdom Accreditation Service (UKAS), Standard Operating Procedure (SOP) 3104) with aqueous solutions analysed directly on a Perkin Elmer DV 4300. Anions nitrate, chloride and sulphate were

c

b

Fig. 1. (a/b) Location of Hodgson Lake on the Antarctic Peninsula together with regional locations cited in the text. Ablation and Moutonne´e Lakes are situated in the Ablation Point Massif. (c) Topographic map of Citadel-Bastion – Corner Cliffs together with selected geomorphological features of the cirque and Hodgson Lake.

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Fig. 2. (a) Oblique aerial photograph of Hodgson Lake looking approximately NNW from c. 3000 m highlighting some of the features from Fig. 1c. For scale, Citadel Bastion is 465 m. The lake is approximately 2 m altitude above WGS84, and its dimensions are 1.87 km (NE–SW) by 1.6 km (NW–SE). (b) Small bird-foot delta at the margin of the contemporary lake (c) Arcuate lobes of ice from the Saturn Glacier discharging into Hodgson Lake over c. 1.3 km of the glacier margin (d) The contact between buried/remnant lake ice and overlying sedimentary deposits in the raised delta at 6.5 m.

determined from direct analysis of aqueous solutions by Ion Chromatography (UKAS SOP 3103 on a Dionex DX100). Total dissolved nitrogen was determined on a Shimadzu TNM-1 analyser equipped with a thermal conductivity detector (UKAS method 3113, not accredited). DOC and TOC analyses were carried out on filtered water (pre-combusted Whatman GF/F filter paper). Samples for DOC analysis were acidified with 2 M HCl and sparged with oxygen, with inorganic carbon converted to CO2 and removed, and the remaining non-purgeable organic carbon regarded as DOC (method applicable in waters where no volatile organic carbon compounds are present). DOC and TOC were determined by

Shimadzu TOC-Vcph analyser (UKAS method 3112a, not accredited), with a detection limit of 0.5 mg/l. Nutrients phosphate and ammonium were determined by colorimetry using a SEAL AQ2 discrete analyser (UKAS method 3115). Alkalinity was determined within 1 week of sample collection via Gran titration (Golterman et al., 1978). To determine the provenance of the lake water, stable isotopes 18 O, 2H and 13CTDIC (Total Dissolved Inorganic Carbon) were measured in two litres of water from 4, 44 and 88 m water depths. These were filtered (pre-combusted Whatman GF/F filter paper) and stored in 125 ml acid-washed Nalgene bottles. Air was

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excluded from the sample bottles, the caps sealed with PVC tape, and they were stored frozen. To determine the provenance of the lake ice, bulk samples of ice collected from the modern lake and from relict ice beneath the 6.5 m raised delta at the southern shore of the lake were analysed for 18O and 2H. Stable isotope analysis of water and ice was determined by CO2 equilibrium (Epstein and Mayeda, 1953) for 18O and zinc reduction (Kendall and Coplen, 1985) for 2H, with isotope compositions reported in standard delta (d) notation (e.g., d18O, d2H, d13CTDIC) using units per mil (vs. Vienna Standard Mean Ocean Water (VSMOW) (Craig, 1961). The analytical precision (1 standard deviation) was better than 0.05 and 2& for d18O and d2H, respectively. To determine the provenance of Dissolved Inorganic Carbon (DIC), 13C/12C ratios were measured on DIC precipitated from water samples in the field by the addition of BaCl2 þ NaOH solution. Samples were filtered and washed then reacted with anhydrous phosphoric acid in vacuo overnight, at a constant 25  C (McCrea, 1950), and the isotopic ratio of the evolved CO2 measured on a VG Optima Mass Spectrometer. Isotopic results for carbonate (precipitated total dissolved inorganic carbon) are reported in standard d13C notation in per mil (& vs. VPDB), based on calibration of laboratory standards against NBS-19. Analytical reproducibility was normally better than 0.1 & (2 sigma). Isotope values were compared with similarly analysed reference data collected elsewhere on the Antarctic Peninsula (Smith et al., 2006) To study biogeochemical interactions at the sediment water interface, lake surface sediments were retrieved using a UWITEC gravity corer from a coring site 93.4 m below the ice surface, within a deep benthic trough (72 00.2560 S, 68 29.0220 W, Fig. 1c) and close to the deepest measured point. Surface sediments were analysed for particle size, water content, carbonate, organic carbon, nitrogen (from which C/N is derived) bulk organic carbon isotopic ratios (d13Corg) and radiocarbon dating of bulk organic carbon following the methods described in Hodgson et al. (2009) Direct biological analyses included microscopic enumeration of water column plankton in two litres of water collected from 4, 44 and 88 m depth. These were preserved in Lugol’s iodine and remained frozen between collection and analysis. Sub-samples of 200 ml of well mixed water were stained with Bengal rose, concentrated in a sedimentation chamber and subsequently screened for the presence of phyto- and zooplankton using a Zeiss Axiovert 135 inverted microscope (magnification 10  100). To detect components of the biota that do not leave morphological remains, filtrates (Whatman GF/C filter paper) of two litres of water from 4, 44 and 88 m water depth were analysed for photosynthetic pigments (chlorophylls, bacteriochlorophylls, carotenoids) using High Performance Liquid Chromatography with diode array detection (HPLC–DAD) following standard methods (Hodgson et al., 2005). Results from the chemical and biological analyses of the lake water, ice and sediments were compared with limnological surveys of other lakes in the region, snow and ice sources and biological reference data from catchment soil, moss, cyanobacteria and other organic material, including N, C, C/N and d13Corg measurements on allochthonous end-members. Reference lakes included Ablation Lake and Moutonne´e Lake, epishelf lakes in the Ablation Point Massif which are hydraulically connected, beneath their impounding ice shelf dams, to the marine water in George VI Sound and have stratified water columns with freshwater overlying marine water (Smith et al., 2006), ‘Cannonball Lake’, the unofficial name for a small 5 m deep freshwater lake situated just between the lateral moraine of George VI Ice Shelf and Cannonball Cliffs, 29 km north of Citadel Bastion (Hodgson, 2001) and other lakes in the published literature.

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4. Results 4.1. Topographic and geomorphological mapping The topographic map of Citadel Bastion and Corner Cliffs is presented in Fig. 1c. From this the dimensions of Hodgson Lake were measured as 1.87 km (NE–SW) and 1.60 km (NW–SE). Echo sounders recorded a maximum depth of 93.4 m. The map includes the first accurate spot heights for the summits of Citadel Bastion (465.4 m) and Corner Cliffs (238 m). The cirque occupied by Hodgson Lake is one of two north-facing cirques abutting the southern margin of the Saturn Glacier. The western cirque between Hall Cliff and Citadel Bastion (Fig. 2a) remains filled with locally derived ice that abuts the Saturn Glacier. In contrast, the cirque at Citadel Bastion is in a more advanced state of deglaciation and includes a number of ice-free slopes. Most of the ridges at the rim of the cirque are ice-free. There are two small remnant catchment glaciers discharging into the southwest corner of the lake (unofficially named ‘Citadel Bastion Glacier’ and ‘Corner Cliffs Glacier’, Fig. 1c). GPS surveys of surface topography to constrain the vertical offsets between Hodgson Lake, the Saturn Glacier, George VI Ice Shelf and sea level in George VI Sound show that the lake ice surface is only 2.3–3.9 m above WGS84. As the maximum measured depth of the lake is 93.4 m this means that the bottom of the lake is c. 89.5 m below WGS84, and therefore below sea level. Key geomorphological features of Hodgson Lake are overlain on the topographic map (Fig. 1c). Two flat-topped relict raised deltas are present on the southern shore of the lake at 8.4 and 6.5 m above the present lake ice surface (Fig. 1c). These consist of laminated fine sands, silts and interbedded gravel layers. A glacier melt water stream has cut a gully into the 6.5 m delta exposing c. 2 m of diamict overlying clean, transparent relict ice (Fig. 2d). The topmost 30 cm of the sediment is cryoturbated and is capped by a coarse gravel lag on the upper surface. This pattern of sediments overlying lake ice can be seen in some areas at the contemporary margin of the lake where small bird-foot deltas are present (Fig. 2b). It seems likely, therefore, that the buried ice is remnant lake ice and formed as a delta deposited on top of the lake ice when the lake ice surface, and possibly water levels, were higher. These raised deltas therefore record former higher levels of the lake ice surface and possibly water levels. Isotopic data (d18O and d2H) confirms that the buried ice is of likely lacustrine origin with values (mean 16.1&, 132& respectively) more similar to contemporary lake ice at Hodgson

Table 1 Isotope data (d18O, d2H and d13CTDIC) from Hodgson Lake and regional reference data sets. Sample description Hodgson Lake Water column 4 m Water column 44 m Water column 88 m Modern lake ice Relict ice from below 6.5 m raised delta Relict ice from below 6.5 m raised delta Relict ice from below 6.5 m raised delta Relict ice from below 6.5 m raised delta Regional reference data George VI Ice Shelf ice – adjacent to Ablation Valley George VI Ice Shelf ice – adjacent to Moutonne´e Valley Fresh snow – Moutonne´e Valley Winter snow pack – Moutonne´e Valley Fresh snow – Ablation Valley Winter snow pack – Ablation Valley

d18O &

d2H &

(VSMOW)

(VSMOW)

d13C & (TDIC)

20.35 20.43 20.38 17.70 16.49 16.05 16.04 16.03

160.8 163.4 161.5 144.6 134.5 130.9 130.6 131.7

15.5 12.7 11.0 – – – – –

18.74

148.6

–

18.78

147.6

–

12.92 19.40 14.88 14.27

99.3 151.3 117.8 113.6

– – – –

2300

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-80

δ2H ‰ VSMOW

-100

MV fresh snow AV snow pack AV fresh snow

-120

Hodgson Lake relict delta ice

Hodgson Lake modern lake ice

-140 MV snow pack

George VI Ice Shelf ice

-160

Hodgson Lake water colum

-180 -22

-20

-18

-16

-14

-12

δ18O ‰ VSMOW Fig. 3. Isotopes d18O and d2H values for the water column, modern lake ice and relict deltas ice at Hodgson Lake compared with regional reference data for snow and ice. The dotted line is the Global Meteoric Water Line (GMWL). MV ¼ Moutonne´e Valley, AV ¼ Ablation Valley.

Lake (17.7&, 144.6&) than contemporary lake water (20.35&, 160.8& at 4 m water depth), and dissimilar to regional reference data for compressed snow, glaciers and ice shelf ice (Table 1, Fig. 3). Some near-horizontally bedded units of the Fossil Bluff Group are exposed on the south-eastern shore of the lake. These are dominantly sandstone, but are mantled in glacial debris around much of the cirque. At the southeast corner of the lake, a prominent and continuous horizontal break in the slope is present at an altitude of 13.4 m, possibly marking a former ice limit or shoreline. Other, more discontinuous near-horizontal benches of debris are also visible around the lake but may owe their origin to underlying sub-horizontal bedrock outcrops. A large landslide is present on the eastern side of Corner Cliffs (Fig. 1c), similar to those seen elsewhere along the coast of Alexander Island such as at Two Step Cliffs and Flatiron Valley (in the Ablation Point Massif), possibly related to the withdrawal of the LGM ice sheet, early Holocene collapse of George VI Ice shelf between ca. 9600–7500 years BP (Smith et al., 2007b; Roberts et al., 2008) (not applicable to Flatiron Valley) or slope instability during a later Holocene warm period. Both the southern margin of the Saturn Glacier and the terminus of ‘Citadel Bastion Glacier’ calve arcuate lobes of glacial ice into the lake (Figs. 1c and 2c), features that are consistent with ice recession. In both cases, these icebergs merge with the surface of the lake ice and their vertical topography becomes progressively lower with distance away from the glaciers. The margin of the Saturn Glacier is steep and might be expected to penetrate further into the Hodgson Lake catchment, but does not. One possibility is that there is

a subglacial bedrock sill that prevents the Saturn Glacier from flowing into the lake basin. A similar subglacial bedrock sill is present at Moutonne´e Lake preventing the ingress of the George VI Ice Shelf. The orientation of the lake is also transverse to the main flow direction of the glacier such as occurs at Lake Wilson in the McMurdo Dry Valleys (see Fig. 1 in Webster and Webster, 1997). In such cases penetration is likely prevented by the limited plasticity of the ice. Instead it continues to flow in more or less a straight line past the entrance to the cirque. On the southern margin of the lake there is a small seasonal surface meltwater inflow stream fed by the ‘Corner Cliffs Glacier’ (Fig. 1c). Although Hodgson Lake is essentially a closed-basin lake system, it is likely that some surface meltwater from snow on the lake ice surface is able to discharge into a depression on the eastern shores of the lake between the lake ice surface, the margin of the Saturn Glacier and Corner Cliffs (indicated by a small arrow, Fig. 1c), but this was not observed during our mid-summer field campaigns. This depression is likely to be a palaeo spillway. During the peak of the summer thaw, several ephemeral melt water streams occupied gullies on steeper bedrock slopes, particularly on the east face of Citadel Bastion. Bedrock failures, or avalanching, are possible origins for the gullies (Fig. 1c). The eastern margin of ‘Corner Cliffs Glacier’ terminates on land and is flanked by ice-cored lateral moraines that are several metres high and composed of diamict consisting primarily of angular clasts. We did not observe any arcuate icebergs in front of this glacier terminus. 4.2. Exposure history In common with regional ice sheet reconstructions (Bentley et al., 2006), evidence of formerly thicker glacier ice is found throughout the catchment. Bedrock surfaces in the col to the South of Citadel Bastion are striated and these are best preserved at the edge of freshly exposed till. There are two intersecting striation trends: 060-240 and 085-265 (Fig. 1c). The former striation set generally consists of deeper cut grooves that cut into the latter set. The 085-265 set are seen on bedrock outcrops all the way up to the summit of Citadel Bastion, but are confined to the high points of the cm-scale topography of bedrock slabs. There are also numerous erratics in the col, and on the summit of Citadel Bastion. These erratics are commonly striated, faceted and some are bullet-shaped. Some elongate boulders are aligned 300-120 . Erratics are mostly composed of sandstone but there are also some erratics of a conglomerate containing cobble-sized granitic clasts. Erratics also occur sporadically along the ridge of Corner Cliffs. Cosmogenic 10Be isotope dating of the pair of erratics from the col to the south of Citadel Bastion (c. 297 m altitude, Fig. 1c) shows that they were exposed ca 13.5 ka, and the pair of erratics at the Citadel Bastion summit (465.4 m altitude) were exposed sometime later at ca 10.2 ka (Table 2).

Table 2 Cosmogenic surface exposure age dating data for Citadel Bastion boulder samples. 10Be production rates were calculated using a value of 5.1  0.3 at g1 yr1 scaled to altitude and latitude for each site (Stone, 2000). Sample

Location

CB.2001.1 CB.2001.2 CB.2001.3 CB.2001.4

Citadel Citadel Citadel Citadel

a

Bastion Bastion Bastion Bastion

summit summit col col

Latitude (S)

Longitude (W)

Altitude (m above WGS84)

STa (cm)

TSCb

Quartz mass (g)

10 Be prod. Rate (at g1 yr1  1s)

10 Be conc. (at g1  1s)

10 Be age (yr  1s)

71 71 72 72

068 068 068 068

465 465 297 297

5 2 5 5

1 0.9999 0.8172 0.8172

17.8217 34.7049 18.0735 22.6967

9.7079  0.595 9.9493  0.595 6.7367  0.505 6.7367  0.505

102,129  6065 98,989  4300 91,801  5398 92,464  5623

10,546  900 9972  737 13,670  1303 13,769  1330

59.6370 59.6370 00.0800 00.0800

31.2080 31.2080 32.0850 32.0850

ST is sample thickness. TSC is the topographic shielding correction, which was calculated using the online Geometric Shielding Calculator written by G. Balco (http://hess.ess.washington.edu/ math/general/skyline_input.php). We used a sample density of 2.65 g cm3, an attenuation length of 160 g cm3, and an erosion rate of 0 g cm2 yr1. A procedural blank gave 10Be/9Be ¼ 2.072  1014  3.361072  1015. Samples were prepared at Edinburgh University, and the AMS measurements were made at the Scottish Universities Environmental Research Centre, East Kilbride, UK, using the analytical standard NIST SRM 4325, which has 10Be/9Be ¼ 3.06072  1011. b

m.a.p.l.l. ¼ metres above present lake level. In-situ percentage water content with 10% error; attenuation factors in Aitken (1998) applied to dry dose-rate values. K2O data was obtained by XRF; U, Th, Rb by ICP-MS. Methods are described in detail in Sanderson (1998) and Sanderson et al. (2001) and Roberts et al. (in press). c

a

3.374  0.059 0.192  0.019 16  1 8.4 CB8.4 1464

b

3.210  0.065 0.136  0.014

2.237  0.094 (n ¼ 2) 2.548  0.056 (n ¼ 4) 1.982  0.075 2.572  0.035 2.363  0.090 2.510  0.035 1.038  0.024 1.312  0.016 1.247  0.028 1.311  0.016 7.797 9.394 9.709 9.474 1.791 2.215 2.081 2.401 61.62 – 97.16 – 2.29 2.385 2.71 2.274 (1) (2) (1) (2) 81 Hodgson Lake raised deltas 1460 CB6.5 6.5

Dry beta dose rate Dry TSBC dose rate Cosmic dose rate Mean total dose rate (mGya1  1s) (mGya1  1s) (mGya1  1s) (mGya1  SE) Dry gamma dose rate (mGya1  1s) Rb (ppm) U (ppm) Th (ppm) K2O/K (%)

SUTL no. Sample Height (m.a.p.l.l.)a Waterb content (%) Sample geochemistryc

Table 3 Summary luminescence dose-rate data for coarse gravel samples collected from relict raised lake deltas on the southern shore of Hodgson Lake.

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Samples for OSL dating were taken from two raised deltas; one at 6.5 m above the current lake level (sample CB6.5) collected at the interface between relict lake ice and overlying delta sediment (Fig. 2d), and one at 8.4 m (sample CB8.4) above the current lake level near the upper surface of the delta. The dating of these and other regional deltas, and their mode of formation are described in detail elsewhere (Roberts et al., in press) together with a detailed discussion of the OSL methods applied. In brief, equivalent dose (De) values for the lower delta CB6.5 were well defined and, excluding statistical outliers, unimodally distributed. Eight subsamples passed experimental validity (recycling and recuperation) tests and have a weighted mean age of 4.5  0.6 ka (Tables 3 and 4). Discs that did not pass the recycling tests have similar De values to those that did. In contrast, De values from the upper delta (sample 1464, Table 4) were larger and more scattered. We identified two potential age clusters for the CB8.4 sample: (B) 37–26 ka, and (A) 18–11 ka, and other samples beyond quartz saturation age. A single disc produced a Holocene age (5.5  8.6 ka), but we do not consider age to be valid because of the large error (Tables 3 and 4). The wide scatter of ages from the CB8.4 sample could be related to environmental factors, such as reworking of the delta sediment during deglaciation resulting in multiple bleaching histories. We regard the youngest age constraint of c. 11 ka as a maximum age estimate for the upper delta, with older ages likely representing sediment that was incompletely bleached. We discuss the impact of incomplete bleaching for dating glaciogenic sediments in further detail in Roberts et al. (in press). From the cosmogenic 10Be isotope and OSL data we infer that at 13.5–10.2 ka the catchment ice was at least 295 m thick, covering the cirque rim until 10.2 ka. Then, sometime after 11 ka (the maximum SAR-OSL age for the upper delta deposit) the upper delta was formed implying that the lake ice surface must have been at least 8.4 m above its present level. By 4.4 ka the ice had further thinned to 6.5 m above its present level (the weighted mean SAROSL age for the lower delta deposit) and then thinned progressively to its present configuration. 4.3. Limnology The bathymetry of the central and northern part of Hodgson Lake includes a central trough (Fig. 1c, inset) that deepens to the north. Lake ice thickness is 3.6–4.0 m, which is substantially thicker than that recorded on lakes further north on Alexander Island (Smith et al., 2006) and slightly thicker than the range of 2.2–3.8 m recorded at other perennially ice-covered lakes in Antarctica such as Lake Untersee in Dronning Maud Land (Wand et al., 1997). Our observations, including GPS measurements of 1.6 m of variable lake ice surface topography, and the absence of any flat refrozen moat areas confirms that the lake ice is perennial and that the lake has yet to be fully deglaciated. Vertical profiles of the water column (Fig. 4) show that the lake is not stratified and is fresh water with a minor increase in conductivity from 0.021 to 0.923 mS cm1 at the sediment water interface. There is a steady increase in temperature from 0.28  C near the surface to 0.41  C in the deeper waters, declining oxygen concentrations with depth and a small increase in pH from 7.5 to 8.0, with a step at about 60 m (Fig. 3), but water column alkalinity is similar at 3 m and 52 m (Table 5). Although there are relatively minor increases in all cations and sulphate with lake depth, there is no evidence in the water column conductivity profiles or in the ionic composition of the water column for sea water penetration into the lake via a connection with the marine waters of George VI Sound beneath the Saturn Glacier, even though the bottom of the lake is 89.5 m below WGS84, and therefore below sea level. The

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Table 4 Summarised equivalent dose SAR-OSL measurements, validity tests, weighted mean ages and age ranges for luminescence samples collected from relict raised lake deltas on the southern shore of Hodgson Lake from Roberts et al., in press. SUTL Lab no.

Sample ID

Depth (cm) or (m.a.l.l.)

na

1460 1464

CB6.5 CB8.4

62.5 8.4

8/16 1/16 5/16 4/16 1/16 5/16

Cluster

Deb (Gy)

Recycle ratioc

Total dose rated (mGya1)

Weighted mean agee (103 yr)

Age ranges (103 yr)

A B

14.5  5.3 20  29 38–64 86–126 196  27 >saturation

1.06  0.13 – – – – –

3.210  0.065 3.374  0.059 3.374  0.059 3.374  0.059 3.374  0.059 3.374  0.059

4.4  0.7 – – – – –

2.5–6.2 5.5  8.6 11–19 26–37 58  8 >100

a

Number of discs used to construct weighted mean age/total number of discs examined. Weighted mean equivalent dose value. c Weighted mean ratio of samples with recycling ratios in the range 0.9–1.1. Only samples with recycling ratios in this range were used for age calculations (Spencer and Owen, 2004). d See Table 3 for full dose-rate data and Roberts et al. (in press) for a description of methods and calculations. e Weighted mean age of discs that passed recycle ratio test. Age errors are derived from weighted means of 1-sigma standard errors and 5% uncertainty in beta source calibration (Sanderson, 1998). b

absence of tide cracks, formed in lakes where tidal forcing compresses lake ice against the shore, similarly rules out any hydraulic connection to the sea. This is in contrast with the epishelf Ablation Lake and Moutonne´e Lake 135 km to the north in the Ablation Point Massif (Fig. 1b), which have both tide cracks and saline monimolimnia as a result of their hydraulic connection with George VI Sound (Smith et al., 2006). Water column d18O and d2H values are amongst the most depleted for the region (Table 1) but lie close to the global meteoric water line (Fig. 3). Water column d18O is in the range 20.35 to 20.43& which is within the range of values for the closest regional ice cores at Beethoven Peninsula 210 km to the west (min 22.47, max. 4.504, mean 15.09&) and the Dyer Plateau to 200 km to the northeast (min. 27.7, max. 23.09, mean 25.65&), the latter mean plotting to the left of our co-isotope plot (Fig. 3). Ionic dominance in Hodgson Lake is by Na (ionic order Na > Ca > Mg > K) and Cl (ionic order Cl > SO4). In contrast, in the lower (marine-influenced) waters of epishelf Ablation Lake the cation order is Mg > Na > Ca > K with anions also Cl > SO4 (Table 5). Water column concentrations of the major ions Ca, Mg, Na, K, Cl and SO4 (Table 5) are just slightly higher than that typical of continental rain (see Table 10.5 in Wetzel, 2001). The concentrations of these ions and of Si are also present at concentrations notably lower than other Alexander Island lakes. They are also markedly more dilute than surficial streams in the region, where, for example, concentrations of SO4 may exceed 240 mg/L in snowmelt percolating through gypsum-encrusted rudimentary soils (A. Hodson, 0

Lake depth (m)

20

40

60

80

100 0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.4

Temp. (0C)

0.8

Cond. (mS/cm)

65 70 75 80

DO (%)

7.6 7.8 8.0 8.2

pH

Fig. 4. Water column profiles from Hodgson Lake showing variations in temperature ( C), conductivity, dissolved oxygen (DO) and pH with lake depth.

Unpublished data, 2008). However, the lake is enriched with respect to its Al and Fe concentrations (at least when compared with continental rain), suggesting that rock-water interaction is taking place somewhere within the system. Greater concentrations of most solutes at 88 m water depth are also evident, and might indicate that the rock-water interactions take place within sediment pore waters at the bottom of the lake. Hodgson Lake waters are therefore very dilute and comparable to subglacial melt water that has undergone a low degree of rock-water contact. The lake is ultra-oligotrophic. Low to undetectable levels of total nitrogen, nitrate, ammonium and phosphate measured in the lake are at lower concentrations than those recorded in other perennially ice-covered Antarctic Lakes such as the moat waters of Lake Wilson (Darwin Mountain region) where enrichment of inorganic nutrients, for example NO 3 of stratospheric origin, is attributed to freeze concentration processes (Vincent and Howard-Williams, 1994), and Lake Untersee (Dronning Maud Land) where elevated concentrations of NH4 and PO4 in anoxic bottom waters are attributed to diffusion from the sediment where they have been formed by the degradation of organic matter (Wand et al., 1997). Instead, in Hodgson Lake the system is closed and carbon poor, and the nutrient values more similar to the ranges found in the accreted lake ice of subglacial Lake Vostok (Karl et al., 1999), the largest Antarctic subglacial lake. Total organic carbon and dissolved organic carbon are present in the lake water but at lower levels than the median organic contents of global precipitation (TOC 1.1 mg/l, DOC 1.0 mg/l (Wetzel, 2001)) and oligotrophic lakes (TOC 2.2 mg/l, DOC 2.0 mg/ l (Wetzel, 2001)). Nevertheless, measurable quantities of organic carbon are present (Table 5), but they vary little down the water column. However, the d13C values of total dissolved inorganic carbon (d13CTDIC) in the water column are lower than atmospheric and carbonate end-members and increase with depth from 15.5 to 11.0& (Table 1). Lake surface sediment measurements of N, C, C/N, and d13C were compared with measurements on soils, gravels, mixed benthos, algae, cyanobacteria, moss and fossil wood in the Hodgson Lake catchment and other reference sites on Alexander Island to help distinguish various sources of organic matter deposited in the lake (Tables 6 and 7, Fig. 5). This showed that the C/N vs. d 13C in the surface sediments was most similar to values recorded for catchment gravels and fine grained sediments. Reference data set C/N and d13C values are generally within the range of 6.7–14.8 and 14.1 to 26.6 respectively (Fig. 5); with the exception of gravel clasts from the 6.4 m delta and the fossil wood sample which have elevated C/N ratios (79.8 and 34.8 respectively, Table 6). The lake surface sediments consisted of silty clay with a water content of

D.A. Hodgson et al. / Quaternary Science Reviews 28 (2009) 2295–2309

2303

Table 5 Hodgson Lake and reference data set lake water column chemistry including cations, anions, alkalinity, total N, TOC, DOC, and nutrients nitrate, ammonium and phosphate measured at fixed depths in the water column. Notes (1): There is a slight systematic bias between the DOC and TOC methods whereby DOC > TOC in some cases. The differences between the two datasets are of the same order as the detection limit (0.5 mg/l). One interpretation is that there is no measurable particulate organic matter as we have been unable to detect TOC > DOC. (2) It is interesting to note that in the lower (marine-influenced) waters of Ablation Lake (which exchanges water with the marine environment under the ice shelf in George VI Sound, the cation order is Mg > Na > Ca > K with anions also Cl > SO4. Typically Mg is the third most abundant ion in seawater, behind sodium and chloride. In this case the dominance of Mg over Na and Ca suggests that these salts have been rejected during congelation ice formation (Tison et al., 1993) at the base of George VI Ice Shelf. Studies have shown that CaC03.6H2O is the first salt to precipitate from seawater at a temperature close to the freezing point and that Na2SO4.lOH2O and CaS04.2H20 soon follow, at temperatures of 8  C and 10  C respectively. In contrast Mg and K ions in seawater at sub-freezing temperatures, under quasi equilibrium conditions, do not change until the temperature falls to 34  C (Richardson, 1976). Cations inc. Si Lake

Sample depth (m)

Laboratory identifier

Al mg/l

Fe mg/l

Mg mg/l

Ca mg/l

K mg/l

Na mg/l

Si mg/l

Hodgson Hodgson Hodgson Ablation Ablation Moutonne´e Moutonne´e Cannonball

4 44 88 5 70 15 35 2

EE64601 EE64602 EE64603 EE64604 EE64605 EE64606 EE64607 EE64608

0.027 0.034 0.167 <0.002 0.22 0.015 0.023 0.005

0.02 0.022 0.091 <0.001 0.005 0.004 0.003 <0.001

0.653 0.624 2.02 6.7 207 4.73 8.44 1.39

1.18 1.17 1.69 4.35 9.84 7.66 8.98 13.6

0.331 0.324 1 2.32 7.12 1.75 2.89 0.458

6.5 5.68 18.3 54.8 171 37.5 66.6 2.04

0.205 0.178 0.422 0.104 0.276 0.574 0.518 0.731

0.018

28.945

Mean Anions, NH4, PO4

0.062

Lake

Sample depth (m)

Laboratory identifier

Cl mg/l

SO4–S mg/l

Hodgson Hodgson Hodgson Ablation Ablation Moutonne´e Moutonne´e Cannonball

4 44 88 5 70 15 35 2

EE65001 EE65002 EE65003 EE65004 EE65005 EE65006 EE65007 EE65008

9.7 8.93 34.2 105 311 71.8 123 1.62

0.74 0.65 1.79 5.91 16.6 7.06 9.54 9.07

Mean Total N, TOC & DOC

83.156

Lake

Sample depth (m)

Laboratory identifier

Hodgson Hodgson Hodgson Ablation Ablation Moutonne´e Moutonne´e Cannonball

4 44 88 5 70 15 35 2

EE65401 EE65402 EE65403 EE65404 EE65405 EE65406 EE65407 EE65408

Mean Nutrients NH4, PO4

TN mg/l

TOC mg/l 0.41 0.36 0.43 0.62 0.45 0.5 0.45 1.2

0.67 0.49 0.54 0.63 0.54 0.65 0.57 1.39

na

0.55

0.69

Sample depth (m)

Laboratory identifier

NO3–N mg/l

NH4–N mg/l

PO4–P mg/l

4 44 88 5 70 15 35 2

EE65001 EE65002 EE65003 EE65004 EE65005 EE65006 EE65007 EE65008

<0.100 <0.100 <0.100 <0.100 <0.100 <0.100 <0.100 <0.100

0.012 0.01 <0.0 0.012 0.018 <0.0 0.014 0.032

<0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

na

na

na

Lake

Sample depth (m) 3 52

Mean

Laboratory identifier

45.303

0.376

DOC mg/l

<0.04 <0.04 <0.04 <0.04 <0.04 <0.04 0.04 0.3

Lake

Hodgson Hodgson

2.024

6.420

Hodgson Hodgson Hodgson Ablation Ablation Moutonne´e Moutonne´e Cannonball Mean Alkalinity

6.059

Alkalinity meq. I1 0.052 0.054 0.053

38% (Table 7). Total organic carbon content was 0.5%. Radiocarbon dating of the surface sediment bulk organic fraction gave an age of 17,000  70 years Before Present (uncorrected). Direct analyses of biological material consisting of light microscopic analyses of particulates in preserved water column samples showed that no protists or zooplankton taxa were present, implying that primary production by eukaryotic algae is below the detection limits of light microscopy. This was supported by solvent extractions of photosynthetic pigments in water column filtrates

that had no colour, and HPLC analyses where pigments in all samples were below pigment detection limits. In contrast, the biological reference data set samples from the catchment meltwater streams and wet seepages has a biota similar to that observed elsewhere on Alexander Island (Smith et al., 2006), and included a range of aerophilic diatom taxa including Gomphonema, Stauroneis, Muelleria, Hantzschia, Luticola, Pinnularia, Diadesmis, and Psammothidium, filamentous green algae and abundant cyanobacterial mats.

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Table 6 N, C, C/N and d13C measurements on soils, gravels, mixed benthos, algae, cyanobacteria, moss and fossil wood at Hodgson Lake and other reference sites on Alexander Island. ML ¼ Moutonne´e Lake, AB ¼ Ablation Lake, GVIIS ¼ George VI Ice Shelf. C/N

d13C

34.8 6.7 10.4 6.7 9.3 8.0 12.7

25.6 25.1 24.9 – 25.0 26.6 25.4

0.6 1.5 4.1 3.9 10.5 2.2 3.8

13.9 9.7 14.8 14.0 12.6 10.1 12.52

25.4 18.6 24.3 23.2 17.8 17.9 21.2

9.5 9.0 1.5 6.4 – 0.9 3.8 13.3 6.3

9.8 12.6 12.5 10.5 – 9.6 10.0 12.0 11.0

18.3 14.1 19.2 17.8 17.3 22.9 18.9 19.1 18.45

0.5

5.6

11.7

23.5

0.1

5.4

79.8

23.6

Site/environment

Material

%N

Soils and gravels Hodgson Lake 6.5.m raised delta Hodgson Lake 8.4.m raised delta Hodgson Lake 8.4 m raised delta ML Valley – melt pool ML AB Valley – soil from frost sorted polygon Mean

Gravel clasts assorted Gravel clasts assorted Gravel clasts (<2 mm fraction) Fine grain seds Gravel clast >10 mm (Mudstone) Soil

0.0 0.0 0.0 0.1 0.0 0.0 0.0

0.1 0.1 0.1 0.3 0.2 0.3 0.2

Mixed Benthos ML Valley, NE meltpool on ice shelf moraine ML Valley, NE meltpool on ice shelf moraine ML Valley, NE stream between meltpools on ice shelf ML, Main inflow stream AB Valley, wet seepage adjacent to camp AB Valley, 1Km upstream of camp nr. Cairn Mean

Mixed Mixed Mixed Mixed Mixed Mixed

0.0 0.2 0.3 0.3 0.8 0.2 0.3

Algae and cyanobacteria and moss ML Valley, South Inflow Stream ML Valley, North Inflow Stream ML Valley, North Inflow Stream upstream of moraine ML Valley, NW Inflow Stream ML Valley ML Valley, South Inflow (ice shelf proximal) ML, South Inflow stream ML Valley – floor Mean

Green filamentous benthic cyanobacteria Green filamentous benthic cyanobacteria Benthic cyano. mat Filamentous benthic cyano. Red/pink benthic cyano. Benthic cyano. mat Epilithic cyano. Black epilithic cyano.

1.0 0.7 0.1 0.6 – 0.1 0.4 1.1 0.6

Moss GVI-IS, marginal moraine

Moss in wet seepage

Fossil material Citadel Bastion – Fossil Wood

Fossil Wood

benthos benthos benthos benthos benthos benthos

5. Discussion 5.1. Geomorphology

overridden by ice at the LGM (Bentley et al., 2006) when a major ice stream occupied George VI Sound (Smith et al., 2007a). The nearby exposure ages from the east coast of Alexander Island show that thinning of outlet glaciers was underway by 15–10 ka (Moutonne´e Valley) and in the southern part of Alexander Island by 7.4 ka (Two

-14 -16 -18

δ13C

Numerous geomorphological features such as glacial striae, plucked bedrock and the exposure ages of glacial erratics on the cirque ridge above Hodgson Lake are evidence that Citadel Bastion and Corner Cliffs were overridden by ice at some time during the last glacial cycle and that this ice persisted there until after 13.5 ka. This is consistent with the known deglaciation history of the Antarctic Peninsula Ice Sheet in eastern Alexander Island and western Palmer Land where the presence of striations in cols and on summits, and 29 cosmogenic 10Be/26Al exposure ages have shown that most of the uplands adjacent to George VI Sound were

Table 7 Physical and chemical properties of Hodgson Lake surface sediment, sampled at a depth of 93.4 m.

-20 -22 -24 -26

Surface sediment properties Sand Silt Clay Water content Organic carbon by LOI Carbonate by LOI Total organic carbona Nitrogen Carbon: Nitrogen d13Corg AMS 14C radiocarbon age

%C

1.08 59.35 39.57 37.81 5.13 2.37 0.477 0.048 9.94 25.10 17,000  70

Note: sand silt and clay measured at 4 cm sediment depth. a Preferred estimate of how much organic material is present.

% % % % % % % % ratio & yrBP

-28 6

8

10

12

14

16

18

20

C/N Catchment gravel and fine grained sediments Mixed benthos from streams and meltpools Cyanobacteria from streams and meltpools Moss in wet seepage Soil from frost sorted polygon Hodgson Lake surface sediment

Fig. 5. C/N and d13C in Hodgson Lake surface sediments compared with regional reference data.

D.A. Hodgson et al. / Quaternary Science Reviews 28 (2009) 2295–2309

Step Cliffs) (Bentley et al., 2006). Evidence from the nearest relative sea level curves in Marguerite Bay (although 470 km distant) suggests that, regionally, the Holocene deglaciation was likely monotonic with no evidence of major glacial readvance events (Bentley et al., 2005). However, on a local valley scale, glacier fluctuations are evident from moraines in the Ablation Point Massif (135 km to the North), but these remain undated (Sugden and Clapperton, 1980; Clapperton and Sugden, 1982). The striations and erratics show that Citadel Bastion has been over-run by ice on at least one occasion. Ice flow was directed along an axis aligned approximately ENE–WSW during glacial over-riding (Fig. 1c). The cross-cutting striation directions may be related to different glacial episodes or to evolving ice dynamics during a single glacial event. Cosmogenic isotope ages show that Holocene ice recession was underway from the col site by 13.5 ka and that Citadel Bastion summit was deglaciated sometime later at ca 10.2 ka (Table 2). This suggests that a vestigial ice cap may have been present on the summit for ca 3.3 ka following ice sheet deglaciation; a potentially analogous ice cap is the unnamed plateau ice cap at 800 m a.s.l directly east of the Batterbee Mountains which is described in Bentley et al. (2006). Furthermore, the Hall Cliff cirque to the immediate west of Citadel Bastion (Fig. 2a) is still filled with locally derived ice, which suggests that the Citadel Bastion catchment is close to the threshold between net ice ablation and accumulation. Factors that may have promoted the earlier onset of deglaciation at Citadel Bastion relative to the Hall Cliff cirque are the steep headwall, which in places is too steep to allow accumulation, and the well-defined cirque rim, which has acted as a barrier to further ice inflow from the west. Being still ice-bound, the adjacent Hall Cliff cirque (Fig. 2a) is probably a good analogue for the configuration of Hodgson Lake immediately prior to deglaciation. It is not known if the Hall Cliff cirque contains a subglacial lake, but this could easily be tested in future with radio echo-sounding. The cosmogenic isotope data therefore suggest that Hodgson Lake remained under an ice mass at least 295–400 m thick until sometime after 13.5 ka when the col became ice-free. During the early to mid-Holocene the catchment and Saturn Glaciers would have undergone progressive thinning. This would also have resulted in a gradual lowering of the spillway of the lake where the Saturn Glacier meets Corner Cliffs and a gradual decline in the thickness of ice over the lake. Evidence for this includes the discontinuous horizontal benches and breaks of slope, both of which could also be attributed to the decreasing thickness of catchment ice. It is possible that the water level in the lake at this time was higher than present, as a result of thicker bounding glaciers during deglaciation. The raised deltas, overlying relict lake ice, are the first good evidence that higher lake ice surfaces were present in the catchment. Against a background of Holocene deglaciation, these higher lake surfaces were possibly maintained by increased meltwater availability and/or snow and snow and ice accumulation during the well-documented early (11–9.5 ka) and mid-late Holocene (c. 4.5–2 ka) warm periods (Hodgson et al., 2004a; Bentley et al., 2009; Roberts et al., in press). The OSL data provide only weak evidence that the upper delta formed in the early Holocene warm period, but the weighted mean OSL date from the lower delta of 4.4  0.7 ka is coeval with the mid to lateHolocene warming on the Antarctic Peninsula. The geomorphological and geochemical evidence suggests that the bounding ice dam of the Saturn Glacier has remained thick enough to isolate the lake from the influence of the marine incursions both during the early Holocene retreat of George VI Ice Shelf (Bentley et al., 2005), the Holocene marine high stand (41 m in Marguerite Bay), and the mid to late-Holocene (c. 4.5–2 ka) warm period. The sequential dating of these geomorphological features (erratics, upper and lower deltas) is consistent with the gradual

2305

deglaciation of the catchment, particularly in the late-Holocene when ice readvances would have likely removed or reworked the ephemeral deltas. From the OSL age of the lower delta, we conclude that the lake ice surface was at an altitude of 6.5 m above the present lake ice sometime after 4.5–2 ka, impounded by a thicker Saturn Glacier. At this time it is likely that the ‘Citadel Glacier’ (and possibly the ‘Corner Cliffs Glacier’) still likely covered a greater surface area of the lake compared with their present configuration, which includes features such as the arcuate lobes of calved glacial ice associated with catchment ice retreat. 5.2. Limnology Hodgson Lake has now emerged from under the retreating ice of the local catchment glaciers, but remains perennially ice covered. Geomorphological evidence suggests that the lake remains essentially a closed-system with only minor seasonal inflows from local snowmelt and glacier decay which discharge via the shoreline deltas onto the lake ice surface, and possibly some surface outflows down the spillway onto the Saturn Glacier, though the latter were not observed. The old surface sediment radiocarbon age (Table 7) also supports continued closure of the lake with respect to the modern atmosphere and atmospheric carbon. Further tests dating base-insoluble (humin) compounds in the surface sediments (to exclude the influence of old particulate carbon in the sediments) did not result in a younger age for the surface sediment and suggests that there is currently no ‘zero age’ material in the lake (see Hodgson et al., 2009). 5.3. Biogeochemistry Vertical profiles of the water column of Hodgson Lake show only weak stratification features and an absence of the structures seen in epishelf lakes in the Ablation Point Massif (Smith et al., 2006). The lower water temperatures recorded directly beneath the lake ice are a feature of many polar lakes and the result of water being in contact with the lake ice causing inverse stratification (Laybourn-Parry et al., 2001). Deeper in the water column temperatures increase; this may be a response to the geothermal heat flux and the insulating properties of the overlying ice. Hodgson Lake anion and cation concentrations are also lower than other freshwaters in the reference data set, with values of the four major cations Ca, Mg, Na, K and anions Cl and SO4 being just slightly higher than that recorded in continental rain. The ionic order, dominated by Na and Cl (Na > Ca > Mg > K; Cl > SO4), is therefore consistent with the water being derived from meteoric precipitation acquired from the ocean (Wetzel, 2001), deposited up in the catchment as snow, and accumulating as firn and ice, then entering the lake through subglacial basal melting (Fig. 6). The water column d18O and d2H values are amongst the most depleted for the region (Table 1), are similar to values for compressed winter snow pack in Moutonne´e Valley, lie close to the global meteoric water line (Fig. 3) and are within the ranges recorded in regional ice cores. This, together with the very low conductivity, provides additional evidence that the source water is primarily derived from subglacial melting and has not had prolonged access to the atmosphere, which would have resulted in a net loss of water from the basin, concentrating the ions in the water column. Similarly, solute exclusion during continuous formation and ablation loss of the ice cover would have the effect of driving d18O and d2H off the Global Meteoric Water Line. Isolation from the atmosphere is further supported by water chemistry measurements showing an apparent absence of freezing and evaporation concentration mechanisms which are responsible for increased salt concentrations elsewhere in Antarctica (Hodgson

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Fig. 6. Summary diagram illustrating a number of the physical and chemical attributes that define the constraints on life in Hodgson Lake, including evidence for the abiotic and biotic processes that may be occurring in the water column and sediments. This cross section is oriented approximately between ‘a’ to ‘c’ on Fig. 1c.

et al., 2001b) and have been cited as a primary source of nutrient (nitrate) enrichment in polar lakes (Vincent and Howard-Williams, 1994). Loss of water through sublimation of lake ice also appears to be rather limited compared with perennially ice-covered lakes such as Lake Untersee in Dronning Maud Land where water loss through sublimation has resulted in elevated lake surface water salt concentrations approximately 10 times (Na) and 5 times (Cl) higher than those measured in Hodgson Lake (Hermichen et al., 1985). Inputs from surface streams also appear to be negligible because the marginally higher conductivity measured in catchment melt water (0.035 mS/cm) has no effect on the water column conductivity profile (which was uniformly between 0.021 and 0.022 mS/cm above 76 m, Fig. 4). The lower relative concentrations of Ca, HCO3 (alkalinity) and SO4 in the lake also suggest a minor influence of catchment geology on the ion chemistry, in contrast to Lake Untersee where values are more than 40 (Ca) and 213 (HCO3) times higher. Certainly, there is no evidence for the influence of secondary gypsum and CaCO3 precipitates upon the aqueous chemistry, as observed at Mars Oasis and other flowpaths on Alexander Island (Miekeljohn and Hall, 1997; A. Hodson, Unpubl. Data) and in the reference data set at Cannonball Lake (Table 5). Collectively this biogeochemical evidence suggests that the thick glacier cover over the site during the last glacial has been replaced by perennial lake ice in the late-Holocene, and the lake has experienced continued isolation from the atmosphere and no significant inflows.

5.4. Evidence of life Subglacial chemical weathering is well known for being able to take place whilst decoupled from the atmosphere (e.g. Tranter et al., 2005). In these instances, proton provision from the oxidation of organic carbon and sulphide mineral surfaces can readily fuel rock weathering reactions (Tranter et al., 2002). Since both processes may be attributed to microbially mediated reactions, they form a basis for prospecting for subglacial life using chemical fingerprints (Hodson et al., 2008). However, sulphide oxidation may initially be achieved by purely abiotic reactions using O2 derived indirectly from the atmosphere via glacier melt. Therefore it is when sulphide oxidation is thought to be achieved in anoxic environments that bacterially mediated reactions can be invoked more reliably (Tranter et al., 2005). The O2 demand that is discernable from Hodgson Lake is therefore a potential indicator of bacterial oxidation of organic carbon, but the possibility that inorganic oxidation of sulphides is occurring means the subject requires closer scrutiny. Inorganic oxidation of sulphide minerals furnishes SO4 and protons to solution and at the same time depletes 15 moles of O2 for every 8 moles of SO4 produced (Equation (1)). Oxic, pH 7–8 conditions typical of subglacial environments and observed in Hodgson Lake mean that Fe is precipitated as Fe-oxyhydroxides, rather than being solubilised. Equation (1) depicts sulphide oxidation when coupled to carbonate dissolution: a pairing that is necessary in order to explain cation enrichment and the absence of

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low pHs in subglacial environments. In principle, any mineral susceptible to acid hydrolysis can therefore be substituted for CaCO3 in Equation (1).

4FeS2ðsÞ þ 16CaCO3ðsÞ þ 15O2ðaqÞ þ 14H2 Oð1Þ  2 ¼ 16Ca2þ ðaqÞ þ 16HCO3ðaqÞ þ 8SO4ðaqÞ þ 4FeðOHÞ3ðsÞ

(1)

Evidence for sulphide oxidation in Hodgson Lake data includes the increase in SO4 and cation concentrations at depth whilst O2 concentrations decline. The oxidation of trace sulphides settling within the water column after delivery from streams and ice flow could account for some or all of these changes. However, even if all SO4 were attributed to Equation (1), concentrations of SO4 are trivial when compared with the O2 change (which is itself rather modest). This most likely rules out a water column process because no other obvious O2 sink exists (e.g. DOC oxidation – levels barely vary and are low). It is also likely that water column chemistry is a poor indicator of the biogeochemical cycling of sulphur and organic carbon that might be occurring elsewhere in the system: especially in the lake sediments. The diffusion of pore waters into the lake from its benthic sediments is far more likely to be responsible for the increased concentrations of ions at depth within the lake. These sediments are also ideal habitats for bacteria and so the likelihood that anoxia is maintained here by bacterial oxidation of organic carbon deserves consideration. Under these circumstances, sulphide oxidation is most likely achieved by Equation (2) (Tranter et al., 2005). 2þ FeS2 þ 14Fe3þ þ 8H2 O/2SO2 þ 16Hþ 4 þ 15Fe

(2)

This mechanism can also explain the elevated Fe concentrations at depth in the lake because Fe2þ is soluble and can thus diffuse into the water column. However, it requires anoxia and it is not at all clear from the lake column O2 profiles whether anoxia actually prevails within the sediment pore waters below. Tentatively, the þ absence of NO 3 in the presence of NH4 could indicate that denitrification is occurring, as is now known to occur beneath High Arctic glaciers (Wynn et al., 2007). Further, the d13C composition of total dissolved inorganic carbon (d13CTDIC) is lower (15.5 to 11.0&) than atmospheric and carbonate-derived end-members, and might indicate the production of microbial CO2. The problem here however, is that the greatest d13CTDIC values lie nearest the lake sediments (Table 1). Also, carbonate weathering can produce low d13CTDIC values relative to the d13C of the host mineral in glacial environments due to fractionation effects associated with glacial crushing (Skidmore et al., 2004), although these effects are pronounced in higher energy glacial environments that are almost certainly characterised by shorter rock-water contact times than Hodgson Lake. Lastly, since there are also potential inorganic oxidation mechanisms that can operate in anoxic conditions (e.g. contact between MnO2 and FeS2: (Schippers and Jørgensen, 2001)), the chemical markers of life in Hodgson Lake are inconclusive and at best betray very minor bacterial demand upon O2 and other oxidants that result in small, perhaps undetectable changes in the carbon biogeochemistry. This is perhaps unsurprising, given the low energy, ultra-oligotrophic nature of the lake. Despite being essentially a closed-system the heterotrophic potential of the lake is nevertheless supported by measurable dissolved organic carbon (cf. Christner et al., 2006) in both the water column and the sediments. However, direct microscopic analyses of particulates in Lugol’s preserved water column samples revealed no protists or zooplankton taxa, implying that primary production by eukaryotic algae is below microscopic detection limits. Particularly puzzling is the absence of cyanobacteria, which are the dominant

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phototrophs in many high latitude lakes (Vincent, 2000; Hodgson et al., 2004b). These are common in the catchment meltwater streams, seepages and snow-melt patches together with filamentous green algae and diatoms. However, they appear to have limited access to the water column. We cannot rule out their presence under the ice in the littoral zone as this was not directly sampled. Similarly, pigments including chlorophylls, bacteriochlorophylls and carotenoids, could not be detected in the water column using ultra-sensitive HPLC–DAD methods, which suggests that the biota is more limited than that measured in so called ‘end-member’ east Antarctic lakes at comparable latitudes such as >435 m deep Beaver Lake in the Amery Oasis (70–72 S, 64–69 E) where chlorophyll a concentrations were measured up to 4 mg L1 (Laybourn-Parry et al., 2001). The absence of lacustrine diatoms in the water column is also similar to Beaver Lake where a diatom phytoplankton and benthic diatom flora is absent (Cremer et al., 2004) and coupled with extremely low concentrations of bacteria and phototrophic nanoflagellates (Laybourn-Parry et al., 2001). As a result of the paucity of biological activity in the water column, surface sediment organic content is also very low (TOC, 0.5%) and just marginally higher than water column TOC. However, this carbon cannot reliably be attributed to autochthonous production and may simply be accounted for by the presence of carbon-bearing sediments or various volatile components (Meyers and Teranes, 2001). This is supported by the C/N and d13C (Fig. 5) and TOC data (Table 6), which shows that the surface sediments share similar values with catchment gravels (mean 0.2% TOC) and fine grained sediments from the lake deltas. Some of this additional sedimentary carbon could therefore be derived from kerogen, a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. Although we found no direct evidence of life using these basic reconnaissance methods, it is certainly possible that a proportion of this carbon, both in catchment gravels water column and lake sediments, is associated with the presence of bacteria including sulphate reducers iron oxidisers and methanogens which have been predicted by geochemistry to exist in other subglacial environments (Skidmore et al., 2000). For example, bacterial communities have been found in subglacial waters and sediment laden ice under Canadian High Arctic glaciers (Cheng and Foght, 2007) with subglacial drainage systems at the glacier bed providing organic and inorganic nutrients for microbial activity. However, there is no evidence that these communities are abundant in Hodgson Lake as, despite the apparent oxygen demand implied by declining oxygen concentrations in the water column both the iron and the sulphate values increase with depth in the water column (Table 5) suggesting that sulphate reducers and iron oxidisers are not having a measurable impact on the water column chemistry (methane was not measured). A number of physical and chemical attributes define the constraints on abundant life in Hodgson Lake (Fig. 6). These include the perennial ice and snow cover which are likely to explain the low productivity, low temperature, limited lake circulation (Hawes, 1983), low light penetration (Palmisano and Simmons, 1987), low sediment deposition rates (Doran et al., 1994) and the absence of atmosphere–water column interactions (Wharton et al., 1993) including nutrient supply. Despite the strong evidence of isolation of the water column from the atmosphere a number of the prerequisites for life are present including the biologically utilisable sulphides and organic carbon and/or kerogen, and oxidising agents in subglacial water such as found in subglacial Lake Vostok (Wadham et al., 2004). Thus, we cannot rule out that some of the measurements in the water column and sediments are associated with biological activity or are the product of in situ microbial decomposition. These include d13CTDIC values that decline with

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depth (Table 1) which in studies of Arctic glaciers has been interpreted as an indicator of chemoautotrophic microbial oxidation of organic carbon (Wadham et al., 2004). At best, this suggests that a nano/pico plankton might be present in the photic zone. Similarly, declining oxygen values in the water column may be associated with a biological oxygen demand from microbial (bacterial) activity in the surface sediments. Related to this, it is worth noting that the surface sediment of TOC and nitrogen values (Table 7) are marginally higher than those measured in subglacial sediments from some temperate southern hemisphere glaciers in New Zealand (Foght et al., 2004). Surface sediment nitrogen is also high (relative to carbon) and suggests an absence of nitrogen fixation but does not rule out the presence of nitrate reducers. Further ultra-sensitive studies of the lake’s microbiology and water column carbon chemistry are warranted, together with molecular analyses to determine what life is present and to allow correlation of water and sediment biogeochemistry with microbial populations. Culture-based and molecular biological studies including gene probes are being applied to identify the biogeochemical activity and therefore the types of organisms that might be present in the lake (e.g., bacteria, Archaea, lower Eukaryota, or viruses). 6. Conclusions 1) Subglacial lakes at retreating margins of the Antarctic ice sheet provide a valuable, relatively low cost opportunity to test the technologies and hypotheses that will be employed in the exploration of deep continental subglacial lakes. 2) Hodgson Lake remained under an ice mass at least 295 m thick until sometime after 13.5 ka. Progressive thinning of the ice mass occurred during the Holocene, leaving behind evidence such as discontinuous horizontal benches, breaks of slope and raised deltas. From the OSL age of the lower relict delta, we conclude that the ice overlying the lake was at an attitude of 6.5 m above the present lake ice sometime after 4.6 ka. The thick glacial ice cover over the site has been replaced by perennial lake ice in the late-Holocene. 3) The lake remains isolated from the atmosphere by perennial lake ice up to 4 m thick and has no significant inflows. Evidence of isolation from the atmosphere is provided by radiocarbon dating and water column ion concentrations, the latter being lower than other freshwaters on Alexander Island and only slightly higher than is typically recorded in continental rain. Water column d18O and d2H values are amongst the most depleted for the region and lie close to the global meteoric water line. This together with the ionic order is consistent with the water being derived from meteoric precipitation acquired from the ocean accumulated as catchment ice and subsequently melted into the water column. Relatively low concentrations of calcium and bicarbonate ions suggest only a minor influence of catchment geology on the ion chemistry. 4) Hodgson Lake is ultra-oligotrophic. Nutrients are broadly within the ranges of those found in the accreted lake ice of subglacial Lake Vostok. Total organic carbon and dissolved organic carbon are present but at lower concentrations than the median organic content reported for global precipitation. This suggests that the carbon in the lake might be ‘legacy’ carbon from older carbon-bearing sediments and meteoric precipitation. 5) The chemical markers of life in Hodgson Lake are inconclusive and at best betray very minor bacterial demand upon O2 and other oxidants that result in small, perhaps undetectable changes in the carbon biogeochemistry. Microscopic analyses of water samples revealed no microscopic organisms which

suggests that if a biota is present, then its biomass is more limited than that measured in other ‘end-member’ east Antarctic lakes such as Beaver Lake. This is perhaps unsurprising, given the low energy, ultra-oligotrophic nature of the lake. 6) The next phase of this research will be to carry out a detailed molecular analysis of the lake water and benthos to determine what life is present, and a palaeolimnological study of the lake sediments to see what they can reveal about the history of the lake in its subglacial state (Paper II). Acknowledgements This work was funded by the UK Natural Environment Research Council SAGES-10K and CACHE-PEP projects led by Eric Wolff and DH and a NERC AFI funded project led by DS, MB and DH. EV and WV are funded by Belgian Science Policy project HOLANT and the Fund for Scientific Research – Flanders. The British Antarctic Survey provided the logistic access to the study site and a field assistant, Adam Hunt. Hilary Blagborough (BAS) is thanked for assistance with cosmogenic sample preparation. Staff at University of Edinburgh, the SUERC and the SUERC AMS facility are thanked for their assistance with OSL and cosmogenic sample preparation and measurement. Staff at the UK Natural Environment Research Council Centre for Ecology and Hydrology are thanked for their water chemistry analytical services. Martyn Tranter and an anonymous reviewer provided invaluable suggestions. This work forms part of the proof of concept studies for the exploration of subglacial Lake Ellsworth led by Martin Siegert. References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. ISBN 0 19 854092. Bentley, M.J., Fogwill, C.J., Kubik, P.W., Sugden, D.E., 2006. Geomorphological evidence and cosmogenic 10Be/26Al exposure ages for the Last Glacial Maximum and deglaciation of the Antarctic Peninsula Ice Sheet. GSA Bulletin 118 (9/10), 1149–1159. Bentley, M.J., Hodgson, D.A., Smith, J.A., Cox, N.J., 2005. Relative sea level curves for the South Shetland Islands and Marguerite Bay, Antarctic Peninsula. Quaternary Science Reviews 24, 1203–1216. Bentley, M.J., Hodgson, D.A., Smith, J.A., O´ Cofaigh, C., Domack, E.W., Larter, R.D., Roberts, S.J., Brachfeld, S., Leventer, A., Hjort, C., Hillenbrand, C.-D., Evans, J., 2009. Mechanisms of Holocene palaeoenvironmental change in the Antarctic Peninsula region. The Holocene 19 (1), 51–69. Cheng, S.M., Foght, J.M., 2007. Cultivation-independent and -dependent characterization of bacteria resident beneath John Evans Glacier. FEMS Microbial Ecology 59, 318–330. Christner, B.C., Royston-Bishop, G., Foreman, C.M., Arnold, B.R., Tranter, M., Welch, K.A., Lyons, W.B., Tsapin, A.I., Studinger, M., Priscu, J.C., 2006. Limnological conditions in Subglacial Lake Vostok, Antarctica. Limnology and Oceanography 51 (6), 2485–2501. Clapperton, C.M., Sugden, D.E., 1982. Late Quaternary glacial history of George VI Sound area, West Antarctica. Quaternary Research 18, 243–267. Craig, H., 1961. Isotopic variations in meteoric waters. Science 133, 1833–1834. Cremer, H., Gore, D., Hultzsch, N., Melles, M., Wagner, B., 2004. The diatom flora and limnology of lakes in the Amery Oasis, East Antarctica. Polar Biology 27, 513–531. Doran, P.T., Wharton Jr., R.A., Lyons, W.B., 1994. Paleolimnology of the McMurdo Dry Valleys, Antarctica. Journal of Paleolimnology 10, 85–114. Elliott, M.H., 1975. The Stratigraphy and Sedimentary Petrology of the Ablation Point area, Alexander Island. University of Birmingham. Ellis-Evans, J.C., Wynn-Williams, D., 1996. Antarctic: a great lake under the ice. Nature 381, 644–646. Epstein, S., Mayeda, T.K.G.C.A., 1953. Variations of the 18O/16O ratio in natural waters. Geochimica et Cosmochimica Acta 4, 213. Foght, J., Aislabie, J., Turner, S., Brown, C.E., Ryburn, J., Saul, D.J., Lawson, W., 2004. Culturable bacteria in subglacial sediments and ice from two southern hemisphere glaciers. Microbial Ecology 47 (4), 329–340. Golterman, H.L., Clymo, R.S., Ohnstad, M.A.M., 1978. IBP Handbook No 8. Methods for Physical and Chemical Analysis of Fresh Waters. Blackwell Scientific Publications, Oxford, 214 pp. Hawes, C.R.I., 1983. Turbulent mixing and its consequences on phytoplankton development in two ice-covered lakes. British Antarctic Survey Bulletin. No. 60, 69–82.

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