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Glacio-Lacustrine Aragonite Deposition Antarctic Science 19 (3), 365–372 (2007) & Antarctic Science Ltd 2007 Printed in the UK DOI: 10.1017/S0954102007000466 Glacio-lacustrine aragonite deposition, meltwater evolution and glacial history during isotope stage 3 at Radok Lake, Amery Oasis, northern Prince Charles Mountains, East Antarctica IAN D. GOODWIN1 and JOHN HELLSTROM2 1Environmental and Climate Change Research Group, School of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW 2308, Australia 2School of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia [email protected] Abstract: The late Quaternary glacial history of the Amery Oasis, and Prince Charles Mountains is of significant interest because about 10% of the total modern Antarctic ice outflow is discharged via the adjacent Lambert Glacier system. A glacial thrust moraine sequence deposited along the northern shoreline of Radok Lake between 20–10 ka BP, overlies a layer of thin, aragonite crusts which provide important constraints on the glacial history of the Amery Oasis. The modern Radok Lake is fed by the terminal meltwaters of the alpine Battye Glacier. The aragonite crusts were deposited in shallow water of ancestral Radok Lake 53 ka BP,during the A3 warm event in Isotope Stage 3. Oxygen isotope (d18O) analysis of the last glacial-age aragonite crusts 18 indicates that they precipitated from freshwater with a d OSMOW composition of -36%, which is 8% more depleted than the present water (-28%) in Radok Lake. A regional oxygen isotope (d18O) and elevation relationship for snow is used to determine the source of meltwater and glacial ice in Radok Lake during the A3 warm event. This relationship indicates that Radok Lake received meltwater from the confluence of both Battye Glacier ice and an expansion of grounded western Lambert Glacier ice in the Amery embayment. Received 31 July 2006, accepted 8 November 2006, first published online 29 June 2007 Key words: carbonate, glacial meltwater, lacustrine, Lambert Glacier grounding, oxygen isotope Introduction has established the pattern of glacial retreat from the Radok Lake is a large, glacial lake (20 km2 in area) in the northern shore of Radok Lake basin. Grounded ice Amery Oasis of the Northern Prince Charles Mountains retreated from a position at 125 m elevation on the slope (Bardin et al. 1990) (Figs 1 & 2). The water level in the above the northern Radok Lake shoreline at 20.0 ka Æ modern lake lies 7 m above sea level and the nearby, tidal 2.2 ka, to a position at 70 m elevation, close to the thrust Beaver Lake which is an epishelf lake formed in an moraine sequence by 10.5 Æ 1 ka. On the basis of these embayment adjacent to the Amery Ice Shelf (Adamson paired cosmogenic surface exposure ages, Fink et al. et al. 1997). The modern Radok Lake lies in an over- (2006) confirmed that the thrust moraine sequence was deepened basin that is up to 346 m deep (339 m below deposited during the Late Glacial–Holocene transition, as a modern sea level), and is surrounded by rock cliffs up to result of the shrinking of the glacial-age Battye Glacier. 400 m above the lake surface (Fig. 3). The lake is presently The fabric of the sediments in the moraine (M. Hambrey, fed by glacial meltwater from the small Battye Glacier personal communication 1999) and the pattern of surface (13 km long) which drains the local ice plateau of the exposure indicate that the moraine was deposited by ice McLeod Massif (Fig. 4). Radok Lake overflows at times of flowing from Radok Lake onto the northern slope. However, peak meltwater inflow, and drains to Pagodroma Gorge and it is unknown whether the ice in Radok Lake during the last Beaver Lake, via a shallow sill, at the eastern edge of the lake. glacial cycle originated from an alpine catchment in the Adamson et al. (1997) mapped the present and former northern Prince Charles Mountains or whether it originated shoreline geomorphology of Radok Lake and reported a from a Lambert Glacier ice stream overriding the Amery glacial thrust moraine sequence at an elevation of Oasis. The latter would require a substantial thickening and 50–80 m (after McKelvey et al. 1995, 2001) and grounding zone advance of the Amery Ice Shelf–Lambert mapped a raised shoreline at 70–80 m above the present Glacier system to at least the locality of the Amery Oasis, northern shoreline of Radok Lake. They suggested a which is 200–300 km inland from the ice shelf front in Holocene age for the thrust moraines based on their Prydz Bay. The modern Lambert Glacier Amery Ice Shelf relatively recent weathering, when compared to the grounding zone is located approximately 200 km further surrounding sediments. Subsequently, cosmogenic surface inland (Fricker et al. 2002). Adamson et al. (1997) reported dating using 10Be and 26Al isotopes by Fink et al. (2006) that glacial and fluvial landforms, degree of surface and 365 366 IAN D. GOODWIN & JOHN HELLSTROM Fig. 3. Topographic cross section through the Radok Lake basin from the Jetty Peninsula ice ridge in the south to the northern basin slopes. sediment weathering and the presence of surface sediments rich in Pliocene fauna indicated that the Radok Lake basin and Amery Oasis had not been overridden by Lambert Glacier ice streams since the Pliocene. On the north-eastern corner of Radok Lake, a modern melt-stream has incised into the surface sediments and has cut a number of braided channels down to the modern Radok Lake shoreline. A conspicuous feature in the main channel (Fig. 5) is an outcrop of carbonate crusts. This outcrop is located 10–20 m elevation above most of the thrust moraine sediments. These carbonate crusts were sampled in the 1988/89 summer field season for isotope geochemistry analysis, to provide an insight into their Fig. 1. Location map showing the Amery Oasis, northern Prince origin and possible evolution of meltwater in the Radok Charles Mountains, Amery Ice Shelf and Lambert Glacier Lake basin, and the associated glacial history of the Amery system. Oasis. The characteristics of the carbonates and their isotope geochemistry are discussed below, together with the glacial history of the Radok Lake catchment. Fig. 2. Topographic map showing the Radok Lake basin and the modern boundary of the Battye Glacier catchment. GLACIAL HISTORY DURING ISOTOPE STAGE 3 AT RADOK LAKE 367 Fig. 4. Aerial photograph showing the Battye Glacier trough Fig. 6. Photomicograph of an aragonite crust section. and Radok Lake basin. Carbonate sedimentology, isotopic composition, slight (28) dip towards the modern lake shoreline (Fig. 5a). depositional age and environment The discontinuous and discoid-shaped carbonate crusts, are typically 5–10 cm long, and occur as single layers or as The 5 to 10 mm thick, carbonate crusts were collected from clusters. (Figs 5b & 6). This layer forms a discontinuity the dissected channel section through the surface and is overlain by 2–3 m of sediments comprising semi- sediments. They outcrop along a defined stratum, with a stratified and massive basal diamicton, alternating with laminated silty sand. Downslope and to the west of the outcrop, Hambrey & McKelvey (2000) reported the thrust moraine sequence, comprising discontinuous slabs of stratified glacio-lacustrine or fjordal sediments, with massive silty sand and laminated clayey silts, and silty sands, with dropstones. Hambrey & McKelvey (2000) proposed that these sediments were distal turbidites, subglacial debris flows, and ice rafted deposits possibly of early Miocene to Pliocene-age Pagodroma Group that had been reworked by late Quaternary glacial events and deposited with preserved stratigraphy, in the ice marginal moraine, during ice retreat. The carbonate crusts were analysed using X-Ray Diffraction (XRD) techniques and consist entirely of aragonite. The inorganic aragonite is micritic with thin laminae and some peloids, together with some algal filaments from microbial mats. Borings, and shell castings (gastropods) were present in a few samples. Low concentrations (0 to 10%) of halite were measured by XRD on some samples of the surrounding silty sand sediments. The composition and structure of these aragonitic layers are similar to the Late Quaternary lake aragonites and algal limestones from the Taylor Valley, Transantarctic Mountains (Hendy 2000). Hence, the Radok Lake carbonate crusts are interpreted to have formed in Fig. 5a. Photograph of vertical exposure cut by gullying in the thrust association with algal mats in a shallow water environment. moraine sequence at 50 m elevation on the basin slope above Stable isotope mass spectrometry on six samples of the northern shoreline of modern Radok Lake. The exposure shows layer of aragonite crusts, outcropping beneath the aragonite (Table I) yielded a mean carbon isotopic 13 diamicton. b. Close-up photograph showing a cluster of the composition d CPDB ¼ 1.59 Æ 0.28%, and a mean oxygen 18 discoid and draped aragonite crusts, outcropping in Fig. 5a. The isotope composition d OSMOW ¼ -3.49 Æ 0.08%. The 18 stratigraphy and the whole preservation of the shallow water calculated d OSMOW value of the parent water body from crusts, indicates in situ deposition at this location. which the aragonites were precipitated under assumed 368 IAN D. GOODWIN & JOHN HELLSTROM Table I. Carbon and oxygen stable isotopic analyses of aragonites and snow/ice. 13 18 18 18 Sample d CPDB % d OSMOW % d OSMOW % d OSMOW % aragonite aragonite meltwater snow/ice Aragonite (mean n ¼ 6) 1.59 Æ 0.28 -3.49 Æ 0.08 Radok Lake @ 52 ka BP -36.0 Radok Lake Modern -28.4 Æ 0.3 Ice sheet snow @ 1100 to 2600 m -31 to -41 Modern Battye Glacier snow @ 1100 to 1300 m -25 to -31 18 Note that all values are measured, except for the calculated d OSMOW % value for Radok Lake meltwater at 52 ka BP.
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