Downloaded from gsabulletin.gsapubs.org on March 10, 2014 Garwood Valley, Antarctica: A new record of Last Glacial Maximum to Holocene glaciofl uvial processes in the McMurdo Dry Valleys Joseph S. Levy1,†, Andrew G. Fountain2, Jim E. O’Connor3, Kathy A. Welch4, and W. Berry Lyons4 1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA 2Department of Geology, Portland State University, Portland, Oregon 97201, USA 3U.S. Geological Survey Oregon Water Science Center, Portland, Oregon 97201, USA 4The School of Earth Sciences and Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210, USA ABSTRACT surface (Sugden et al., 1993). Determining the from fl uvial deltas perched on valley walls. The dynamics of ice-sheet advance and retreat in lake levels required to have formed these deltas We document the age and extent of late Qua- Antarctica during the Pleistocene-Holocene are higher than the overfl ow heights for mod- ternary glaciofl uvial processes in Garwood transition has become a topic of interest because ern McMurdo Dry Valleys closed basin lakes Valley, McMurdo Dry Valleys, Antarctica, of its potential for informing predictions of (Doran et al., 1994). Accordingly, for the ele- using mapping, stratigraphy, geochronology, ice-sheet response to future episodes of warm- vated deltas and shorelines to have formed, the and geochemical analysis of sedimentary and ing (Conway et al., 1999). The McMurdo Dry grounding line for the Ross Sea ice sheet must ice deposits. Geomorphic and stratigraphic Valleys are an ideal environment in which have been north of the McMurdo Dry Valleys, evidence indicates damming of the valley at to date the record of Last Glacial Maximum allowing the ice sheet to plug the lower ends of its Ross Sea outlet by the expanded Ross Sea (LGM) ice-sheet processes because, while the valleys, resulting in the formation of glacier- ice sheet during the Last Glacial Maximum. they are currently ice-sheet free, the McMurdo dammed lakes. For the lakes to have drained, the Damming resulted in development of a pro- Dry Valleys preserve glacial drift and till units grounding line must have retreated south of the glacial lake in Garwood Valley that persisted from multiple glacial periods and from mul- McMurdo Dry Valleys, allowing local base level from late Pleistocene to mid-Holocene time, tiple ice-sheet sources (including both East and to return to sea level (Stuiver et al., 1981). and in the formation of a multilevel delta West Antarctic Ice Sheets) (Brook et al., 1995; This glacial dam model has motivated exten- complex that overlies intact, supraglacial till Denton et al., 1989; Hall and Denton, 2000; sive mapping and dating of shorelines and paleo- and buried glacier ice detached from the Ross Stuiver et al., 1981). lake delta deposits in the McMurdo Dry Valleys Sea ice sheet. Radiocarbon dating of delta In order for glacial till and/or stranded glacier in order to decipher the late Quaternary history deposits and inferred relationships between ice to be deposited within the McMurdo Dry of the Ross Sea ice sheet. Pleistocene and Holo- paleo lake level and Ross Sea ice sheet ground- Valleys, the Ross Ice Shelf (Fig. 1), fed by the cene paleolakes have been mapped using this ing line positions indicate that the Ross Sea East and West Antarctic Ice Sheets, must have method in the northern McMurdo Dry Valleys: ice sheet advanced north of Garwood Valley been thicker during glacial periods. In particu- Taylor Valley (Hall and Denton, 2000; Hall et al., at ca. 21.5 ka and retreated south of the valley lar, the ice sheet needs to have been grounded 2000a; Hendy et al., 1979; Higgins et al., 2000), between 7.3 and 5.5 ka. Buried ice remaining (here, referred to as the Ross Sea ice sheet when Wright Valley (Hall and Denton, 2005), Vic- in Garwood Valley has a similar geochemical in a grounded state) with suffi cient thickness to toria Valley (Hall et al., 2002); and in the south- fi ngerprint to grounded Ross Sea ice sheet drive ice fl ow upslope into the McMurdo Dry ern McMurdo Dry Valleys abutting the Royal material elsewhere in the southern Dry Val- Valleys (Stuiver et al., 1981). Society range: Marshall Valley (Dagel, 1985) leys. The sedimentary sequence in Garwood Direct dating of glacial tills can be chal- and Miers Valley (Clayton-Greene et al., 1988). Valley preserves evidence of glaciofl uvial lenging in the absence of exogenous markers Notably, Garwood Valley (Figs. 1 and 2), located interactions and climate-driven hydrologi- like volcanic ash (e.g., Marchant et al., 2002). north of Marshall and Miers Valleys, has not cal activity from the end of the Pleistocene Consequently, research in the Ross Sea region been mapped in detail, although deltaic deposits through the mid-Holocene, making it an un- and Transantarctic Mountains has focused were observed by Péwé (1960), who named the usually complete record of climate activity primarily on dating the lacustrine effects of paleolake inferred to have produced the depos- and paleoenvironmental conditions from the ice sheets—the formation of lakes and their its “Glacial Lake Howard.” Interestingly, Hendy terrestrial Antarctic. deposits during times when valley mouths were (2000) reported that no evidence had yet been blocked by the expanded Ross Sea ice sheet identifi ed supporting the existence of an LGM- INTRODUCTION (e.g., Clayton-Greene et al., 1988; Conway age paleolake dammed by the Ross Sea ice sheet et al., 1999; Dagel, 1985; Denton et al., 1989; in Garwood Valley, suggesting that delta deposits The McMurdo Dry Valleys of Antarctica pre- Hall and Denton, 2000; Hall et al., 2000a, 2002; identifi ed by Péwé (1960) in Garwood Valley are serve an ~14-m.y.-old record of the complex Hendy et al., 1979; Péwé, 1960; Prentice et al., likely of Holocene, rather than late Pleistocene, interactions between glaciations and the land 2008; Stuiver et al., 1981)—and the isostatic age (Hendy, 2000; Stuiver et al., 1981). responses—marine shell deposits uplifted by Here, we report on the stratigraphy of recently †Present address: Institute for Geophysics, Uni- isostatic rebound postglaciation (Hall et al., exposed glaciofl uvial deposits on the fl oor of versity of Texas, Austin, Texas 78758-4445, USA. 2000a). This fi rst dating strategy largely relies Garwood Valley, Antarctica (Figs. 2 and 3). We E-mail: [email protected]. on dating algal mats and lacustrine carbonates show that the Garwood Valley paleolake and GSA Bulletin; September/October 2013; v. 125; no. 9/10; p. 1484–1502; doi: 10.1130/B30783.1; 18 fi gures; 6 tables. 1484 For permission to copy, contact [email protected] © 2013 Geological Society of America Downloaded from gsabulletin.gsapubs.org on March 10, 2014 Garwood Valley, Antarctica: A new record of Last Glacial Maximum to Holocene glaciofl uvial processes in the McMurdo Dry Valleys 160˚W 160˚E valley between the modern Garwood Glacier and the Ross Sea. Regionally, the bedrock underlying the southern McMurdo Dry Valleys (Salmon, Gar- wood, Marshall, Miers) is composed of Skelton Group metasediments and igneous intrusives 80˚S TAM of Precambrian to Cambrian age (Dagel, EAIS AIS 1985). The Royal Society Range that forms W the head of Garwood Valley is composed of VV 85˚S Devonian to Triassic Beacon Supergroup sand- TV stones, intruded by sills of the Ferrar Dolerite RIS (Dagel, 1985). WV Glaciologically, Garwood Valley has been affected both by local alpine glaciers, such as the Garwood, Joyce, and nearby Koettlitz Gla- ciers, as well as by the East/West Antarctic Ice Sheet or Ross Sea ice sheet invading up the SV valley from the Ross Sea. At least two distinct EAIS widespread tills are present above 500 m in the RSR GV lower valley, dated by cosmogenic methods to 272 ± 7 ka and 104 ± 3 ka (Brook et al., 1995). BP These old till ages are interpreted to refl ect MaV inheritance of cosmogenic nuclides from till parent rocks, or to represent tills emplaced by MV grounded ice during the preceding glacial peri- ods (Brook et al., 1995). Initial observations by KG Péwé (1960) of the glaciofl uvial deposits along the valley fl oor (those analyzed here) led him to infer glaciation of Garwood Valley by an expanded lobe of the nearby Koettlitz Glacier Figure 1. Context map showing the location of Garwood Valley (GV) in the McMurdo Dry at ca. 6 ka. Valleys, Antarctica. Base map is LIMA Landsat image mosaic (Bindschadler et al., 2008). Regionally, the presence of porphyritic TV—Taylor Valley, VV—Victoria Valley, WV—Wright Valley, MV—Miers Valley, EAIS— an ortho clase phonolite (“kenyte”) in the widely East Antarctic Ice Sheet, WAIS—West Antarctic Ice Sheet, TAM—Trans-Antarctic Moun- distributed (and commonly ice-cored) Ross I tains, RSR—Royal Society Range, SV—Salmon Valley, MaV—Marshall Valley, BP—Brown drift unit, which fringes the coastal margin of Peninsula, RIS—Ross Ice Shelf, and KG—Koettlitz Glacier. Inset map shows the location of the McMurdo Dry Valleys from Taylor south to the McMurdo Dry Valleys (box). Miers (Stuiver et al., 1981), has led to a broad consensus that the source of this glacial deposit was Ross Sea ice sheet material that trans- delta deposits are unusual in the McMurdo Dry GEOLOGICAL CONTEXT ported Ross Archipelago volcanics (typifi ed Valleys in that they span the complete sequence by “kenyte” from Ross Island) to the south and from ice damming of Garwood Valley, through Garwood Valley (78.026°S, 164.144°E) is a west. On the basis of radiocarbon-dated algal paleolake growth and decay, to modern-climate coastal Dry Valley located between the Royal mats, lake carbonates, and marine mollusks modifi cation processes in a single outcrop.