Solute and Isotope Geochemistry of Subsurface Ice Melt Seeps in Taylor Valley, Antarctica
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Downloaded from gsabulletin.gsapubs.org on January 24, 2010 Solute and isotope geochemistry of subsurface ice melt seeps in Taylor Valley, Antarctica Katherine J. Harris† University of North Carolina at Chapel Hill, 400 McCauley Street, Chapel Hill, North Carolina 27516, USA Anne E. Carey‡ School of Earth Sciences, Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, Ohio 43210-1398, USA W. Berry Lyons Kathleen A. Welch Byrd Polar Research Center, Ohio State University, 108 Scott Hall, 1090 Carmack Road, Columbus, Ohio 43210-1002, USA Andrew G. Fountain Departments of Geology and Geography, Portland State University, Portland Oregon 97207-0751, USA ABSTRACT features in the dry valleys are potential ter- by low moisture, cold temperatures, high salt restrial analogs for the geologically young concentrations, and low productivity (Cour- The McMurdo Dry Valleys of Antarctica gullies observed on Mars, which are thought tright et al., 2001). Mosses and algal and cyano- are a polar desert region with watersheds to be evidence of groundwater seepage and bacterial mats persist from summer to summer dominated by glacial melt. Recent ground surface runoff. in ephemeral streams (McKnight et al., 1998). exploration reveals unusual surface-fl ow-seep Lakes support a plankton community domi- features not directly supplied by glacial melt. Keywords: Antarctica, groundwater, stable iso- nated by algae and bacteria, but with some pro- Much of this seep water is potentially derived topes, dry valleys, ice, hydrology. tozoans and rotifers present (Priscu et al., 1999). from permafrost, snow patches, refrozen Liquid water is the primary limiting condition precipitation accumulated in the subsurface, INTRODUCTION for life in Antarctica (Kennedy, 1993). For this buried glacier ice, or even groundwater from reason, processes that affect the formation and the deep subsurface. Flow features that lack The McMurdo Dry Valleys, located from distribution of liquid water strongly infl uence obvious glacier melt sources were identifi ed 76°30′ to 78°30′S lat and 160° to 164°E long, biodiversity in Taylor Valley (Fountain et al., in archived aerial photographs of Taylor Val- compose the largest relatively ice-free region 1999). The hydrologic regime in Taylor Valley ley. This valley was surveyed for extant and in Antarctica, with an approximate area of is based upon glacial melt. During the austral extinct seeps, and the locations of geomorphic 4800 km2. This condition exists because the summer, streams are fed by liquid water from features in fi ve active seeps were documented. Transantarctic Mountains dam the fl ow of the thawing glacier margins. Depending on summer Water samples from seeps were analyzed for East Antarctic Ice Sheet. Moreover, glaciers temperatures, these streams fl ow for periods major ions and stable isotopes of hydrogen do not form in the valley bottoms because the of 4–12 weeks. Streams discharge into closed and oxygen. Solute chemistry and isotopic sublimation and melt of snow and ice exceed basin lakes with 3–6-m-thick permanent ice signatures of seeps are distinct from those of snow accumulation in all seasons (Fountain et covers, and the lakes lose water only through nearby streams and glaciers, with the seeps al., 1999). Taylor Valley, in the middle of the evaporation and sublimation (Fountain et al., having elevated solute concentrations. McMurdo Dry Valleys region, is a landscape 1999). Ice-cemented permafrost occurs in the All but one seep had water isotopically featuring a mosaic of glaciers, exposed soils upper 1 m of the Taylor Valley soil surface and heavier than water from nearby glaciers and bedrock, ephemeral streams, and peren- extends downward to depths of several hundred and streams, suggesting that seep waters nially ice-covered lakes (Fig. 1). The valley is meters (Bockheim, 2002; McGinnis and Jensen, have been substantially modifi ed if they had oriented NE-SW and extends from the terminus 1971). At elevations generally below 300 m in been derived originally from the same mete- of Taylor Glacier at the western end to the coast the McMurdo Dry Valleys, the active layer is oric sources that supply local glaciers and of the Ross Sea at the eastern end, a distance ~0.5–1 m, with the base of the active layer being streams. The seeps are important because of ~35 km. Precipitation is negligible at <5 mm the top of the permafrost (Campbell et al., 1998). they compose a previously overlooked com- per year (water equivalent), and mean annual Campbell et al. (1998) point out that the extent ponent of the desert hydrological cycle. Seep temperatures range from −16 °C to −20 °C to which moisture is lost from, or accumulates (Doran et al., 2002). within, the permafrost is unknown. Despite the harsh climatic conditions in Tay- Since 1993 the McMurdo Dry Valleys †Present address: University of Oxford, Parks Road, Oxford OX1 3PR, UK. lor Valley, the region is host to a polar desert have been the site of a Long Term Ecological ‡Corresponding author e-mail: carey@geology. ecosystem. Soils are characterized by extremely Research (LTER) study. During an unseason- ohio-state.edu. low invertebrate biodiversity, with life restricted ably warm austral summer in 2001–2002, an GSA Bulletin; May/June 2007; v. 119; no. 5/6; p. 548–555; doi: 10.1130/B25913.1; 4 fi gures; 3 tables; Data Repository item 2007115. 548 For permission to copy, contact [email protected] © 2007 Geological Society of America Downloaded from gsabulletin.gsapubs.org on January 24, 2010 Taylor Valley seeps, Antarctica 77° 34´ 48˝ S 0 2 4 8 77° 34´ 48˝ S 161° 5´ 60˝ E 163° 27´ 36˝ E Fryxell Red Streak Figure 1. Map of relevant seep features in Taylor Val- Mummy Pond seeps ley, McMurdo Dry Valleys, Antarctica. Glaciers are depicted Pearse dry flow in white, exposed ground in brown, and ponds in blue. Wormherder Creek (outlet) Wormherder Creek (source) 77° 40´ 12˝ S 77° 40´ 12˝ S 161° 5´ 60˝ E 163° 27´ 36˝ E ephemeral water seep, now named Wormherder (2) analyze seep water samples for major ions Valleys soils are without ice in the top 1 m, and Creek, was discovered in Taylor Valley, where and δD and δ18O isotopes to ascertain seep water the rest have ice within the top 10–50 cm (Bock- fl ow had not been observed previously, although origins; and (3) compare the seeps to similar heim, 2002). Campbell and Claridge (1982) surface morphology suggested a relict stream Martian landforms. noted that moisture movement in the soils was channel (Csathó et al., 2005; Lyons et al., 2005). primarily in the form of vapor, but there was lim- Without any obvious water source (e.g., snow HYDROGEOLOGIC BACKGROUND ited migration of snowmelt. Owing to the hyper- patches, glacier), the seep water was attributed to aridity of the climate, the sublimation rates can the melting of subsurface ice. Although the pres- The valley fl oors are covered with Quater- be extremely high (Ng et al., 2005). Thus, the ent study revealed the meltwater source to be a nary glacial, alluvial, and lacustrine deposits. polar desert climate restricts groundwater fl ow, large snow patch with subsurface fl ow paths (see In Taylor Valley the glacial deposits have been particularly shallow groundwater fl ow, because below), this work drew attention to other unstud- produced by the infl ow of the West Antarc- there is limited snowfall and minimal to nonex- ied wet spots commonly observed in the region. tic Ice Sheets during glacial periods and the istent recharge. Other hyperarid landscapes, including polar advance and retreat of Taylor Glacier (the east- Cartwright and Harris (1981) recognized that ones, show evidence of past and present subsur- ern extent of the East Antarctic Ice Sheet) and some shallow subsurface fl ow occurs in Taylor face water seepage. For example, in the Atacama local alpine glaciers during interglacials (Hall Valley and Wright Valley (to the north of Tay- Desert of northern Chile, large quebradas were and Denton, 2000; Hendy, 2000). The primary lor Valley), which takes place at the base of the produced as the climate dried out and ground- water source is glacier melt from the alpine gla- active layer at rarely more than 1 m deep. They water sapping occurred (Hoke et al., 2004). On ciers that descend from the surrounding moun- speculated that one or a combination of the fol- Axel Heiberg Island in the Canadian High Arc- tains (Fig. 1), because the snow (~5 cm water lowing agents recharged these shallow fl ow sys- tic (79°26′N), mineralized springs occur that equivalent) typically sublimates before making tems: surface water (e.g., glaciers and perennial are not associated with meteoric water recharge a hydrologic contribution (Gooseff et al., 2003). snowfi elds), snowfall, ground ice (i.e., perma- (Pollard et al., 1999). Thermal springs occur on Melt occurs from November to February (Foun- frost), and buried ice. They estimated that only Svalbard that are related to deep crustal faulting tain et al., 1998) and drains to permanent stream 1% of the McMurdo Dry Valleys contain shal- (Banks et al., 1998). However, these examples channels, which fl ow intermittently for ~10 low-subsurface-fl ow environments, but on the differ importantly from the McMurdo Dry Val- weeks each summer (McKnight et al., 1998). basis of their examination of aerial photographs leys seep occurrences either in their scale or their The glacial melt and streamfl ow are highly vari- these features in Wright Valley were persistent. probable sources. The Chilean seeps are very able on daily, seasonal, and interannual time Since their work, no research has been under- large-scale features of the landscape, whereas scales (McKnight et al., 1999). taken to investigate these features.