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Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem

Joseph S. Levy1,†, Andrew G. Fountain1, Michael N. Gooseff2, Kathy A. Welch3, and W. Berry Lyons4 1Department of Geology, Portland State University, Portland, Oregon 97210, USA 2Department of Civil and Environmental Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA 3Byrd Polar Research Center and the School of Earth Sciences, Ohio State University, Columbus, Ohio 43210, USA 4The School of Earth Sciences and Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210, USA

ABSTRACT sent a new geological pathway that distrib- 1999). The goal of this paper is to integrate utes water, energy, and nutrients in Antarctic permafrost processes that route water through Water tracks are zones of high soil mois- Dry Valley, cold desert, soil ecosystems, pro- active-layer soils into the Antarctic cold desert ture that route water downslope over the viding hydrological and geochemical connec- hydrogeological paradigm. ice table in polar environments. We present tivity at the hillslope scale. Do Antarctic water tracks represent isolated physical, hydrological, and geochemical evi- hydrological features, or are they part of the dence collected in Taylor Valley, McMurdo INTRODUCTION larger hydrological system in Taylor Valley? Dry Valleys, Antarctica, which suggests that How does water track discharge compare to previously unexplored water tracks are a sig- The polar desert of the McMurdo Dry Val- Dry Valleys stream discharge, and what roles do nifi cant component of this cold desert land leys, the largest ice-free region in Antarctica, water tracks play in the Dry Valleys salt budget? system and constitute the major fl ow path is a matrix of deep permafrost, soils, glaciers, What infl uence do water tracks have on Antarc- in a cryptic hydrological system. Geologi- streams, and lakes that together form the geo- tic ice-covered lakes? cal, geochemical, and hydrological analyses logical context for the southernmost functioning In this paper, we present the case that, dur- show that the water tracks are generated terrestrial ecosystem (Kennedy, 1993; Lyons et ing summer months, the shallow permafrost/ by a combination of infi ltration from melt- al., 2000). Recent observations in Taylor Valley soil environment of Taylor Valley supports a ing snowpacks, melting of pore ice at the ice (Fig. 1) of shallow groundwater discharge from spatially extensive system of interconnected table beneath the water tracks, and melting the active layer (the seasonally thawed portion water tracks that result in the appearance of of buried segregation ice formed during win- of the permafrost) as seeps during peak sum- linear and sublinear dark soil patches at the ter freezing. The water tracks are enriched mer warming have generated new interest in ground surface. We test the hypotheses that in solutes derived from chemical weathering examining permafrost-related groundwater pro- (1) these water tracks are part of a morphologi- of sediments as well as from dissolution of cesses (Harris et al., 2007; Lyons et al., 2005). cal and hydrological continuum that includes soil salts. The water tracks empty into ice- These new observations revive early reports of previously described seeps and streams; covered lakes, such as Lake Hoare, resulting groundwater activity in nearby Wright Valley (2) water tracks contribute an ecologically sig- in the interfi ngering of shallow groundwater (Cartwright and Harris, 1981; Wilson, 1981). nifi cant volume of water to Taylor Valley soils solutions and glacier-derived stream water, These seeps appear to be supplied by melting that originates from snow and ground-ice melt, adding complexity to the geochemical pro- snow and ground ice, suggesting that they may rather than from glacier melt; and (3) water fi le. Approximately four orders of magnitude be similar to “slope streaks” and gully landforms tracks are a major pathway for solute transport less water is delivered to Lake Hoare by any observed in neighboring Wright Valley (Head et through Dry Valleys soils and are an important given water track than is delivered by surface al., 2007; Levy et al., 2008), and may be mor- component of the Lake Hoare (a closed-basin runoff from stream fl ow; however, the sol- phologically or genetically similar to slope lake) salt budget. ute delivery to Lake Hoare by water tracks streaks or transient slope lineae on Mars (Mush- equals or may exceed the mass of solutes kin et al., 2010; McEwen et al., 2011). Linear SITE DESCRIPTION delivered from stream fl ow, making water or downslope-branching zones of enhanced soil tracks signifi cant geochemical pathways. moisture and downslope water transport, often The McMurdo Dry Valleys span an ice- Additionally, solute transport is two orders occurring in the absence of well-defi ned chan- sheet–free region between the Trans-Antarctic of magnitude faster in water tracks than in nels, are hallmarks of “hillslope water tracks”— Mountains of southern Victoria Land and the adjacent dry or damp soil, making water features that route much of the water and nutri- Ross Sea. Taylor Valley, one of the McMurdo tracks “salt superhighways” in the Antarctic ent fl ux through Arctic, permafrost-dominated Dry Valleys, is an ~50-km-long valley cen- cold desert. Accordingly, water tracks repre- soils (Hastings et al., 1989; McNamara et al., tered on 77.7°S, 162.6°E (Fig. 1). Taylor Valley

†E-mail: [email protected]

GSA Bulletin; November/December 2011; v. 123; no. 11/12; p. 2295–2311; doi: 10.1130/B30436.1; 13 fi gures; 3 tables.

For permission to copy, contact [email protected] 2295 © 2011 Geological Society of America Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Levy et al. has been the focus of the McMurdo Dry Val- Taylor Valley Hydrological Setting to east, Lakes Bonney, Hoare, and Fryxell) leys Long-Term Ecological Research project (Fig. 1) that are fed principally from glacier (MCM-LTER) since 1993. The valley is fi lled The dominant hydrological pathway in the melt (Lyons et al., 1998a). Thirty named to the west by the Taylor glacier, a polythermal McMurdo Dry Valleys connects glaciers to streams route glacier meltwater to the lakes outlet glacier of the East Antarctic Ice Sheet, perennially ice-covered lakes via a network of (Alger et al., 1997), although more than 60 and it opens to the east into McMurdo Sound in stream channels in which hyporheic exchange channels wider than ~6 m across (and thus, the Ross Sea. moves water, salts, and nutrients into and out of clearly resolvable in IKONOS satellite and air- An extremely simple, nematode-dominated the stream- and lake-proximal soils (Barrett et photo data) are present along the valley walls soil ecosystem (Priscu, 1998) persists in Taylor al., 2009; Cartwright and Harris, 1981; Doran (McKnight et al., 1999). Valley, despite mean annual temperatures of et al., 2008; Gooseff et al., 2002, 2007; Green et ~–18 °C (Doran et al., 2002a), 3–50 mm water- al., 1988; Lyons and Mayewski, 1993; Matsub- Taylor Valley Soils and Permafrost equivalent annual precipitation (Fountain et al., aya et al., 1979; McKnight et al., 1999; Nezat 2009), and extreme aridity produced by desic- et al., 2001). Neither infi ltration of snowmelt The portions of the surface of Taylor Valley cating winds (Clow et al., 1988). Liquid water nor groundwater movement (shallow or deep) that are not characterized by lakes, glaciers, or availability is thought to be the primary limit- has been considered to be a major hydrological bedrock are covered by a heterogeneous soil ing factor in Antarctic soil ecosystems (Foun- process (Cartwright and Harris, 1981; Chinn, composed of glacial drift, valley-wall collu- tain et al., 1999; Kennedy, 1993). Accordingly, 1993; Hunt et al., 2010). This limited hydrologi- vium, marine sediments, and paleo–lake beds the simplicity of the ecological processes in cal activity results from the extreme aridity of (Bockheim et al., 2008). With a mean annual Dry Valleys soils has been shown to refl ect the the McMurdo Dry Valleys and the persistence temperature of ~–18 °C (Doran et al., 2002a), limited geohydrological pathways that distrib- of continuous, ice-cemented permafrost that Taylor Valley soils are perennially frozen, form- ute liquid water to organisms during the brief extends from the surface to depths of hundreds ing continuous permafrost to a depth of ~200– summer season when surface temperatures of meters (Bockheim et al., 2007; Campbell et 600 m (Decker and Bucher, 1980; McGinnis exceed 0 °C (Conovitz et al., 1998; Courtright al., 1997; Clow et al., 1988; Decker and Bucher, and Jensen, 1971). Much of this permafrost is et al., 2001; Gooseff et al., 2003a, 2007; McK- 1980). The paucity of shallow groundwater in ice-cemented; however, the low atmospheric night et al., 1999; Virginia and Wall, 1999). Dry Valleys permafrost has been inferred to water vapor pressure in Taylor Valley (Clow Liquid water in Dry Valleys soils is largely result in minimal transport of salts and nutrients et al., 1988) results in the removal of shal- limited to the upper ~1 m—the active layer of through soils (Claridge et al., 1997). low ground ice by vapor diffusion (Hagedorn the permafrost column that thaws during warm Three large, ice-covered, closed-basin et al., 2007). Accordingly, based on observa- summer conditions. lakes are located in Taylor Valley (from west tions of the upper ~1 m of Taylor Valley soils, approximately half have been mapped as dry frozen, while half are shallowly ice-cemented (Bockheim et al., 2007). Summer warming of the soil surface results in thawing of the upper ~10–60 cm of the permafrost, resulting in the formation of an active layer (Bockheim et al., 2007). In some locations, the active layer is meltwater-saturated, while in other locations, it is nearly dry (Campbell et al., 1997; Marchant and Head, 2007). The presence of both water-rich and water- poor permafrost in Taylor Valley results from the fact that it spans a microclimate transition between the relatively warm and wet coastal thaw zone and the cooler and drier intermediate mixed zone (Marchant and Head, 2007). Across this microclimate transition, precipitation drops from ~40–50 mm water-equivalent per year to <10 mm water-equivalent per year (Fountain et al., 2009). Accordingly, the hydrogeological landforms present in Taylor Valley, along with other permafrost features such as sand- and ice- wedge polygons and solifl uction lobes, repre- Figure 1. Location map of Taylor Valley. TG—Taylor glacier, LB—Lake Bonney, LH— sent important examples of climate-controlled Lake Hoare, CA—Canada glacier, and CO—Commonwealth glacier. LTME—Long-Term “equilibrium landforms” that are diagnostic of Manipulation Experiments (described in the ecological implications section). Numbers these transitional climate conditions (Marchant 1–3 indicate the locations of water tracks considered in the water budgets section. Water and Head, 2007). tracks with internal dots are included in the geomorphic analysis. Inset shows location of the Water tracks analyzed in this study are McMurdo Dry Valleys (MDV), with box indicating the vicinity of Southern Victoria Land, located in the Lake Hoare, Lake Fryxell, and and the arrow indicating the location of the McMurdo Dry Valleys. Latitude and longitude Lake Bonney basins. Most are located on the are demarcated in 10° increments. Taylor Valley is located at 77.7°S, 162.6°E. south sides of the basins (Fig. 1). The north

2296 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem sides of the basins are characterized by steep ice-cement table (measured as the depth to deviations 0.9 and 0.1, respectively). Light- slopes (20°–25°), with thin colluvial soil covers refusal of a 1-cm-diameter steel rod inserted toned salt-effl orescences on the fringes of resulting from debris fl ows and/or exposed bed- manually into the soil). VWC and EC were some water tracks are inferred to result from rock. The south sides of the basins have more measured using a Decagon Devices 5TE probe desiccation of brine-bearing surface soils gentle slopes (5°–10°) with thicker soils and with a Taylor Valley soil-specifi c calibration. along the margins of the water tracks. Surface do not show evidence of energetic mass wast- During December–January 2009–2010, water darkening is inferred to result from reduc- ing. Taylor Valley soils below ~300 m eleva- track surface soils had surface volumetric tion in visible wavelength surface albedo tion (in which the water tracks are located) are water contents three times higher than adja- with increasing water content (e.g., Liu et al., predominantly sandy skeletal calcic haplorthels cent, light-toned soils (15% ± 2% VWC vs. 2002). The depth to the ice-cement table in (typically 75%–80% sand by mass), and there is 5% ± 2% VWC; standard deviations: 9.0% water tracks was more than twice the depth a surface of desert pavement of coarse cobbles and 1.0%, respectively). Water track surface of refusal for adjacent light-toned soils (45 ± and pebbles (Campbell, 2003). soils had electrical conductivities approxi- 1 cm versus 19 ± 1 cm; standard deviations mately ten times higher than adjacent soils 11 cm and 9 cm, respectively). These compari- FIELD OBSERVATIONS: METHODS (0.53 dS/m vs. 0.05 dS/m ± 10%; standard sons are summarized in Table 1. AND RESULTS

During the 2009–2010 austral summer, we observed and measured the properties of Antarc- tic water tracks. The morphological, sedimento- logical, hydrological, and in situ geochemical properties of the water tracks are described here. Laboratory geochemical analyses are described in a subsequent section.

Water Track Surface Characteristics

Several dark, linear, or downslope- branching surface features (water tracks) were observed during reconnaissance of Taylor Valley. These darkened soil surfaces extend downslope, typically oriented along the steepest local gradient, over lengths of ~200–1900 m (Figs. 2 and 3). The dark soil surfaces range from ~1 to 3 m in width and are commonly observed in sublinear depres- sions, or in local low ground between knobby terrain composed of ice-cemented, degraded moraines (Figs. 2 and 3). In situ measurements were collected along the length of several water tracks and from adjacent, light-toned soils. Measurements included: soil volumetric water content (VWC), electrical conductivity (EC, a measure of salinity), and depth to the

Figure 2. Spatial relationships among seeps, fl uvial channels, and water tracks. (A) Gla- cier-fed channel (white arrows) on the north shore of Lake Bonney (LB). Black arrows show dark, linear patches of soil adjacent to the incised channel. Image is ~150 m across. (B) Water track on the south shore of Lake Hoare. Arrow shows water track downslope fl ow direction. Box indicates the extent of part C. Image is ~5 m across. (C) Surface seep (white arrow) downslope of a boulder (black arrow). The seep emerges from the water track. Image is ~1 m across. Images have been stretched to enhance contrast.

Geological Society of America Bulletin, November/December 2011 2297 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Levy et al.

Figure 3. Taylor Valley water tracks. (A) Water track 1 (Fig. 1) extends downslope from snowpack sources to ice-covered Lake Hoare. (B) Close-up view of the water track surface shown in part A. Backpack and 1 m tile probe are shown for scale. (C) Water track 2 (Fig. 1) on the south shore of Lake Hoare. Several linear, dark patches (water tracks) are visible (white arrows), which inte- grate into a higher-order water track that empties into Lake Hoare. (D) Close-up view of the water track shown in part C. Pick at lower left is for scale. Images have been stretched to enhance contrast.

Water Track Profi le Characteristics (Fig. 4). This is typical of active-layer soils in from the upslope face, and draining from the Taylor Valley (Campbell, 2003; Campbell et downslope face of the excavation. The thickest Water track soils are predominantly clas- al., 1997). Saturated horizons were absent in saturated horizons were observed where con- sifi ed as sandy skeletal calcic haplorthels soil pits into light-toned, dry (off-track) soils. cavities existed in the ice table and where ice (Campbell, 2003), and they are composed Water-saturated soil was typically encoun- table slopes were low. These observations sug- largely of unsorted silt, sand, and pebbles, tered above the ice table in soil pits excavated gest transport of water track melt solutions over although clay-sized particles can constitute into water tracks. Saturated layer thicknesses the ice table and ponding at local lows. As noted several weight percent in some samples (Levy ranged from <1 mm (surface dampness) to in Table 1, wet surface soils are most common et al., 2008). Like most Antarctic gelisols, ~20 cm (Fig. 5). Removal of wet soil down to along water tracks where the saturated horizon water track soils lack well-defi ned soil hori- the ice table resulted in the fl ow of water out is thick (~10 cm) and where active-layer thaw is zons (Bockheim et al., 2008). of the lower portion of the upslope face of the deep (~40–50 cm). Excavations into the water tracks revealed pit. Water slowly fi lled the excavation to the increasing soil moisture with increasing depth height of the saturated level, entering the pit Water Track Landscape Position: Channels and Seeps

TABLE 1. WATER TRACK VERSUS NON-WATER-TRACK SOILS Water tracks in Taylor Valley are present in Parameter On-track Off-track N association with several landforms related to VWC (%) 15 ± 2 5 ± 2 34, 20 water transport. In some locations, water tracks EC (dS/m) 0.53 ± 10% 0.05 ± 10% 31, 16 were located within incised stream channels in DTR (cm) 45 ± 1 19 ± 1 28, 20 Note: Comparison of surface soil volumetric water content (VWC), surface soil electrical conductivity (EC), which no overland fl ow was present at the time and depth to refusal (DTR) for water tracks (on-track) and adjacent soils (off-track). N—number of samples for of observation (Fig. 2). This distinguishes water on-track, and off-track, respectively. Errors indicate actual measurement experimental uncertainties. track–related water transport from hyporheic

2298 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem

break in slope, resulting from upslope damming of colluvium (e.g., Putkonen et al., 2007). In such locations, the thickness of saturated soil above the ice table is greater than the available soil column height in the downslope lee of the boulder, resulting in the percolation of water from the active layer as a seep (similar to famil- iar spring processes).

Water Track Soil Hydraulic Conductivities

The hydraulic conductivity of wet and dry soil was measured in Taylor Valley using a Decagon Devices Minidisk Infi ltrometer. Mean hydraulic conductivity was 0.02 cm/s (N = 6). Hydraulic conductivity was strongly dependent on soil grain size (spanning cobbles to silt) and on the thermal conditions of the soil. Hydrau- lic conductivity was found to be low for muddy soils (0.002 cm/s), as well as for hard-frozen soils (0.005 cm/s), and to be greatest for dry, Figure 4. Plot showing volumetric water content (VWC), temperature (T), and elec- unfrozen soils (up to 0.06 cm/s). The observed trical conductivity (EC) with depth in a water track soil column. differences in hydraulic conductivity between frozen and unfrozen, nonsaturated surface soils are consistent with the 1–2 orders of magnitude transport, in which water is exchanged between or downslope-branching patches of darkened, reduction in hydraulic conductivity predicted a fl owing stream and the streambed (Gooseff damp surface soil, and they can be extremely by Kleinberg and Griffi n (2005) for frozen soils et al., 2002; McKnight et al., 1999). In addi- salt-rich (resulting in seasonal bright salt effl o- that are 40%–60% saturated. tion, liquid water seeps with identical mor- rescences), making it possible to map seep Hydraulic conductivities in water tracks are phological characteristics (i.e., dark, damp soil locations from air-photo and satellite image also likely to be temporally variable due to not associated with a glacial runoff channel, data (Fig. 2) (Harris et al., 2007). In fact, water changes in volumetric water content during the through which small volumes of groundwater tracks can be observed in their present locations summer season. Permeability of water track discharge occurred through gentled channel- in aerial photographs extending back to 1959, soils will increase as soil moisture increases ized topography) to those described by Lyons et suggesting that they are not a “new” feature of during thaw and snowmelt addition periods, and al. (2005) and Harris et al. (2007) were present the McMurdo Dry Valleys. it will decrease as water drains prior to winter within some water tracks (Fig. 2). Taylor Val- Seeps typically discharge at the water track freezing (although the presence of ice-saturated ley seeps are commonly associated with linear surface at points where boulders produce a local and excess ice permafrost suggests that drainage

Figure 5. (A) A thick saturated horizon fi lling the pit from the ice table (~35 cm depth) to ~15 cm depth on a water track. (B) Snow- banks commonly accumulate in water track troughs (windblown snow) at several locations along water tracks. The water track (water track 2) is ~500 m from left to right. Images have been stretched to enhance contrast.

Geological Society of America Bulletin, November/December 2011 2299 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Levy et al. of water track soils in summer is minimal). grains was located several centimeters off the Quickbird satellite image data for comparison To capture these complexities, we assume an axis of the current sand wedge, suggesting the with spatial and hydrological analyses con- average water track hydraulic conductivity of relict formation of composite-wedge polygons ducted using a 2 m/pixel digital elevation model 0.02 cm/s. Future analyses will focus on sea- in some Taylor Valley water tracks (Ghysels and (DEM). The DEM was produced from airborne sonal changes in Antarctic permafrost hydro- Heyse, 2006; Marchant and Head, 2007; Mur- LiDAR scanning in 2001–2002 (Schenk et al., logical characteristics. ton, 1996). Buried massive ice deposits have 2004). Surface darkening of soils along water also been observed in Taylor Valley in some tracks (e.g., Fig. 3) and surface brightening due Water Track Melt Sources locations, and these could serve as additional to salt effl orescence are visible in panchromatic sources of water track meltwater (Bockheim et 60 cm/pixel Quickbird images (Fig. 6). Satellite Most water tracks are clearly associated al., 2008; Prentice et al., 2008). image analysis was used to identify the inferred with surface ice in the form of snow. Seasonal water track locations marked on Figure 1. snowbanks commonly accumulate in topo- Water Track Thermal Properties DEM analysis of water track topography was graphic troughs or depressions associated with conducted using methods typically applied to water tracks (Fig. 5)—similar to observations Given the importance of soil moisture in Arctic hillslope watersheds (e.g., McNamara et of snow accumulation in polygon troughs and determining the thermal properties of soils al., 1999). DEMs for eastern and western por- gully channels in neighboring Wright Valley (Baver et al., 1972; Ikard et al., 2009), mea- tions of Taylor Valley were fi rst mosaicked in (Levy et al., 2008; Morgan, 2009). During sum- surements of soil apparent thermal diffusivity ArcMap 9.3 to produce a seamless DEM cover- mer, snowbanks contribute water to the water (ATD) were collected. Two strings of Campbell ing the full extent of Taylor Valley (the DEM tracks as snowmelt infi ltrates into the soil. Sur- Scientifi c 107-L thermistors were buried in Tay- covers Taylor Valley from the valley fl oor to face soil moisture in the vicinity of snowbanks lor Valley soils starting in December 2009 (four 1306 m and extends from the coast to Taylor typically exceeds 10% VWC, and the depth to probes per string, spaced 10 cm apart, record- glacier). The mosaicked DEM was then pro- the ice table is typically 5–10 cm (likely owing ing hourly). One string was buried in a wet soil cessed using the ArcToolbox Fill function to to insulation of the ice-cement by the overly- patch (mean soil moisture = 21% by volume), remove sinks in the DEM (sinks are DEM pix- ing snowbank), even when nearby water track while the second string was buried in a dry soil els that have no downslope outlet). Although active-layer thicknesses are greater. This source patch (mean soil moisture = 7% by volume). the maximum fi ll depth was left unconstrained, of meltwater, coupled with a thin active layer ATD was determined by the graphical fi nite dif- 46% of sinks identifi ed in the DEM were (limited capacity to store or transport the snow ference method (Pringle et al., 2003) using tem- <10 cm deep, and 99% were less than 70 cm runoff), accounts for the high soil moisture and perature data from the thermistor strings, and a deep (Fig. 7). Many of the sinks were located relatively low EC associated with water track moving 48 h calculation window (i.e., the ATD within dark water track surfaces visible in IKO- soils in the vicinity of snowbanks (see geochem- for a given time was calculated using the change NOS image data, suggesting that they were not istry section). of temperature with depth for the preceding and artifacts in the data but, rather, represent sites Water tracks are also associated with ground subsequent 24 h relative to a data point). ATD of localized internal drainage, consistent with ice, raising the possibility of nonsnow contribu- calculations are summarized in Table 2. ATD is observations of ponding along water tracks. tions to water track fl ow. Pore ice (commonly highly variable in Taylor Valley. Taylor Valley The presence of sinks and pond sites along called “ice-cement”) is the dominant form of soils show diverse thermal characteristics, but water tracks may indicate the existence of a ground ice in Taylor Valley, based on 62 soil they are consistent with sandy soils with moder- critical active-layer saturation depth required to pits excavated during 2009–2010. Ice content in ate porosity containing <40% water by volume achieve water track hillslope connectivity (e.g., the upper ~10 cm of the ice table averages 29% (Baver et al., 1972; Ikard et al., 2009). Stieglitz et al., 2003). A fl ow direction raster by volume (n = 20, 16% by mass, assuming 1.8 was then computed using the ArcToolbox Flow g/cm3 sediment bulk density, i.e., the mean for DEM AND SATELLITE Direction function (to produce a raster show- 58 Taylor Valley soils collected during 2009– IMAGE ANALYSIS ing the maximum downslope drainage direction 2010). Mean skeleton density for Taylor Valley from each DEM pixel). From the fl ow direction samples is 2.73 g/cm3 (n = 26), suggesting an A light detection and ranging (LiDAR) digi- raster, the Flow Accumulation and Flow Length average porosity for Taylor Valley soils of 36%. tal elevation model and coregistered Quickbird ArcToolbox tools were used to create rasters Excavations within water tracks also provided satellite image data were used to test whether cataloging the number of upslope DEM pixels examples of excess ice (ice exceeding pore the geochemical linkages among water tracks, that contributed fl ow to each point in the DEM, space) (French, 2007). Excess ice in water track seeps, and the other surface hydrological fea- and the maximum upslope distance from which excavations was present as ice lenses with and tures in Taylor Valley outlined here resulted fl ow could be traced to each DEM pixel. without laminations, suggesting ice segrega- from hydrological linkages at the hillslope scale. Inspection of fl ow accumulation rasters cou- tion processes during autumn freezing. Excess Water tracks in Taylor Valley were identifi ed in pled with satellite image data shows the presence ice was also present as friable, granular, poorly consolidated wedges 1–2 cm in width, suggest- ing localized ice-wedge formation (Leffi ngwell, 1915). For comparison, sand-wedge polygons TABLE 2. WATER TRACK SOIL SUMMERTIME APPARENT THERMAL DIFFUSIVITY Soil type Mean apparent thermal diffusivity Apparent thermal diffusivity range are the dominant thermal-contraction-crack (mm2/s) (mm2/s) polygon variety in Taylor Valley, consistent with 0–20 cm depth wet soil 0.46% ± 10% 0.38–0.51 generally dry soil conditions outside of water 0–20 cm depth dry soil 0.71% ± 10% 0.56–0.87 20–40 cm depth wet soil 0.78% ± 10% 0.68–0.92 tracks (Marchant and Head, 2007; Pewe, 1959). 20–40 cm depth dry soil 0.49% ± 10% 0.31–0.61 At the margin of one water track excavation, an Note: Calculations of apparent thermal diffusivity (ATD) for water track (wet) and off-track (dry) soils as a ~10-cm-wide wedge of foliated ice and sand function of depth in Taylor Valley.

2300 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem

Figure 6. Dark water track soils are spatially correlated with locations of enhanced, topography-driven, potential water accumula- tion. Water track 1 traverses the south wall of Taylor Valley, downslope toward Canada glacier and Lake Hoare. (A) Panchromatic Quickbird image data showing darkened soil with bright salt effl orescences. (B) Image data with overlay showing digital elevation model pixels with >0.04 km2 upslope accumulation area. Portion of Quickbird image 09FEB01213752. Images have been stretched to enhance contrast. of linear fl ow paths that strongly correlate with troughs that are depressed up to ~1 m relative to gons with 1 m widths (a minimum width for the locations of dark and linear patches of soil the surrounding soil surface. typical water tracks visible at the surface). The identifi ed as water tracks—particularly at con- Water track DEM analyses were used to combined surface area of these high-confi dence tributing areas exceeding ~104 pixels (0.04 km2) evaluate the estimate by Cartwright and Har- water track polygons was 1% of the DEM (Fig. 6). Water tracks identifi ed in the DEM fl ow ris (1981) that no more than 1% of McMurdo area (287 km2—the entire valley fl oor below accumulation rasters defl ect around topographic Dry Valleys surfaces support shallow ground- 1300 m elevation, which is the DEM limit; as obstacles, such as ice-cemented moraines and water movement (i.e., water tracks). Flow seen in Fig. 6, DEM-delimited water tracks ridges, suggesting that the downslope transport accumulation raster pixels with >0.04 km2 of extend at least this far upslope in Taylor Valley). of water through the active layer is responsible upslope accumulating area (strong water track Extending the accumulation criteria to smaller for surface darkening visible from the ground candidates) from a subset of the Taylor Val- water track tributary pixels (DEM pixels with and from orbit (Fig. 6). Topographic transects ley DEM (focusing on the Lake Hoare basin) >0.02 km2 of upslope accumulation area), the orthogonal to water tracks identifi ed from sat- were converted into vector features in ArcMap. portion of the DEM potentially underlain by ellite image data show that water tracks fl ow A 0.5 m buffer was created surrounding each water track–related shallow groundwater rises through broad, downslope-oriented surface water track vector to produce water track poly- to 4%. Water tracks that can be clearly resolved in 60 cm panchromatic Quickbird images are considerably wider than 1 m, suggesting that these spatial estimates are minimum values for the extent of shallow groundwater activity in Taylor Valley.

WATER TRACK GEOCHEMISTRY METHODS AND RESULTS

Major ion analyses have been used exten- sively to characterize the linkages between Antarctic surface water reservoirs, including glaciers, streams, and lakes (Gooseff et al., 2006, 2002; Harris et al., 2007; Lyons et al., 2005, 1998a, 1998b; Welch et al., 1996; Wil- Figure 7. Histogram showing depth of digital elevation model son, 1981). Water samples were collected from sinks fi lled in by the Fill tool. Most sinks are less than 0.2 m the active layer in Taylor Valley using sterile deep, suggesting that internal drainages in the landscape may polyethylene syringes. Samples were fi ltered in be real, and can fi ll through water track ponding, resulting in the fi eld using 0.45 μm HT Tuffryn membrane connectivity on the Taylor Valley hillslopes. syringe fi lters into precleaned polyethylene

Geological Society of America Bulletin, November/December 2011 2301 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Levy et al. bottles. Bottles were fully fi lled and were kept was collected using cleaned picks and scoops, Major ion data for Taylor Valley water track refrigerated at all times. When soil moisture and was thawed in Whirl-Pak bags before being systems are presented in Figure 8. Analyzed 2+ 2+ + + – 2– levels were well below saturation, making syringe-fi ltered. Snow was collected using clean ions include Ca , Mg , K , Na , Cl , SO4 , – – direct water-sampling impossible, soil samples scoops into Whirl-Pak bags and was melted NO3 , and F . We analyzed (1) samples of active- were collected in 1 L Whirl-Pak bags and were prior to fi ltering. Major ion concentrations were layer groundwater from within the water tracks, kept refrigerated until the samples could be determined by ion chromatography as described (2) ice-cemented permafrost (both pore ice centrifuged to separate water from soil grains. in Welch et al. (1996), resulting in a total ana- and excess ice) from beneath the water tracks, Permafrost (ice-cemented soil, excess ice, etc.) lytical error of <4%. (3) snow trapped in water track troughs,

Figure 8. Major ion versus Cl– concentration for Taylor Valley water track–related soil (WT), ice-cement (IC), excess ice (Seg), source snow (Snow), ponded fl ow (Pond), and contacts between Lake Hoare and water tracks (Lake).

2302 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem

TABLE 3. WATER TRACK δ18O, δD, AND D-EXCESS downslope over distances of several hundred δ18O δD D-excess* meters (Fig. 9, steps 10–12). Water tracks deliver Water track component (‰) (‰) (‰) water from the active layer to the Taylor Valley Snow −32.08 ± 4.27 −255.79 ± 32.24 10.84 ± 3.96 Active-layer groundwater −22.68 ± 3.57 −201.37 ± 16.87 −9.93 ± 12.48 lakes, as evidenced by the solute-rich, water Ice-cemented permafrost −16.72 ± 3.72 −172.75 ± 17.02 −28.97 ± 28.97 track–like chemistry of samples collected from Water track–fed pond −14.10 ± 7.29 −163.66 ± 40.09 −40.85 ± 18.51 Note: Stable isotope ratios for water track geomorphic components. Errors are standard deviations of the the shore waters of Lake Hoare at the mouth of sample. water tracks (e.g., Fig. 9, step 12). This shore- *D-excess is calculated after Craig (1961). water chemistry is approximately ten times more concentrated (by solute mass) than typical Lake Hoare interior water (Lyons et al., 2000). (4) water track–fed ponds, and (5) water from in Table 3 and are reported relative to standard Vertical transects of ice-cemented perma- the shore of Lake Hoare where water tracks mean ocean water (SMOW) with an analytical frost beneath water tracks provide insight into emptied into the lake. Major ion concentra- precision of ±0.25‰ for δ18O and ±1.0‰ for the interactions between active-layer solutions tions in the water tracks generally fall within δD. Stable isotope ratios for snow melting into and the ice table. Typical, non-track-related, the range of typical Taylor Valley surface waters the water tracks are close to the local meteor- ice-cemented permafrost at low elevations in (glaciers, streams, lakes) reported by Lyons et itic water line for Taylor Valley (Gooseff et al., Taylor Valley (below ~300 m) has low TDS al. (1998a). Cl– (considered a conservative ion), 2006). Snowmelt feeding water tracks is the iso- (<200 mg/L), which is largely uniform in the Ca2+, Mg2+, Na+, and K+ concentrations in water topically lightest water in the system, followed upper 10 cm of the ice-cement. In contrast, the tracks fall within the Taylor Valley continuum by water track active-layer groundwater, ice- ice-cement underlying water tracks is highly reported by Lyons et al. (1998a), but in rare cement underneath water tracks, and fi nally, by concentrated with solutes—over 4600 mg/L of cases, concentrations are more concentrated water track–fed pond water. Some isotopic val- solutes (notably Ca2+ and Cl–) were found at the than the Blood Falls hypersaline discharge ues of the active-layer groundwater values asso- ice surface. The concentration tapers off rapidly – (Fig. 8). NO3 in water track–related snow and ciated with water tracks fall close to the mete- with depth. TDS in 5–10 cm beneath the ice- ice-cement is comparable to Taylor Valley gla- oric water line, while others do not. We interpret cement table is only slightly more salt-rich than cier ice concentrations (Lyons et al., 1998a), these results to indicate that water that has been ice-cement located in non-track locations (~260 but other water track components (active-layer in the water track system through several freeze- mg/L), suggesting freeze-concentration at the water, segregation ice, etc.) exceed these values thaw cycles (e.g., ice-cement), and that has ice-table surface. For comparison, these ice- by several orders of magnitude. Ca2+/Na+ molar experienced prolonged evaporation (e.g., water cement solute concentrations are slightly more ratios in the water tracks fall largely between track pond water), is strongly depleted in D rela- than half the peak TDS concentrations (6900 0.1 and 1 (Fig. 8), a pattern interpreted by Wil- tive to 18O. This suggests that water track solu- mg/L) reported by Dickinson and Rosen (2003) son (1981) as indicative of largely atmospheric tions are a mixture of more evolved ground ice for high-elevation (>1000 m) Taylor Valley wall (precipitation) sources for the water, rather than melt (ice-cement, seasonally frozen pond water) permafrost. However, solute concentrations in deep groundwater sources, based on their simi- and fresh snowfall melt and infi ltration. the ice at the ice-table–water track interface are larity to glacier Ca2+/Na+ ratios. considerably higher than solute concentrations Water track fl uids have low Na+/Cl– molar Water Track Geochemistry found in most water track solutions. Freezing ratios on average (0.66, compared to 0.86 in sea- Spatial Patterns point depression resulting from typical water water), suggesting the possibility of cryogenic track solution concentration averages ~0.5 °C; concentration of salts during seasonal freezing A transect down one water track is exemplary however, it can reach up to 6.8 °C in the con- (Starinsky and Katz, 2003). However, the high of water track geochemical spatial relationships centrated solutions found in water track 3. These Ca2+/Na+ ratio (0.72 on average) more strongly in Taylor Valley (Fig. 9). Snow sources for water values represent minimum values, as samples suggests humidity-driven salt separation at the track moisture are relatively solute-poor; how- were collected at peak water track dilution permafrost surface as a primary driver of sol- ever, water track moisture in the active layer (maximum snowmelt input) during December ute enrichment (Wilson, 1979). Likewise, the develops total dissolved solids (TDS) loads in and January 2009. K+/Na+ ratios of water track fl uids average 0.17, excess of 1 g/L within 10 m of snowbanks (mean which is very similar to Taylor Valley stream snowbank TDS is 9 mg/L, N = 6), analogous to Comparison of Water Track and Seep ratios (~0.16), which have not been shown to the increase in stream solute concentrations from Geochemistry experience cryogenic concentration. Further hyporheic weathering noted by Gooseff et al. analysis of water track fl uids for Br content will (2002). Water track moisture undergoes evapora- The geochemical linkages between water resolve the precise origin of the salt enrichment tive concentration in ponds where fl ow through tracks and Taylor Valley groundwater seeps are relative to other Dry Valleys waters. the water track intersects the surface (commonly of particular interest (Harris et al., 2007; Lyons Stable isotope (δ18O and δD) ratios were mea- upslope of ice-cemented ridges on the valley et al., 2005). Figure 10 plots water track ground- sured for fl uids in water tracks using a Picarro walls); however, variability in the slope of ion- water major ion data along with geochemical WS-CRDS Analyzer for Isotopic Water, Model to-Cl– plots (Fig. 8) suggests that differential salt data for seeps and adjacent (glacier-fed) streams L1102-i. Samples analyzed for stable isotope dissolution in water track soils is also a major reported by Harris et al. (2007). For all elements ratios are splits from the major ion samples factor in determining water track solution com- analyzed, water fl owing through the active layer that were separated prior to ion chromatogra- position. Water track solutions are diluted by in water tracks spans the range of concentrations phy (IC) analysis. Identical sample collection meltwater contributions from snowbanks pres- measured in seeps and streams (~10−7–0.3 M). and curation procedures were followed for both ent in the water track trough (e.g., Fig. 9, steps However, water track concentrations also exceed sample sets. The δ18O, δD, and D-excess values 4 and 7). Water track solute concentrations solute concentrations in seeps by 1–2 orders of + – – for Lake Hoare basin water tracks are reported can remain relatively constant as water moves magnitude in all cases except F and NO3 . NO3

Geological Society of America Bulletin, November/December 2011 2303 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Levy et al. is the only ion for which seep concentrations are and their role in geological, hydrological, and area of 3.0 km2 (range: 0.77–8.3 km2). Taken typically similar to water track concentrations. ecological processes. together, these data reveal that water tracks The presence of seep-like features in water Do water tracks represent isolated hydro- and named streams/creeks in Taylor Valley fall tracks, coupled with the similarities in their logical features, or are they part of the larger along the same channel length/accumulation chemistry, suggests that seeps are an emergent hydrological system in Taylor Valley? Water area curve L = aAm (where L is channel length, process associated with water track activity, and tracks identifi ed in the fi eld (Fig. 1) have regu- A is accumulating area, and a and m are empiri- they are locations where saline solutions mov- lar length/accumulation-area trends when mea- cal parameters) (Mandelbrot, 1982; McNamara ing downslope through water tracks discharge at sured in the 2 m/pixel DEM. The ten water et al., 1999). For Taylor Valley water tracks and the surface due to abrupt changes in slope. tracks identifi ed in the Lake Hoare basin have streams, L = 30A0.33 (r2 = 0.89) (Fig. 11). a mean contributing length of 1.8 km (range: THERMAL AND HYDROLOGICAL 1.3–2.7 km) and a mean contributing area of Water Track Role in Taylor Valley MODELING AND DISCUSSION 0.37 km2 (range: 0.06–1.2 km2) based on the Water Budgets fl ow length and accumulation rasters. For com- Water track in situ and laboratory geochemi- parison, nine of the streams in the Lake Fryxell If water tracks are a part of the near-surface cal data, in situ thermal and physical data, and basin were also analyzed using the DEM, and hydrological system in Taylor Valley, what is DEM hydrological data can be combined to they have a mean contributing length of 4.1 km the water fl ux through them, and how does it assess the nature of water tracks in Taylor Valley (range: 2.4–7.0 km) and a mean contributing compare to the discharge from streams? Taking (mg/L) 4 2− (mg/L) − 3 Concentration of Concentration F, NO F, Concentration of Concentration 0.5 Ca, Mg, Na, K, Cl, SO

Figure 9. Geochemical transect down water track 1. Lower image shows sample site locations 1–13. S—source snow, W—water track solutions, P—surface ponding, LS—shore of Lake Hoare at the water track mouth, and LI—lake interior (a sample from 4 m depth collected in 2006; portion of Quickbird image 09FEB01213752). Image has been stretched to enhance contrast.

2304 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem the largest water track observed during 2009– wet soils and 0.3 m/yr for dry soils. Field obser- Delta Stream, the closest gauged stream to the 2010 (Figs. 3A–3B), here termed water track 1 vations of water track 1 show that it is ~3 m water track is 33 L/s, or 2.9 × 106 L/d (based (Fig. 1), the feature drops 170 m in elevation wide on average, with a typical saturated (fl ow- on mean daily values 2000–2009), draining into over 1900 m of fl ow length, yielding an aver- ing) depth of 10 cm. Taking fl ux times cross- nearby Lake Fryxell. age gradient of 5°. Taking the average hydraulic sectional area leads to a discharge through the Accordingly, water tracks do not appear to conductivity measured in Taylor Valley soils, water track of 5.4 cm3/s, or ~470 L/d. The next be a major contributor of water to McMurdo we compute an average fl ux of 1.8 × 10−3 cm/s largest water track observed in the Lake Hoare Dry Valleys lakes, as compared to streams. using Darcy’s fl ow law. Interestingly, this rate basin during 2009–2010 (Figs. 3C–3D), water However, satellite image and fi eld analysis is two orders of magnitude greater than ionic track 2, has a similar slope (4°) and cross- suggest that water tracks are a major source migration rates measured in non-water-track sectional area, suggesting a similar discharge. of water to McMurdo Dry Valleys soil ecosys- soils by Claridge et al. (1997), i.e., 1.7 m/yr for For comparison, mean summer discharge of tems. Unlike McMurdo Dry Valleys streams,

Figure 10. Major ion versus Cl– concentration for Taylor Valley water tracks (WT) (this paper) and seeps/streams reported by Harris et al. (2007).

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water track 1 is 1.84 × 10−2 (Fig. 9), yielding a of 0.033 S/m and 0.231 S/m (measured in situ), Cl– fl ux to Lake Hoare from water track 1 of 8.6 respectively. In contrast, Andersen Creek water mol/d by multiplying the molarity (mol/L) by entering Lake Hoare has a mean temperature the discharge (L/d). Track 2 is considerably less of 0.2 °C and a mean electrical conductivity of briny than water track 1 (1.64 × 10−3 mol/L Cl– 0.005 S/m (based on 2000–2010 gauge data), at the Lake Hoare shore), resulting in approxi- yielding a mean density of 999.9 kg/m3. Den- mately an order of magnitude less Cl– fl ux than sity variability from temperature uncertainty is from track 1. In contrast, the briniest water ±~0.01 kg/m3. These calculations illustrate the track in the Lake Hoare basin, water track 3, strong dependence of solution density on TDS, has a Cl– concentration that reaches 7.31 × 10−1 relative to temperature, for small temperature mol/L, suggesting the possibility of large fl uxes differences and high solute loads. Based on con- (~340 mol/d). However, fi eld observations sug- ductivity-temperature-density measurements gest that water track 3 is not currently discharg- through the Lake Hoare water column collected ing into Lake Hoare due to damming by an ice- by the MCM-LTER on 3 January 2010, Andersen Figure 11. Plot of feature length (L) ver- cored ridge—solute transport into Lake Hoare Creek water would fi nd an equal-density level at sus accumulation area (A) for water tracks would require cryptic fl ow through fractures 3.3 m depth, saline waters from water track 1 and named streams/creeks in Taylor Val- (e.g., Dickinson and Rosen, 2003). For compar- would be neutrally buoyant at 6.3 m depth, and ley. Lengths and accumulation areas were ison, the Cl– concentration of Andersen Creek water track 3 solutions would sink to the bot- determined from analysis of a fl ow direction (one of the two major streams in the Lake Hoare tom of the lake. This slight density imbalance raster generated from a 2 m digital elevation drainage) is highly variable, but it averages may permit water track solutions to dip below model. Water tracks and creeks fall along ~110 μM (Lyons et al., 2003). Andersen Creek surface water input by stream fl ow, to produce the same length/accumulation area curve has an average discharge of ~30 L/s (based on Cl–-rich, interfi ngered pockets of water under 0.33 2 (gray line), L = 30A (r = 0.89). 2000–2010 gauge data), yielding an average current conditions. Water track–lake intersec- Cl– fl ux of 285 mol/d. tion points are also the locations of subsurface Next, we calculate the effect of Cl– fl ux from anoxic channel features incised into the lake bed the duration of water track contribution to water tracks on Taylor Valley lake salt budgets. (Peter Doran, 2010, personal commun.). A shal- lakes is mainly controlled by the source water Lake Hoare has an average Cl– concentration of low injection and interfi ngering mechanism like dynamics (how much snow and/or ground ice ~5 mM and a volume of 17.5 × 106 m3 (Lyons et this was inferred by Wilson (1981) as a means is available for melt) and surface energy bal- al., 2000), yielding a Cl– reservoir of 87.5 × 106 for injecting bomb-generated tritium into deep, ance (that drives melting). Once water track moles of Cl–. Assuming constant (year-round) stratifi ed, and ancient lake waters in Wright and saline solution transport is initiated, however, fl ow, the water tracks could account for the Lake Taylor Valleys. The presence of density-layered it is expected that their hydroperiod is limited Hoare Cl– reservoir given 28 k.y. of fl ow from solutions raises intriguing questions about lake by colder temperatures than the stream hydro- water track 1 alone (considerably longer than stability versus composition-dependent overturn period (see geochemistry section). the age of the lake) (Doran et al., 1999, 1994), (e.g., Lyons et al., 1996). Based on the similarity These fl ux calculations indicate that saline or ~700 yr of fl ow from water track 3 alone. between water track Ca2+/Na+ ratios measured solutions moving along the ice table through Depending on water track salinity variations in the Lake Hoare basin (0.72) and Lake Hoare water tracks may take ~75–150 yr (mean: 104 yr, with time and location, water tracks may be an bottom water (0.27), it appears that water track N = 10) to move from the highest reaches of the important contributing pathway in the Taylor solutions may contribute signifi cantly through- water track to the lake shore, assuming constant Valley lake solute budget. out the Lake Hoare water column; however, fl ow. This suggests that there may be ample time water track fl uids are not likely to be a major for water track solutions to leech soluble species Water Track Discharge into Lakes infl uence on the ionic ratio of Lake Fryxell from surrounding soils—a chemical weathering (Ca2+/Na+ = 0.03) and Bonney (Ca2+/Na+ = 0.01) pathway strongly suggested by the exception- Analysis of water-column geochemical pro- bottom water (the latter are thought to be com- – ally high NO3 concentrations observed at water fi les reveals the presence of high-salinity waters posed of relict, evapoconcentrated paleobrines track termini (Harris et al., 2007) (see geochem- at depth in Lakes Fryxell and Bonney, and, to a that are denser than water track solutions and istry section). lesser extent, Lake Hoare (Doran et al., 1999, that may have experienced Ca2+ loss from cal- 1994; Lyons et al., 2000). These profi les have cite precipitation; such brines are absent in Lake Water Track Role in Taylor Valley been interpreted to indicate desiccation of the Hoare; Lyons and Mayewski, 1993). Salt Budgets lakes (complete, in the case of Lake Hoare) and refi lling by stream water with little to no evapo- Water Tracks and the Permafrost Although water tracks may not be a signifi - concentration within the past ~1 k.y., based on Thermal Regime cant factor in the Taylor Valley water budget, Cl– diffusion rates and Li+/Na+ ratios (Lyons and their high salinity raises the possibility that they Welch, 1997; Lyons et al. 1998b). Water track Next, we consider the thermal effect of water are signifi cant sources of solutes to McMurdo physical and chemical data suggest that water track fl uid transport on permafrost in Taylor Val- Dry Valleys lakes. Cl– is commonly used as a track solutions during 2009–2010 had a den- ley. Apparent thermal diffusivity (ATD) is the conservative element in Taylor Valley hydrolog- sity of 1000.2 kg/m3 (for water track 1) and up ratio between soil thermal conductivity (its abil- ical studies because it is unlikely to precipitate to 1001.7 kg/m3 (for water track 3). Densities ity to transport heat) and soil heat capacity (the out of streams and lakes due to halite or antarc- were determined using the equations of state product of soil density and specifi c heat capac- ticite saturation (Lyons et al., 2005, 1998a). The in Fofonoff (1985) for surface water at 2.8 °C ity, or the tendency of the soil to store heat). In Cl– molarity of water entering Lake Hoare from (measured in situ) and electrical conductivities the cold and ice-cemented soils of Antarctica,

2306 Geological Society of America Bulletin, November/December 2011 Downloaded from gsabulletin.gsapubs.org on October 11, 2011 Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem conductive heat transfer provides an important of latent heat of fusion in limiting water track track soils, may result in enhanced soil creep fi rst-order measure of heat movement (Hage- thaw depth. Consider a typical 1 m2 patch of that transports sediments downslope faster than dorn et al., 2007; McKay et al., 1998). water track soil that thaws to 45 cm depth from dry soils. Such a soil creep mechanism could, in As in previous studies of Taylor Valley an average winter temperature of −30 °C (e.g., part, provide a feedback that lowers the topog- seeps (Lyons et al., 2005), we employed an Table 1) and has an average porosity of 30% raphy around water tracks, enhancing the gentle elementary thermal diffusion model that ana- (and that is entirely ice-saturated). The volume topographic channels through which they fl ow. lytically determines temperature with depth initially contains 0.14 m3 of ice and 0.32 m3 of Creep erosion in water tracks might account for and time based on a sinusoidal function that sediment. Assuming a volumetric heat capac- the “overabundance” of channels in the Taylor prescribes surface temperature (Campbell, ity of 1.5 × 106 J/m3•°C for the sediment (as Valley landscape reported by McKnight et al. 1977; Yershov, 1998). measured in the fi eld) and 1.9 × 106 J/m3•°C (1999) relative to the rarity of overland fl ow. for ice, to raise the intermingled ice and sedi- –z/D ω 7 T(z, t) = To + A(0)e sin( t – z/D), (1) ment to 0 °C would require 2.2 × 10 J of net Evaporative Concentration and heat addition (8.0 × 106 J for the ice; 1.4 × Sublimation in Water Tracks 7 where T is temperature, To is mean annual sur- 10 J for the sediment). Once at 0 °C (neglect- face temperature, z is depth, t is time, A(0) is ing the role of water track salts in depressing What is the role of evaporative concentration mean annual temperature at the surface, ω is the freezing point), assuming a latent heat of of water track solutions, and how does sublima- the angular frequency of temperature oscillation fusion of water of 334 kJ/kg, an additional tion of seasonally frozen water track ice affect (annual), and D is the damping (or e-folding) 4.3 × 107 J of net heat would be required to salt and water budgets within the water tracks? depth. D = (2κ/ω)1/2, where κ is the apparent melt the water track ice to a liquid. This sug- These questions are motivated by the season- thermal diffusivity (ATD). A major shortcoming gests that heat added to the water track is par- ally and interseasonal persistence of dark, wet of this model is that it does not account for heat titioned ~34% into warming and ~66% into water track soils as well as the observations of exchange, nor does it account for the chang- thawing water track ice. In contrast, an adja- light-toned salt deposits at some water track ing ATD with depth observed for Taylor Valley cent (off-track) dry soil patch thawing to 20 cm margins. Evaporation/sublimation rates in the soils. Accordingly, it tends to dramatically over- depth from −30 °C requires only 9 × 106 J of McMurdo Dry Valleys have been calculated to predict the depth of thaw. net heat addition, suggesting the possibility of be ~30 cm/yr for ice-covered lakes (Clow et The sinusoidal-analytic model does not accu- vastly different energy budgets for (wet) water al., 1988), or 6.17 mm/d for open-pan evapora- rately predict the depth of thaw in Taylor Valley track soils and (dry) off-track soils. tion (Gooseff et al., 2003b; comparable to an water tracks. However, it is possible to use it to Despite analytical shortcomings, however, ~50 d warm season with peak evaporation rates). illustrate the importance of water track moisture the sinusoidal-analytic model and a simple Applying the 6.17 mm/d evaporation rate, this in changing surface soil thermal conditions. We energy budget calculation clearly illustrate that suggests that for the water tracks to remain vis- used a manual fi t to climate data (mean tempera- increased ATD, resulting from high soil mois- ibly wet (dark) at the surface, for water track ture −18 °C, amplitude 25 °C), and ATD values ture, coupled with heat advected from warm 1 (~3 m width, ~1000 m long), a volume of for sandy soil from Baver et al. (1972) of 0% water traveling along the water track, and from ~20 m3/d of water is being delivered from the VWC and 15% VWC (i.e., dry, off-track soil a decrease in water track albedo with wetting, saturated active layer beneath the water track to and wet, water track soil = 0.2 and 0.8 mm2/s, has the potential to drive thaw depths (measured the surface to meet evaporative demand, inte- respectively). In this scenario, thaw occurs at as depth to refusal) tens of centimeters deeper in grated over the full length of the feature. This 60 cm depth in wet soils but is arrested above water tracks than in off-track (“dry”) soils. This evaporation rate should be considered a maxi- 60 cm in dry soils. These thawing depths exceed enhanced depth of thaw in water tracks likely mum, because it does not consider small changes the on-track/off-track ice-table depths observed mobilizes water that is seasonally trapped as in evaporation rate due to solute-driven depres- during 2009–2010. The discrepancies between pore and excess ice within water track channels, sion of water track fl uid vapor pressures (not modeled thaw depth and observed thaw depth contributing to water track fl ow, while leaving more than ~1%). Where water tracks exhaust result from a combination of complex thermal ground ice “locked” in permafrost outside of the water table (defi ned by the saturated active inputs (the r2 of the sinusoidal models relative water tracks. layer) at the margins of the water track, intense to the actual temperature data is ~0.84, owing to Given the importance of surface tempera- soil salination occurs, resulting in the deposition winter katabatic warming). Other error sources tures in driving the liberation of ground ice in of visible salt effl orescences. In addition, winter include the inability of the sinusoidal-analytic water tracks, it is possible that these features are sublimation rates are found to be small in water model to partition energy into melt generation emerging from a multidecadal period of qui- tracks. Follow-on observations of water track or to change ATD with depth. Albedo differ- escence resulting from ~0.7 °C of cooling per ice-table depths in October 2010 revealed that ences on and off water tracks may also locally decade in the McMurdo Dry Valleys over the water track soils were commonly ice-cemented enhance summer warming on water tracks, 1986–2000 period (Doran et al., 2002b), and within 1 cm of the ground surface, in contrast to while winter sublimation of soil moisture may that active-layer transport of salts and nutrients depths to the ice table of ~10 cm for off-track locally enhance cooling on water tracks. Deep to McMurdo Dry Valleys lakes may become soils. This suggests minimal ice removal from active layers in some water tracks suggest that more vigorous. Future warming of only 1–2 °C within water tracks during Antarctic winter. over-the-ice-table fl uid transport may be advect- in the McMurdo Dry Valleys could signifi cantly What is the role of evaporation on water track ing heat down slopes (Yershov, 1998). increase the volume of water moving through solution chemistry? An ideal site to measure What is the role of latent heat in controlling Taylor Valley water tracks by the end of this evaporative concentration of water track solu- thaw depth in water track soils? Although a century (Arblaster and Meehl, 2006; Chapman tions is in the stretch of water track 1 imme- complete thermodynamic treatment of water and Walsh, 2007; Shindell and Schmidt, 2004). diately above Lake Hoare (Fig. 9, steps 9–11). track soils is beyond the scope of this study, In addition, the enhanced depth of thaw in water Here, over ~25 m of downslope fl ow (~2 wk a simple energy budget can illustrate the role tracks, coupled with the saturation of water transit time, based on fl ow rates calculated

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earlier), water track solution chemistry remains survive in 1 dS/m solutions (Nkem et al., 2006). contributions to larger, more established water nearly unchanged. In fact, the water track These laboratory studies have been verifi ed by tracks. In contrast, the Lake Bonney long-term solutions become more dilute with distance fi eld observations in Taylor Valley showing manipulation experiment is located in the water downslope, likely refl ecting small additions species-dependent and asymptotic reduction in track associated with the Wormherder Creek of low-solute water from the incorporation of live nematodes as soils exceed salinities of 1 or seep (Harris et al., 2007; Lyons et al., 2005). snowmelt and/or buried excess ice in the water 2 dS/m (Poage et al., 2008). These data suggest Monitoring at this site is currently assessing the track during summer heating. This suggests that that saline portions of some water tracks may response of soil ecosystems to the emergence of on daily/weekly time scales, evaporation is not not be suitable habitats for some nematodes, and a water track. a major determinant of water track chemistry. they may act as barriers to nematode coloniza- In contrast, where water track solutions pond tion (Fig. 12). In addition, new water tracks may CONCLUSIONS (either at the surface, or in the subsurface), for wick saline water (2–4 dS/m) into established example, in the water track 1 pond (Fig. 9, step nematode communities, with potentially lethal Water tracks are a spatially extensive water 5), or at the terminus of water track 3, solute consequences. transport pathway in the McMurdo Dry Valleys concentrations can reach values up to 50 times Soil moisture has a more complicated effect that can move meltwater derived from snow and greater than the solute concentrations of water on soil ecology. High soil moisture (up to 16% ground-ice sources through active-layer soils, track fl uids found immediately upslope. This by mass or 29% by volume, assuming average and ultimately to Dry Valleys lakes. In this man- suggests that where fl ushing with freshwater Taylor Valley bulk densities) is strongly corre- ner, water tracks represent a signifi cant source (snow, or ground ice melt) is minimal, evapo- lated with E. antarcticus presence; however, S. of hydrological, chemical, and ecological con- rative concentration can have a large effect on lindsayae is most commonly found in drier soils nectivity between Dry Valleys soils and lakes. water track solution chemistry. (<6% by mass, or 11% by volume) (Virginia Water tracks are the primary conduits of fl ow in How strong is the evaporative concentration and Wall, 1999). These data suggest that water this cryptic, shallow, groundwater system. signal in water track stable isotope values? Tak- track location may strongly infl uence nematode First, water tracks are a source of salts to ing the evapoconcentration–stable isotope rela- species distribution at the hillslope scale, as (low McMurdo Dry Valleys lake and soil ecosystems. tionships of Gremillion and Wanielista (2000), salinity) water track soils may be suitable for E. Water tracks transport atmospherically depos- +1‰ δ18O per 1% reduction in water volume antarcticus, while off-track soils are more suit- ited and rock-weathering–derived solutes to by evaporation, and +0.62‰ δD per 1% reduc- able for S. lindsayae (see geochemistry section). lake water columns in a quantity comparable to, tion in water volume, it is possible to estimate Water tracks have had a systematic effect on and potentially exceeding, stream inputs. Water evaporation losses during transitions between some of the long-term manipulation experiments track solute transport rates exceed non-water- different components of the water track system conducted in Taylor Valley (designed to test the track soil transport rates by two orders of mag- (Table 3). Considering only δ18O, the snow to response of soil ecosystems to changing envi- nitude, suggesting that water tracks are a “solute water track active-layer fl uid transition results ronmental conditions; Freckman and Virginia, superhighway” through Taylor Valley soils. in a loss of ~9% water volume, the water track 1997; Virginia and Wall, 1999). The Lake Hoare Second, some water track fl uids interfi nger fl uid to ice-cement transition results in an ~6% long-term manipulation experiments conducted with stream water, producing the potential for water loss, and the evaporative concentration by the MCM-LTER are not affected by water complex lake chemistry with depth. The dens- of water track fl uids in ponds refl ects an ~9% tracks. They are located on topographic highs est water track fl uids are predicted to sink to the volume loss. Comparable calculations using between water tracks (Fig. 13). Any meltwater bottom of Lake Hoare, suggesting a new pos- δD values result in percentage water losses 5– accumulating at the ice table in these experimen- sible explanation for the source of Lake Hoare 10 times greater. These calculations neglect tal sites is likely to drain downslope, providing bottom waters. Water track fl uids appear to kinetic fractionation effects on water track stable isotope values resulting from freezing, sublima- tion, and vapor diffusion. The pan evaporation experiments of Gooseff et al. (2003b) suggest that when peak evaporation occurs, stable iso- tope values for water track fl uids may change rapidly, on hourly to daily time scales.

ECOLOGICAL IMPLICATIONS

Permafrost landforms provide structure to the nematode-dominated ecosystem in Taylor Valley by controlling the distribution of liquid water, salts, and heat in the soil (Barrett et al., 2004; Nkem et al., 2006; Poage et al., 2008; Virginia and Wall, 1999). Laboratory studies report that even Antarctic-adapted, endemic nematodes (the top predator in the Dry Valleys) Figure 12. Water track solution electrical conductivity (EC) cannot tolerate salinities in excess of 4.1 dS/m, versus soil volumetric water content (VWC). Some water track but that some Scottnema lindsayae nematodes soils may be inhospitable environments for nematodes, includ- are able to tolerate ~2 dS/m solutions, and some ing Scottnema lindsayae and Eudorylaimus antarcticus, due to Eudorylaimus antarcticus nematodes are able to high salinity and/or high soil moisture.

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Figure 13. (A) Location of the Lake Hoare McMurdo Dry Valleys Long-Term Ecological Research project (MCM-LTER) long- term manipulation experiment (LTME) relative to water tracks (dark soil) and digital elevation model overlay showing pixels with >0.04 km2 upslope accumulation area. The LTME is isolated from water track infl uence. (B) Location of the Lake Bonney LTME relative to a water track associated with Wormherder Creek. The Bonney LTME is seasonally inundated by the water track. Portion of Quickbird images 09FEB01213752 and 09DEC25204653.

REFERENCES CITED interfi nger at all depths in the Lake Hoare water both suitable (because of locally elevated soil column, including the base, but they are not moisture content), and, in cases, nonsuitable likely candidates for the sources of the brines at (because of high salinities) for nematode- Alger, A.S., McKnight, D.M., Spaulding, S.A., Tate, C.M., Shupe, G.H., Welch, K.A., Edwards, R., Andrews, the bases of Lakes Bonney and Fryxell. dominated ecosystems. E.D., and House, H.R., 1997, Ecological processes in a Third, seeps identifi ed in Taylor Valley, and cold desert ecosystem: The abundance and species dis- across the Dry Valleys, represent locations ACKNOWLEDGMENTS tribution of algal mats in glacial meltwater streams in Taylor Valley, Antarctica: Institute of Arctic and Alpine where the saturated thickness of a water track Research Occasional Papers, v. 51, p. 1–118. is greater than the depth to the ice-cement table, This work is supported by the Antarctic Organisms Arblaster, J.M., and Meehl, G.A., 2006, Contributions of producing a localized discharge of active-layer and Ecosystems Program in the Antarctic Sciences external forcings to southern annual mode trends: Division of the National Science Foundation (NSF) Journal of Climate, v. 19, p. 2896–2905, doi:10.1175/ water. Accordingly, seeps are an emergent phe- JCLI3774.1. (grant ANT-0851965 to Levy). Quickbird image data nomenon associated with water track activity. Barrett, J.E., Virginia, R.A., Wall, D.H., Parsons, A.N., Pow- were provided courtesy of Paul Morin and the Antarc- ers, L.E., and Burkins, M.B., 2004, Variation in bio- Fourth, when taken together, Dry Valleys tic Geospatial Information Center. Light detection and geochemistry and soil biodiversity across spatial scales streams, seeps, and water tracks represent a ranging (LiDAR) topography was kindly made possi- in a polar desert ecosystem: Ecology, v. 85, no. 11, morphological continuum between a high-fl ow ble through a joint effort from the NSF, National Aero- p. 3105–3118, doi:10.1890/03-0213. nautics and Space Administration (NASA), and the Barrett, J.E., Gooseff, M.N., and Takasc-Vesbach, C., 2009, regime (producing overland fl ow in streams) U.S. Geological Survey (USGS), with basic postpro- Spatial variation in soil active-layer geochemistry and a low-fl ow regime (where only shallow cessing from the Byrd Polar Research Center. Diana across hydrologic margins in polar desert ecosystems: Hydrology and Earth System Sciences, v. 13, no. 12, active-layer fl ow occurs through water tracks). Moseman and Devin Castendyk at State University p. 2349–2358, doi:10.5194/hess-13-2349-2009. These results highlight the intense degree of of New York (SUNY) College at Oneonta provided Baver, L.D., Gardiner, W.H., and Gardiner, W.R., 1972, Soil connectivity among the cryosphere, hydro- water track solution density calculation assistance, Physics: New York, New York, Wiley, 498 p. and Martin Lafrenz at Portland State University pro- Bockheim, J.G., Campbell, I.B., and McLeod, M., 2007, sphere, atmosphere, and biosphere in the vided fi eld geographic information system expertise Permafrost distribution and active-layer depths in the McMurdo Dry Valleys. and equipment. Many thanks go to Deb Leslie at the McMurdo Dry Valleys, Antarctica: Permafrost and Finally, water tracks are a permafrost land- Byrd Polar Research Center for water track stable iso- Periglacial Processes, v. 18, p. 217–227, doi:10.1002/ tope analysis. This project has benefi ted greatly from ppp.588. form that structures ecosystems in the Ant- Bockheim, J.G., Prentice, M.L., and McLeod, M., 2008, discussion with McMurdo Dry Valleys Long-Term arctic cold desert. Water tracks are an impor- Distribution of glacial deposits, soils, and permafrost in Ecological Research project (MCM-LTER) team Taylor Valley, Antarctica: Arctic, Antarctic, and Alpine tant source of moisture to soil ecosystems members (www.mcmlter.org), and from the thought- Research, v. 40, no. 2, p. 279–286, doi:10.1657/1523 and produce patchy expanses of soil that are ful comments of three anonymous reviewers. -0430(06-057)[BOCKHEIM]2.0.CO;2.

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