Hydraulic conductivity of active layer soils in the McMurdo Dry Valleys, Antarctica: Geological legacy controls modern hillslope connectivity

Geomorphology 283 (2017) 61–71

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Hydraulic conductivity of active layer soils in the McMurdo Dry Valleys, Antarctica: Geological legacy controls modern hillslope connectivity Logan M. Schmidt, Joseph S. Levy ⁎ University of Texas Institute for Geophysics, 10100 Burnet Rd., Austin, TX 78758, USA

a r t i c l e

i n f o

Article history: Received 25 July 2016 Received in revised form 30 January 2017 Accepted 30 January 2017 Available online 2 February 2017 Keywords: Hydrology Hillslope Permafrost Antarctica Water track Ecology

a b s t r a c t Spatial variability in the hydraulic and physical properties of active layer soils influences shallow groundwater flow through cold-desert hydrological systems. This study measures the saturated hydraulic conductivity and grain-size distribution of 90 soil samples from the McMurdo Dry Valleys (MDV), Antarctica—primarily from Taylor Valley—to determine what processes affect the spatial distribution of saturated hydraulic conductivity in a simple, mineral-soil-dominated natural hillslope laboratory. We find that the saturated hydraulic conductivity and the grain-size distribution of soils are organized longitudinally within Taylor Valley. Soils sampled down-valley near the coast have a higher percentage of fine-sized sediments (fine sand, silt, clay) and lower saturated hydraulic conductivities than soils collected up-valley near Taylor Glacier (1.3 × 10−2 vs. 1.2 × 10−1 cm/s). Soils collected mid-valley have intermediate amounts of fines and saturated hydraulic conductivity values consistent with a hydrogeologic gradient spanning the valley from high inland to low near the coast. These results suggest the organization of modern soil properties within Taylor Valley is a relict signature from past glaciations that have deposited soils of decreasing age toward the mouth of the valley, modified by fluvial activity acting along temporal and microclimate gradients. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In permafrost environments, shallow groundwater flow through the seasonally thawed active layer above the ice table is the primary flow path for the snowmelt and glacier runoff that ultimately feeds regional hydrological systems (Kane and Stein, 1983; Hinzman et al., 1991; Kane et al., 1991; McNamara et al., 1999). Preferential active layer flow paths, commonly called water tracks, provide structure to hillslope permafrost and ecological processes by controlling the thermal characteristics of the soil (Kane and Stein, 1983; Hinzman et al., 2007; Levy and Schmidt, 2016), the distribution and metabolism of soil biota (Chapin et al., 1988; Hastings et al., 1989; Oberbauer et al., 1991; Levy et al., 2013a,b; Ball and Levy, 2015), and the flux of solutes and weathering products through the soil (Williams et al., 2006; Harris et al., 2007; Caine, 2010; Ball and Virginia, 2012). Accordingly, water tracks are a critical linkage in polar watersheds that provide connectivity between regional geological and soil conditions and downstream biogeochemical processes (Gooseff et al., 2011b). Because water tracks flow through the seasonally thawed active layer, the flux of water, solutes, and heat is expected to increase in polar and alpine regions as active layers thicken (Gooseff et al., 2011a, 2013) and decrease where ⁎ Corresponding author. E-mail address: [email protected] (J.S. Levy).

http://dx.doi.org/10.1016/j.geomorph.2017.01.038 0169-555X/© 2017 Elsevier B.V. All rights reserved.

active layers thin (e.g., Ramos et al., 2016), producing changes to physical and ecosystem functioning downstream. Despite the importance of water tracks in routing water, solutes, and heat through active layer soils, relatively little is known about the processes that determine water track routing and overall discharge (McNamara et al., 1999; Levy et al., 2011). This is, in part, because of the difficulty of measuring subsurface properties (particularly saturated hydraulic conductivity) across large watersheds containing many heterogeneous landforms. Past measurements of saturated hydraulic conductivity in Arctic water tracks range from ~ 1.0–1.9 × 10− 2 cm/s in the organic soil horizon down to ~ 0.9–1.4 × 10−3 cm/s in underlying mineral horizons below 15 cm depth (Hinzman et al., 1991). Clay-rich watersheds can have active layers with extremely low saturated hydraulic conductivities, e.g., ~ 10−7 cm/s (Benson and Othman, 1993), while watersheds with sorted periglacial features can have highly variable saturated hydraulic conductivities, spanning 1.2 × 10− 4 to 1.2 × 100 cm/s, for soils in frost mound interiors and intermound troughs, respectively (Hodgson and Young, 2001). Compared to the Arctic, few measurements of saturated hydraulic conductivity from ground-based or satellite measurements have been made in ice-free regions of Antarctica and tend to be averaged over whole water tracks (e.g., 2 × 10−2 to ~ 1 × 10−1 cm/s, Levy, 2012a; Levy et al., 2011). As coastal margins of Antarctica begin to experience glacial retreat and exposure of new land surface (e.g., Cook et al., 2005), Antarctic active layer

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hydrology is expected to become an even more important driver of landscape processes (e.g., Gooseff et al., 2011b). In order to help make sense of the variability in hillslope hydraulic properties, we measured the saturated hydraulic conductivity of soil samples from 90 locations in the McMurdo Dry Valleys of Antarctica (Fig. 1) in order to test two hypotheses: i) geological history controls saturated hydraulic conductivity, and ii) position in the hydrological system (e.g., on a water track flow path vs. off the water track flow path) controls saturated hydraulic conductivity. Hypothesis 1 presents two possible and potentially contradictory predictions: older soils could be enriched in weathering products (fines), resulting in poorer sorting and lower saturated hydraulic conductivity than younger soils. Or, removal of fines from older soils through water track flow could produce increasingly high saturated hydraulic conductivity with age through enhancements in sorting. Hypothesis 2, that position in the hydrological system controls ksat, is motivated by studies like Hodgson and Young (2001), which indicate that small-scale sorting features such as frost boils have strong influences on hydraulic processes. Again, two apparently contradictory predictions emerge from hypothesis 2: water tracks could have either higher saturated hydraulic conductivity than off-track soils due to removal of fines during water track flow, or they could have a lower saturated hydraulic conductivity as a consequence of accumulation of fines in the water track that have been transported in from adjacent off-track soils. 1.1. Regional setting The McMurdo Dry Valleys (MDV) are the ideal environment in which to test these hypotheses regarding how hillslope geology affects water track flow. At 4500 km2, the MDV are the largest ice-free area in Antarctica (Levy, 2012b). They are a cold desert with a mean annual temperature of − 18 °C (Doran et al., 2002) and 3–50 mm of water-

equivalent precipitation, all of which falls as snow and most of which sublimates (Hunt et al., 2007; Fountain et al., 2009). A lack of vascular plants in the MDV means that soils are almost exclusively shaped by physical (geological, cryogenic) processes, rather than by the distribution of biota, allowing us to determine how location, age, and geological legacy (parent material and climate/surface processes history; Lyons et al., 2000) combine to determine the spatial distribution of saturated hydraulic conductivity in the active layer. Active layer thicknesses in the MDV span 0–70 cm, with ~ 20–30 cm being most common in Taylor Valley (Bockheim et al., 2007). Active layers form over ice-cemented permafrost (Bockheim et al., 2007) with a typical temperature of ~ −14 to −19 °C at 10 m depth (Decker et al., 1982)—similar to the long-term annual average temperature in Taylor Valley (−18 °C, Doran et al., 2002). Active layers are typically very dry, with b 1–2% volumetric water content in the upper 10–20 cm, except for where water tracks are present, allowing soil moisture to rise to saturation (Campbell, 2003; Levy et al., 2013a,b). We sampled soils primarily in Taylor Valley, an ~50-km-long valley centered on 77.7° S, 162.7° E that links Taylor Glacier, an outlet glacier of the East Antarctic Ice Sheet, to the Ross Sea. Taylor Valley is comprised of a mosaic of distinct soils that record evidence of several Quaternary glaciations dominated by the eastward expansion of Taylor Glacier (Higgins et al., 2000a,b; Bockheim et al., 2008) and the westward advance of Ross Sea grounded ice-sheets (e.g., Stuiver et al., 1981; Hall et al., 2000). The Bonney basin (Fig. 1) is characterized by a variety of complex landforms of glaciolacustrine, fluvial, and aeolian origin (Higgins et al., 2000a). These include several distinct soil textures: from coarse, glacier-marginal talus deposits, to sand-dominated glaciofluvial and glaciolacustrine facies, to silt-dominated lacustrine deposits and meltout tills (Higgins et al., 2000a). Silty landforms are concentrated along the valley floor and can derive from lacustrine or morainal

Fig. 1. (A) Location of soil sample sites in Taylor Valley. Soils map adapted from Bockheim et al. (2008) to show the location of soil groups. TG is Taylor Glacier, LB is Lake Bonney, LH is Lake Hoare, LF is Lake Fryxell. (B) Location map showing Beacon, Taylor, and Garwood Valleys in the context of the Ross Sea region. (C) Location of soil sample from Beacon Valley. (D) Location of soil samples from Garwood Valley. Base map is LiDAR hillshade.

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processes; while coarse till, channel, and talus deposits are commonly found higher along the valley walls (Higgins et al., 2000a). The Lake Fryxell and New Harbor basins (Fig. 1) are occupied by Ross Sea Drift and associated deposits, and like Bonney basin soils, are composed of a variety of landform types, including several varieties of tills, paleolake delta deposits, lake-reworked tills, channels, etc. (Denton and Marchant, 2000; Hall et al., 2000). In all basins studied, primary glacial deposits and glaciolacustrine deposits are commonly found below ~ 300 m asl (above sea level), with colluvial soils found above this boundary (Hall et al., 2000; Higgins et al., 2000a). To provide endmember environmental comparisons, soils were also sampled from the mouth of Garwood Valley (78.0° S, 164.2° E), a comparatively warm, coastal thaw zone site (Marchant and Head, 2007), where December/January average temperatures exceed 0 °C (Fountain et al., 2014). The Garwood sample sites are dominated by thick deposits of fully intact Ross Sea Drift marine-influenced till (Levy et al., 2013b), and MIS 5-age melt-out till found farther upvalley (Levy et al., 2016). A second endmember sample was collected in Beacon Valley (77.8° S, 160.6° E), a site in the upland frozen zone in which almost no active layer forms in freeze-dried, Miocene-aged tills (Marchant and Denton, 1996) owing to average summer temperatures below −10 to −15 °C (Fountain et al., 2014). The Beacon Valley site is within the Granite Drift, a terrestrial till that dominates this site and that has been extensively modified by sublimation/sand-wedge polygon formation and aeolian processes (Marchant et al., 2002; Kowalewski et al., 2011). These two sites provide endmember climatological conditions (cold, dry, and inland versus warmer, wetter, and coastal) as well as endmember age/ composition conditions. 2. Methods Soil samples were collected in 2012–2013 and 2015–2016. The goal of these opportunistic (2012) and coordinated (2015) sampling campaigns was to generate a representative, if not exhaustive, collection of soil properties from water track-affected soils across Taylor Valley. Sample locations, elevations, slope, and orientation data are included in the Supplementary data table. Dry active layer conditions dominate Taylor Valley, and so many soil types were not sampled because they were not within the studied water track watersheds. Soils collected in 2012–2013 were collected from the upper 10 cm of the soil column immediately beneath the desert pavement (which was scraped off with a trowel). This sampling depth was chosen because it is the horizon through which the first flows of the season move (i.e., when the ice table is within 1–2 cm of the surface; Levy et al., 2011), which makes it the most relevant for comparison to orbital measurements of hydraulic conductivity (which captures only early season flow). Soils were sampled from on- and off-track sites along the lengths of 2–3 water tracks per valley segment during foot traverses from the bottom to the top of water tracks. Samples were collected from representative reaches of each water track. Samples collected in 2012–2013 were procured by scooping ~ 500 mL of soil into a Whirl-Pak bag. Soils collected in 2015 were selected to sample a depth profile through a water track and adjacent off-track soils at ~10 cm increments from the soil surface immediately beneath the desert pavement to the top of the ice table (typically 4–5 samples), as well as a near-surface (0–10 cm) longitudinal profile of samples along the length of the water track at adjacent on- and off-track points (~ 4 pairs of on- and off-track samples, plus one pair of depth profiles at on- and off-track sites). Samples collected in 2015–2016 were collected using UMS steel ring samplers pressed or pounded into the ground and capped in the field. Soils were sampled from several units, spanning ages from 12.4 to 120 ka and elevations from 0 to 450 m (Fig. 1 and Supplementary dataset; Hendy et al., 1979; Hall et al., 2000; Higgins et al., 2000b;

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Bockheim et al., 2008). Soils were sampled from three major regions of the valley: up-valley in the Lake Bonney watershed, mid-valley in the Lake Hoare watershed, and down-valley in the Lake Fryxell and Ross Sea watersheds. Up-valley soils predominantly are from the 113 to 120 ka aged Bonney till and associated soils (Higgins et al., 2000b). Mid-valley soils dominantly are from the 12.4 to 120 ka aged Bonney, Ross Sea Drift, and undifferentiated tills. And down-valley soils predominantly are representative of the Ross Sea Drift (Ross I), a 12.4 to 23.8 ka aged till deposited by the grounded Ross Sea Ice Sheet (Stuiver et al., 1981; Denton and Marchant, 2000; Hall and Denton, 2000). Samples were collected from dry active layer soils and from visibly wetted water tracks. Although Taylor Valley has experienced a wide range of post-glacial modification processes including valley wall debris flows (Levy et al., 2011), aeolian deposition (Šabacká et al., 2012), soil inflation (Bockheim, 2010), and thermal contraction cracking (Péwé, 1959; Sletten et al., 2003), no evidence for significant, widespread cryoturbation (present or relict, e.g., Schaefer et al., 2016) is present in the valley owing to the extremely low liquid water availability (Campbell, 2003). Although water tracks in other valleys can be routed by nonsorted patterned ground (e.g., Levy et al., 2008), soils in this study were collected from nonfractured, minimally turbated soils representing the majority of Taylor Valley water track surfaces. To ensure this, samples were collected no closer than 3 m to a polygon fracture if present. Soils were stored in Whirl-Pak bags and were oven dried at 105 °C for 24 h before being shipped to the University of Texas, Austin, USA. Nonringed (e.g., 2012–2013) sediments were then homogenized in a riffle splitter to eliminate sorting caused by shipping and pretreatment and were repacked into a stainless steel ring to field density (~1.8 g/cm3, achieved through the application of light, uniform pressure to the sample, as in Levy and Schmidt, 2016). The largest clasts sampled were pebble-sized, and no N 3 cm clasts were included in the experimental sample set. To saturate soils without trapping air, samples were placed on porous plates in a trough that was then filled with deionized water. Soil constant head saturated hydraulic conductivity, ksat, was then measured using a UMS KSAT instrument (UMS, Munich, Germany). The KSAT uses a pressure sensor beneath the soil sample to measure pressure head while water is percolated vertically (perpendicular to cross section) through the fully saturated soil sample at room temperature. The KSAT makes constant head measurements by evaluating Darcy's law: ksat ¼ Q=A  L=H

ð1Þ

where the length L and area A of the soil sample are known, and the constant pressure head difference H and steady-state flow rate Q are determined by linear regression. The device uses the time and the cumulative percolated water volume at selected pressure heads for computation of saturated hydraulic conductivity. Five measurements were conducted on each sample. Average results by valley reach are shown in Table 1, and all sample results are included in the Supplementary material. For three samples, LVWT1, LVOFFA (both down-valley samples), and UVWT3A (an up-valley sample), unsaturated hydraulic conductivity measurements were made using a UMS HYPROP instrument (UMS, Munich, Germany) in order to provide preliminary constraints on changes to hydraulic conductivity within the water track soil column. These samples were measured subsequent to measurements of ksat but before ringed samples were disaggregated for grain size and sorting measurements. After saturated hydraulic conductivity measurements were made, soils were oven dried and dry sieved to determine grain-size distributions. A receiver pan and five sieves with mesh sizes 2000, 1000, 500, 125, and 63 μm, were used with a Ro-Tap Sieve Shaker to obtain distributions. Mesh sizes were chosen to capture variation in the sand-size fraction of soils. Results were analyzed in GRADISTAT to obtain grain-

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Table 1 Average saturated hydraulic conductivity, ksat, grain-size distribution overview, and sorting index for sediments from each study location; uncertainty is reported as one standard deviation by valley reach (standard deviation of average experimental values—note, Beacon Valley has only one sample, so no standard deviation is given). Site

ksat [cm/s]

% fine

% sand

% coarse

Sorting index [σG]

Beacon Valley Up-valley Mid-valley Down-valley Garwood Valley

0.01 0.12 0.04 0.01 0.02

3.3 4.7 ± 2.1 8.0 ± 8.4 21.6 ± 18.5 10.0 ± 9.7

68.0 41.1 49.4 57.2 55.9

28.7 61.7 38.3 26.1 34.1

2.33 2.0 ± 2.3 ± 2.5 ± 3.3 ±

± ± ± ±

0.19 0.10 0.02 0.01

size statistics. Geometrically (metric) calculated Folk and Ward parameters were used for analysis and textural classification (Folk and Ward, 1957; Blott and Pye, 2001). For the purposes of the discussion below, particles b 125 μm (fine sand, silt, clay) are considered ‘fine,’ and particles N 1 mm (coarse sand, gravel, etc.) are considered ‘coarse.’ In order to evaluate how the saturated hydraulic conductivity of soils affects water track flow paths and drainage density, surface slopes were analyzed from a 1 m/pixel airborne LiDAR digital elevation model (DEM; Fountain et al., 2017) using the methods described by McNamara et al. (1999) and implemented in ArcMap 10.2. Glacier-fed stream channels were excluded from this analysis by masking out glacier areas. Closed sinks b1 m in depth in the DEM were filled in order to reduce flow path truncation. Median sink depths in Taylor Valley are 0.1 m, consistent with a surface characterized by trenches and pitting from thermal contraction crack polygons (Levy et al., 2011). Flow direction was computed using a D8 algorithm that identifies the steepest path from each DEM cell into any of its neighboring eight cells. Flow accumulation (watershed area) is computed by summing the area of pixels that flow into each cell. Water tracks in Taylor Valley are spatially associated with topographic flow paths with an area N 0.02 km2 (Levy et al., 2011). Accordingly, in order to estimate water track location, we generated vector flow path polylines for DEM pixels with N 0.02 km2 upslope accumulating area. We then calculated drainage density (km flow path per km2) for the valley in order to determine the spatial distribution of drainage density to compare with slope and saturated hydraulic conductivity.

± ± ± ±

21.1 14.2 17.6 16.3

± ± ± ±

20.1 16.7 17.0 13.4

0.4 0.4 0.6 1.1

3. Results Soils analyzed in this study are moderately to poorly sorted and are comprised predominantly of sand-sized mineral particles (on average ~ 70% by mass) with the remainder primarily gravel-sized fractions and relatively small silt/clay fractions (Fig. 2). Soil sorting values range from 0.8 (very well sorted) to 4.9 (very poorly sorted). The silt/clay fraction (b63 μm) of sampled soils ranges from 0.05% to 16.1%, while the gravel fraction (N2 mm) of soils ranges from 0.03% to 95%. Texturally, soils range from sand to sandy gravel. Saturated hydraulic conductivity values for the soils range from 1.6 × 10− 4 to 6.2 × 10− 1 cm/s and are strongly influenced by physical soil properties (Figs. 3-4). As expected, well-sorted soils have higher measured hydraulic conductivities than poorly sorted soils. Likewise, soils with smaller fine fractions, especially fine sand, exhibit higher measured hydraulic conductivities than soils enriched in fines. The inverse is true for the presence of coarse-sized particles. Illustratively, the sample with the highest saturated hydraulic conductivity, 123012-1, has ksat = 6.2 × 10− 1 cm/s and is also the most well sorted. It contains the lowest amount of fine-sized particles and the greatest amount of coarse-sized particles of any measured soil (Fig. 3). Soil hydraulic conductivities are organized with spatial position in Taylor Valley (Figs. 4–5, Table 1). Up-valley soils generally have higher measured saturated hydraulic conductivity than down-valley soils. The average saturated hydraulic conductivity measured in up-valley,

Fig. 2. Ternary plot of gravel, sand, and mud percentages for all samples (after Folk, 1980). Colors indicate study location within the MDV. TV indicates Taylor Valley.

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Fig. 3. Normalized saturated hydraulic conductivity of each sample as a function of grain-size properties (coarse fraction, fine fraction, and sorting index). Normalization divides each ksat value by the maximum measured ksat value in order to highlight relative differences in sample hydraulic conductivity and to allow clearer plotting. Particles with diameter N 1 mm are considered coarse. Particles with diameter b 0.063 mm are considered fine. Sorting index calculated using GRADISTAT software (modified from Folk and Ward, 1957), with larger indices corresponding to lesser sorting.

Fig. 4. Saturated hydraulic conductivity (measured at left and normalized by dividing by maximum ksat at right). Note: Beacon Valley result is plotted in Fig. 5, but not here, to improve clarity. UVT is Up-Valley Taylor, MVT is Mid-Valley Taylor, DVT is Down-Valley Taylor, and GV is Garwood Valley.

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Fig. 5. Soil properties as a function of longitude. All values normalized to maximum measured value to enhance clarity. Taylor Valley is bounded by ~162° E (up-valley) to ~164°E (downvalley): BV is Beacon Valley, UVT is Up-Valley Taylor, MVT is Mid-Valley Taylor, DVT is Down-Valley Taylor, and GV is Garwood Valley.

mid-valley, and down-valley soils are 0.12 ± 0.19, 0.04 ± 0.10, and 0.01 ± 0.02 cm/s, respectively. While each valley reach has a high internal variability, one-way ANOVA analysis of ksat for the three valley reaches results in between-group variance exceeding within-group variance with a P level of 0.006. Soil physical properties are also organized with spatial position within Taylor Valley (Figs. 4–5). Soils become more poorly sorted with position down-valley. Average sorting values σG calculated for up-valley, mid-valley, and down-valley soils are 2.0 ± 0.4, 2.3 ± 0.4, and 2.5 ± 0.6, respectively. Likewise, the fine fraction of soils tends to increase with position down-valley, while the coarse fraction tends to decrease. Fines constitute 4.7%, 8.0%, and 21.6% of soil mass in average upvalley, mid-valley, and down-valley soils, respectively. Coarse particles constitute 54.2%, 42.6%, and 21.3% of up-valley, mid-valley, and downvalley soils, respectively (Fig. 5). These results are mirrored in in the unsaturated hydraulic conductivity results: soils become abruptly less conductive (increased matric potential) within a change in soil moisture content of ~ 3 volumetric percent (e.g., from saturation at 35 vol% to 34 vol%, matric potential rises approximately tenfold). Fine-grained down-valley soil exhibits a second increase in matric potential (and associated reduction in hydraulic conductivity) at ~17 vol% water; while coarse-grained, up-valley soil has a single log-linear decrease in hydraulic conductivity with decreasing water content (see Supplementary materials).

Environmental end-member Beacon Valley and Garwood Valley soils have mean hydraulic conductivities of ~1 and ~2 × 10−2 cm/s, respectively. The Beacon Valley soil has a sorting value of 2.3, with a fine fraction of 3% and a coarse fraction of 29%. In contrast, Garwood Valley soils have a mean sorting value of 2.8, a fine fraction of 10%, and a coarse fraction of 34% (Figs. 2–3). Comparing ksat values for samples collected on water tracks with those collected in adjacent, off-track soils, we see small differences in the mean saturated hydraulic conductivity for each reach of the valley, with average ksat on water tracks higher than in adjacent, off-track soils. However, these differences are not statistically significant (P = 0.28, 0.46, and 0.41, for up-valley, mid-valley, and down-valley samples, respectively, likely owing to high variability in on- and off-track soils crossed by water tracks; Fig. 1). Saturated hydraulic conductivity is variable with depth for on- and off-track soils (Fig. 6). The down-valley water track decreases in ksat with increasing depth, while the up-valley profile shows the opposite trend. Qualitatively, water track drainage density correlates well with saturated hydraulic conductivity. Calculated drainage density is plotted in Fig. 7 and reaches as high as ~20 km/km2. Drainage density is highest up-valley and generally decreases in the down-valley direction. Drainage density is higher on south-facing slopes than on north-facing slopes. While valley walls are steeper up-valley than down-valley (Fig. 7), local

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2012a; Levy et al., 2011). Measured values for ksat are bracketed by active layer saturated hydraulic conductivity values from the arctic, which vary from ~ 1.0–1.9 × 10−2 cm/s for near-surface, organic-rich horizons (McNamara et al., 1999) to ~ 0.9–1.4 × 10−3 cm/s in deeper mineral horizons (Hinzman et al., 1991). Interestingly, they are also bracketed by extensively cryoturbation-sorted sediments such as those described by Hodgson and Young (2001), which vary from 1.2 × 10−4 to 1.2 × 100 cm/s over meter length scales in frost mounds. As expanded upon in the following sections, we interpret these MDV values to reflect a largely inherited ksat signature that has been minimally modified by subsequent geological processes since the soils were emplaced, providing a rare opportunity to examine the depositional properties of cold region soils. Differences in ksat for soils collected in 2012–2013 versus those collected in 2015–2016 are small, but not negligible (Figs. 4–5), reflecting potential disturbances to soil flow properties resulting from grab sampling and disturbance. Ringed samples span a subset of the range of saturated hydraulic conductivity measured in each valley reach, commonly overlapping the middle of the grab sample range, although 2015–2016 samples generally have a lower measured ksat than grab samples collected in nearby water tracks. Notably, however, ringed samples (2015– 2016) were collected from different water track locations than grab samples, ringed samples include soil samples from depth (which, in the down-valley samples, means lower ksat values than surface samples), and there are fewer ringed samples than grab samples. Importantly, the key trends identified in the results section, high ksat upvalley with lower ksat values in the mid-valley and lower valley, coupled with no clear on/off-track differences, are reproduced by the 2015–2016 samples when considered separately from the grab samples. 4.1. Geographic controls on k

Fig. 6. Saturated hydraulic conductivity vs. depth at three Taylor Valley sites.

maxima in drainage density are found in areas with steep slopes and with high saturated hydraulic conductivity. 4. Discussion Our measurements suggest that the mineral soils of Taylor Valley have markedly different hydraulic conductivities and grain-size distributions based on their position in the valley. We interpret these spatial differences in soil properties to be caused by a geologic legacy from past glaciations (hypothesis 1). Here we discuss the relationship between sediment properties and the glacial history and environmental setting of Taylor Valley and interpret how geologic legacy shapes past and present landscape changes in the MDV. Future investigations of saturated and unsaturated hydraulic conductivity are clearly motivated by this reconnaissance data set. However, the sharp dropoffs in hydraulic conductivity at low water contents (see Supplement) coupled with the abrupt reductions in soil moisture content above the saturated horizon in water tracks (e.g., Levy et al., 2011) suggest that generalizations about groundwater flow through MDV water tracks under saturated conditions may capture most of the water moving through these soils (in contrast to unsaturated conditions that dominate groundwater flow in the arctic; Hinzman et al., 1991). With an overall MDV mean of ~4 × 10−2 cm/s, these detailed measurements of ksat are broadly consistent with ksat values measured via remote sensing in the MDV, e.g., 2 × 10− 2 to ~ 1 × 10− 1 cm/s (Levy,

The soils measured in this study were predominantly sampled from three major regions of Taylor Valley—up-valley, mid-valley, and downvalley—each representing a separate watershed and containing a particular suite of soils. Up-valley soils are comprised of Bonney till and associated soils, a glacial drift sheet deposited by the penultimate advance of Taylor Glacier (Higgins et al., 2000a,b; Bockheim et al., 2008). Up-valley soils are the highest elevation and the oldest soils studied and are most sedimentologically consistent with the talus deposits, glaciofluvial group sediments, glaciolacustrine, and windblown deposits described by Higgins et al. (2000a) as Sediment Group 4 (except UVWTC samples that were collected closer to the valley floor and that have the highest fines content in the up-valley group, although not as high as would be expected in Bonney meltout till, consistent with sample location). Notably, we did not sample any mud/silt-dominated soils in the Bonney basin floor, which would be expected to have high fines contents and concomitant low ksat. Instead, the up-valley group soils exhibit the best sorting and the highest hydraulic conductivities and contain the smallest fraction of fines. Mid-valley soils are comprised of Bonney till and undifferentiated soils comprised of mixed glacial deposits (Hall et al., 2000; Higgins et al., 2000a; Bockheim et al., 2008). Mid-valley Bonney soils exhibit high saturated hydraulic conductivity, while mid-valley undifferentiated soils have lower hydraulic conductivities. Down-valley soils are comprised of Ross Sea Drift and associated soils, a marine drift sheet deposited by the grounded Ross Sea Ice Sheet in the late Pleistocene (Stuiver et al., 1981; Hall and Denton, 2000). Down-valley soils are the lowest elevation and the youngest soils measured. Of the soils measured, down-valley soils exhibit the poorest sorting, the lowest hydraulic conductivities, and contain the highest fraction of fines (P b 0.001 for these properties intercomparisons via one-way ANOVA). Our results suggest that spatial variability in the saturated hydraulic conductivity of soils in Taylor Valley is largely determined by soil origin

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Fig. 7. Water track flow path density determined from DEM analysis. Panels A–C show computed water track flow paths (white lines) for the Lake Bonney (up-valley), Lakes Hoare and Fryxell (mid-valley), and Ross Sea (down-valley) watersheds. Water track line density (km/km2) is plotted in color. Panel D shows overall Taylor Valley water track drainage density plotted with 100-m elevation contours (white lines). Water track drainage density decreases down-valley, although locally high densities can be found on steep and low-angle slopes.

and post-depositional processing. Differences in soil properties, derived from differences in soil source and modulated by age and groundwater flow, are the dominant control on the variability in active layer hydraulic properties. Together, these variables set the weathering environment and initial conditions for MDV soils, although we did not explicitly test for the effects of differences in pedogenic properties such as clay mineralogy, cohesion, or the presence of soil aggregates. We interpret the spatial distribution of saturated hydraulic conductivity values to reflect deposition of the Bonney soils as a relatively finedepleted, hydraulically conductive drape extending from present Taylor Glacier to Canada Glacier during eastward expansion of the Taylor Glacier. In contrast, Pleistocene expansion of the Ross Sea Ice sheet into Taylor Valley (~18,700 to 12,800 YBP; Hall et al., 2015) resulted in the emplacement of drift and morainal soils with abundant fines and low saturated hydraulic conductivity in a more poorly sorted deposit that fills the mouth of the valley. Today, the differences in soil properties resulting from sedimentological differences between terrestrial and marine till deposits (high ksat, vs. lower ksat) are retained in the modern up-valley and down-valley soil columns. Notable differences exist between the canonical sedimentology of Bonney and Ross Sea Drift tills (Higgins et al., 2000a; Hall et al., 2000)—in particular, most sediments sampled in this study are finespoor compared to muddy drifts common to valley bottom sectors of Taylor Valley. We interpret the differences between the sediment properties of the shallow active layer (sampled in this study) and canonical values of till sedimentology (sampled in deep pits in the dry active layer between hydrological features; Higgins et al., 2000a) to reflect changes to the upper few centimeters to 10s of centimeters of the soil column since emplacement of the tills. For example, many water tracks occupy relict stream channels that may have been partially winnowed of fines during high discharge periods (Levy, 2015). Despite the potential for post-emplacement modification of MDV sediments, what is notable is the valley-scale relationship between present day soil hydrological properties and the original paleo-properties of tills, soils, and other deposits. We interpret this to result from the stability of the MDV landscape in which chemical and physical weathering rates are extremely low (Campbell and Claridge, 1987).

The limited availability of water in the MDV limits physical, chemical, and biotic disturbance. As a result, the properties of ancient soil deposits may largely shape the hydraulic properties of modern MDV soils. However, differences in slope and microclimate conditions between the landscape environments of up-valley, mid-valley, and down-valley soils may also have combined to enhance the relative differences between the original soil properties over time. The up-valley soil column was deposited as a relatively hydraulically conductive medium in a steeply sloped part of the valley. Through time, initially high ksat may have combined with high hydraulic gradient to enhance saturated hydraulic conductivity of up-valley soils to create a net-erosional drainage network in which water is able to percolate down steep slopes rapidly, efficiently flushing fines from water tracks and increasing sorting. Further, because these soils are in the intermediate mixing zone (IMZ) microclimate where greater evaporation and sublimation maintains low soil-moisture (Marchant and Head, 2007), high wind speeds may have further enhanced aeolian removal of fines. In contrast, down-valley, fines-rich, low ksat, glaciomarine tills deposited over flat terrain may have experienced feedbacks working to maintain their low saturated hydraulic conductivity by trapping fines in poorly sorted, shallow, low-discharge water tracks. Longer water residence times in low ksat soils, combined with the warmer and wetter conditions associated with the coastal thaw zone (CTZ; Marchant and Head, 2007) in eastern Taylor Valley may also have enhanced chemical weathering and fines production. It is possible that more weathering products are produced down-valley where these fine-sized sediments can then be transported downslope by water tracks, solifluction, and aeolian processes. Alternatively, fines in down-valley soils may be at least partially exogenous, having entered the soil column through the inflation of desert pavements (Bockheim, 2010). Although the fine-grained vesicular horizon is hypothesized to grow in thickness with age, we note that the down-valley sediments are from the youngest surfaces studied in this investigation. This is consistent with a lack of a statistically significant change in fine-grained vesicular layer horizon thickness with age in Taylor Valley (Bockheim, 2010). Instead, we infer that any exogenous additions of fine material to the near-surface in the lower reaches of Taylor Valley result from the local wind regime, which loses

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velocity and sheds sediment in the mouth of the valley (Šabacká et al., 2012). 4.2. End member comparisons To further investigate how climate setting and age modifies active layer soil properties in the MDV, we compared soils from Beacon Valley and from the mouth of Garwood Valley to those from the Taylor Valley system. Beacon Valley soil is composed of Miocene-aged glacial till and is broadly similar in history to up-valley tills in eastern Taylor Valley (e.g., deposition by cold-based and/or polythermal glaciers). The sample we examined is less fines-rich than canonical descriptions of the Granite Drift (Marchant et al., 2002) and may reflect aeolian reworking (winnowing of silts, emplacement of sands) in the shallow soil column. Its sedimentological similarity to up-valley samples makes it reasonable to expect the hydraulic and physical soil properties of Beacon Valley till would be similar to those measured for up-valley soils (i.e., high ksat, moderately sorted, high coarse fraction, low fine fraction). However, the Beacon Valley soil has a significantly lower saturated hydraulic conductivity than up-valley soils, and while the Beacon Valley soil contains a low fines fraction similar to the up-valley soils, it does not contain the high coarse fraction measured in most up-valley soils. What accounts for this discrepancy in hydraulic and textural soil properties between the similarly old glacial tills? It may be that the upland frozen zone microclimate regime that characterizes Beacon Valley has allowed for less fluvial processing of soils than for those in the intermediate thaw zone in upper Taylor Valley. In addition, drainage winds may have worked to remove fines in Beacon Valley over million-year timescales, but without seasonal thawing to create runoff, the coarsest clasts have not been mobilized downslope by colluviation to increase soil permeability. The resulting soil column is composed of poorly sorted, gravelly sand with lower saturated hydraulic conductivity than the more fluvially reworked or groundwater-processed tills in the intermediate mixed zone of the upper Taylor Valley. In contrast, the soils filling the mouth of Garwood Valley represent almost fully-intact marine Ross Ice Sheet till deposited in the warm coastal thaw zone. Garwood Valley soils are similar to down-valley soils in Taylor Valley in terms of glacial origin, age, and microclimate history. They would be expected to have correspondingly low hydraulic conductivities and poor sorting. Indeed, Garwood Valley soils have low ksat and high σG values consistent with those measured for down-valley soils and consistent with their glaciomarine geologic history. 4.3. Hydrological controls on k Our analysis suggests that at a regional scale, position in the hydraulic system (on water tracks versus off—hypothesis 2) is not the most important control on soil properties. Indeed, when analyzed by soil reach, differences between on- and off-track saturated hydraulic conductivities are not statistically different. This likely results from the large variance within each valley reach and the small sample size when samples are further subdivided by hydraulic position. Intriguingly, the difference between mean on-track vs. off-track soil properties is greater for up-valley soils than for down-valley soils. This may be an effect of the historically higher ksat and stronger hydraulic gradient of up-valley water tracks, which allows for more effective fluvial fines removal and shorter water/rock reaction times, causing less weathering and fines production. Because water track discharge increases with depth to the icetable, it is also possible that most fluvial modification of soil properties occurs beneath the upper 10 cm of the soil column and is not captured by this study. Detailed cross sections of water track soils and adjacent off-track soils are needed to completely evaluate this hypothesis; however, our reconnaissance sampling suggests that while potentially a factor in controlling soil ksat, hydraulic position is not the dominant factor. Likewise, because mostly massive, minimally structured sediments were

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sampled, this study does not consider effects of anisotropic flow through MDV soils (e.g., longitudinal flow in water tracks, vertical wicking through water tracks, and vertical infiltration in water track/seep systems), which may augment or retard shallow groundwater flow in some locations (e.g., where bedding dominates saturated hydraulic conductivity). Intriguingly, our pilot investigations of ksat vs. depth do not show broadly generalizable patterns for either on- or off-track soils. While on-track ksat increases with depth for the down-valley profile, the opposite is true for the up-valley profile (and little to no variability is observed with depth for the mid-valley profile (Fig. 6)). Off-track depth profiles show a similar lack of clear patterns. These depth profiles show a similar ~1 order of magnitude variability to the ksat variability observed within each valley reach, suggesting that location-to-location differences in soil properties produce highly localized flow conditions, even in the context of broad patterns of decreasing saturated hydraulic conductivity from up-valley to down-valley. 4.4. Geomorphic, hydrological, and ecological implications How do the measured differences in soil hydraulic properties affect the Taylor Valley landscape? Surficial geology strongly affects hillslope drainage networks (Perron et al., 2008). In Taylor Valley, water track morphology varies spatially. Up-valley hillslopes are characterized by densely spaced, parallel, linear water tracks; while hillslopes down-valley are characterized by water tracks that are sparsely spaced, dendritic, and sinuous (Fig. 7). We suggest that the observed spatial variability between up-valley and down-valley water track spacing arises at least in part from differences in saturated hydraulic conductivity between upvalley and down-valley soils. This is because water track spacing as measured via DEM is responsive to incision (advective erosion), where greater incision leads to closer spacing. Incision is determined by discharge, which is proportional to saturated hydraulic conductivity, gradient, and meltwater availability. Thus, the higher ksat up-valley soils allow for greater advective incision than lower ksat down-valley soils, leading to closer, more parallel water track spacing up-valley, even on equally sloped surfaces (where present). In contrast, deeper active layer thaw in down-valley soils resulting from longer wetted seasons and elevated thermal conductivity (Levy and Schmidt, 2016) may increase soil diffusion rates in down-valley hillslopes (they are muddier and more regionally wetted), resulting in the formation of sinuous and branching water tracks. These spatial differences in soil ksat may also affect how water moves through water tracks in different parts of the valley—a hydrological process that has geochemical and biological implications. High ksat soils on steep up-valley slopes can move active layer solutions more rapidly downslope, resulting in short transit times and rapid drainage of the active layer when summer melting ceases. This may, in part, explain the presence of double-tailed water tracks in the Lake Bonney basin (as well as other regions in the MDV, see Supplementary materials)—wetting features that consist of a down-slope region of darkened soil linked to two bands of soil darkening that grow narrower upslope (Levy, 2012a). Such water tracks may represent pulses of hydrological activity in which a slug of meltwater propagates downslope through the high-ksat soils typical of up-valley environments, leaving behind a narrow wetted trail along the margins. In contrast to pulsed flow associated with high ksat in up-valley soils, slow percolation of water track fluids through mid-valley and downvalley soils may cause a longer annual wetting period. Geochemically, this suggests that water tracks in mid-valley and down-valley soils may have longer hydroperiods in which evaporation can occur, resulting in enhanced evaporative fractionation of water track solutions (see Supplementary materials). Long-term wetting in low ksat water tracks may also help explain spatial patterns of biological activity in the MDV, where water availability is the limiting factor for biological productivity (Kennedy, 1993).

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Longer persistence of wetted soil conditions resulting from slow drainage of the active layer are consistent with lower bacterial biovolume in up-valley water tracks than in down-valley water tracks (Ball and Levy, 2015) and with decreasing stocks and turnover rates of organic carbon in MDV soils with distance up-valley (Burkins et al., 2000, 2001; Levy et al., 2013a). This study suggests that saturated hydraulic conductivity, and the role it plays in changing the rates of discharge through water tracks, may act as an additional overlay on valley-scale climate processes (e.g., cooling inland and with increasing elevation) that shape the distribution and functioning of terrestrial biogeochemical processes in Antarctic active layers. 5. Conclusions We used MDV soils to test two hypotheses posed to address the question: does parent material or hydrological/pedogenic history matter more for determining active layer hydraulic properties. Broadly, the physical and hydraulic properties of active layer soils through which the studied water tracks flow vary spatially as a function of distance inland away from the Ross Sea—although moraines, patchy drift sheets, deltaic deposits, etc. make for a complex overall sedimentary environment. Based on this reconnaissance survey, coastal soils are generally more poorly sorted, consist of a higher fraction of fine particles, and a lower fraction of coarse particles than soils farther inland. Consistently, measured saturated hydraulic conductivity of soils increases with distance inland as soils become older, coarser, and less clogged with fines. This trend is consistent across disturbed (grab sample) and intact (ring sample) measurements, although future measurement campaigns should focus on analysis of minimally disturbed samples. The hydrogeologic gradient is interpreted to exist primarily as the result of the spatial distribution of glacial, glaciomarine, and glaciolacustrine-sourced sediments associated with past environmental conditions. Older, higher elevation, more inland, fines-depleted soils deposited as a hydraulically conductive medium persist today as a hydraulically conductive unit, having been further sorted by enhanced fluvial and groundwater activity over ~ 100 ka timescales. Meanwhile, fines have been retained or accumulated in the low-conductivity soils formed by younger, lower, fines-enriched, glaciomarine-sourced soils located near the coast. Together, this suggests that in this environment, geological history exerts a stronger influence on the saturated hydraulic conductivity of soils than position in the hydrological system (e.g., on a water track flow path vs. off the water track flow path). Accordingly, we find that the ancient distribution of soils of different sedimentology and age, possibly modified by differences in environmental regime, determines the modern distribution of soil properties in Taylor Valley, and together serve as a critical control on landscape hydrological and biogeochemical processes throughout the MDV. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.geomorph.2017.01.038. Acknowledgements This work was supported by NSF Antarctic Integrated Systems Science award PLR-1245749 to JSL. Thanks to six anonymous reviewers for their constructive reviews. Thanks also to Jaclyn Watters and Steve Chignell for field assistance. References Ball, B.A., Levy, J.S., 2015. The role of water tracks in altering biotic and abiotic soil properties and processes in a polar desert in Antarctica. J. Geophys. Res. Biogeosci. 120: 270–279. http://dx.doi.org/10.1002/(ISSN)2169-8961. Ball, B.A., Virginia, R.A., 2012. Meltwater seep patches increase heterogeneity of soil geochemistry and therefore habitat suitability. Geoderma 189–190:652–660. http://dx. doi.org/10.1016/j.geoderma.2012.06.028. Benson, C.H., Othman, M.A., 1993. Hydraulic conductivity of compacted clay frozen and thawed in situ. J. Geotech. Eng. 119, 276–294.

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