Rock glaciers in Pearse Valley, Antarctica record outlet and alpine glacier advance from MIS 5 through the Holocene

Geomorphology 336 (2019) 40–51

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Rock glaciers in Pearse Valley, Antarctica record outlet and alpine glacier advance from MIS 5 through the Holocene Kate M. Swanger a,⁎, Esther Babcock b, Kelsey Winsor a,c, Rachel D. Valletta d a

Department of Environmental, Earth and Atmospheric Sciences, University of Massachusetts, Lowell, MA 01854, United States of America Logic Geophysics and Analytics, LLC, Anchorage, AK, United States of America School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011, United States of America d Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104, United States of America b c

a r t i c l e

i n f o

Article history: Received 13 November 2018 Received in revised form 15 March 2019 Accepted 15 March 2019 Available online 18 March 2019 Keywords: McMurdo Dry Valleys Stable isotopes Ground-penetrating radar Permafrost

a b s t r a c t Rock glaciers and buried ice are common in the McMurdo Dry Valleys, Antarctica. In central Taylor and Pearse valleys rock glaciers cover at least 10% of the valley walls and occur at elevations of 300–800 m above sea level. We investigate the origin and geomorphology of a ~1.5 km2 rock glacier in northern Pearse Valley, the westernmost extension of Taylor Valley. The rock glacier is cored by sporadic deposits of clean ice that are covered by sand-rich, stratified sediments and dissected by five glacial meltwater streams. Ground-penetrating radar data indicate that the clean buried ice ranges from 1 to 14 m thick and contains dipping sediment-rich bands. Water stable isotopes from five cores extracted from the buried ice support multiple ice sources: (1) recent ice from alpine glaciers and (2) ancient, stagnant ice from East Antarctic outlet Taylor Glacier. The eastern half of the rock glacier lies directly downslope from alpine Fountain Glacier, which is actively feeding ice, sediments and water to the rock glacier via ~1 km-long ice falls. The buried ice in this section of the rock glacier is relatively heavy isotopically and similar to Fountain Glacier (δ18O of −36‰ to −32‰). The ice that cores the western half of the rock glacier is isotopically light and similar to Taylor Glacier (δ18O of −43‰ to −40‰). The last documented advance of Taylor Glacier that was sufficient to reach the rock glacier position occurred during Marine Isotope Stage 5 (70–125 ka), implying long-term preservation of the ice. The age and origin of buried ice in Pearse Valley has implications for rock glaciers throughout the Antarctic. Rock glaciers (1) are potentially long-term archives of glacial ice with complex depositional histories and (2) could be used to map previous advances of both outlet and alpine glaciers. © 2019 Published by Elsevier B.V.

1. Introduction The McMurdo Dry Valleys are the largest unglaciated region in Antarctica, yet they contain significant ground ice, in the form of interstitial ice in sediments as well as massive clean ice. This buried ice often cores lobate flow features that have been described as debris-covered glaciers, rock glaciers and gelifluction lobes (Hassinger and Mayewski, 1983; Marchant and Head III, 2007; Swanger et al., 2010; Bockheim, 2014). These varying labels are important because they indicate formation under a range of climatologic, hydrologic, and glaciologic conditions, and therefore denote very different geomorphic histories. For example, gelifluction lobes generally contain thin ice lenses emplaced as groundwater freezes (Matsuoka, 2001), whereas debris-covered glaciers are active alpine glaciers with till-covered ablation zones (Marchant et al., 2002). The term rock glacier was first used by Capps Jr (1910) to describe flowing ice-rich talus in Alaska. More recently researchers have expanded ⁎ Corresponding author. E-mail address: [email protected] (K.M. Swanger).

https://doi.org/10.1016/j.geomorph.2019.03.019 0169-555X/© 2019 Published by Elsevier B.V.

the term rock glacier to include all flowing mixtures of ice and sediment, with two end-member origins: 1) glacigenic, which are cored by clean ice of glacial origin and 2) permafrost, composed of colluvium with sufficient interstitial ice to flow (Hamilton and Whalley, 1995; Clark et al., 1998; Humlum, 2000). Debris-covered glacial ice occurs throughout the Dry Valleys, most notably near the coast and in the Quartermain Range (Fig. 1). In coastal Taylor and Garwood valleys, there are N10-m thick lenses of glacial ice, deposited when the ice sheets expanded northward in the Ross Sea during the Last Glacial Maximum (LGM) (Hall and Denton, 2000; Pollard et al., 2002; Levy et al., 2013a). Given its location in the warmest region of the Dry Valleys, this buried ice is actively melting (Levy et al., 2013b, 2018). Buried glacier ice in the high-elevation interior of the Dry Valleys, such as Beacon Valley, is comparatively stable, experiencing limited sublimation and melting. At this location, buried ice may have been preserved for N1 million years (Schaefer et al., 2000; Yau et al., 2015). In this paper, we discuss buried ice and rock glaciers in Pearse Valley, which is located geographically and climatologically between the two end-members discussed above (Garwood and Beacon valleys). The

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Fig. 1. Location map of Pearse Valley in the central McMurdo Dry Valleys. The eastern valley mouth is filled by a lobe of Taylor Glacier and the northern valley wall is home to cold-based alpine glaciers that flow from the Asgard Range. TV = Turnabout Valley and SV = Simmons Valley. Left lower inset: Topographic map of Pearse Valley showing four alpine glaciers, Schlatter (SG), Nylen (NG), Fountain (FG) and Catspaw (CG) and two lakes, House (LH) and Joyce (LJ). Flowing ice-rich sediments are common along the base of Pearse Valley walls, labeled according to location, northern (NP), western (WP), and southern Pearse (SP) rock glaciers. Two thick dashed lines indicate the transects for the MIS 5 advance of Taylor Glacier into central and northern Pearse Valley, shown in Fig. 9. Lower and upper right insets: Dry Valley location in Antarctica and McMurdo Sound. MDV = McMurdo Dry Valleys, GV = Garwood Valley. Public domain Landsat7 imagery courtesy of NASA Goddard Space Flight Center and U.S. Geological Survey.

age, origin and modern stability of buried ice in Pearse Valley is poorly understood. A major goal of this study was to determine if the Pearse Valley rock glaciers formed via glacigenic processes, permafrost processes, or a mixture of the two. The origin and age of these rock glaciers could provide insights into the climatic conditions that foster long-term preservation of buried ice, as well as constraining the extent and timing of glacial advances and/or permafrost development.

2. Background and setting

extension of Taylor Valley; however, Taylor Glacier currently acts as a geomorphic boundary between the two. The Dry Valleys are presently a cold polar desert, with mean annual atmospheric temperatures (MAAT) ranging from −17 to −35 °C (Doran et al., 2002). All modern precipitation comes in the form of snowfall, with rates of 20–50 mm/yr (water equivalent) in Pearse Valley, and 100–200 mm/yr in the adjacent Asgard Range, the source region for the local alpine glaciers (Fountain et al., 2010). Mean summer air temperatures (December through February) in Pearse Valley are −2 to −5 °C, and daily soil surface temperatures in Pearse Valley can exceed 0 °C for N30 days per year (Doran et al., 2002, Marchant and Head, 2007).

2.1. Geomorphology and climate of the Dry Valleys 2.2. Buried ice distribution in the Dry Valleys The McMurdo Dry Valleys, located between 77 and 78°S and 160–164°E, are in the Transantarctic Mountains and comprise the largest ice-free region on the continent at ~4500 km2 (Levy, 2013). Consisting of three main east-west trending valleys (Victoria, Wright and Taylor) separated by ~1500-m elevation mountain ranges, the Dry Valleys are home to one of the longest terrestrial records of climate in Antarctica (Lewis et al., 2008). The southernmost of these valleys, Taylor Valley, contains the largest number of glaciers and lakes, with dozens of alpine glaciers flowing north and south down the valley walls. Additionally, Taylor Glacier, an outlet from the East Antarctic Ice Sheet (EAIS) fills the western half of Taylor Valley, terminating ~35 km from the McMurdo Sound (Fig. 1). Geologically, Pearse Valley is the western

Subsurface ice is common throughout the Dry Valleys, occurring as interstitial ice (in pore spaces) and clean buried ice (cm- to m-scale horizontal ice lenses). The depth/duration of summer thaw and the proximity to moisture sources control both the depth to the ice table and the gravimetric water content of interstitial ice (McKay, 2009), which is more common than clean ice. Buried ice and rock glaciers are concentrated in a few areas: (1) the coastal regions, such as Garwood Valley, (2) central Taylor, Pearse and Wright valleys, and (3) the Quartermain Range (Fig. 1) (Hassinger and Mayewski, 1983; Marchant et al., 2002; Pollard et al., 2002; Levy et al., 2013b; Swanger, 2017). In central Taylor and Pearse valleys, rock

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glaciers cover at least 10% of the valley walls and are generally located at elevations of 300–800 m above sea level (m asl). Despite their ubiquity, the age and origin of the rock glaciers in Taylor and Pearse valleys is poorly constrained relative to those in the Quartermain Range (Marchant et al., 2002; Swanger, 2017) and the coastal valley regions (Hall et al., 2000; Hendy et al., 2000).

These fans cover an area of 0.2 km2. The water in the ephemeral meltwater streams eventually flows into two unnamed ponds (b0.04 km2 of combined surface area and b3 m deep), dammed by a ~30 m bedrock high to the south. These ponds, in turn, drain to Lake Joyce to the southeast. 3. Methods

2.3. Geomorphic setting of Pearse Valley and rock glaciers 3.1. Sedimentology and stratigraphy Pearse Valley (77°43′ S, 161°30′ E) is a 36 km2 ice-free region, bounded to the west and east by Taylor Glacier, to the north by the Asgard Range and to the south by the Friis Hills. As Taylor Glacier flows past the Friis Hills and then east to flow into Taylor Valley, a lobe of the glacier flows northwest for ~3 km into Pearse Valley. This lobe presently terminates at the valley mouth at an elevation of ~300 m asl, damming Lake Joyce (Fig. 1). Ice-rich lobate rock glaciers occur extensively along the base of the Asgard Range and the Friis Hills. We label rock glaciers by location: northern, western and southern Pearse (Fig. 1). The focus of this investigation is the northern Pearse rock glacier, which flows from the bedrock cliffs of the Asgard Range southward onto the floor of Pearse Valley. The northern Pearse rock glacier is a piedmont-type, covering 1.5 km2, with a width of ~1.7 km and a maximum length of ~0.8 km. It flows from an elevation of 700 m asl and terminates at 450–500 m asl. Due north of the northern Pearse rock glacier and directly above the bedrock cliffs, the cold-based Fountain Glacier flows for ~7 km, terminating at 1150 m asl. The glacier terminates along the bedrock cliffs above the northern Pearse rock glacier, and in one section feeds a ~1km long ice fall that cascades down the bedrock gullies (Figs. 2 and 3a). Five main meltwater streams flow from Fountain Glacier, down the bedrock cliffs and then along the surface of the rock glacier. In many locations, these streams are actively incising the rock glacier, cutting channels up to 5 m deep, and then depositing sediments in a series of five fans directly in front of the rock glacier terminus (Figs. 2 and 3a).

During four field seasons between 2004 and 2015, 17 pits were excavated in/near the northern Pearse rock glacier for sedimentological analyses (Fig. 2). Eleven pits were excavated into the rock glacier surface, two were excavated directly north of the rock glacier into mixed talus and fluvial sediments below the bedrock slope, and four were excavated into the fluvially-deposited fans due south of the rock glacier (Fig. 2). Pits were dug by hand and in all cases limited to b70 cm depth. Stratigraphic variations were visually documented in the field and where distinct bedding occurred each bed was gathered as a separate sample (pits KSE-06-21 and 15-03). The samples were stored in Whirlpak bags for grain size analyses. Silt and clay (mud) fractions were calculated by measuring the dry weight of the entire sample, then removing the mud fraction through wet sieving with the +4-phi mesh, and re-weighing the sample after drying it at 105 °C for 24 h. For the KSE-06 samples, the remaining gravel and sand fraction was dry sieved for N–4 phi and then at 0.5-phi intervals from −2.5 to +4 phi (at Boston University). All other samples were dry sieved for N−4 phi and then at 1.0-phi intervals from −2 to +4 (at University of Massachusetts Lowell). Statistics of grain size distributions were conducted using graphical methods and equations defined by Folk and Ward (1957): (1) graphic mean, (2) inclusive graphic standard deviation (sorting), and (3) inclusive graphic skewness. Median grain size corresponds to the 50th percentile on the cumulative curve. The level of sediment sorting

Fig. 2. Locations of ice cores, ice samples, sediment samples, and ground-penetrating radar scans from northern Pearse rock glacier. Fountain Glacier is directly north of this image and is feeding the ice apron in the upper right. Public domain aerial photograph from U.S. Geological Survey, TMA2480-V0118 taken in 1983, courtesy of the Polar Geospatial Center.

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Fig. 3. (a) Northern Pearse rock glacier with sediment fans and two small ponds distal to the rock glacier terminus. (b) Rock glacier surface with contraction cracks near sample KSE-14-01. (c) Excavation KSE-06-23 (SIC-05): poorly stratified, poorly sorted sands. (d) Excavation DVX-14-02: stratified, moderately-sorted sands. (e) Excavation KSE-15-03: 0–25 cm is poorly sorted and unstratified and 25–40 cm depth is stratified sands dipping north. All images courtesy of K.M. Swanger, reprinted with permission.

(as defined by labels such as poorly-sorted, moderately-sorted, etc.) are based on definitions in Folk and Ward (1957). 3.2. Ground-penetrating radar We imaged the rock and ice stratigraphy below the ground surface via ground-penetrating radar (GPR). We collected GPR data from four transects on the northern Pearse rock glacier using a Geophysical Survey Systems, Inc. (GSSI) commercially available GPR system. Three transects were roughly parallel to rock glacier flow direction (N-S), whilst the fourth was perpendicular to rock glacier flow direction (E-W). The EW transect crossed one of the N-S transects (Fig. 2). GPR locations were chosen to coincide with ice core locations. We present two profiles here, associated with ice cores PVI-01 and PVI-02 (see Section 3.3). Three different frequencies of antennas were used: (1) 400MegaHertz (MHz), (2) 200-MHz, and (3) 100-MHz. All three antennas were shielded. The higher frequency antennas are bistatic in a common housing, but the 100-MHz antenna is a monostatic system which

combines the transmitter and receiver electronics in a single housing. Maximum depth of penetration was approximately 10–15 m. Here we present data from only the 200-MHz and the 400-MHz antennas. Table 1 provides settings for the acquisition.

Table 1 Settings for GPR data acquisition. Antenna Central Frequency

200-MHz

400-MHz

Antenna configuration

Single Housing unit 2048 4 350–700 ns 0.05 m –20 dB

Single Housing unit 4096 4 150–500 ns 0.1 m –20 dB

Samples per trace Stacking Time Window Trace Spacinga Trace Gainb a b

Governed by calibrated system odometer. Set to minimum allowable in GSSI system.

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Due to excessive surface roughness, we removed surface clasts greater than ~0.1 m3 before data acquisition, maintaining a straight line as much as possible (the largest boulders could sometimes not be moved). Smoothing the surface in this way before acquisition enhanced coupling between the antennas and the ground, thereby increasing signal penetration. The smooth surface also reduced the need for smallscale topographic corrections to recover the true geometry of subsurface reflectors. After acquisition, we surveyed the line start and end points with a handheld GPS receiver and recorded surface topography using a Brunton pocket transit to provide data for static corrections during subsequent GPR data processing. All disturbed clasts were placed back in their original locations after data acquisition. Initial data processing consisted of a standard GPR workflow in ReflexW (Sandmeier, 2008): (1) dewow to remove low-frequency signal content that is an unavoidable consequence of GPR electronics, (2) time-zero correction to move the first break to zero time, and (3) background average subtraction to remove the direct arrival while preserving other reflectors. Additional steps included the following: (1) appropriate bandpass filters depending on frequency, (2) velocity analyses using diffraction hyperbola in the data, (3) topographic correction, and (4) topographic migration using the velocity derived in step 3. Finally, we applied gain for visualization purposes. 3.3. Ice coring and ice sampling Five ice cores were extracted from buried ice using a SIPRE auger with a core diameter of 7.6 cm (3 in.). At all coring locations, we excavated through a 30–50 cm thick surface sediment layer to expose the contact between dry sediments and clean buried ice (Figs. 2 and 4). The five ice cores from west to east were labeled: SIC-05 (extracted in 2006), PVI-01, -03A, \\03B, and -02 (all extracted in 2015). The depth of coring was limited by bands of sediment-rich ice. Maximum depth (from the top of the ice surface) of core recovery was: 225 cm (SIC05), 80 cm (PVI-01), 20 cm (PVI-03A), 125 cm (PVI-03B), and 145 cm (PVI-02). In addition to the five ice cores, fifteen hand samples

(b500 g each) were gathered from the upper ~5 cm of the buried ice, from ice veins and from ice-cemented sediments. Hand samples refers to ice gathered with a pick rather than with a core auger. Hand samples were packed in sterile Whirlpak bags. Ice samples (labeled PVI-04) were also gathered from the ice apron of Fountain Glacier (Figs. 2 and 3a). All samples were kept frozen in the field (air temperatures ranged from −20 to −5 °C) before being transported to McMurdo Station, after which they were kept at below −18 °C. 3.4. Stable isotopic sampling and analyses Ice cores were cut, photographed and catalogued at National Ice Core Laboratory (if sediment-free) and University of Massachusetts Lowell. All ice cores were cut vertically, with one half preserved as an archive. The other half was then sliced into targeted 1 cm thick horizontal sections and the outer 5 mm of the core surface was removed. The remaining inner core ice was thawed in a sealed, sterile container and then transferred to a 4 ml HDPE bottle that was sealed with thread tape and stored in a −20 °C freezer until analyzed. Before bottling the water samples, HDPE bottles were rinsed in a 10% nitric acid bath for at least 24 h and then triple-washed with deionized water and allowed to dry. Stable isotopic analyses (n = 67) were conducted on the isotope ratio mass spectrometer at the Boston University Stable Isotope Laboratory. Analyses for δ18O were done via CO2 equilibration; deuterium analyses were done via pyrolysis using a GVI ChromeHD™ system. Analytical precision for both measurements is ±0.1‰. Isotope values are presented as per mil (‰) relative to Vienna Standard Mean Ocean Water (VSMOW). Deuterium excess (d) values were calculated as d = δD − 8δ18O. 4. Results 4.1. Geomorphology and sedimentology of the rock glacier The surface of the northern Pearse rock glacier is a patchwork of fluvial, colluvial, and eolian sediments. The northern section of the rock

Fig. 4. (a) Top of buried ice at sampling location, PVI-03B. Clean ice was covered by 50 cm of sand- and boulder-rich sediments. (b) PVI-02 ice core at 72–104 cm below the ice surface. The ice core varied from clean ice to sediment rich (shown here). (c) Ice core PVI-01, depth of 50–80 cm below the ice surface, which was covered by 30 cm of dry sediments. Note sharp contact between clean ice and ice-cemented sediments at the base of the core. (d) Sampling site 15–02. Ice vein in ice-cemented sediments, excavated ~50 m south of PVI-01. All images courtesy of K.M. Swanger, reprinted with permission.

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glacier consists of fans of fluvial sands and fine gravels, with isolated boulders that have fallen from the cliffs above. These fans slope 15–20° to the south for ~200 m and then grade into the main rock glacier. The main body of the rock glacier is characterized by boulder-rich deposits of poorly- to moderately-stratified sands and gravels. Contraction-crack polygons ~15 m in diameter are also common in this region (Fig. 3b), indicative of ice at depth (Marchant and Head III, 2007). The main rock glacier slopes south at an average of 15°, finally terminating along a 35° slope that averages 4–5 m high. The surficial sedimentology of the northern Pearse rock glacier is dominated by sands, gravels and boulders. Eleven of the 17 excavations were on the rock glacier, of which nine exhibited cobble pavement development (poor to moderate). At five excavations, sediments were unstratified. Three excavations yielded poorly- to moderately-stratified sands/gravels. The final three excavations (KSE-06-21, -06-24, and -15-03) had unstratified coarse sediments in the upper 15–20 cm underlain by moderately-stratified sands. Four of the stratified deposits were characterized by N2-cm thick beds of sand/gravel with lenses of finer and lighter-colored sands (Fig. 3d,e). Where stratified, bedding was usually horizontal or gently dipping with the surface slope. There were exceptions, like samples 14-31 and 15-03, in which sediments were dipping ~30° W and ~45° N respectively. See Supplementary figures and tables for latitude, longitude, field photographs, descriptions, and cumulative grain size graphs of all 17 excavations. Table 2 presents grain size graphical statistics for 19 samples from all 17 excavations. Silt and clay fractions in all rock glacier samples (n = 13) was b3% by weight (wt%). Ratios of gravel:sand varied from 50:50 to 1:99. Median grain sizes ranged from −1.7 to +1.3 phi and graphical mean sizes ranged from −1.3 to +1.1 phi. Of the 13 rock glacier samples, many (n = 8) had graphic mean grain sizes of “coarse sand,” and many (n = 7) were poorly-sorted (with inclusive standard deviations of 1.0–2.0 phi) (Table 2). The four sediment samples from the fluvial fans directly south of the rock glacier (Figs. 2 and 3) were sand-rich (88–100 wt%) and moderately-sorted. Graphic mean grain sizes ranged from +0.3 to +1.3 phi (coarse to medium sands). The fan deposits were statistically

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similar to the finer-grained sediments excavated from the rock glacier (such as samples 14-02, 06-22, 15-03B, and 14-31). Finally, we analyzed talus and fluvial deposits from below the bedrock slope, just north of the rock glacier. These samples (14-14 and 14-15) were very-poorly sorted with mixed gravels and sands (Table 2). 4.2. Buried ice morphology and ice core descriptions Buried ice occurs sporadically in the northern Pearse rock glacier and was encountered in b50% of field excavations (maximum depth of excavation ~70 cm). Where located, clean buried ice exhibits variable gas inclusions and in situ sediment concentrations (Fig. 4). Descriptions of the ice cores and sediment cover are presented in Table 3. At the westernmost site, SIC-05, the core lacked sediments for the entire 225-cm length. At core site PVI-01, the buried ice was heterogeneous with respect to gas inclusions, ranging from bubble-free to bubble-rich ice. At 66 cm depth below the ice surface, the core transitioned from clean ice to ice-cemented sediments along a sharp contact dipping at 35° (Fig. 4c). At the easternmost core site, PVI-02, the upper 130 cm of the buried ice were clean, except for dispersed sands from 90 to 100 cm depth (Fig. 4b). At 130 cm depth within the core, the clean ice transitioned into ice-cemented sediments along a sharp contact, dipping at 50°. Cores PVI-03A and \\03B were excavated within 50 m of each other from isolated hummocks near the bedrock cliffs (Figs. 2 and 4a). Core 03A was 20 cm in length and free of sediments. Core 03B was 125 cm in length and mostly sediment free, with occasional sediment-rich lenses (Table 3). 4.3. Stable isotopes Buried ice in northern Pearse rock glacier exhibited variable stable isotopic values, with δ18O ranging from −32 to −46‰ and δD from −270 to −350‰ (Fig. 5). When δ18O is plotted against δD, all of the Pearse ice-core data fall on a slope of 6.9, with an r2 value of 0.97. Ice core samples fall into two distinct groupings: the eastern samples are

Table 2 Sedimentology of rock glacier, distal fans and talus in northern Pearse Valley. g:s:m (wt%)a

Median (phi)b

Graphic mean (phi)c

Graphic mean (label)c

Inclusive graphic standard deviationd

Sortingd

Inclusive graphic skewnesse

On rock glacier surface KSE-14-01 30 DVX-14-02 30 KSE-15-03A 15 KSE-15-03B 30 KSE-14-13 15 KSE-06-20 20 KSE-06-21A 10 KSE-06-21B 25 KSE-06-22 20 KSE-06-23 15 KSE-06-24 20 KSE-14-31 20 KSE-14-33 20

30:70:0 1:99:0 33:66:1 3:97:0 13:86:1 13:86:1 11:88:1 4:95:1 5:95:0 6:94:0 15:83:2 1:99:0 51:48:1

+0.1 +0.4 +0.2 +0.7 +1.3 +0.4 +0.8 +1.1 +0.8 +0.8 +0.8 +0.4 −1.7

−0.6 +0.6 −0.6 +0.7 +1.0 +0.2 +0.7 +1.1 +0.7 +0.7 +0.5 +0.5 −1.3

v. coarse sand coarse sand v. coarse sand coarse sand medium sand coarse sand coarse sand medium sand coarse sand coarse sand coarse sand coarse sand granule

2.1 0.9 2.6 1.1 1.5 1.5 1.6 1.1 1.0 1.1 1.8 0.9 2.4

Very poor Moderate Very poor Poor Poor Poor Poor Poor Moderate Poor Poor Moderate Very poor

−0.4 +0.3 −0.4 0.0 −0.5 −0.3 −0.3 −0.2 −0.2 −0.2 −0.3 +0.1 +0.2

Fluvial sediment fans distal to rock glacier KSE-14-08 10 KSE-14-09 10 KSE-14-11 10 KSE-14-12 10

2:98:0 0:100:0 12:88:0 0:100:0

+0.6 +1.3 +0.3 +0.7

+0.7 +1.3 +0.3 +0.8

coarse sand medium sand coarse sand coarse sand

0.9 0.8 1.5 1.0

Moderate Moderate Poor Moderate

+0.2 −0.1 −0.2 +0.2

Talus and stream channel directly north of rock glacier (beneath bedrock slope) KSE-14-14 15 41:58:1 −0.2 −0.7 KSE-14-15 15 23:76:1 +0.8 0.0

v. coarse sand coarse sand

2.4 2.3

Very poor Very poor

−0.2 −0.5

Sample

a b c d e

Sample depth (cm)

g:s:m = weight percent of gravel:sand:mud fractions. Mud includes silt and clay. phi (ϕ) size of 50%ile by weight (ϕ50). Statistical analyses follow Folk & Ward (1957). v. coarse sand = very coarse sand. Very poor: N2.0 phi, poor: 1.1–2.0 phi, moderate: 0.71–1.0 phi. Negative values = coarse skewed. Positive values = fine skewed.

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Table 3 Northern Pearse rock glacier ice core descriptions. Sample

Latitude, longitude, elevationa

Overlying sediments, thickness and description

Ice core descriptionb

SIC-05 PVI-01

77°42′11′′ S, 161°35′11′′ E, 610 masl 77°42′11′′ S, 161°35′46′′ E, 650 masl

55 cm thick. Poorly-stratified sands. 30 cm thick. Poorly-stratified sands and fine gravels.

PVI-02

77°42′14′′ S, 161°36′46′′ E, 630 masl

35 cm thick. Unstratified coarse sands.

PVI-03A

77°42′10′′ S, 161°36′26′′ E, 680 masl

45 cm thick. Unstratified sands, gravels, pebbles, and cobbles.

PVI-03B

77°42′09′′ S, 161°36′28′′ E, 680 masl

50 cm thick. Poorly-stratified, poorly sorted, sands to boulders.

Ice veins

77°42′10.5′′ S, 161°35′41′′ E, 635 masl

40 cm thick. Moderately-stratified, sands and fine gravels.

0–225 cm: clean, gas inclusions. 0–70 cm: clean, gas inclusions. 70–80 cm: ice-cemented sands. 0–40 cm: clean, gas inclusions. 40–100 cm: dispersed sands, gas inclusions. 100–130 cm: clean, gas inclusions. 130–145 cm: ice-cemented sands. 0–15 cm: clean, gas inclusions. 15–20 cm: dispersed sands, gas inclusions. 0–30 cm: clean, gas inclusions 30–38 cm: dispersed sands, gas inclusions. 38–95 cm: clean, gas inclusions. 95–100 cm: dispersed sands, gas inclusions. 100–112 cm: clean, gas inclusions. 112–125 cm: ice-cemented sediment veins. No ice core extracted. Ice-cemented sediments with clean ice veins, no gas inclusions, 2–4 cm thick.

a b

masl = meters above sea level. Clean ice ≤ 5% sand by volume. Dispersed sands = 5–50% sediment. Ice-cemented sediments ≥50% sediments.

two ice cores extracted from the western part of the rock glacier, PVI01 and SCI-05, have δ18O values of −43 to −40‰, much lighter isotopically than Fountain Glacier (see Supplementary table for all stable isotopic data).

isotopically heavy and the western samples are isotopically light. The samples from the Fountain Glacier ice apron yield δ18O values ranging from −36 to −34‰, and plot with the buried ice from the eastern part of the rock glacier, PVI-02, -03A and \\03B (Figs. 2 and 5). The -260

Taylor Glacier / Taylor Dome Taylor Valley alpine glaciers

-280 WL GM

Western NP rock glacier PVI-01 SIC-05 Ice veins Eastern NP rock glacier PVI-02 PVI-03A PVI-03B Fountain Glacier

Depth (cm)

a)

-320

-340 -44

-42

-40

-36

-38

0

0

40

40

80

80

120

160

200 -44

b)

-34

-32

δ18O (‰)

Depth (cm)

δD (‰)

-300

120

160

-42

-40

-38

δ18O (‰)

-36

-34

200

-32

c)

-10

-5

0

5

10

15

Deuterium excess (‰)

Fig. 5. Stable isotopic data from Pearse Valley buried ice and Fountain Glacier. (a) δD vs δ18O from five ice cores extracted from the northern Pearse rock glacier. PVI-01 and SIC-05 come from the western section of the rock glacier, PVI-02, 03A, and 03B come from the eastern section, near the modern Fountain Glacier ice apron (Fig. 2). Ice veins are isolated ~1-cm-thick refrozen meltwater in ice-cemented sediments near PVI-01, site 15-02 (Figs. 2 and 4d). GMWL = global meteoric water line. Isotopic range for Taylor Valley alpine glaciers comes from Gooseff et al. (2006). Taylor Glacier and Dome isotopic range come from Steig et al. (2000) and Grootes et al. (2001). In legend NP = northern Pearse. (b) Depth vs. δ18O and (c) depth vs. deuterium excess from the five buried ice cores.

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Of the refrozen meltwater veins that were sampled 50 m south of PVI-01 (Figs. 2 and 4d), half of the samples are isotopically similar to PVI-01. The other half of the samples fall below the local meteoric water line (LMWL). All of the ice vein samples fall on a slope of 5.0 (r2 value of 0.96), and are consistent with an initial water source that is isotopically light, but that has experienced evaporation fractionation. Deuterium excess values for all of the sampled ice range from −10‰ to +15‰. In general, D-excess values for the western buried ice (cores PVI-01 and SIC-05) are more positive, averaging + 9.5‰, whereas the eastern buried ice (cores PVI-02, -03A, and\\03B) average + 1.8‰. Buried ice isotopic values (δ18O and deuterium excess) plotted versus depth show variable relationships. For example, PVI-01 and\\03B exhibit consistent isotopic values with depth (δ18O values vary by b1‰). Conversely, PVI-02 yields consistent δ18O values from 10 to 100 cm depth, but the uppermost 10 cm of the ice become systemically heavier toward the surface (Fig. 5b). These values also correspond to systemically more negative deuterium excess values (Fig. 5c). These data are consistent with either in situ sublimation fractionation of the upper ~10 cm of the ice (Lacelle et al., 2011) or an upper layer of ice that has experienced melt and evaporation fractionation at some point in the past. 4.4. Ground-penetrating radar Fig. 6 shows two of the four GPR profiles collected, from ice core locations PVI-01 and PVI-02 (Fig. 2). Radar facies are characterized by

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abrupt transitions between what is likely ice-rich material and sediment-rich material (Fig. 6). In the vicinity of ice core PVI-01, interpretation of the radar facies indicates the presence of clean ice (little to no internal reflectors) with interspersed sediment layers (reflectors denoted with arrows in Fig. 6). In the longer GPR transect shown, the interpreted buried ice lens reaches a maximum thickness of ~14 m at the northern (start) of the transect. The interpreted ice lens then consistently thins to the south, finally reaching a thickness of ~2 m (Fig. 6a). The ice lens feature is near-surface (covered by b1 m of dry sediments) and extends for 50 m of the profile distance. At this location, the radar facies are indicative of multiple dipping reflectors in the buried ice, dipping to the north or up rock glacier flow direction (arrows in Fig. 6). From 50 to 85 horizontal m of the profile, there is an area of poor signal content that we attribute to localized salinity rather than clean ice (Fig. 6a). This interpretation is based on (1) the presence of dozens of salt nodules in the surface sediments at this location and (2) the lack of any buried clean ice within 60 cm of the ground surface based on field excavations. Near PVI-02, the interpreted buried ice lens is thinner (1–5 m thick) with many north- and south-dipping reflectors (likely to be sediment layers) throughout the profile (Fig. 6b). Although only two profiles are shown in Fig. 6, all four profiles demonstrated variable occurrence, thickness and sediment concentrations for buried ice in the rock glacier. At both GPR transects shown, the occurrence of buried ice with interspersed sand-rich lenses is supported by the ice cores PVI-01 and -02.

Fig. 6. GPR profiles from Pearse Valley. (a) Data collected with 200-MHz antennas near PVI-01. The black line shows an interpreted demarcation between mostly clean ice (above) and sediment-rich materials (below). The box highlights an area where near-surface salinity may have precluded GPR signal penetration. Downslope from that area (line position 85 m and beyond), sediment-rich materials likely extend to the surface with little clean ice present. (b) 200-MHz data collected near PVI-02. In this profile, sediment-rich material is close to the surface. The box highlights an area of interpreted cleaner ice. In both (a) and (b), sediment layers (arrows) are likely interspersed with cleaner ice.

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At both ice-core locations (which serve as “ground truth” for the GPR data) clean ice is buried under a ~50-cm cap of sediments. The GPR radar facies show this near-surface, sediment-ice transition as a prominent reflector within 50 cm of the surface (Fig. 6). Further upholding our GPR-derived interpretations of the inter-ice stratigraphy, in both ice cores we encountered ice-cemented sediment layers with abrupt, dipping contacts with the overlying clean ice (Fig. 4). 5. Discussion 5.1. Rock glacier ice sourced from both alpine and outlet glaciers Based on the stable isotopic data and geomorphic mapping, we conclude that the northern Pearse rock glacier is glacigenic, with some melting and reworking of the glacier ice during and after its deposition. The ice that cores the western part of the northern Pearse rock glacier is possibly sourced from Taylor Glacier, whereas the ice in the eastern part of the rock glacier is sourced from local Fountain Glacier. Most published stable isotopic values for snow, ice and meltwater come from central and eastern Taylor Valley (Gooseff et al., 2006) and Taylor Dome (Steig et al., 2000, Grootes et al., 2001), with very few samples from Pearse Valley (Heldmann et al., 2012). Regardless, these data provide general constraints on regional stable isotopic values. Summer snowfall is isotopically heavier at the coast than in the high-elevation interior; δ18O of snow and ice from near-coastal glaciers is around −25‰ (Gooseff et al., 2006). Stable oxygen isotopic values at Taylor Dome vary from −36‰ during the interglacial intervals, such as Marine Isotope Stage (MIS) 5e, to −45‰ during the LGM (Steig et al., 2000; Grootes et al., 2001). The sampled buried ice from northern Pearse rock glacier falls into two distinct isotopic groups: lighter samples in the west (cores PVI-01 and SIC-05) and heavier samples in the east (cores PVI-02, -03A, and \\03B). The isotopically heavy samples (δ18O of −34‰) correlate with the Fountain Glacier ice apron, supporting an alpine glacier source for the eastern part of the rock glacier (Fig. 5). The buried ice in the western part of the rock glacier is much lighter isotopically (δ18O of −42‰) than the Pearse and Taylor valley alpine glaciers, and is instead isotopically similar to Taylor Glacier (Fig. 5), which sources from Taylor Dome of the East Antarctic Ice Sheet. Interestingly, cores PVI-01 and SIC-05 are also isotopically similar to “buried snow patches” sampled by Heldmann et al. (2012) (δ18O = −43‰), which were much lighter than the modern snowfall (δ18O = −28‰) or the local glaciers (δ18O = −34‰), possibly indicating that the these “buried snow patches” were fed by windblown snow from the ice sheet. The D-excess values from cores PVI-02 and\\03B become systematically more negative toward the surface of the ice compared to ice at depth (Fig. 5c). These values indicate increased evaporation fractionation of the upper 10 cm of the ice, possibly due to in situ melting of the buried ice during peak summer warming or incorporation of meltwater that has infiltrated through the surface sediments to the top of the buried ice. 5.2. Rock glacier formation by burial of ice aprons Previous work in the Dry Valleys has demonstrated that interstitial ice is recharged via meltwater infiltration and occasionally by vapor condensation (McKay, 2009; Lacelle et al., 2011). However, the processes that cause burial of clean ice may vary by region. For example, in Beacon Valley thick deposits (up to ~100 m) of glacier ice are “buried” under a cap of sublimation till, forming debris-covered glaciers (Marchant et al., 2002; Shean and Marchant, 2010). Our data indicate that this process is not responsible for the buried ice in Pearse Valley. At this location, buried ice occurrence is patchy and ice lenses are b20 m thick. Additionally, the Pearse Valley ice is capped by sand- and gravel-rich deposits that are often poorly- to moderately-stratified, consistent with fluvial and alluvial processes, rather than till.

Given the stratigraphy, geochemistry and geomorphic setting of the buried ice in Pearse Valley, we developed a conceptual model for ice burial and preservation. The present-day Fountain Glacier ice apron acts as the modern analog for this conceptual model. Nearly all glaciers in the Dry Valleys have ice aprons at their termini, formed as cold-based ice calves at the glacier margin. These ice aprons are a mélange of glacier ice chunks, refrozen meltwater, boulders, and sediments (Fig. 7) (Fitzsimons et al., 2008). We conclude that the ice that cores the northern Pearse rock glacier formed as an ice apron that was buried by fluvial and colluvial sediments, allowing for preservation of the ice for N104 year timescales (Fig. 8). Preservation of ice aprons in this way requires specific geomorphic and hydrologic conditions, and therefore not all Dry Valley ice aprons become rock glaciers. Based on the data from Pearse Valley, ice-apron burial and preservation occurs near major slope breaks (such as below bedrock cliffs) where (1) calved ice can accumulate and (2) entrained sediments in meltwater streams are deposited due to the slope decrease, thus allowing for rapid burial of the ice apron.

5.3. Age of buried ice: MIS 5 to Holocene The northern Pearse rock glacier lies up to 2.5 km up valley from the modern margin of Taylor Glacier, and at an elevation 200–250 m higher (Fig. 1). Taylor Glacier has advanced multiple times during the late Pleistocene (Brook et al., 1993; Higgins et al., 2000; Swanger et al., 2011). The most recent major advance of Taylor Glacier beyond its present configuration occurred during MIS 5 (70–125 ka), depositing a series of moraines and proglacial lakes in central Taylor Valley that are ~250 m above the modern glacier terminus (Higgins et al., 2000). The MIS 5 advance of Taylor Glacier is not well-mapped in Pearse Valley. Therefore, we attempted to (roughly) reconstruct the advance at this location. This reconstruction was based on the general morphology (both ice height and bed topography) of modern Taylor Glacier lobes in Turnabout, Beacon, Simmons and Pearse valleys (Fig. 1). All four of these ice-lobes are N3 km in length (from the glacier central flowlines) and at all locations the subglacial bed slopes upward in the direction of ice flow (Kavanaugh et al., 2009). All four glacier lobes exhibit ice surface slopes of 2–3°, increasing to ~10° about 500 m upflow from the glacier margin. Based on the consistency of modern ice-lobe morphologies, we reconstructed the MIS 5 ice-lobe advance into Pearse Valley based on: (1) ice-surface slope of 3° which increases abruptly ~500 m from the margin, (2) a vertical height increase of 250 m for the main trunk of Taylor Glacier (Higgins et al., 2000), and (3) modern topographic data for Pearse Valley from our field mapping campaigns and Fountain et al. (2017). Based on this reconstruction, the MIS 5 glacier lobe likely advanced 3–4 km farther into central Pearse Valley than its modern extent. Advance into northern Pearse Valley would have been less due to the higher bedrock slope, with ice advance ranging from 2 to 3 km (Fig. 9). Our reconstructed glacier advance during MIS 5 is in general agreement with a series of undated moraines along the western wall of Pearse Valley (see Fig. 1 lower left inset for location of moraines and profiles for Taylor Glacier reconstructions). In northern Pearse Valley, an MIS 5 advance of 2.5 km (consistent with our reconstruction) would place the Taylor Glacier margin at the modern rock glacier location. If the northern Pearse rock glacier contains ice from both outlet and alpine glaciers, these results indicate a long and complex geomorphic history for a single rock glacier. Additionally, the ice that cores the western section of the rock glacier is tentatively dated to MIS 5 (70–125 ka), implying long-term preservation of some of the buried ice in Pearse Valley. The buried ice in the eastern part of the rock glacier is likely much younger and/or still forming today, as it is fed by calved ice, sediments and meltwater from Fountain Glacier.

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Fig. 7. (a) Fountain Glacier terminates at the crest of a bedrock slope above the rock glacier, calving to a 1-km long ice apron below. The ice apron is composed of ice, sediment, and meltwater. (b) Freshly calved ice blocks covering previously-deposited sand and ice. Fountain Glacier terminus is visible at the top of the bedrock gully. (c) Lower section of the ice apron, showing blocks of glacier ice within a matrix of sediment-rich meltwater-sourced ice. All images courtesy of K.M. Swanger, reprinted with permission.

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Fig. 8. Conceptual model of ice-cored drift formation as either: connected ice-cored moraines that are deposited during retreat or from sediment burial of an extensive ice apron. (a) Cold-based glacier advance. Glacier lacks significant englacial debris and terminates along an ice cliff that calves to an ice apron below. The ice apron is covered by a drift of boulders and stratified sands/gravels. (b) Initial glacier retreat. Previous buried ice apron is preserved while a new ice apron forms. (c) Present glacier terminus with an extensive ice-cored rock glacier distal to its margin. Buried ice is covered by a mixture of fluvial sands, rock-fall boulders and till.

6. Conclusions Buried ice and rock glaciers are common in Taylor and Pearse valleys, however the age, origin and geomorphic stability of these landforms is poorly constrained. Based on field excavations and GPR scans, the 1.5km2 northern Pearse rock glacier contains sporadic deposits of clean bubble-rich ice, up to at least 14 m thick. The ice is capped by a patchwork of stratified sand-rich deposits, indicative of fluvial processes, as well as unstratified colluvium and/or drift. Based on stable isotopic, geomorphic and GPR results, we conclude that the rock glacier is glacigenic, forming from the burial of ice aprons along the margins of previously

advanced cold-based glaciers. The eastern part of the rock glacier is a recent feature fed by calved ice chunks, meltwater and sediments from local alpine glacier, Fountain Glacier. The western part of the rock glacier is cored by ancient ice likely deposited during the MIS 5 advance of East Antarctic outlet glacier, Taylor Glacier. The results are significant because they reveal the complexity of rock glacier origins and degradation in ice-free regions of Antarctica. Some rock glaciers in the Dry Valleys might be ancient features that provide information on past glacial advances. Rock glaciers, which are common not only to the Dry Valleys but throughout the Antarctic, are important archives of glaciological

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Fig. 9. Advance of Taylor Glacier lobe during the last interglacial period, MIS 5 (70–125 ka). Here we show lobe advance in (a) central and (b) northern Pearse Valley based on (1) a 250-m vertical increase in the ice surface relative to present (Higgins et al., 2000), and (2) the modern relationships between ice-surface and bed topography for Taylor Glacier lobes (see Fig. 1 for Pearse Valley profile locations). Reconstructed advance is in general agreement with undated moraines in western Pearse Valley and the northern Pearse rock glacier.

and climatological data, as well as important landforms to monitor with future potential warming.

Acknowledgments This work was funded by the National Science Foundation Office of Polar Programs Antarctic Earth Sciences award #1341284. The authors would like to thank the McMurdo Station support staff, especially those at the Berg Field Center, Petroleum Helicopters International, and the Crary Laboratory, for assistance during the field season. Also thank you to James Dickson and Myles Danforth for field assistance. Geoff Hargreaves was of great assistance in subsampling ice cores at the National Ice Core Laboratory, as was Robert Michener (Boston University) with stable isotopic analyses of water. Thank you to two anonymous reviewers who added clarity and depth to the manuscript with their helpful comments. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.geomorph.2019.03.019.

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