Geochemical and Isotopic Evidence for the Escape of Ice Sheet Fluids in an East Antarctic Ice Sheet Outlet Glacier, Taylor Valley, Antarctica

Home , Blood Falls, Lake Bonney (Antarctica), Suess Glacier, Taylor Valley

Geochemical and isotopic evidence for the escape of ice sheet fluids in an East Antarctic Ice Sheet outlet glacier, Taylor Valley, Antarctica

1 1 1 1 1 Michael G. Scudder , Graham H. Edwards , Gavin G. Piccione , Terrence Blackburn , Slawek Tulaczyk 1Earth & Planetary Science Department, University of California Santa Cruz, Santa Cruz, CA, USA 5 Correspondence to: Michael Scudder ([email protected])

Abstract: The isotopic similarities between fluids beneath the East Antarctic Ice Sheet (EAIS) and the subglacial discharge found at the snout of Taylor glacier, one of the outlet glaciers located hundreds of kilometers away, implies that subglacial fluids may transit across this incredible distance. Such a model is consistent with the imaging, by airborne transient electromagnetic (ATE), of high salinity fluids at the 10 base of Taylor glacier to at least 5.75 km up-glacier from the snout, where ATE resolution fails (Mikucki et al., 2015). The transit of subglacial discharge could also be detected through the examination of basal ice, which has been hypothesized to form by subglacial freeze on of fluids at the ice-rock interface. We test these ice formation models as well search for direct evidence for the passage of subglacial discharge beneath Taylor glacier through examination of the elemental and U-Series isotopic compositions of basal 15 ice from Taylor Valley and compare with the compositions for regional lakes, streams, bedrock, and subglacial brines. For example, the basal ice from Suess Glacier, an alpine glacier draining a local accumulation zone in the Asgard Mountains, has 234U/238U and major element compositions nearly identical to surface waters and local silicates indicating mechanical entrainment of surface salts into basal ice by a cold-based formation mechanism. In contrast, the 234U/238U isotopic and major element 20 composition of basal ice from Taylor Glacier is consistent with a mixture of the same surface salts and the englacial brine known as Blood Falls, confirming the passage of EAIS-derived subglacial brines beneath Taylor Glacier. Fluids maturate beneath the EAIS for hundreds of thousands of years and emanate at Taylor Glaciers snout. Our major, minor, and trace element data confirm these fluids come from beneath the ice and link their presence to the movement of ice and the formation of basal ice implying 25 Taylor Valley is not just an outlet for Taylor glacier, but also for EAIS fluids.

1. Introduction

Basal ice is comprised of sediment-laden layers of ice in the basal portions of glaciers and is distinct from overlying meteoric ice chemically, structurally, and texturally (Souchez & Tison, 1981). The firnification of snowfall creates meteoric ice containing bubbly white ice with <0.05% debris by volume (Samyn et al., 2005), while the underlying basal ice forms from 30 processes at or near the glacier base. The variety of basal ice facies observed include amber ice marked by a uniform bubbly amber-coloured ice layer (Holdsworth, 1974; Samyn et al., 2005), stratified ice characterized by alternation between debris rich and free layers with a small grain size (<0.4 cm) (D. E. Lawson, 1979) (Samyn et al., 2005). Basal ice facies can exhibit differing isotopic compositions indicating different origins within the same facies (Sleewaegen et al., 2003). At the interface between the Earth’s surface and the cryosphere, the basal ice layer and hydrology of glaciers and ice sheets holds implications 35 for their stability, dynamics, and response to climatic forcing (Knight, 1997). Basal conditions such as pressure, temperature, and salinity control whether frozen basal conditions promote friction and limited ice flow or wet basal conditions lubricate the base and facilitate fast ice flow (Clarke, 2005; Pattyn, 2010). Efforts to model the stability of ice masses are improved by illuminating the mechanisms of these basal processes (Marshall & Clark, 2002).

40 The mechanisms of basal ice formation can be grouped into either cold- or warm-based processes. Mechanisms operating beneath warm-based or temperate ice include regelation, congelation, and glacio-hydraulic super cooling (Daniel E. Lawson et al., 1998; Weertman, 1957). Regelation involves ice flow around an obstacle producing a stoss-side pressure excess driving melting, while a lee-side pressure deficit forms basal ice through refreezing (Weertman, 1957). Congelation involves freezing on non-locally derived water to the base of the glacier (Knight, 1997). Glacio-hydraulic super cooling requires subglacial (i.e. 45 at the glacier bed) or englacial water rising at a sufficiently steep bed slope creating a pressure decrease that drives the freeze- on accretion of basal ice (Lawson et al., 1998). Cold-based or polar basal ice formation occurs when sediments are incorporated via a sub-solidus mechanism. One such mechanism termed the apron model, proposes permafrost or calved ice contaminated with sediment is mechanically incorporated into the glacier base as it advances (Holdsworth & Bull, 1968; Hooke, 1973; Shaw, 1977). Another mechanism of polar basal ice formation invokes entrainment of dry sediments into sub-solidus ice by melt- 50 refreeze occurring across nm-scale interfacial water films at the ice-sediment interface leaving bulk ice unaltered (Cuffey et al., 2000). Despite the presence interfacial water films, there is no bulk refreezing of liquid water. While not necessarily mutually exclusive, these processes operate in different pressure-temperature regimes within glaciers. Those glaciers where both temperate and polar processes occur at separate locations are termed polythermal. One such polythermal system, Taylor Glacier (Hubbard et al., 2004; Robinson, 1984; Samyn, et al., 2008), is predicted to reflect both warm- and cold-based basal 55 conditions and offers a case study of the interactions between its glacial and subglacial environments via the mechanisms of basal ice formation.

t n e t Commonwealth x E Glacier n Canada r Seuss Glacier u Glacier Lake Hoare b Lake Fryxell h s a W e k Taylor Glacier la Blood Falls o Lake Bonney le a P McMurdo Sound N

0 km 10 km 20 km Figure 1: Regional Map Sample locations and main geologic features shown. Shaded area traces the 336 m contour which is

the approximate extent of paleolake Washburn.

Taylor Glacier is an outlet glacier of the East Antarctic Ice Sheet (EAIS) that occupies upper Taylor Valley, one of the 60 McMurdo Dry Valleys (MDVs) and one of few places in Antarctica where basal ice is exposed above sea level. Taylor Valley is a hyper arid polar desert with mean annual temperature and precipitation of -15 to -30 °C and <100 mm respectively, with glacier melt as the primary source of liquid water in the valley lake system (Doran et al., 2002). Taylor Valley is flanked by the Asgard Range in the north and the Kukri Hills in the south, out of which flow alpine glaciers, and minor melt from these glaciers form streams that feed into three perennially frozen lakes (Fig. 1) (Welch et al., 2010). The ice-free lower valley 65 terminates in the Ross Sea (Fig. 1). Thick layers of stratified and englacial basal ice facies crop out near Taylor Glacier’s terminus (Appendix B3)(Samyn et al., 2005). The regional climate predicts a frozen glacier bed for Taylor Glacier with basal ice temperatures < -7°C (Hubbard et al., 2004). Yet, a hypersaline iron-rich brine, known as Blood Falls, periodically emanating from the terminus of Taylor Glacier (Lyons et al., 2019) and extensive basal debris imply an active subglacial hydrology and at least localized warm-based processes of basal ice formation, such as congelation. This is further evidenced 70 by remote sensing measurements that detect a subglacial brine reservoir feeding Blood Falls and thermal conditions at the pressure melting point ~6 km upstream of Taylor Glacier’s terminus (Badgeley et al., 2017; Foley et al., 2016; Hubbard et al., 2004; Mikucki et al., 2015; Robinson, 1984). Stable isotopes are consistent with this interpretation, showing Taylor Glacier basal ice has undergone melting and refreezing of meteoric ice evidenced by an enrichment in heavy isotopes (Mager, Fitzsimons, Frew, & Samyn, 2007). The alpine glaciers of the valley hold no evidence for sub-solidus basal conditions like 75 Taylor Glacier, although thin bands of massive to stratified basal ice do occur. Locally sourced Suess Glacier contains amber

basal ice and is predicted to be frozen at its bed evidenced by shallow ice thickness and no subglacial brine, and to interact with the regional lakes (Sleewaegen et al., 2003).

Taylor Glacier and Taylor Valley alpine glaciers advance out of phase with the nearby Ross Ice Shelf, such that Taylor Valley 80 glaciers have advanced during interglacial periods and receded during glacial periods (Brook et al., 1995; Hendy et al., 1979). Taylor Valley is covered with a mix of glacial, lacustrine, and fluvial deposits primarily derived from local granitoids, sandstones, Ferrar sill tholeiites, and McMurdo Volcanic Group rocks (Toner et al., 2013). Taylor Glacier has left a geomorphologic record of at least five Quaternary advances into Taylor Valley (Denton et al., 1970; Hendy et al., 1979), advancing with the expansion of the EAIS as far as present-day Canada glacier (Bockheim et al., 2008; Brook et al., 1993). 85 Taylor Glacier terminates into Lake Bonney, which is comprised of eastern and western lobes. Bonney Narrows (Fig. 1) limits mixing between East and West Lake Bonney (Spigel & Priscu, 1998). The composition of waters found at the bottom of Lake Bonney mirror that of Blood Falls, while waters from Lake Hoare and Fryxell resemble that of McMurdo Sound seawater and have not interacted with Taylor Glacier. No longer present today, Paleolake Washburn (Fig. 1) formed when the Ross Sea Ice 90 dammed the mouth of Taylor Valley at the Last Glacial Maximum and was ~336 m deep (Hall et al., 2000). The glaciers, lakes, streams and brines of this region reflect unique geochemical signatures (Henderson et al., 2006; W. B. Lyons et al., 1998). One approach to explore relationships between water and ice reservoirs in Taylor Valley is by comparing the chemical signatures of the various waters, both solid and frozen. The McMurdo Dry Valleys Long Term Ecological Research (LTER) project monitored the major element chemistry of the meteoric glacial ice, streams, lakes of Taylor Valley finding that solute 95 concentrations increase up to ~6 orders of magnitude between the glaciers, streams, and lakes respectively (W. B. Lyons et al., 1998). This exchange was interpreted as salts forming in soils and being picked up by streams, deposited in the lakes, and blown into the glacier. The high salt content in the streams was not only attributed to the dissolution of previously deposited salts, but it was also suggested that active chemical weathering also contributes to this signal (W. B. Lyons et al., 1998). This study explores how these sources interact with basal ice using major element as well as Uranium isotopic compositions. 100 Uranium isotopes have been measured in various water masses in Taylor Valley and provide additional insight into the provenance of dissolved cations in Taylor Valley. 238U decays with a half-life of ~4.5 billion years to 234U via 234Th (half-life = 24.1 d) and 234Pa by one alpha and two beta decays. The short half-lives of 234Th and 234Pa are generally geologically negligible, whereas 234U lasts far longer with a half-life of ~245 thousand years. Secular Equilibrium (SE) occurs in long- 105 lived geologic systems where the ingrowth and outgrowth of intermediate daughter products converge on equilibrium values. The ratio of any two members in the decay chain for a system in SE is defined by the reciprocal ratio of their respective decay constants, such that a system in secular equilibrium has an activity ratio of (234U/238U)= 1, where parentheses denote an activity ratio. While chemical fractionation of U isotopes is generally small (≤ 2%) and relegated to extreme redox environments (Dauphas & Schauble, 2016), physical fractionation of 238U and its radioactive daughter product 234U readily results from α-

110 recoil-injection of 234U into waters from 238U housed in sediment surfaces (Fireman, 1986; Fleischer, 1988; Ku, 1965). Such deviations from secular equilibrium may provide insight into the origin and weathering history of basal ice in the dry valleys. The α-recoil-injection of 234U occurs in nearly all terrestrial waters (Andersen, Stirling, Zimmermann, & Halliday, 2010; Vigier, Bourdon, Turner, & Allègre, 2001), and the prolonged nature of ice-rock-water contact of glaciers coupled with low weathering rates result in 234U/238U above secular equilibrium for high latitude glacial landscapes (Pogge von Strandmann et 115 al., 2006). Such a pattern has also been observed in Taylor Valley such that the englacial brine along with the proglacial lakes exhibit (234U/238U) compositions 2-5 times higher than SE and seawater (Henderson et al., 2006). This brine is of marine origin with alteration from prolonged ice-rock contact causing enrichment in elemental concentrations and a 234U/238U a factor of ~6 above SE (W. B. Lyons et al., 2019).

120 Fluids enriched with 234U have also been detected beneath the EAIS through the analysis of subglacial precipitates which form from subglacial waters prior to be exhumed at intraglacial moraines. U-Th dating of the precipitates provides the 234U/238U at the time of mineral formation on a reveals that secular evolution 234U/238U of EAIS fluids, which in locations of the Wilkes Basin is shown to evolve from marine-like values (1.147) to 20 times higher over a time frame from 400 ka to the present, which was interpreted by Blackburn et al., as recording the collapse of the Wilkes Basin at MIS 11 followed be the readvance 125 of ice at MIS 10 which is recorded to remain stable until the present day (Blackburn et al., 2020). Surprisingly, the 234U/238U evolution of these waters is shared by the subglacial discharge into Taylor Valley, which is recorded by composition of carbonates that form in proglacial lakes that occupied Taylor Valley during past interglacials. Collectively these data imply that the subglacial discharge found at Taylor Glacier is sourced from the EAIS and that the marine geochemical and biological characteristics of these fluids was inherited during a marine incursion at ~400 ka. If Taylor Glacier is an outlet glacier of the 130 EAIS, the fluids that reside and mature beneath the EAIS accompany this transit.

Elevated 234U/238U compositions in both Blood Falls and Lake Bonney deep waters and the shared secular evolution of 234U/238U between EAIS and MDV fluids imply a shared provenance of these water masses both in time and space, the latter of which is supported by airborne resistivity measurements which remotely detect subglacial fluids beneath Taylor Glacier to 135 at least 5.75 km up-glacier to where increasing ice thickness prevents resolution (Mikucki et al., 2015). The direct detection of high salinity fluids via geochemistry beneath the outlet glacier remains. To fulfil this we explore the elemental and U-Series compositions of basal ice from Taylor Valley to directly detect the interaction between the subsurface and surface hydrologic systems, as well as glaciers in Taylor Valley.

2. Methods

140 2.1. Field work and sample handling

Basal ice was collected from Taylor, Suess, and Canada Glaciers in the austral spring of 2017 (Fig 1; Appendix A1). Ice samples were chipped off of glaciers using a steel ice-chisel, targeting bands of sediment-laden basal ice (Appendix B3). Before any material was collected, surficial portions of ice characterized by a dusty appearance due to sublimation were removed by chisel (Appendix B1). Samples were collected in plastic bags that were stored in canvas bags for transport, 145 shipment, and storage. Samples were stored out-of-doors (mean temperature <-10 °C) for the duration of fieldwork in Taylor Valley, stored at -20 °C upon return to McMurdo Station, and shipped to UC Santa Cruz frozen in insulated boxes in <0 ºC refrigerated shipping units. No evidence of melt was observed upon receipt at UC Santa Cruz, where samples have since been stored at -20 °C.

2.2. Laboratory methods 150 2.2.1 Melting procedure

A total of seventeen ~25 g aliquots were extracted for analysis from samples of Taylor Glacier (17-TV- 14, -18, and -20) and Suess Glacier (17-TV-28) basal ice, targeting varying amounts of sediment content (Appendix A2). All labware was cleaned

155 prior to sample contact (PFA fluxed progressively in HF, HNO3, HCl, and rinsed with Milli-Q; polypropylene centrifuge tubes

rinsed with ~2N HNO3 followed by Milli-Q at room temperature) and all acid reagents were triple distilled in PFA stills. Aliquots were separated from original samples and each aliquot was rinsed away with 18 megohm  cm deionized ultra-pure water to melt away the outer ~5 mm of ice. To separate ice and sediment components, samples were melted at room temperature in 100 mL PFA beakers. The liquid and suspended fraction was immediately decanted and centrifuged for 20 minutes at ~1500 160 RPM in a 50 mL polypropylene centrifuge tube. The supernatant was removed and centrifuged a second time with a clean centrifuge tube, and the second supernatant was pipetted into a 30 mL PFA beaker. The extracted melt fractions were

evaporated to dryness on a 100 °C hot plate and re-dissolved in 4 mL of 7N HNO3. The sediment fraction was rinsed into a 50 mL centrifuge tube then filled with ultra-pure water, centrifuged, and decanted three times to remove water-soluble salts, then

dried in an oven at ~90 °C. The dry masses of the three sediment “pellets” were summed for each aliquot of basal ice. 165 2.2.2 Measuring major, minor and trace element compositions

Major element concentrations were measured with Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) on a Thermo iCAP 7400 in the Plasma Analytical Lab at University of California Santa Cruz. One ml of the acidified melt

170 fraction was diluted in 4 mL 5% HNO3 with trace HF. Samples were dosed with an internal Yttrium-spike upon introduction to account for plasma effects and concentrations were calculated relative to an in-house calibrated Multi-Element Standard (MES) to obtain concentrations.

Minor and trace element compositions were measured on a Thermo Scientific X-Series quadrupole Inductively Coupled 175 Plasma Mass Spectrometer (ICP-MS) in the Plasma Analytical Lab at University of California Santa Cruz. One ml of the

acidified melt fraction was extracted and diluted in 2 ml 0.8N HNO3 with trace HF and ~1 ml of 200 ppb Rh spike (in 0.8N

HNO3) to internally account for concentration and plasma effects. Concentrations of Na, Mg, Ca, Mn, Fe, and Sr were measured relative to an in-house calibrated MES, capable of reproducing accepted values of NIST SRM1640 to within 1σ standard deviation. Concentrations were obtained using three-point calibration curves which varied ~10% proportionally. A 180 total procedural blank was measured with concentrations averaging three orders of magnitude below concentrations of each element.

2.2.3 Measuring U isotopic compositions

To prepare melt fractions for high-precision isotopic measurements, 2 mL of the acidified melt fractions were evaporated to

185 dryness and re-dissolved in 1 mL of 7N HNO3. This is loaded onto a pre-conditioned 1 ml column of AG 1-X8 (200-400 dry

mesh) cleaned of U with twice repeated rinses of 3 ml 6N HCl and ultra-pure water. Matrix was eluted with 2 ml 7N HNO3 followed by 2.5 ml 6N HCl, and U was eluted and collected in 2 ml ultra-pure water. The elution was evaporated to dryness,

rehydrated in 1 ml 7N HNO3, and the purification was repeated to improve U ionization. The final U elution was evaporated

to near dryness, fluxed for one hour in ~200 μl of ultra-pure 30% H2O2, and finally evaporated just to dryness with trace H3PO4.

190 Purified U was loaded onto 99.99% purity Re ribbon with a Si gel-0.035 M H3PO4 activator and measured by Thermal Ionization Mass Spectrometry on the UCSC IsotopX X62 Thermal Ionization Mass Spectrometer using a dynamic Faraday- Daly measurement technique after Blackburn and others (2020). The isotopes 235U, 236U, 238U are measured statically as oxides on Faraday cup detectors connected to 1012 Ω resistance amplifier cards. The isotope 234U is measured concurrently on a Daly- Photomultiplier complex. Within the run 235U and 238U peaks are swapped between Faraday-Faraday and Daly-Faraday 195 configurations. The corresponding 235U/238U measurements are used to calculate a Fara-Daly gain that is applied to correct the measured 234U/238U for detector bias. Total procedural blanks were measured and were ~80 pg U, to which sample measurements ranging 40-150 ng were insensitive.

2.3 McMurdo Dry Valleys LTER & literature sourced data

Major elemental data displayed in Figures 2 – 4 for Taylor Valley Lakes, Streams, Blood Falls, and Meteoric Ice were obtained 200 from the McMurdo Dry Valleys LTER at mcm.lternet.edu, reflecting datasets published in (Doran, 2014; Lyons & Welch, 2014, 2015). West and East Lake Bonney were divided into surface and deep categories at a depth of 15 and 12 m respectively based on chemical and limnological indicators of stratification at those depths (Spigel & Priscu, 1998). Taylor Valley Streams were sorted into upper and lower valley streams; we identify upper drainages as those west of the Nussbaum Riegel high stand, and lower drainages east of the high stand. For the streams to pair a single isotopic measurement with many major element 205 data points, the median and 2σ standard deviation was calculated for the LTER data and paired with the isotopic measurements (Fig. 4). The Blood Falls Runoff and lake data due to large major element compositional range was paired with a single uranium isotopic composition and error (Fig. 4).

The Blood Falls Englacial value was obtained with a thermoelectric melting probe sampling a brine conduit within Taylor Glacier (W. B. Lyons et al., 2019). Soluble salts were obtained from Taylor Valley soils (Toner et al., 2013) and local silicate 210 rocks include mean compositions of local granitoids (Smillie, 1992), generalized passive margin sandstones (Bhatia, 1983), Ferrar sill tholeiites (Antonini et al., 1999), and McMurdo Volcanic Group rocks (Cooper et al., 2007), collectively representative of regional geologies that source Taylor Valley sediments. McMurdo Sound Seawater [U] and isotopic composition values are from Henderson and others (2006) while major element composition is from Lyons and others (2019).

2.4 Elemental and isotopic mixing models

215 Two component mixing models were used to identify the magnitude of mixing between possible end-member water sources in both elemental and isotopic space (Fig. 2-4 and Appendix C1). In figures 2-4 we assume endmember mixing between the ‘Englacial’ composition for Blood Falls (Lyons et al., 2019) and surface waters.

3. Results

The highest salinity endmember is 220 the englacial Blood Falls brine (Fig. 2; Lyons et al., 2019), which is similar in elemental composition to high salinity Blood Falls Runoff. More dilute sources 225 such as the streams and lakes span lower [Na] and Na/Ca values, while still trending toward an Figure 2: Comparison of Na (mmol/L) to Na/Ca tracks the englacial salinity and composition. salinity of different waters within Taylor Valley (Na) and defines Local silicate rock sources and water compositions (Na/Ca).230 The Taylor Valley soils share similar elemental mixing model shows how different sources are mixing. Na/Ca compositions with nearby Streams and Suess Basal Ice compositions. McMurdo Sound Seawater and Lake Fryxell, the lake closest to McMurdo Sound, have comparable salinity and 235 Na/Ca compositions that follow a separate trend of mixing than Blood Falls brines (Fig.2). The Lakes and Streams are all more calcic than seawater. The surface of East and West Lake Bonney have similar salinity to Taylor Basal Ice while Lake Hoar & Fryxell trend towards McMurdo Sound Seawater in accordance with proximity. Taylor Glacier Basal Ice falls between Blood Falls Englacial and Taylor Valley Streams in both [Na] and Na/Ca space. The lowest salinity endmember, Taylor Meteoric

Ice, is similar in composition to Suess Basal Ice, streams, and 240 local silicate sources. Mixing models match the observed trend between Blood Falls Englacial, Blood Falls Runoff, Taylor Basal Ice, and streams. Upper and Lower Taylor Valley Streams closely resemble Suess Basal Ice in composition and salinity. West and East Lake Bonney deep 245 waters are omitted from Figure 2 due to underlying trends irrelevant to relationships being examined. From the mixing model’s trajectory there is a missing endmember source at surface water compositions and Taylor Glacier Basal Ice salinity. 250 Figure 3 explores the U systematics of the valley features. The highest 234U/238U endmember in Figure 3 is Blood Falls Englacial, with Blood Falls Runoff being the next most Figure 3: Taylor Valley waters and ice 1/ppb [U] and isotopic enriched source. Blood Falls Runoff varies in [U] yet is composition. Inset shows entire parameter space.

255 consistently high in 234U/238U. West & East Lake Bonney Surface values trend between Taylor Valley Streams and Taylor Basal Ice isotopic compositions and [U]. Taylor Glacier Basal Ice falls along mixing lines between Blood Falls Englacial and Taylor Valley Streams. Lake Fryxell 260 closely resembles McMurdo Sound Seawater, but with slight increases in both [U] and (234U/238U). Upper Taylor Valley Streams are elevated in 234U/238U compared to seawater and span a wide range of [U]. Taylor Basal Ice falls on mixing lines with Blood Falls Englacial, Blood 265 Falls Runoff, Lake Bonney Surface, Upper Valley Streams, Lake Fryxell, and McMurdo Sound Seawater. As shown in Figure 2, there is also a trend between the Englacial Brine, Blood Falls Runoff, Taylor Basal Ice,

and Upper Taylor Valley Streams in uranium elemental Figure 4: Taylor Valley waters and ice Na/Ca and 234U/238U 270 and isotopic space. compositions.

In Figure 4 the trends observed in separate analysis of major element and U-Series isotopic data are reinforced and clarified by a paired comparison of the two systems. The Englacial Brine, Blood Falls Runoff, Taylor Glacier Basal Ice, Upper Taylor Valley Streams, and local silicate sources fall on the same mixing line. West & East Lake Bonney moat and deep waters nearly 275 fall on the same mixing line as Lake Fryxell. Lake Bonney Surface values do not fit on a mixing trend.

4. Discussion

4.1 Mixing Relationships and basal ice formation

Elemental and isotopic compositions of basal ice can be used to identify possible mixing relationships between the various 280 solute sources in Taylor Valley. In Figures 2-4, two-endmember mixing lines fixed at a blood Falls Englacial endmember and a range of plausible low-salinity endmember values illuminate these relationships. The salinities and compositions predicted by these models (Fig. 2) exposes several control processes including mixing, dilution, and concentration. The mixing lines in Figure 2 suggest that basal ice reflects a mixture between Blood Falls Englacial and salts with the same elemental composition (e.g. Na/Ca) as the streams but at higher concentrations. East & West Lake Bonney deep measurements are omitted from 285 Figure 2 because although they follow similar trend to surface East & West lake Bonney but at a higher salinity, Lake Bonney is undersaturated with respect to calcite due to calcite precipitation (Neumann et al., 2001) pushing the Na/Ca ratio of some deep Bonney measurements beyond the scope of the mixing relationships being examined. Lake Hoar and Fryxell also reflect processes unrelated to our investigation, trending towards a seawater composition in accordance with their proximity to McMurdo Sound. 290 Consistent with the major element data, uranium isotopic data (Fig. 3) shows Blood Falls Englacial and surface waters fall on the same mixing lines, but a large range of [U] in surface waters makes definite conclusions difficult. The wide span in surface water and blood falls runoff concentrations likely reflects dilution between measurement sites and times. The heterogeneity of Blood Falls runoff results from incorporation of solutes from meteoritic ice, lake ice, snow. Further, surface waters are 295 transiting various drainages indicated by very low [U] and a 234U/238U of snowmelt.

Although when compared in paired elemental and isotopic space (Fig. 4), the Blood Falls Englacial and surface waters fall precisely on the same mixing line with Taylor Glacier Basal Ice as an intermediate composition. This demonstrates that basal ice is a mixture that can be traced chemically between subglacial brines and surface waters. Elevated 234U/238U from long term 300 water-rock contact and shared Na/Ca compositions of surface waters in Taylor Basal Ice support its forming from a mixture of surface waters and brines (Fig. 4). This requires a warm-based basal ice formation mechanism operating in Taylor Glacier such as regelation or glacio-hydraulic supercooling.

4.2 Comparison to Suess Glacier 305 The non-EAIS derived control basal ice from Suess Glacier matches the salinity of streams and shares the Na/Ca composition of meteoric ice, streams, and local silicate sources (Fig. 2). This indicates surface waters incorporating cations plausibly derived from local silicate sources are being mechanically incorporated into Suess Basal without melting and refreezing fractionating these elements. Additionally, Th concentrations in Suess Basal Ice are an order of magnitude higher than in Taylor Basal Ice. 310 This further supports a cold based mechanism for Suess Glacier as insoluble thorium is excluded from subglacial waters, whereas mechanical entrainment of Suess Basal Ice without fluids more readily permits thorium incorporation. This evidence makes a cold based basal ice formation mechanism such as the Apron model more defensible for Suess Basal Ice.

4.3 Source of Taylor basal ice salts. 315 Although the composition of Taylor basal ice reflects a mixture between Blood Falls with surface water like compositions but more concentrated than modern surface waters, the hydrologic head produced by glaciostatic pressure beneath Taylor Glacier prevents the active transit of stream waters to the subglacial brine reservoir. What could explain both the increased concentration and occurrence beneath the glacier is the incorporation of saline permafrost or dry salt residues left behind by 320 Paleolake Washburn prior to readvance of Taylor Glacier. As Paleolake Washburn drained and/or evaporated it likely left behind salts and permafrost that were overrun as Taylor Glacier advanced into the valley. Subsequent pressure-melting and the influx of saline brines would have permitted these brines to melt and mix with this Washburn residue, incorporating that mixture into its basal ice. Since these paleolake Washburn salts would be concentrated in the residue, this facilitates the elevated salinity of surface water elemental compositions required in the mixing models of Figure 2. If Paleolake Washburn 325 solutes are a major source of the salinity on valley sediments, this suggests that much of the local stream salinities may be deriving from the recycling of these solutes rather than high rates of chemical weathering as suggested by (Lyons et al., 1998).

4.4 Implications for subglacial fluid travel

330 Blackburn and others (2020) demonstrated that subglacial fluid compositions are preserved in subglacial precipitates from the Wilkes Basin in the EAIS over time in the form of interbedded opal and carbonate layers. The α-recoil-injection of 234U during long-term ice-rock contact causes 234U accumulation in these fluids, resulting in 234U/238U enrichment in subglacial brines up to 20 times higher than marine waters. This 234U/238U enrichment signal is preserved in Blood Falls and a modified enrichment signal is preserved in the basal ice of Taylor Glacier (Table A2), suggesting these fluids observed ~700 km inland are passing 335 beneath the EAIS to the glacier snout. This differs from other hypotheses of brine origin such as a localized salt dome or the recycling of Lake Bonney fluids in that a subglacial brine reservoir hundreds of kilometers away is feeding this subglacial brine river flowing out at the surface. Although the brine is known to be of marine origin (W. B. Lyons et al., 2019), our results

provide further evidence of the large regional extent of this reservoir of marine-derived fluids residing within the Wilkes Basin in East Antarctica.

340 5. Conclusion

Elemental and Isotopic trends indicate that the basal ice of Taylor Glacier forms via a warm-based formation mechanism such as regelation or glacio-hydraulic supercooling from fluids that compositionally are defined as a mixture of surface water salts and subglacial brines. Major element mixing trends point to Paleolake Washburn as a major contributor of these salts, which also is the likely source of high salinity in Taylor Valley’s soils. This is in contrast to the basal ice of locally sourced polar 345 glaciers: The basal ice from Suess Glacier is interpreted to form through the mechanical entrainment of solutes without melting or refreezing, suggesting a subsolidus basal ice formation mechanism such as the Apron model. A similar 234U/238U enrichment signal in subglacial precipitates from the Wilkes Basin in East Antarctica ~700 km inland is observed in the basal ice of Taylor Glacier suggesting a marine derived subglacial brine reservoir beneath the EAIS feeds subglacial and englacial fluids that span this distance and emanating from the glacier snout. This implies Taylor Valley is an outlet not just for Taylor Glacier but also 350 for EAIS fluids.

6. Appendices

Appendix A – Tables

Sample Type Latitude Longitude Elevation (m, a.s.l.) 17-TV-14 Basal Ice -77.64853 162.73294 162 17-TV-18 Basal Ice -77.72827 162.26992 109 17-TV-20 Basal Ice -77.72226 162.27587 77 17-TV-28 Basal Ice -77.72566 162.28169 59 Table A1: Basal ice sample, location, and elevation.

355

360

365

Glacier

Taylor Taylor Taylor Taylor Taylor Taylor Taylor Taylor Taylor Taylor Taylor Taylor Taylor Taylor

Suess Suess

Suess

17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17

------

Sample Sample

TV TV TV TV TV TV TV TV TV TV TV TV TV TV TV TV

------

20.1.3 18.1.6 14.1.1 28.1.4 28.1.3 28.1.2 28.1.1 20.1.4 20.1.2 20.1.1 18.1.5 18.1.4 18.1.3 18.1.2 18.1.1 14.1.3 370 14.1.2

Sample

14.43 24.24 18.66 58.65 19.85 15.11 15.85 31.35 21.23 12.14 40.11 21.68 18.33 22.81 25.50 24.26 16.48 Bulk

(g)

Sediment

10.29 13.34 20.51 13.22 13.05 14.66

4.44 8.42 4.45 5.61 4.20 4.39 2.31 7.03 7.71 9.04

375 7.77

(g)

Aqueous

13.96 38.14 11.43 10.66 10.24 27.14 16.83 26.88 14.65 10.62 13.77 12.45

9.99 5.32 9.83 9.60

8.71

(g)

380

Ice:Rock

2.25 1.36 0.40 1.86 1.36 2.40 1.83 6.46 3.83 4.26 2.03 2.08 1.38 1.52 0.95 0.66

1.12

(mM)

39.46 70.63 13.11 22.87 21.15 23.21 14.57 13.97 46.39 29.52 27.94

21.17

0.15 3.33 6.57 0.10 0.29 Na Na

385 (mM)

4.03 5.74 0.05 1.19 1.79 1.66 1.84 0.56 1.70 1.66 3.80 2.31 0.63 2.68 2.12 0.04 0.11 Mg Mg

(mM)

1.39 0.09 1.01 0.61 1.25 0.67 0.68 1.12 1.35 1.08 0.06 0.07

K K

- - - - -

(mM)

8.11 9.56 0.15 5.11 3.14 4.43 4.85 0.40 6.93 7.45 7.67 6.55 0.84 5.71 4.08 0.12 0.23 Ca Ca

390

(μM)

1.10 3.24 0.24 0.70 0.68 0.83 0.74 5.93 0.82 0.67 2.71 1.80 0.23 0.90 0.66 0.12 0.45 Mn

10.83 10.75 10.42 16.88 23.53

(μM)

9.62 0.28 4.96 0.73 3.50 6.86 1.11 1.10 0.17 4.87 5.84 5.98 Fe

29.34 22.88 19.38 21.84 23.53 20.60 23.92 16.14 11.08 23.12 18.13

395 (μM)

0.97 7.57 3.53 5.83 0.72 1.45 Sr

(ppb)

0.02 0.00 0.06 0.05 0.24 0.08 0.02 0.06 0.12 0.11 0.02 0.00 0.34 0.04 0.15 0.65 0.13

400

(ppb)

13.88

4.36 1.18 2.46 2.35 2.42 3.73 2.63 2.32 1.82 9.99 5.53 5.85 3.50 0.06 0.15 0.11

8.17E 3.08E 8.63E 2.18E 1.11E

2σ ±

------

- - - - -

07 06 07 06

405

234

3.39 3.86 3.73 3.99 3.92 3.92 3.19 3.58 3.14 3.76 3.71 3.56 3.70 3.17

- - -

238

6.88E 2.70E 2.09E 2.43E 2.24E 2.83E 1.72E 2.10E 1.89E 1.99E 1.88E 3.42E 2.44E 4.43E

2σ ±

- - -

------

03 02 02 02 02 02 02 02 02 02 02 02 02 02

410

Table A2: Elemental and Isotopic data.

Appendix B – Images A B

415 Image B1: Basal ice sample site 17-TV-18. Photographs of the southern flank of Taylor Glacier near the site of sample 17-TV-18. Shown in (A), a ~2 m thick band of debris-laden basal ice (1.8 m tall scientists in the left of image for scale). An unaltered dusty ice surface is shown to the right of the glove (B), showing ice loss by sublimation. To the left of the glove is where this alteration had been chipped away to expose ‘fresh’ ice that was then sampled. Glove is 28 cm long.

420 Image B2: Basal ice sample site 17-TV-20. Ice covered moraine near the northern end of Taylor Glacier. Shovel is ~1.2 m long. A B

Image B3: Basal ice sample site 17-TV-28. Located between samples 18 & 20, debris laden basal ice at wide (A), close (B), 425 and ultra-close (C) angles. In image C stratified layers of basal ice are visible. Trowel is 33 cm long.

Appendix C – Equations

Equation C1: Parabolic Mixing Equation 430

(퐶표푛푐퐴 × 퐶표푛푐퐵)(푅퐵 − 푅퐴) 퐶표푛푐퐴 푅퐴 − 퐶표푛푐퐵푅퐵 푅푚푖푥 = + 퐶표푛푐푀 × (퐶표푛푐퐴 − 푁푎퐵퐹퐸) 퐶표푛푐퐴 − 퐶표푛푐퐵

Adapted from (White, 2013)

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