The Geochemistry of Englacial Brine from Taylor Glacier, Antarctica

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

RESEARCH ARTICLE The Geochemistry of Englacial Brine 10.1029/2018JG004411 From Taylor Glacier, Antarctica Key Points: W. Berry Lyons1,2 , Jill A. Mikucki3 , Laura A. German1,2, Kathleen A. Welch1,2,4 , • Englacial brine was extracted from 1,2 1,2 5 6,7 Taylor Glacier, Antarctica, using Susan A. Welch , Christopher B. Gardner , Slawek M. Tulaczyk , Erin C. Pettit , clean‐entry techniques Julia Kowalski8 , and Bernd Dachwald9 • This is the ﬁrst direct sampling of recently discovered subglacial brines 1School of Earth Sciences, The Ohio State University, Columbus, OH, USA, 2Byrd Polar and Climate Research Center, The beneath Taylor Valley, Antarctica 3 • Ohio State University, Columbus, OH, USA, Department of Microbiology, University of Tennessee, Knoxville, Knoxville, TN, Geochemistry suggests that brine is 4 5 of marine origin and has been USA, Now at Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, CO, USA, Earth & Planetary 6 subsequently altered by chemical Sciences Department, University of California, Santa Cruz, CA, USA, CNMS Department of Geosciences, University of weathering products Alaska Fairbanks, Fairbanks, AL, USA, 7Now at College of Earth Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA, 8AICES and Faculty of Georesources and Materials Engineering, RWTH Aachen University, Aachen, Germany, 9Faculty of Aerospace Technology, FH Aachen University of Applied Sciences, Aachen, Germany

Correspondence to: W. B. Lyons, ‐ [email protected] Abstract Blood Falls is a hypersaline, iron rich discharge at the terminus of the Taylor Glacier in the McMurdo Dry Valleys, Antarctica. In November 2014, brine in a conduit within the glacier was penetrated and sampled using clean‐entry techniques and a thermoelectric melting probe called the Citation: fi Lyons, W. B., Mikucki, J. A., IceMole. We analyzed the englacial brine sample for lterable iron (fFe), total Fe, major cations and anions, German, L. A., Welch, K. A., Welch, S. nutrients, organic carbon, and perchlorate. In addition, aliquots were analyzed for minor and trace A., Gardner, C. B., et al. (2019). The elements and isotopes including δD and δ18O of water, δ34S and δ18O of sulfate, 234U, 238U, δ11B, 87Sr/86Sr, geochemistry of englacial brine from δ81 Taylor Glacier, Antarctica. Journal of and Br. These measurements were made in order to (1) determine the source and geochemical evolution Geophysical Research: Biogeosciences, of the brine and (2) compare the chemistry of the brine to that of nearby hypersaline lake waters and 124, 633–648. https://doi.org/10.1029/ previous supraglacially sampled collections of Blood Falls outflow that were interpreted as end‐member 2018JG004411 brines. The englacial brine had higher Cl− concentrations than the Blood Falls end‐member outflow;

Received 19 JAN 2018 however, other constituents were similar. The isotope data indicate that the water in the brine is derived Accepted 25 OCT 2018 from glacier melt. The H4SiO4 concentrations and U and Sr isotope suggest a high degree of chemical Accepted article online 2 FEB 2019 weathering products. The brine has a low N:P ratio of ~7.2 with most of the dissolved inorganic nitrogen in Published online 28 MAR 2019 + the form of NH4 . Dissolved organic carbon concentrations are similar to end‐member outﬂow values. Our results provide strong evidence that the original source of solutes in the brine was ancient seawater, which has been modiﬁed with the addition of chemical weathering products.

1. Introduction The subglacial environment of major ice sheets has become of important scientific interest (e.g., Laybourn‐Parry & Wadham, 2014; Siegert et al., 2011, 2016). Yet little direct evidence exists on the ecology and biogeochemistry of these environments and the processes that control them. Although there have been both modeling efforts (Wadham et al., 2008) and the analyses of refrozen subglacial waters (Christner et al., 2006; Priscu et al., 1999; Royston‐Bishop et al., 2005; Siegert et al., 2003), to date only a few direct samplings and analyses have occurred, most recently along the Whillans Ice Stream in West Antarctica (Achberger et al., 2016; Christner et al., 2014; Michaud et al., 2016; Mikucki et al., 2016; Purcell et al., 2014; Skidmore et al., 2010). Geophysical and remote sensing investigations have demonstrated that Antarctic subglacial aquatic environments are diverse. Subglacial lakes range in size from over 10,000 km2, such as Lake Vostok, to lakes less than 1 km2. Observations include lakes and ponds connected in series by subglacial streams, allowing movement of water between lakes at timescales less than a year, saturated sediments that act as deformable beds under flowing glacier ice, and, undoubtedly, groundwater‐type systems (Ashmore & Bingham, 2014; Fricker et al., 2014; Kyrke‐Smith & Fowler, 2014; Mikucki et al., 2015; Siegert, 2016; Wright & Siegert, 2012). All of these subglacial aquatic systems remain poorly understood due to the lack of direct sampling—the logistical challenges associated with drilling through hundreds of meters of ice presents a fi ‐ ‐ ©2019. American Geophysical Union. signi cant barrier. Differences in water residence times, water rock ratios, refreezing at the water ice All Rights Reserved. interface, basal geologic conditions, and biological processes could produce water chemistries that vary

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greatly (Wadham et al., 2010). Besides the obvious ecological interest in these subglacial environments, they can be extremely important in regard to glacier dynamics as well (i.e., Siegfried et al., 2016; Stearns et al., 2008). Although it is not clear currently how the brine affects the movement of Taylor Glacier, the hypersaline nature of the Taylor Glacier subglacial system keeps the water from freezing and thus could impact glacier dynamics (Hubbard et al., 2004). Polythermal glaciers in the Arctic are known to release subglacial waters, particularly at the start of the melt season. These waters have long been studied to better understand the biogeochemistry below ice. Because glaciers in the Antarctic that terminate on land are primarily cold based, there is limited opportunity for the surface collection of any released subglacial materials. Blood Falls, the system we describe here, is currently the only known exception. In addition to the interest in subglacial environments on Earth, the subglacial brine that we describe within this paper is of great interest from a planetary geobiological perspective as well. Pluto has just become the seventh body in our solar system thought to harbor an “ocean” beneath an ice cover (Nimmo et al., 2016). The existence of subcryospheric “oceans” on such bodies within our solar system including Enceladus, Titan, and Europa has generated great interest (e.g., Mitri et al., 2014; Postberg et al., 2011), while most recently evidence of subglacial water has been detected below the ice of the South Polar Layered Deposits on Mars (Orosei et al., 2018). Whether these subcryospheric oceans can sustain life today is a major question in astrobiology and planetary exploration. Interest in the subglacial waters on Earth is also at least partially driven by examining the lifeforms in these environments and describing the structure and function of the ecosystems present. For example, in the recent Scientific Committee on Antarctic Research Horizon Scan exercise, one of the questions posed was “How do subglacial systems inform models for the development of life on Earth and elsewhere?” (Kennicutt et al., 2015). It has been demonstrated that the geochemistry of the subglacial environment influences the microbial diversity in these systems (Mitchell et al., 2013) and that coupled biogeochemical processes can allow microbial growth (Mikucki et al., 2009). The McMurdo Dry Valleys are the largest ice‐free region in Antarctica and represent a location where both the West Antarctic Ice Sheet (WAIS) and East Antarctic Ice Sheet (EAIS) have advanced and retreated throughout the Pleistocene (Denton et al., 1989). The Taylor Glacier, which terminates in Taylor Valley (Figure 1), is one of the easternmost portions of the EAIS. Blood Falls, a hypersaline, iron‐rich subglacial flow, discharges at the terminus of Taylor Glacier in the westernmost portion of Taylor Valley (Mikucki et al., 2009). The source of this discharge is believed to be a subglacial aquifer (Mikucki et al., 2015) several kilometers upstream that flows toward the terminus until it is connected to the surface through a perennial englacial fracture (Badgeley et al., 2017). In this paper we describe for the first time, the biogeochemical composition of this hypersaline subglacial environment acquired through direct clean‐entry sampling techniques.

1.1. Description of the Taylor Glacier Subglacial Brine System Blood Falls was first described chemically by Black et al. (1965). These authors concluded that the existence of the subglacial brine had “far‐reaching consequences” and acknowledged that the extent and origin of the brine were important scientific questions. Taylor Glacier is described as frozen to its bed and surface melt water does not penetrate to the base (Fountain et al., 1999). Robinson (1984) first hypothesized that subglacial brine may be sealed below the lower several kilometers of Taylor Glacier, causing what was described as polythermal glacier behavior. Using ice‐penetrating radar, Hubbard et al. (2004) detected a zone 3–6km upglacier from the terminus that they predicted to contain saturated sediments or ponded liquid. They estimated this zone to be approximately 400–1,000 m in width (Hubbard et al., 2004). More recent work using airborne transient electromagnetic techniques (Foley et al., 2015; Mikucki et al., 2015) has detected wide- spread hypersaline groundwater of less than 0 °C below Taylor Glacier that connects with the west lobe of Lake Bonney (WLB). Taylor Glacier terminates into WLB, a perennially ice‐covered lake whose monimolimnion is hypersaline (Lyons et al., 2000; Lyons et al., 2005). This hypersaline groundwater extends tens of meters below the glacier‐sediment interface (Foley et al., 2015; Mikucki et al., 2015). Evidence of this groundwater brine system was detected as far as ~5.75 km upglacier from the terminus, at which point the instrument reached its limits of detection of ~350 m of ice thickness (Mikucki et al., 2015). This was a significant discovery, as it indicated that an extensive subglacial aquifer extends from underneath the lower Taylor Glacier to both Blood Falls and WLB. Mikucki et al. (2015) conservatively suggest that the subglacial

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Figure 1. Map of Taylor Valley, Antarctica. The clean tent was located 100 m, upglacier from the Blood Falls discharge site. The valley is separated into two main basins by a topographic high near the terminus of the Suess Glacier.

brine volume in the saturated sediments below the lower reaches of the Taylor Glacier is probably the largest volume of liquid water in Taylor Valley, larger than all surface lakes combined. The EAIS has advanced and retreated within Taylor Valley over at least the past 300 kyr (Hendy, 2000) and probably through the Pleistocene and into the late Tertiary (Levy et al., 2012). The monimolimnion of WLB has elevated concentrations of both 4He and 40Ar derived from U/Th and K radioactive decay (Poreda et al., 2004; Warrier et al., 2015). Warrier et al. (2015) and Hall et al. (2017) argue that some input of these gases are due to subglacial discharge from beneath the Taylor Glacier because this input appears at depths of 25–35 m, near the depth of expected subglacial outflow. They have estimated the maximum residence of 4He and 40Ar to be ~250 kyr or during Marine Stage 7 (160–240 kyr ago). The geochemistry of the Blood Falls outflow brine has been well documented with what we are referring to as end‐member, or unaltered subglacial source brine, defined as having Na+ and Cl− concentrations >1 M, 2− 2+ 2+ SO4 ≃ Ca , and Ca > carbonate alkalinity (Lyons et al., 2005). Lyons et al. (2005) and others have proposed that these data suggest the brine may originate either from previously deposited marine salts (e.g., NaCl

and CaSO4) or highly evaporated marine waters that had been resolubilized via subglacial melt. Under the marine water hypothesis, the original seawater providing these salts were speculated to originate from the late Tertiary period when Taylor Valley was a fjord (Lyons et al., 2005). A subsequent change in sea level would have trapped any marine water where it would undergo evaporation and cryoconcentration (Lyons et al.,

2005). However, both high concentrations of H4SiO4 and more radiogenic (than Miocene to present day seawater) 87Sr/86Sr values indicate that any trapped seawater has been modified by the input of chemical weathering products from aluminosilicate minerals (Lyons et al., 2005; Mikucki et al., 2004). Microbiological work on the end‐member Blood Falls outflow brine (originally collected in 1999 and 2004) indicated a unique consortium that facilitates what the authors called a catalytic sulfur cycle (Mikucki et al., 2009). This process is now more widely known as cryptic sulfur cycling (Canfield et al., 2010; Mills et al., 2016) and has been detected in a variety of marine sediments (i.e., Mills et al., 2016). The outflow brine collected for the 2009 Blood Falls outflow study was anoxic but not sulfidic (Mikucki et al., 2009).

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The geochemical/biogeochemical part of our overall work was designed to assess three questions: (1) What is the overall composition of the brine? (2) What is the original source of the solutes? And (3) is the geochemistry of brine conducive to hosting life? Much previous work has documented the geochemistry of the monimolimnion of WLB (e.g., Lyons et al., 2000, 2005; Poreda et al., 2004; Warrier et al., 2015), as well as characterized the biogeochemistry and biology of Blood Falls (Lyons et al., 2005; Mikucki et al., 2004, 2009; Mikucki & Priscu, 2007), and it will not be repeated here in its totality. These early works on Blood Falls and Lake Bonney speculated on the biogeochemistry and the origin of the brine. In this paper, we present new data on the biogeochemistry of the actual brine as well as the measurement of a variety of new ana- lytes (isotopes as well as elemental composition) that provide further information on its derivation. The new data can be compared to, and contrasted with, the previous data to better establish the overall origin of, as well as the connectivity between these unusual waters. Using an in situ, clean‐entry sampling technique designed for exobiological/biogeochemical investigations (Dachwald et al., 2014; Kowalski et al., 2016), we have directly sampled brine from the englacial conduit that is believed to connect the Blood Falls surface feature with its source beneath of the Taylor Glacier (Badgeley et al., 2017). We present a comprehensive geochemical analysis of this englacial brine, compare it to previous Blood Falls outﬂow brine and WLB water analyses, and discuss both its origin and signiﬁcance. Throughout the text, we will refer to this brine as the englacial brine.

2. Materials and Methods The outﬂow of Blood Falls has been routinely sampled since the inception of the McMurdo Long Term Ecological Research site (MCM‐LTER) in 1993, providing over a decade of chemical data. During certain sea- sons, more comprehensive analysis of the outﬂow has been conducted (Mikucki et al., 2004; Lyons et al., 2005; Mikucki et al., 2009; http://www.mcmlter.org). In November 2014, the Taylor Glacier ice was penetrated and the englacial brine sampled at ~17‐m depth within the glacier using the IceMole melting probe (Dachwald et al., 2014; Konstantinidis et al., 2015; Kowalski et al., 2017, 2016). The clean tent, from which the IceMole was deployed, was established approximately 100 m from the terminus, directly upglacier from the visible Blood Falls discharge. The tent and drilling direction was oriented perpendicular to the englacial conduit crevasse that was described in Badgeley

et al. (2017). Englacial brine was pumped to the surface through precleaned (3% H2O2 and nanopure water flushed; Mikucki et al., 2016) polyurethane tubing and collected into appropriate precleaned containers (Teflon or polyethene for most constituents) following established protocols from the MCM‐LTER (Fortner et al., 2011). Trace element samples were filtered in a “clean tent” on the surface of the glacier, while samples for major elements, nutrients, and dissolved organic carbon (DOC) were filtered within 48 hr in the Crary Laboratory at McMurdo Station. All inorganic constituents were filtered through Whatman 0.4‐μm polycarbonate filters, the major ions and nutrients using precleaned plastic filter towers, and the trace elements using precleaned polypropylene syringes. Aliquots for DOC analysis were filtered through acid rinsed and precombusted Whatman GF/F filters into precleaned amber glass bottles. Filtered samples were placed into precleaned LDPE or HDPE containers as described in Welch et al. (2010), Fortner et al. (2011), and Yang et al. (2015). Additional total iron analysis was performed on samples of unfiltered englacial brine and an archived Blood Falls outflow brine sample from 2004. These samples were collected to overflowing in acid‐washed borosilicate glass serum vials and sealed with a butyl rubber stopper. The Blood Falls outflow sample from 2004 had also been “killed” with ~2% benzalkonium chloride. Blanks of our best quality deionized water were processed in the same manner as the englacial brine. In addition, as the IceMole melted through the glacier to the englacial brine, melted glacier ice samples were collected and filtered for later analyses. The filtered englacial brine samples were analyzed for a large number of constituents. These are listed in Table 1 along with the techniques utilized and related references. For major ionic components, the englacial brine samples were diluted with our best ultrapure deionized water (DI) before analysis at McMurdo Station. For trace metals, samples were analyzed upon return to the lab at The Ohio State University with standards that were matrix matched to the salinity of the englacial brine using Na:Cl solutions (German, 2015). For the other analyses listed in Table 1, the samples were analyzed directly as brines using standard calibration techniques.

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Table 1 Methods for Analysis of Englacial Brine Samples Analyte Method Reference/laboratory

Solutes Cl Ion chromatography Welch et al. (2010) Br F SO4 Li Na K Mg Ca Fe Inductively Coupled Plasma Optical This work Emission Spectrometry (ICP‐OES) Co Sector Field Inductively Coupled Plasma Fortner et al. (2011) Mo Mass Spectrometry (SF‐ICP‐MS) U V NO3 +NO2 as N Continuous Flow Analyzer Welch et al. (2010) Si SRP NH3 ClO4 Ion Chromatography‐Tandem mass spectrometry Jackson et al. (2012) Isotopes δ18O Picarro CRDS Leslie et al. (2017) δD δ11B Negative ion Thermal Ionization Leslie et al. (2014) Mass Spectrometry (TIMS) 34 δ S‐SO4 Isotope Ratio Mass Spectrometry Isotope Tracer Technologies, Inc., 18 δ O‐SO4 coupled with Elemental Analyzer Waterloo, Ontario, Canada δ81Br Isotope Ratio Mass Spectrometry coupled with Gas Chromatography 234U/238U Alpha Spectroscopy UNC, Chapel Hill 87Sr/86Sr Thermal Ionization Mass Spectrometry (TIMS) Lyons et al. (2016) Note. CRDS = Cavity Ring‐Down Spectroscopy; UNC = University of North Carolina.

For total iron analysis, samples of unﬁltered englacial brine and Blood Falls outﬂow brine from 2004 were

acidiﬁed with Optima™ HNO3 and trace metal grade HCl and heated to 85 °C to bring precipitated iron fully into solution. Samples were then analyzed using Inductively Couple Plasma Optical Emission Spectrometry at multiple dilutions (i.e., 50 times, 100 times, and 200 times) to investigate possible matrix effects. The relative standard deviations of the multiple dilutions were 1–3%. The relative standard deviation calculated from multiple replicates of diluted brine was <1%. Accuracy of the iron measurements were compared against the National Institute of Standards and Technology Standard Reference Material 1643e (Trace Elements in Water) and TMDA 64.2 (Environment Canada), and results were within 7% and 1%, respectively. The higher value of 7% from the National Institute of Standards and Technology 1643e standard can be attributed to its low certiﬁed concentration, which fell below our calibration range, close to the detection limit of the method.

3. Results and Discussion 3.1. Salinity and Major Ions Data generated for the englacial brine as well as available data for the Blood Falls end‐member outﬂow and WLB are shown in Table 2. The total dissolved solids (TDS) of the englacial brine was 126 g/L. This is close to the observed range of TDS for Blood Falls outﬂow waters (100–110 g/L) collected by the MCM‐LTER team, and within the range of the TDS measured in deepest portions (~24–25 m) of WLB (110–140 g/L; Lyons et al., 2005).

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Table 2 Geochemistry of the Englacial Brine, Blood Falls Outflow Brine, West Lake Bonney Hypolimnion (30 m), and Seawater Blood Falls outflow brine “Enrichment” factors (englacial Englacial West Lake brine/ Analyte Units brine 8 January 2010 24 January 2002 3 December 2004 Bonney (30 m) Seawatera seawater) − Cl mM 2,043 1,320 1,450 1,347 2,172b 559 3.65 − Br mM 2.18 1.76 1.9 1.6 4.41b 0.86 2.53 − F μM 124 —— —53b 70 — 2− b SO4 mM 58.5 50.9 48.7 48 44.2 28.9 — DIC mM —— — 55c 78b 2.08 — −d HCO3 mM 108 —— — ——— Li+ μM 681 429 412 — 748b 25 27.2 Na+ mM 1,683 1,117 1,104 1,106 1,535b 480.6 3.50 K+ mM 30.5 19.4 18.0 19.8 34b 10.5 2.90 Rb+ μM 2.12 1.24 ——1.82e 1.4 1.51 Mg2+ mM 199 135 127 129 344b 54.1 3.68 Ca2+ mM 78.3 54.4 52.2 52.7 54.1 10.5 7.46 Sr2+ μM 681 211 ——575e 92 7.40 Ba2+ μM 0.59 0.2 ——1.46e 0.1 5.90 B mM 2.17 1.34 ——2.00f 0.42 5.17 Si μM 484 — 313 — 177g 100 4.84 Fe (total) μM 476 —— 418 ——— fFe μM 351 —— —71b —— + c g NH4 μM 8.6 — 168 94 303 —— − − NO2 +NO3 as N μM 0.49 — 1.3 ———— SRP μM 1.27 — 0.7 ND 0.3g —— DOC (NPOC) μM 592 ——420c 845g —— h ClO4 nM <5 —— —13 0 — δ18O ‰ −40.41 ——−39.5c −41.68 −0.5 — δD ‰ −324.3 —— —−329.96 —— δ11B ‰ 44.4 46.9 ——48.5i 39.6 — 34 c j δ S‐SO4 ‰ 21.1 20.1 — 21.0 20.6 21 — 18 c j δ O‐SO4 ‰ −1.6 −1.9 — 3.3 −1.8 8.6 — δ81Br ‰ −0.2 −0.23 ——0.44j —— 234U/238U 5.91 — 4.47k — 4.54k 1.145 — 87Sr/86Sr 0.71223 0.71235 ——0.71236l 0.70917 — Temperature °C −7.1 ——−5.2c −4.4 —— Note. Three Blood Falls outflow events are reported, all representing the “end‐member,” or outflow brine. A dash (—) indicates that analysis was not performed. ND indicates that the analyte was not detected. DOC = dissolved organic carbon; NPOC = nonpurgeable organic carbon; SRP = soluble reactive phosphate. aAll seawater data from Pilson (2013). bSample collected from West Lake Bonney at 30 m in 2014. cMikucki et al. (2009); mean of outflow samples collected in December 2004. dBicarbonate calculated by difference. eWitherow and Lyons (2011). fLyons et al. (2005). g(McMurdo Long Term Ecological Research site database, 2011 data). hJackson et al. (2012). iSample collected at 35 m 12 December 2008. jSample collected at 35 m 30 November 2014. kHenderson et al. (2006); collected January 2002. lSample collected at 38 m 25 November 2007.

The hypersaline hypolimnion of WLB and the Blood Falls outﬂow have been described previously as being derived from ancient seawater that is possibly as old as Miocene in age, originating from when Taylor Valley was a fjord (Lyons et al., 2005; Mikucki et al., 2009). The englacial brine shares some similarities with these end‐member Blood Falls outﬂow collections but has distinct chemical characteristics (Table 2). Major ion ratios of the englacial brine are similar to seawater but with subtle and important differences. For example,

the Ca:SO4 and Ca:Cl are greater than seawater, while the Br:Cl, Na:Cl and SO4:Cl ratios are lower than sea- 2+ + 2− water (Figure 2). We have calculated the “excess” Ca , and “deficient” Na and SO4 concentrations, relative to evaporated seawater, by using the current seawater Ca:Cl, Na:Cl, and SO4:Cl ratios and the englacial brine Cl− concentrations. The englacial brine had 40 mM of “excess” Ca2+ and is 71 and 47 mM “deficient” + 2− in Na and SO4 , respectively. These calculations suggest one of two possible mechanisms for the modification to the origin seawater: (1) either there was a potential 2‐to‐1 substitution of Na+ for Ca2+ via cation 2− exchange, and a loss of SO4 by some other mechanism, such as bacterial sulfate reduction, or (2) the pre- + cipitation of mirabilite (Na2SO4·10H2O) from the concentrated seawater has occurred. The exchange of Na

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Figure 2. Major ion molar ratios in Taylor Valley, including a depth proﬁle of West Lake Bonney, and seawater, englacial brine and Blood Falls outﬂow plotted as vertical lines. (a) Ca/SO4, (b) Ca/Cl, (c) Br/Cl; (d) Na/Cl; (e) SO4/Cl.

for Ca2+ is observed in many basinal brines, in both sedimentary and igneous lithologies (Davisson & Criss, 2− 1996), and this might contribute to the SO4 loss via gypsum (CaSO4·2H2O) precipitation. The Blood Falls brine system is devoid of detectable oxygen, and the reduction potential suggests it is suboxic and not sulfidic (Mikucki et al., 2009). However, there is evidence for sulfate reduction to sulfite via cryptic sulfur cycling. While it is believed that the majority of sulfite is reoxidized to sulfate, it is possible that some is lost as sulfide although there is no strong isotopic or molecular evidence for this process.

Although the stoichiometry does not align perfectly (e.g., Na1.5SO4), we favor the second proposed mechan- + 2− + ism, mirabilite precipitation, to explain the loss of Na and SO4 , relative to concentrated seawater Na . Research has shown that at temperatures beginning at −6.4 °C, mirabilite precipitates from seawater (Butler et al., 2016; Marion et al., 1999; Sohl et al., 2010). The temperature of the englacial brine was measured at −7 °C, below the onset of mirabilite precipitation. Mirabilite is a common evaporitic mineral in the McMurdo Dry Valleys region and is also associated with the Blood Falls outﬂow (Keys & Williams, 1981). These deposits, observed throughout the McMurdo Dry Valleys, are from seawater precipitation, as

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Table 3 evidenced by their isotopic composition (Bowser et al., 1970). Here we uti- Trace Metal Concentrations in Englacial Brine and Hypersaline West Lake lized the FREZCHEM model (Marion & Kargel, 2008) to simulate the evo- Bonney water (μM) lution of a brine with the initial major ion geochemistry of seawater that West Lake Bonney has been concentrated to the ﬁnal Cl− concentration observed in the eng- Analyte Brine (depth of highest concentration) lacial brine. At −6 °C, mirabilite begins to precipitate with no accumula- Co 1.8 1.15 (35 m)a tion of ice (i.e., the refreezing of water within the brine to ice). By −8 °C, a Mo 0.046 0.074 (19 m) 3.4 × 10−2 moles of mirabilite have been produced, as well as 9.4 moles of U 0.54 0.20 (“bottom”)b water lost to ice. These results strongly support the notion that mirabilite V 0.02 no data has been lost from the englacial brine and exists somewhere in the a b From Ward et al. (2003). From Henderson et al. (2006). subglacial/englacial system.

Because CaCO3 and Na2SO4·10H2O are the two least soluble binary salts potentially produced in this brine system, slight variations in temperature, ionic composition, and TDS may allow for precipitation‐dissolution of these salts at various times of brine development or even at different locations within the brine system. For example, recently measured subglacial inflow into WLB at 20‐m depth has a similar TDS to the englacial brine. As noted above, the deepest portions of the WLB can have TDS as great as 140 g/L. Perhaps these differences imply that the subglacial brine system has a more heterogeneous composition than we have been able to sample, and there are “pockets” of brine with slightly differing compositions. Another important piece of evidence suggesting that the primary salts of the englacial brine were derived 34 from seawater is δ SSO4 (Table 2). This value (21.1‰) is similar to modern seawater (21‰), as well as Blood Falls outflow brine collected in 2004 and 2010 (21‰ and 20.1%; Mikucki et al., 2009, and this work), and from deep waters of the WLB (20.5‰ and 20.6‰; Anglen, 2005, and this work). We therefore conclude 2− that all these parts of the subglacial brine system have SO4 derived from a seawater source. While the 34 δ SSO4 is unchanged relative to seawater through time, the oxygen isotopic composition of sulfate can provide insights into microbially mediated sulfur dynamics (Lloyd, 1967). Previous measurements of Blood 18 18 Falls outflow brine (Mikucki et al., 2009) reported a depletion δ OSO4 relative to marine δ OSO4 values reported from over the past 5 Ma (Turchyn & Schrag, 2004). Equilibration of brine sulfate with highly nega- 18 tive in situ δ OH2O is unlikely to occur, even on the order of tens of millions of years, particularly given the low temperatures and circumneutral pH of the brine (Chiba & Sakai, 1985). Mikucki et al. (2009) proposed that the depletion was due to catalytic sulfur cycling, also known as net‐zero sulfate reduction or the cryptic sulfur cycle (Mills et al., 2016). When sulfate is reduced to sulfite, it readily equilibrates with in situ water. These authors supported this claim with the characterization of microbial genes that mediate this key reduction step. The cryptic nature of the sulfur cycling requires that all sulfate reduced must be quantitatively 18 reoxidized. Values of δ OSO4 between −10‰ and 10‰ would indicate that some fraction of sulfate was turned over by this process. We observed a value of −1.6‰ in the englacial brine, again within the range pro- 18 18 posed by Mikucki et al. (2009). These authors observed a δ OSO4 of 3.3‰. The findings of δ OSO4 of −1.6‰ support this process and suggest cryptic sulfur cycling has occurred to a greater extent in the englacial brine. 3.2. Addition of Weathering Products to the Englacial Brine Although the englacial brine originates from seawater, it also contains a number of constituents that have

been derived from inputs from the chemical weathering of aluminosilicate minerals. The H4SiO4 concentration of the englacial brine is 484 μM (Table 2). If we assume that the surface ocean has a value of 100 μM − (Pilson, 2013) and the Cl concentration factor is 3.6 (englacial brine/seawater), the observed H4SiO4 is >100 μM higher than expected from cryoconcentration of the original seawater alone. In addition, the 87Sr/86Sr ratios of the englacial brine, Blood Falls outflow brine, and WLB water at 38 m are 0.712230, 0.71235, and 0.71236, respectively (Table 2). Modern‐day seawater has a value of 0.7092, and it was even less radiogenic in the late Tertiary. These data suggest that the original seawater has acquired a significant amount of radiogenic 87Sr. The chemical weathering of glacial environments usually produces Sr higher in 87Sr/86Sr than expected from whole‐rock values as a result of mechanical grinding and preferential dissolution of micas (Anderson et al., 1997), suggesting that glacial advance and retreat within the Taylor subglacial basin has aided in the preferential leaching of the 87Sr from minerals such as biotite. In addition to the Si and Sr addition, the dissolved U concentration of the englacial brine is 0.54 μM (Table 3) with a 234U/238U ratio of 5.91 (Table 2), while Blood Falls outflow brine and associated ice (not known whether it is end‐ member outflow brine or not) has a 234U/238U ratio of 4.47–4.49, and Lake Bonney has a 234U/238U ratio

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of 4.54 (Henderson et al., 2006). Current seawater has a 234U/238U of 1.145 (Henderson et al., 2006). The higher ratios we observed are due to the high mobility of 234U brought about from α recoil in the minerals of the rocks in contact with the brine subglacially, a process that is likely enhanced by glacial grinding and the resulting decrease in grain size, which increases the amount of 234U in waters after α recoil (Robinson et al., 2004). Recent work has suggested that water released subglacially during the beginning of the last major deglaciation signiﬁcantly increased the global ocean 234U/238U ratio (Chen et al., 2016). Our work certainly supports the notion that subglacial drainage can have very high 234U/238U values due to the increased residence time of the water in contact with subglacial materials that have been ground by glacier advance and retreat. 3.3. Rock‐Water Interactions and Modiﬁcations of the Englacial Brine To better understand how the brine has been altered subglacially or englacially relative to a seawater system, we have calculated “enrichment” factors between the englacial brine and seawater of a number of minor elements as well as Ca2+ and K+ assuming that Cl− behaves conservatively in the system (Table 2). As noted above, the enrichment factor of Cl− in the englacial brine is 3.65 relative to seawater. If other elements also behave conservatively, they should have a similar enrichment value. This is not the case, however, for any of these elements (Table 2). K+ and Rb+ are depleted relative to Cl−, while the other measured elements are enriched by varying amounts. Li+ is the most highly enriched at 7.5 times greater than would be expected from the Cl− enrichment alone. Sr2+ and Ca2+ are enriched 2 times more, while Ba2+ and B3+ are enriched ~1.5 times more, compared to Cl−. Lyons and Welch (1997) have previously noted the high concentrations of Li+ and the high Li+:Na+ ratios in Lake Bonney and attributed the Li+ source primarily to chemical weathering inputs. The δ7Li signature of the deep water in Lake Bonney is lighter than seawater indicating a contribution of Li+ from a weathering source (Witherow et al., 2010), and these authors suggested that this source was ~25% of the total concentration, with the remaining 75% derived from a marine source. The deep

waters of Lake Bonney are supersaturated with respect to BaSO4 (Witherow & Lyons, 2011), and the enrichment of Ba2+ in the englacial brine may represent the subglacial solubilization of previously deposited

BaSO4. The hypersaline waters of Lake Bonney have similar B:Cl ratios as seawater (Lyons et al., 2005), and the enrichment of B to Cl in the englacial brine reflects an enhanced source of B3+ from the subglacial basin. Previous work on desert playa and sedimentary basin hypersaline systems have demonstrated that K+ can be removed from solution, either through adsorption and ion exchange, or through diagenetic clay mineral formation (Connolly et al., 1990; Jones et al., 2009). Rb+ also has demonstrated removal during evaporative concentration through incorporation into such minerals as jarosite (Long et al., 1992). Although it is very unlikely that jarosite occurs subglacially in this environment as the pH of the end‐member outflow brine is 6.2, and jarosite only forms in environments of very low pH (1.5–3.0; Jamieson et al., 2005), we suggest that like K+,Rb+ is removed through uptake by clay minerals in the subglacial sedimentary environment. 3.4. Source of the Water to the Englacial Brine As pointed out by both Lyons et al. (2005) and Mikucki et al. (2009), the Blood Falls outflow brine has 18 18 δ OH2O and δDH2O values similar to the local glacier ice. The englacial brine δ OH2O value of −40.4‰ is slightly more depleted than the Blood Falls outflow brine (Table 2), which is within the range observed in modern snow and older (up to 2 kyr) ice measured along a 12‐km transect from the mouth of the Taylor 18 Glacier northwestward (Neumann et al., 2005). Our δ OH2O value is similar to the mean value of ice 9 km from the ice divide (Neumann et al., 2005) and within the shallower depths in Lake Bonney

(Gooseff et al., 2006). Our englacial brine δDH2O value of −324‰ is within the range of Taylor Glacier surface sample ice collected ~13–25 km upglacier from the terminus, corresponding to ~15–11.5 kyr (Aciego et al., 2007). The original seawater was evaporated and lost and the water replaced with local glacier melt, as previously concluded by Mikucki et al. (2009) for the Blood Falls outﬂow brine. 3.5. Other Evidence of Seawater Evolution/Modiﬁcation We have also analyzed the englacial brine for δ81Br and δ11B (Table 2), and there are previously published data available for the δ37Cl and δ11B signatures of the deep water from WLB at 35‐m depth (Leslie et al., 2014; Lyons et al., 1999). All of these data also strongly suggest that these brines are sourced from evaporated seawater. δ37Cl values between −0.9‰ and 0‰, δ81Br values between −0.3‰ and +0.27‰, and δ11B values

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between 40‰ and 70‰ are all consistent with seawater values (Bagheri et al., 2014; Vengosh et al., 1992). Interestingly, the δ37Cl at 25‐m depth in Lake Bonney has a more positive value (0.32‰), perhaps suggesting a contribution of Cl− from halite dissolution (Bagheri et al., 2014; Shouakar‐Stash et al., 2007). The 0.44‰ δ81Br value for the hypolimnion of Lake Bonney may suggest some removal of 79Br within the system (Hanlon et al., 2017).

3.6. Trace Metal Concentrations Unfiltered water from the englacial brine had a total Fe concentration of 476 μM, and filtered englacial brine had fFe concentration of 351 μM. These values are significantly lower than the value of 3.45 mM for Blood Falls outflow brine collected on 3 December 2004 originally reported by Mikucki et al. (2009), who measured total iron concentrations of 3.2–3.76 mM. While not unreasonable for anoxic brines such as the Red Sea (Anschutz et al., 2000), these values are high when compared to our englacial sample. Therefore, we analyzed an archived sample of Blood Falls outflow brine (collected on 14 November 2004) for total Fe by Inductively Couple Plasma Optical Emission Spectrometry and measured a value of 418 μM (Table 2). The 3 December 2004 sample reported by Mikucki et al. (2009) was measured at McMurdo Station using the spectrophotometric ferrozine assay (Viollier et al., 2000) to determine total and dissolved iron by measuring the Fe2+–ferrozine complex prior to and following reduction with hydroxylamine. It is possible that there was excess iron in the 3 December 2004 sample through additional iron oxide entrainment from surface iron deposits. The new measurements are the same order of magnitude, but higher on average, than concentrations detected in Lake Vida brine (256–308 μM; Murray et al., 2012). Thus, we find the originally reported value for 3 December 2004 to be either erroneous or an anomalously high sample. We conclude that total Fe concentrations reported here (418–476 μM) provide a more accurate reflection of the end‐member brine. We recommend more routine measurement of total Fe at Blood Falls and Lake Bonney in the future to better deconvolute the variation in Fe release. Using surface discharge estimates from Keys (1979) of ~2,000 m3, we recalculate potential iron release during discharge events to be on the order of ~47 kg. We also measured filterable (i.e., passing 0.4‐μm filter) Co, Mo, U, and V. Like Fe, Mo and U have been measured in the deep waters of WLB (Henderson et al., 2006; Ward et al., 2003), and all these data are shown in Table 3. Measuring concentrations of trace metals at the nanomolar level is extremely difficult in hypersaline solutions, and finding that the Co, Mo, and U concentrations of the englacial brine are similar to the earlier work published on WLB suggests these data are robust, and these waters have a similar source. In addition, these low values also support recent work indicating that in general, chloride‐rich saline lakes have lower Mo, V, and U than seawater (Mochizuki et al., 2018).

3.7. Biogeochemistry of the Englacial Brine + − − The englacial brine has concentrations of NH4 ,NO2 +NO3 , and soluble reactive phosphate of 8.6, 0.49, − − and 1.27 μM, respectively (Table 2). We suspect that at least a portion of the NO2 +NO3 is an oxidation + artifact of the short storage period of the sample prior to analysis and that it may have originated as NH4 ; thus, we choose to use the term total dissolved inorganic nitrogen (DIN). Its concentration in the englacial brine is 9.1 μM, and N:P ratio is 7.2:1. This DIN value is an order of magnitude lower than the end‐member Blood Falls outflow brine (DIN = 94 μM) measured in 2004 (Mikucki et al., 2009); these authors were unable to detect P in this sample. The DOC (nonpurgeable organic carbon) of the englacial brine is 592 μM, which is higher than the Blood Falls outflow water, (420 μM) but lower than the deep water of WLB (845 μM; Table 2). The cause for these higher values in the monimolimnion of WLB is likely due to contributions of organic carbon from photosynthetic production in the surface water of WLB. Regardless, the concentrations of dissolved N, P, and C measured in the englacial brine, coupled with previous reports documenting the presence of microbial life, support the notion that this environment hosts active microbiological processes, and our findings are consistent with a milieu that is conducive to life.

3.8. Origin and Evolution of the Brine The discovery of subsurface saline waters throughout most of Taylor Valley (Foley et al., 2015; Mikucki et al., 2015) has forced a reevaluation of the origin of the brine system. Although we only have geochemical data from the brines discussed above, the englacial Taylor Glacier brine sampled by us, and the end‐member Blood Falls outﬂow, the extensive elemental and isotopic analyses of these samples has allowed us to

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conﬁrm and/or eliminate a number of potential origins and evolution of the brine system in the western portion of Taylor Valley.

We will evaluate three potential origins: (1) cryoconcentration of past proglacial lake waters, (2) solubilization of evaporite deposits, and (3) the modiﬁcation and cryoconcentration of ancient seawater. 3.8.1. Cryoconcentration of Past Proglacial Lake Waters During the Pleistocene, the current lakes in Taylor Valley waxed and waned as the climate changed between glacial and interglacial conditions (e.g., Hall & Denton, 2000; Hendy, 2000). This process distributed salts from the original waters within the valley soils in response to concentration and precipitation. During the Last Glacial Maximum (LGM), a mega lake, Glacial Lake Washburn, existed throughout the Taylor Valley, including in the Bonney Basin. This mega lake may have reached an elevation of 300 m, as the WAIS blocked any outﬂow into McMurdo Sound (Hall & Denton, 2000). Recent work suggests that the WAIS may have penetrated as far inland as midway through the valley to the current location of the Suess Glacier, which separates the Lake Hoare and Lake Bonney Basins (Toner et al., 2013). These authors contend that the similarity of Na:Cl of both water leached from the soils of the Bonney Basin and WLB bottom waters indicate a common origin, further suggesting that these waters represent the original composition of the LGM mega lake in the Bonney Basin when accounting for cryoconcentration to its present brine composition. The waters in the Bonney Basin would have been physically separated from the rest of Taylor Valley to the east at an elevation of 116 m above sea level (masl) near the current terminus of the Suess Glacier. Hence, Glacial Lake Washburn would have formed two closed‐basin lakes at the time when the

lake levels decreased to this elevation (Toner et al., 2013). The decrease of Ca:Cl and SO4:Cl ratios with elevation below 116 masl is further evidence for closed‐basin evaporation in the Bonney Basin. The Ca:Cl and

SO4:Cl ratios in the water leached from the soils below this 116‐masl elevation are higher than the WLB 2+ 2− deep waters, indicating loss of Ca and SO4 through gypsum and calcite precipitation, as the WLB brine was concentrated into its present form. The Mg:Cl and K:Cl are similar in both the soil leachate and the WLB deep waters (Toner et al., 2013).

Water soluble salts in these Bonney Basin soils have also undergone what Toner et al. (2013) termed “post- depositional” leaching, as the highest Cl− concentrations are usually found at depth in the soil proﬁle (10– 50 cm below the surface), suggesting to them the potential for movement of the initial lake waters downward through what is now the active layer of these soils. There are locations in Siberia and Canada where the downward movement of meteoric water, which was sequentially frozen to form permafrost, left a highly saline residual water (Alexeev & Alexeeva, 2003; Stotler et al., 2011). Stuiver et al. (1981) analyzed carbonate minerals to a depth of 100 m from core material in sediment next to the Commonwealth Glacier in Taylor Valley, collected by the Dry Valleys Drilling Project. Their analysis demonstrated that the carbonate δ18O values were associated with formation water of δ18Oof−28.1‰, where the associated permafrost was −29‰. These data indicate that the formation water was derived from the local alpine glaciers (e.g., Gooseff et al., 2006). Stuiver et al. (1981) suggested that this water was initially “trapped” in the valley when sea level was lower, and then as sea level rose, seawater intruded from below. As noted above, advances and retreats of Taylor Glacier are well documented over time, and the lacustrine carbonates associated with proglacial lakes in what is now central Taylor Valley have δ18O formed from Taylor Glacier‐derived melt (Higgins et al., 2000). These advances of Taylor Glacier and the development of more eastward Taylor Glacier proglacial lakes occurred during Marine Isotopic stages 5a, 5c, 5e, 7, 9, and 11 (Higgins et al., 2000). Foley et al. (2015) have also suggested that permafrost development at depth in eastern Taylor Valley (i.e., Frxyell Basin) is due to the draining of Glacial Lake Washburn and this interpretation is consistent with resistivity data.

We have previously used the FREZCHEM model to evaluate the chemical evolution of the surface (i.e., very low TDS) waters of both Lakes Fryxell and Hoare in the eastern portion of Taylor Valley upon freezing (Lyons et al., 2005). We have used these two solutions as the closest in composition to the original fresh lake water of the LGM high stands. Unlike Lake Bonney, these two lakes do not appear to have input from a brine source developed from current surface waters. Modeled results show that Lake Hoare waters evolve into an

Mg‐Na‐Cl brine with extremely low Ca, SO4, and HCO3 concentrations, very much unlike what we observe in the englacial brine. On the other hand, the modeling results show that freezing of Lake Fryxell surface waters produce a brine that is dominated by Na and Cl, but the Na:Cl and K:Cl ratios are higher and the

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Ca:Cl is lower than WLB or Blood Falls (Lyons et al., 2005). Previous work in permafrost regions has shown that high TDS and depleted δ18O/δD subsurface brines can be formed from infiltration and freezing (Stotler et al., 2011). The geochemistry of the englacial brine in our modeled results are distinct, suggesting that infiltration and freezing of fresh water are not the source of the englacial brine. Because there appears to be no subsurface connection to the aquifer below Taylor Glacier, the brines identified in the central and eastern Taylor Valley may have a different origin and evolutionary path, and thus a different geochemistry than the englacial brine. They could have developed by the influx of fresher, glacial maximum‐aged lake water and subsequently frozen from the surface downward. 3.8.2. Solubilization of Evaporite Deposits The notion of solubilization of evaporite deposits has not been expounded since the earliest days of geochemical investigations in the McMurdo Dry Valleys (Angino et al., 1964). The Na:Cl and Br:Cl ratios all argue against the idea that all the Na‐Cl present in these brines are derived from the dissolution of halite (Lyons et al., 2005). The Li:Cl ratio of the englacial brine also argues against this source (Bagheri et al., 2014), as do the δ37Cl and δ81Br values observed in the englacial brine. Therefore, we conclude that this is not a viable source of the observed salts. 3.8.3. Modification of Ancient Seawater Numerous authors have argued that Taylor Valley and Wright Valley to the north have previously been flooded by seawater. However, few agree on the timing. Sediment core data from both valleys and McMurdo Sound indicate a waxing and waning of both the EAIS, and ice coming from the Ross Sea to the south and east during the late Tertiary, similar to what has been described for the Pleistocene (Ehrmann & Polozek, 1999; Ishman & Rieck, 1992; Levy et al., 2012; Porter & Beget, 1981; Powell, 1981). Webb (1972) suggested that Wright Valley was a fjord 3.4–3.8 Myr ago. Prentice et al. (1993) later suggested that the primary basis for this interpretation, the Mesa Gravel deposit, was 5.5 ± 0.4 Myr old. Currently, Taylor Valley is lower in elevation than Wright Valley, and it is assumed that when Wright Valley was a fjord, Taylor Valley was as well (Porter & Beget, 1981). Others have suggested that latest marine incursion into Taylor Valley was as late as only 100,000–300,000 years ago (Hendy et al., 1977) or as recent as the LGM (Higgins et al., 2000). The former range of age values are in line with the recent ages calculated by noble gas accumulation for the hypoliminion waters of WLB (i.e., 160–240 kyr BP; Warrier et al., 2015). The latter or more recent input of seawater through glacier advance at the LGM should probably also be con- sidered, as Toner et al. (2013) now suggest that the Ross Sea Ice Sheet advance may have gone further west that previously thought—to the present location of the Suess Glacier terminus. Therefore, the seawater could have flowed down slope into the Bonney Basin. Interestingly, the timing of this occurrence is similar to the DIC‐14C age in the bottom waters of WLB at ~27,000 14C years BP (Doran et al., 2014). From the new data presented here the origin of the original water that is now the englacial brine is most likely ancient seawater. The majority of the chemical data point to this source. 3.8.4. Cryoconcentration of the Seawater Multiple lines of evidence, including what is presented above, indicate the end‐member brine that exists under the Taylor Glacier was derived from seawater (e.g., Lyons et al., 1999, 2005; Mikucki & Priscu, 2007). Clearly, we can only speculate on the mechanism controlling cryoconcentration that led to the production of this hypersaline water. Theoretically, the water could have evolved from a proglacial lake that, during a very cold period, lost its ice cover and the fluid was cryoconcentrated much like the bottom of the east lobe of Lake Bonney (Doran et al., 2014). As noted above, it was initially a fjord that became an inland sea as sea level declined. The fjord water was then concentrated to dryness, or very close to dryness, or continually diluted with glacier melt as supported by the isotopic data. Eventually, the brine was overrid- den by the Taylor Glacier as it waxed and waned with the climate in the Pleistocene (Hendy, 2000; Higgins et al., 2000), and in the Tertiary as well (Sugden et al., 1995). Proglacial melt could have been the source of water to solubilize the brine/evaporite deposit during an advance, probably during an interglacial (Swanger, 2017).

4. Conclusions Our new geochemical data from the englacial brine conﬁrms previously published data that the Taylor Glacier subglacial brine is of marine origin that has been extensively altered by extended rock‐water interactions. Recent ﬁndings that show that hypersaline groundwater exists beneath much of Taylor Valley

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(Mikucki et al., 2015) and that cryogenic brines have been important diagenetic fluids in the Victoria Land Basin (Frank et al., 2010; Staudigel et al., 2018) make the understanding of the biogeochemistry and origin, distribution, and movement of these fluids important. Blood Falls is the only known surface manifestation of the recently detected extensive aquifers in the McMurdo Dry Valleys and provides a snapshot of deep groundwater conditions in Antarctica. Prior to using the IceMole for direct collection of the englacial brine, all Blood Falls brine was collected at the surface point of outflow. By comparing these events of surface‐ collected outflow to the englacial brine, we can imagine two possible scenarios to explain the brine geochemistry. First, the brine that is released from the subglacial realm is variable although consistently high in TDS, and the observed variations in salinity, organic carbon, and nutrient concentration and other elemental and isotopic constituents are due to transport of brine from different points within a distributed drainage system. Alternatively, the brine end‐member geochemistry can only be observed during active outflow discharge events. Our englacial sample was collected ~6–7 months after a discharge event was observed. Thus, the englacial brine may represent residual, postdischarge brine that is slowly freezing into the englacial conduit. A major conclusion of this work is that concentrations of fixed nitrogen, phosphate, and DOC are in the range that could support microbial activity. Thus, these data, coupled with the high iron and sulfate concentrations, provide an environment that is condusive to the coupled biogeochemical processes originally described in Mikucki et al. (2009).

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