Paleohydrology of Southwest Nevada (USA) Based on Groundwater 234U/238U Over the Past 475 K.Y

Paleohydrology of southwest Nevada Paleohydrology of southwest Nevada (USA) based on groundwater 234U/238U over the past 475 k.y.

Kathleen A. Wendt1,†, Mathieu Pythoud2, Gina E. Moseley1, Yuri V. Dublyansky1, R. Lawrence Edwards2, and Christoph Spötl1 1Institute of Geology, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria 2Department of Earth Sciences, University of Minnesota, 116 Church St. SE, Minneapolis, Minnesota 55455, USA

234 238 ABSTRACT Understanding this region’s hydrological vari- ity ratio [( U/ U)ACT] is reported here in delta ability over longer timescales is required by notation, where δ234U is the deviation in permil 234 238 Subaqueous calcite deposited on the federal regulations to confidently assess the of ( U/ U)ACT from its secular equilibrium 234 234 238 walls of Devils Hole 2 cave (Nevada, USA) long-term risk of future radionuclide migration value of 1 [δ U = (( U/ U)ACT – 1) × 1000]. represents a unique archive for geochemi- (10 CFR 963). The state of disequilibrium for Under closed system conditions, measured δ234U 234 234 cal variations within the regional aquifer. the naturally occurring uranium isotopes U (δ Um) can be corrected for the decay of ex- Here, we present a 475,000-year record of and 238U in groundwaters has been widely used cess 234U since the time of sample formation (t) initial 234U/238U activity ratios in delta nota- to investigate the modern hydrological param- in order to determine the initial value of δ234U 234 234 tion (δ U0). Results show a range in values eters of the AMGFS and surrounding aqui- (δ U0): 234 from 1851–1616‰. Variations in δ U0 coin- fers (Paces et al., 2002; Neymark et al., 2005; 234 234 λ234t cide with interglacial-glacial cycles over the Bushman et al., 2010; Paces et al., 2013; Paces δ U0 = δ Um e , (1) 234 past 475,000 years. Maximum δ U0 values and Wurster, 2014). Uranium can be added to 234 correspond to the last five glacial intervals, groundwater systems through (in)congruent where λ234 represents the decay constant of U –6 –1 234 during which southwest Nevada experienced dissolution of bedrock and overlying sediments (2.82206 × 10 a ; Cheng et al., 2013). δ U0 234 cool, pluvial conditions. Minimum δ U0 where, in the presence of oxidizing conditions, recorded in secondary carbonates has been used values correspond to interglacial intervals, it readily forms uranyl ions that complex with to investigate past changes in regional rainfall during which this region experienced warm, other ligands (i.e., uranyl carbonate; Langmuir, amount (Robinson et al., 2004), surface run-off arid conditions. We propose that an elevated 1978 and references therein). Disequilibrium (McGee et al., 2012), and infiltration frequency water table during glacial periods inundated occurs when 234U enters waters preferentially and/or amount (Ayalon et al., 1999; von Gunten previously dry bedrock and basin sediments, due to processes linked to the energetic alpha- et al., 1996; Bonotto and Andrews, 2000; Hell- thereby leaching excess 234U accumulated in decay of parent 238U, such as the ejection of 234U strom and McCulloch, 2000; Zhou et al., 2005; these materials. We interpret Devils Hole 2 into pore space during alpha-recoil or acceler- Cross et al., 2015; Maher et al., 2014; Oerter 234 cave δ U0 as a proxy for water-rock inter ated diffusion via alpha-recoil tracks (Rosholt et al., 2016). actions in this regional aquifer, which is et al., 1963; Cherdyntsev, 1971; Kigoshi, 1971; Paleo-hydrological research in Nevada in- 234 ultimately tied to the surface moisture con- Osmond and Cowart, 1976; Osmond and Ivano cludes the study of δ U0 from fossil spring ditions at recharge zones. The mechanism vich, 1992; Stirling et al., 2007). The degree of deposits (Quade et al., 1995), soil carbonates proposed here serves as a testable hypothesis disequilibrium is a function of numerous hydro- (Maher et al., 2014), and dripstone speleo- and possible analogue for future subaqueous geologic parameters, including but not limited thems (Cross et al., 2015). Yet, these deposits speleothem studies in similar hydrogeologic to, groundwater source, recharge amount and are frequently complicated by fragmented settings. Due to its unprecedented duration, frequency, water-rock interactions, and flow preservation, lack of continuous deposition, or 234 the Devils Hole 2 cave δ U0 record provides rate (e.g., Osmond and Cowart, 1976; Andrews difficulties in dating due to detrital contamina- the first paleo-moisture record in southwest and Kay, 1982; Ivanovich et al., 1991; Kronfeld tion. Considering these limitations, we focus Nevada for marine isotope stages 10–12. In et al., 1994; Toulhoat et al., 1996; Johannesson on the calcite coatings found in Devils Hole addition, high-precision δ234U measurements et al., 1997; Roback et al., 2001; Paces et al., and Devils Hole 2 caves, located ~200 m apart of modern groundwaters sampled from 2002; Neymark et al., 2005; Maher et al., 2006; in the discharge zone of AMGFS. Pioneering Devils Hole 2 cave are presented. Bushman et al., 2010; Paces et al., 2013; Paces work by Ludwig et al. (1992) revealed varia- 234 and Wurster, 2014; Priestley et al., 2018). Thus, tions in δ U0 associated with the last six gla- INTRODUCTION studying changes in AMGFS groundwater cial-interglacial cycles. An investigation into 234U‑238U disequilibrium over long timescales the mechanisms driving these variations has so The Ash Meadows groundwater flow system provides valuable insight into the long-term far not been made. Here, we combine recently (AMGFS) is a large carbonate aquifer located hydrological variability of this region. published data from Moseley et al. (2016) with 234 downstream of a potential radioactive waste re- Secondary carbonates, such as spring deposits new high-precision δ U0 measurements from pository site in southwest (SW) Nevada, USA. and speleothems, reflect the234 U-238U disequilib- Devils Hole 2 cave in order to study variations rium of the groundwater from which they precip- between 475 and 5 thousand yr B.P. (ka) and †kathleen.wendt@uibk.ac.at itated. Measured 234U-238U disequilibrium activ- propose a possible mechanism driving these

GSA Bulletin; March/April 2020; v. 132; no. 3/4; p. 793–802; https://doi.org/10.1130/B35168.1; 7 figures; 1 table; Data Repository item 2019299; published online 25 July 2019.

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changes. In addition, we present high-precision Geological and Hydrogeological Setting Timber Mountain-Oasis valley caldera complex measurements of the modern groundwater δ234U overlay the carbonate sequence (Frizzell and sampled in Devils Hole 2 cave. DH and DH2 caves are located in the discharge Shulters, 1990; Fridrich et al., 1994). Quater- zone of the AMGFS, a large (~12,000 km2) nary alluvial and lacustrine sediments fill most STUDY SITE Paleozoic limestone aquifer dominated by the low-lying basins in the region. Cambrian Bonanza King Formation (Winograd Regional groundwater movement and inter- Devils Hole (DH) and Devils Hole 2 (DH2) and Thordarson, 1975). In the mid-Tertiary, basin exchange occur within the lower carbon- caves are located in the Ash Meadows Oasis E-W extension generated widespread normal ate aquifer (Fig. 1), which is alternately confined in SW Nevada (36°25′N, 116°17′W; 719 m block faulting and N-S oriented fractures in the by young and partly indurated sediments in ba- above sea level). The caves are a set of tectonic brittle carbonate sequence (Riggs et al., 1994). sins and unconfined beneath ridges (Belcher and fissures (Riggs et al., 1994) that were later Prevailing NW-SE extension over the past ~10 Sweetkind, 2010). Groundwater flow is con- modified by condensation corrosion (Dubly- million years produced subsequent NE-SW ori- trolled by variations in fracture transmissibil- ansky and Spötl, 2015). The entrance to DH2 ented fractures, resulting in a highly fractured ity and structural heterogeneity (Winograd and is 200 m NNE and approximately +25 m in carbonate rock that provides regional-scale Thordarson, 1975). Additional regional hydro- vertical height from DH. DH and DH2 inter- drainage from high-elevation recharge zones geological units include upper carbonate aqui- sect the regional water table at –15 m (DH) to low-elevation discharge zones through an fers disjointed by basin-fill sediment sequences, and –40 m (DH2) below the surface. Survey- extensive network of subterranean openings lower clastic aquitards, and volcanic tuff aqui ing by the authors suggests identical water (Winograd and Pearson, 1976; Riggs et al., tards (Winograd and Thordarson, 1975). table elevations in DH and DH2 within 8 cm 1994). To the north of our study region, rhyolitic The AMGFS is primarily recharged by infil- uncertainty. and quartz latitic Tertiary volcanic rocks of the tration of snowmelt and rainfall in the upper ele

White River 37°0 ′ 0 ″ N A Yucca Flow System B Flat 37°0 ′ 0 ″ N Yucca ins Mountain Frenchman a Flat nt Amargosa Desert ? u X

Sheep Mo

Spring Mo Y

untai California Nevada ns 36°0 ′ 0 ″ N 36°0 ′ 0 ″ N Devils Hole Caves Specter Range C Springs Amargosa Flat X 1000 m 37°0 ′ 0 ″ N Y

0 m

DH/DH2 Caves –1000 m

(A) Altitude (m)(B) Hydrogeologic units (C) Groundwater δ2 34 U –85–860 Paleozoic-Precambrian Clastic Rocks 7000 860–1280 Paleozoic Carbonate Rocks 2000 500 1280–1640 Paleozoic Undi erentiated Groundwater ow 1640–2030 Mesozoic Sedimentary Rocks direction Quaternary-Tertiary Volcanic Rocks/Tu 2030–3620 Fault; direction of Quaternary Alluvial/Lacustrine Deposits relative movement 36°0 ′ 0 ″ N Figure 1. Southern Nevada, USA study area. (A) Map of the Ash Meadows groundwater flow system (AMGFS) region. Blue arrows indicate regional groundwater flow direction based on Winograd and Thordarson (1975), Thomas et al. (1996), and Bushman et al. (2010). Thicker arrows indicate the principal groundwater flow direction from Spring Mountains to Devils Hole caves. Thinner arrows represent minor groundwater inputs to the AMGFS. (B) Hydrogeological units based on Belcher and Sweetkind (2010). Line X-Y indicates location of vertical transection (vertical exaggeration of ~2.5). (C) δ234U of groundwater sampled within the study area (Thomas et al., 1991; Paces et al., 2002; Cizdziel et al., 2005). Size of symbols scaled in proportion to value. Values range from 500 to 7500‰. Location of Devils Hole and Devils Hole 2 (located ~200 m apart) indicated by yellow star.

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vations of the Spring Mountains (~500 mm a–1; tic rock aquifers is between 1900 and 2100‰. were hand-drilled along the growth axis of the Winograd and Thordarson, 1975; Thomas Groundwater δ234U from the Miocene volcanic halved core using 0.3–0.4 mm carbide-tipped et al., 1996; Winograd et al., 1998; Davisson rock aquifers in the Fortymile Wash and Crater drill bits. Powdered sample sizes ranged be- et al., 1999). Groundwater flows northwest Flat areas, in the upland recharge area of Pahute tween 30–50 mg of calcite. 230Th dating was toward Frenchman Flat, merging with minor Mesa north of Yucca Mountain, and in down- performed at the University of Minnesota, groundwater inputs from the White River flow gradient areas of Amargosa valley are commonly Minneapolis, Minnesota USA. Samples were

system sourced from the mountainous region of between 3000 and 5000‰. Anomalously high digested in HNO3 and spiked with a mixed central Nevada (Fig. 1; Winograd and Thordar- δ234U between 5000 and 7500‰ have been mea- 233U-236U-229Th spike similar to that described son, 1975; Thomas et al., 1996). Additional mi- sured in volcanic aquifers in the Yucca Mountain in Edwards et al. (1987). Spiked samples were

nor groundwater inputs sourced from Emigrant region (Paces et al., 2002). Total U concentra- fumed with concentrated HClO4, co-precipi- Valley north of Yucca Flats have also been pro- tions in groundwater in the AMGFS region are tated with Fe, centrifuged, and loaded into an- posed (Fig. 1; Davisson et al., 1999; Winograd between 0.02 and 10 µg L–1 (Thomas et al., 1991; ion exchange columns following the methods and Thordarson, 1975). From Frenchman Flat, Paces et al., 2002; Cizdziel et al., 2005). described by Shen et al. (2002, 2012). Separate the groundwater follows a high-transmissivity U and Th liquid extracts were measured using zone southward and ultimately discharges in Modern and Past Climate Regimes a ThermoFisher Neptune Plus multicollector– high volume (38,000 L min–1) along a fault- inductively coupled plasma–mass spectrom- controlled spring line located in Ash Meadows SW Nevada is currently one of the driest eter (MC-ICP-MS) via a secondary electron Oasis and ~1 km south from DH and DH2 caves regions in North America. High mean annual multiplier on peak-jumping mode (Shen et al., (Winograd and Thordarson, 1975). The curved temperatures (20 °C), high mean evapotrans- 2012; Cheng et al., 2013). Chemical blanks flow path of the AMGFS reflects the presence piration rates (2600 mm a–1) and low mean were measured with each set of 10–15 samples of an aquitard block at the northwestern end of precipitation rates (15–120 mm a–1) contrib- and were found to be negligible (<100 ag 230Th, the Spring Mountains (Fig. 1). Bushman et al. ute to year-long arid conditions and a lack of <50 ag 234U, <0.5 pg 232Th and 238U). Ages were (2010) proposed an alternative groundwater groundwater recharge to the AMGFS along val- calculated using the 230Th and 234U half-lives pathway (Fig. 1) sourced from recharge in the ley floors Laczniak( et al., 2001). Vegetation at of Cheng et al. (2013). The 234U-238U disequi- 234 Yucca Mountain region that infiltrates the un- valley floors largely consists of desert sage and librium at the time of deposition (δ U0) was saturated zone of volcanic rock before reach- barrel cacti. The higher elevations of the Spring determined by back-calculating the measured ing the lower carbonate aquifer, where it flows Mountains (above 3000 m above sea level) re- δ234U using its associated 230Th age. southward through major north-south oriented ceive more precipitation (mean 500 mm a–1), To measure the δ234U of modern groundwater faults toward Ash Meadows Oasis. Hydro have lower mean annual temperatures (6 °C), within DH2 cave, two water samples were col- geologic conditions have remained static since and support predominately alpine arctic forbs lected at approximately –0.2 and –1.3 m below the Pliocene (Hay et al., 1986). and grasses (Winograd et al., 1998). the water table. 4 L of water were sampled us- Evidence from both DH calcite δ18O and 14C The SW United States, including SW Ne- ing a sterile polyethylene hand pump and sepa- ages from dissolved organic carbon fractions vada, underwent drastic hydroclimate changes rated into 2 L collection bottles at each depth. suggests groundwater transit times of <2000 throughout the Quaternary, as best illustrated by Both pump and collection bottles were soaked years from the Spring Mountains to DH and the repeated expansion and desiccation of plu- in dilute HCl and tested for possible background DH2 caves (Winograd et al., 2006). Due to the vial lakes and wetlands on orbital to millennial contamination prior to collection. Samples were long flow path (>60 km), prolonged residence timescales (e.g., Oviatt, 1997; Springer et al., transported to the Death Valley Park Aquatic time, and the retrograde solubility of calcite, the 2015). Proxy-constrained model simulations Ecology laboratory in Pahrump, Nevada and

groundwater flowing southwest through DH and suggest that a southward displacement and in- acidified using 1 mL of HNO3 per 1 L of water DH2 caves is slightly supersaturated with re- tensification of the Pacific storm track increased approximately two hours after collection. 2 L spect to calcite (SI = 0.2; Plummer et al., 2000). the amount of wintertime precipitation over from each depth were passed through a 0.2 µm Calcite has been continuously depositing upon the SW United States during glacial intervals pore-sized filter. Water sample uranium mea- the submerged walls of both caves in the form of (COHMAP Members, 1988; Oster et al., 2015). surements were performed at the University of dense mammillary crusts over the past 500 k.y. Increased rainfall contributed to an increase in Minnesota. 2 g subsamples, corresponding to at a very slow rate of roughly 1 mm ka–1 (Wino- local moisture availability, defined here as an 6 ng 238U, were spiked with a 233U-236U tracer grad et al., 1992, 2006; Moseley et al., 2016). approximate measure of precipitation minus (Cheng et al., 2013). Uranium was collected fol- The groundwater transit time between DH to evaporation (P–E), which prompted increased lowing chemical methods described by Chen DH2 cave is ~5 years based on parameters out- water table elevations in the AMGFS and sur- et al. (1986) and isotope ratios were measured lined in Winograd et al. (2006). rounding regions (Szabo et al., 1994; Springer on a ThermoFisher Neptune Plus MC-ICP-MS et al., 2015; Wendt et al., 2018). During inter- via a secondary electron multiplier on peak- δ234U of Modern AMGFS Groundwater glacial intervals, a northward recovery of the Pa- jumping mode. Total procedural blanks includ- cific storm track contributed to decreased P–E, ing filtering were found to contribute ~1 pg 238U Saturated-zone groundwater δ234U and total U resulting in low water table elevations (Szabo and <0.3 fg 234U, and were well within instru- concentrations in the AMGFS region are largely et al., 1994; Wendt et al., 2018) similar to today. mental uncertainties. dependent on the rock type of the aquifer (Fig. 1). Waters in Paleozoic carbonate rock aquifers from MATERIALS AND METHODS RESULTS Oasis valley, Amargosa valley, Spring Mountains, 234 234 and the easternmost Nevada Test Site have δ U A 90 cm-long core was drilled from the A total of 100 δ U0 values were calculated between 500 and 3000‰. Groundwater δ234U in hanging wall of DH2 cave at +1.8 m above the along the DH2 core. Results from the first 230 234 Quaternary alluvial and Precambrian siliciclas- modern water table. Samples for Th dating 51 δ U0 values were previously published

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DISCUSSION A 2200 B 2.90 R² = 0.02 Link to Regional Hydroclimate 2150 2.85 234 The DH2 δ U0 time-series reveals a close 2.80

2100 (‰) link with local moisture availability over the past 0 475 k.y. Periods of elevated δ234U values (de- U 0 2.75 fined here as 234U values greater than modern 234 δ 2050

δ values) coincide with periods of increased P–E 2.70 in SW Nevada, as indicated by DH2 water- 2000 table elevations greater than +5.5 m relative to 2.65 the modern water table (r.m.w.t.) between 391 1950 ± 7 and 342 ± 5, between 320 ± 3 and 250 ± 2, 2.60 (‰) and between 218 ± 1 and 157.8 ± 0.7 ka (Fig. 3; 0 00.001 0.0020.003 0.0040.005 0.006 0007

U –1 Wendt et al., 2018). During the last glaciation, 1/[U] (g ng ) 1900 234 23 4 elevated δ U0 values (>1760‰) between 83.1

δ 2.90 and 13.0 (±0.3) ka coincide with increased 1850 P–E in SW Nevada, as indicated by marsh 2.85 and spring deposits in the Las Vegas valley 1800 (Quade, 1986; Quade and Pratt, 1989; Quade 2.80 et al., 1995; Springer et al., 2015), speleothem (‰)

0 R² = 0.02 growth in Pinnacle Cave (Lachniet et al., 2011),

1750 U 2.75 spring deposits in Indian Springs valley (Quade 234 and Pratt, 1989), paleo-ecological reconstruc-

δ 2.70 1700 tions from southern Nevada pack-rat middens (Thompson et al., 1999), and water table eleva- 2.65 1650 tions above +5 m r.m.w.t. in DH2 and DH caves (Szabo et al., 1994; Wendt et al., 2018). During 2.60 234 0246810 12 14 16 18 interglacial intervals, decreased δ U0 values 1600 DH2 DH Growth Rate (mm ka–1) (<1760‰) coincide with periods of decreased P–E, as indicated by water table low-stands (below +1.8 r.m.w.t.) at 410 ± 6, 327 ± 3, 239.2 234 Figure 2. (A) Box plot of δ U0 from Devils Hole (DH) cave (calculated from data by Ludwig ± 1.5, and 128.8 ± 0.4 ka. We argue that DH2 et al., 1992) versus Devils Hole 2 (DH2) cave (this study) southwest Nevada, USA. (B) Upper 234 δ U0 reflects hydrological variability in SW 234 234 panel: δ U0 (n = 97) versus the reciprocal U concentration. Lower panel: DH2 δ U0 versus Nevada over glacial-interglacial timescales. calcite growth rate. The timing and duration of elevated DH2 234 δ U0 is coincident with periods of elevated lake levels (e.g., Ku et al., 1998; Bacon et al., 2006; 1 234 in Moseley et al. (2016) (Table DR2 ). The (0.85%) with the previously published δ U0 Benson et al., 2013; McGee et al., 2012; Ovi- 234 remaining 49 δ U0, U concentrations, and data from DH cave (Fig. 2), which spans be- att, 1997) and increased vadose-zone infiltration 230Th ages are presented here for the first time tween 630 ± 109 ka and 5.2 ± 0.2 ka (Ludwig (Maher et al., 2014; Cross et al., 2015) recorded 230 234 (Table DR1; see footnote 1). Th ages range et al., 1992). Five maxima in DH2 δ U0 are in a diverse array of archives across the wider from 4.89 ± 0.45 ka to 476 ± 11 ka. A single identified at ca. 475 ± 11, 374 ± 6, 278 ± 2, SW United States. Curiously, however, the in- 234 234 calculated δ U0 value was identified as a sta- 185.1 ± 0.7, and 43.2 ± 0.2 ka corresponding terpretation of δ U0 recorded in DH2 speleo- 234 tistical outlier (Q test) and omitted from the to glacial marine isotope stages (MIS) 12, 10, thems is opposite to the interpretation of δ U0 234 234 data set. DH2 δ U0 during this time ranged 8, 6, and 2, respectively. DH2 δ U0 minima recorded in subaerial speleothems in this region from 1851–1616‰. Uranium concentrations occurred at 410 ± 6, 327 ± 3, 239.2 ± 1.5, and (e.g., Cross et al., 2015; Denniston et al., 2007; of DH2 calcite averaged 566 ± 270 (1σ) ng 128.8 ± 0.4 and 4.90 ± 0.05 ka corresponding Shakun et al., 2011; Lachniet et al., 2011, 2014), g–1 over the past 475 k.y. Growth rates were to interglacial MIS 9, 7, 5, and the Holocene, and thus merits discussion. The mechanisms calculated using a linear interpolation. Nega- respectively (Fig. 3). controlling the δ234U in subaerially formed tive rates due to occasional age reversals were The δ234U of modern groundwater col- speleothems (i.e., dripstones) are attributed to 234 omitted. Results show that DH2 δ U0 does lected in DH2 cave is 1762 ± 2‰ and U con- the frequency and amount of surface recharge not correlate with U concentration (R2 = 0.02) centrations were measured as 3.085 ± 0.005 (Cross et al., 2015 and references therein). Be- or growth rate (R2 = 0.02) over the past 475 k.y. µg L–1 (Table 1). DH2 modern groundwater cause 234U is produced in the unsaturated zone (Fig. 2). The range and average value of the δ234U values falls within the observed range at a constant rate, 234U accumulates in soil and 234 234 DH2 δ U0 data agree within mean uncertainty of calcite δ U0 variations. All past and mod- bedrock during periods of infrequent and/or low ern measurements presented in this study recharge amount (i.e., low P–E), such that infil- 1GSA Data Repository item 2019299, uranium- series data, is available at http://www .geosociety are within the range of modern groundwater trating waters during these dry periods contain .org/datarepository/2019 or by request to editing@ δ234U collected throughout the AMGFS region high δ234U values. During periods of frequent geosociety.org. (Fig. 1). and/or high recharge 234U is diluted, resulting in

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1900

1850

1800 (‰) 0

1750

23 4 δ

1700

1650

1600 10 9

8 Wa

7 ter T 6 (r able Elevation .m. w. 5

4 t. ) 4 2 1

0 –1 0 50 100 150 200 250 300 350 400 450 500 Age (ka BP)

234 234 Figure 3. δ U0 vs. water table changes in Devils Hole caves, located in southwest Nevada. DH2 δ U0 (‰) in red with associated 2σ uncertainties (this study) versus DH2 and DH water-table elevation record (black) plotted relative to the modern water table (r.m.w.t.) with 2σ uncertainties (Szabo et al., 1994; Wendt et al., 2018). DH—Devils Hole cave; DH2—Devils Hole 2 cave.

lower δ234U values. Overall, a negative correla- table. The relatively constant U concentration water δ234U (up to 7500‰) values (Paces 234 tion between δ U0 values and P–E is observed and growth rate recorded in DH2 calcite argue et al., 2002). in subaerial speleothems from the SW United against past changes in the chemical or physi- During glacial periods, increased recharge in States (Cross et al., 2015; Denniston et al., cal properties of groundwater (e.g., pH, redox lower-altitude regions (relative to Spring Moun-

2007; Shakun et al., 2011; Lachniet et al., 2011, state, temperature, or partial pressure of CO2). tains) may have affected the hydraulic head and 234 234 2014). Conversely, δ U0 recorded in DH and The synchronicity between DH2 δ U0 and mixing proportion of different groundwater flow DH2 calcite (subaqueously deposited) demon- hydroclimatic variations in SW Nevada argue paths contributing to AMGFS. For example, in- strates a positive correlation with regional P–E. against mechanisms associated with tectonic ac- creased recharge in the northern volcanic ter- Our results clearly demonstrate that previously tivity, as these changes would result in random- rains may have increased this region’s ground- 234 234 described interpretation of speleothem δ U0 ized changes in δ U0 over time. The remainder water input to the AMGFS, thereby elevating 234 does not universally apply to all depositional of this discussion will investigate mechanisms groundwater δ U0. Under this mechanism settings. In order to understand the link between (3) and (4). alone a direct correlation would be expected 234 234 DH and DH2 δ U0 and regional hydroclimate between DH2 δ U0 and P–E in volcanic ter- changes, we propose and discuss the following Changes in Groundwater Source rains north of DH and DH2 caves (e.g. Yucca 234 possible controlling mechanisms. The modern groundwater in DH2 cave Mountain). Instead, DH2 δ U0 reach maxi- (δ234U = 1762 ± 2‰) is largely sourced from mum values mid-glacial cycle (ca. 43.2, 185.1, 234 Mechanisms Controlling δ U0 Spring and Sheep Mountain recharge (Fig. 1; 278, and 374 ka) followed by a gradual decrease Winograd and Thordarson, 1975), whose toward interglacial values despite continued Possible mechanisms controlling ground- groundwater δ234U signatures range from 500 to DH2 water table high stands (Wendt et al., water δ234U in DH and DH2 caves include: 3000‰ (Thomas et al., 1991; Paces et al., 2002; 2018) and high mean annual precipitation in (1) changes in the chemical or physical proper- Cizdziel et al., 2005), typical of carbonate-rock the Yucca Mountain region (Thompson et al., ties of groundwater, (2) changes in groundwater aquifers in this region. Additional groundwater 1999) until the termination of the associated flow rate or path length due to tectonic activity, pathways to the AMGFS from northern regions glacial cycle (Fig. 4). For example, during the 234 (3) changes in groundwater source, and (4) ex- have been proposed (see Study Site section), last glaciation, DH2 δ U0 commenced decreas- posure to previously unsaturated bedrock and/or including inputs originating from volcanic ter- ing toward interglacial values at 43.2 ± 0.2 ka, sediments due to fluctuations of the local water- rains, which show anomalously high ground- despite maximum wet conditions of the Yucca

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Age (ka BP) Termination II and ~30 k.y. prior to Termina- 020406080 100 120140 160 180 200 tion I (Fig. 4). The DH2 δ234U time series cannot account for 1850 –14.75 past changes to the input of southeast-flowing

DH2 δ O (‰VPDB) groundwaters from the White River Flow Sys- tem or Yucca and Frenchman Flats, because 1800 –15.25 18 (‰) these regions contain groundwaters with similar

0 U concentrations and δ234U signatures to Spring

U 1750 –15.75 and Sheep Mountain ranges (Thomas et al., 234

δ 1991; Paces et al., 2002; Cizdziel et al., 2005) 1700 –16.25 (see supplementary information for further 234 discussion). Overall, the timing of DH2 δ U0 1650 decline during glacial periods does not support –16.75 changes in groundwater inputs from volcanic (in proportion to modern values ) YM annual precipitation 1600 terrains as a primary mechanism for recorded 3 234 DH2 δ U0 variations, and suggests that the contribution of excess 234U is sourced locally. 2 A commonly used hydrologic tracer not in-

1 cluded in this study is the strontium isotopic composition. The mean 87Sr/86Sr value for the 0 larger springs at Ash Meadows (accounting 9 for 83% of discharge) is 0.7125 ± 0.0002 (2σ) 8 (Stuckless et al., 1991). Previous studies on the 7 Sr isotope budget of regional groundwater indi- 6 cate indistinguishable 87Sr/86Sr values between 5 the AMGFS carbonate aquifer (0.7084–0.7191) 4 and in the volcanic aquifers in the vicinity of (r.m.w.t.) 3 Yucca Mountain (0.7093–0.7119) (Bushman et al., 2010; Stuckless et al., 1991). Assigning 2 a most probable source of water based on po- 1 tential variations in Sr isotopes recorded in DH 0

DH/DH2 Water Table Elevation calcite would therefore require a more in-depth –1 investigation. 020406080100 120 140 160 180 200 Age (ka BP) Exposure of Previously Unsaturated Bedrock Figure 4. Paleohydrologic changes in SW Nevada over the past 200 k.y. DH2 δ234U with associ- and/or Basin Sediments ated 2σ uncertainties (red; this study), DH2 δ18O (blue; Moseley et al., 2016), Yucca Moun- An alternative mechanism involves the inun- tain (YM) region annual precipitation plotted in proportion to modern annual precipitation, dation of previously unsaturated bedrock and/or such that “3” represents annual precipitation three times greater than modern values (green; basin-fill sediments during periods of high P–E. Thompson et al., 1999), and DH and DH2 water-table elevations (black; Szabo et al., 1994; Wendt et al. (2018) document >9.5 m rises of Wendt et al., 2018). Gray bars indicate estimated brief water-table high-stands as described the DH2 water table during cool and wet glacial in Wendt et al. (2018). Arrows indicate the onset of δ234U depletion during the last two glacial cycles over the past 350 k.y. Considering this, cycles. DH—Devils Hole cave; DH2—Devils Hole 2 cave; VPDB—Vienna Pee Dee belemnite. the AMGFS lower carbonate aquifer can be di- vided into two zones: a lower zone that remains saturated throughout both glacial (high water ta- Mountain region from 27 to 11.5 ka (Thompson enriched δ234U signatures while simultaneously ble) and interglacial (low water table) intervals, et al., 1999) and continuous water table high- depleting the δ18O of discharging waters (and of and an upper zone that is saturated exclusively stands (+5.5 m r.m.w.t.) until 19.85 ± 0.04 ka calcite precipitating from this water). Instead, during glacial intervals. During interglacial in- 234 18 (Wendt et al., 2018). DH2 δ U0 decoupled from DH2 δ O ~40 k.y. tervals, we propose that the unsaturated upper Additional evidence against this mechanism prior to the enrichment of δ18O associated with zone undergoes a build-up of 234U along min- includes the observed decoupling between 18 234 DH2 δ O and DH2 δ U0 during glacial periods. Recharge from the Yucca Mountain re- TABLE 1. RESULTS OF GROUNDWATER MEASUREMENTS, SOUTHWEST NEVADA, USA 234 238 234 234 238 gion that reached the lower carbonate aquifer Collection depth [treatment] U/ U δ U* U conc. U conc. (m) (atomic ×10–6) (‰) (pg L–1) (μg L–1) would have likely mixed with southward-flow- 0.2 [filtered] 151.84 ± 0.111762.2 ± 2.0460 ± 1 3.079 ± 0.005 ing groundwaters sourced from Pahute Mesa, 0.2 [unfiltered] 152.01 ± 0.101765.3 ± 1.9461 ± 1 3.088 ± 0.005 which are depleted in 18O relative to discharg- 1.3 [filtered] 151.79 ± 0.111761.5 ± 2.1461 ± 1 3.089 ± 0.005 δ 1.3 [unfiltered] 151.82 ± 0.111761.9 ± 2.1460 ± 1 3.083 ± 0.005 ing AMGFS waters by 1–2‰ (Davisson et al., Notes: Uranium concentration (conc.) and isotopic composition of groundwater sampled in the Devils Hole 2 1999). An increase in groundwater contribution cave. Samples were collected in February 2016. All uncertainties are 2σ. 234 234 238 from this region is therefore expected to have *δ U = ([ U/ U]ACT – 1) × 1000.

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eral grain and fracture surfaces (Fig. 5). Due to the absence of passing groundwaters, mobile Sediment Glacial Figure 5. Diagram illustrating 234U atoms remain in damaged lattice sites or water table 234 are ejected directly into the neighboring pore the production of mobile U space where they remain associated to mineral Bedrock Interglacial near the surfaces of bedrock surfaces. During glacial intervals, a high water water table fractures. During interglacial table re-exposes the upper zone to oxidizing (low water table) periods, mobile 234U accumulates in unsatu- groundwaters, such that the accumulated mobile Saturated Zone rated bedrock and overlaying 234U is transported downstream. The resulting Unsaturated increase in groundwater 234U concentrations is fracture space (air) sediments, where it remains in ultimately registered in DH2 calcite as increased the damaged crystal lattice sites 234 238 and/or associated to fracture δ U0 without significant changes to U concentrations. α surfaces. During glacial (high Fracture 234 234 water table) periods, a rise in During periods of high P–E, it is additionally surface U U possible that: (1) Quaternary-Tertiary lacustrine Water the local water table trans- α 234 sediments, alluvial sediments, and volcanic tuffs ports the accumulated U NOT TO SCALE downstream. that overlay the primary carbonate rock aquifer Near-surface build-up of would become partially saturated (exact volume leachable 234 U unknown) due to an increased water table elevation and (2) local recharge to the AMGFS may occur in Quaternary sediments and volcanic tuff (MIS 8), between 1825 ± 6‰ and 1828 ± 5‰ reconstructions from Spratt and Lisiecki (2016) filling basin floors. Under this scenario the avail- (MIS 6), and 1822 ± 3‰ (MIS 4–2). Indeed, the which show a close agreement with the tim- 234 ability of mobile U would further increase, as timing of DH2 local Umax associated with MIS ing of DH2 water table fluctuations on glacial- the fraction of mobile 234U that is susceptible to 8 at 278 ± 3 ka agrees within age uncertainties interglacial timescales (Wendt et al., 2018). For preferential leaching increases in proportion to with the timing of water table maxima (+8 m the purposes of this study, we set the interglacial decreasing grain size. The previously unsatu- r.m.w.t.) at 273 ± 2 ka. The timing of DH2 lo- sea level boundary to ≥–5 m relative to modern

rated upper bedrock and overlying Quaternary cal Umax associated with MIS 6 between 197.2 day. Using this, the duration of the last four inter sediments will henceforth be referred to as the and 185.1 ± 2 ka coincides with the timing of glacials are approximated to 16 ka (MIS 11c), “upper zone,” and are assumed constant in water table maxima (+9.5 m r.m.w.t.) at 190.0 9 ka (MIS 9e), 8 ka (MIS 7a–c), and 5 ka (MIS

physical composition over this study interval. ± 0.7 ka. Finally, the timing of DH2 local Umax 5e). By comparing each interglacial duration to

In order to evaluate the compatibility of the associated with MIS 4.2 at 43.2 ± 0.2 ka co- the following glacial Umax value (selecting 1828 234 proposed mechanism to the DH2 δ U0 record, incides with the timing of water table maxima ± 5‰ as absolute local Umax for MIS 6), a posi- we present a simple conceptual model that es- (+9 m r.m.w.t.) at 44.0 ± 1.0 ka (Fig. 4). tive correlation (R2 = 0.98) is observed (Fig. 7). timates the nonlinear response of groundwater (2) According to the proposed model, higher This result supports the hypothesis that longer δ234U variations to water-table fluctuations dur- water table maxima (i.e., H) associated with a ing a glacial-interglacial cycle (Fig. 6). Follow- single glacial cycle would increase the volume ing the end of an interglacial interval, rising of upper-zone saturation and thereby capture U MA X water table elevations during a glacial inception more 234U, which would result in higher ground-

δ 234 would increase the exposure of the upper zone water δ234U. The maximum height of DH2 wa- U

to groundwater and thus increase groundwater ter table in a glacial cycle is similar (>+8 m 0 234 (‰) δ U0. Water table elevations first reach maxi- r.m.w.t.) between MIS 8, 6, and 4–2. An ex- 234 mum height (H) as δ U0 attain peak values ception is the short pluvial interval associated

(Umax). After reaching maximum height, water with MIS 7d (ca. 230–220 ka), during which tH table elevations remain at or near maximum the DH2 water table reached above +3.1 m but Wa

height throughout the remainder of the gla- no higher than +6.5 m r.m.w.t. (Wendt et al., ter

234 234 T

cial interval (tH). Following Umax, δ U0 values 2018). Corresponding DH2 δ U0 values did able Elevation gradually decrease throughout the interval t in not exceed 1748‰ during this time, suggesting H H 234 response to a depletion of mobile U in the up- that lower water table maxima at MIS 7d did not tI per zone. In total, the conceptual model shown saturate a sufficient volume of the upper zone to in Figure 6 resembles the triangle-shaped peaks shift δ234U toward higher values. 234 observed in the DH2 δ U0 time series during (3) According to the proposed model, pro- glacial intervals. longed periods of low water table levels would Time

234 234 234 Three lines of evidence from the DH2 δ U extend the accumulation interval of U within UMA X = glacial δ U δ 0 maximum U t I = duration of interglacial

height of maximum duration of water table time series support this mechanism. (1) Accord- the upper zone such that U values are expected H = t H = max water table at/near maximum height ing to the proposed model, the timing of DH2 to scale proportionally to the duration of the 234 δ U0 maxima (i.e., Umax) is expected to co- previous interglacial (tI). Determining the dura- incide with periods in which DH2 water table tion of the last four interglacials using the DH2 Figure 6. Conceptual model explaining the elevations first reached maximum height (H) water table record is complicated by the low nonlinear response of Devils Hole 2 cave 234 within a glacial cycle. We define DH2 local Umax data resolution during MIS 9e and lack of data δ U0 to water table fluctuations over an values as 1851 ± 32‰ (MIS 10), 1835 ± 13‰ during MIS 11. Instead, we use global sea level idealized interglacial-glacial cycle.

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234 interglacial intervals provide a longer period of timescales. We interpret DH2 δ U0 as a proxy 83.1 and 13.0 (±0.3) ka coincide with periods 234U accumulation in the upper zone, which in for the interaction of groundwaters with previ- of high local water table elevations. Likewise, 234 234 turn results in higher δ U0 maxima during the ously unsaturated bedrock and sediments. The decreased δ U0 (<1760‰) correlate to periods following glacial interval. amount and nature of water-rock interactions in of low local water table elevations at 410 ± 6, Overall, we argue that the timing and ampli- the AMGFS are a function of water table ele 327 ± 3, 239.2 ± 1.5, and 128.8 ± 0.4 ka (Fig. 4). 234 tude of δ U0 variations point to the inundation vations, which in turn reflect recharge amount We propose that elevated water table levels in- of the upper zone as the primary driving mecha- and surface P–E conditions at recharge zones. creased the exposure of upper zone bedrock and 234 234 nism of DH2 δ U0. Future groundwater hy- Intervals of anomalously low and high δ U0 sediments to groundwater during glacial inter- drology and geochemical modeling is required values may therefore shed light on past periods vals, resulting in the leaching of mobile 234U that to test this hypothesis. Budget calculations are of extreme P–E conditions. The lowest recorded accumulated during the previous interglacial 234 largely limited by the unknown surface area of DH2 δ U0 values (1615 ± 3‰) between 128.8 interval. Evidence for this mechanism includes 234 234 bedrock fractures, through which U is con- and 121.4 (±0.4) ka coincide with DH2 water similar timing in maximum DH2 δ U0 and the tributed to the groundwater system. Additional table low stands that reached below modern DH2 water table elevations during glacial inter- major unknowns must also be addressed, such elevations (Wendt et al., 2018) and the highest vals, as well as a correlation between maximum 18 234 as the differential contribution of uranium from DH/DH2 δ O values recorded over the past glacial DH2 δ U0 values and the duration of bedrock and overlying sediments, potential 500 k.y. (Winograd et al., 1992, 2006; Moseley the previous interglacial interval. We interpret 234 past variations in hydraulic head and flow rate, et al., 2016), suggesting exceptionally low P–E DH2 δ U0 as a proxy for the interaction of changes in preferential flow routes over time, conditions in the AMGFS recharge regions as- groundwaters with previously unsaturated bed- and the extent and effect of calcite deposition sociated with MIS 5e. The highest recorded rock and sediments, which is ultimately tied to 234 along fracture surfaces in the aquifer. DH2 δ U0 values (1850 ± 30‰) coincide with the surface P–E conditions at recharge zones. 18 234 the lowest DH δ O values over the past 500 k.y. Due to its unprecedented length, the DH2 δ U0 Interpreting DH2 δ234U (Winograd et al., 1992, 2006), suggesting ex- record provides the first hydroclimate record ceptionally high P–E associated with MIS 10. in SW Nevada to encompass MIS 10–12. The 234 234 Variations in DH2 δ U0 over the past Curiously, modern groundwater δ U and cal- mechanism proposed serves as a testable hy- 234 475 k.y. provide insight into hydrological cite δ U0 during the Holocene have relatively pothesis that may be applicable to future sub- changes in SW Nevada on glacial-interglacial high interglacial values (Fig. 3) despite low aqueous speleothem studies in similar climatic water table elevations and enriched (~–15.3‰) and hydrogeologic settings. 1890 δ18O (Moseley et al., 2016; Wendt et al., 2018). R² = 0.98 This may be due to the partial opening of the ACKNOWLEDGMENTS 1880 MSWD = 0.84 cave (via roof collapse) at ca. 4.5 ka (Winograd This work was supported by the Austrian Science et al., 2006), which potentially introduced sur- Fund project number FP263050 (to C.S.) and by the 1870 face waters that contained higher δ234U due to National Science Foundation project number 1602940 interactions with upper zone sediments. (to R.L.E.). This research was conducted under re- 1860 More importantly, the DH2 δ234U record pro- search permit numbers DEVA-2010-SCI-0004 and 0 DEVA-2015-SCI-0006 issued by Death Valley Na- X 11c vides the first record of hydroclimate conditions tional Park. We thank K. Wilson for assistance in the

MA 1850 in SW Nevada from MIS 10–12. Maximum field and P. Zhang for assistance in the laboratory. 234

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