Earth and Planetary Science Letters 300 (2010) 287–298

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Earth and Planetary Science Letters

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Limited hydrologic response to Pleistocene climate change in deep vadosezones — Yucca Mountain, Nevada

James B. Paces a,⁎, Leonid A. Neymark a, Joseph F. Whelan a, Joseph L. Wooden b,Steven P. Lund c, Brian D. Marshall a

a U.S. Geological Survey, Box 25046, MS963, Denver Federal Center, Denver, CO 80225, USAb U.S. Geological Survey, Stanford University, Stanford, CA 94305, USAc Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA

⁎ Corresponding author. Tel.: +1 303 236 0533; fax;E-mail address: jbpaces@usgs.gov (J.B. Paces).

0012-821X/$ – see front matter. Published by Elsevierdoi:10.1016/j.epsl.2010.10.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 June 2010Received in revised form 1 October 2010Accepted 5 October 2010Available online 3 November 2010

Editor: T.M. Harrison

Keywords:vadose zonepaleohydrogeologypaleoclimateU-series datingsecondary ion mass spectrometryYucca Mountain

Understanding the movement of water through thick vadose zones, especially on time scales encompassinglong-term climate change, is increasingly important as societies utilize semi-arid environments for bothwaterresources and sites viewed as favorable for long-term disposal or storage of hazardous waste. Hydrologicresponses to Pleistocene climate change within a deep vadose zone in the eastern Mojave Desert at YuccaMountain, Nevada, were evaluated by uranium-series dating of finely layered hyalitic opal using secondaryion mass spectrometry. Opal is present within cm-thick secondary hydrogenic mineral crusts coating floors oflithophysal cavities in fractured volcanic rocks at depths of 200 to 300 m below land surface. Uraniumconcentrations in opal fluctuate systematically between 5 and 550 μg/g. Age-calibrated profiles of uraniumconcentration correlate with regional climate records over the last 300,000 years and produce time-seriesspectral peaks that have distinct periodicities of 100- and 41-ka, consistent with planetary orbital parameters.These results indicate that the chemical compositions of percolating solutions varied in response to near-surface, climate-driven processes. However, slow (micrometers per thousand years), relatively uniformgrowth rates of secondary opal and calcite deposition spanning several glacial–interglacial climate cyclesimply that water fluxes in the deep vadose zone remained low and generally buffered from the largefluctuations in available surface moisture during different climates.

+1 303 236 4930.

B.V.

Published by Elsevier B.V.

1. Introduction

Hydrologically unsaturated zones (vadose zones) more than 100 mthick are common in arid and semi-arid regions and preserve regionalclimate variations as changes in rates andcompositionsof infiltratingporewater (Edmunds and Tyler, 2002; Glassley et al., 2002; Walvoord et al.,2004). An understanding of the long-term movement of water throughthick vadose zones is increasingly important as limitedwater resources inthese environments are evaluated in terms of both quantity and quality,and as these environments are being considered as sites for long-termisolation of toxic waste. However, the flow of small amounts of waterthrough thick vadose zones, usually as thin films, is difficult to measure,especially at time scales required for waste isolation (thousands tohundreds of thousands of years). In particular, chronologies of porewaterrecords are poorly constrained and may not extend beyond the last full-glacial episode (10 to 20 ka). In contrast, long and detailed historiesspanning hundreds of thousands of years are common in speleothemsand travertine veins formed in karst environments (Fairchild et al., 2006;

Winograd et al., 1992). These types of mineral records that allowevaluation of paleohydrologic responses to climate have not beenreported fromnon-karst settings due to the apparent absence ofmaterialsthat clearlypreserve thehistoryof unsaturatedflowin thickvadose zones.

Secondary fracture coatings exposed in underground workings atdepths of 200–300 m within Yucca Mountain, Nevada (Fig. 1), offer aunique opportunity to investigate long-termhydrologic responses of athick vadose zone to Pleistocene climate change. Although themorphologies and growth histories are distinct from speleothemsdeposited in karst settings, they formed from similar depositionalprocesses involving downward-percolating vadosewater (Paces et al.,2001; Whelan et al., 2002). An understanding of the hydrologicresponse to climate-induced changes in moisture availabilitythroughout the Pleistocene is critical to evaluations of sites such asYucca Mountain, which was studied as a geologic repository for high-level radioactive waste (Department of Energy, 2008; Levich andStuckless, 2007; Sharpe, 2007; Stuckless and Dudley, 2002, p. 2-6).Deep percolation represents the primary mechanism for release andtransport of long-lived radionuclides. The Yucca Mountain site is nolonger being considered as a viable option for long-term storage ofspent nuclear fuel, in part, because of the perception that the fracturedrock may become saturated with water during wetter climates (e.g.,

Fig. 1. A) Shaded-relief map showing the locations of underground tunnels at Yucca Mountain, Devils Hole, and pluvial lakes in Death Valley and adjacent basins (Lowenstein et al.,1999; Smith and Bischoff, 1997). B) Shaded-relief map of central Yucca Mountain with traces of the Exploratory Studies Facility (black) and East–West Cross Drift (red) tunnels.Locations of secondary mineral coatings used in this paper are shown as blue-filled circles. Locations of coatings sampled for other studies are shown as white-filled circles (Whelanet al., 2008). Block-bounding normal faults are shown as solid gray lines with symbols on downthrown side (Day et al., 1998). C) East–west section across Yucca Mountain showingmajor hydrogeologic units, high-angle normal faults, proposed repository horizon, and static water table (Day et al., 1998).

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NEI Nuclear Notes at http://neinuclearnotes.blogspot.com/2010/03/why-not-yucca-mountain.html; accessed on October 1, 2010).

This paper focuses on determining the last 300,000 years (300 ka) ofopal growth in samples of secondary mineral coatings from fracture andlithophysal cavities present in the deep vadose zone. Because of the veryslow growth rates of these coatings (micrometers per thousand years, orμm/ka; Neymark et al., 2002; Paces et al., 2004), U-series disequilibriumages were determined at spatial resolutions of ~20 μm using secondary-ion mass spectrometry (SIMS). If variations in Pleistocene climateproduced large differences in infiltration and deep percolation, vadosezone minerals are likely to preserve systematic variations in growth ratecaused by fluctuations in solute supply or degree of mineral saturation inpercolatingwater. Importantly, variations in growth rates at depth shouldbe correlated with fluctuations in surface moisture known to haveaffected the region (Sharpe, 2007, and references therein). Such

correlations have been observed in speleothems along with chemicaland isotopic data that reflect climate-induced variations in vadosehydrochemistry (Fairchild et al., 2006).

2. Hydrogeologic setting

Yucca Mountain, located on the eastern side of the Mojave Desertapproximately 145 km northwest of Las Vegas, Nevada, consists of a 1–3-km-thick sequence of gently dipping, ~12.8-million-year (Ma) oldtuffswith a 400–700-m-thick vadose zone (Keefer et al., 2007; StucklessandDudley, 2002). The repositoryhorizonwasproposed tobe located inthe lower of two densely welded and fractured ash-flow tuffs separatedby 25 to N100 m of nonwelded ash-flow tuffs (Fig. 1C). The tuffsconstituting the vadose zone have been grouped into informalhydrogeologic units based on their physical properties (Montazer and

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Wilson, 1984). The moderately to densely welded Tiva Canyon andTopopah Spring welded units (Fig. 1C) have relatively high fracturepermeability and low matrix permeability. The intervening, largelynonwelded to partially welded Yucca Mountain and Pah Canyon Tuffs,along with basal Tiva Canyon Tuff and uppermost Topopah Spring Tuff,have been grouped together as the Paintbrush Tuff nonwelded (PTn)hydrogeologic unit (Moyer et al., 1996)whichhas relatively highmatrixpermeability and low fracture permeability.

Widely accepted conceptual models of infiltration in arid settings(Flint et al., 2001a,b) posit that infiltration is directly related to availablesurfacemoisture, that infiltrationpassing beneath the root zone (zoneofevapotranspiration) recharges to the water table, and that infiltrationfluxes were substantially larger than present-day values duringPleistocene pluvial climate conditions when temperatures were asmuch as 10–15 °C cooler and precipitationwas 1.4–3 times wetter thanpresent conditions (Forester et al., 1999; Quade et al., 2003; Sharpe,2007; Spaulding, 1985; Thompson et al., 1999). Increased surfacemoisture during the last several glacial episodes supported largefreshwater lakes in Death Valley and other basins in the region(Fig. 1A; Forester et al., 2005; Lowenstein et al., 1999; Smith andBischoff, 1997) andmay have produced substantially higher amounts ofinfiltration compared to modern estimates (N100 mm/a versus 0.6–10 mm/a; Faybishenko, 2007). Despite the clear evidence for increasedsurfacemoisture in the past, the response of deep vadose percolation toPleistocene climate fluctuations has remained difficult to quantify.

3. Samples and analytical methods

3.1. Sample description

Secondary mineral coatings consist of 1- to 30-mm-thick irregularcrusts of coarse, sparry calcite often interlayered with lesser clear hyaliticopal (Paces et al., 2001; Whelan et al., 2002). Coatings are present on asmall percentage of open fracture footwalls and cavity floors exposedthroughout the Exploratory Studies Facility (ESF) and East–West CrossDrift (EWCD) tunnels (Fig. 1B). Samples used here are from the upperlithophysal and middle nonlithophysal zones of the Topopah Spring Tuffin the northern part of the underground workings. This area containssome of the thicker,more delicate coatingswith relatively abundant opal;however, coatings in this area are fundamentally similar to othersexposed elsewhere in the ~10 km of tunnels excavated.

The sporadic distribution of mineral coatings, their restriction toslopingfloors rather than ceilings or hangingwalls ofmineralized cavities,and their highly irregular and convoluted textures are interpreted asevidence that water never ponded within lithophysae. Instead, physicalcharacteristics of the deposits are consistentwith precipitation fromfilmsof water that descended through fracture pathways and seeped into air-filled cavities (Paces et al., 2001; Whelan et al., 2002). Precipitation ofcalcite can be caused by changes in temperature as water flowsdownward, by changes in the partial pressure of CO2 within a gas-filledcavity, by increases in Ca concentrations in solution, and by evaporation(Dreybrodt, 1980; Marshall et al., 2003). In contrast, warming ofdescending fluids causes opal dissolution rather than precipitationwhereas loss of water vapor by evaporation results in opal precipitationregardless of dissolved silica species:

SiO2ðOHÞ2−2 þ 2Hþ↔SiO2 þ 2H2O↑as vapor

SiOðOHÞ3− þ Hþ↔SiO2 þ 2H2O↑as vapor

SiðOHÞ4↔SiO2 þ 2H2O↑as vapor

The absence of other silicate phases within secondary mineralcoatings is considered to be evidence that evaporation, rather thansilicateweathering, is the primary cause of opal precipitation. Therefore,

evaporative loss of water from thin films drawn up along the outersurfaces of minerals by capillary forces (Whelan et al., 2005) is capableof driving simultaneous precipitation of both calcite and opal. Thin(b3 mm) coatings of calcite on steep-dipping fractures invariably lackopal and likely formed by geothermalwarming of descending solutions.However, thicker coatings (5–30 mm) on shallow-dipping cavity floorscommonly contain calcite and opal, often with textures indicatingcoeval growth of both phases (Paces et al., 2001; Whelan et al., 2002).The thickest coatings record the most complete history of seepage andgrowthwith basal layers that are 10 Ma or older (Neymark et al., 2002;Whelan et al., 2008; Wilson et al., 2003).

3.2. Analytical methods

Isotopes of Th and U were analyzed by secondary ion massspectrometry (SIMS) using the Stanford-U.S. Geological Survey Sensi-tive High-Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG)following methods described elsewhere (Paces et al., 2004). An 18–40 nA primary beam of 16O− ions was used to sputter and ionize U andTh or their oxides from spots on polished cross sections of hyalitic opal.Individual analyses required approximately 25 min to count ions atmasses 28Si416O7, 234U, 238U, 230Th16O, 232Th16O, 235U16O, 238U16O, andbackground masses. Analyses resulted in steep-walled, flat-bottomedpits approximately 25 μm in diameter and 10 μm deep (see Fig. S1 inonline supplementary material). Measured 234U/238U and 230Th16O/238U16O (equivalent to 230Th/238U) atomic ratios for opal in radioactivesecular equilibrium typically are within analytical uncertainty of valuescalculated using accepted decay constants (see Fig. S2 in onlinesupplementary material). Nevertheless, ratios measured in YuccaMountain opal were normalized to the weighted mean values for thesame ratios measured in a 1.9-Ma-old opal standard (M21277; Amelinand Back, 2006) to correct for systematic instrument bias and to obtainactivity ratios (AR). Measured 232Th abundances in Yucca Mountainhyalitic opal are extremely small (232Th16O/230Th16O atomic ratiostypicallymuch less than 1.0) and essentially all 230Thmeasured in theseopals is derived from in situ decay of 238U. Two-sigma uncertainties forisotope ratios are derived from counting statistics (within-run error)plus external error derived from the reproducibility of frequent analysisof the M21277 opal standard.

Ages based on 230Th/U disequilibrium were calculated using conven-tional iterative 230Th–234U–238U dating equations (Broecker, 1963;Ivanovich and Harmon, 1992; Ludwig, 2003). However, 230Th/U agesolder than ~200–250 ka have large uncertainties due to large analyticalerrors associated with 234U and (especially) 230Th. To take advantage ofthe longer half-life of 234U (~245 ka versus ~76 ka for 230Th), model234U/238U ages (Ludwig and Paces, 2002) were calculated for spots olderthan ~200 ka assuming that 234U/238U AR in solutions at a givensite remained constant through time and equal to values obtained from230Th/U-dating results for younger layers from the same grain.Model-ageuncertainties were propagated using within-run error for the measured234U/238U AR combined with the 2σ range in initial 234U/238U ARcalculated for younger layers.

Concentrations of U were estimated by normalizing 238U16O beamintensities in analyses of unknown opals to values measured forM21277 spots determined in the same analytical session andmultiplying by the median value of 981.5 μg/g for M21277 Uconcentrations reported in Amelin and Back (2006). Errors reportedfor opal U concentrations are large (±30% 2σ) because of theheterogeneous U distributions in the standard. Quantification of Uconcentrations is not necessary for U-series dating.

Cathodoluminescence (CL) images of calcite and opal revealalternating bright and dark bands that reflect growth (Fig. 2). Thislayering is important for tracing internal microstratigraphy and forrecording compositional variations that reflect changes in percolationchemistry or vadose-flow conditions. Profiles of relative pixelbrightness on CL images were made using an image-processing add-

Fig. 2. Cathodoluminescence images of growth-zoned opal in samples HD2055,HD2059, and HD2074 from three different lithophysal cavities showing locations ofspots used to determine 238U16O beam intensities (red circles) and U-series ages(yellow circles). Orange lines trace growth layering from spots to measured opalprofiles.

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in to Adobe Photoshop™ (The Image Processing Tool Kit™, v4.0,Reindeer Graphics, Inc.). To compensate for differences in brightnesslevels between images, pixel values were normalized to themaximumvalue present in each profile (“relative brightness”). Profile distanceswere converted to depths in micrometers using scale bars included onCL images and confirmed by petrographic measurements. The centersof SIMS ablation pits were projected along CL-defined growth layersuntil they intersected profile lines overlain on the CL image. Distancesfrom spot centerlines to the outer surface of the mineral grain,measured in micrometers, were used as apparent microstratigraphicdepths. To evaluate relations between CL brightness and U concen-tration, a mean value of relative brightness was calculated for pixelswithin a 10-μm interval on either side of each SIMS spot centerlineprojected to the CL profile line.

Grain mounts were oriented to minimize angular corrections fordipping layers. Verification that layering intersects the polished surfaceat high angles was made by petrographic examination. However,because polished surfaces are not necessarily perpendicular to growthlayering, measured microstratigraphic depths represent apparentthickness that may overestimate true microstratigraphic thickness. Assuch, calculated growth rates represent maximum values. Correctionsfor layer dip angle would decrease calculated growth rates by less than30% for layers dipping up to 45 degrees from vertical.

4. Results

4.1. Cathodoluminescence intensity and uranium concentration

Yucca Mountain hyalitic opal fluoresces bright yellow–green undershort-wavelength ultraviolet light. This characteristic coloration has beenattributed to activation caused by trace amounts of U that maintains anear-linear O-U-O molecular bond after inclusion in the solid phase(Hanchar, 1999). To test if UO2

2+ also causes CL response in YuccaMountain hyalitic opal, a traverse of 44 SIMS spots across growth layeringin opal sample HD2055 (red circles in Fig. 2) yielded measured 238U16Obeam intensities of 136–13,938 counts per second and relativemean spotbrightness values of 34.2–99.0% (Fig. 3; data in Table S1 of onlinesupplementary material). CL brightness and 238U16O beam intensity arepositively correlated (R2 value of 0.955). U concentrations determined forU-series age analyses also are positively correlated with CL brightness.Spot analyses of other trace elements (Mg, K, Na, Ca, Al, Mn, Fe, Ba) weredetermined to evaluate potential correlationswithU concentrations or CLbrightness; however, no consistent trends were observed (see section onSHRIMP-RG Elemental Abundances in online supplementary material).Therefore, CL intensity in the hyalitic opal is attributed to the amount of Upresent. This allowed conversion of measured CL intensities to Uconcentrations using linear relations determined for each opal grain.Resulting U concentrations ranged from5.4 to 550 μg/g (data in Tables S1and S2 of online supplementary material).

4.2. Age—depth relations

Anaccurate age framework is essential forprovidingapaleohydrologiccontext for these samples. Both 230Th/Uages and initial 234U/238UARwerecalculated for most analyses, as well as model 234U/238U ages for spotsolder than~200 ka (Table S2 inonline supplementarymaterial). Resultingages range from41 to 1290 ka and pass three important geochronologicaltests: (i) ages increase with increasing microstratigraphic depth,(ii) coeval layers have similar ages, and (iii) secular equilibrium 234U/238U and 230Th/238U values are obtained for ancient opal (e.g., 1.9-Ma-oldopal standard 21277 and deepest layers of Yucca Mountain opal).

Relations between age and microstratigraphic depth are remark-ably linear over the last 300 ka and indicate that average opal growthrates are extremely slow, ranging between 0.42 and 1.54 μm/ka(Fig. 4). These rates are consistent with those noted previously forPleistocene opal dated by U-series (Paces et al., 2004) and U–Pb

Fig. 3. Relation between relative mean spot brightness and 238U16O beam intensity forSIMS spots analyzed across an opal profile from sample HD2055 (red circles in topimage of Fig. 2; data from Table S1). Error bars represent 5% of the given value for spotbrightness and 2σ counting errors for beam intensities (if greater than symbol size).

Fig. 4. Relations between microstratigraphic depth and uranium-series age (±2σ) foropal profiles shown in Figure. 2. Numbered points correspond to spots in Figure 2 andanalyses in Table S2. Regression slopes represent growth rates in μm/ka.

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(Nemchin et al., 2006) methods. Given these slow rates of growth, a~20-μm-diameter spot incorporates opal deposited over a 13–48 katime period. Furthermore, spots that include the outermost layers ofopal have 230Th/U ages ranging between 31 and 41 ka (Table S2 inonline supplementary material). Regressions of age versus micro-stratigraphic depth yield high linear correlation coefficients(R2=0.972 to 0.995) and low MSWD (mean square of weighteddeviates, a measure of the ratio of observed scatter of points from thebest-fit line to the scatter expected from assigned errors; Ludwig,2003) values (0.46 to 0.71), both indicating that long-term averageopal growth rates in individual grains remained essentially constantover the last 300 ka. The relatively large age uncertainties for spotsthat include layers formed over substantial time spans allow for thepossibility of shorter term (less than ~10 ka) variations in growthrates. Nevertheless, deviations from linear growth are not apparentover the last 300 ka in these profiles (Fig. 4) even though layers spanseveral climate cycles thought, on the basis of hydrologic modelingstudies, to have resulted in 10- to 100-fold variations in infiltration(Faybishenko, 2007).

Unlike opal, U concentrations in Yucca Mountain calcite are toolow for SIMS dating. However, opal interlayered within calcite (Fig. 5)can be dated. Results from the tips of several calcite blades yieldaverage calcite growth rates of 0.84 and 1.21 μm/ka (Fig. 6). Theserates are the same as those obtained for opal-only profiles, and withMiocene growth rates determined for cm-thick calcite-dominatedcoatings from the same vadose horizon (Neymark et al., 2002). Eventhough opal is typically present as discrete patches within calciteblades, textural relations along lateral opal/calcite contacts as well ascompositional banding at the same spatial scales in both phases areinterpreted as evidence of coeval growth (Paces et al., 2001; Whelanet al., 2002; Wilson et al., 2003). Therefore, growth rates determinedfrom opal profiles are applicable to calcite deposited over the sametime periods.

5. Hydrologic responses to climate change

5.1. Continuity of depositional records

The ability of the dating method to capture and resolve detailedgrowth histories is important for using these records to evaluatehydrologic responses to Quaternary climate. In particular, thepresence of non-zero age intercepts for regressions shown in Figure 4implies that opal was not deposited on these grains during the last 24to 33 ka. A depositional hiatus, which includes both the wettest and

driest conditions during the last 30 ka, may be an indication thatwater did not seep into the cavities over this time period. Therefore, itis important to understand how faithfully the secondary mineralsrecord a history of percolation and seepage in the vadose zone.

Although the calcite-opal coatings formby general outward growthfrom basal layers into open cavities, detailed growth histories can becomplex and difficult to deconvolute. Microstratigraphic discontinu-ities within opal have been observed from textural relations whereopal layers separated by substantial thicknesses of calcite can be tracedon CL images back to areas lacking the intervening calcite. Measuredage-depth relations in these cases yield very different growth ratesdepending on whether the intervening calcite is included or excluded

Fig. 5. Cathodoluminescence images of the tips of secondary calcite blades from samples HD2310 (A) and HD2313 (B) containing inclusions of opal formed on growth surfaces.Orange lines trace growth layering from spots dated by SIMS (yellow circles) to measured calcite+opal profiles used to calculate mineral growth rates shown in Figure 6 (data inTable S2).

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(i.e., where the profiles are measured). In the several cases wherethese types of profiles were dated, resulting ages indicate depositionalhiatuses on the order of several hundred thousand years. Shorterhiatuses might not be readily recognized.

Non-zero age intercepts are also dependent on where the origin ofa profile is located. In all three cases shown in Figure 2, outermostsurfaces of opal hemispheres where chosen for zero depths. However,the opal grain in sample HD2059 (middle image) has approximately50 μmof calcite lying on top of the outermost layer of dated opal (lightgray material above spots 2 and 6). Given the 1.54 μm/ka growth ratefor this opal, the youngest period of calcite deposition matches the33 ka hiatus missing from the HD2059 depth profile shown inFigure 4. Nevertheless, opal deposition appears to have ceased onthe HD2059 grain after about 33 ka for crystallographic or chemicalreasons that are not fully appreciated, although calcite depositionappears to have continued until recently.

If non-zero age intercepts from dated opal profiles truly reflected anabsence of seepage into these cavities, all opal growth should haveceased. Therefore, no opal younger than about 24 ka (X-intercept inbottom plot of Fig. 2) should be present on outer surfaces of opalhemispheres from sample HD2074. However, the outermost several μmof opal from 6 other hemispheres from this same cavity have ages

determined by in situmicrodigestions that range from 4±1.9 to 11.6±0.8 ka (Table 5 of Paces et al., 2004). Furthermore, previous efforts todetermine age-depth relations for two opal hemispheres from HD2074resulted in age intercepts of 8±23 ka and 8.9±8.3 (determined bySIMSdating; Fig. 9 of Paces et al., 2004), and 7.7±6.3 ka (determined byin situmicrodigestion dating; Fig. 13 of Paces et al., 2004). Cumulatively,these results indicate that seepage into the HD2074 cavity did not stopduring the current interglacial climate; however, they also indicate thatnot all opal grains represent a simple and continuous record of seepagehistory.

5.2. Numerical simulations of opal growth rates

A series of simple numerical simulations of opal growth and spotanalysis were constructed to evaluate the sensitivity of the SIMS age-profiling methods used in this study (see section on Simulations ofOpal Growth Rates in online supplemental material). In thesesimulations, opal layers were deposited in 1-μm layers at constantgrowth rates from seepagewater with constant 234U/238U AR. U-seriesisotopic compositions were calculated for each layer assuming closed-system evolution since the time that each layer was formed. Ages for ahypothetical SIMS analyses were then simulated by averaging the

Fig. 6. Age-depth relations for profiles of the outermost layers of two secondary calciteblades containing interlayered opal shown in Figure 5. Numbered points refer to spotsshown in Figure 5 and data in Table S2. Errors represent 2σ uncertainties.

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isotopic compositions of layers included within a 25-μm-wide “spot”.Growth rates, hiatus durations, data density, and U concentrationswere varied to evaluate the effects on simulated profiles.

Simulations of a continuously growing layered opal indicate that the230Th/238U and 234U/238U values obtained using a 20-μm-diameter spotyield a close approximation of the true average age for the layersanalyzed. 230Th/U ages calculated using weighted average isotopiccompositions (layers constituting the central part of the spot having thegreatest influence) have a small, but systematic bias toward youngervalues due to the non-linear effects of radioactive decay (Neymark andPaces, 2000; Neymark et al., 2000). However, these effects are typicallynegligible relative to the 10% or larger analytical uncertainties obtainedfrom ion microprobe counting statistics (Fig. S9 of online supplementalmaterial). Theseeffects are greatest for young samplesdeposited at slowrates (b0.5 μm/ka).

Results of simulations that included systematic depositionalhiatuses (i.e., 10 to 50 ka periods of non-deposition out of every100 ka) indicate that detailed growth histories are likely to beobscured at slow rates of growth (b1 μm/ka). Increased data density(i.e., overlapping spots) improves the chances of identifying growthhiatuses. However, for simulations using growth rates less than 1 μm/ka, it is not possible to discriminate between (1) episodic depositionat higher growth rates interspersed with hiatus of 20 ka or less, versus(2) continuous growth at slower time-averaged growth rates (Fig. S12of online supplemental material). Deposits formed at higher growth

rates between 1 and 2 μm/ka are more likely to discriminate betweenthese two scenarios and yield accurate growth histories, especially ifspots are closely spaced.

5.3. Age-calibrated uranium profiles

Regardless of the geochronological limitations and possible gaps inthe depositional record, the high spatial resolution (micrometer-scale) of the distinct CL banding present in vadose zone opals capturesdetailed compositional information at millennial or shorter timescales. In order to evaluate whether the variations in U concentrationcausing this banding are dependent on cavity-specific conditions ormore site-wide causes, profiles need to be calibrated using theavailable age information. To do this, ages were assigned to each pixelof a CL profile using the slope and intercept values derived by linearregression of age versus depth data (Fig. 4). Resulting age-calibratedprofiles were checked against age-depth-intensity relations forindividually dated horizons to ensure that ages for dated SIMS spotswere honored. Overall uncertainties for ages estimated for CL profilesare based on uncertainties calculated for regression slopes.

Resulting age-calibrated U profiles have several distinct U-concentration peaks present between 40 and 50 ka, 120–140 ka,230–280 ka, and 300–350 ka (Fig. 7). The timing of peaks is consistentamong profiles from different cavities and generally correlate with theprecisely dated δ18O record of regional paleotemperature preserved inDevils Hole calcite (Winograd et al., 1992, 2006), located approx-imately 45 km south–southeast of Yucca Mountain (Fig. 1A). For eachopal profile, four primary peaks (labeled 1–4) and four secondarypeaks (labeled a–d) were assigned ages at either their maximumvalues or midpoints of peak width (Fig. 7). The calcite δ18O profilefrom Devils Hole was treated similarly using peaks with higher δ18O(warmer climate). Mean ages for high-U opal layers closely correlatewith the record of warm interglacial climates, matching ages of high-δ18O peaks from the Devils Hole profile within b10 ka (Table 1). Theclose correspondence between individual opal profiles and thecontinuous record of calcite deposition at Devils Hole is viewed asevidence that these opal grains did not suffer depositional disconti-nuities long enough to compromise the climate-proxy record.Discrepancies in timing between peaks in the three opal profiles aremost noticeable for the oldest, least reliably dated layers, particularlyfor profile HD2055, which has the poorest geochronological con-straints of the three.

5.4. Time-series analyses

Astronomical forcing has been identified as a likely cause foroscillatory cycles of climate change throughout the Pleistocene(Milankovitch cycles). Paleoclimate records commonly have periodi-cities of 100, 41, and 24 to 19 ka corresponding to the Earth's orbitalparameters of eccentricity (departure from a circular orbit), obliquity(angle of tilt of the pole), and precession (rotation of the pole about itscentral axis) (Muller and MacDonald, 1997). Time-series analysis(spectral analysis) of paleoclimate signals incorporating these types ofperiodic features will yield spectrums with a small number offrequencies that can be related to orbital periods (Muller andMacDonald, 2000).

Time-series analyses were made of age-calibrated opal U profilesusing both the multi-taper method (MTM) and maximum entropymethod (MEM). Both methods evaluate total time series, but estimatespectral content in different ways. In addition, both methods assumethat the SIMS-derived ages used to calibrate CL brightness profiles aretrue ages — no “tuning” of the age profiles was performed prior toanalysis. Therefore, only the best-dated profiles were tested byspectral analysis.

Results of time-series analyses show spectra with strong internalconsistency and frequencies indicating periodicities greater than 10 ka

Fig. 7. Age-calibrated uranium concentration profiles for opals shown in Figure 2 alongwith calcite δ18O profile for the Devils Hole climate record (Winograd et al., 1992,2006). Estimates of 2σ uncertainties are based on errors calculated for slopes in Figure 4and are shown as error bars for peaks with ages at approximately 50 and 350 ka. Peaksfor uranium concentration and calcite δ18O are sequentially labeled from 1 to 4 forprimary peaks, and a–d for secondary peaks and their ages are given in Table 1.Interglacial periods based on marine oxygen isotope stages are shown as gray bandslabeled 3, 5e, 7e, 9c, and 11c in italics (Winograd et al., 1992).

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(Fig. 8). Themost prominent spectral peak in all profiles has a frequencyof approximately 1×10−5 cycles a−1 corresponding to a periodicity of100 ka (Fig. 8). This is true for both MTM and MEM methods, butspectral peaks are better defined andmore stable usingMEM results. Inaddition, higher frequency peaks are present indicating periodicities atshorter time scales. The most prominent higher frequency spectralpeaks fall between 2.19×10−5 and 2.55×10−5 a−1 corresponding toperiodicities of 46–38 ka. These peaks bracket the 41,000-yearperiodicity attributed to orbital obliquity. Additional high-frequencyspectral peaks at 4.2×10−5 to 4.9×10−5 a−1 353 (24–20 ka) are closeto periodicities expected from orbital precession.

The relatively large uncertainties in the non-tuned ages used forspectral analyses limit the abilities of these profiles to define accurateorbital periodicities. However, the two dominant peaks in the spectrafrom the best-dated opal profiles are similar to spectral patternsobserved in the Devils Hole calcite δ18O record (Winograd et al.,2006). Although the spectral analysis of YuccaMountain opal does notconclusively demonstrate astronomical forcing as the cause ofvariations in U concentration, observation of the 100- and 41-kaperiods in the best-dated profiles is consistent with the presence of aclimate signal deep within the Yucca Mountain UZ.

5.5. Deep vadose zone responses to climate change

The correlation between increased U concentrations in opal andinterglacial stages (along with cyclic variations of Mg concentration insimilar-aged calcite; Wilson et al., 2003) are consistent with greater

Table 1Ages of peaks in uranium concentrations labeled on the opal profiles shown in Figure 7.

Peak label Age of profile feature (ka)

1 a 2 b

HD2055 53±13 88±21 127±30 155HD2059 41±3 77±5 131±9 156HD2074 51±5 83±8 122±12 167Weighted mean 44±14 79±4 128±7 159Devils hole δ18O 47±7 81 129±11 163

Uncertainties for individual peaks are calculated from 2σ slope uncertainties reported for relevel and include an additional weighting factor (Student's t times the square root of themeaindividual errors. Ages for Devils Hole are derived from the data given in Winograd et al. (1duration of warm periods based on the widths of the dark bands shown in Figure 7.

evaporative concentration of solutes in soil and fracture water duringwarmer, drier climates. Profiles of calcite abundance and δ18O in theYucca Mountain subsurface indicate that the effects of evaporationextend into the fracture network to depths up to 100 m (Whelan et al.,2002). Studies of other thick vadose zones have recognized theimportance of subsurface evaporation and upward vapor fluxes, aswell as the long periods (10–100 ka) required for equilibration tonear-surface conditions (Glassley et al., 2002; Walvoord et al., 2002,2004).

Because of the strong climate signal observed in vadose zone opalformed over the last 300 ka, mineral growth rates might be expectedto fluctuate along with climate-induced changes in surface moisture.Studies of speleothems from North America, Europe, and Australiahave found that faster growth rates are correlated with Pleistoceneperiods of increased surface moisture (Ayliffe et al., 1998; Baker et al.,1993; Musgrove et al., 2001). Growth rates in modern speleothemsalso were shown to correlate well with rainfall and excess water (i.e.;drip rate; Banner et al., 2007; Genty and Quinif, 1996). Althoughgrowth rates of Yucca Mountain minerals are ~100,000 times slowerthan speleothems in karst environments (Fairchild et al., 2006),calcite precipitation is likely to be controlled by similar processes inboth settings. Consequently, estimates of water-to-calcite volumeratios for the deep Yucca Mountain vadose zone are similar to valuesreported for speleothems and laboratory experiments where bothwater and calcite volumes are known (Marshall et al., 2003).

Calcite and amorphous silica have remained abundant in soils atthe surface of Yucca Mountain throughout at least the middlePleistocene (Section 10.4 of Ludwig and Paces, 2002). Therefore, it islikely that water infiltrating into the vadose zone has been chemicallysaturated with respect to both phases over the last 300 ka. Thislikelihood is supported by data from modern water seeping fromseveral fractures in shallow parts of the ESF tunnel during the wetwinter of 2005 as well as water perched in the deep vadose zone withδ13C-corrected radiocarbon ages of 2 to 7 ka (p. 37 of Yang et al.,1996), all of which have compositions that were saturated withrespect to calcite and silica gel (Oliver and Whelan, 2006; Fig. 36 ofPaces et al., 2001). Furthermore, effects of substantial variations insurface temperature during the Pleistocene are likely to be negligiblewithin individual cavities at depths of 200 to 300 m.

Because both saturation indices and temperature likely remainedconstant in the deep vadose zone at Yucca Mountain, percolation fluxand associated seepage into cavities is interpreted to be the dominantfactor controlling rates of mineral growth. Thicker coatings havehigher growth rates and are a likely consequence of greater and morecontinuous average seepage fluxes. Thinner coatings have lowergrowth rates and are interpreted to have formed in cavities thatreceived smaller or less frequent seepage fluxes. Water-film thicknessand flow velocity are likely critical factors that explain differencesbetween thinner, opal-poor coatings lining steeply dipping fracturescompared to thicker, opal-rich coatings flooring shallow dippinglithophysae. Within individual lithophysal cavities, water filmssupplying solutes to outer mineral surfaces may only represent a

c 3 d 4

±37 189±45 269±64 298±71 350±84±10 192±12 232±15 269±17 307±20±16 189±18 230±22 294±29 349±34±8 191±10 233±12 277±36 319±58

193 238±13 287 328±10

gressions in Figure 4. Uncertainties for weighted means are given at the 95% confidencen square of the weighted deviates; Ludwig, 2003) if a set of peaks exhibit scatter beyond992, 2006) with uncertainties for major interglacial periods representing approximate

Fig. 8. Results of time-series spectral analyses for four well-dated CL profiles of opal from Yucca Mountain. Age-calibrated CL profiles were analyzed using the maximum entropymethod with several different filters (section 3.8 of Muller and MacDonald, 2000). Dashed vertical lines shown at 100-, 41-, and 23 ka represent cycles attributed to majorastronomical forcing factors (eccentricity, obliquity, and precession, respectively). CL images for opal samples are shown in Figure 2, except for HD2074, IP1, grain #1 (not shown).

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fraction of total water flux flowing through that cavity. Nevertheless,the presence or absence of water films as well as the thicknesses ofwater filmswill remain proportional to total seepage flux. Therefore, ifclimate change produced fluctuations in percolation and seepagefluxes in the deep vadose zone, mineral growth rates also should havefluctuated.

Alternatively, mineral growth rates may be limited by the rates ofCO2 and water vapor removal. Although gas loss is an importantmechanism for deposition, a water-limiting mechanism for mineralgrowth is considered more likely for several reasons. Air permeabilityin the deep vadose zone would tend to decrease during pluvialclimates if increased percolation filled fractures and reduced theoverall air circulation within the deep vadose zone. As a consequence,mineral growth rates would likely be lower during wetter periods ifgas flow was the driving factor for mineral precipitation. In addition,age-calibrated profiles from different cavities are strongly correlated(Fig. 7) indicating that water films were closely connected to gravitydriven percolation regardless of the different geometries involved. Ifgas-flow was the dominant factor limiting mineral precipitation,coherency between profiles from different cavities would be lesslikely given the variety of fracture geometries affecting gas perme-ability. Finally, the dependency of mineral precipitation on waterrather than gas fluxes is supported by geochemical and isotopic datafrom the host rocks themselves. Whole-rock samples subjacent tolithophysal cavities that host 10–30-mm-thick mineral coatings showgreater U losses and 234U/238U disequilibrium than rock samplessubjacent to cavities hosting little or no secondary minerals (Paces etal., 2006), reflecting greater degrees of water-rock interactionassociated with more-mineralized cavities.

Despite the strong evidence for a meteoric source of vadose zonewater, the effects of large variations in effective surface moistureknown to have occurred throughout the Pleistocene are not apparentin these deep vadose-mineral records. Although the available spatialand geochronological resolutions do not allow detailed growthhistories to be defined at century or even millennial time scales,these data are consistent with more-or-less uniform percolation andseepage fluxes at depth despite the large differences in surface-moisture availability between glacial and interglacial time periods.Variations in surface moisture over the time period recorded by opalprofiles is manifested by the presence or absence of pluvial lakesthroughout the region, including a very wet penultimate glacial cycle(marine oxygen isotope stage 6) that resulted in pluvial Lake Manlyreaching depths up to 180 m at between ~180 and 140 ka followed bya long, warm interglacial period (stage 5e, Fig. 7) from about 130 to110 ka that allowed Lake Manly to evaporate (Forester et al., 2005;Lowenstein et al., 1999). These records of climate change form thebasis for large fluctuations in model infiltration simulated betweenglacial and interglacial climates (Faybishenko, 2007).

The uniform rates of mineral growth within the deep vadose zoneat YuccaMountain are interpreted as strong evidence that the amountof water flowing through the deep vadose zone was buffered fromvariations in surface moisture and shallow infiltration. Hydrogeologicprocesses contributing to this buffering may include: down-slopediversion of water at the contacts between colluvium andwelded TivaCanyon Tuff and between the Paintbrush nonwelded tuff and theunderlying welded Topopah Spring Formation (Fig. 1C) as firstproposed by Montazer and Wilson (1984); increased evapotranspi-ration by plant communities that adapt to cooler, wetter climates;

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reduction of water fluxes in shallow fractured tuffs by evaporationand upward water flux; uniform release of percolation stored innonwelded tuffs overlying fractured repository units; and reductionof deep fracture flux by evaporation and/or imbibition into theweldedtuff matrix of the deep vadose zone. These processes, most likelyworking in combination, may be effective at limiting the amount ofwater that infiltrates into the subsurface, reducing the percolation fluxas it migrates downward, and buffering the percolation flux thatenters into the deep vadose zone.

5.6. Meteoric versus upwelling water sources

Age-calibrated profiles of U concentration in opal that correlatewith Pleistocene regional climate variations demonstrate that waterflowing through the deep vadose zone was derived from a meteoricwater source. This conclusion strongly contrasts with recentlyreported interpretations of measured hydrogen and calculatedoxygen isotope data for fluid inclusions in calcite coatings fromsimilar depths within Yucca Mountain (Dublyansky and Spötl, 2010).Those authors concluded that an apparent enrichment of δ18O inwater from calcite-hosted fluid inclusions (calculated using measuredδ2H values, fluid inclusion homogenization temperatures, and theassumption of O isotopic equilibrium between calcite and water)required water/rock exchange and a deep-seated hydrothermal watersource. However, elevated temperatures are only applicable to theoldest parts of themineral coatings (Whelan et al., 2008;Wilson et al.,2003): secondaryminerals formed in the last 2 to 4million years weredeposited under modern thermal conditions. If the isotope measure-ments reported by Dublyansky and Spötl were from inclusions formedduring this younger time frame, then temperature corrections areinappropriate. Without temperature corrections, measured δ2H andδ18O values (Table 1 of Dublyansky and Spötl, 2010) scatter on eitherside of the global meteoric water line.

Other than the well-documented hydrothermal alteration of tuffswell below the water table at Yucca Mountain that are associated withTimberMountain caldera eruptions at ~10.5 Ma (e.g., Bish and Aronson,1993; Whelan et al, 2008), there is no credible evidence requiringhydrothermal activity. Preservation of fragile fumarolic δ18O signaturesin vadose zone nonwelded tuffs precludes post-depositional hydro-thermal modification (Holt, 2002). Furthermore, seismic pumping, themechanism that Dublyansky and Spötl (2010) invoke to inject calcite-andopal-depositinghydrothermalfluidshundredsofmeters up into thevadose zone is not widely accepted (Bredehoeft, 1992; Carrigan et al.,1991; Rojstaczer, 1999). In contrast, the opal climate-proxy recordspresented here are consistent with a simple mechanism of gravity-driven, vadose zone flow, and a large body of other evidence from bothsurface and subsurface hydrogenic deposits (Marshall et al., 2005;National Research Council, 1992; Neymark et al., 2005; Paces et al.,2001; Sonnenthal et al., 2005; Stuckless et al., 1991, 1998; Vaniman andWhelan, 1994; Vaniman et al., 1994, 2001;Whelan and Stuckless, 1992;Whelan et al., 1994, 2002, 2005, 2008; Wilson and Cline, 2005; Wilsonet al., 2003) that overwhelmingly supports a meteoric source.

6. Conclusions

The hydrologic response of deep vadose zones to changes inclimate-induced surface moisture is an important aspect of under-standing travel times, recharge rates, and potential water fluxes inarid environments, especially at sites chosen for isolating toxic wastesover long periods of time. The accepted paradigm of a directcorrelation between effective surface moisture and deep fractureflow is shown to be questionable at Yucca Mountain within the aridMojave Desert of southern Nevada. Instead, data from secondaryhydrogenic minerals indicate that water fluxes through the deepvadose zone are at least partly decoupled from surface moisture and

shallow infiltration, both of which have experienced large fluctuationsover the past million years.

Data supporting this conclusion were obtained from detailedgeochronological studies of U-rich opal formed in natural cavities atdepths of 200 to 300 m below land surface. Profiles across individualhemispherical grains were dated by U-series methods using secondaryionmass spectrometrywith spatial resolutions of approximately 20 μm.Ages obtained for 1-mm-thick grains range from 41 to 1290 ka. Opalgrains also exhibit intricate growth banding that is manifested mostnotably in cathodoluminescence images. Oscillatory CL intensity isshown to be a function of U concentration which varies from 5.4 to550 μg/g. Opal growth rates calculated over the past 300 ka areextremely slow, ranging from 0.42 to 1.54 μm/ka for different grains.Growth rates of associated calcite are shown to be similar. In spite of thevariations in growth rates between different opal grains, rates remainedmore-or-less constant within individual profiles over time given thecurrent degree of spatial and geochronological resolution. Ages are usedto calibrate the more detailed U concentration profiles which show aremarkable correspondence with climate-proxy records and withfrequencies indicative of planetary orbital forcing factors. Major peaksin U concentration correspond closelywith interglacial episodes and areinterpreted to be the result of increased amounts of evaporativeconcentration in soil water and shallow infiltration.

The uniform, linear rates of opal growth indicate that hydrologicconditions in the deep vadose zone remained stable throughout thePleistocene despite multiple climate cycles, including the particularlycold and wet penultimate glacial episode (Sharpe, 2007). Long-termhydrologic stability also is impliedby similar growth rates for older partsof the same coatings (Neymark et al., 2002;Whelan et al., 2008) despiteMiocene and Pliocene climates thatwere warmer andwetter relative toPleistocene conditions (Thompson, 1991; Winograd et al., 1985). Theseresults imply that seepage fluxes in the deep vadose zone are decoupledfrom effective surface moisture and buffered from large variations inshallow infiltration. A number of hydrogeologic processes likelycontribute to this long-term hydrologic stability including runoff anddown-slope diversion at contacts between soil and bedrock andbetween nonwelded and welded tuffs, changes in evapotranspirationrates, evaporation and upward flux of shallow fracture water, storageand uniform releases of percolation from nonwelded tuffs, andreduction of deep fracture flux by evaporation and imbibition. Otherdeep vadose settings likely contain similarmineral records that could beused to evaluate hydrologic response to climate change.

Acknowledgements

We thank L. Benson, K. Maher, R. Roback, I. Winograd, M. Cosca, S.Ewing and three anonymous journal reviewers for technical reviews ofthismanuscript and earlier drafts. Funding and field support for thisworkwas provided by the U.S. Department of Energy (DOE) under InteragencyAgreement DE-AI28-07RW12405; however, DOE had no involvement inthe study design, analyses, or interpretation of results. Author contribu-tions are as follows: Paces hadprimary responsibility for the study design,data collection, interpretation, and report writing. Neymark andWoodenwere instrumental in helping collect U-series data. Lund contributedinsights into the comparisonof opal andDevilsHole profiles andprovidedtime-series analyses and interpretations. Neymark,Whelan, andMarshallcontributed to interpretations of vadose zone mineral genesis andhydrologic processes, and helped refine the final presentation. Any useof trade, firm, or product names is for descriptive purposes only and doesnot imply endorsement by the U.S. Government.

Appendix A. Supplementary data

Supplementary data to this article can be found online atdoi:10.1016/j.epsl.2010.10.006.

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References

Amelin, Y., Back, M., 2006. Opal as a U–Pb geochronometer: Search for a standard.Chem. Geol. 232, 67–86.

Ayliffe, L.K., Marianelli, P.C., Moriarty, K.C., Wells, R.T., McCulloch, M.T., Mortimer, G.E.,Hellstrom, J., 1998. 500 ka precipitation record from southeastern Australia:evidence for interglacial relative aridity. Geology 26, 147–150.

Baker, A., Smart, P.L., Ford, D.C., 1993. Northwest European palaeoclimate as indicatedby growth frequency variations of secondary calcite deposits. Palaeogeogr.Palaeoclimatol. Palaeoecol. 100, 291–301.

Banner, J.L., Guilfoyle, A., James, E.W., Stern, L.A., Musgrove, M., 2007. Seasonalvariations in modern speleothem calcite growth in central Texas, USA. J. Sediment.Res. 77, 615–622.

Bish, D.L., Aronson, J.L., 1993. Paleogeothermal and paleohydrologic conditions in silicictuff from Yucca Mountain, Nevada. Clays Clay Miner. 41, 148–161.

Bredehoeft, J.D., 1992. Response of the ground-water system at Yucca Mountain to anearthquake. National Research Council, 1992, Ground water at Yucca Mountain,how high can it rise? Final report of the Panel on Coupled Hydrologic/Tectonic/Hydrothermal Systems at Yucca Mountain. National Academy Press, Washington,D.C., pp. 212–222.

Broecker, W.S., 1963. A preliminary evaluation of uranium-series inequilibrium as atool for absolute age measurement on marine carbonates. J. Geophys. Res. 68,2817–2834.

Carrigan, C.R., King, G.C.P., Barr, G.E., Bixler, N.E., 1991. Potential for water tableexcursions induced by seismic events at Yucca Mountain, Nevada. Geology 19,1157–1160.

Day, W.C., Dickerson, R.P., Potter, C.J., Sweetkind, D.S., San Juan, C.A., Drake II, R.M.,Fridrich, C.J., 1998. Bedrock geologic map of the Yucca Mountain area, Nye County,Nevada. U.S. Geol. Surv. Geol. Invest. Ser. I-2627.

Department of Energy, 2008. Safety Analysis Report, Chapter 2: repository safety afterpermanent closure. U.S. Department of Energy, Office of Civilian Radioactive WasteManagement, Yucca Mountain Repository Licence Application, DOE/RW-0573,Update No. 1, Docket No. 63-001.

Dreybrodt, W., 1980. Deposition of calcite from thin films of natural calcareoussolutions and the growth of speleothems. Chem. Geol. 29, 89–105.

Dublyansky, Y., Spötl, C., 2010. Evidence for a hypogene paleohydrogeological event atthe prospective nuclear waste disposal site Yucca Mountain, Nevada, USA, revealedby the isotope composition of fluid-inclusion water. Earth Planet. Sci. Lett. 289,583–594.

Edmunds, W.M., Tyler, S.W., 2002. Unsaturated zones as archives of past climates:toward a new proxy for continental regions. Hydrogeol. J. 10, 216–228.

Fairchild, I.J., Smith, C.L., Baker, A., Fuller, L., Spötl, C., Mattey, D., McDermott, F., E.I.M.F.,2006. Modification and preservation of environmental signals in speleothems.Earth Sci. Rev. 75, 105–153.

Faybishenko, B., 2007. Climatic forecasting of net infiltration at Yucca Mountain usinganalogue meteorological data. Vadose Zone J. 6, 77–92.

Flint, A.L., Flint, L.E., Kwicklis, E.M., Bodvarsson, G.S., Fabryka-Martin, J.T., 2001a.Hydrology of Yucca Mountain, Nevada. Rev. Geophys. 39, 447–470.

Flint, A.L., Flint, L.E., Bodvarsson, G.S., Kwicklis, K.M., Fabryka-Martin, J.T., 2001b.Evolution of the conceptual model of unsaturated zone hydrology at YuccaMountain, Nevada. J. Hydrol. 247, 1–30.

Forester, R.M., Bradbury, J.P., Carter, C., Elvidge-Tuma, A.B., Hemphill, M.L., Lundstrom,S.C., Mahan, S.A., Marshall, B.D., Neymark, L.A., Paces, J.B., Sharpe, S.E., Whelan, J.F.,Wigand, P.E., 1999. The climatic and hydrologic history of southern Nevada duringthe late Quaternary. U.S. Geol. Surv. Open-File Rep. 98–635 63 pp..

Forester, R.M., Lowenstein, T.K., Spencer, R.J., 2005. An ostracode based paleolimnologicand paleohydrologic history of Death Valley: 200 to 0 ka. Geol. Soc. Am. Bull. 117,1379–1386.

Genty, D., Quinif, Y., 1996. Annually laminated sequences in the internal structure ofsome Belgian stalagmites— importance for paleoclimateology. J. Sediment. Res. 66,275–288.

Glassley, W.E., Nitao, J.J., Grant, C.W., 2002. The impact of climate change on thechemical composition of deep vadose zone waters. Vadose Zone J. 1, 3–13.

Hanchar, J.M., 1999. Spectroscopic techniques applied to uranium in minerals. In:Burns, P.C., Finch, R. (Eds.), Uranium: mineralogy, geochemistry and theenvironment: Mineral. Soc. Amer. Reviews in Mineralogy, 38, pp. 499–519.

Holt, E.W., 2002. 18O/16558 O evidence for an early, short-lived (10 yr), fumarolicevent in the Topopah Spring Tuff near the proposed high-level nuclear wasterepository within Yucca Mountain, Nevada, USA. Earth Planet. Sci. Lett. 201,559–573.

Ivanovich, M., Harmon, R.S., 1992. Uranium-series disequilibrium: applications to earth,marine, and environmental sciences. Clarendon Press, Oxford, U.K.

Keefer, W.R., Whitney, J.W., Buesch, D.C., 2007. Geology of the YuccaMountain site area,southwestern Nevada. In: Stuckless, J.S., Levich, R.A. (Eds.), The Geology andClimatology of Yucca Mountain and Vicinity, Southern Nevada and California: Geol.Soc. Amer. Memoir, 199, pp. 53–103.

Levich, R.A., Stuckless, J.S., 2007. Yucca Mountain, Nevada — a proposed geologicrepository for high-level waste. In: Stuckless, J.S., Levich, R.A. (Eds.), The Geologyand Climatology of Yucca Mountain and Vicinity, Southern Nevada and California:Geol. Soc. Amer. Memoir, 199, pp. 1–7.

Lowenstein, T.K., Li, J., Brown, C., Roberts, S.M., Ku, T.-L., Luo, S., Yang, W., Forester, R.M.,1999. 200, 000 year paleoclimate record fromDeath Valley salt core. Geology 27, 3–6.

Ludwig, K.R., 2003. User's Manual for Isoplot 3.00 a geochronological toolkit forMicrosoft Excel. Berkeley Geochronology Center Spec. Pub., 4. 73 pp.

Ludwig, K.R., Paces, J.B., 2002. Uranium-series dating of pedogenic silica and carbonate,Crater Flat, Nevada. Geochim. Cosmochim. Acta 66, 487–506.

Marshall, B.D., Neymark, L.A., Peterman, Z.E., 2003. Estimation of past seepage volumesfrom calcite distribution in the Topopah Spring tuff, Yucca Mountain, Nevada. J.Contam. Hydrol. 62–63, 237–247.

Marshall, B.D., Neymark, L.A., Peterman, Z.E., 2005. Reply to “Commentary: Assessmentof past infiltration fluxes through Yucca Mountain on the basis of the secondarymineral record— Is it a viable methodology?” by Y.V. Dublyansky and S.Z. Smirnov.J. Contam. Hydrol. 77, 219–224.

Montazer, P., Wilson, W.E., 1984. Conceptual hydrological model of flow in theunsaturated zone, Yucca Mountain, Nevada. U.S. Geol Surv. Water-Resour. Invest.Rep. 84–4345 55 pp.

Moyer, T.C., Geslin, J.K., Flint, L.E., 1996. Stratigraphic relations and hydrologicproperties of the Paintbrush Tuff nonwelded (PTn) hydrologic unit, YuccaMountain, Nevada. U.S. Geol. Surv. Open-File Rep. 95–397 151 pp.

Muller, R.A., MacDonald, G.J., 1997. Specrum of 100-yr glacial cycle: orbital inclination,not eccentricity. Proc. Natl Acad. Sci. 94, 8329–8334.

Muller, R.A., MacDonald, G.J., 2000. Ice ages and astronomical causes; data, spectralanalyses, andmechanisms. Chichester, UK, Praxis Publishing, London, UK, Springer-Verlag, 318 pp.

Musgrove, M., Banner, J.L., Mack, L.E., Combs, D.M., James, E.W., Cheng, H., Edwards, R.L.,2001. Geochronology of late Pleistocene to Holocene speleothems from centralTexas: implications for regional paleoclimate. Geol. Soc. Am. Bull. 113, 1532–1543.

National Research Council, 1992. Ground water at Yucca mountain, how high can itrise? — final report of the panel on coupled hydrologic/tectonic/hydrothermalsystems at Yucca Mountain. Natl. Acad. Press, Washington, D.C.. 231 pp.

Nemchin, A.A., Neymark, L.A., Simons, S.L., 2006. U–Pb SHRIMP dating of uraniferousopals. Chem. Geol. 227, 113–132.

Neymark, L.A., Paces, J.B., 2000. Consequences of slow growth for 230Th/U dating ofQuaternary opals, Yucca Mountain, Nevada, USA. Chem. Geol. 164, 143–160.

Neymark, L.A., Amelin, Y.V., Paces, J.B., 2000. 206Pb–230Th–234U–238U and 207Pb–235Ugeochronology of Quaternary opal, YuccaMountain, Nevada. Geochim. Cosmochim.Acta 64, 2913–2928.

Neymark, L.A., Amelin, Y., Paces, J.B., Peterman, Z.E., 2002. U–Pb ages of secondary silicaat YuccaMountain, Nevada: implications for the paleohydrology of the unsaturatedzone. Appl. Geochem. 17, 709–734.

Neymark, L.A., Paces, J.B., Marshall, B.D., Peterman, Z.E., Whelan, J.F., 2005. Geochemicaland C, O, Sr, and U-series isotopic evidence for the meteoric origin of calcrete atSolitario Wash, Crater Flat, Nevada, USA. Environ. Geol. 48, 450–465.

Oliver, T.A., Whelan, J.F., 2006. Characterization of seepage in the exploratory studiesfacility, Yucca Mountain, Nevada. Proceed. Internatl. High-Level Rad. WasteManagement Conf. 2006. Las Vegas, NV, April 30–May 4, 2006. Amer. NuclearSoc., LaGrange Park, IL, pp. 286–292.

Paces, J.B., Neymark, L.A., Marshall, B.D., Whelan, J.F., Peterman, Z.E., 2001. Ages andorigins of calcite and opal in the Exploratory Studies Facility tunnel, YuccaMountain, Nevada. U.S. Geol. Surv. Water-Resour. Invest. Rep. 01–4049 95 pp.

Paces, J.B., Neymark, L.A., Wooden, J.L., Persing, H.M., 2004. Improved spatial resolutionfor U-series dating of opal at Yucca Mountain, Nevada, USA, using ion-microprobeand microdigestion methods. Geochim. Cosmochim. Acta 68, 1591–1606.

Paces, J.B., Neymark, L.A., Ghezzehei, T., Dobson, P.F., 2006. Testing the concept of driftshadow at Yucca Mountain, Nevada. Proceed. 11th Internatl High-Level Rad. WasteManagement Conf. Las Vegas, NV, April 30–May 4, 2006. Amer. Nuclear Soc., LaGrange Park, Illinois, pp. 278–285.

Quade, J., Forester, R.M., Whelan, J.F., 2003. Late Quaternary paleohydrologic andpaleotemperature change in southern Nevada. In: Enzel, Y., Wells, S.G., Lancaster, N.(Eds.), Paleoenvironments and Paleohydrology of the Mojave and Southern GreatBasin Deserts: Geol. Soc. of Amer. Spec. Paper, 368, pp. 165–188.

Rojstaczer, S., 1999. Stress dependent permeability and its political consequences atYucca Mountain (abst.). Eos Trans. Amer. Geophys. Union 80, S4.

Sharpe, S.E., 2007. Using modern through mid-Pleistocene climate proxy data to boundfuture variations in infiltration at Yucca Mountain, Nevada. In: Stuckless, J.S.,Levich, R.A. (Eds.), The Geology and Climatology of Yucca Mountain and Vicinity,Southern Nevada and California: Geol. Soc. Amer. Memoir, 199, pp. 155–205.

Smith, G.I., Bischoff, J.L., 1997. Core OL-92 from Owens Lake: project rationale, geologicsetting, drilling procedures, and summary. In: Smith, G.I., Bischoff, J.L. (Eds.), An800,000-year Paleoclimatic Record from Core OL-92, Owens Lake, SoutheastCalifornia: Geol. Soc. Amer. Spec. Paper, 317, pp. 1–8.

Sonnenthal, E., Xu, T., Bodvarsson, G., 2005. Reply to commentary “Assessment of pastinfiltration fluxes through Yucca Mountain on the basis of the secondary mineralrecord — is it a viable methodology?”, by Y.V. Dublyansky and S.Z. Smirnov. J.Contam. Hydrol. 77, 225–231.

Spaulding, W.G., 1985. Vegetation and climates of the last 45,000 years in the vicinity ofthe Nevada Test Site, south-central Nevada. U.S. Geol. Surv. Prof. Pap. 1329 83 pp..

Stuckless, J.S., Dudley, W.W., 2002. The geohydrologic setting of Yucca Mountain,Nevada. Appl. Geochem. 17, 659–682.

Stuckless, J.S., Peterman, Z.E., Muhs, D.R., 1991. U and Sr isotopes in ground water andcalcite, Yucca Mountain, Nevada — evidence against upwelling water. Science 254,551–554.

Stuckless, J.S., Marshall, B.D., Vaniman, D.T., Dudley, W.W., Peterman, Z.E., Paces, J.B.,Whelan, J.F., Taylor, E.M., Forester, R.M., O'Leary, D.W., 1998. Comments on “Overviewof calcite/opal deposits at or near the proposed high-level nuclear waste site, YuccaMountain, Nevada, USA — pedogenic, hypogene or both?” by C.A. Hill, Y.V.Dublyansky, R.S. Harmon, C.M. Schluter. Environ. Geol. 34, 70–78.

Thompson, R.S., 1991. Pliocene environments and climates in the western UnitedStates. Quat. Sci. Rev. 10, 115–132.

Thompson, R.S., Anderson, K.H., Bartlein, P.J., 1999. Quantitative paleoclimaticreconstructions from late Pleistocene plant macrofossils of the Yucca Mountainregion. U.S. Geol. Surv. Open-File Rep. 99–338 38 pp.

298 J.B. Paces et al. / Earth and Planetary Science Letters 300 (2010) 287–298

Vaniman, D.T., Whelan, J.F., 1994. Inferences of paleoenvironments from petrographic,chemical and stable-isotope studies of calcretes and fracture calcites. Proceed. FifthInternatl. High-Level Rad. Waste Management Conf. Las Vegas, NV, May 22–26,1994. Amer. Nuclear Soc, LaGrange Park, IL, pp. 2730–2737.

Vaniman, D.T., Chipera, S.J., Bish, D.L., 1994. Pedogenesis of siliceous calcretes at YuccaMountain. Geoderma 63, 1–17.

Vaniman, D.T., Chipera, S.J., Bish, D.L., Carey, J.W., Levy, S.S., 2001. Quantification ofunsaturated-zone alteration and cation exchange in zeolitized tuffs at YuccaMountain, Nevada, USA. Geochim. Cosmochim. Acta 65, 3409–3433.

Walvoord, M.A., Plummer, M.A., Phillips, F.M., Wolfsberg, A.V., 2002. Deep arid systemhydrodynamics 1: equilibrium states and response times in thick desert vadosezones. Water Resour. Res. 38, 1308–1323.

Walvoord, M.A., Stonestrom, D.A., Andraski, B.J., Striegl, R.G., 2004. Constraining theinferred paleohydrologic evolution of a deep unsaturated zone in the AmargosaDesert. Vadose Zone J. 3, 502–512.

Whelan, J.F., Stuckless, J.S., 1992. Paleohydrologic implications of the stable isotopiccompositionof secondary calcitewithin theTertiaryvolcanic rocks of YuccaMountain,Nevada. Proceed. Third Internatl. High-Level Rad.WasteManagementConf. LasVegas,NV, April 12–16, 1992. Amer Nuclear Soc., LaGrange Park, IL, pp. 1572–1581.

Whelan, J.F., Vaniman, D.T., Stuckless, J.S., Moscati, R.J., 1994. Paleoclimatic andpaleohydrologic records from secondary calcite — Yucca Mountain, Nevada.Proceed. Fifth Internatl. High-Level Rad. Waste Management Conf. Las Vegas, NV,May 22–26, 1994. Amer. Nuclear Soc., LaGrange Park, IL, pp. 2738–2745.

Whelan, J.F., Paces, J.B., Peterman, Z.E., 2002. Physical and stable-isotope evidence forformation of secondary calcite and silica in the unsaturated zone, Yucca Mountain,Nevada. Appl. Geochem. 17, 735–750.

Whelan, J.F., Paces, J.B., Peterman, Z.E., Marshall, B.D., Neymark, L.A., 2005. Erratum to‘Reply to the comment on “Physical and stable-isotope evidence for formation ofsecondary calcite and silica in the unsaturated zone, YuccaMountain, Nevada” by Y.V.Dublyansky, S.E. Smirnov, and G.P. Palyanova'. Appl. Geochem. 20, 1039–1050.

Whelan, J.F., Neymark, L.A., Moscati, R.J., Marshall, B.D., Roedder, E., 2008. Thermalhistory of the unsaturated zone at Yucca Mountain, Nevada, USA. Appl. Geochem.23, 1041–1075.

Wilson, N.S.F., Cline, J.S., 2005. Reply to the Comment on “Origin, timing, andtemperature of secondary calcite-silica mineral formation at Yucca Mountain,Nevada” by Y. V. Dublyansky, S. Z. Smirnov, and G. P. Palyanova. Geochim.Cosmochim. Acta 69, 4391–4395.

Wilson, N.S.F., Cline, J.S., Amelin, Y.V., 2003. Origin, timing, and temperature ofsecondary calcite-silica mineral formation at Yucca Mountain, Nevada. Geochim.Cosmochim. Acta 67, 1145–1176.

Winograd, I.J., Szabo, B.J., Coplen, T.B., Riggs, A.C., Kolesar, P.T., 1985. Two-million-yearrecord of deuterium depletion in Great Basin ground waters. Science 227, 519–522.

Winograd, I.J., Coplen, T.B., Landwehr, J.M., Riggs, A.C., Ludwig, K.R., Szabo, B.J., Kolesar, P.T.,Revesz, K.M., 1992. Continuous 500, 000-year climate record from vein calcite inDevils Hole, Nevada. Science 258, 255–260.

Winograd, I.J., Landwehr, J.M., Coplen, T.B., Sharpe, W.D., Riggs, A.C., Ludwig, K.R.,Kolesar, P.T., 2006. Devils Hole, Nevada, δ18O record extended to the mid-Holocene.Quat. Res. 66, 202–212.

Yang, I.C., Rattray, G.W., Yu, P., 1996. Interpretation of chemical and isotopic data fromboreholes in the unsaturated zone at Yucca Mountain, Nevada. U.S. Geol. Surv.Water-Resour. Invest. Rep. 96–4058 58 pp.