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Isotopic Studies of Yucca Mountain Soil Fluids and Carbonate Pedogenesis

Ted A. McConnaughey, Joseph F. Whelan, Kimberley P. Wickland^and Richarj

U.S. Geological Survey Box 25046 SEP 2 8 «95 Denver, CO 80225 OST/ ABSTRACT into the unsaturated zone, and hydrogenic minerals precipi• tated along the way likewise reflect the soil genesis of parent Secondary carbonates occurring within the soils, faults, fluids1,2, ' . Soil fluids are therefore key to understanding and subsurface fractures of Yucca Mountain contain some of die origins and movements of subsurface fluids, and for the best available records of paleoclimate and paieohydrology interpreting the isotopic compositions of hydrogenic miner• for the potential radioactive waste repository site. This article als (mainly calcite, opal, and clays) deposited from those discusses conceptual and analytical advances being made fluids. with regard to the interpretation of stable isotope data from pedogenic carbonates, specifically related to the 13C content Climatic change adds an important temporal dimension of soil C02> CaCO, precipitation mechanisms, and isotopic to the problem. The abundance and isotopic composition of fractionations between parent fluids and precipitating car• meteoric waters has changed over time. So have the species bonates. composition and densities of vegetation, which affect surface water balance, the isotopic composition of soil fluids, and The C content of soil carbon dioxide from Yucca weathering rates. Soil and subsurface carbonates record these Mountain and vicinity shows most of die usual patterns changes, providing useful mineral repositories of climatic expected in such contexts: decreasing C content with information at Yucca Mountain. depth (due mainly to increased importance of respired CO, ), decreasing 3Cwith altitude (partially due to relative• Faults in and around Yucca Mountain accumulate par• ly more C-3 vegetation), and reduced C during spring ticularly impressive carbonate and silica infillings because of (due again to higher rates of respiration, and reduced gas their effects on surface and subsurface hydrology. permeability of wet soils.) These patterns exist within the Understanding how these carbonates form and attain their domain of a noisy data set; soil and vegetationai heterogenei• structural, chemical, and isotopic properties therefore illumi• ties, weather, and other factors apparently contribute to iso• nates important aspects of fault hydrology. As dating pro• topic variability in tlie system. cedures for the carbonates become better developed, the tectonic history of the mountain may also become clearer. Several soil calcification mechanisms appear to be important, involving characteristic physical and chemical A. Carbon-13 in Soil Gas Carbon Dioxide environments and isotopic fractionations. When CO, loss from thin soil solutions is an important driving factor, car• Soil gas sampling sites in the vicinity of Yucca bonates may contain excess heavy isotopes, compared to Mountain are shown in Figure 1. Seven sites span more than equilibrium precipitation with soil fluids. When root calcifi• 1 km of elevation, encompassing substrates ranging from cation serves as a proton generator for plant absorbtion of desert colluvium to volcanic tuff, and vegetation types soil nutrients, heavy isotope deficiencies are likely. Successive ranging from desert scrub to pinon-juniper forest. Steel cycles of dissolution and reprecipitation mix and redistribute sampling tubes were forced into the soils to depths ranging pedogenic carbonates, and tend to isotopically homogenize from 10 cm to 3 m, usually limited by rocks in the soils. and equilibrate pedogenic carbonates with soil fluids. Deeper samples were also obtained using gas sampling tubes previously installed at the USGS low-level waste research 1. INTRODUCTION facility near Beatty, and from a caisson facility in jackass Flats. Samples from near the crest of Yucca Mountain were The shallow soils of Yucca Mountain and vicinity form obtained from shallow holes drilled into the volcanic tuffs. a skin over the landscape, through which gases and liquids pass before crossing from the atmosphere to the deep unsatu• Separate gas samples were generally withdrawn by rated zone, or vice versa. This skin is far from passive; soil syringe for analysis of C02 concentrations (by portable processes, including interactions between meteoric precipita• infrared gas analyzer) and pumped into mylar balloons, for tion and soil minerals and biota, determine the chemical and later extraction of CO, for isotopic analysis. In-line desicc- isotopic properties of fluids at the air-land interface. Soil ant traps were used during collection of some balloon fluids tend to retain their properties as they penetrate deeper samples, but had little effect on the 5 Cor 81 O values of »"?» •*, f"*,

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*.-•' Figure 1. Soil gas sampling sites in the vicinity of Yucca Mountain. USE = U.S. Ecology; YC = Yucca Crest; FR = Fran Ridge; 40M = Fortymiie Wash; PWT = Pagany Wash; PIA = roadcut on side of Rainier Mesa; and RMl = top of Rainier Mesa.

extracted CO,. Soil samples for analysis of soil moisture, of however been noted over periods of several mondis, pre• the H and O isotopic composition of soil water, and 5 C of sumably reflecting CO, leakage through the mylar balloons associated organic and inorganic soil carbon, were also col• or their septa. Other limitations on die data stem from the lected by auger, and frozen for later analysis. creation of flow fields within the soils during sampling, and from spatial heterogeneities related to nearby vegetation, Extraction of CO, for isotopic analysis was accom• animal burrows, etc. For samples collected between March plished by cryogenic trapping in the laboratory. Replicate and September 1993, soil gas 5 C ranges from about -22 extractions from single balloons made within a few days typi• to -8 %o PDB, with the following trends (Fig. 2): cally agreed within mass spectrometric precision for both 513C and 6180 (about .05 %oa ), and replicate balloons 1) Soil gas 8 C decreases with depth within die top typically agreed within 0.2 %o. Some isotopic drift has meter, while CO, concentrations increase. These observa• tions are consistent with larger contributions of isotopically and dO values arc reported as the %o (parts per thousand) light CO, from soil respiration, deeper within die soils. deviations of the unknowns from the isotopic compositions of the About half of the depth-dependent variation often occurs 13 International Standards PDB (for C) and SMOW (for O). within die top 10 cm, while die 8 C minimum or asymp-

# 2586 RADIOACTIVE WASTE MANAGEMENT

AIR on North Yucca Mountain, Yucca Crest near UZ-6, and RM. 1 and PIA on Rainier Mesa) contain about 10%o lower Si3C than the low elevation sites (such as USE, near Bcatty). At least as interesting as these trends are the large isoto- pic variations observed at individual sites. This is especially pronounced within the stream-terrace alluvium sampled at -40-i Fortymile Wash, where seasonal isotopic variations at a single depth sometimes span nearly the entire isotopic range observed at all sites, for all depths and seasons. Shallow hola drilled into tuffs on Yucca Crest also illustrate this variability (Fig. 3). Factors controlling the range of 813C at a particular | -80 H site are currendy under investigation. B. Isotopic Mixtures. Q. Soil CO, derives from two main sources: atmospheric Q -120H CO, and CO- respired by plant roots and soil organisms. n PWT Each has a characteristic carbon isotopic composition. 13 Atmospheric C02 has a 8 C of about -8.5 %o, down some• Beat what from the pre-industrial value of about -6.5 %o which prevailed during the time most existing recent soil car• -160-1 bonates precipitated. Respired CO, resembles surrounding vegetation, which ranges from about -27 to -12 %o, depend• ing on the local mix of C-3 .and C-4 plants, respectively. This in turn correlates with altitude and other factors. Carbonate-rich dusts derived largely from Paleozoic lime• -200- 1 • '' I • IT-I-P stones and playa sediments supply much of the calcium in -25 -20 -15 -10 7 soil carbonates, but their contributions to carbon are minor compared to atmospheric and respired C02- 13 5 C (%o) Carbonates precipitated in isotopic equilibrium with

soil C02 should be about 8%o increased in 8 C, depending Figure 2. Carbon-13 content of soil gas carbon dioxide to some extent on temperature. Based on the data presented from vertical profiles at two sites in the Yucca Mountain above, this defines a contemporary equilibrium range o region, March-September 1993. The PWT site (open around -14 to +0%o. The lower values apply to high eleva• squares), near the head of Pagany Wash, is the highest soil tion sites and deeper than 50 cm in the soil column, and the gas sampling site on Yucca Mountain. The USE site near heavier values apply where atmospheric C02 contributes Beatty (filled circles) is located in the Amargosa Desert most of the carbon to the mixture. Atmospheric CO, was southwest of Yucca Mountain. about 2%o heavier before the industrial revolution, ana con• sidering the uncertainties in environmental temperatures at the time of soil calcification, and in the associated equilibri• tote usually occurs at about the depth (30 to 50 cm) where um isotope fractionation factors, it is reasonable to extend plant roots appear to be concentrated. the range for equilibrium pedogenic carbonates to about +3%o. Most carbonates from the Yucca Mountain region fall 2) Most sites show a seasonal progression from isotopi• within this range , and equilibrium models shed consid cally light (low 8 C) in the spring to isotopically heavy in b m 5,

The existence of anomalously heavy and light carbon HCO3', and isotopically heavy carbonate precipitates. A Mopes in carbonates, and their association with distinctive similar kinetic isotope fractionation can affect oxygen anate morphologies, suggests that such carbonates pre- isotopes. in isotopically distinctive microenvironments anoVor Second, CO- evasion sets" up Rayleigh-type isotope ecipitate under non-equilibrium conditions. What follows separations, even in the absence of kinetic disequilibria la framework for interpreting these phenomena. during CO, generation, since CO, at equilibrium at K C. Calcification Mechanisms and Isotopic ambient surface temperatures is lowered in 513C by about 7- onsequences 8%o compared to HCO,". Consequently, as CO, evasion progresses, the remaining HCOj" becomes progressively Calcification is a two-step process. First, calcium bicar- heavier, sometimes leading to large l3C enrichments12. If rnate rich solutions form. In the climatic context of Yucca kinetic fractionations during HCO^" —• C02 reactions (as lountain, this is likely to occur mainly during early spring, above) are added to the picture, larger 3C enrichments in en soils are wet, plants are active, and soil respiration precipitating carbonates result. nerates high CO, partial pressures in the soils. Conditions change to encourage CaCO, precipitation. Soil dewa- The association of soil calcification with plant roots has ring during late spring and summer has been considered to possible relevance to occurrences of isotopicaliy light car• the dominant process driving pedogenic calcification5; bonates. Calcified roots are ubiquitous around Yucca ithcr physical processes contributing to CaCO, supersatura- Mountain, forming particularly impressive fossil gardens pons within bulk soils include temperature increases and loss within fault zones, as for example where the Paintbrush fault CO, from soil waters. Biological processes also appear to crosses the sand ramps of Busted Butte, and along the Ghost |be important. Dance and Solitario Canyon faults. Well-preserved calcified roots also occur in paleodischarge deposits, as near the south• Soil CO, concentrations vary among sites near Yucca ern end of Crater Flat and Ash Meadows, and in cracks ^Mountain from less than atmospheric {currently about 350 within non-welded volcanic tuffs throughout the region. fppm) to more than 40,000 ppm. The subatmospheric values Calcified rootlets and apparent fungal products also occur Iwere observed transiently during cold, wet periods, and widely within massive pedogenic carbonates '. The abun• ^probably resulted from wetting of alkaline soils to form solu• dance and diversity of such features suggests that roots and tions which temporarily absorbed CO,. High soil CO, fungi contribute to soil calcification in many settings which values result from respiration by soil organisms (including are not as clear and which have not been closely examined. ;. plant roots), and are most pronounced within the root zone, in spring, at higher elevations. Most of this CO, is lost as Root calcification involves several components. soils dry out in the summer. CO, loss from soil fluids Transpiration draws water toward the root, concentrating (evasion)should induce CaCO, precipitation in a manner ions there and encouraging calcification. Roots also produce analogous to what frequendy occurs during travertine and conduits for downward water flow horn the surface. Finally, speieothem deposition, as represented by the reaction roots often exhibit external pH banding , i.e. zones around the roots, usually near the root tips, which are more acidic or Q2+ 2HCO, C0 + CaC0 H 0. + 2 3+ 2 alkaline than surrounding soil fluids. The acidic zones appar• CO, evasion probably contributes to calcification mainly ently contribute to the solubilization and uptake of elements during late spring and summer, as soils dry out and become such as P, Fe, and Mn from the soils. The roots obtain more permeable ro exchange with atmosphere, and as soil protons for acid secretion either from soil fluids some dis• respiration diminishes in response to desiccation. Degassing tance away, creating external alkaline zones which may contributes to calcification in the deep unsaturated zone as become calcified, or from the intracellular (vacuolar) precipi• well; CO, concentrations below the soils within die unsatu• tation of carbonates and oxalates: rated zone are lower than many soil CO, concentrations, CaCO, precipitation: allowing downward percolating fluids to lose CO, and pre• .2* + cipitate CaCO,. •CaC01 + :.4

When C02 evasion from solution causes CaCO, super- Oxalate precipitation:

saturation and precipitation, some interesting disequilibrium z+ 8,9,10,11 Ca + H,C,0, -* CaC,0. + 2H* isotopic fractionations can occur . These disequilibri• 2 2 4 2 -i um effects commonly enrich the precipitate by several "/cm. Root calcification is particularly iikek alkaline and There are at least two components to these enrichments. calcareous soils, since more protons are riv :.red to titrate soil bases and free the acid soluble nutrients. The vacuoles of First, molecules bearing C suffer a kinetic disadvan• root ceils sometimes become completely fii'i:' with oxalates tage during HCO," debydroxylation and H,CO, dehydra• or carbonates creating microscopicaii\ gnizable soil tion. In tnin, alkaline solutions, such as those that occur on wet soil particles in Yucca Mountain region, the l5C deplet• fossils. The regions of proton extrusion dii. - -•> the root tip ed CC^ so formed can partially escape from solution before are also likely to become silicified, as is often observed in fully equilibrating with HCO " This leads to 13C-enriched desert soils. Microbes may also metabolize plant oxalates to 2588 RADIOACTIVE WASTE MANAGEMENT carbonates. Internat. Conf., Las Vegas, Nevada, (this volume^ (1994). '^ The physiology of root calcification appears to be similar to that described for aquatic plants ' in which case 2. STUCKLESSr J.S., Z.E. PETERMAN, and D&M molecular CO- provides much of the carbon for calcifica• MUHS, "U and Sr isotopes in ground water ami 3 tion. CaC-O, deposition probably involves a mechanism calcite, Yucca Mountain, Nevada: Evidence against •! similar to that previously described for calcium salts of odicr upwclling water", Science, 254, 551-554 (1991). If anions lacking uncharged forms (analogous to CO.) at phys• iological pH. If so, oxalate reaches the calcification site 3. VANIMAN, D.T. and J.F. WHELAN, "Inferences of | dirough an anion exchange process which is absent in paleoenvironment from chemical and stable isotope • '« CaCO, precipitating systems , accounting for the fact that studies of calcretes and fracture calcites", in. IHRLWM '" ? CaCO, and CaC-O^ do not normally occur together in the Proc, ASCE and ANS, 5di Internat. Conf., Las Vegas, same calcified cell . Nevada, (this volume) (1994). 4. PETERMAN, Z.E., J.S. STUCKLESS, B.D. ; Calcification associated with pH banding by plant roots MARSHALL, SA. MAHAN, and KA.. FUTA, may display nonequilibrium isotope fractionations that are "Strontium isotope geochemistry of calcite fillings in opposite in character to those discussed above, i.e. lighter deep core, Yucca Mountain, Nevada - a progress than isotopic equilibrium, analogous to those observed in ra report", in IHRLWM Proc., ASCE and ANS, 3rd many other biological carbonates . Applying that model, Internat. Conf., Las Vegas, Nevada, 1582-1586 (1992). C02 diffuses across the calcifying membrane into adjacent alkaline solutions, ionizes, and precipitates as CaCO,. 5. QUADE, J., T.E. CERLING, and J.R. BOWMAN, CO, molecules are kinetically disadvantaged especially "Systmatic variations in the carbon and oxygen isotopic during CO, hydroxylation, resulting in C depletions in composition of pedogenic carbonate along elevation precipitating CaCO,. O kinetic fractionations usually transects in the Southern Great Basin, United States", accompany the C effects, often giving rise to positive corre• Geol. Soc. Amcr. Bull., 101,464-475 (1989). lations between 6 C and O. 8 C decreases of over 10%o, and O decreases of about 4%o are possible. 6. WOOD, W.W. and M.J. PETRAITIS, "Origin and distribution of carbon dioxide in the unsaturated zone Numerous published reports ^ contain data suggestive of the southern high plains of Texas", Water Res. Res., of the presence of such kinetic C depletions in pedogenic 20, 1193-1208. carbonates. Unpublished data by Jaillard is particularly 7. CERLING, T.E., "Carbon dioxide in the atmosphere: impressive. The isotopic composition of soil C02 is poorly constrained in most cases however, and other processes could Evidence from Cenozoic and Mesozoic paleosols", Am. influence the isotopic composition of precipitating car• J. Sci., 291,377-400(1991). bonates in the general manner observed. Continuing investi• 8. GONFIANTINI, R., C. PANICHI, and E. gations will attempt to pin down roles played by such TONGIORGI, "Isotopic disequilibrium in travertine mechanisms in determining the isotopic compositions of soil deposition". Earth Plan. Sci. Let., 5, 55-58 (1968). and UZ carbonates. 9. FRIEDMAN, I., "Some investigations of die deposition The probability that isotopic disequilibrium will exist of travertine from hot springs — I. The isotopic chemis• during some of the more probable forms of carbonate pre• try of a travertine-depositing spring. Geochim. cipitation at Yucca Moutain raises the obvious question of Cosmochim. Acta, 34, 1303-1315 (1970). why equilibrium models appear to have worked rather well. Part of the answer undoubtedly rests in the fact that we have 10. HENDY, C.H., "The isotopic geochemistry of spe- only demanded rather general and low resolution models up leothems - I. The calculation of the effects of different to this point. Probably more important however, is the fact modes of formation on the isotopic composition of that soil carbonate deposits represent the accumulated pro• speleothems and their applicability as paleoclimatic ducts of repeated cycles of precipitation and dissolution, and indicators", Geochim. Cosmochim. Acta, 35: 801-824 these cycles encourage homogenization and equilibration of (1971). the carbonates. This complex genesis is of course part of the 11. MICHAELIS, J., E. USDOWSKI, and G. reason why isotopic dating of soil carbonates is complica- 13 12 22 23 MENSCHEL, "Partitioning of C and C on the ted ' . degassing of C02 and the precipitation of calcite -- Ravleigh-thpe fractionation and a kinetic model", Am. J. Sci., 285, 318-327 (1985). References: 12. STILLER, M, J.S. ROUNICK, and S. SHASTA, 1. WHELAN, J.F., D.T. VAN1MAN, J.S. STUCKLESS, "Extreme carbon-isotope enrichments in evaporating and R.J. MOSCATI, "Paleoclimatic and paleo hydro log• brines", Nature, 316, 434-435 (1985). ic records from secondary calcite: Yucca Mountain, Nevada", in IHRLWM Proc, ASCE and ANS, 5th 13. MARSHALL, B.D., Z.E. PETERMAN, K. FUTA, J.S. ISOTOPIC STUDIES 2589

JCKLESS, SA MAHAN, J.S. DOWNEY, and ^D. GUTENTAG, "Origin of carbonate deposits in vicinity of Yucca Mountain, Nevada: Preliminary fiesults of strontium-isotope analyses", in IHRLWM oc, Proc. ANS, Las Vegas, Nevada, v. 2, 921-923 1990). WEN, J A. "Terrestrial rhizophytes and H+ currents culating over at least a millimetre: an obligate rela- onship?", New Phytologist, 117: 177185 (1991). jtjAILLARD, B., A. GUYON, and A.F. MAURIN, Structure and composition of calcified roots, and their icntification in calcareous soils", Geoderma, 50: 197- iiO (1991). ICCONNAUGHEY, TA and R.H. FALK, "Calcium- proton exchange during algal calcification", Biol. Bull., J80,185-195 (1991) iCCONNAUGHEY, T., "Calcification in Chara cor-

lina: C02 hydroxylation generates protons for bicar- inate assimilation", Limnol. Oceanogr., 36, 619-628 10991). I S 83 ?a 'fcfCCONNAUGHEY, T., "Biomineralization mechan- »S jisms1', p 57-73 in Origin, Evolution, and Modern !. B. ? 3 it [Aspects of Biomineralization in Plants and Animals (ed. TtL E. Crick), Plenum, New York, 1989. s

rWCCONNAUGHEY, T. "13C and 180 isotopic dise• 3. a- 3 quilibrium in biological carbonates. 1. Patterns. O 00

">Geochim. Cosmochim. Acta, 53, 151-162, (1989) s §1 ° * r± M MCCONNAUGHEY, T. "13C and 180 isotopic dise- S. g a. B ; quilibrium in biological carbonates. 2. In vitro simula• >3 g C } s. tion of kinetic isotope effects", Geochim. Cosmochim. !1! g K b 3 oW Acta, 53, 163-171 (1989) •< rt w o 3 C ^* eft oa <** ^^ o s, CM r» C- ftff »O 8.8 ^ CERLING, T.E. and R.L HAY, "An isotopic study of oa. % i 1»2 a o P t paieosol carbonates from Olduvai Gorge", Quat. Res., H .25,63-74(1986). 1 "" I. * a -2. ~s S'& S AMUNDSON, R.G., O.A CHADWICK, j.M. S g* » B.a £3^ SOWERS, and H.E. DONER, "The stable isotope •*» « £ .as Sgn 2 « chemistry of pedogenic carbonates at Kyle Canyon, II — 3 Nevada. Soil Sci. Soc. Am. J., 53, 201-210 (1989). I III |& PENDELL, E.G., J.W. HARDEN, S.E. TRUMBORE, s ' and O.A. CHADWICK, "Isotopic approach to soil- 09 3 carbonate dynamics: Implications for paieodimatic ?S2 iir interpretations. ??? §JU09 . » H Rsfssji S3 < 3 •• -1 HIGH LEVEL RADIOACTIVE WASH MANAGEMENT Proceedings oftoe Fifth Annual International Conference Las Vegas, Nevada, May 22-26,1894 VOLUME 1 Sponsored by the American Society of Civil Engineers American Nuclear Society

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