Appendix C

The Effects of Pluvial Climates In The Vicinity of Yucca Mountain: A Summary

Prepared for the National Research Council's Panel on Coupled Hydrologic/Tectonic/Hydrothermal Systems at Yucca Mountain

W. Geoffrey Spaulding

Dames & Moore

Las Vegas, Nevada 89119

Revised September 6, 1991

INTRODUCTION

Evidence of former, wetter climatic conditions is widespread throughout the southwestern United States. Among the most striking examples are the wave-cut terraces on the margins of closed valleys; evidence that these basins once supported vast lakes. In the late 19th century Gilbert (1890) was among the first to correlate high stands of these lakes, and the “pluvial” climates that they indicated, with glacial ages. The correlation between pluvial climatic episodes and glacial ages (or stades) has lent a basic time scale to major climatic fluctuations in North American deserts. Although the correlation does not hold true in other of the world's great deserts (in North Africa, for example, the last pluvial climatic episode is broadly correlated



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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Appendix C The Effects of Pluvial Climates In The Vicinity of Yucca Mountain: A Summary Prepared for the National Research Council's Panel on Coupled Hydrologic/Tectonic/Hydrothermal Systems at Yucca Mountain W. Geoffrey Spaulding Dames & Moore Las Vegas, Nevada 89119 Revised September 6, 1991 INTRODUCTION Evidence of former, wetter climatic conditions is widespread throughout the southwestern United States. Among the most striking examples are the wave-cut terraces on the margins of closed valleys; evidence that these basins once supported vast lakes. In the late 19th century Gilbert (1890) was among the first to correlate high stands of these lakes, and the “pluvial” climates that they indicated, with glacial ages. The correlation between pluvial climatic episodes and glacial ages (or stades) has lent a basic time scale to major climatic fluctuations in North American deserts. Although the correlation does not hold true in other of the world's great deserts (in North Africa, for example, the last pluvial climatic episode is broadly correlated

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? with the beginning of the present interglacial; Ritchie et al., 1985; Spaulding, 1991), it applies well throughout the western U.S., from the Great Basin and Mojave Desert, to the Sonoran and Chihuahuan Deserts. Other important issues have been resolved more recently, including what constitutes a pluvial climatic regime in this region, and how much of an impact do pluvial climatic episodes have in terms of increased recharge to the aquifer. The answers to these questions affect assessments of the suitability of Yucca Mountain, or any other proposed locality in the vicinity of the Nevada Test Site, as a potential sub-surface repository for high-level nuclear waste. They also reflect on the validity of assertions concerning the magnitude of impact of pluvial climates on the water table. Chronological Framework The design-life of the proposed Yucca Mountain repository is 10,000 yr (10 ka), a time span that lends itself well to consideration within the context of evidence for late Quaternary climatic fluctuations (those that have occurred over the last ca. 130 ka). The historic meteorological record is, of course, too brief to encompass the major changes that take place on such a time span, as is the dendroclimatic record. The end of the last glacial age, the Wisconsin, is placed at 10 ka (radiocarbon years before present) by international convention (Olausson, 1982). It was characterized by the Early and Late Wisconsin stades, periods of maximum expansion of Northern Hemisphere ice sheets and maximum depression of global temperatures, and the Middle Wisconsin interstade. The two major episodes of global climate change encompassed by the radiocarbon time scale (roughly the last 50 ka) are the Middle/Late Wisconsin transition at 23 ka, and the Late Wisconsin/Holocene transition at 10 ka. The first involved a change to colder and effectively wetter environments, while the second involved a shift to a warmer and effectively drier climate accompanying worldwide deglaciation. The full glacial, or Wisconsin-maximum, at ca. 18 ka witnessed the maximum extent of continental glaciers and maximum lowering of global temperatures (see contributions in Porter, 1983). Geographic and Paleohydrologic Framework Located within the southern hydrographic Great Basin, the Nevada Test Site lies astride the climatic and vegetation transition between the warm-temperate Mojave Desert to the south and the cold-temperate Great Basin Desert to the north (Cronquist et al., 1972;

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Beatley, 1975). Sparse creosote bush (Larrea divaricata) 1 desertscrub occupies the valleys of the Mojave, while relatively productive sagebrush (Artemisia subgen. Tridentatae) and sagebrush-bunchgrass (Artemisia - Agropyron spp.) vegetation typifies many valleys to the north. This environmental transition from arid conditions in the south to relatively moist conditons in the north has apparently persisted through the late Quaternary. In their study of pluvial (dating to former, effectively wetter climatic intervals) lakes in Nevada, Mifflin and Wheat (1979) note that south of 38° N lat. very few closed basins possess wave cut terraces and, therefore, it is unlikely that they supported pluvial lakes. This observation also applies to basins in adjacent California. The only known exceptions are lakes that were fed by runoff from the east slopes of the Sierra Nevada and Transverse Ranges (such as Pluvial Lakes Mojave and Searles; Figure 1), and a small basin on the northwest end of the Sheep Range, ca. 100 km east-southest of Yucca Mountain. The widespread paleohydrologic evidence for a drier ice-age climatic regime in the vicinity of the Nevada Test Site, relative to the central and northern Great Basin, conflicts with many popular conceptions of the effect of the last pluvial episode. But there are several lines of evidence that indicate that the last pluvial climatic regime was actually relatively arid. The data from two sites within the southern Great Basin are relevant to understanding the chronology of paleohydrologic changes: Searles Lake in southeastern California (Smith, 1979; Smith and Street-Perrott, 1983) and the springs of Las Vegas Valley in southern Nevada (Haynes, 1967; Quade, 1986; Quade and Pratt, 1989). Both localities have received careful study, and their chronologies are well controlled through stratigraphy and multiple radiometric dates. Benson et al (1990) have proposed an alternative chronology for Searles Lake that may affect the correlations apparent in Figure 1. However, the data currently available are insufficient to discard the previously established chronology of Smith (1979). Artesian springs are end-points of a hydrologic system that differ from that of a pluvial lake. High stands of a pluvial lake largely reflect runoff (Enzel et al., 1989), and spring discharge is affected by aquifer recharge (chiefly snow melt in the high mountains). In this case each system should be sensitive to significant pluvial episodes. The Paleozoic carbonate aquifer of the Las Vegas Valley is a confined 1   See Appendix C Supplement for descriptions and explanations of plant species used as indicators of moisture and temperature in paleoclimate studies.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 1 Comparison of paleohydrologic chronologies from the southern Great Basin (Searles Lake from Smith and Street-Perrott, 1983; Las Vegas Valley from Quade, 1986) with the temporal distribution of midden samples from Yucca Mountain and vicinity.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? system, and increased recharge in the highlands of the Spring and Sheep Ranges should steepen the hydrologic gradient, resulting in a rapid (relative to radiocarbon-based chronological control which is expressed with standard deviations of up to a thousand years or more) increase in spring discharge at the end of that gradient. Thus, the apparent correlation of major “pluvial” episodes evident in a comparison of the Searles Lake and Las Vegas Valley records (Figure 1) indicates that they may provide a reliable chronology of late Quaternary paleohydrologic changes in the southern Great Basin. Ancient packrat (Neotoma spp.) middens provide much of the data discussed here, and descriptions of these deposits and methods used in their analysis are offered by Betancourt et al. (1990). Composed primarily of mummified plant fragments and fecal pellets encased in a matrix of crystallized packrat urine, they are common in the hills of the region. Plant macrofossils from a midden represent an autochthonous accumulation; most workers assume that a 30 to 50 m radius around the den encompasses nearly all packrat foraging activities. Although normally dominated by the remains of one or two plant species, the assemblages also contain a diverse array of other plants. These assemblages are used to infer climatic conditions at a given time, established by radiocarbon dating organic material selected from the midden sample. THE PALEOECOLOGICAL RECORD The paleocological record provided by both fossil pollen and plant macrofossils from radiocarbon dated packrat middens provides direct evidence of environmental conditions during the last glacial age. Before the development of a comprehensive packrat midden record, the most detailed paleoecological evidence for the region was provided by fossil pollen studies from Tule Springs in the Las Vegas Valley. Wisconsin-age sediments there yielded abundant pine pollen, and were used to reconstruct glacial-age vegetation zonation in which pinyon-juniper woodland (Pinus monophylla - Juniperus osteosperma) and pondersa pine - white fir forest (Pinus ponderosa - Abies concolor) extended down into the valleys some 1000 m below their current lower elevational limits (Mehringer, 1967). This reconstruction was in accord with paleoclimatic models that were most widely accepted at that time (e.g. Leopold, 1955; Antevs, 1948): a mild and wet pluvial climatic regime with average annual precipitation perhaps double today's meager amount and a decline in average annual temperature of less than 5°C (∆Ta<5°C). Fossil sites that provide information on environmental change in

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? the Nevada Test Site area span ca. 2000 m of relief, from low elevations in the Amargosa Desert and Las Vegas Valley, to high elevations in the Sheep Range. Fossil records from such an extensive gradient can be used to illustrate some principal features of the late Quaternary environments of the region, and to demonstrate that the area was relatively dry during the late Quaternary regardless of whether a “pluvial” or “interpluvial” climatic regime prevailed. The assertion that pluvial climates lead to radical rises (>10 m) in water table elevation during the late Quaternary presupposes that those climates were characterized by substantial (>100 percent above modern) increases in average annual rainfall. The most efficient means of testing this hypothesis is to refer to the existing paleoecological record for evidence of (1) full-glacial climatic conditions, and (2) plants that require wet-ground to exist (phreatophytes) regardless of when they occurred. The first set of data yields insight into climate during the time when conditions were most different from those of the present. The second set of data yields evidence for the extent and timing of effluent ground water where none now exists. Selected Fossil Records During the last glacial age juniper woodland extended to the valley bottoms while the driest sites supported shrubs such as shadscale (Atriplex confertifolia) and sagebrush (Artemisia subgen. Tridentatae). Nowhere in the Wisconsin fossil record is there evidence for the desert shrubs that typify the current Mojave Desert. Above ca. 1600 m on tuffaceous substrate, and above 1800 m on calcareous rocks, there existed subalpine conifer woodland dominated by limber pine (Pinus flexilis). No macrofossil evidence has been found for ponderosa pine-white fir forest during the last glacial age (Spaulding, 1985, 1990; Spaulding et al., 1983). Such forest vegetation would indicate wetter, milder conditions than those which appear to have prevailed. The Skeleton Hills-1 (Sk-1) midden, from 910 m elevation in the Amargosa Desert, is a low-elevation fossil record where the response of vegetation to increased effective moisture can be expected to have been most pronounced. In addition, it is from a north-facing slope and therefore not subject to the drying conditions found on south-facing slopes. The Sk-1A(12) assemblage, dated at 36.25 ±.91 ka, contains abundant Utah juniper, and woodland and steppe shrubs (Figure 2). The full-glacial Sk-1A(2) assemblage is broadly similar in that it is also dominated by juniper, but there are significant differences. The abundance of steppe shrubs in the Sk-1A(2) sample, relative to Sk-1A(12) (Figure 2), indicates a dry and cold climate at ca. 17

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 2 Relative abundance of indicator plant taxa in samples from the Skeleton Hills-1 packrat midden, Amargosa Desert. Abbreviations are: LADI, Larrea divaricata; JUOS, Juniperus osteosperma; MOUT, Mortonia utahensis; SCRI, Scopulophila rixfordii. ka. In particular, the abundance of shadscale (60% of the total number of identified specimens excluding juniper) on this north-facing slope indicates that relative aridity was still an important feature of this environment. The subsequent early and late Holocene assemblages from the Sk-1 site reflect the importance of desert shrubs in the surrounding vegetation and contain no woodland species. Similar patterns are seen at higher elevations. Willow Wash-4 (WW-4), at 1580m elevation in the Sheep Range, was near the upper limit of glacial-age juniper woodland. This site, on a north-facing slope, yields two samples that are of probable Middle Wisconsin age. The WW-4E (>44 ka) and WW-4C(1) (24.4 ±.760 ka) midden assemblages contain abundant Utah juniper, but fewer steppe shrubs than do the full-glacial WW-4C(2) and WW-4D samples (19.020 ±.750 and 17.7 ±.740 ka, respectively; Figure 3). Montane and subalpine species also are more abundant in the Middle Wisconsin samples. The abundance of steppe shrubs on this north facing slope, at a relatively high elevation, indicates that full-glacial climates were typified by cold, dry conditions.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? A single early Holocene sample (WW-4B) contains abundant juniper like the Wisconsin-age assemblages, but with a suite of desert shrubs that sets it off from the older records (Figure 3). The modern WW-4 sample contains a larger proportion of steppe shrubs than the early Holocene assemblage. The late Holocene climate therefore appears to have been drier and colder than that of the terminal Wisconsin or early Holocene (Spaulding and Graumlich, 1986). Evidence of Phreatic Environments The extensive Wisconsin-age spring deposits in the larger valleys of southern Nevada attest to a considerable expansion of wet-ground habitats during the last glacial age (Quade, 1986; Quade and Pratt, 1989). But this geological evidence is restricted to valley-bottom settings. Figure 3 Relative abundance of indicator plant taxa in samples from the Willow Wash-4 packrat midden, southeastern Sheep Range. Abbreviations are: CELA, Ceratoides Lanata.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? What of the larger drainage systems of the Spring and Sheep Ranges and those of the highlands of the Nevada Test Site? Wet-ground species appear only rarely in the southern Great Basin midden record. Five records are of “long-distance transport taxa, ” primarily the isolated seed shells of net-leaf hackberry (Celtis reticulata). Hackberry seeds are durable and were probably transported to the midden sites by birds. This tree requires perennially moist conditions to survive in the Great Basin today, and these remains may have been transported from expanded and more numerous springs and water courses. To date there are but two sites that provide unequivocal records of wet-ground habitat in uplands where none now exists. Abundant hackberry seeds from Deadman Canyon-2 in the Sheep Range indicate the presence of perennial water at 2075m elevation 9.8 ka. The tree is now extinct in the range (Spaulding, 1981). The other site is in Fortymile Canyon (FMC), the major drainage of the western Nevada Test Site. The FMC-7 midden site lies at ca. 1250 m elevation and is a small rock shelter ca. 60 m above the canyon floor. Samples from the top and bottom strata of the FMC-7 midden yielded dates of 47.2 ±3.0 and >52.0 ka (FMC-7(1) and FMC-7(3), respectively). The twigs and seeds of wild-rose (Rosa woodsii) and willow (Salix sp.) were found in FMC-7(1) and FMC-7(3). In addition, FMC-7(1) contained the seeds of marsh knotweed (Polygonum lapathifolium-type). Willow and marsh knotweed are obligate wet-ground species while wild-rose is restricted to perennially moist environments in the Great Basin. The relative abundance of all wet-ground taxa is higher in FMC-7(1) than in any other assemblage from the locality (Figure 4). The general composition of the FMC-7 samples suggests a warmer environment than during the full-glacial, based on prior studies (Spauld-ing et al., 1984). Evidence for perennial water during a warmer climatic episode also applies to the other fossil records of the wet-ground plants, which all date to the early Holocene ( Table 1). PALEOCLIMATIC RECONSTRUCTIONS Important contrasts between Middle and Late Wisconsin fossil records are (1) wet-ground plant species appear to have been more abundant during the Middle Wisconsin, and (2) steppe shrubs appear to have been dominant during the Late Wisconsin. These suggest that effective moisture and temperature may have been lower during the Late Wisconsin, and particularly during the full glacial. This makes sense on meteorological grounds alone. Enhanced temperatures can lead

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 4 Indicator taxa in packrat midden samples from Fortymile canyon, immediately east of North Yucca Mountain, as percentages of the total number of taxa (N) encountered in those samples. Note the relative abundance of riparian plants in the oldest two assemblages, and the paucity of these species in the younger assemblages. to increased precipitation through increased atmospheric competence. The ability of the atmosphere to evaporate and transport water ( “competence”) is strongly affected by air temperature. Full-Glacial Climates The idea of a relatively cold and dry full glacial climate is not new (Galloway, 1983) but it conflicts with a model of a full glacial characterized by equable temperatures and substantially increased annual precipitation (Van Devender et al., 1987). Issues that are important in this conflict include whether key macrofossil assemblages actually relate to full-glacial climate, and whether paleoclimatic reconstructions derived from one area in the Southwest can be extended to another, bioclimatically distinct area. Paleoecological data indicating a substantial reduction in winter temperature (−∆Tw) in the study area during the Wisconsin have been discussed by Spaulding (1985). The exclusion of warm-desert plants from even the lowest and most arid sites is consistent with a −∆Tw of

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Table 1 Packrat midden records of hydrophilic species from the Nevada Test Site and vicinity. Area & Site N. lat. W. long. Elev. (m) Sample Species Source 14-C date (ka) Amargosa Desert Skeleton Hills-1 36° 32' 116° 20' 910 Sk-1B(2) Celtis reticulata seed long distance 9.2 ± 0.14 Skeleton Hills-2 36° 38' 116° 17' 940 SK-2(2) muskrat tooth long distance 8.17 ± 0.1 Sheep Range Willow Wash-4 36° 28' 115° 15' 1585 WW-4B Populus sp. twig local (?) 9.82 ± 0.11 Flaherty Shelter 36° 30' 115° 14' 1650 Unit 3/125cm Celtis reticulata seed long distance * Deadman-2 36° 37' 115° 16' 2075 Dm-2 Celtis reticulata seeds local 9.56 ± 0.18 North Yucca Mountain Fortymile Canyon-7 36° 57' 116° 22' 1250 FMC-7(1) Salix sp., Rosa woodsii Polygonum lapathifolium-type local 47.2 ± 3.0   36° 57' 116° 22' 1250 FMC-7(3) several phreatophytes local >52 Sandy Valley Sandy Valley-2 35° 53' 115° 42' 935 SaV-2(3)3 Celtis reticulata seed long distance 9.4 ± 0.09 * Sample from bioturbated cave sediment, 75 cm below a radiocarbon date of 6.95 ± 0.32 ka (A-1297).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? at least 6°C. The prevalence of steppe shrubs and drought-adapted conifers suggests similarities between the full-glacial fossil records from this area and the flora of the northern Great Basin. Most reconstructions call for a depression of full-glacial summer temperatures (−∆Ts) in excess of −∆Tw, (Spaulding et al., 1983). Values of −∆Ts derived from elevational depressions of key plant taxa in the Nevada Test Site region range from 6.4° to 9°C. The decline in average annual temperature (−∆Ta) in the study area during the Late Wisconsin is estimated to have been ca. 7°C (Spaulding, 1985). These reconstructions accord well with the distribution of relict features indicating permanently frozen ground on Great Basin mountain ranges (Dohrenwend, 1984). Moisture-loving, montane species such as ponderosa pine and white fir were expected in the fossil record when the model of a mild, moist glacial-age climate was thought to apply (e.g., Mehringer, 1967). Subsequent research shows that these plant species were actually rare (white fir) or apparently absent (ponderosa pine) from the fossil record of the southern Great Basin (Spaulding, 1990). An estimated increase in average annual precipitation (Pa) of 40 percent is all that is required to account for the paleobiotic record in this region (Spaulding, 1985). With the fossil record dominated by drought-and coldtolerant species it is difficult to see how the increase could have been greater. General analogs and model simulations suggest a reduction in summer precipitation below today's meager amounts (presently <25 percent of the annual total in the Yucca Mountain area), and a substantial increase in winter precipitation to account for the apparent increase in Pa. This strong winter-seasonality of precipitation during the last glacial age is analogous to present conditions in the central and northern Great Basin. What changes in atmospheric circulation forced increased winter precipitation? The modeled position of full-glacial (18.0 ka) airflow over North America (Figure 5) shows pronounced southward displacement of the mean annual position of the westerly jet stream (COHMAP, 1988). It currently enters North America over the Pacific Northwest, but was deflected southward during the full-glacial by the massive Laurentide ice sheet and an enhanced pole-equator pressure gradient. The fact that it may have entered the continent south of the Nevada Test Site area (Figure 5) could account for evidence of mild-moist conditions at sites in southern Arizona, and colder, drier conditions to the north (Spaulding and Graumlich, 1986). Suppression of summer rainfall as a consequence of reduced Ts is well simulated by climate models (Spaulding and Graumlich, 1986; COHMAP, 1988).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 5 Model simulations of atmospheric circulation at (A) 18 ka (B) 12 ka and (C) 9 ka (COHMAP, 1988). Note that the model resolution is such that the outline of the North American continent is only approximate. Prescribed ice sheet extent indicated by hachured area, modeled mean position of the westerly jet indicated by stippled area. Arrows indicate stronger surface winds than present in winter (W) and summer (S). ∆, Position of Yucca Mountain on the model grid. The numbers are geographic reference points: (1) 35.5° N. Lat., 123.75 ° W. Long; (2) 26.6° N. Lat., 116.25° W. Long.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? The Terminal Wisconsin-Early Holocene The southern Great Basin paleohydrologic record indicates that there was general decline in effective moisture between ca. 17 and 13 ka (Figure 1), even given the meager amounts that characterized the full glacial. This is consistent with model simulations of late-glacial climate change (Figure 5). Model simulations indicate that the northward retraction of the westerly jet between 18 and 12 ka was largely due to the retreat of the North American ice sheets (COHMAP, 1988). The result in the Nevada Test Site area should have been a decline in the frequency and intensity of winter precipitation events. The later (13 to 14 ka) high stand of Pluvial Lake Lahontan to the north (Benson and Thompson, 1987) may have been caused by increased precipitation associated with the repositioning of the prevailing westerlies at this more northerly latitude. Despite evidence for the northward retreat of the westerly jet, effective moisture continued to exceed that of the present until the close of the early Holocene in Southern Nevada (Spaulding, 1985; Van Devender et al., 1987). The Searles Lake and the Las Vegas Valley records even indicate episodes of increased effective moisture (Figure 1). What could have caused this? Some suggest that it was due to the persistent southward displacement of the mean position of the prevailing westerlies (Van Devender et al., 1987). An alternative explanation is that a rather different precipitation regime prevailed in the Southwest between ca. 12 and 8 ka (Spaulding, 1990; Spaulding and Graumlich, 1986). At most sites there was near-complete turnover in community composition between 13 and 11 ka. This involved the reduction or elimination of steppe shrubs and low-elevation conifer populations. Increases in summer-seasonality of precipitation and warmer winter temperatures may account for these changes. Warmer winters also are indicated by the expansion of desert shrubs at higher elevations (e.g. Figure 3). Increased summer precipitation is indicated by the importance of grasses and succulents (agave, yucca, cacti) in low-elevation fossil records between ca. 12 and 8 ka (Spaulding, 1990). Variations in incoming solar radiation forced by perturbations in the Earth's orbit have ultimate effects on global climate (regulation of the glacial-interglacial cycle; Berger et al., 1984) and more immediate effects on regional climate (e.g., Kutzbach and Street-Perrott, 1985). Among the best documented immediate effects are climate changes associated with the last summer insolation maximum. At about 10 ka orbital perturbations resulted in a ca. 8 percent increase in Northern Hemisphere summer insolation, and a corresponding decrease in

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? winter insolation. The modeled effects of enhanced summer radiation in the northern subtropics are an increase in monsoonal (summer) rainfall, and extension of the monsoonal rainfall regime north of its present seasonal limit. These predictions are confirmed by the global paleohydrologic record (Kutzbach and Street-Perrott, 1985; Ritchie et al., 1986). Model simulations of seasonal precipitation over an area-averaged Southwest also show increased summer precipitation beginning by 12 ka (Spaulding and Graumlich, 1986). Intensified thermal lows would result in enhanced advective flow of maritime tropical air into the desert interior as moister, colder air would be drawn from the ocean into the region of low pressure. The strengthening and northward displacement of subtropical high pressure systems would have provided additional moisture aloft during the summer (Figure 5). Could a “monsoonal pluvial” account for the record of increased lake levels and the reactivation of artesian springs between 12 and 8 ka? Perhaps not. Today runoff and recharge are critically dependent on winter precipitation (Enzel et al., 1989; Winograd and Thordarson, 1975). Presently, summer rainfall has little effect on the annual hydrologic budget of dry lake basins, and is not thought to contribute significantly to recharge. Thus it is necessary to entertain some alternative possibilities. Perhaps both summer and winter precipitation exceeded present amounts between 12 and 8 ka. It also is possible that non-analogous seasonal insolation regimes (COHMAP, 1988) and ocean-land thermal gradients resulted in non-analogous runoff and recharge conditions. Under these conditions, analogies based on historic hydrologic data may provide imperfect measures of what could have occurred during the terminal Wisconsin and early Holocene. Given these and other uncertainties, it is premature to offer specific climatic reconstructions for the terminal Wisconsin and early Holocene. Early Holocene records of wet-ground plant species suggest that recharge was still sufficient to support expanded springs and water courses, despite the development of increasingly arid vegetation on upland slopes. The contrasting lack of wet-ground plants from full-glacial middens is suggestive, but may be simply due to sample distribution (Figure 1). Climates of the Last 8000 Years Essentially modern vegetation and climatic conditions were established in the Southwest between 7.8 and 7 ka. Within the last seven millennia it appears unlikely that ∆Ta exceeded 1.5°C and Pa varied

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? more than 20 percent from current long-term means. However, there were marked variations within these limits. Much of the first half of the middle Holocene (from ca. 7.5 to 5.5 ka) appears to have been effectively more arid than the present (Hall, 1985; Spaulding, 1991). And much of the late Holocene (after 3.5 ka) appears to have been characterized by levels of effective moisture equal to or slightly exceeding those of the present (Cole and Webb, 1985; Spaulding, 1990). CONCLUSIONS AND INFORMATION NEEDS There is virtually no evidence in the glacial-age fossil record for an increase in average annual precipitation exceeding 40 percent of modern amounts. The nature of full-glacial vegetation can be attributed to a relatively cold and dry climatic regime with increased average annual precipitation approximately 40 percent above that of the present, coupled with mean annual temperatures approximately 7°C below those of the present. The paucity of records of wet-ground vegetation, and their absence during the full glacial, is consistent with the assertion that pluvial climates in this region were much drier than that which would be inferred from the standard application of the word “pluvial.” The one record of local wet-ground vegetation in Fortymile Canyon, dated to the Middle Wisconsin, is consistent with modeled responses of ground-water to increased recharge north of a steep drop in the calculated position of the potentiometric surface north of Yucca Mountain (Czarnecki, 1985). This record deserves further consideration in part because it lies ca. 60 m above the present floor of Fortymile Canyon, and it points to the need for additional investigations of paleohydrologic conditions in the vicinity. However, it is not evidence for a radical change in the elevation of the water table in the vicinity of the Yucca Mountain, south of the break in elevation of the potentiometric surface. Moreover, full-glacial middens from the same area provide no evidence of wet-ground vegetation. Along with a need for continued search for evidence of perennially moist conditions in presently-dry water courses in the area, there is also need for high-elevation (>2000 m) fossil records contemporaneous with a possible latest Wisconsin-early Holocene pluvial episode to resolve the apparent contradiction between increased discharge (presumably as a consequence of increased high-elevation recharge) and evidence for increased aridity in existing fossil records. There is also a need for information and techniques that would allow more sophisticated climatic interpretation of the fossil data. The known climatic affinities of plant species within a given fossil

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? assemblage could be used for quantitative derivation of paleoclimatic parameters, if standardized data existed. Establishment of a database relating species' ranges to measured climatic parameters, and its application to the macrofossil record, would provide a great deal of new information on climatic stability of the Nevada Test Site and vicinity. REFERENCES Beatly, J.C. 1975. Climates and vegetation pattern across the Mojave/Great Basin Desert transition of southern Nevada American Midland Naturalist. 93: 53-70. Beatly, J.C. 1976. Vascular plants of the Nevada Test Site, and central-southern Nevada National Technical Information Service Rpt. TID-26881. Benson, L. V. and R. S. Thompson. 1987. The physical record of lakes in the Great Basin, North America and adjacent oceans during the last deglaciation In W. F. Ruddiman and H. E. Wright, Jr., eds. 241-260. Geological Society of America. Boulder. Benson, L. V., D. R. Currey, R. I. Dorn, K. R. Lajoie, C. G. Oviatt, S. W. Robinson, G. I. Smith, and S. Stine. 1990. Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years Palaeogeography, Palaeoclimatology, Palaeoecology 78: 241-286. Berger, A. L., J. Imbrie., J. Hays, G. Kukla, and B. Saltzman, eds. 1984. Milankovitch and climate: Understanding the response to astronomical forcing Dordrecht, D. Reidel. Betancourt, J. L., T. R. Van Devender and P. S. Martin, eds. 1990. Fossil packrat middens: The last 40,000 years of biotic change in the American southwest Univ. Arizona Press Tucson. COHMAP Project Members. 1988. Climatic changes of the last 18,000 years: Observations and model simulations Science. 241: 1043-1052. Cole, K. L., and R. H. Webb. 1985. Late Holocene vegetation changes in Greenwater Valley, Mojave Desert, California Quaternary Research. 23: 227-235. Czarnecki, J.B. 1985. Simulated effects of increased recharge on the ground-water flow system of Yucca Mountain and vicinity, Nevada-California: U.S. Geological Survey Water-Resources Investigations Report. 84-4344 Dohrenwend, J. C. 1984. Nivation landforms in the western Great Basin and their paleoclimatic significance Quaternary Research. 22: 275-288. Enzel, Y., D. R. Cayan, R. Y. Anderson, and S. G. Wells. 1989. Atmospheric circulation during Holocene lake stands in the Mojave Desert: Evidence of regional climate change Nature. 341: 44-48. Galloway, R. W. 1983. Full-glacial southwestern United States, mild and wet or cold and dry? Quaternary Research. 19: 236-248. Gilbert, G.K. 1890. Lake Bonneville. U.S. Geological Survey Monograph 1. Hall, S. A. 1985. Quaternary pollen analysis and vegetational history of the Southwest In Pollen records of late Quaternary North American sediments, V.M. Bryant, Jr., and R.G. Holloway, eds. 95-124. American Association of Stratigraphic Palynologist Foundation Dallas, TX. Kutzbach, J. E. and F. A. Street-Perrott. 1985. Milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr BP Nature. 317: 130-134.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Mehringer, P. J., Jr. 1967. Pollen Analysis of the Tule Springs site, Nevada. In Pleistocene studies in southern Nevada, H. M. Wormington and D. Ellis, eds. Nevada State Museum Anthropological Papers. 13: 129-200. Olausson, E., Ed. 1982. The Pleistocene/Holocene boundary in southwestern Sweden. Sveriges Geologiska Undersoking, Serie C. NR. 74. Stockholm. Peng, T.-H., J. G. Goddard, and W. S. Broeker. 1978. A direct comparison of 14C and 230Th ages at Searles Lake, California Quaternary Research. 9: 319-329. Porter, S. C., ed. 1983. The Late Pleistocene. University of Minnesota Press, Minneapolis. Quade, Jay. 1986. Late Quaternary environmental changes in the Upper Las Vegas Valley, Nevada Quaternary Research. 26: 340-357. Quade, Jay and W. L. Pratt. 1989. Late Wisconsin groundwater discharge environments of the southwestern Indian Springs Valley, southern Nevada Quaternary Research. 31: 351-370. Smith, George I. 1979. Subsurface stratigraphy and geochemistry of Late Quaternary evaporites, Searles Lake, California U. S. Geological Survey Professional Paper.1043. Spaulding, W. G. 1981. The late Quaternary vegetation of a southern Nevada mountain range Ph.D. thesis, University of Arizona, Tucson. Spaulding, W. G. 1985. Vegetation and climates of the last 45,000 years in the vicinity of the Nevada Test Site, south-central Nevada U. S. Geological Survey Professional Paper. 1329. Spaulding, W. G. 1990. Vegetational and climatic development of the Mojave Desert: The last glacial maximum to the present In Packrat middens: The Last 40,000 years of biotic change J. L. Betancourt et al., eds. 166-199. University of Arizona Press Tucson. Spaulding, W. G. 1991. A middle Holocene vegetation record from the Mojave Desert and its paleoclimatic significance Quaternary Research. 35: 427-437. Spaulding, W. G., E. B. Leopold, and T. R. Van Devender. 1983. Late Wisconsin paleoecology of the American Southwest In The Late Pleistocene, S.C. Porter, ed. 259-293. University of Minnesota Press. Minneapolis. Spaulding, W. G., S. W. Robinson, and F. L. Paillet. 1984. Preliminary assessment of climatic change during Late Wisconsin time, southern Great Basin and vicinity, Arizona, California, and Nevada U. S. Geological Survey Water-Resources Investigations Rpt. 84-4328 Spaulding, W. G., and L.J. Graumlich. 1986. The last pluvial climatic episodes in the deserts of southwestern North America Nature. 320: 441-444. Van Devender, T. R., Betancourt, J. L., and R. S. Thompson. 1987. Vegetation history of the deserts of southwestern North America: The nature and timing of the Late Wisconsin-Holocene transition In North America and adjacent oceans during the last deglaciation W. F. Ruddiman and H. E. Wright, Jr., eds. 323-352. Geolological Society of America. Boulder. Winograd, I. J. and W. Thordarson. 1975. Hydrogeologic and hydrochemical framework, south-central Great Basin, with special reference to the Nevada Test Site U. S. Geological Survey Professional Paper 712-C.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? APPENDIX C SUPPLEMENT Indicator Plant Species in Paleoclimate Studies Warm desert plant species: Frost-sensitive plants generally restricted to elevations between 1700 m and sea level, and to latitudes south of 38°N in the Mojave Desert. These species were absent from the Yucca Mountain region before 10 ka. They are indicative of a warm-temperate, desert environment. creosote bush (Larrea divaricata). The most widespread shrub in the warm deserts of North America, creosote bush did not arrive in the Yucca Mountain area until sometime after 8.2 ka. Its absence from even the most arid habitats during the last glacial age is attributed to minimum winter temperatures that were at least 7°C lower than present winter minima (Spaulding, 1985). Cold desert or steppe plant species: Frost-tolerant plants generally restricted to elevations above 1000 m, and to latitudes north of 38°N in the Great Basin. These species were present in the Yucca Mountain region during the last glacial age, and persist there today, although they are not as widespread. They are indicative of a cold-temperate desert environment. bunchgrass (Agropryon spp.). A large grass frequently associated with sagebrush. sagebrush (Artemisia subgen. Tridentatae). In the southern Great Basin this subgenus includes A. tridentata (big sagebrush) which is common on alluvial fans and sandy bottoms of the higher (>1500 m) valleys of the Great Basin, and A. nova (black sagebrush) which is common on rocky substrate and is frequently associated with dry woodland communities (pinyon-juniper and subalpine conifer woodland). These two species are indistinguishable based on the leaves and twigs preserved in macrofossil assemblages. shadscale (Atriplex confertifolia). This desert shrub is commonly found today near valley bottoms at elevations above 1000 m. It is not known to occur in woodland vegetation today. However, glacial-age macrofossil assemblages indicate that, on calcareous substrate, it commonly occurred in juniper woodland. This anomalous association is attributed to shadscale's extreme tolerance of freezing temperatures (relative to other desert shrubs; Beatley, 1975), and the absence of many other shrub species

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? during the last glacial age that likely compete with it today (Spaulding, 1985). Woodland plant species: Woodland is a term applied to plant communities dominated by short trees, less than 10 m in height, that typically display open spacing and, therefore, possess a diverse shrub component in the well-lighted areas. There are two types of woodland in this region: pinyon-juniper woodland that occupies intermediate elevations immediately above desert scrub vegetation, and subalpine conifer woodland that occupies the highest-elevation peaks and ridges. limber pine (Pinus flexilis). Today limber pine is found above 2600 m elevation only in the highest mountain ranges of the region, and extensive populations are known to occur only in the Spring Range, with more limited populations in the Sheep Range and the Groom Range. This tree was the most common subalpine conifer in the region during the last glacial age, and subalpine woodland dominated by limber pine occurred above 1800 m elevation in the Yucca Mountain area. Elsewhere, on mountain ranges with calcareous soils derived from limestone and dolomite, bristlecone pine (Pinus longaeva) shared dominance with limber pine in ice-age subalpine woodlands. Limber pine is drought tolerant, and the widespread occurrence of subalpine conifer woodland dominated by this tree, usually associated with sagebrush, indicates a relatively dry and cold climate. pinyon pine (Pinus monophylla). Although there are other species of pinyon pine in western North America, single-needle pinyon pine is the widespread pinyon of the Great Basin, and it is the pinyon species that occurs in the glacial-age fossil record of the Yucca Mountain area. Today restricted to elevations above ca. 1700 m, it occurred as low as 940 m in the vicinity of Yucca Mountain during the Late Wisconsin. It is less drought tolerant than Utah juniper, and is less frequently encountered in the fossil record than juniper, its common modern associate. Ranging as far north as ca. 42°N lat. today, the northern-most record of pinyon for the full-glacial comes from the Skeleton Hills on the eastern border of the Amargosa Desert (36° 38 N lat.; Spaulding, 1990). Utah juniper (Juniperus osteosperma). Together with pinyon pine, this tree species comprises the pinyon-juniper woodland which covers vast areas at middle elevations in the hills and mountains of the Great Basin. Presently restricted to elevations above

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? ca. 1600 m, it occurred as low as 400 m during the last glacial age. Although it often shares dominance with pinyon in today's woodlands, the apparent scarcity of pinyon during the last glacial age indicates that juniper woodland, rather than pinyon-juniper woodland, occupied the hills and valleys of the region that today support only desertscrub vegetation. Forest plant species: Forest is rare in the region today, and is restricted to the higher elevations (>ca. 2200 m) of the Spring and Sheep Ranges in southern Nevada. Initial reconstructions of vegetation and climate based on pollen analysis during the 1960's portrayed forest vegetation as occupying the valley flanks of the Mojave Desert (Mehringer, 1967), consistent with the concept of a pluvial climatic regime characterized by greatly increased precipitation (perhaps 100% of present average annual amounts) and a modest decline in temperature (<4°C below present average annual temperature). Subsequent research based on plant macrofossil assemblages from packrat middens indicates that forest may have been absent, and drought-adapted woodland was widespread. ponderosa pine (Pinus ponderosa). This tree is unknown in the glacial-age macrofossil record of the region. The oldest record of ponderosa pine is dated to ca. 10 ka and comes from a site within its modern elevational range at 2400 m elevation. white fir (Abies concolor). Occasional glacial-age fossil records of this species are usually associated not with ponderosa pine, its common modern associate, but with the subalpine conifer limber pine (Pinus flexilis). A single needle of white fir from a packrat midden dated 15.9 ka. from the eastern flank of North Yucca Mountain indicates that this tree probably occupied higher elevations in the area. Wet-ground plant species: Also known as phreatophytes, these plants require perennially moist soils in order to grow and reproduce. Obviously, they are rare in the Mojave Desert today. Tests for their presence in macrofossil assemblages provide one means of assessing the impact of climates of the last glacial age on the hydrology of this area. net-leaf hackberry (Celtis reticulata). This plant occurs either as a tree or shrub near springs and along rivers in the less arid portions of the Southwest. Not recorded by Beatley (1976) for the Nevada Test Site and vicinity, it was recently discovered grow-

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? ing on a small seep in the middle reaches of Fortymile Canyon. The seeds of this tree are widely transported by birds, and are occasionally present in early Holocene macrofossil assemblages from the region, but have not been encountered in glacial-age macrofossil assemblages. marsh knotweed (Polygonum lapathifolium). An herb found only on moist ground, it is not known to occur in the region today. Its macrofossils have been recovered from one site, located in Fortymile Canyon immediately east of North Yucca Mountain at 1250 m elevation. The carbon isotope dates for this record are 47.2 and >52 ka, placing it well before the last glacial maximum (ca. 18 ka). wild rose (Rosa woodsii). A thorny shrub found today along springs and seeps above 1650 m elevation, its macrofossils have been recovered from only one locality, the same one that yielded the remains of marsh knotweed (above). willow (sp.). A shrub or small tree that occurs around many springs in the area, its macrofossils were recovered from the same assemblages that yielded the remains of wild rose (above).