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Ages of travertine deposits in eastern Grand Canyon National Park, Arizona
QUATERNARY
RESEARCH
34,2C32
(19%)
Ages of Travertine Deposits in Eastern Grand Canyon National Park, Arizona BARNEY J. SZABO U.S. Geological
Survey, Box 25046, MS 963, Denver, Colorado 80225 Received December 27, 1988
Travertine deposits in eastern Grand Canyon National Park, Arizona, are inferred to have formed under conditions of effective wetness that were greater than present. Uranium-series dating of 17 samples indicates that the deposits formed at about 15,000, 71,000, 111,000, 171,000, and 338,000 yr B.P. Intervals of travertine deposition are essentially contemporaneous with dated high paleolake levels, montane glacial maxima, and high paleowater-table levels from several Great Basin localities. 8 1!390University of Washington.
sources stays dissolved in groundwater until it coprecipitates with calcium carbonate during accumulation of travertine (Winograd et al., 1988). In contrast, thorium tends to be present in negligible concentration in authigenic carbonates because thorium ions are effectively removed from groundwater by their sorption on solid surfaces of organic or inorganic particulates during transport. Therefore, measurements of 23”Th growth toward radioactive equilibrium with respect to its parent 234U permits the calculation of time elapsed since deposition in unaltered travertine. Presenting the results of uranium-series dating of the Grand Canyon travertine samples and their interpretation is the purpose of this report.
INTRODUCTION
When CaCO,-saturated groundwater emerges from conduits of aquifer rocks, CO* outgassing and evaporation result in precipitation of travertine. Rate of production of travertine depends on a supply of CO,-enriched water which acquired high a CO2 content at the recharge area as a result of vegetal and microbial activities (Gunn and Trudgill, 1982; Wood, 1985). Livnat and Kronfeld (1985) reported that springdeposited travertine can yield paleoclimatic information in now-arid regions because, in the absence of geologic processes affecting the rate of groundwater discharge, they presumably represent more effectively wetter episodes than the present. Travelers through the Grand Canyon, Arizona, have observed at several localities travertine deposits that apparently precipitated from groundwater occurring as a series of perched water bodies in permeable strata (Ford et al., 1976). Water no longer flows at many of these localities; therefore, travertine is no longer deposited. Samples of fossil travertine were collected for this study in 1978 and 1983 to investigate their suitability for uranium-series age determination. The uranium-series method for age determination seems appropriate for dating travertine because uranium from host-rock
SAMPLING AND SAMPLE PREPARATION
Travertine samples collected at seven localities (Fig. 1) display a variety of textures and structures. Travertine cements or drapes over slope rubble, occurs as paleospring mounds, or forms massive cliffs (Table 1). Most samples were collected from the outermost surfaces of the travertine deposits. The clean, dense portions of each sample were broken into smaller fragments and scrubbed in an ultrasonic bath. The fragments were then examined under magniti24
0033-5894190 $3.00 Copyright 0 1990 by tbe University of Washington. All rights of reproduction in any form reserved.
GRAND CANYON 11 Z”30’
112-15
I
I
25
TRAVERTINES 112”OO’
11 1’45’
I
I Lees Fern
Area of report
36-45
ARIZONA
36”30
10
36”00’
15
20 KILOMETERS
-
FIG. 1. Map showing Colorado River in Grand Canyon National Park and localities of travertine
cation and as much associated detritus as possible was eliminated. The cleaned material was crushed to a fine powder and ignited at 900°C for about 8 hr to convert CaCO, to CaO. Samples were dissolved in 8 N HCl and were spiked with appropriate amounts of combined 236U, 228Th, and 22?Th standard solution. Uranium and thorium isotopes were separated and purified by an anionexchange procedure (Szabo et al., 1981). Some of the travertine samples contained a substantial admixture of detrital materials. The carbonate fractions in these samples were dissolved in dilute nitric acid (about 0.1425 N), and the acid-soluble carbonate and acid-insoluble detrital fractions were separated by centrifugation (Szabo and Lindsey, 1986). The acid-insoluble frac-
samples.
tions were dissolved by repeated heating in concentrated HF and HClO, mixtures. Both acid-soluble carbonate and acidinsoluble residue fractions were then spiked, and uranium and thorium were separated from each fraction and purified by anion-exchange. Separated uranium of both carbonate and detritus fractions was electroplated on platinum discs and counted in an OLspectrometer. Thorium separates of both carbonate and detritus fractions were extracted by TTA (dissolved in benzene) and evaporated on stainless-steel discs. The uranium and thorium on the discs were then counted in an (Y spectrometer. RESULTS
The analyses of 17 travertine
samples
100
100
Cardenas CARD- 1
CARD-2
Explorers Monument EM-IV 30
100
Comanche COM-1
99.5
AWA78-2
100
Awatubi AWA78-3
99
99.5
KWA-3
AWA78- 1
100
Carbonate (%I
1. ANALYTICAL
Kwagunt KWA-1
Sample number
TABLE
0.466 * 0.010
3.06 + 0.06
6.66 0.14
6.69 2 0.13
0.778 2 0.016
0.890 0.018
2.09 + 0.03
0.936 + 0.018 1.83 -+ 0.04
1.07 f 0.02
2.63 f 0.03
2.77 t 0.04
2.55 * 0.04
1.000 + 0.015
1.004 + 0.015
1.96 + 0.03
1.64 + 0.03 1.49 2 0.02
23qJ/238u
AND CALCULATED
Uranium (mm)
DATA
23%P34U
0.688 2 0.018
1.46 ” 0.04
0.745 * 0.019
0.693 * 0.015 Elves Chasm area
>200
620 f 100
412 2 200
0.570 2 0.013
1.3
3.2
110 f 4
22 * 12’
3.4
3.0
-2
-2
108 k 5
83 k 3
>280’
46 0.966 +9 + 0.039 Tanner Canyon Area
32 +6
2.2
2.2
318 +39 -29 69 f 3
2.1
(*wPw)fLo
FROM GRAND
178 f 10
>300”
0.492 + 0.015
0.874 + 0.020 1.048 + 0.022
1.11 + 0.05
416 + 83
136 + 50 25 21
SAMPLES
Uranium-series age” W yd
AGES OF TRAVERTINE ARIZONA
Marble Canyon area
=‘?ti3*Th (activity ratio)
URANIUM-SERIES PARK,
CANYON
Trav . draping over older trav.
Trav. from paleo-spring mound do.
Trav. cementing slope rubble
Coarsely crystallized trav. Massive trav. draping slope rubble. do.
Trav. cementing slope rubble do.
Description’
NATIONAL
100
99
100
EC-l
EC78-5
H-8
2.72 2 0.04
1.26 -+ 0.02 0.978 2 0.015 2.02 * 0.02 1.47 + 0.02
1.87 t 0.03 2.34 f 0.04 1.13 2 0.01 2.10 2 0.03
1.36 -c 0.03
2.54 k 0.04
0.881 2 0.013 1.06 f 0.02 0.825 k 0.013 1.31 2 0.02 0.920 ” 0.013
2.49 f 0.04
1.01 f 0.02 0.443 + 0.010 0.690 2 .014 2.62 + 0.05 0.456 k 0.009 0.123 * 0.006
1.44 * 0.03 0.671 0.023 1.48 k 0.04 1.11 f 0.03 1.15 * 0.03
6.40 0.807 * 0.32 k 0.032 1.50 2.05 k 0.06 -+ 0.08 139 0.859 * 20 k 0.017 95 1.07 2 20 2 0.04 Havasu area 13 0.0703 *2 f 0.0021
29 22
1.26 2 0.02 2.63 0.13 1.31 * 0.03 160 ? 20 >lOO
a Calculated using half-lives of 23@Thand 234U of 75,200 and 244,000 yr, respectively. b Initial 234UP38U activity ratio. Calculated from the respective *s@Thage. ’ Estimated to be older than about 700,000 yr based on measured 234UP38U activity ratios. d Acid-insoluble detritus. e Isochron-plot age using data for both acid-soluble and acid-insoluble dervied residue fractions f Trav. = travertine. B -, not applicable.
-
EC78-4-Dd
100
EM-5
77
100
EM-6
EC78-4
-
EM-4-Dd
100
28
EM-4
Elves Chasm EC78-6
-
EM-IV-Dd
7.9 f 0.3
358 +‘l” -60
164 f 7
-
116 * 15’
14 + 2
>300’
s400
-
61 f 10’
-8 1.2
2.6
2.7
-2.3
-
1.4
2.5
-2
-2
-
-
Trav. near Beaver Falls
Trav. cementing slope rubble Trav. cementing slope rubble
Trav. over cemented slope rubble Vug ffling trav . -
Massive trav. deposit do.
Trav. vein in older trav. -
-
Y
28
BARNEY
J. SZABO
and three associated acid-insoluble residue fractions are reported in Table 1. For the set of all samples, uranium concentrations range from 0.44 to 6.7 ppm. All pure travertine samples (less than 1% detrital fraction) contained negligible amounts of 232Th relative to 238U and 23?lYh. The measured values of 23@Th/232Thrange between 13 and 620, indicating that no significant initial 230Th contamination had occurred (Ku, 1976). Assuming that all detritus-free travertine samples remained a closed system with respect to uranium isotopes and 23@Th, I calculated uranium-series ages from measured 23@Ib/234U and 234U/238U and report them in Table 1. Samples EM-IV, EM-4, and EC 78-4 contain abundant detrital residues, between 33 and 62%. For these impure samples, both acid-soluble carbonate and acid-insoluble residue fractions were analyzed and ages were obtained using an isochron-plot technique for extraneous 23”Ih correction (Szabo and Rosholt, 1982; Szabo ef al., 1989). The calculated uranium-series ages fall into five groups (Fig. 2). Three samples yielded ages between about 8000 and 30,000 yr (average is 15,000 yr), three samples yielded ages between about 55,000 and 85,000 yr (average is 71,000 yr), analysis of three samples resulted in ages between about 100,000 and 130,000 yr (average is 111,000 yr), two samples yielded ages beAGE RANGE
-8-30 A
~55-85*100-130 h h
tween about 155,000 and 190,000 yr (average is 171,000 yr), and two (possibly three) samples yielded ages between about 290,000 and 460,000 yr (average is 338,000 yr). Initial 234U/238U activity ratios were backcalculated from 230Th/234U ages and measured 234U/238U. Results show that the groundwater from which the pure travertine samples precipitated was enriched in 234U relative to 238U between about 210 and 340% (Table 1). Three of the pure travertine samples (AWA 78-l) AWA 78-2, and EM-5) have measured 234U/238U near unity. Assuming that the 234U/238U remained relatively constant and >2 in paleo-groundwater, the ages of the three travertine accumulations that have measured uranium ratios near unity can be estimated to be at least 700,000 yr old. DISCUSSION
Results of the age determinations indicate episodes of travertine accumulation in Grand Canyon National Park area between the time interval of 8000 and about 700,000 yr B.P. These episodes of travertine deposition are presumed to have been controlled by long-term fluctuations in regional climatic patterns. Fault movement that changes groundwater discharge, subterranean groundwater piracy, or plugging of conduits with rock debris or carbonate in-
-155-190 h
-290-460 A
r
\
I\
* $JMC
23
TC-
32 u)
EC--@-
s# +
+
T-----
+3-
?
H-x 0
I
I
I
I+
50
100
150
200
I 300
I
I
400
500
(lo3 YR BP.) 2. Uranium-series dates of travertine samples grouped by sampling areas. Errors shown are propagated 1 standard deviation from the counting statistics and from mathematical manipulations. Note change of scale. MC, Marble Canyon area samples from Kwagunt and Awatubi localities, TC, Tanner Canyon area samples from Comanche and Cardenas localities; EC, Elves Chasm area samples from Explorers Monument and Elves Chasm localities; H, sample from Havasu area. FIG.
GRAND
CANYON
filling also can affect travertine formation; however, if any of these processes occurred one would not expect to find evidence for multiple deposition at the same discharge localities indicated in Figure 2. The dated travertine, which presumably formed during intervals more humid than present, may be compared with the dated sedimentary sequences at Searles Lake in southwestern Great Basin that provide a semi-continuous proxy climatic record for the Pleistocene (Smith et al., 1983; Smith, 1984; Bischoff et al., 1985). The reconstructed lake-level fluctuations in Searles Lake during the last 150,000 yr are shown in Figure 3 (after Smith, 1984) together with the dated travertine depositional intervals at Grand Canyon. The two records correlate well: periods when lake levels were high and when travertine was deposited are in phase. The two older travertine deposits (at about 171,000 and 320,000 yr B.P.) may have formed during deposition of unit A + B at Searles Lake (Smith, 1984); that depositional period was characterized by fluctuating, semi-continuous lake levels without major lake expansion or salt deposition. The oldest and most massive travertine deposits in the Grand Canyon area may have been deposited during the wet cycle of hydrologic regime VI at Searles Lake, or be-
29
TRAVERTINES
tween about 1.3 and 1.0 myr ago (Smith, 1984). Many playas and dry lake basinsthroughout the Great Basin commonly contain shorelines and sedimentological evidence of former lake cycles (Smith and StreetPerrott, 1983; Benson and Thompson, 1987). Oscillation of these lake levels is generally assumed to relate to oscillation in regional climate (Forester, 1987). Quade (1986) and Quade and Pratt (1989) investigated the extent of marshes in southern Nevada and found that marshes and shallow lakes reached their maximum extent between 30,000 and 10,000 yr B.P., probably an effectively wetter climatic interval. Recent results indicate that there were two major and one minor period of high lakelevel stands in the Bonneville and Lahontan basins during the last 150,000 yr (McCoy, 1987; Lao and Benson, 1988, and references therein). The two major high paleolake stands, which occurred between about 150,000 and 100,000 and 30,000 and 11,000 yr B.P., and the minor lacustrine cycle (intermediate in age, ca. 50,000 yr B.P.) may correlate with the three youngest travertine depositional episodes at Grand Canyon. The three earlier high lake-level stands in the Bonneville basin (at about 200,000 yr, between about 600,000 and 200,000 yr, and
s* 0 EF
;;7l >B SW l-0
-
m
-
(103 YR B.P.) FIG. 3. History of salinity levels in Searles Lake, modified from Smith (1984), and waver-tine depositional intervals in eastern Grand Canyon National Park, Arizona.
30
BARNEY
sometime before 600,000 yr; McCoy, 1987) may correlate with the three earlier travertine depositional intervals, dated at about 171,000, 338,000, and >700,000 yr in the Grand Canyon area (Table 1 and Fig. 2). Radiocarbon dates of rock varnish in eastern Sierra Nevada (Dom ef al., 1987) and of peat and wood in deposits of Tulare Lake (Atwater et al., 1986) indicate that the last glaciation of the Sierra Nevada (Tioga) and associated moisture-effective period occurred between 26,000 and 13,000 yr ago. Cation-ratio dates for rock varnish of the earlier (Tahoe) glaciation yielded ages of about 150,000 and 185,000 yr (Dorn et al., 1987). These major Sierra Nevada glaciations may correlate with travertine depositional intervals 8000-30,000 and 155,000190,000 yr B.P., respectively. The best studied and dated stratigraphic record is that of the last lake-level fluctuation cycle in the Great Basin area (Benson, 1978; Peng et al., 1978; Scott et al., 1983; Thompson et al., 1986; Benson and Thompson, 1987; Bradbury et al., 1989). Most age data for the last 40,000 yr of closed basin lake history were obtained by radiocarbon dating of organic and inorganic forms of carbon, packrat-midden fossils, and rock varnish on landform surfaces. The accuracy of a radiocarbon date, however, is limited by sample reliability. Radiocarbon age estimates can be too young if addition of modem 14C occurs, or too old if components of older sediments are incorporated. These problems, related both to dating and to stratigraphic interpretation, resulted in significantly different interpretations of the timing of lake-level changes. The available age data from four major Great Basin lake systems (Lahontan, Bonneville, Mono, and Searles) were critically reexamined by Benson et al. (in press). Their assignment of ages to lake-level and lake-surface area variations indicates that the smaller lake systems (Mono Lake and Lake Searles) had more frequent lake-level oscillations than the larger lake systems (Lahontan and Bonneville). There are also
J. SZABO
significant similarities among these four major lake systems. All experienced major periods of lake-level changes with high stands occurring before about 13,000 yr B.P. and the lakes stood at moderate or high levels between about 30,000 and 10,000 yr B.P. The travertine depositional interval 8000 to 30,000 yr and the high lakelevel cycle in the Great Basin dating 10,000 to 30,000 yr B.P. appear synchronous. Direct evidence for conditions wetter than present during late Pleistocene time in southern Nevada comes from uraniumseries dating of carbonates in the Devils Hole cave system at Ash Meadows. The walls of the cave are marked by travertine that precipitated during former higher stands of the water table when conditions wetter than modern resulted in greater groundwater recharge. Travertine ages indicate that paleo-water level was at or higher than 6 m above present level between about 110,000 and 17,000 yr ago and that the peak paleo-water level, or corresponding paleo-groundwater recharge, occurred between about 40,000 and 20,000 yr ago (B. J. Szabo and co-workers, unpublished data). These speleothem dates at Devils Hole show that increase in effective moisture conditions were essentially contemporaneus with deposition of the younger travertines at eastern Grand Canyon localities. Travertine deposition in the Grand Canyon appears to be episodic and to be controlled by long-term climatic oscillations, although this interpretation is based on limited data and field observation at this time. Future investigation using improved sampling technique is planned so as to improve the data base. Dated Grand Canyon travertine depositional episodes correlate reasonably well with other climate-controlled geologic records across the Great Basin; however, attempts to correlate different regional records suffer from lack of reliable dating, large errors in reported age estimates, variations in sampling interval, and difficulties of stratigraphic interpretation.
GRAND CANYON
Nevertheless, the results suggest that climatic change in the Great Basin area may be regional in scale and in phase. Based on stable isotope measurements of precipitation and groundwater samples from southern Nevada, Benson and Klieforth (in press) suggest that past effectively wetter conditions were caused by yeararound proximity of the mean position of the jetstream and associated atmospheric frontal systems over the southern Great Basin. The near contemporaneity of several proxy records that imply climatic conditions effectively wetter than present suggest that past increases in effective moisture encompassed extensive areas of the Great Basin. ACKNOWLEDGMENTS The assistance of J. P. Schafer, P. A. O’Malley, and J. N. Rosholt in collecting some of the travertine samples is greatly appreciated. I thank J. C. Tinsley, J. L. Bischoff, and L. V. Benson for reviewing the manuscript.
REFERENCES Atwater, B. F., Adam, D. P., Bradbury, J. P., Forester, R. M., Mark, R. K., Lettis, W. R., Fisher, G. R., Gobalet, K. W., and Robinson, S. W. (1986). A fan dam for Tulare Lake, California, and implications for the Wisconsin glacial history of the Sierra Nevada. Geological Society America Bulletin 97, 97-109. Benson, L. V. (1978). Fluctuation in the level of pluvial Lake Lahontan during the last 40,000 years. Quaternary Research 9, 300-318. Benson, L. V., Currey, D. R., Dom, R. I. Lajoie, K. R., Oviatt, C. G., Robinson, S. W., and Smith, G. I. (in press). Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years. In “Paleolakes and Paleooceans” (E. P. L. Meyers and L. V. Benson, Eds.). Special issue of Paleogeography, Paleoclimatology, Paleoecology. Benson, L. V., and Klieforth, H. (in press). Stable isotopes in precipitation and ground water in the Yucca Mountain region, southern Nevada: Paleoclimatic implications. In “Aspects of Climate Variability in the Pacific and the Western Americas.” Pacific Climate Workshop’s Monograph, American Geophysical Union. Benson, L. V., and Thompson, R. S. (1987). The physical records of lakes in the Great Basin. In “North America and Adjacent Oceans during the
TRAVERTINES
31
Last Deglaciation” (W. F. Ruddiman and H. E. Write, Jr., Eds.), pp. 241-260. The Geology of North America, v. K-3, The Geological Society of America, Boulder, CO. Bischoff, J. L., Rosenbauer, R. J., and Smith, G. I. (1985). Uranium-series dating of sediment from Searles Lake: Differences between continental and marine climatic records. Science 227, 1222-1224. Bradbury, J. P., Forester, R. M., and Thompson, R. S. (1989). Late Quatemary paleolimnology of Walker Lake, Nevada. Journal of Paleolimnology 1, 249-267. Dorn, R. I., Turrin, B. D., Jull, A. J. T., Linick, T. W., and Donahue, D. J. (1987). Radiocarbon and cation-ratio ages for rock varnish on Tioga and Tahoe morainal boulders of Pine Creek, eastern Sierra Nevada, California, and their paleoclimatic implications. Quaternary Research 28, 38-49. Ford, T. D., Huntoon, P. W., Billingsley, G. H., Jr., and Breed, W. J. (1976). Rock movement and mass wastage in the Grand Canyon. In “Geology of the Grand Canyon” (W. T. Breed and E. C. Roat, Eds.), pp. 116128. Museum of Not-them Arizona, Flagstaff, and Grand Canyon Natural History Association. Forester, R. M. (1987). Late Quatemary paleoclimate records from lacustrine ostracodes. In “North America and Adjacent Oceans during the Last Deglaciation” (W. F. Ruddiman and H. E. Write, Jr., Eds.), pp. 261-276. The Geology of North America, v. K-3, The Geological Society of America, Boulder, CO. Gunn, J., and Trudgill, S. T. (1982). Carbon dioxide production and concentrations in the soil atmosphere: A case study from New Zealand volcanic ash soils. Catena 9, 81-94. Ku, T. L. (1976). The uranium-series methods of age determination. Annual Review of Earth and Planetary Sciences 4, 347-379. Lao, Y., and Benson, L. (1988). Uranium-series age estimates and paleoclimatic significance of Pleistocene tufas from the Lahohtan Basin, California and Nevada. Quaternary Research 30, 165-176. Livnat, A,, and Kronfeld, J. (1985). Paleoclimatic implications of U-series dates for lake sediments and travertines in the Arava Rift Valley. Quaternary Research 24, 164-172. McCoy, W. D. (1987). Quatemary aminostratigraphy of the Bonneville basin, western United States. Geological Society of America Bulletin 98, 99-112. Peng, T. H., Goddard, J. G., and Broecker, W. S. (1978). A direct comparison of 14C and 23“Th ages at Searles Lake, California. Quaternary Research 9, 319-329. Quade, J. (1986). Late Quatemary environmental changes in the upper Las Vegas Valley, Nevada. Quaternary Research 26, 340-357. Quade, J., and Pratt, W. L. (1989). Late Wisconsin
32
BARNEY
groundwater discharge environments of the southwestern Indian Springs Valley, Southern Nevada. Quaternary Research 31, 351-370. Scott, E. W., McCoy, W. D., Shroba, R. R., and Rubin, M. (1983). Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, westem United States. Quaternary Research 20, 261285. Smith, G. I. (1984). Paleohydrologic regimes in the southwestern Great Basin, O-3.2 myr ago, compared with other long records of “global” climate. Quaternary Research 22, 1-17. Smith, G. I., Barczak, V. J., Moulton, G. F., and Liddicoat, .I. C. (1983). “Core KM-3, a Surface-toBedrock Record of Late Cenozoic Sedimentation in Searles Valley, California.” U.S. Geological Survey Professional Paper 1256, 24 pp. Smith, G. I., and Street-Perrott, F. A. (1983). Pluvial lakes of the western United States. In “Late Quaternary Environments of the United States” (H. E. Wright, J., Ed.), Vol. 1. pp. 190-214. Univ. of Minnesota Press, Minneapolis. Szabo, B. J., Carr, W. J., and Gottschall, W. C. (1981). Uranium-thorium dating of Quatemary carbonate accumulations in the Nevada Test Site region, southern Nevada. U.S. Geological Survey Open-File Report 81-119, 35 pp.
J. SZABO Szabo, B. J., and Lindsey, D. A. (1986). Estimating limiting age for Pleistocene erosional surfaces in central Montana by uranium-series dating of associated travertines. Earth Surface Processes and Landforms 11, 223-228. Szabo, B. I., McHugh, W. P., Schaber, G. G., Haynes, C. V., Jr., and Breed, C. S. (1989). Uranium-series dated authigenic carbonates and Acheulian sites in southern Egypt. Science 243, 1053-1056. Szabo, B. J., and Rosholt, J. N. (1982). Surticial continental sediments. In “Uranium-Series Disequihbrium: Application to Environmental Problems” (M. Ivanovich and R. S. Harmon, Eds.), Chapt. 10, pp. 246-267. Oxford Univ. Press, Oxford. Thompson, R. S., Benson, L. V., and Hattori, E. M. (1986). A revised chronology for the last Pleistene lake cycle in the central Lahonton Basin. Quaternary Research 25, l-9. Winograd, I. J., Szabo, B. J., Coplen, T. B., and Riggs, A. C. (1988). A 250,000 year climatic record from Great Basin vein calcite: Implications for Milankovitch theory. Science 242, 1275-1280. Wood, W. W. (1985). Origin of caves and other solution openings in the unsaturated (vadose) zone of carbonate rocks: A model for CO, generation. Geology 13, 822-824.
RESEARCH
34,2C32
(19%)
Ages of Travertine Deposits in Eastern Grand Canyon National Park, Arizona BARNEY J. SZABO U.S. Geological
Survey, Box 25046, MS 963, Denver, Colorado 80225 Received December 27, 1988
Travertine deposits in eastern Grand Canyon National Park, Arizona, are inferred to have formed under conditions of effective wetness that were greater than present. Uranium-series dating of 17 samples indicates that the deposits formed at about 15,000, 71,000, 111,000, 171,000, and 338,000 yr B.P. Intervals of travertine deposition are essentially contemporaneous with dated high paleolake levels, montane glacial maxima, and high paleowater-table levels from several Great Basin localities. 8 1!390University of Washington.
sources stays dissolved in groundwater until it coprecipitates with calcium carbonate during accumulation of travertine (Winograd et al., 1988). In contrast, thorium tends to be present in negligible concentration in authigenic carbonates because thorium ions are effectively removed from groundwater by their sorption on solid surfaces of organic or inorganic particulates during transport. Therefore, measurements of 23”Th growth toward radioactive equilibrium with respect to its parent 234U permits the calculation of time elapsed since deposition in unaltered travertine. Presenting the results of uranium-series dating of the Grand Canyon travertine samples and their interpretation is the purpose of this report.
INTRODUCTION
When CaCO,-saturated groundwater emerges from conduits of aquifer rocks, CO* outgassing and evaporation result in precipitation of travertine. Rate of production of travertine depends on a supply of CO,-enriched water which acquired high a CO2 content at the recharge area as a result of vegetal and microbial activities (Gunn and Trudgill, 1982; Wood, 1985). Livnat and Kronfeld (1985) reported that springdeposited travertine can yield paleoclimatic information in now-arid regions because, in the absence of geologic processes affecting the rate of groundwater discharge, they presumably represent more effectively wetter episodes than the present. Travelers through the Grand Canyon, Arizona, have observed at several localities travertine deposits that apparently precipitated from groundwater occurring as a series of perched water bodies in permeable strata (Ford et al., 1976). Water no longer flows at many of these localities; therefore, travertine is no longer deposited. Samples of fossil travertine were collected for this study in 1978 and 1983 to investigate their suitability for uranium-series age determination. The uranium-series method for age determination seems appropriate for dating travertine because uranium from host-rock
SAMPLING AND SAMPLE PREPARATION
Travertine samples collected at seven localities (Fig. 1) display a variety of textures and structures. Travertine cements or drapes over slope rubble, occurs as paleospring mounds, or forms massive cliffs (Table 1). Most samples were collected from the outermost surfaces of the travertine deposits. The clean, dense portions of each sample were broken into smaller fragments and scrubbed in an ultrasonic bath. The fragments were then examined under magniti24
0033-5894190 $3.00 Copyright 0 1990 by tbe University of Washington. All rights of reproduction in any form reserved.
GRAND CANYON 11 Z”30’
112-15
I
I
25
TRAVERTINES 112”OO’
11 1’45’
I
I Lees Fern
Area of report
36-45
ARIZONA
36”30
10
36”00’
15
20 KILOMETERS
-
FIG. 1. Map showing Colorado River in Grand Canyon National Park and localities of travertine
cation and as much associated detritus as possible was eliminated. The cleaned material was crushed to a fine powder and ignited at 900°C for about 8 hr to convert CaCO, to CaO. Samples were dissolved in 8 N HCl and were spiked with appropriate amounts of combined 236U, 228Th, and 22?Th standard solution. Uranium and thorium isotopes were separated and purified by an anionexchange procedure (Szabo et al., 1981). Some of the travertine samples contained a substantial admixture of detrital materials. The carbonate fractions in these samples were dissolved in dilute nitric acid (about 0.1425 N), and the acid-soluble carbonate and acid-insoluble detrital fractions were separated by centrifugation (Szabo and Lindsey, 1986). The acid-insoluble frac-
samples.
tions were dissolved by repeated heating in concentrated HF and HClO, mixtures. Both acid-soluble carbonate and acidinsoluble residue fractions were then spiked, and uranium and thorium were separated from each fraction and purified by anion-exchange. Separated uranium of both carbonate and detritus fractions was electroplated on platinum discs and counted in an OLspectrometer. Thorium separates of both carbonate and detritus fractions were extracted by TTA (dissolved in benzene) and evaporated on stainless-steel discs. The uranium and thorium on the discs were then counted in an (Y spectrometer. RESULTS
The analyses of 17 travertine
samples
100
100
Cardenas CARD- 1
CARD-2
Explorers Monument EM-IV 30
100
Comanche COM-1
99.5
AWA78-2
100
Awatubi AWA78-3
99
99.5
KWA-3
AWA78- 1
100
Carbonate (%I
1. ANALYTICAL
Kwagunt KWA-1
Sample number
TABLE
0.466 * 0.010
3.06 + 0.06
6.66 0.14
6.69 2 0.13
0.778 2 0.016
0.890 0.018
2.09 + 0.03
0.936 + 0.018 1.83 -+ 0.04
1.07 f 0.02
2.63 f 0.03
2.77 t 0.04
2.55 * 0.04
1.000 + 0.015
1.004 + 0.015
1.96 + 0.03
1.64 + 0.03 1.49 2 0.02
23qJ/238u
AND CALCULATED
Uranium (mm)
DATA
23%P34U
0.688 2 0.018
1.46 ” 0.04
0.745 * 0.019
0.693 * 0.015 Elves Chasm area
>200
620 f 100
412 2 200
0.570 2 0.013
1.3
3.2
110 f 4
22 * 12’
3.4
3.0
-2
-2
108 k 5
83 k 3
>280’
46 0.966 +9 + 0.039 Tanner Canyon Area
32 +6
2.2
2.2
318 +39 -29 69 f 3
2.1
(*wPw)fLo
FROM GRAND
178 f 10
>300”
0.492 + 0.015
0.874 + 0.020 1.048 + 0.022
1.11 + 0.05
416 + 83
136 + 50 25 21
SAMPLES
Uranium-series age” W yd
AGES OF TRAVERTINE ARIZONA
Marble Canyon area
=‘?ti3*Th (activity ratio)
URANIUM-SERIES PARK,
CANYON
Trav . draping over older trav.
Trav. from paleo-spring mound do.
Trav. cementing slope rubble
Coarsely crystallized trav. Massive trav. draping slope rubble. do.
Trav. cementing slope rubble do.
Description’
NATIONAL
100
99
100
EC-l
EC78-5
H-8
2.72 2 0.04
1.26 -+ 0.02 0.978 2 0.015 2.02 * 0.02 1.47 + 0.02
1.87 t 0.03 2.34 f 0.04 1.13 2 0.01 2.10 2 0.03
1.36 -c 0.03
2.54 k 0.04
0.881 2 0.013 1.06 f 0.02 0.825 k 0.013 1.31 2 0.02 0.920 ” 0.013
2.49 f 0.04
1.01 f 0.02 0.443 + 0.010 0.690 2 .014 2.62 + 0.05 0.456 k 0.009 0.123 * 0.006
1.44 * 0.03 0.671 0.023 1.48 k 0.04 1.11 f 0.03 1.15 * 0.03
6.40 0.807 * 0.32 k 0.032 1.50 2.05 k 0.06 -+ 0.08 139 0.859 * 20 k 0.017 95 1.07 2 20 2 0.04 Havasu area 13 0.0703 *2 f 0.0021
29 22
1.26 2 0.02 2.63 0.13 1.31 * 0.03 160 ? 20 >lOO
a Calculated using half-lives of 23@Thand 234U of 75,200 and 244,000 yr, respectively. b Initial 234UP38U activity ratio. Calculated from the respective *s@Thage. ’ Estimated to be older than about 700,000 yr based on measured 234UP38U activity ratios. d Acid-insoluble detritus. e Isochron-plot age using data for both acid-soluble and acid-insoluble dervied residue fractions f Trav. = travertine. B -, not applicable.
-
EC78-4-Dd
100
EM-5
77
100
EM-6
EC78-4
-
EM-4-Dd
100
28
EM-4
Elves Chasm EC78-6
-
EM-IV-Dd
7.9 f 0.3
358 +‘l” -60
164 f 7
-
116 * 15’
14 + 2
>300’
s400
-
61 f 10’
-8 1.2
2.6
2.7
-2.3
-
1.4
2.5
-2
-2
-
-
Trav. near Beaver Falls
Trav. cementing slope rubble Trav. cementing slope rubble
Trav. over cemented slope rubble Vug ffling trav . -
Massive trav. deposit do.
Trav. vein in older trav. -
-
Y
28
BARNEY
J. SZABO
and three associated acid-insoluble residue fractions are reported in Table 1. For the set of all samples, uranium concentrations range from 0.44 to 6.7 ppm. All pure travertine samples (less than 1% detrital fraction) contained negligible amounts of 232Th relative to 238U and 23?lYh. The measured values of 23@Th/232Thrange between 13 and 620, indicating that no significant initial 230Th contamination had occurred (Ku, 1976). Assuming that all detritus-free travertine samples remained a closed system with respect to uranium isotopes and 23@Th, I calculated uranium-series ages from measured 23@Ib/234U and 234U/238U and report them in Table 1. Samples EM-IV, EM-4, and EC 78-4 contain abundant detrital residues, between 33 and 62%. For these impure samples, both acid-soluble carbonate and acid-insoluble residue fractions were analyzed and ages were obtained using an isochron-plot technique for extraneous 23”Ih correction (Szabo and Rosholt, 1982; Szabo ef al., 1989). The calculated uranium-series ages fall into five groups (Fig. 2). Three samples yielded ages between about 8000 and 30,000 yr (average is 15,000 yr), three samples yielded ages between about 55,000 and 85,000 yr (average is 71,000 yr), analysis of three samples resulted in ages between about 100,000 and 130,000 yr (average is 111,000 yr), two samples yielded ages beAGE RANGE
-8-30 A
~55-85*100-130 h h
tween about 155,000 and 190,000 yr (average is 171,000 yr), and two (possibly three) samples yielded ages between about 290,000 and 460,000 yr (average is 338,000 yr). Initial 234U/238U activity ratios were backcalculated from 230Th/234U ages and measured 234U/238U. Results show that the groundwater from which the pure travertine samples precipitated was enriched in 234U relative to 238U between about 210 and 340% (Table 1). Three of the pure travertine samples (AWA 78-l) AWA 78-2, and EM-5) have measured 234U/238U near unity. Assuming that the 234U/238U remained relatively constant and >2 in paleo-groundwater, the ages of the three travertine accumulations that have measured uranium ratios near unity can be estimated to be at least 700,000 yr old. DISCUSSION
Results of the age determinations indicate episodes of travertine accumulation in Grand Canyon National Park area between the time interval of 8000 and about 700,000 yr B.P. These episodes of travertine deposition are presumed to have been controlled by long-term fluctuations in regional climatic patterns. Fault movement that changes groundwater discharge, subterranean groundwater piracy, or plugging of conduits with rock debris or carbonate in-
-155-190 h
-290-460 A
r
\
I\
* $JMC
23
TC-
32 u)
EC--@-
s# +
+
T-----
+3-
?
H-x 0
I
I
I
I+
50
100
150
200
I 300
I
I
400
500
(lo3 YR BP.) 2. Uranium-series dates of travertine samples grouped by sampling areas. Errors shown are propagated 1 standard deviation from the counting statistics and from mathematical manipulations. Note change of scale. MC, Marble Canyon area samples from Kwagunt and Awatubi localities, TC, Tanner Canyon area samples from Comanche and Cardenas localities; EC, Elves Chasm area samples from Explorers Monument and Elves Chasm localities; H, sample from Havasu area. FIG.
GRAND
CANYON
filling also can affect travertine formation; however, if any of these processes occurred one would not expect to find evidence for multiple deposition at the same discharge localities indicated in Figure 2. The dated travertine, which presumably formed during intervals more humid than present, may be compared with the dated sedimentary sequences at Searles Lake in southwestern Great Basin that provide a semi-continuous proxy climatic record for the Pleistocene (Smith et al., 1983; Smith, 1984; Bischoff et al., 1985). The reconstructed lake-level fluctuations in Searles Lake during the last 150,000 yr are shown in Figure 3 (after Smith, 1984) together with the dated travertine depositional intervals at Grand Canyon. The two records correlate well: periods when lake levels were high and when travertine was deposited are in phase. The two older travertine deposits (at about 171,000 and 320,000 yr B.P.) may have formed during deposition of unit A + B at Searles Lake (Smith, 1984); that depositional period was characterized by fluctuating, semi-continuous lake levels without major lake expansion or salt deposition. The oldest and most massive travertine deposits in the Grand Canyon area may have been deposited during the wet cycle of hydrologic regime VI at Searles Lake, or be-
29
TRAVERTINES
tween about 1.3 and 1.0 myr ago (Smith, 1984). Many playas and dry lake basinsthroughout the Great Basin commonly contain shorelines and sedimentological evidence of former lake cycles (Smith and StreetPerrott, 1983; Benson and Thompson, 1987). Oscillation of these lake levels is generally assumed to relate to oscillation in regional climate (Forester, 1987). Quade (1986) and Quade and Pratt (1989) investigated the extent of marshes in southern Nevada and found that marshes and shallow lakes reached their maximum extent between 30,000 and 10,000 yr B.P., probably an effectively wetter climatic interval. Recent results indicate that there were two major and one minor period of high lakelevel stands in the Bonneville and Lahontan basins during the last 150,000 yr (McCoy, 1987; Lao and Benson, 1988, and references therein). The two major high paleolake stands, which occurred between about 150,000 and 100,000 and 30,000 and 11,000 yr B.P., and the minor lacustrine cycle (intermediate in age, ca. 50,000 yr B.P.) may correlate with the three youngest travertine depositional episodes at Grand Canyon. The three earlier high lake-level stands in the Bonneville basin (at about 200,000 yr, between about 600,000 and 200,000 yr, and
s* 0 EF
;;7l >B SW l-0
-
m
-
(103 YR B.P.) FIG. 3. History of salinity levels in Searles Lake, modified from Smith (1984), and waver-tine depositional intervals in eastern Grand Canyon National Park, Arizona.
30
BARNEY
sometime before 600,000 yr; McCoy, 1987) may correlate with the three earlier travertine depositional intervals, dated at about 171,000, 338,000, and >700,000 yr in the Grand Canyon area (Table 1 and Fig. 2). Radiocarbon dates of rock varnish in eastern Sierra Nevada (Dom ef al., 1987) and of peat and wood in deposits of Tulare Lake (Atwater et al., 1986) indicate that the last glaciation of the Sierra Nevada (Tioga) and associated moisture-effective period occurred between 26,000 and 13,000 yr ago. Cation-ratio dates for rock varnish of the earlier (Tahoe) glaciation yielded ages of about 150,000 and 185,000 yr (Dorn et al., 1987). These major Sierra Nevada glaciations may correlate with travertine depositional intervals 8000-30,000 and 155,000190,000 yr B.P., respectively. The best studied and dated stratigraphic record is that of the last lake-level fluctuation cycle in the Great Basin area (Benson, 1978; Peng et al., 1978; Scott et al., 1983; Thompson et al., 1986; Benson and Thompson, 1987; Bradbury et al., 1989). Most age data for the last 40,000 yr of closed basin lake history were obtained by radiocarbon dating of organic and inorganic forms of carbon, packrat-midden fossils, and rock varnish on landform surfaces. The accuracy of a radiocarbon date, however, is limited by sample reliability. Radiocarbon age estimates can be too young if addition of modem 14C occurs, or too old if components of older sediments are incorporated. These problems, related both to dating and to stratigraphic interpretation, resulted in significantly different interpretations of the timing of lake-level changes. The available age data from four major Great Basin lake systems (Lahontan, Bonneville, Mono, and Searles) were critically reexamined by Benson et al. (in press). Their assignment of ages to lake-level and lake-surface area variations indicates that the smaller lake systems (Mono Lake and Lake Searles) had more frequent lake-level oscillations than the larger lake systems (Lahontan and Bonneville). There are also
J. SZABO
significant similarities among these four major lake systems. All experienced major periods of lake-level changes with high stands occurring before about 13,000 yr B.P. and the lakes stood at moderate or high levels between about 30,000 and 10,000 yr B.P. The travertine depositional interval 8000 to 30,000 yr and the high lakelevel cycle in the Great Basin dating 10,000 to 30,000 yr B.P. appear synchronous. Direct evidence for conditions wetter than present during late Pleistocene time in southern Nevada comes from uraniumseries dating of carbonates in the Devils Hole cave system at Ash Meadows. The walls of the cave are marked by travertine that precipitated during former higher stands of the water table when conditions wetter than modern resulted in greater groundwater recharge. Travertine ages indicate that paleo-water level was at or higher than 6 m above present level between about 110,000 and 17,000 yr ago and that the peak paleo-water level, or corresponding paleo-groundwater recharge, occurred between about 40,000 and 20,000 yr ago (B. J. Szabo and co-workers, unpublished data). These speleothem dates at Devils Hole show that increase in effective moisture conditions were essentially contemporaneus with deposition of the younger travertines at eastern Grand Canyon localities. Travertine deposition in the Grand Canyon appears to be episodic and to be controlled by long-term climatic oscillations, although this interpretation is based on limited data and field observation at this time. Future investigation using improved sampling technique is planned so as to improve the data base. Dated Grand Canyon travertine depositional episodes correlate reasonably well with other climate-controlled geologic records across the Great Basin; however, attempts to correlate different regional records suffer from lack of reliable dating, large errors in reported age estimates, variations in sampling interval, and difficulties of stratigraphic interpretation.
GRAND CANYON
Nevertheless, the results suggest that climatic change in the Great Basin area may be regional in scale and in phase. Based on stable isotope measurements of precipitation and groundwater samples from southern Nevada, Benson and Klieforth (in press) suggest that past effectively wetter conditions were caused by yeararound proximity of the mean position of the jetstream and associated atmospheric frontal systems over the southern Great Basin. The near contemporaneity of several proxy records that imply climatic conditions effectively wetter than present suggest that past increases in effective moisture encompassed extensive areas of the Great Basin. ACKNOWLEDGMENTS The assistance of J. P. Schafer, P. A. O’Malley, and J. N. Rosholt in collecting some of the travertine samples is greatly appreciated. I thank J. C. Tinsley, J. L. Bischoff, and L. V. Benson for reviewing the manuscript.
REFERENCES Atwater, B. F., Adam, D. P., Bradbury, J. P., Forester, R. M., Mark, R. K., Lettis, W. R., Fisher, G. R., Gobalet, K. W., and Robinson, S. W. (1986). A fan dam for Tulare Lake, California, and implications for the Wisconsin glacial history of the Sierra Nevada. Geological Society America Bulletin 97, 97-109. Benson, L. V. (1978). Fluctuation in the level of pluvial Lake Lahontan during the last 40,000 years. Quaternary Research 9, 300-318. Benson, L. V., Currey, D. R., Dom, R. I. Lajoie, K. R., Oviatt, C. G., Robinson, S. W., and Smith, G. I. (in press). Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years. In “Paleolakes and Paleooceans” (E. P. L. Meyers and L. V. Benson, Eds.). Special issue of Paleogeography, Paleoclimatology, Paleoecology. Benson, L. V., and Klieforth, H. (in press). Stable isotopes in precipitation and ground water in the Yucca Mountain region, southern Nevada: Paleoclimatic implications. In “Aspects of Climate Variability in the Pacific and the Western Americas.” Pacific Climate Workshop’s Monograph, American Geophysical Union. Benson, L. V., and Thompson, R. S. (1987). The physical records of lakes in the Great Basin. In “North America and Adjacent Oceans during the
TRAVERTINES
31
Last Deglaciation” (W. F. Ruddiman and H. E. Write, Jr., Eds.), pp. 241-260. The Geology of North America, v. K-3, The Geological Society of America, Boulder, CO. Bischoff, J. L., Rosenbauer, R. J., and Smith, G. I. (1985). Uranium-series dating of sediment from Searles Lake: Differences between continental and marine climatic records. Science 227, 1222-1224. Bradbury, J. P., Forester, R. M., and Thompson, R. S. (1989). Late Quatemary paleolimnology of Walker Lake, Nevada. Journal of Paleolimnology 1, 249-267. Dorn, R. I., Turrin, B. D., Jull, A. J. T., Linick, T. W., and Donahue, D. J. (1987). Radiocarbon and cation-ratio ages for rock varnish on Tioga and Tahoe morainal boulders of Pine Creek, eastern Sierra Nevada, California, and their paleoclimatic implications. Quaternary Research 28, 38-49. Ford, T. D., Huntoon, P. W., Billingsley, G. H., Jr., and Breed, W. J. (1976). Rock movement and mass wastage in the Grand Canyon. In “Geology of the Grand Canyon” (W. T. Breed and E. C. Roat, Eds.), pp. 116128. Museum of Not-them Arizona, Flagstaff, and Grand Canyon Natural History Association. Forester, R. M. (1987). Late Quatemary paleoclimate records from lacustrine ostracodes. In “North America and Adjacent Oceans during the Last Deglaciation” (W. F. Ruddiman and H. E. Write, Jr., Eds.), pp. 261-276. The Geology of North America, v. K-3, The Geological Society of America, Boulder, CO. Gunn, J., and Trudgill, S. T. (1982). Carbon dioxide production and concentrations in the soil atmosphere: A case study from New Zealand volcanic ash soils. Catena 9, 81-94. Ku, T. L. (1976). The uranium-series methods of age determination. Annual Review of Earth and Planetary Sciences 4, 347-379. Lao, Y., and Benson, L. (1988). Uranium-series age estimates and paleoclimatic significance of Pleistocene tufas from the Lahohtan Basin, California and Nevada. Quaternary Research 30, 165-176. Livnat, A,, and Kronfeld, J. (1985). Paleoclimatic implications of U-series dates for lake sediments and travertines in the Arava Rift Valley. Quaternary Research 24, 164-172. McCoy, W. D. (1987). Quatemary aminostratigraphy of the Bonneville basin, western United States. Geological Society of America Bulletin 98, 99-112. Peng, T. H., Goddard, J. G., and Broecker, W. S. (1978). A direct comparison of 14C and 23“Th ages at Searles Lake, California. Quaternary Research 9, 319-329. Quade, J. (1986). Late Quatemary environmental changes in the upper Las Vegas Valley, Nevada. Quaternary Research 26, 340-357. Quade, J., and Pratt, W. L. (1989). Late Wisconsin
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BARNEY
groundwater discharge environments of the southwestern Indian Springs Valley, Southern Nevada. Quaternary Research 31, 351-370. Scott, E. W., McCoy, W. D., Shroba, R. R., and Rubin, M. (1983). Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, westem United States. Quaternary Research 20, 261285. Smith, G. I. (1984). Paleohydrologic regimes in the southwestern Great Basin, O-3.2 myr ago, compared with other long records of “global” climate. Quaternary Research 22, 1-17. Smith, G. I., Barczak, V. J., Moulton, G. F., and Liddicoat, .I. C. (1983). “Core KM-3, a Surface-toBedrock Record of Late Cenozoic Sedimentation in Searles Valley, California.” U.S. Geological Survey Professional Paper 1256, 24 pp. Smith, G. I., and Street-Perrott, F. A. (1983). Pluvial lakes of the western United States. In “Late Quaternary Environments of the United States” (H. E. Wright, J., Ed.), Vol. 1. pp. 190-214. Univ. of Minnesota Press, Minneapolis. Szabo, B. J., Carr, W. J., and Gottschall, W. C. (1981). Uranium-thorium dating of Quatemary carbonate accumulations in the Nevada Test Site region, southern Nevada. U.S. Geological Survey Open-File Report 81-119, 35 pp.
J. SZABO Szabo, B. J., and Lindsey, D. A. (1986). Estimating limiting age for Pleistocene erosional surfaces in central Montana by uranium-series dating of associated travertines. Earth Surface Processes and Landforms 11, 223-228. Szabo, B. I., McHugh, W. P., Schaber, G. G., Haynes, C. V., Jr., and Breed, C. S. (1989). Uranium-series dated authigenic carbonates and Acheulian sites in southern Egypt. Science 243, 1053-1056. Szabo, B. J., and Rosholt, J. N. (1982). Surticial continental sediments. In “Uranium-Series Disequihbrium: Application to Environmental Problems” (M. Ivanovich and R. S. Harmon, Eds.), Chapt. 10, pp. 246-267. Oxford Univ. Press, Oxford. Thompson, R. S., Benson, L. V., and Hattori, E. M. (1986). A revised chronology for the last Pleistene lake cycle in the central Lahonton Basin. Quaternary Research 25, l-9. Winograd, I. J., Szabo, B. J., Coplen, T. B., and Riggs, A. C. (1988). A 250,000 year climatic record from Great Basin vein calcite: Implications for Milankovitch theory. Science 242, 1275-1280. Wood, W. W. (1985). Origin of caves and other solution openings in the unsaturated (vadose) zone of carbonate rocks: A model for CO, generation. Geology 13, 822-824.