Isaac J. Winograd

Continuous 500,000-Year Climate Record from Vein Calcite in Devils Hole, Nevada

Oxygen-18 (δ18O) variations in a 36-centimeter-long core (DH-11) of vein calcite from Devils Hole, Nevada, yield an uninterrupted 500,000-year paleotemperature record that closely mimics all major features in the Vostok (Antarctica) paleotemperature and marine δ18O ice-volume records. The chronology for this continental record is based on 21 replicated mass-spectrometric uranium-series dates. Between the middle and latest Pleistocene, the duration of the last four glacial cycles recorded in the calcite increased from 80,000 to 130,000 years; this variation suggests that major climate changes were aperiodic. The timing of specific climatic events indicates that orbitally controlled variations in solar insolation were not a major factor in triggering deglaciations. Interglacial climates lasted about 20,000 years. Collectively, these observations are inconsistent with the Milankovitch hypothesis for the origin of the Pleistocene glacial cycles but they are consistent with the thesis that these cycles originated from internal nonlinear feedbacks within the atmosphere-ice sheet-ocean system.

Mass-Spectrometric 230Th-234U-238U Dating of the Devils Hole Calcite Vein

The Devils Hole calcite vein contains a long-term climatic record, but requires accurate chronologic control for its interpretation. Mass-spectrometric U-series ages for samples from core DH-11 yielded 230Th ages with precisions ranging from less than 1,000 years (2σ) for samples younger than ∼140 ka (thousands of years ago) to less than 50,000 years for the oldest samples (∼566 ka). The 234U/238U ages could be determined to a precision of ∼20,000 years for all ages. Calcite accumulated continuously from 566 ka until ∼60 ka at an average rate of 0.7 millimeter per 103 years. The precise agreement between replicte analyses and the concordance of the 230Th/238U 234U/238U ages for the oldest samples indicate that the DH-11 samples were closed systems and validate the dating technique in general.

A 250,000-Year Climatic Record from Great Basin Vein Calcite: Implications for Milankovitch Theory

A continuous record of oxygen-18 (δ18O) variations in the continental hydrosphere during the middle-to-late Pleistocene has been obtained from a uranium-series dated calcitic vein in the southern Great Basin. The vein was deposited from ground water that moved through Devils Hole—an open fault zone at Ash Meadows, Nevada—between 50 and 310 ka (thousand years ago). The configuration of the δ18O versus time curve closely resembles the marine and Antarctic ice core (Vostok) δ18O curves; however, the U-Th dates indicate that the last interglacial stage (marine oxygen isotope stage 5) began before 147 � 3 ka, at least 17,000 years earlier than indicated by the marine δ18O record and 7,000 years earlier than indicated by the less well dated Antarctic δ18O record. This discrepancy and other differences in the timing of key climatic events suggest that the indirectly dated marine δ18O chronology may need revision and that orbital forcing may not be the principal cause of the Pleistocene ice ages.

Deep oxygenated ground water: anomaly or common occurrence?

Contrary to the prevailing notion that oxygen-depleting reactions in the soil zone and in the aquifer rapidly reduce the dissolved oxygen content of recharge water to detection limits, 2 to 8 milligrams per liter of dissolved oxygen is present in water from a variety of deep (100 to 1000 meters) aquifers in Nevada, Arizona, and the hot springs of the folded Appalachians and Arkansas. Most of the waters sampled are several thousand to more than 10,000 years old, and some are 80 kilometers from their point of recharge.

Two-million-year record of deuterium depletion in great basin ground waters.

Fluid inclusions in uranium series-dated calcitic veins from the southern Great Basin record a reduction of 40 per mil in the deuterium content of ground-water recharge during the Pleistocene. This variation is tentatively attributed to major uplift of the Sierra Nevada Range and the Transverse Ranges during this epoch with attendant increasing orographic depletion of deuterium from inland-bound Pacific storms.

500,000-year stable carbon isotopic record from devils hole, nevada.

The record of carbon-13 (δ13C) variations in DH-11 vein calcite core from Devils Hole, Nevada, shows four prominent minima near glacial terminations (glacial-interglacial transitions) V to II. The δ13C time series is inversely correlated with the DH-11 oxygen isotope ratio time series and leads it by as much as 7000 years. The δ13C variations likely record fluctuations in the δ13C of dissolved inorganic carbon of water recharging the aquifer. How such variations are transported 80 kilometers to Devils Hole without obliteration by water-rock reaction remains an enigma. The record may reflect (i) global variations in the δ13C of atmospheric CO2 and, hence, the δ13C of continental biomass or (ii) variations in extent and density of vegetation in the southern Great Basin. In the latter case, δ13C minima at 414, 334, 246, and 133 thousand years ago mark times of maximum vegetation.

Deuterium as a Tracer of Regional Ground-Water Flow, Southern Great Basin, Nevada and California

Hydrogeologic studies of the southern Great Basin differ widely in their conclusions about the origin of several groups of major springs discharging at valley level from Paleozoic carbonate rocks. Delineation of watersheds feeding these springs is complicated by interbasin movement of ground water through areally extensive and highly fractured Paleozoic carbonate rocks and by the structural complexity of, and sparse subsurface hydrogeologic data for, the carbonate aquifer. The mean, median, and standard deviation of the deuterium content of ground water at, and the number of samples collected from, four of these major discharge areas and one major recharge area (Spring Mountains–Sheep Range) are given below. The deuterium data are expressed in δ D units (permil deviations from SMOW); the number of samples is given in parentheses. Pahranagat Valley: −113, −113, 1, (9); Spring Mountains–Sheep Range: −102, −102, 3, (12); Ash Meadows: −106, −107, 3, (15); Death Valley: −109, −109, 1, (4); Muddy River: −100, −100, 1, (5). A nonparametric test (Kolmogorov–Smirnov statistic) of the data for the first three areas indicates different deuterium populations at the 0.01 level of significance. The deuterium data indicate that Ash Meadows springs are fed by a mixture of recharge derived from the Spring Mountains–Sheep Range uplands, 30 to 60 mi east of the springs, with underflow from Pahranagat Valley and vicinity, 90 mi northeast of the springs. Mass balance considerations suggest that the underflow amounts to about 35 percent of the spring discharge. Underflow to Ash Meadows from Pahrump Valley, a source area proposed in the literature, is not supported by the data. The Death Valley springs may be fed, in part, by water derived from Ash Meadows. The Muddy River springs may originate as precipitation on the Spring Mountains–Sheep Range area. The suggested sources for the Ash Meadows and Death Valley springs are in agreement with sources previously identified by hydrogeologic and hydrochemical studies of the region. The source proposed for the Muddy River springs, however, is not supported by water-budget studies. A major assumption in utilization of deuterium as a natural tracer of regional ground-water flow is that the mean deuterium content of recharge to proposed source areas remained relatively constant during the residence time of water in the aquifer of interest. Available data are insufficient to test this assumption for the southern Great Basin region.

Yucca Mountain Revisited

Despite hundreds of studies and dozens of workshops and panels, Yucca Mountain remains controversial as a repository for radioactive wastes.

Comment on “Testing the Interbasin Flow Hypothesis at Death Valley, California”

In the 1960s, a major hydrogeologic investigation was conducted at the Nevada Test Site (NTS, Figure 1) that included drilling, hydraulic testing, and hydrogeochemical studies in conjunction with geologic mapping and geophysical surveys. This work demonstrated that a large part of south central Nevada is underlain by thick (several kilometers) highly fractured Paleozoic carbonate rocks that typically act as an aquifer. The aquifer flanks and underlies most of the intermontane basins from east central Nevada southward, through the NTS, to the southern Funeral Mountains east of Death Valley (Figure 1). Water levels measured in many test holes demonstrate that the potentiometric surface in the carbonate aquifer generally is uninterrupted by the ridges that separate the many topographically closed basins of the region.

Interbasin groundwater flow in south central Nevada: A further comment on the discussion between Davisson et. al.. [1999a, 1999b] and Thomas [1999]

In their response to the comments by Thomas [1999], Davisson et al. [1999a] dismiss a large set of potentiometric measurements pertinent to an understanding of the hydrogeology of Yucca and Frenchman Flats in southcentral Nevada. This commentary is submitted to demonstrate, first, that their dismissal of this data set is unfounded and, second, that these potentiometric data call into question the central thesis of the original paper by Davisson et al. [1999b].