Alan C. Riggs

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.

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.

Major carbon-14 deficiency in modern snail shells from southern nevada springs.

Carbon-14 contents as low as 3.3 � 0.2 percent modern (apparent age, 27,000 years) measured from the shells of snails Melanoides tuberculatus living in artesian springs in southern Nevada are attributed to fixation of dissolved HCO3- with which the shells are in carbon isotope equilibrium. Recognition of the existence of such extreme deficiencies is necessary so that erroneous ages are not attributed to freshwater biogenic carbonates.

Influence Of Depositional Environment On Devil's Hole Calcite Morphology And Petrology

Devils Hole, a steeply dipping (δ80°) tectonically formed planar fissure >165 m deep (Riggs et al., 1994), has been accumulating calcite speleothems that contain at least two different paleoclimate records. Below water table deposits contain a 560,000 year paleoclimate isotopic record (Winograd et al., 1992), while above water table deposits record 120,000 years of changing water table elevation (Szabo et al., 1994). The different depositional environments in Devils Hole control the form of the speleothems.