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미국

Mixed-230Th/U formation ages and secondary uranium concentrations of above-water carbonates, and primary uranium concentrations of continuously submerged calcite at Devils Hole, Nevada

For more than a half million years, dense vein calcite has been precipitating in Devils Hole, located about 115 km northwest of Las Vegas, Nevada. This process also occurs in Devils Hole Cave 2, which is hydrologically connected and situated about 200 meters north of the original Devils Hole cave. Calcite has been precipitating in this natural laboratory in oxygen isotopic equilibrium at constant temperature to a depth of at least 140 meters. This vein calcite, also called mammillary calcite because of its morphology, is suitable for high-accuracy uranium-series dating. Continuously submerged calcite contains an unbroken record of the sequential variation of the oxygen isotopic composition of water recharging this well-mixed environment. A record of stable oxygen isotopes in the continuously submerged calcite provides climate variations spanning 563 thousand years. This record displays warm and cold climate cycles. Paleoclimate modelers use the Devils Hole oxygen-isotope record to validate climate models. Model prediction is particularly relevant for the parched southwestern United States. This high-accuracy oxygen-isotope time series was used to correct published uranium-series ages of non-continuously formed calcite in two cores (see Table S1 and Figure S1), cyclically exposed by water-table decline during glacial-interglacial transitions. This method relies on the premise that the oxygen isotopic compositions of coevally precipitated calcite are identical, allowing matching calcite oxygen isotopic composition values to establish formation ages. Uranium-series ages (based on matching oxygen-isotope values) differ by thousands of years because of mobility of uranium in samples from cores collected at or above the modern water table (Table S1). As a part of this project, samples of bat guano and soil samples were analyzed for uranium concentration (see Table S2 and Figure S2). The mean uranium-238 concentration of original formation uranium within continuously submerged vein calcite (termed primary uranium) can be estimated from uranium-238 concentrations of vein calcite within samples collected from cores deeper than 20 meters below the modern water table. The mean uranium-238 mass fraction within these deep samples (Table S3) is 456 ± 100 nanograms per gram (2 sigma uncertainty, n = 45). Uranium added to calcite samples after their formation is termed secondary uranium. Secondary uranium in a calcite sample can decrease the apparent uranium-series age of that sample if its secondary uranium concentration is not accounted for. The estimated mass fraction of secondary uranium-238 within 46 samples from nine cores collected at or above the modern water table having uranium-series ages less than 296,000 years ranges from 30 to 37,046 nanograms per gram (Table S4). The estimated secondary uranium-238 concentrations within 14 samples from six cores having uranium-series ages greater than 296,000 years is shown in Table S5. Nineteen samples from eight cores (Table S6) have uranium-238 concentrations substantially lower than that of primary uranium (Table S3). Based on an evaluation of oxygen isotopic compositions of samples from core DH2-D (Table S1), the uranium-series ages between approximately 140 and 120 thousand years ago need to be increased by between 4 and 8 kyr as shown in Figure S3. Based on a comparison of oxygen isotopic compositions of folia, flowstone, and calcite from Brown’s Room, a subaerial room in Devils Hole, with those of cores DHC2-8 or DH-2 (Table S1), many formation ages of Brown’s Room samples are as much as 11,600 years too young (Table S7 and Figure S4). Figure S5 is a cross-sectional sketch of Devils Hole Cave 2 showing the estimated minimum zone of cessation of precipitation of vein (mammillary) calcite since approximately 18 to 20 thousand years ago.