T103 Yucca Mountain Update: Recent Advances from Scientific Investigations of the Unsaturated Zone II

Monday, 28 October 2002
1:30 PM - 5:30 PM

C-O-SR-U ISOTOPE EVIDENCE FOR THE METEORIC ORIGIN OF CALCRETE AT SOLITARIO CANYON WASH, CRATER FLAT, NEVADA

Paper number: T103-4
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

Carbonate-rich soil (calcrete) and colluvium at Solitario Canyon Wash, Crater Flat, Nevada, contain authigenic calcite and silica for which a meteoric water source commonly is accepted by analogy with similar aridic soils elsewhere. Nevertheless, Szymanski and Dublyansky (1995, GSA Abstracts, v. 27, no. 6, p. 419) and Hill and Dublyansky (1999, Environ. Geol., v. 38, p. 77-81) proposed a ground-water-discharge origin for these soil deposits.  They interpreted variations in C and O isotopes as evidence of elevation-dependent CO2 degassing, evaporation, and cooling as spring water flowed downhill. New d13C, d18O, and 87Sr/86Sr data for the carbonate fractions of 31 soil samples from three sites at Solitario Canyon Wash and U-series data for 8 samples of silica-rich material at one site are inconsistent with this model, and instead indicate a meteoric water source.

Values of d13C (–5.1 to –7.8‰),  d18O (22.7 to 17.2‰), and 87Sr/86Sr (0.71270 to 0.71146) for these samples are within published ranges for southern Nevada soils. In addition, the Sr data in calcrete are incompatible with 87Sr/86Sr values <0.7105 measured in ground water from two boreholes near the sampled area. Initial 234U/238U activity ratios for authigenic silica of about 1.6 also are identical to regional soil values and are substantially lower than the >2.7 values measured in local ground water. Each sample site also shows considerable variability of C, O, and Sr isotopic ratios. Although average d13C and  d18O values slightly decrease at lower elevations (the difference in elevations between highest and lowest sampled sites is about 135 m), the data are consistent with regional elevation dependency in soils (Quade et al., 1989, GSA Bull., v. 101, p. 464-475). More importantly, the compositional ranges observed at each site are as large as the entire Hill and Dublyansky “trend.”  Four U-series ages from a finely layered clast rind increase progressively from 64 ka to >1 Ma, indicating that the calcretes can have long depositional histories. Combined age and isotope data indicate that observed C and O isotopic variability in Crater Flat calcrete likely is caused by precipitation from meteoric water under variable Quaternary climate conditions over hundreds of thousands of years.