PLEISTOCENE CLIMATE VARIATION IDENTIFIED BY ION-MICROPROBE DATING OF VADOSE-ZONE OPAL, YUCCA MOUNTAIN, NEVADA, USA

Continuous records spanning 100,000-yr time scales are important for evaluating hydrologic responses to climate change at the proposed high-level nuclear waste repository at Yucca Mountain, Nevada, which likely will experience numerous climate cycles before radioactivity in the waste decays to background levels (>1 m.y.). Secondary coatings of calcite and opal in the ~500-m-thick vadose zone preserve a record of water percolating through fractures and seeping into cavities. Opal contains high U concentrations (5 to 500 µg/g) and high U/Th, allowing U-series dating using the USGS/Stanford ion microprobe. Resulting analyses for 25-µm-diameter spots traversing the outermost 1 mm of opal grains indicate that: (1) both ²³⁰Th/U ages for layers younger than 300 ka and model ²³⁴U/²³⁸U ages for layers between 300 and 1,500 ka increase with microstratigraphic depth, (2) multiple analyses of texturally synchronous layers yield identical ages, and (3) analyses of opal older than 1,500 ka have ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U ratios in radioactive secular equilibrium. Age profiles across individual grains indicate that opal growth rates were extremely slow (0.1 to 2 µm/k.y.) and, at the depth of the potential repository, remarkably constant over the last 300 to 400 k.y., despite substantial differences in effective moisture at the surface between glacial and interglacial climates.

These age determinations were used to calibrate cathodoluminescence (CL) images of opal that reveal distinct growth layering caused by oscillatory variations in U concentration. Relative intensities of U-rich (bright CL) and U-poor (dim CL) layers were quantified from digital images. Resulting age-calibrated CL profiles indicate that U-rich layers correlate with interglacial climates. U-rich layers are less frequent in opal younger than 400 ka and show a 100-k.y. periodicity (orbital eccentricity), whereas U-rich layers are more frequent in opal older than 600 ka, reflecting increased influence of 40-k.y. periodicity (obliquity). Opal data indicate that percolation deep within the vadose zone responded to climate cycles; however, the uniformity of growth rates indicates that hydrogeologic processes effectively buffered seepage at the potential repository horizon from climate-induced changes in effective moisture.