EVIDENCE FROM SECONDARY CALCITE d¹³C VALUES OF UNSATURATED-ZONE DEPOSITIONAL CONDITIONS AND PAST CLIMATE AT YUCCA MOUNTAIN, NEVADA

The thick unsaturated zone (UZ) at Yucca Mountain, proposed site of a high-level nuclear waste repository, consists largely of welded and fractured Miocene-age ash-flow tuffs. A small percentage (<10%) of fractures and lithophysal cavities in the welded tuffs host secondary deposits of calcite and silica formed from meteoric water percolating down through the UZ along fracture flow paths. The deposits form coatings and crusts from <1 mm to ~5 cm thick on the footwalls of fractures and the floors of cavities. The deposits consist of an early stage (ES) of calcite (±fluorite) locally capped by chalcedony±quartz followed by an intermediate stage (IS) of calcite (often bladed) and a late stage (LS) of calcite (mainly as overgrowths); opal locally accompanies both IS and LS calcite. U-Pb dating of associated silica indicates that the stage boundaries occurred about 6 to 8 Ma (ES-IS) and 3 to 4 Ma (IS-LS). The d¹³C values of calcite (~1100 analyses) in the UZ range from –8.3 to +10.1‰ and vary with depositional stage. This range reflects, in part, climate-controlled changes in the mix of C3 (–27‰) and C4 (–13‰) plant matter in the overlying soils, resulting in dissolved inorganic carbon in the percolating water with d¹³C values ranging from ~–12‰ (100% C3) to ~+2‰ (100% C4). During the ES, d¹³C values ranged from –6.5 to +10.1‰, with a median of +4.7‰. Although the values as low as –6.5‰ suggest a C3 plant community at the surface, the ¹³C-enrichment indicates widespread anaerobic conditions and bacterial methanogenesis in the UZ. Anoxic conditions during the ES are supported by CH₄–rich gases in calcite-hosted fluid inclusions and by depositional temperatures of 40 to 70°C that were favorable for thermophilic methanogens. The d¹³C values of IS calcite range from –7.6 to 8.1‰, with a median of –2‰ that suggests more C4 plants with wetter summers and (or) warmer winters and which is consistent with regional and global increases in the abundance of C4 plants during the early Pliocene. Calcite d¹³C values decreased sharply in the LS, where they range from –8.3 to –0.6‰, with a median of –6.5‰, and indicate a C3-dominated plant community. This change, at 3 to 4 Ma, is roughly coincident with the colder winter and drier summer conditions that characterized regional climate during the glacial/interglacial cycles of the late Pliocene and Pleistocene.