Other data sources - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 186

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Other data sources

6.1 DEVIL'S HOLE

6.1.1 Devil's Hole data

Devil's Hole is an open fault zone adjacent to a major groundwater discharge area
in south-central Nevada; it is located approximately 115 km westnorthwest of Las
Vegas, Nevada. This open fissure is lined with a thick ( 0.3m) layer of dense

calcite that has precipitated continuously from calcite-supersaturated groundwater.
These deposits exist down to depths in excess of 130m below the water table
(which is 15m below land surface) and are believed to correspond to a time
span of more than the past 500,000 years (Winograd et al., 1992).
Winograd et al. (1992) claimed that the d 18 O variations in calcite from the
cave most likely reflect isotopic variations in atmospheric precipitation falling on
groundwater recharge tributary areas to Devil's Hole. Isotopic variations in atmo-
spheric precipitation are believed to reflect changes in average winter-spring land
surface temperature, the season during which recharge is most likely to have
occurred. Higher d 18 O values reflect warmer temperatures, and lower values reflect
a colder climate.
Winograd et al. (1992) reported on results from a 36 cm long core of vein
calcite that was recovered from about 30m below the water table. This core con-
tained pure calcite, and there were no apparent interruptions to the deposition
process. They analyzed samples for 18 O and 13 C at 285 points along the core. The
sampling interval (1.26mm) represented an average time interval of about 1,800
years. Absolute ages were established at 21 places along the core by measuring the
ratios of the radioactive isotopes of uranium and thorium. ( 238 U decays to form
230 Th, which decays with a half-life of 77,000 yr; the ratio gives a measure of the
age.) Ages between these points were estimated by interpolation. Dating was
accomplished radiometrically with high precision using thermal-ionization mass-

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