Ranges (amplitudes) of isotopic ratios in speleothems, with particular reference to samples from the centre of North America

Abstract

This paper compares unstable and stable isotope ratios measured in a speleothem core from the Nevada desert (the well-known DH 11) with speleothems at the centre of the continent in the Black Hills of South Dakota. All samples grew over most of the past 500,000 y. The thermal calcite in the large Nevadan desert basin displays a range of the initial U ratio (234U/238U0) of only 0.25 and is much enriched in 234U, reflecting selective leaching in a very low energy weathering environment. Similar thermal calcite from a much smaller basin the Black Hills (Wind Cave) is less homogenised (U/U0 range is ~0.5) and less enriched as a consequence of its more vigorous sem-arid weathering environment. Both display glacial-interglacial cyclicity in the ratio. A vadose speleothem from Jewel Cave, Black Hills, has a U/U0 range of 0.75 and many erratic features attributable to rapidly changing local conditions in the overlying tall grass- coniferous forest vegetal transition. O and C stable isotopes are again homogenised in the large Nevada basin: the range is 2.8 ‰ 18O and 1.2 ‰ 13C, encompassing all glacial-interglacial cyclicity. In the Wind Cave thermal calcites the range again is roughly double, an effect of lesser homogenisation taking place in the smaller aquifer. There is also a perturbation of the signals of ~100 ky duration that may have been caused by release of deep CO2. 18O and 13C measurements in the Jewel Cave vadose sample form a broad envelope that encloses the Black Hills thermal calcite data, indicating that the full range, glacial – interglacial, of speleothem 18O was probably 10 ‰ here in the centre of the continent, which may be compared to 1.2 ‰ estimated at the sources of the precipitation in the Gulf and Caribbean. 13C of the vadose speleothem ranged 7.0 ‰ in the Black Hills, oscillating between moderate and very low (desert-like) levels of soil CO2 activity.