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

• Autorzy: Ford D.
• Języki publikacji: EN

Abstrakty

EN

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 promile ^18 O and 1.2 promile ^13 C,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 aquiter. There is also a perturbation of the signals of ~ 100 ky duration that may have been caused by release of deep CO2. ^18 O and ^13 C 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 ^18 O was probably 10 promile here in the centre of the continent, which may be compared to 1.2 promile estimated at the sources of the precipitation in the Gulf and Caribbean. ^13 C of the vadose speleothem ranged 7.0 promile in the Black Hills, oscillating between moderate and very low (desert-like) levels of soil CO2 activity.

Słowa kluczowe

EN

oxygen and carbon isotope ratios; thermal water speleothems; uranium isotope ratios; vadose speleothems

• Wydawca: Faculty of Geology of the University of Warsaw ; Komitet Nauk Geologicznych PAN
• Czasopismo: Acta Geologica Polonica
• Rocznik: 2002
• Tom: Vol. 52, nr 1
• Strony: 31--34
• Opis fizyczny: Bibliogr. 8 poz.,

Twórcy

autor

Ford D.

• School of Geography and Geology, McMaster University, Hamilton, ON L8S 4K1, Canada

Bibliografia

• 1. BAKALOWICZ, M, FORD D.C., MILLER T.E., PALMER A.N., PALMER N.V. 1987. Thermal Genesis of solution caves in the Black Hills, South Dakota. Bulletin, Geological Society of America, 99, 729-738.
• 2. FORD , D.C., LUNDBERG J., PALMER A.N., D REYBRODT W., & S CHWARCZ H.P. 1993. Uranium-series dating of the draining of an aquifer: The example of Wind Cave, Black Hills, South Dakota. Bulletin of the Geological Society of America, 105, 241-250.
• 3. FORD D.C. & WILLIAMS P.W. 1989. Karst Geomorphology and Hydrology. 601 pp. Chapman and Hall; London.
• 4. HENDY, C.H. & WILSON, A.T. 1968. Paleoclimatic data from speleothem. Nature, 219, 48-51.
• 5. IVANOVICH , M & R.S. H ARMON 1992. Uranium-series Disequilibrium. 910 p. Oxford Science Publications; London.
• 6. LUDWIG , K.R, S IMMONS K.R., S ZABO B.J., WINOGRAD I.J., LANDWEHR J.M., RIGGS A.C., & HOFFMAN R.J. 1992. Mass-Spectrometric 230Th-234U-238U Dating of the Devils Hole Calcite Vein. Science, 258, 284-287.
• 7. TARHULE-LIPS , R.F.A 1999. Karst processes on Cayman Brac, a small oceanic carbonate island. McMaster University PhD thesis. 249 pp.
• 8. WINOGRAD , I.J , C OPLEN , T.B., L ANDWEHR , J.M, R IGGS , A.C., LUDWIG , K.R., S ZABO , B.J., KOLESAR , P.T., & R EVESZ , K.M. 1992. Continous 500,000-Year Climate Record from Vein Calcite in Devils Hole, Nevada. Science, 258, 255-260.