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|Title:||Speleochronology and Late Pleistocene Climates Inferred O, C, H, U and Th Isotopic Abundances in Speleothems|
|Abstract:||<p>The Th²³⁰/U²³⁴ and U²³⁴/U²³⁸ ratios of speleothems (calcareous deposits) from caves developed in limestones of the Greenbrier Series, S.E. West Virginia, was measured by alpha-particle spectrometry to assess the deficient Th²³⁰ (ionium) and excess U²³⁴ dating methods respectively. Pure, non-porous, very coarsely crystalline deposits showing no signs of submersion or re-solution were found to be suitable for Th²³⁰/U²³⁴ dating. This method was used to date deposits between about 2000 and 300,000 years old. The decay of excess U²³⁴, which in principle can be used to date deposits 50,000 to 1,500,000 years old, could not be used as a routine method because the initial excess of U²³⁴ could not be reliably determined. Concordant Pa²³¹/Th²³⁰ and Th²³⁰/U²³⁴ ages were measured for two samples from West Virginia.</p> <p>Norman-Bone Cave and Grapevine Cave, both in Greenbrier Co., were studied in detail. Norman-Bone Cave contains speleothems at least 300,000 years old. Based on the decay of U²³⁴, one stalagmite appeared to have been deposited between about 760,000 years and 1,250,000 years B.P. Speleothem ages from caves in the Crowsnest Pass area of Alberta, the Nahanni region of the North West Territories and the Ciudad Valles area of Mexico were also obtained. With the exception of the single Mexican sample, deposition appears to stop during prolonged periods of cold climate; therefore a continuous record of CaCO₃ deposition is not preserved in the northern caves. Based on the gaps in the recota, the coldest periods of the Wisconsin, Illinoian and Kansan (?) glacial ages occur between 20-35,000, 135-160,000 and 240-270,000 years B.P. respectively. The intervening periods are times of cool to warm, moist climate which favour speleothem deposition.</p> <p>The ratio O¹⁸/O¹⁶ (δºct) and c¹³/C¹² (δºct) in young and fossil speleothems deposited in isotopic equilibrium with parent waters was measured in an attempt to construct detailed climate curves for the Late Pleistocene. Five young speleothems all appeared to be deposited in isotopic equilibrium: an isotopic temperature of 8.6±1.6ºc was calculated, 2.3ºC lower than the present mean annual surface air temperature. The discrepancy could be real, for cave temperatures are often slightly lower than mean annual surface air temperatures.</p> <p>Variations inδºct and δºct in two dated stalagmites from Norman-Bone Cave (NB4 and NB10) and a flowstone deposit from Grapevine Cave (GV2) are commonly uncorrelated within individual depositional layers, satisfying Hendy's criteria for CaCO₃ deposition in oxygen isotopic equilibrium with water. Parts of NB4 and GV2, which were deposited contemporaneously about 100,000 years ago record the same temperature maximum.</p> <p>Cyclic secular variations δºct cannot be interpreted in terms of climat change without knowledge of variations in 60 of waters depositing the CaCO₃. An attempt was made to measure these variations directly by extracting water from fluid inclusions in the fossil deposits. The D/H (δᵢD) ratio of the water was measured and the Craig-Dansgaard relationship (δᵢD =86ᵢº + 10) applied to calculate δºᵢ (≡δºw). Absolute temperatures calculated from δºw and δºct are, in general too low but with further refinements the method may become a valuable tool for paleoclimatologists.</p> <p>In general, δºct is found to increase in calcite deposited at lower temperatures. This is attributed to the deposition of CaCO₃ from waters of relatively constant isotopic composition, so that changes in δºct are determined by the effect of temperature on the CaCO₃-H₂O fractionation factor. (0.24‰ increase per 1ºC temperature drop).</p> <p>The relative constancy of δºw values of cave drip-waters is attributed to seasonal CaCO₃ deposition from waters whose isotopic composition is biassed toward mean summer values. As mean annual temperatures decrease, CaCO₃ deposition is from waters which are progressively biassed in isotopic composition toward the mean July value.</p> <p>Temperature maxima are recorded in GV2 at 80.000±3000 years B.P. and 100.000±4000 years B.P.; a lower maximum is recorded at 60.000±3000 years B.P. Temperature maxima in NB10 are recorded at 163,000±6800, 170,000±7000 and 180,000±9700 years B.P. An intensely cold period is recorded between 180,000 and 170,000 years B.P. which was inferred to be colder than any event recorded in GV2 between 100,000 and 60,000 years B.P.</p> <p>δºct also increases as regional temperatures decrease. probably as a result of the decreased rate of plant respiration, microbial activity and CO₂ production in the soil during cold periods. The δºct variations are not well enough defined to be used as a reliable indicator of climate change.</p> <p>Good correlation is observed with the time-temperature curve for Wisconsin glaciation and the Sangamom interglacial, as obtained from deep-sea cores and raised coral reefs. Correlation with the penultimate (Illinoian) glaciation and preceding (Yarmouth) interglacial is unclear.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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