Isotopic composition of palaeowaters as inferred from speleothem and its fluid inclusions

Speleothem precipitated in the deep interior of karstic caves is commonly found to have been deposited in isotopic equilibrium with the water from which it was precipitated. The temperature of deposition can in such cases be determined from the oxygen isotopic fractionation between the calcite and the water. For this purpose the composition of the water can be determined from analyses of the water trapped as fluid inclusions in the stalagmites, flowstones and other speleothems. However, because of the possibility of isotopic exchange between the calcite and the oxygen of the water, D/H ratios of the inclusions have been used to infer the original 18O/16O ratio, assuming a relation between oxygen and hydrogen isotope ratios comparable to that existing at present in both meteoric precipitation and drip waters in caves. Some shift in the position of the meteoric water line is inferred to have occurred during glacial periods, however, as shown by measurements of ice cores from Antarctica. If account is taken of this shift, then palaeotemperatures for Wisconsin stage speleothems are found to be consistent with their ages relative to the time scale of the glacial stage. Variations in delta D of meteoric palaeowaters can be directly inferred from fluid inclusion data; using the uranium series dates on the host speleothem, it is possible to reconstruct the history of the isotopic composition of rain and snow at a site over the past 300,000 years. During the last two glacial stages in North America, precipitation was on the average about 12 per mille lighter than its modern value at each site. (author)