Changes of ²H and ¹⁸O enrichment of meteoric water and Pleistocene glaciation

Abstract

Recent theoretical modelling1 has suggested that the well documented δD–δ¹⁸O relationship for modern meteoric waters may have changed during the Pleistocene glacial periods in response to increased ocean surface air humidity. Here we present isotopic data on fluid inclusions and host speleothem which support this hypothesis. Depositional temperatures can be determined for speleothems formed in equilibrium conditions from the temperature dependence of the calcite–water ¹⁸O fractionation by isotopic analysis of both entrapped fluid inclusions and host calcite. Palaeotemperatures for five areas of east-central North America and Bermuda, calculated assuming the present δD–δ¹⁸O meteoric water relationship for fluid inclusion waters, are observe dto be too low during late Pleistocene glacial periods, in some instances falling below 0°C. By contrast, interglacial palaeotemperatures are largely equivalent to those in the areas at present. As speleothem deposition cannot occur at subzero temperatures, a possible solution to this dilemma is a shift in the intercept of the meteoric water relationship.