D/H ratios in speleothem fluid inclusions: a guide to variations in the isotopic composition of meteoric precipitation

D/H ratios of fluid inclusion waters extracted from 230Th/234U-dated speleothems that were originally deposited under conditions of isotopic equilibrium should provide a direct estimate of the hydrogen isotope composition of ancient meteoric waters. D/H ratios are presented for 47 fluid inclusion samples from thirteen speleothems deposited over the past 250,000 years at cave sites in Iowa, West Virginia, Kentucky and Missouri. At each site glacial-age waters are depleted in deuterium relative to those of interglacial age. The average interglacial/glacial shift in the hydrogen isotopic composition of meteoric precipitation over ice-free areas of east-central North America is estimated to be -12%. This shift is consistent with the present climatic models and can be explained in terms of the prevailing pattern of atmospheric circulation and an increased ocean-continent temperature gradient during glacial times which more than compensated for the increase in deuterium content of the world ocean. (Auth.)