Detailed isotopic (δ18O, δ13C, 87Sr/86Sr) analyses have been performed on aragonitic fresh-water, brackish-water, and marine gastropod shells of latest Paleocene–earliest Eocene age from northwestern Europe. Fresh-water shells (e.g., Viviparus, Lymnaea) from Sparnacian deposits in the Paris basin show relatively high δ18O values, in the range −4‰ to −0.1‰. Large intrashell δ18O variability, typically 3‰, is consistent with significant seasonal variation in precipitation and/or temperature. We also present intrashell isotopic profiles for 22 recent fresh-water shells, mainly Viviparidae, representing the major middle- to low-latitude climate zones (e.g., Sweden, France, Portugal, Lake Chad, Ceylon, Gambia, Congo, Lake Victoria, New Guinea). Distinct isotopic patterns, reflecting humidity and temperature, characterize each of the climate zones. Comparing the results for recent and fossil shells suggests that the climate in northern France during Sparnacian time was warm subtropical with a pronounced seasonal drought. The δ18O composition of Sparnacian lake water has been estimated from the fossil fresh-water shell δ18O and by using seasonal temperatures derived from δ18O profiles of fully marine shells. The best estimate of the δ18O composition of Sparnacian lake water (compared with Eocene mean ocean water) is −1‰. This is consistent with paleorain δ18O values of ∼–3‰ to −5‰, similar to present mean rain δ18O (standard mean ocean water) values close to −4‰ in Portugal at ∼40°N, the paleolatitude of the Paris basin during earliest Eocene time. The evaporated nature of Sparnacian lake water contradicts hypotheses of globally equable humid conditions in the warm early Eocene and instead suggests that dry subtropical highs occurred over the eastern parts of the oceans, similar to today. The results of this study demonstrate the potential of fossil fresh-water carbonates to reconstruct the development of subtropical highs and Hadley cell circulation in the past.