Air humidity and lake {delta}18O during the latest Paleocene-earliest Eocene in France from recent and fossil fresh-water and marine gastropod {delta}18O, {delta}13C, and 87Sr/86Sr

摘要

Detailed isotopic (18O, 13C, 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 18O values, in the range –4 to –0.1. Large intrashell 18O 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 18O composition of Sparnacian lake water has been estimated from the fossil fresh-water shell 18O and by using seasonal temperatures derived from 18O profiles of fully marine shells. The best estimate of the 18O composition of Sparnacian lake water (compared with Eocene mean ocean water) is –1. This is consistent with paleorain 18O values of –3 to –5, similar to present mean rain 18O (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.