Evidence for periods of wetter and cooler climate in the Sahel between 6 and 40 kyr BP derived from groundwater

Concentrations of noble gases, stable isotopes and 14 C in samples from the Continental Terminal groundwaters of Niger provide evidence for more humid and cooler climate phases in West Africa in the Holocene and the late Pleistocene. During humid phases, even within the Holocene, the soil temperature was up to 5.5degreesC cooler than today, which is partly attributed to atmospheric cooling, but also to a change in the relationship between air and soil temperature due to increased vegetation. Intense rainfall events and increased groundwater recharge are consistently indicated by stable isotope data and excess air concentrations, i.e., the component of dissolved atmospheric gases in excess of solubility equilibrium. This finding encourages the use of excess air as an additional, humidity-related climate indicator.