Improving noble gas based paleoclimate reconstruction and groundwater dating using 20Ne/22Ne ratios

The interpretation of noble gas concentrations in groundwater with respect to recharge temperature and fractionated excess gas leads to different results on paleo-climatic conditions and on residence times depending on the choice of the gas partitioning model. Two fractionation models for the gas excess are in use, one assuming partial re-equilibration of groundwater supersaturated by excess air (PR-model, Stute et al., 1995), the other assuming closed-system equilibration of groundwater with entrapped air (CE-model, Aeschbach-Hertig et al., 2000). In the example of the Continental Terminal aquifers in Niger, PRand CEmodel are both consistent with the data on elemental noble gas concentrations (Ne, Ar, Kr, and Xe). Only by including the isotope ratio Ne/Ne it can be demonstrated that the PR-model has to be rejected and the CE-model should be applied to the data. In dating applications He of atmospheric origin (Heatm) required to calculate H-He water ages is commonly estimated from the Ne excess presuming that gas excess is unfractionated air (UA-model). Including in addition to the Ne concentration the Ne/Ne ratio and the concentration of Ar enables a rigorous distinction between PR-, CEand UA-model and a reliable determination of Heatm and of H-He water ages. Copyright © 2002 Elsevier Science Ltd