Oxygen isotope exchange unravels million-year long hydrogeochemical evolution of a deep carbonate aquifer in the Swiss Molasse Basin

The stable isotope composition of groundwaters is essential to characterize their origin and processes of their hydrogeochemical evolution. In carbonate aquifers, the interaction between groundwater and carbonates enriched in 18 O may shift groundwater δ 18 O values to the right of the global meteoric water line. Although experimental studies showed that the exchange of oxygen isotopes between groundwater and carbonates may proceed rapidly under ambient conditions, there is little information regarding the kinetics of the exchange under geological conditions. To better quantify the relevance of carbonate-groundwater interaction on oxygen isotope signatures of both groundwater and carbonates, we present an extensive hydrogeochemical characterization of deep groundwaters collected during a deep drilling campaign in the Swiss Molasse Basin. Combining information from chemical and isotopic analyses, groundwater residence time tracers, the geological evolution of the area, as well as geochemical modelling we quantitatively demonstrate that the exchange of oxygen isotopes is mainly relevant for carbonate-dominated aquifers consisting of fractured rock with a low porosity and a large solid to liquid ratio. For such conditions, isotopic equilibrium is reached in more than a few hundred thousand but less than 20 Mio years at temperatures below 70 °C. It follows that the equilibration of groundwater δ 18 O values may serve as independent proxy for the residence time of very old groundwaters. Moreover, our study shows that for rock buffered systems the oxygen isotope exchange has only minor effects on the oxygen isotope composition of rock-forming calcite. Accordingly, it does not affect the use of the δ 18 O signature of calcite as environmental proxy.