Stable isotope composition of bentonites from the Swiss and Bavarian Freshwater Molasse as a proxy for paleoprecipitation

The hydrogen and oxygen isotope compositions of bentonites and smectite-rich tuffs from the North Alpine Foreland Basin were measured for samples dated between 9 and 21 Ma. Assuming clay mineral formation shortly after the deposition of the volcanic ash layers and at surface temperatures estimated from paleoclimatic studies, the H and O isotope composition of water in equilibrium with the clay minerals can be calculated. A comparison with values derived from other proxies of paleoprecipitation in the Molasse Basin (such as pedogenic carbonates and mammal remains) and δ¹⁸O values of carbonate in the bentonite beds, suggests that the δ¹⁸O values of water estimated by this method are typical for foreland precipitation at this time and for this region. Therefore, the δ¹⁸O value of smectite in bentonites and altered tuff layers can be a useful tool for studies of paleoclimate and -topography. There are, however, indications that the clay minerals did not retain their initial hydrogen isotopic composition, but that a postformational exchange of H isotopes has occurred.