OLIGO-MIOCENE STABLE ISOTOPE PALEOCLIMATE RECORDS FROM PALEOSOLS IN THE NORTH ALPINE FORELAND BASIN

The development of the North Alpine Foreland Basin has been strongly influenced by the tectonic evolution of the adjacent Alpine orogen. Detailed knowledge of the stratigraphy, facies relationships, and chronology of sedimentation provides an exceptional case to establish a detailed terrestrial stable isotope record for the climatic evolution in Central Europe. We present detailed oxygen and carbon isotopic records for Oligo-Miocene terrestrial paleosols developed in the North Alpine Foreland Basins of Switzerland, including the Oligo-Miocene boundary and the mid-Miocene climatic optimum (MMCO). These paleosols consist of reddish pedogenic mudstones containing abundant carbonate concretions and calcified roots. These soils developed in overbank settings of the Hörnli, Napf, Kronberg and Speer alluvial fan systems at the Alpine thrust front and have been dated by magnetostratigraphy with ca. 100 ky precision. δ¹⁸O values for the calcrete are relatively constant at 25 ‰ (SMOW) between 30 and 21 Ma, decrease to 20 ‰ between 21 and 17 Ma and show the lowest values in the entire section between 17 and 14.5 Ma. Miocene pedogenic mudstones show a similar isotopic trend than their associated nodules. The reason for exceptionally low δ¹⁸O values during the MMCO is not completely clear. One possible interpretation is that the decrease to exceptionally low values reflects the formation of significant Alpine topography responsible for reorganizing the air circulation patterns. The subsequent isotopic increase at 14.5 Ma in the paleosols mimics the general trend of the global oceanic oxygen isotope records and also corresponds to a general rapid cooling event, well documented in Central Europe, following the MMCO. We therefore interpret these measurements to record a global climatic variation in the mid-Miocene. This isotopic record from the North Alpine Foreland Basin provides a detailed database for climate changes in Central Europe consistent with paleontological observation and suggests that the growth of the Alpine topography in the Central Alps has been responsible for local climate change and reorganization of the air circulation patterns.