Oxygen Isotopic Composition of Miocene Meteoric Water (δ ¹⁸ O _w ) in Alpine Foreland Basins

Abstract

The reconstructed stable oxygen isotopic composition of meteoric water (δ ¹⁸ O _w ) is widely used for paleoclimate reconstructions. Understanding past environmental factors that drove changes in δ ¹⁸ O _w is essential for interpreting past climate conditions. Here, we investigate the spatial and temporal variations of δ ¹⁸ O _w in the Miocene Alpine foreland basin from multiple proxy materials with isotope‐enabled climate model simulations. Based on δ ¹⁸ O values and clumped isotope‐based temperatures of pedogenic carbonate nodules, we compare Middle Miocene δ ¹⁸ O _w records from the Northern Alpine Foreland Basin (NAFB) in Switzerland and its counterpart in the Mediterranean Alpine foreland (Digne‐Valensole Basin, SE France). Additionally, we analyze isotope‐tracking atmospheric general circulation model outputs of δ ¹⁸ O in precipitation for the Middle Miocene and present‐day to assess changes in distributions. Our results highlight a latitudinal δ ¹⁸ O _w gradient in the Middle Miocene that is similar to the present‐day. The occurrence of distinct N‐S differences in δ ¹⁸ O _w values during the Miocene are likely the result of a similar‐to‐present climate divide (North‐Atlantic vs. Mediterranean climates). Concurrently, we suggest that shifts in atmospheric circulation over the Alpine region during certain time intervals of the Middle Miocene led to rapid high‐amplitude variations in δ ¹⁸ O _w . We note significant differences in δ ¹⁸ O _w values between proxy‐based reconstructions and climate model simulations that we attribute to (a) proxy biases toward higher δ ¹⁸ O _w values and (b) inappropriate model resolution and topography for the Miocene Alpine region. Our study advances the use of multiproxy data‐model comparison for paleoclimate and paleoaltimetry reconstructions and points out the importance of integrating proxy and modeling approaches.