Refining paleoelevation estimates of the European Alps by simulating Middle Miocene climate and δ18O responses to diachronous surface uplift scenarios

Abstract

Estimates of past surface elevations are essential for understanding the evolution of the Earth's physiography and biodiversity distribution. Stable isotope paleoaltimetry is widely used to infer paleoelevation due to a robust systematic inverse relationship between elevation and isotopic composition (${\delta }^{18}O$, $\mathrm{\delta D}$) of meteoric waters (i.e., isotopic lapse rate). The difference in ${\delta }^{18}O$ of paleo-meteoric water reconstructed from coeval proxy materials between adjacent low- and high-elevation sites ($\Delta {\delta }^{18}{O}_p$) is transformed into paleoelevation changes using such isotopic lapse rates ($\delta$-$\delta$ approach). Most often, the isotopic lapse rate is assumed to be stationary through time and space and, therefore, relies on modern estimates to constrain paleoelevation changes. This study employs model-based sensitivity analysis to assess the spatio-temporal variability of the isotopic lapse rate of the European Alps and to quantify the magnitude of uncertainties in paleoelevation estimates associated with the use of modern isotopic lapse rates. We use the high-resolution isotope-tracking climate model to simulate the ${\delta }^{18}O$ in precipitation (${\delta }^{18}{O}_p$) response to Middle Miocene conditions (e.g., atmospheric CO2, palaeogeography) and diachronous west-to-east surface uplift propagating along the Alpine orogen. The simulated isotopic lapse rates become shallower by $\sim$1.0 ‰ km⁻¹ in response to Middle Miocene conditions compared to the Pre-Industrial period and vary within the range of $\pm$1.5 ‰ km⁻¹ for the diachronous surface uplift scenarios of the Alps. Applying the simulated isotopic lapse rates to Miocene $\Delta {\delta }^{18}{O}_p$ proxy reconstructions suggests an overestimation of Central Alps paleoelevation by $\sim$1.5 km when using modern rainfall-based isotopic lapse rate across the Alps. However, the simulated Miocene isotopic lapse rates estimate aligns more closely with modern global river-based lapse rates, suggesting they are more suitable than rainfall-based estimates when a paleoclimate-constrained isotopic lapse rate is unavailable.