Orographic precipitation, fossil leaf wax δ2H, and the paleotopography of the Eocene Sierra Nevada, California, USA

The stable isotopic composition of clay minerals, volcanic glass, and leaf waxes preserved in paleochannels of the northern Sierra Nevada (California, USA) shows higher than modern elevations in the Paleogene, yet this result remains debated due to difficulty reconciling the topography and geomorphology of the modern and ancient landscapes. Here, we present new leaf wax hydrogen isotope data (δ2ΗnC29) from individual fossil leaves preserved in Eocene−Oligocene alluvial gravels of the northern Sierra Nevada to reconstruct past precipitation δ2Η. We use modern water samples and the Orographic Precipitation and Isotopes model to estimate the distribution of precipitation isotopes across the western North American Cordillera and use this result as a basis for interpreting Eocene precipitation isotopes after removing the Coast Ranges and correcting for a warmer Eocene climate. In combination, these data are used to constrain changes to Sierra Nevada topography between the Eocene and present. We show the following: (1) Eocene δ2ΗnC29 values decrease systematically across the ancient Sierra Nevada, consistent with Rayleigh distillation during orographic lifting and rainout. (2) Bulk sediment and individual leaf δ2ΗnC29 show good agreement, and heterogeneity within individual leaf δ2ΗnC29 data indicates isotopic signatures were locally derived and primary rather than reset. (3) The topography of the northern Eocene Sierra Nevada was ∼30% higher than today. To estimate paleotopography, we evaluate the observed hydrogen isotope fractionation in the Eocene relative to the present (Φlift) and show that this is proportional to the ratio of the past topography to present topography, and we base our conclusion on the average Φlift values of ∼1.29. New stable isotope and modeling results closely align with previous estimates and are consistent with a high Late Cretaceous magmatic arc that decayed through the Cenozoic and experienced renewed uplift in the late Cenozoic.