REVISITING UNCERTAINTY IN KAOLINITE PALEOALTIMETRY AND PALEOTHERMOMETRY WITH OXYGEN ISOTOPE CONSTRAINTS FROM THE EOCENE SIERRA NEVADA

Despite the broad interest in determining the topographic and climatic histories of mountain ranges, the evolution of California’s Sierra Nevada remains actively debated. Prior stable isotope-based studies of Sierra Nevada have relied exclusively on hydrogen isotopes in kaolinite, hydrated volcanic glass and leaf n-alkanes. Additional constraints from the oxygen isotope composition of phyllosilicates increase the robustness of findings from a single isotope system and allow for the reconstruction of paleotemperatures. Here, we reconstruct the temperature and elevation of the Early Eocene Sierra Nevada using the oxygen isotope composition of kaolinitized granite clasts from the ancestral Yuba and American Rivers. We evaluate the possible contributions of hydrogen isotope exchange by direct comparison with more robust oxygen isotope measurements. Next, we utilize differences in the hydrogen and oxygen isotope fractionation in kaolinite to constrain paleotemperature.

Oxygen isotope geochemistry of in-situ kaolinites indicates upstream (eastward) depletion of ¹⁸O in the northern Sierra Nevada. δ¹⁸O values ranging from 11.4 – 14.4 ‰ at the easternmost localities correspond to paleoelevations as high as 2400 m when simulating the orographic precipitation of moisture from a Pacific source using Eocene boundary conditions. This finding is consistent with stable isotope studies of the northern Sierra using fossil leaves and volcanic glass, but oxygen isotope based paleoelevation estimates are systematically ~500 – 1000 m greater than those from hydrogen-based estimates from the same samples. Kaolinite geothermometry from 16 samples measured in duplicate or triplicate range from 13 to 37 °C, with an average Early Eocene temperature of 24.2 ± 2.0 °C (1s). For most samples we find that the uncertainty due to the observed scatter is less than the expected scatter based on analytical error from the δ¹⁸O and δD measurements (MSWD <1). This kaolinite temperature reconstruction is in agreement with paleofloral and soil biomarker constraints from the California interior and Eocene general circulation model simulations. Our results confirm prior hydrogen isotope-based paleoelevations and further substantiate the existence of a hot and high Eocene Sierra Nevada.