ANCIENT ELEVATION OF NORTHERN SIERRA NEVADA MOUNTAINS DETECTED FROM STOMATAL ANALYSES OF 16 – 21 MILLION YEAR OLD FOSSIL LEAVES

The topographic history of the Sierra Nevada has been the focus of an increasing number of studies, but because of conflicting results derived from different methods, the actual altitudinal history remains unresolved. Quantitative paleoelevation studies based on oxygen and hydrogen isotopes, thermochronology and paleobotanical proxies argue that the Sierra Nevada were high-standing during the early Cenozoic and may have lost elevation since. In contrast, river incision and stratigraphic tilt studies claim that the main elevation gain was not obtained until the Pliocene, associated with the delamination of an eclogite root under the southern Sierra Nevada. In order to resolve the current discrepancies in the topographic development of the northern Sierra Nevada, we have estimated the paleoelevation of two early-middle Miocene sites in the northern Sierra Nevada, using a new paleobotanical proxy based on the response in stomatal density on leaves to decreasing CO₂ partial pressure with altitude.

Stomatal analysis of fossil Quercus pseudolyrata leaves indicates a paleoelevation for Gold Lake of 2152 ± 519 m, 139 m higher than present, and for Mohawk of 2051 ± 286 m, 665 m higher than present. The uncertainty in coeval sea-level CO₂ estimations provides the largest source of potential error in this novel method. The lowest estimated elevations, using the maximum likely CO₂ concentration, were at least 1500 m a.s.l. during the early-middle Miocene for both sites. Paleoelevation estimates using established paleobotanical methods (Leaf Margin Analysis, CLAMP and the Nearest Living Relative method) for the Gold Lake and Mohawk floras are remarkably consistent with stomata-based estimates. The paleobotanical data reported here from the Mohawk and Gold Lake floras indicate that a previously suggested post-Eocene uplift of ~500 m had already been gained by the Miocene, strongly arguing against a significant Pliocene uplift phase linked to mantle delamination under the northern Sierra Nevada, without unprecedented Late Miocene erosion to remove previously established relief.