Paleoelevation Reconstruction of the Sierra Nevada from Compound-Specific Stable Isotopes of Fossil Leaves

Fossil leaf materials provide an important record of terrestrial paleoclimatic and environmental conditions. In particular, fossil angiosperm assemblages have offered important information on the paleoelevation of mountain-belts by providing measures of past temperature and precipitation. However, paleobotanical elevation estimates are often hampered by uncertainties in local terrestrial lapse rates, adequate sample size, and or are reliant on taxon-specific data. Hydrogen and carbon isotope analyses of individual organic compounds in ancient leaf cuticle provides a potential means of quantifying changes in the isotopic composition of leaf water that may reflect changes in elevation or water-use efficiency, and thus an independent measure of paleoelevation. We measured the hydrogen and carbon isotope compositions of individual n-alkanes from ancient angiosperm leaf cuticle collected in the northern Sierra Nevada to test the utility of this paleoelevation reconstruction approach. Here, Eocene to recent sediments fill ancient drainages of the Sierra Nevada and contain abundant and exquisitely preserved fossil leaf materials of the early Eocene Chalk Bluffs flora. Hydrogen isotope analyses of n-alkanes extracted from bulk sediments and individual fossil leaves within these Eocene channels show a systematic decrease (>25‰) in δD with distance from the Eocene shoreline, suggesting the presence of a large elevation gradient. Furthermore, isotopic shifts in individual n-alkanes extracted from ancient leaf cuticle across the Sierra Nevada are similar to isotopic shifts observed in Eocene-aged authigenic minerals from these same channels, demonstrating the utility of this paleoelevation reconstruction approach.