HYDROGEN ISOTOPE RATIOS OF LEAF WAX LIPIDS FROM SIERRA NEVADA LAKE SEDIMENTS RECORD DECADAL HYDROCLIMATE VARIABILITY DURING THE MEDIEVAL PERIOD

Persistent drought is one of the greatest risks of climate change facing the western United States. Highlighting this risk is evidence from paleo hydrologic reconstructions that during the medieval period (~900-1400 AD) this region was witness to droughts of greater duration than any experienced in recorded history. However, much remains to be learned about these “mega-droughts,” including their impact on precipitation and snowpack in the Sierra Nevada Mountain range, which currently serve as a crucial source of fresh water to southern California. Stable hydrogen isotope ratios (δD) of plant leaf wax lipid compounds preserved in lake sediments show promise as a proxy for recording past hydroclimate variability in the Sierra Nevada Mountains. Previous work demonstrates that the δD of these compounds reflects that of lake water and/or shallow ground water--reservoirs both fed by local precipitation. Lake sediment δD can therefore reflect the processes that determine δD of meteoric water, including temperature, humidity, moisture source and storm track. We have measured δD of aquatic and terrestrial plant leaf wax fatty acids extracted from a suite of sediment cores collected at Swamp Lake (elevation: 1553m), in Yosemite National Park, along the Sierra Nevada crest. Measurements at annual to interannual resolution were made for two time periods: the 20^th century and the 12^th-15^th centuries. Comparison of 20^th century results with instrumental records of Sierra Nevada hydroclimate reveals a significant relationship at decadal scales between Swamp Lake leaf wax lipid δD variability and wintertime precipitation, April 1^st snow water content and Palmer Drought Severity Index (PDSI). In agreement with these relationships, medieval δD variability, which ranges on the order of 30‰, mimics that of regional tree ring PDSI reconstructions at decadal frequencies. This δD record provides an independent means of capturing decadal scale hydroclimate variability and offers a unique perspective on the mega-drought intervals in the Sierra Nevada Mountains.