University of California Water Resources Center

Persistent drought presents one of the greatest risks of climate change faced by the western United States. Highlighting this risk is from paleo hydrological evidence that during the medieval period (~900-1400 AD) this region experienced two extensive droughts of greater duration than any experienced in recorded history. However, much remains to be learned about these “mega droughts,” including the severity of deficit in precipitation and snowpack in California’s Sierra Nevada Mountain range, which currently serve as a crucial source of fresh water to California agriculture, industry and domestic users. To possibly extract a record of the hydrological conditions in the Sierra Nevada during this era, cores of annually laminated sediment from Swamp Lake, in the northwest portion of Yosemite Park were collected. Stable hydrogen isotope (δD) ratios of plant leaf-wax lipid compounds preserved in the lake sediments and the thickness of the annual sedimentary laminations (varves) show promise as potential proxies for recording past hydroclimate variability in the Sierra Nevada Mountains. δD measurements at annual to interannual resolution were made for two time periods: the 20th century and the 13th-15th centuries while varve thickness was measured over the entire period 1150-2006 AD. We find significant negative correlations between 20th century δD fluctuations and instrumentally recorded variability in total precipitation, snow water equivalence (SWE) and Palmer Drought Severity Index (PDSI). Medieval δD values correspond to independent regional hydrologic reconstructions at the decadal to multi-decadal scale, which appear to delineate relatively dry medieval episodes. An issue that is still unclear is that the mean δD during this period is little different from that during the relatively moist contemporaneous period. This finding is at odds with evidence from submerged tree stumps for two prolonged droughts separated by about 200 years. Varve thickness measurements are on going, but results thus far reveal difficulties in using this proxy for reconstructing past Sierra Nevada hydroclimate variability, the most significant being the complexity of the seasonal deposition cycle and sub-millimeter thicknesses. Overall, geochemical and geomorphological information gleaned from Swamp Lake sediment cores provide an independent means of reconstructing hydroclimate in the Sierra Nevada and provide a unique perspective on the mega-drought intervals in the Sierra Nevada Mountains.