PALEOLIMNOLOGICAL EVIDENCE OF THE YOUNGER DRYAS EVENT IN THE SIERRA NEVADA, CA

Paleolimolimnology has enjoyed rapid growth as a result of: 1) improved coring and sampling techniques that have allowed for higher temporal resolution; 2) the use of more sensitive proxies, such as diatoms and chironomids, and 3) better statistical techniques and greater computing power, which have allowed for quantitative calibration of the climate signal from paleolimnological proxies. These advancements have allowed us to re-examine the Sierra Nevada, CA, for evidence of climate and environmental change during the last glacial-interglacial transition. A chironomid-inference model for temperature developed from a 60-lake calibration set was applied to subfossil chironomid assemblages from sediments from two mid-elevation lakes that spanned ~15,000 to 10,000 calendar years BP. This temperature reconstruction revealed that lakewater temperatures warmed by approximately 5 degrees C during this time. The warming was interrupted by a short cooling episode (~2-5 degrees C) which reached maximum intensity at ~12,200 cal years BP, coincident with the middle of the Younger Dryas event. Diatom analyses indicate that planktonic diatoms first increase, then decrease and finally increase during the Younger Dryas cooling. Diatom-inference models for depth, total Kjeldahl nitrogen (TKN) and salinity applied to fossil diatom assemblages during the Younger Dryas suggest that the increases in planktonic diatoms are the result of increased effective moisture and/or decreased ice cover. Although these results are preliminary, they indicate that rapid climate and environmental variations occurred in the Sierra Nevada during the glacial-interglacial transition. This data, in tandem with other paleolimnological studies from North America, Russia and Siberia and along the western coast of North America, contribute to a growing body of research that supports a teleconnection of Younger Dryas cooling from northeastern Atlantic to the northeastern Pacific via northern Eurasia. The rapid climate shifts documented here, and in other paleolimnological studies of the last glacial-interglacial transition, provide clues as to how lake and terrestrial ecosystems will be impacted with future climate change.