600 YEARS OF CLIMATIC AND ANTHROPOGENIC INFLUENCES RECORDED IN FALLEN LEAF LAKE, SIERRA NEVADA, CA

Multi-proxy analyses of a gravity core from Fallen Leaf Lake, an oligotrophic sub-alpine lake in the southern Lake Tahoe Basin, show a climatic response to the Little Ice Age (LIA), subsequent warming, and anthropogenic effects commencing in the 1800s. Diatom analyses define two zones of climate variability. High abundances of the diatoms Stephanodiscus alpinus and Aulacoseira subarctica indicate cooler and windier conditions between 1385 and 1807 AD than during the past 200 years. The period between 1385 and 1807 AD corresponds to the Little Ice Age and the neoglacial advances in the southern Sierra Nevada. For the ~140 years after the LIA, Fallen Leaf Lake experienced a period of warming and both the sedimentologic and diatom records were impacted by anthropogenic factors. The transitional period of warming and development for 140 years after the LIA is characterized by increases in Cyclotella rossii and Discostella stelligera and decreases in Pseudostaurosira brevistriata, and also shows increases in elemental cobalt, zinc, and tin that may be attributed to increased coal burning and smelting activities in California and Nevada. Above is a zone of strong anthropogenic influence commencing in the mid-1900s and marked by rapid increases in three planktonic araphids that dominate the modern lake system; Asterionella formosa, Tabellaria flocculosa strain IIIp, and the Fragilaria tenera group. The anthropogenic zone also shows increases in sediment accumulation that may be attributed to increased land development. This lake shows a discernible sensitivity to cooling that affects lake stratification and mixing, and to anthropogenic influences.