EARLY HOLOCENE CLIMATE CHANGE AT EAGLE LAKE, CA LOCATED IN THE TRANSITION ZONE OF THE NORTH AMERICAN DIPOLE

Northern California is located at the current transition zone of the North American climate dipole between the Pacific Northwest and Southwest. Changes in the size and location of this transition zone over the last 10,000 years are poorly constrained. Given the differences in seasonal precipitation between the NW and SW US, the relative sizes of the two regimes and the position of the transition zone will control in part the moisture availability and snow pack of the Sierra Nevada Mountains. A network of carefully placed sites is needed to reconstruct both spatial and temporal changes in this key transition zone. One site is Eagle Lake, CA (40.6540°N, 120.7439°W), which is located on the eastern flank of the Sierra Nevada Mountains within the current transition zone. A sediment core was collected in 2011 in 20 m of water in the southwest basin of the lake. An ~2500 –year long record of climate change was reconstructed from multiple proxies, including total organic carbon (TOC), δ¹³C and δ¹⁵N composition of bulk organic material, and n-alkane distribution. From ~9,500 cal yr BP to 7,500 cal BP the record exhibits moderate values of TOC with a distinct period between 8,000 – 9,000 cal yr BP that has well-preserved laminations and large fluctuations in %TOC. These laminations likely indicate prolonged anoxia, suggesting the Eagle Lake area had a climatic affinity with the Pacific Northwest during the early Holocene. Preliminary normal-alkane analysis indicates that the lake predominantly preserved terrestrial organic matter. The deposition of the Mazama tephra at ~7600 cal yr BP was followed by a doubling of TOC values, suggesting possible fertilization or change in climate.