GULL LAKE, CA: A NEW SEDIMENTARY ARCHIVE OF HISTORICAL CLIMATE CHANGE IN THE EASTERN SIERRA NEVADA

The Sierra Nevada range is a key water source for the state of California. Snowmelt from the high Sierra is needed to recharge rivers, which ultimately provide water for millions of people and support a huge agricultural sector. Climate change is an existential threat to water resources in the Sierra Nevada, as rising temperatures alter the natural seasonal water cycle in complex ways. The purpose of this study was to ascertain the sensitivity of Gull Lake in the eastern Sierra Nevada to climatically-driven environmental changes over the Industrial Era. To this end, we collected two short sediment cores (< 50 cm) that capture the last ~260 years of sedimentation at Gull Lake, and dated them using ¹³⁷Cs and ²¹⁰Pb. We analyzed several biogeochemical variables from these sediments, including biogenic silica (BiSi), organic carbon (C) and nitrogen (N), and their stable isotopes (δ¹³C and δ¹⁵N, respectively). These data will allow us to compare modern Gull Lake to the pre-Industrial Era system to understand how humans have influenced the lake and the broader climate. Initial findings show changes in many geochemical indicators that are consistent with a shift to more algal organic matter sources, including lower C/N values, over the last 90 years. However, BiSi and δ¹³C values are lower in the more recent sediments than in the pre-Industrial Era record, suggesting either a potential decline in primary production, or an increase in nutrient cycling. A decline in production would be consistent with findings from other lakes in the region that have also experienced lower primary production over the post-Industrial Era. Further, a shift in δ¹³C and δ¹⁵N over this time period is consistent with increased fossil fuel combustion and the concomitant fallout of lighter isotopes of these elements, which attests to the role of Gull Lake in recording broader changes. Our findings suggest that Gull Lake is a valuable addition to the suite of lake-sediment records from western North America, which continues to experience historic drought conditions for which more paleoclimate context is essential.