Oxygen and carbon isotope variations in precipitation and speleothem calcite from a northern California cave: Implications for paleoclimate reconstructions during the Late Pleistocene
Weisman, Isabelle Ellis
I have conducted a modern precipitation and cave drip water analysis in Northern California and developed precisely dated, high-resolution records of stable oxygen (δ18O) and carbon (δ13C) isotope variability from a Lake Shasta Caverns speleothem (LSC3) from Lakehead, California (40.8043°N, 122.3040°W). Analysis of six years of weekly rainfall isotope data (2010-2016) indicates that temperature and moisture source are significant controls on the δ18O and δ2H signature of precipitation that falls in Northern California. U-Th dating of the LSC3 stalagmite reveals that it precipitated from ~35.9 to ~14.3 ka, spanning Heinrich Stadials 1-3, and the last deglaciation, a period when regional records suggest large hydroclimatic changes in western North America (WNA). Increased δ18O and δ13C in LSC3 during the last glacial period (Marine Isotope Stage 2) including the Last Glacial Maximum suggest an increase in subtropical moisture but overall dry conditions in Northern California. This interpretation is supported by slower stalagmite growth rates during the glacial relative to the deglaciation and Marine Isotope Stage 3. Wetter conditions during the BA are suggested by decreased δ13C values and faster stalagmite growth rates. Comparison of the LSC3 record with coeval paleoclimate records from WNA will assist in refining our understanding of regional hydroclimatic variability and will help to determine the stationarity of spatial patterns of change between the north and south across the western United States. Understanding the driving mechanisms of California precipitation variability is critical in this heavily-populated, water stressed region.