HOLOCENE CLIMATE VARIABILITY IN THE WESTERN SIERRA NEVADA: ORGANIC GEOCHEMICAL EVIDENCE FROM SWAMP LAKE, YOSEMITE NP

Large-scale atmospheric circulation patterns and underlying ocean conditions in the NE Pacific have a strong bearing on continental climate and water availability in California, as demonstrated in the past century of instrumental measurements. However, the nature of the relationship between ocean-atmosphere processes and climate variability in the Sierra Nevada of central California remains unclear on timescales greater than a few decades, and during past climate regimes of the Holocene. The 20,000-yr sedimentary record recovered from Swamp Lake, a mid-elevation (1554 m) lake in Yosemite NP, provides a rare opportunity to reconstruct climatic variability in the central Sierra at <100 yr resolution in relation to known or theorized regime shifts in the North Pacific and global drivers of climate. Here we present the results of organic geochemical and isotopic analyses of sedimentary organic matter from a 10 m core from the deep basin of Swamp Lake. We utilize carbon and nitrogen elemental abundances (TOC, TN, C/N) and isotopic compositions (d¹³C, d¹⁵N) of bulk OM to reconstruct changes in lake productivity, terrigenous input and organic matter sources. Our results indicate that productivity and OM source fluctuations at Swamp Lake, driven largely by wet-season temperatures, have mirrored large-scale regime shifts and centennial-scale fluctuations in the North Pacific. We also explore the utility of compound-specific hydrogen isotope (D/H) measurements on leaf-wax n-alkanes extracted from the sediment as a hydrologic tracer. We find that leaf-wax D/H ratios are most sensitive to changes in the lake water balance and the size of the spring snowpack, which over time have responded to atmospheric circulation changes on multiple timescales. A long-term, Holocene-spanning decline in D/H of terrestrial leaf waxes reflected a gradual increase in cool season (Dec-Feb) precipitation driven by insolation changes. Distinct multi-century oscillations in D/H occurring over the last ~12 kyr were coincident with both circulation regime shifts and drought cycles in California. Increased amplitude and frequency of D/H variability over the past ~6000 years paralleled the development of modern ENSO cycles in the tropical Pacific.