An organic geochemical record of Sierra Nevada climate since the LGM from Swamp Lake, Yosemite

Sediment records from Swamp Lake (SL) in the central Sierra Nevada, California, provide evidence of climatic change on millennial and centennial timescales over the last ∼20,000 years. Total organic carbon (TOC) abundance varied in concert with elemental and isotopic tracers of organic matter (C/N, δ ¹³C _org, δ ¹⁵N), biogenic silica content, total magnetic susceptibility, and sediment lithology. We interpret the down-core proxy records as representing the response of the lake environment, in terms of temperature, seasonal ice cover, mixing regimes, runoff and in situ OM and nutrient cycling, to shifting climate states. These environmental factors in turn drove changes in algal productivity, OM sources, microbial OM regeneration and secondary production, and detrital input. The late Pleistocene (∼19.7-10.8 cal. kyr BP) was dominated by fluctuations between relatively warm/dry intervals with high TOC (17.4-16.5, 15.8-15.0, 13.9-13.2, 11.4-11.0 cal. kyr BP) and cold/wet intervals (16.5-15.8, 14.8-13.9, 13.1-11.6, 11.0-10.7 cal. kyr BP) characterized by low TOC and high detrital input. The Holocene (∼10.7 cal. kyr BP - present) was characterized by three abrupt increases in TOC (after ∼10.8, 8.0, and 3.0 cal. kyr BP) and numerous century-scale fluctuations. TOC increases reflected enhanced lake productivity and OM recycling, and reduced detrital input, in response to changing winter temperature and hydrologic regimes. Inferred environmental changes at SL correlate with other Sierra Nevada paleorecords, and with reconstructed sea surface temperatures along the California margin. Parallel changes in the SL and SST records over the past ∼20,000 years provide new evidence that continental climate in the Sierra Nevada and the California Current system have responded, on multiple timescales, to common drivers in North Pacific ocean-atmospheric circulation.