Productivity and Temperature Variability Over the Past 15000 Years at a Small Alpine Lake in the Southern San Juan Mountains, Colorad

High-resolution records of Holocene temperature change are fundamental to placing modern warming into the context of natural variability. Few studies, however, have examined the full range of Holocene temperature change in the southwestern United States. This study examines four sediment cores from Crater Lake (37.23°N, 106.41°W), Colorado using a combination of traditional and hyperspectral observations to infer changes in productivity over the past 15000 years. The chronology for the sediments is based on nine 14C dates and radionuclide measurements on two cores. Depth matching across two additional sediment cores allowed for replication of inferred productivity variability at multiple coring sites. Magnetic susceptibility and two spectral indicators of minerogenic content were also measured to compare with relative changes in productivity. Aquatic productivity at Crater Lake, inferred from spectral measurements of chl a and phycocyanin, covary with instrumental summer temperature in the region over the past 90 years, consistent with the hypothesis that productivity in alpine lakes is often limited by summer temperature and growing-season length, and therefore appear to be useful proxies for summer temperature at Crater Lake. Spectral reflectance based productivity estimates, and sedimentary organic matter content generally covary with regional and global temperature changes over the Holocene, and with dramatic increases accompanying the Pleistocene-Holocene boundary. Spectrally-inferred productivity measurements peak during the middle Holocene climatic optimum, concurrent with regional estimates of warm, arid conditions. Fluctuations in the concentration of biogenic silica (BSi) do not consistently covary with other productivity indicators, with a distinct interval of low concentrations through the middle Holocene. The similarity of BSi variability to regional lake level reconstructions suggest that changes in lake chemistry may have influenced sedimentary BSi concentrations, although this interpretation remains uncertain. The continuity of this record allows a unique perspective of recent warming in the context of the entire Holocene, capturing rapid increases in productivity since ca. 1850 CE. The apparent warming from the 19th to 20th centuries exceeds any other century-scale change in the Holocene.