Millennial- to annual-scale Holocene climate change in the Alaskan Arctic and tropical Andes inferred from physical sedimentology and geochemical indicators preserved in finely laminated alpine lake sediment archives

High-resolution sediment archives from small alpine lakes in the Alaskan Arctic and tropical Andes were used to investigate Holocene climate change in these climatically sensitive and important regions. Varved minerogenic sediments from glacial-fed Blue Lake, Brooks Range Alaska (1275 m asl), were used to derive a proxy temperature record between AD 730-2005. Cool temperatures characterize the late Holocene (last millennial average, or LMA = 4.2°C) with 20th century warming anomalous within the context of the last 1275 years (0.8°C above LMA). However, temperatures between AD 1350-1450 and AD 1500-1620 approached modern values (0.4 and 0.3°C above LMA, respectively). Prolonged cooling at Blue Lake occurred from AD 1620-1880, during the Little Ice Age (LIA). LIA cooling and 20th century warming correspond to radiative minima and maxima, respectively. However, the relationship between radiative forcing and temperature is not consistent through the record, suggesting that other factors contributed to temperature variability in this region. In the tropics, South American summer monsoon (SASM) variability during the last 2300 years was reconstructed from oxygen isotope ratios of authigenic carbonate (delta18Ocal) preserved in varved sediments from Laguna Pumacocha (4300 m asl), Peru. High delta18O cal values and reduced variability during the Medieval Climate Anomaly (MCA; AD 920-1050) suggest that the SASM and El Nino-Southern Oscillation (ENSO) were weak due to reduced easterly tropospheric flow and cooling within the eastern tropical Pacific (ETP). Low delta18Ocal from during the LIA (AD 1415-1820) and enhanced variability from AD 1415-1770 suggest that the SASM and ENSO were strong as a result of enhanced easterly tropospheric flow and warm, but variable, sea surface temperatures in the ETP, respectively. These ocean-atmosphere responses reflect the tropical Pacific's response to radiative forcing described by the ocean dynamical thermostat model (Clement et al. 1996). Long-term delta18O cal trends at Pumacocha suggest that the SASM was weak during the early Holocene and then strengthened through the Holocene as Southern Hemisphere insolation increased. This confirms the importance of orbital controls on SASM dynamics. However, the Andean SASM strengthened to a greater extent than the SASM over the Amazon basin, indicating that these regions respond differently to similar forcing mechanisms.