Mild Little Ice Age and unprecedented recent warmth in an 1800-year lake sediment record from Svalbard

The Arctic region is subject to a greater amplitude of climate variability than the globe as a whole and is currently undergoing large scale changes due in part to anthropogenic global warming. The Svalbard Archipelago occupies an important location for understanding patterns and causes of Arctic climate variability; however, available paleoclimate records from Svalbard are of restricted use due to coarse sampling resolution, chronologic uncertainties, and limitations of existing climate proxies. Here we present a sub- to multi-decadal scale record of summer temperature for the past 1800 years from lake sediments of Kongressvatnet on West Spitsbergen, Svalbard, based on the first instrumental calibration of the alkenone paleothermometer. The age model for these High Arctic lake sediments, which cannot be dated using radiocarbon, is based on 210Pb, plutonium activity, and the first application of tephrochronology to lake sediments in this region. We find that the summer warmth of the past 50 years recorded in both the instrumental and alkenone records was unmatched in West Spitsbergen in the course of the past 1800 years, including during the Medieval Climate Anomaly (MCA), and that summers during the Little Ice Age (LIA) of the 18-19th centuries on Svalbard were not particularly cold, even though glaciers occupied their maximum Holocene extent. Our results suggest that increased wintertime precipitation, rather than cold temperatures, was responsible for LIA glaciations on Svalbard and (together with marine records) that heat transport into the Arctic via the West Spitsbergen Current (WSC) increased beginning approximately AD 1600.