ABSTRACT: New records of climate variability in the southwest Yukon: Atmospheric circulation change in the North Pacific, paleohydrology and paleohumidity from lacustrine stable oxygen isotopes and a lake level reconstruction.

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Abstract

Analyses of sediment cores from two small lakes in the southwest Yukon, Jellybean Lake (60.35?N, 134.80?W, 730-m a.s.l.) and Marcella Lake (60.074?N, 133.808?W, 697-m a.s.l.) provide records of Holocene changes in atmospheric circulation, hydrology and humidity from millennial time-scales up to twenty to thirty-year resolution. This new climatic information and our proposed Aleutian Low mechanism lays a framework for evaluation of the paleoenvironmental and human response to climate change in the region. Jellybean Lake water reflects the oxygen isotope composition of mean annual precipitation. Thus, the oxygen isotope history of Jellybean Lake inferred from sedimentary carbonate oxygen isotope ratios suggests multi-decadal shifts in the oxygen isotope composition of mean annual precipitation superimposed on century and millennial trends. Recent fluctuations correspond to changes in the North Pacific Index, a measure of the intensity and position of the Aleutian Low pressure system over the Gulf of Alaska. This suggests that the Jellybean oxygen isotope record reflects changes in Aleutian Low intensity and position since ~7500 cal BP. Abrupt shifts to an intensified/eastward Aleutian Low occur ~1200 and 400 cal BP after a period of weakening/westward position after 3000 cal BP. These changes correspond with late Holocene glacial advances in the St. Elias Mountains, changes in North Pacific Salmon abundance and shifts in atmospheric circulation over the Beaufort Sea. Marcella Lake is a small, hydrologically closed, evaporation sensitive lake. Former water levels, driven by regional paleohydrology, were reconstructed by multi-proxy analyses of sediment cores from four sites spanning shallow to deep water. Prior to 10,000 cal BP water levels were low. Between 10,000 and 9,000 cal BP they rose to 3- to 4- m below modern levels. Between 7500 and 5000 cal BP water levels were 5- to 6- m below modern but rose by 4000 cal BP. Between 4000 and 2000 cal BP they were higher than modern. During the last 2000 years, water levels lowered to 1- to 2- m below or near modern levels. Marcella Lake water oxygen isotopes are strongly affected by summer evaporation thereby allowing past humidity changes to be reconstructed from sedimentary calcite oxygen isotope ratios at fifty to two hundred-year resolution. The record from Jellybean Lake accounts for variations related to atmospheric circulation and ambient temperature allowing the calculation of the difference between Marcella and Jellybean records, a parameter from which we can estimate change in humidity. Between 4500 and 2800 cal BP, summer humidity was higher than the modern average of 58% before reaching a maximum between 2800 and 1200 cal BP near 70%. At ~1200 cal BP humidity abruptly decreased to 65% before decreasing again, to ~60%, around 400 cal BP. The following climate patterns are emerging. The early Holocene was warm and dry. Between 9000 and 10,000 cal BP there was a rapid increase in lake level suggesting a shift in the precipitation regime. The early aridity may have prevented the establishment of spruce forest. Between 7500 and 4000 cal BP lake levels were relatively stable 5-m below modern levels and Aleutian Low intensity was predominantly weaker and/or westward. Between ~4500 and 3000 cal BP the Aleutian Low intensified and/or shifted eastward before weakening and/or shifting westward further between 3000 and 2000 cal BP. During this interval lake-level and humidity was higher than modern and late Holocene glacial advances began. Rapid Aleutian Low intensification and/or eastward shifts ~1200 and 400 cal BP correspond with paleohumidity decreases in the interior, Little Ice Age glacial advances and increases in North Pacific salmon abundance. Abrupt changes in Aleutian Low intensity/position and humidity were more frequent during the last ~2000 years than the preceding period.

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