Late Holocene change in climate and atmospheric circulation inferred from geochemical records at Kepler Lake, south-central Alaska

Climate records during the last millennium are essential in placing recent anthropogenic-induced climate change into the context of natural climatic variability. However, detailed records are still sparse in Alaska, and these records would help elucidate climate patterns and possible forcing mechanisms. Here we present a multiple-proxy sedimentary record from Kepler Lake in south-central Alaska to reconstruct climatic and environmental changes over the last 800 years. Two short cores (85 and 101 cm long) from this groundwater-fed marl lake provide a detailed stable isotope and sediment lithological record with chronology based on four AMS C-14 dates on terrestrial macrofossils and Pb-210 analysis. The delta O-18 values of inorganic calcite (CaCO3) range from -17.0 to -15.7 aEuro degrees, with the highest values during the period of 1450-1850 AD, coeval with the well-documented Little Ice Age (LIA) cold interval in Alaska. The high delta O-18 values during the cold LIA are interpreted as reflecting shifts in atmospheric circulation. A weakening of the wintertime Aleutian low pressure system residing over the Gulf of Alaska during the LIA would have resulted in O-18-enriched winter precipitation as well as a colder and possibly drier winter climate in south-central Alaska. Also, elevated calcite contents of > 80 % during the LIA reflect a lowering of lake level and/or enhanced seasonality (warmer summer and colder winter), as calcite precipitation in freshwater lakes is primarily a function of peak summer temperature and water depth. This interpretation is also supported by high delta C-13 values, likely reflecting high aquatic productivity or increased residence times of the lake water during lower lake levels. The lower lake levels and warmer summers would have increased evaporative enrichment in O-18, also contributing to the high delta O-18 values during the LIA. Our results indicate that changes in atmospheric circulation were an important component of climate change during the last millennium, exerting strong influence on regional climate in Alaska and the Arctic.