A 600-YEAR MULTIPLE-PROXY RECORD OF ATMOSPHERIC CIRCULATION AND CLIMATE CHANGE FROM KEPLER LAKE, SOUTH-CENTRAL ALASKA

We present a multiple-proxy reconstruction (calcite C-O isotopes, organic matter (OM) C-N isotopes, and loss-on-ignition data) of an 85-cm core from Kepler Lake, an evaporation-insensitive (based on water isotopes), groundwater-fed marl (~80% carbonate) lake ~45 km NE of Anchorage, AK. Chronology of the 600-year record (1408-2007 AD) was based on three calibrated AMS ¹⁴C dates from terrestrial macrofossils. δ¹⁸O_VPDB values from calcite vary from -17.0‰ to -15.7‰. δ¹⁸O increased beginning at 1440 AD from -16.8‰ to -16.1‰, remained high (above -16.2‰) between 1440 and 1850 AD with two negative excursions of ~0.5‰ (1550 and 1725 AD), and decreased to -16.7‰ at 1880 AD. The relatively high δ¹⁸O values during the Little Ice Age (LIA) from 1460-1850 AD suggest a weakening of the semi-permanent Aleutian Low (AL) pressure system residing over the Gulf of Alaska, resulting in enriched ¹⁸O in precipitation. A weakening of the AL during the LIA would also lead to a colder climate in south-central Alaska, which is supported by documented glacial advance. The negative shift of ~1‰ in δ¹⁸O in the 1840s has been documented in ice cores from Mt. Logan and in marl lake sediments from the southern Yukon, suggesting a broad-scale shift in atmospheric circulation. δ¹⁵N_air shows a steady increase from 1.6‰ to 6‰ (1840 AD - Present), indicating increasing aquatic productivity in a warm, post-LIA climate and/or increased N contribution from industrial pollutants. The δ¹³C records show an increase from 1600 to 1850 AD of 1.5‰ and 3‰ and a decrease from 1850-1900 AD of 1‰ and 1.5‰, from calcite and OM, respectively, seemingly requiring an explanation involving more than productivity changes alone. Both OM and carbonate contents show shifts at 1440 AD, 1600 AD and 1840 AD, which may correlate with the above-mentioned isotopic shifts. Our results demonstrate the utility of multiple isotopes in addressing changes in atmospheric circulation, climate, and ecological responses.