An 800-year multiple-proxy record of atmospheric circulation, climate change, and aquatic productivity from Kepler Lake, South-Central Alaska

A multiple-proxy investigation was undertaken on two short cores (85 cm and 101 cm long) from Kepler Lake, an evaporation-insensitive, groundwater-fed marl lake in South-Central Alaska, with the goal of reconstructing climate and environmental changes in recent centuries. The proxies employed in this study include: calcite Carbon-Oxygen isotopes, organic matter (OM) Carbon-Nitrogen isotopes, and loss on ignition (LOI) analysis. An 800-year chronology for two cores was established based on four calibrated AMS 14C dates on terrestrial macrofossils, 210Pb analysis and core-to-core correlations using LOI results. delta18O VPDB values of inorganic calcite range from -17.0‰ to -15.7‰, with the highest values occurring during the period of 1450 to 1850 AD, a time coeval to the Little Ice Age (LIA). The high delta18O values during the cold LIA contrast with the conventional temperature interpretation using O isotopes and are interpreted to reflect a shift in atmospheric circulation. A weakening of the wintertime Aleutian Low (AL) pressure system residing over the Gulf of Alaska would result in 18O-enriched winter precipitation as well as a colder winter climate in SC Alaska during the LIA. We propose that elevated CaCO3 contents of >80% during the LIA reflect enhanced seasonality (warmer summer and colder winter), as calcite precipitation in freshwater lakes is primarily a function of peak summer temperature. This interpretation is also supported by high delta13C values, likely reflecting high aquatic productivity. The enhanced seasonality during the LIA may explain some complex moisture and temperature changes inferred from studies of glacier, lake and tree-ring records. Since 1850 an increase of 6‰ in delta15N from ca. +2‰ to +8.0‰ is interpreted to be caused by atmospheric transport of 15N enriched N-containing pollutants to the lake since the European Industrial Revolution. Through the use of multiple proxies complex relationships between climate change (shifts in the strength and/or position of the AL) and anthropogenic change (atmospheric deposition of nitrogen) can be examined in the context of aquatic ecosystems in order to yield a more complete understanding of paleo-environmental change during the late Holocene.