STABLE-ISOTOPIC RECORD OF THE YOUNGER DRYAS WARMING AT FOY LAKE, MONTANA

The Younger Dryas is the largest, and most recent, stadial to develop during the current interglacial period that began ~14,000 years ago. In the alpine environment of western North America, high-resolution records of environmental response to Younger Dryas cooling and subsequent warming into the Holocene are rare. A record of the abrupt termination of the Younger Dryas and stepped transition into the Holocene was reconstructed from δ¹⁸O and δ¹³C values of endogenic carbonate from Foy Lake, Montana. The δ¹⁸O shift is approximately 5‰, indicative of a substantial increase in water and/or air temperature. The greatest increase happens in ~100 years. The rapid increases in δ¹⁸O and δ¹³C values correlate with a shift from calcite to aragonite, signaling warmer temperatures and greater evaporative concentration of the water. The transition is also marked by an abrupt increase in planktic diatoms from a more benthic assemblage. However, the timing based on a previous chronological model places the transition to the Holocene at 10,800 cal yr BP, nearly 800 years after the termination in the GISP2 ice core. This discrepancy in timing is either due to a lag in the response of cordilleran lakes or a poor initial chronology.