GREAT BASIN CLIMATE SINCE THE YOUNGER DRYAS

We present a high-resolution and precisely-dated stalagmite record of Great Basin paleoclimate, the Leviathan chronology, that shows a strong summer insolation forcing. Stalagmite growth was slow and oxygen isotope (δ¹⁸O ) values were low during the Younger Dryas, which we interpret as a cool and dry climate, which was followed by a transition to highest δ¹⁸O values during continued slow stalagmite growth at 8000 years before present. This peak 8ka δ¹⁸O represents the warmest and driest conditions of the Holocene, and supports the notion of a distinct altithermal in the Great Basin. Following the altithermal, δ¹⁸O values decreased to near modern levels about 4000 yr BP, at which time growth rates increased dramatically and a decrease in carbon isotope values track more productive soils. The Leviathan chronology suggests that late Holocene climate was cooler and wetter than the mid-Holocene. The trend to late-Holocene cooling and wetter conditions is consistent with the longer-term orbital scale forcing of Great Basin paleoclimate that is tightly coupled to the classic Milankovitch forcing of precessional-scale variations in northern hemisphere summer insolation. However, observations of warming since 1600 yr BP suggests a departure from the orbital signal, which would predict continued cooling for at least another thousand years until the next summer insolation minima. Based on the observation of pluvial lake expansions following the summer insolation minima during the last glacial period, we suggest that this departure from Milankovitch pacing may represent an averted pluviation in the Great Basin, and may be related to the rise in atmospheric carbon dioxide and methane concentrations between 5000 and 8000 years before present.