Neogene stable isotope evidence for long-term stability of large-scale hydroclimate over the North American Great Plains

Abstract:

The North American Great Plains are characterized by a sharp aridity gradient at around the 100^th meridian with a more humid climate to the east and a more arid climate to the west. This aridity gradient shapes the region's agriculture, economy, and its ecosystems, and recent work suggests that arid conditions on the Great Plains may expand eastward with global warming. The abundant and widespread Neogene sediments of the Ogallala Formation that underlie the Great Plains present an opportunity to reconstruct regional hydroclimate conditions at a time when atmospheric CO₂ and global temperatures were higher, providing insight into the aridity and ecosystem response to warming and increases in atmospheric CO₂. We present new (n=366), alongside previously published, paleosol carbonate δ¹⁸O data across more than 50 sites that span the Great Plains to evaluate the long-term hydroclimatic and ecosystem changes in the region since the late Neogene. Carbonate δ¹⁸O demonstrate remarkable similarity between the spatial pattern of paleo-precipitation δ¹⁸O and modern precipitation δ¹⁸O, with low δ¹⁸O in the northwest Plains and high δ¹⁸O in the southeast Plains. Today, modern precipitation δ¹⁸O over the Great Plains is set by the mixing between moist, high-δ¹⁸O moisture delivered by the southerly Great Plains Low-Level Jet and drier, low δ¹⁸O westerly air masses. In the absence of countervailing processes, we interpret this similarity between paleo and modern δ¹⁸O to indicate that the proportional mixing between these two air masses has been minimally influenced by changes in global climate. Thus, any changes in the position of the 100^th meridian aridity gradient has not been forced by dynamical changes in these two synoptic systems. We conclude that, to first order, the modern aridity gradient over the Great Plains and the atmospheric processes that drive it are not strongly sensitive to changes in global climate.