GREAT PLAINS OGALLALA FORMATION STABLE ISOTOPE EVIDENCE FOR THE POSITION OF THE 100TH MERIDIAN ARIDITY GRADIENT IN THE LATE NEOGENE

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 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 when atmospheric CO₂ and global temperatures were higher, providing insight into the aridity and ecosystem response to warming. We present new (n=366), alongside previously published, paleosol carbonate δ¹³C and 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. 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 the 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. In contrast, prior to the widespread appearance of C4 plants on the Plains, paleosol carbonate δ¹³C show an east-to-west gradient, with higher values to the west, reflective of lower primary productivity to the west. Close comparison with modern primary productivity data indicates that primary productivity has declined and shifted eastward since the late Neogene, likely a reduction in CO₂ fertilization during the late Neogene. We conclude that, to first order, the modern aridity gradient and the hydrologic processes that drive it are not strongly sensitive to changes in global climate and any shifts in this aridity gradient in response to rising CO₂ will be towards the west, rather than towards the east.