The Hydroclimate and Environmental Response to Middle Miocene Warming in the Southwestern USA: Stable Isotope Evidence

Predictions for the southwestern US with warming often suggest increased aridity. We investigate the sedimentary record of the Miocene Climate Optimum and Transition (MCO and MCT; similar to 17-14 Ma) in northern New Mexico to understand the impact of warmer global temperatures and higher pCO2 on southwestern US hydroclimate. The MCO and MCT comprised a globally warmer period with elevated pCO2 similar to end-of-the-century (similar to 400-800 ppm) projections. We present new stable isotope (delta 18O and delta 13C) records of vadose-zone and groundwater terrestrial carbonates and of modern precipitation, stream, and groundwater from the Espa & ntilde;ola basin in northern New Mexico and establish a high-resolution age model using new 40Ar/39Ar ages. We interpret delta 18O as reflecting the balance between summertime monsoonal and wintertime precipitation and delta 13C as a reflection of plant productivity. Terrestrial carbonate delta 18O is lowest during the MCO and MCT and is correlated with terrestrial carbonate delta 13C and anti-correlated with the benthic delta 18O record. We interpret these data as recording an overall winter-wet climate during the MCO and MCT, but that precipitation seasonality varied in response to changes in global climate during this period. The further correlation with carbonate delta 13C suggests that plant productivity was driven by the amount of wintertime precipitation. Comparison with middle Miocene climate model simulations reveals that higher CO2 drives a shift toward wintertime precipitation. Though paleogeographic changes may obscure a direct comparison to modern warming, overall, our findings suggest that prolonged global warmth may be associated with increased wintertime precipitation and greater primary productivity in northern New Mexico.