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Abstract 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; ∼17–14 Ma) in northern New Mexico to understand the impact of warmer global temperatures and higherpCO₂on southwestern US hydroclimate. The MCO and MCT comprised a globally warmer period with elevatedpCO₂similar to end‐of‐the‐century (∼400–800 ppm) projections. We present new stable isotope (δ¹⁸O and δ¹³C) records of vadose‐zone and groundwater terrestrial carbonates and of modern precipitation, stream, and groundwater from the Española basin in northern New Mexico and establish a high‐resolution age model using new⁴⁰Ar/³⁹Ar ages. We interpret δ¹⁸O as reflecting the balance between summertime monsoonal and wintertime precipitation and δ¹³C as a reflection of plant productivity. Terrestrial carbonate δ¹⁸O is lowest during the MCO and MCT and is correlated with terrestrial carbonate δ¹³C and anti‐correlated with the benthic δ¹⁸O 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 δ¹³C suggests that plant productivity was driven by the amount of wintertime precipitation. Comparison with middle Miocene climate model simulations reveals that higher CO₂drives 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. 

The US Southwest is projected to get warmer and drier due to increasing atmospheric CO2, which threatens the region’s ability to support its current ecosystem. However, there is high uncertainty in this projection as precipitation and evapotranspiration remain poorly constrained. We use paleoclimate proxy data from the Miocene to gain insights into Southwest climate during periods of higher atmospheric CO2. Today, the southwest US is characterized by two wet seasons: in winter, the mid-latitude westerlies deliver Pacific-derived moisture, whereas summer moisture is predominantly delivered by the North American Monsoon. We present a new high-resolution sedimentary archive of carbon and oxygen stable isotope (d18O, d13Ccarbonate, and d13Corganic) data to constrain the hydroclimate and ecosystem productivity response to higher atmospheric CO2, derived from authigenic carbonates within the Miocene-aged Santa Fe Group of the Rio Grande Rift from the Española and Albuquerque basins. We find substantial spatial and temporal variability in d18O, likely reflecting variability in the strength of the two circulation systems that deliver moisture to the southwest US. Overall, reconstructed precipitation d18O is lower than today throughout much of the Miocene, suggesting potentially a greater influence of the wintertime westerlies in the moisture budget of the southwest US during the Miocene. Sedimentary organic d13C is < -20‰ throughout the Miocene, indicative of little C4 plant influence during this time. Sedimentary carbonate d13C is generally always less than -5‰, and is positively correlated to carbonate d18O. Such coupling may reflect the influence of evaporation on these samples or a strong link between moisture delivery and primary productivity in this arid climate.