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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.