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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 Denver Basin Aquifer System (DBAS) is an important groundwater resource for Front Range communities and is currently experiencing increasing demand as populations grow and surface water supplies remain limited. It is necessary to better constrain aquifer recharge mechanisms to enable sustainable use of this resource. In other sedimentary basin aquifer systems, mountain front recharge has been shown to be a significant contributor to local basin groundwater recharge. In the DBAS, inputs from the mountain block are poorly understood, and previous numerical models have treated large segments of the mountain-front boundary as impermeable. However, there exist potential connections between the mountain block and the DBAS, either by direct contact of permeable units, which would facilitate underflow recharge into the basin, or by surface water infiltration to the aquifer units where they outcrop near the mountain front. To observe spatial and temporal relationships between mountain block water and DBAS water, we use water stable isotopes and characterize the d2H and d18O of monthly precipitation, seasonal surface waters, and groundwaters in and around the Front Range and Denver Basin. The goal of this study is to determine if differences in the isotopic composition of waters across the Front Range permit the use of d18O and d2H as tracers of water flow between Front Range streams and groundwater and the DBAS. We analyzed the unique signature of mountain-block water to compare with DBAS water stable isotope data collected from Castle Rock Water municipal wells. Stable isotope ratios varied spatially and temporally, with the greatest temporal variance observed in precipitation. Streams showed great spatial variance, and less significant seasonal variance between the three seasonal sampling events conducted. Groundwaters showed very little temporal variance but had great spatial variance both between the aquifer units of the DBAS and between different locations within the mountain block crystalline aquifer. The lowest d2H and d18O ratios were measured in winter precipitation, winter streams, and groundwater samples collected from the high-elevation Front Range. Samples of DBAS groundwaters with the lowest d2H and d18O ratios indicate potential hydrogeologic connection to the mountain block. Interpreted mixing lines on a d-excess versus d18O plot support the potential DBAS-mountain block connection. The deepest aquifer units of the DBAS (Arapahoe and Laramie-Fox Hills) show the least relationship with meteoric or surface waters on both a d2H and d18O plot and the d-excess versus d18O plot and have higher d18O values than would be predicted based on their previously measured recharge ages and paleoclimate data from the region. Characterizing the spatial and temporal variations in water stable isotope signatures of the Front Range and DBAS region enhances understanding of the region’s hydrology and hydrogeology. Additionally, these results help to better inform models of aquifer recharge and promote sustainable use of the DBAS resource.