The hydroclimate response to warming in the southwestern US: A study across the Mid-Miocene Climate Optimum

Stable isotopes of inorganic carbonates are commonly used to reconstruct terrestrial paleoclimates. We analyze the 18O and 13C of near-surface, authigenic carbonates formed during the Mid-Miocene Climatic Optimum (MMCO; 17-14.5 Ma) near Espanola, NM, to study the effects of high pCO2 and warming on the hydroclimate of the southwestern United States. Modern climate model projections of a drier southwestern US conflict with paleoclimate data from the middle Pliocene Warm Period which suggests wetter-than-preindustrial conditions in the southwest. The region receives westerly wintertime moisture, which is typically enhanced during an El Nino (ENSO) event, and summertime moisture from the North American Monsoon; these sources are isotopically distinct, suggesting that the 18O of soil and near-surface carbonates may reflect the relative importance of each moisture source. The MMCO is well established in the marine record, but terrestrial records are lacking; the Espanola Basin has a continuous and thick sediment record throughout the mid-Miocene and abundant authigenic carbonates that permit a high-resolution terrestrial record of the MMCO. The abundance of Miocene-aged soil and near-surface carbonates indicates a semiarid to arid environment, but the stable isotope records imply a dynamic hydroclimate across the MMCO. The 18O data record a relative minimumwhich we interpret as an increase in the fraction of wintertime precipitationat ~15.6 Ma, which coincides with relative global cooling in the benthic record. In turn, the 18O and 13C data are positively correlated, indicating that vegetation and primary productivityrecorded in d13C valueswere influenced by the same dynamics that drove changes in d18O, such that greater wintertime precipitation resulted in greater primary productivity. The fossil record at Espanola also indicates a period of rapid evolution and speciation in the faunal assemblage around this time, further supporting the dynamic nature of southwestern US hydroclimate. Taken together, these data suggest that cooler global climates favor greater wintertime moistureperhaps driven by enhanced ENSO conditionswhile warm global climates coincide with higher relative contributions of summertime, monsoonal precipitation.