Hydroclimate and environmental response to warming in the southwestern US, The: a study across the Mid-Miocene Climate Optimum

2022 Summer.Includes bibliographical references.Predictions for the effects of modern climate change on the southwestern US tend to suggest increased aridity, which is incompatible with paleoclimate data from other warm, high pCO2 periods. The Mid-Miocene Climate Optimum (MMCO; ~17-14 Ma) represents a period of warm global temperatures and high pCO2 similar to the projected pCO2 for future decades. We present new stable isotope records of mid-Miocene terrestrial carbonates from the Española basin in northern NM, along with new 40Ar/39Ar ages that establish an updated, high resolution age model for the Miocene-aged basin sediments. Our δ18O and δ13C records span 17-12 Ma, recording the extent of the MMCO and the beginning of late Miocene cooling. We use δ18O as a measure of the balance between summertime and wintertime precipitation and δ13C as a reflection of soil productivity. We find evidence for an increasingly winter-wet climate in the southwest US during the MMCO; when compared to modern precipitation δ18O, the carbonate δ18O record suggests that the region received more westerly-derived, wintertime precipitation than it does today. This indicates that El Niño Southern Oscillation (ENSO) was operating during the MMCO, and may have even been stronger than today; it seems to have been particularly strong during cooler periods during the MMCO, suggesting that cooler temperatures and high pCO2 may be favorable to ENSO. We also find that increases in wintertime precipitation are highly correlated with increases in soil productivity, suggesting that the amount of cool-season precipitation is a main control on vegetation for the region. Changes in the seasonal hydroclimate and soil productivity agree well with the paleontological record at the site, which show a diverse and dynamic faunal assemblage that evolved with the hydroclimate. Collectively our data do not support increased aridity in the southwest US during warm, high pCO2 periods, instead suggesting a shift towards increased cool-season precipitation that drives higher soil productivity, causing dynamic changes in the faunal and floral assemblage of the region