The Hydroclimate and Environmental Response to Warming in the Southwestern US: A Study Across the Mid-Miocene Climate Optimum

Abstract

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 p CO 2 periods. The Mid-Miocene Climate Optimum (MMCO; ~17-14 Ma) represents a period of warm global temperatures and high p CO 2 with estimates similar to the projected p CO 2 for future decades. We present new stable isotope records of mid-Miocene terrestrial carbonates from the Española basin in northern NM, with δ 18 O and δ 13 C records recording the extent of the MMCO and the beginning of late Miocene cooling. New 40 Ar/ 39 Ar ages establish an updated, high-resolution age model for the Tesuque Fm of the Santa Fe Group. We use δ 18 O as a measure of the balance between summertime and wintertime precipitation and δ 13 C 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 δ 18 O, the carbonate δ 18 O record suggests that the region received more westerly-derived, cool-season precipitation than it does today. This indicates that El Niño Southern Oscillation (ENSO) was operating during the MMCO and may have been stronger than today; it seems to have been particularly strong during cooler periods during the MMCO, suggesting that cooler temperatures and high p CO 2 may be favorable to ENSO. The δ 18 O and δ 13 C records are highly correlated, indicating seasonality of precipitation as a main control on soil productivity; increases in soil productivity coincide with increases in cool-season precipitation and with faunal fossils that indicate a wetter environment with large vegetation. 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. During the global cooling immediately following the MMCO Española carbonates display decreasing soil productivity and a more summer-dominant hydroclimate similar to that of the region today, with paleontological records indicating a drier faunal and floral assemblage very different from those that occupied the region during the MMCO. Collectively our data do not support increased aridity in the southwest US during warm, high p CO 2 periods, instead suggesting a shift in the hydroclimate towards cool-season, westerly-derived precipitation, driving higher soil productivity and supporting larger vegetation and dynamic faunal assemblages in the region.