THE HYDROCLIMATE RESPONSE TO WARMING IN THE SOUTHWESTERN US: A STUDY ACROSS THE MID-MIOCENE CLIMATE OPTIMUM

The Española Basin of the northern Rio Grande Rift contains thick and largely continuous exposures of Santa Fe group sediments deposited at relatively fast sedimentation rates with abundant authigenic carbonates and tephra zones; these characteristics allow the basin to serve as a unique high-resolution terrestrial record of mid-Miocene regional paleoclimate. We analyze the δ¹⁸O and δ¹³C of near-surface, authigenic carbonates formed during the Mid-Miocene Climatic Optimum (MMCO; 17-14.5 Ma) in the Española Basin 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. Today, the region receives westerly wintertime moisture, which is typically enhanced during an El Niño (ENSO) event, and summertime moisture from the North American Monsoon; these sources are isotopically distinct, suggesting that the δ¹⁸O of soil and near-surface carbonates may reflect the relative importance of each moisture source. 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. Carbonate δ¹⁸O decreases during the MMCO followed by an increase in values during late-Miocene cooling; these trends are interpreted as an increase and subsequent decrease in the fraction of wintertime precipitation to the region. In turn, the δ¹⁸O and δ¹³C data are positively correlated, indicating that vegetation and primary productivity—recorded in δ¹³C values—were influenced by the same dynamics that drove changes in δ¹⁸O, such that greater wintertime precipitation resulted in greater primary productivity. The fossil record at Española also indicates a period of rapid evolution and speciation in the faunal assemblage during the MMCO, further supporting the dynamic nature of southwestern US hydroclimate. Taken together, these data suggest that the warm global climate of the MMCO favored greater wintertime moisture—perhaps driven by enhanced ENSO conditions—which in turn drove changes in local vegetation and ecosystem evolution.