MIOCENE CLIMATE DYNAMICS OF THE RIO GRANDE RIFT REGION

Climate is thought to provide a first-order control on landscape evolution; yet, there are few quantitative constraints regarding the climatic history of the Rio Grande Rift and how changes in climate might have affected rift evolution. During most of the extension of the Rift, global climate was substantially warmer than the Quaternary, suggesting that there may have been changes in the overall climate of the Rio Grande Rift region. 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. Our δ¹⁸O and δ¹³C records span 17-12 Ma and cover a period of extraordinary warmth during the Neogene—the Miocene Climate Optimum (MCO)—and the onset of late Neogene cooling. Stable isotopes of meteoric waters in this region reflect the influence of the two main circulation systems that deliver moisture to the Rift region: high δ¹⁸O moisture reflects the influence of the North American Monsoon, whereas low δ¹⁸O is delivered by mid-latitude westerly moisture sourced from Pacific extratropical cyclones. In turn, δ¹³C reflects changes in plant productivity and vegetative cover in the region. Overall, δ¹⁸O decreases over the course of the MCO, with local δ¹⁸O minima that correlate with cool episodes observed in the benthic marine δ¹⁸O record. Values of δ¹³C and δ¹⁸O are positively correlated, such that minima in δ¹⁸O are accompanied by minima in δ¹³C. Compared to modern precipitation δ¹⁸O, carbonate δ¹⁸O suggests that the region received more low δ¹⁸O—presumably wintertime—precipitation than it does today. The covariation in δ¹³C suggests that these increases in wintertime precipitation produced increases in soil productivity, likely driven by increasing plant productivity. These hypothesized changes in hydroclimate and soil productivity are supported by the paleontological record at the site, which show a diverse and dynamic faunal assemblage during the MCO, but that becomes increasingly reflective of arid conditions during cooling following the MCO. Collectively, our new data suggest that during periods of active extension, much of the Rift experienced more winter wet conditions and greater vegetative cover than today.