SOUTHWEST CLIMATE AND ECOSYSTEM PRODUCTIVITY IN THE MIOCENE

The US Southwest is projected to get drier in the near future due to increasing atmospheric CO₂, which threatens the region’s ability to support its current ecosystem. However, there is high uncertainty in this projection as the predicted precipitation change is uncertain. Furthermore, regional water loss due to evapotranspiration is highly dependent on vegetation but the plant physiological response to higher CO₂ over geological timescales remains poorly constrained. We use a high-resolution sedimentary archive of past climate and ecosystems—authigenic carbonates within the Miocene-aged Santa Fe Group of the Rio Grande Rift—to inform our understanding of Southwest climate and ecosystem in a warmer world. The middle Miocene represents a deep-time analog for future climate change, exhibiting similar CO₂ and temperature estimates compared to end-of-21^st century projections. We present new carbon and oxygen stable isotope (δ¹⁸O and δ¹³C) data to constrain the hydroclimate and ecosystem productivity response to higher atmospheric CO₂. Today and likely during the Miocene, the Rio Grande Rift ecosystem is water-limited, which suggests plants would respond to rising CO₂ by fixing more carbon. Theoretically, this would mean higher ecosystem primary productivity. Because paleosol δ¹³C is a mixture of high δ¹³C atmospheric CO₂ and low δ¹³C CO₂ from soil respiration (a function of above-ground vegetation productivity), greater ecosystem productivity would result in lower paleosol δ¹³C values. Thus, we expect our new δ¹³C data to be lower during periods of higher CO₂ (and vice versa) if plant response to CO₂ is sustained over long-time scale. We use δ¹⁸O as an indication between summer-wet and winter-wet climate since the modern moisture of the Southwest is sourced from both westerly winter storms (low δ¹⁸O values) and summertime North American monsoons (having distinctly higher δ¹⁸O values). Overall, we find evidence for a shift toward a winter-wet climate for the Southwest during the Mid Miocene Climatic Optimum compared to modern δ¹⁸O data.