Factors Contributing to the Late Cenozoic Cooling and Aridification of Southwestern North America

Climate reconstructions from paleoflora, paleosols, and stable isotopes (δ18O and δD) of authigenic minerals reveal episodes of cooling and drying across southwestern North America during the late Cenozoic (since 23 Ma). This climate transition has been attributed to a number of factors including: 1) global cooling due to drawdown of atmospheric CO2, accompanied by enhanced meridional and tropical zonal SST gradients and the development of polar icecaps; 2) vegetation turnover with deciduous trees, and xerophytic shrub- and grass-lands replacing warm-temperate forests; and, 3) multiple orogenic events that reshaped the continent, uplifting the Rockies and Sierra Nevada Mountains and creating the Basin and Range province. Although studies have investigated some of these potential climate factors, comprehensive comparisons among them and against proxies, especially stable isotope records, are still lacking. We performed a series of seven experiments with a water isotope-enabled global climate model (ECHAM5-JSBACH-wiso) and different boundary conditions in order to identify the influence of these drivers on the late Cenozoic climate transition. Our results indicate that regional cooling is primarily a response to global cooling. The factors that drive hydrological changes, however, differ by region. Expansion of grasslands leads to reduction of precipitation by up to 100 mm/yr and δ18O enrichment of soil water by 1-2‰ across the Great Plain region. In the Basin and Range Province, tectonic subsidence gives rise to as much as a 3‰ enrichment in soil water and reduction of precipitation by up to 200 mm/yr. Across the southwestern coast and Sierra Nevada Mountains, drying primarily results from reduction of the zonal SST gradient. Our results demonstrate the utility of water isotopes for investigating past hydrological regimes, and suggest that a combination of factors conspired to drive late Cenozoic hydrological changes.