Geologic studies suggest that the Pliocene Epoch was both warmer and wetter than today in Mediterranean-type climate (MTC) regions, contrasting with long-term hydroclimatic forecasts seen in most climate models in response to elevated CO2. In western North America, numerous large perennial lakes existed for 10⁵ to 10⁶ years in basins to the east of the Cascades and the Sierra Nevada, but dried out during the Pleistocene. What caused wetter conditions in Pliocene subtropical drylands when theory and model simulations indicate that these regions become drier with current global warming? What were the sources of precipitation and the atmospheric patterns in MTC regions and how did the hydroclimatic regime change with the onset of Pleistocene glacial-interglacial cycles? Was hydroclimate variability mostly controlled by latitudinal changes in the Hadley circulation or by other Ocean-Land-Atmosphere patterns? What were the regional variability dynamics, and impacts of the Pleistocene super interglacials?

Individual Pliocene lake records occur in many areas of the world, but the western North American basins are World Class sites, deep perennial freshwater Pliocene lakes latitudinally arrayed in a MTC region, able to capture a response to Pacific forcing. The cores close spatial and temporal gaps in our records of past climate and environment and contribute to understanding of the formation of critical mineral resources. We apply multiproxy analyses such as: (1) clumped isotopes on carbonates, (2) organic geochemical techniques such as isoprenoid- and branched GDGTs, long chain n-alkanes, compound specific hydrogen and carbon isotopes, PAHs, (3) assemblages of pollen and spores, diatoms, and ostracodes, to obtain millennial resolution or better terrestrial paleoclimate records for the time windows of interest. These records will be used to reconstruct past hydroclimate in close collaboration with climate model experts. Radiometric dating methods, tephras, and magnetostratigraphy will provide chronology.

Keywords: North America, USA, Pliocene, Paleoclimate, Hydroclimate, Paleolakes