Decoupling the ancient hydrologic system from the modern hydrologic system of Pacific Northwest in the United States: Implications for the evolution of topography, climate, and environment

Water is one of the most important elements for functioning of the Earth system, and the Earth's hydrologic cycle controls not only its biogeochemical cycles but also its energy balance. Investigating the ancient hydrologic cycle is, therefore, a key to fully understand terrestrial climate and environmental changes, and to evaluate their variability in the future. Because precipitation patterns are largely affected by regional topography and large-scale atmospheric circulation patterns, this study aims to reconstruct Cenozoic hydrologic patterns in the Pacific Northwest in order to evaluate topographic evolution of the Cascade Range and the Blue Mountains, and the late Quaternary climate and environmental changes in response to the last glaciation.; Estimated paleoclimate parameters (mean annual precipitation and temperature) and stable isotopic compositions of the ancient meteoric water as recorded in authigenic and pedogenic minerals from weathered ash and tuff layers exhibit temporal and spatial variations in the regional hydrologic system of the Pacific Northwest. First, due to tectonic and magmatic processes along the Cascadia subduction zone, elevation of the Cascade Range fell from ∼3250 +/- 350 m to ∼450 +/- 350 m from 45 to 12 Ma and has increased ∼1150 +/- 350 m since the late Miocene. Second, because of magmatic activity associated with eruptions of the mid-Miocene Columbia River Basalt Group, the presence of a pronounced paleotopography of the Blue Mountains induced the spatial variations in the paleopedogenesis and paleoclimate patterns. Third, due to changes in the atmospheric circulation patterns related to the presence and absence of a glacial anticyclone during the late Quaternary, seasonal precipitation patterns in the inland Pacific Northwest have been shifted from 10-50% of winter precipitation and 50-90% of summer precipitation during the last glacial period to the current ∼75% of winter precipitation and ∼25% of summer precipitation. These reconstructed ancient hydrologic systems can provide unique insights of geologic and Earth system processes.