Influence of paleorelief on the Mid-Miocene climate variation in southeastern Washington, northeastern Oregon, and western Idaho, USA

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Terrestrial climate proxy records repeatedly show spatial heterogeneity because regional climate condition is largely influenced by the relationship between large-scale atmospheric circulation patterns and regional topographic features. The paleosols developed on the top of Columbia River Basalt flows emplaced between 15.3 and 15.8 Ma in the northwestern United States show significant spatial variations in the Mid-Miocene soil-forming processes. The paleosol in the northwestern part of the study area has the least degree of chemical weathering, fine scale and very shallow penetrating depth of root traces with the estimated paleoprecipitation and paleotemperature of about 550 mm and 16 °C, respectively. The paleosols in the southeastern part of the study area indicate more intense chemical weathering with the estimated paleoprecipitation and paleotemperature of approximately 1400 to 1500 mm and 17 °C, respectively. The spatial variations in paleo-pedogenesis were predominantly affected by spatial heterogeneity in the Mid-Miocene regional climate and regional paleotopographic features, taking the other ancient soil-forming factors, including time, parent rock materials, and biological activities, into account. We suggest that intrusions of hot Columbia River Basalt magma into the crust were responsible for the regional surface uplift and the paleoclimate heterogeneity because the major vent systems of the Columbia River Basalt eruptions were located in the area where development of the Mid-Miocene high topography existed within the Blue Mountains. Paleotopography of the Earth's surface can be reconstructed by the paleoprecipitation and paleotemperature estimations from several spatially distributed chrono-stratigraphic paleosols although it is difficult to estimate the magnitude of the paleorelief precisely.