Longitudinal Variation in Terrestrial Oxygen Isotope Records of Modern Water and Mid-Miocene Authigenic Minerals: No Mid-Miocene Climatic Optimum in the Inland Pacific Northwest?

The stable isotopic compositions of benthic foraminifera quantitatively indicate that the middle Miocene climatic optimum was a period of global greenhouse conditions within the generally cooling and drying climatic trend in the past 65 million years. There are numerous red-colored weathering sequences preserved in intervolcanic paleosurfaces of the middle Miocene Columbia River Basalt Group. Even though most of the reddened layers were considered as thermally altered `baked' zones by the overlying basalt flows, many of them show much evidence of pedogenesis. Thus, it was suggested that these lateritic weathering horizons are products of intense chemical weathering under humid and temperate paleoclimate conditions in the inland Pacific Northwest during the middle Miocene climatic optimum. However, these weathering sequences are only common around the Blue Mountains region in southeastern Washington, Oregon, and Idaho, where modern mean annual precipitation is relatively higher than the surrounding basins. Therefore, we hypothesize that the observed intense chemical weathering is the result of higher mean annual precipitation due to the orographic effect in the Blue Mountains area. The orographic effect is associated with higher altitude than surroundings in an upwind region. Four mid-Miocene spectacular thick reddish paleosols on top of Columbia River basalt flows were found at nearly the same latitude of 46°N but with different longitudes, ranging from 116°W to 120°W. Authigenic minerals at depths greater than 50 cm below the overlying basaltic flows were collected at each location, and also, for comparison, several surface water samples were collected to determine the modern hydrologic isotopic conditions along the transect. The measured δ18O values of modern surface water show a negative shift of approximately 5‰ from west to east in the study area, and the observed negative shift within the δ18O authigenic mineral values is similar (~4.5‰). Using equilibrium fractionation equations, the estimated paleo-temperatures are also similar to the modern temperatures along the transect. Thus, the regional climate has not been changes significantly since the mid-Miocene. It is likely that topography of the study area in the Blue Mountains region has not been significantly changed either since at least late Miocene. The basalt stratigraphy also indicates that there was a tectonic uplift in the Blue Mountains area during the mid- to late-Miocene.