HR: 1340h
AN: PP43B-0676    [Abstracts]
TI: 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?
AU: * Takeuchi, A
EM: akinorit@gocougs.wsu.edu
AF: Department of Geology, Washington State University, Webster 1228, Pullman, WA 99164-2812 United States
AU: Larson, P B
EM: plarson@wsu.edu
AF: Department of Geology, Washington State University, Webster 1228, Pullman, WA 99164-2812 United States
AU: Suzuki, K
EM: semesuke@hotmail.com
AF: Department of Earth and Ocean Sciences, National Defense Academy in Japan, Hashirimizu 1-10-20, Yokosuka, 239-8686 Japan
AU: Goodwin, A J
EM: angiegoodwin@gmail.com
AF: Department of Geology, Washington State University, Webster 1228, Pullman, WA 99164-2812 United States
AB: 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.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4914 Continental climate records
DE: 8177 Tectonics and climatic interactions
DE: 9350 North America
DE: 9605 Neogene
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005