Abstract

Paleosols preserved between flows of the Columbia River Basalt Province record hiatuses in basalt emplacement and provide detailed information about paleoclimatic and pedogenic conditions during pedogenesis. The relatively long duration of pedogenesis (104 to >106 yr) for these paleosols makes them unlikely to have been influenced by short-duration climate fluctuations and allows them to accurately reflect long-term climate trends related to global climate change. Ten paleosols hosted by the Grande Ronde and Wanapum Formations of the Columbia River Basalts were identified on the basis of pedogenic features such as horizon development, root traces, and saprolitization, and they were studied as paleoclimatic indicators. Relationships between the ages of the paleosols and their clay-mineral compositions suggest that pedogenic processes changed in the eastern Columbia River Basalt Province at ca. 15.8 Ma. Analyses of mass-balance trends and chemical alteration in a ca. 16.0 Ma paleosol indicate that it formed under warm, humid climate conditions. We suggest that the absence of similar warm, humid paleosols and the presence of 2:1 clay-rich paleosols that formed after ca. 15.8 Ma represent a cooling and drying climate change related to the termination of the middle Miocene climate optimum, a development not previously captured in paleosol studies. In addition, the well-constrained dates of the basalts provide better resolution on the timing of this termination. When combined with other middle Miocene paleoclimate estimates from the Columbia River Basalt Province, our results suggest a geographic control on mean annual precipitation in the middle Miocene Columbia River Basalt Province leading to an increasing-to-the-east precipitation gradient.

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