REGIONAL CLIMATIC EFFECTS OF A LARGE ASH FALL ON THE MICROFOSSILS OF THE MIOCENE LATAH FORMATION

The Latah Formation has been known for its plant megafossils since 1897, when clay and siltstone near Spokane, Washington were found to contain fossil leaves. The sediments in the study area were deposited around and behind the basaltic flows of the Priest Rapids member of the Wanapum formation of the Columbia River Flood Basalt. Over a period of ~2 Ma the CRBs added significant amounts of CO₂ to the atmosphere, probably causing in part a climb in global temperature known as the Mid-Miocene Climatic Optimum. Approximately 15 Ma a one meter thick ash fall not related to the Columbia River volcanism was deposited in Miocene Lake Clarkia. Sediments were sampled from below, within, and above the ash and analyzed for their pollen content. Samples were taken every 20 centimeters below and above the ash and every 10 centimeters within the ash. The results of this palynology study indicate that the climate of the Miocene Lake Clarkia was warm temperate to subtropical. This conclusion was reached using the pollen spectrum coupled with the macrofossil assemblage to identify the most likely climate. The effects of ash fall RA-3 did not indicate any long term impact on the regional vegetation of the Clarkia Basin. This study in conjunction with continuing ash chemistry/dating studies helps to tentatively date the Clarkia Floras to ~15 to 15.5 Ma, occurring directly on the apex of the Mid-Miocene Climatic Optimum.