PALYNOLOGICAL EVIDENCE FOR VEGETATION CHANGE DURING THE LATE MIOCENE-PLIOCENE IN THE PACIFIC NORTHWEST

The Late Miocene through the Pleistocene (11.6 Ma – 0.01 Ma) in the Pacific Northwest is bounded by the cessation of the Columbia River Basalt flows (10.5 – 8.5 Ma) and Earth's most recent ice age (2.6 Ma – 0.01 Ma), and is characterized by numerous large scale geological events that no doubt affected local habitats. These geologic events include uplift of the Cascade Volcanic Arc, which led to the development of an extensive rain shadow to the east, and sediment deposition by the meandering Columbia River, which influenced floodplain stability across most of eastern Washington. These effects of shifting regional climate and environments on vegetation are recorded as changes in pollen abundance throughout this interval. In the present study we reconstruct vegetation from the Pacific Northwest during the Late Miocene to early Late Pliocene (8.5 to ~3 Ma) using fossil pollen extracted from two drill core samples from the Hanford site, south-central Washington. The pollen preserved at the Hanford site is one of only two known palynofloral sequences from the Pacific Northwest that spans the Miocene to Pleistocene. Preliminary results from the latest Miocene reveal a decline in a Taxodiaceous association with diverse conifers (Picea, Abies/Keeteleria and Pinus) shifting toward a hardwood association dominated by Pinus, Abies and Cedrus-type pollen. The flora is also rich in herbaceous groups including Asteraceae, Malvaceae, Poaceae, Onagraceae, and Polygonum. Only a few Asian pollen taxa, namely Pterocarya and Keeteleria-type, were discovered in this flora and are from genera that no longer grow in the New World. This flora is clearly indicative of a summer-wet climate typical of the southeastern seaboard where many of these hardwoods grow today. Preliminary work indicates a slight discontinuity in the vegetation across the landscape observed within the two sampled cores. These differences are thought to be due to vegetational variation across the ancient landscape associated with the meandering of the Columbia River. Future work will include sampling for pollen closer to the Plio-Pleisotcene boundary to add to our understanding of vegetation change approaching the onset of most recent Ice Age.