Paleovegetational heterogeneity and the record of Eocene-Oligocene climate change in the interior Pacific Northwest

Fossil plant assemblages from volcanic arc-associated basins are useful for examining global change because arc basin sequences are rapidly deposited and radiometrically datable. However, arc basin sequences reflect complex landscapes with heterogenius and largely successional vegetation, preserved in a range of facies. This study tests whether foliar physiognomic (morphological/anatomical) evidence of Eocene - Oligocene climatic change from fossil leaf assemblages of the interior Pacific Northwest can be reliably isolated from physiognomic differences resulting from ecological and taphonomic variability among the fossil assemblages.; Taxon-based physiognomic analysis in this study used Wolfe's CLAMP, and Wing and Greenwood's multiple regression analysis approaches. Specimen-based physiognomic analysis and floristic analysis provided independent climate information. Physiognomic biases introduced by growth site ecology and taphonomic sorting mechanisms were assessed by facies analysis of coeval assemblages from different growth and preservational facies of the Cedarville Flora, NE California.; Results indicate that parautochthonous assemblages from slowly aggraded fluviodeltaic and braidplain facies preserve physiognomic thermal signals which are high relative to estimated regional climate. Autochthonous and parautochthonous associations from rapidly aggraded, well-drained alluvial facies yield low estimates of temperature and dry-season precipitation relative to estimated regional conditions. Lacustrine assemblages consistently yield physiognomic thermal and precipitation estimates which are low relative to inferred regional paleoclimate.; When ecologically/taphonomically similar facies are compared, the magnitude of Eocene - Oligocene climatic cooling falls below standard error for physiognomic methodology. Results suggest that regional climate of the interior Pacific Northwest experienced a roughly 2{dollar}spcirc{dollar}C decline in mean annual temperature between Middle and Late Eocene time. Late Eocene cooling culminated in an approximately 3{dollar}spcirc{dollar}C decline in mean annual temperature between {dollar}sim{dollar}37-33.6 Ma, and was followed by {dollar}sim{dollar}1-2{dollar}spcirc{dollar}C of warming during the early Oligocene. This climatic record is similar to that of deep sea sediments and terrestrial paleosols, but differs sharply from previous paleobotanical interpretations which ignore ecological and facies relationships of the paleofloras.