New Paleogene paleoclimate analysis of western Washington using physiognomic characteristics from fossil leaves

•The Chuckanut and Manastash formations grew in lowland West Coast settings.•Paleogene leaf physiognomy records approximately 20 Ma of fluctuating climate.•Temperatures were derived from 125 fossil leaf morphotypes from 15 localities.•Temperatures ranged from 12.3° to 29.1 °C; fluctuations match global climate trends.•Results are compared to inland and upland fossil sites of western North America.

The Chuckanut and Manastash formations were deposited in a lowland fluvial environment in western Washington during the Paleocene–Eocene. We provide a thorough paleotemperature analysis using the physiognomic characteristics from the fossil dicotyledonous leaves from these formations using two techniques, Leaf Margin Analysis (LMA) and Climate Leaf Analysis Multivariate Program (CLAMP). This work differs from previous analyses by 1) analyzing more localities and including the assessment of the never-before described Taneum Ridge Body and Manastash Main Body within the Manastash Formation and the Maple Falls Member of the Chuckanut Formation, and 2) using two different techniques to decipher mean annual temperatures through time. In this study, mean annual temperatures (MAT) were determined from 125 distinct morphotypes and are several degrees higher at each locality than previously reported. MAT values derived from LMA ranged from 17.0° to 28.8 °C for the lowermost and 13.4° to 19.5 °C for the uppermost Chuckanut Formation. The Manastash Formation had MATs ranging from 17.1 °C to 29.1 °C. Using CLAMP, paleotemperatures ranged from 15.3° to 19.4 °C for the lowermost and 12.3° to 15.6 °C for the uppermost Chuckanut Formation and 15.8 to 21.3 °C for the Manastash Formation. These data are compared to other Eocene–Paleocene formations in the western US; the Chuckanut and Manastash formations record some of the warmest paleotemperatures in the Paleogene in the western US because of their low-elevation locations. These paleotemperatures may reflect the terrestrial signal of paleoclimate optima defined in marine rocks during the Paleocene–Eocene.