CLIMATE OSCILLATIONS DURING THE EARLY EOCENE CLIMATIC OPTIMUM: CONSTRAINTS FROM ROCK MAGNETIC CYCLOSTRATIGRAPHY OF THE LOWER EOCENE WIND RIVER FORMATION, WYOMING

The Early Eocene Climatic Optimum (EECO, ~52-50 Ma) represents the warmest period of the Earth’s latest greenhouse stage and marks the beginning of long-term cooling in the Cenozoic. Characterization of the climate features of the EECO is an important element of understanding the driving mechanisms of this period of extreme climate. Previous studies have suggested that Milankovitch orbital forcing on Earth’s solar influx has written a signature in early Eocene climate, however, direct terrestrial records of this period are rare. Here we study high-resolution rock magnetic cyclostratigraphy of the lower Eocene Wind River Formation (WRF) in order to characterize the Milankovitch frequencies during the EECO. We also study magnetic polarity stratigraphy in order to improve the chronology of the strata. Near Dubois, Wyoming, the WRF consists of variegated mudstones, siltstones, and paleosols that were deposited in floodplain environments and interbedded sandstones and pebble conglomerates that were deposited in high energy channels. Deposition of the WRF lasted from ~53 Ma to ~51 Ma based on previous mammal fossil studies and lithostratigraphic correlation. The stable input of basement detritus and associated magnetic phases from the surrounding, emerging uplifts, provide the basis for characterizing the orbital forcing parameters during the EECO and improving the chronostratigraphy of the strata. Our detailed sampling of ~280 m of the WRF includes over 2,000 bulk samples used for high-resolution rock magnetic cyclostratigraphy. Our sampling resolution of 10-20 cm per sample is intended to provide data at a temporal resolution of 1-2 ka. Oriented block samples for magnetic polarity stratigraphy were collected from the freshest material possible at varying intervals of the floodplain deposits. Data collected in this study will reconstruct the history and features of early Eocene climate in the Wind River Basin that can be compared to other regional and global records of paleoclimate.