CLIMATIC, TOPOGRAPHIC AND ECOSYSTEM CONSTRAINTS IN THE NORTH AMERICAN CORDILLERA DURING THE MIDDLE EOCENE (~40 MA)

Reconstructing terrestrial paleoclimate during Cenozoic warm periods is critical to evaluating models of past and future climate change. However, it remains difficult to separate local climatic effects on paleoclimate proxy sites from regional or global climatic change because of uncertainties in the paleoelevation histories of paleoclimate proxy localities. One area where this is particularly challenging is the North American Cordillera region, where southward sweeping Paleogene magmatism resulted in dramatic and rapid changes in local climate, topography, and hydrology of many paleoclimate proxy localities. We couple pollen, phytolith and fossil leaf data with compound-specific records of leaf-wax δD and δ¹⁸O of lacustrine and paleosol carbonate to examine climatic and topographic gradients in northeastern Nevada and southwestern Montana during the middle Eocene (~40 Ma). Paleobotanical data reflect strong contemporaneous climatic differences between northeastern Nevada and southwestern Montana. Vegetation of northeastern Nevada reflects a mixed deciduous conifer forest that is inferred to have mean annual temperatures of ~9°C. Phytoliths preserved in floodplain sediments in southwestern Montana show a landscape dominated by forest indicators, with spiral gingers (Costaceae) and palms reflecting considerably warmer and wetter conditions. In addition, Montana contains other biosilica elements (chrysophytes, diatoms, freshwater sponge spicules) that indicate episodically ponded water on the floodplain. Oxygen isotopes of lacustrine and paleosol carbonate are characterized by similar compositions at these two localities. Large differences in local climate but similar isotopic data suggest waters from topographic highlands in northeastern Nevada and Idaho provided source waters draining into southwestern Montana at this time.