PROBLEMS WITH USING FOSSIL FLORAS AS PALEOALTIMETERS, WITH EXAMPLES FROM THE FLORISSANT PALEOFLORA

Paleotemperature estimates from fossil floras have been used to determine paleoelevation values by basically comparing temperatures from high-elevation paleofloras with contemporaneous paleofloras near sea level and then utilizing some value of lapse rate. The late Eocene Florissant paleoflora has been a focus of such studies and published results include paleoelevations just below to very far above the modern elevation of Florissant. The range of published elevation estimates are very broad, with ranges of 2233 m for mean elevations and 3200 m if error estimates are included. This wide range of elevations are caused by the variations in the analysis, including the wasy of deriving paleotemperature estimates, utilizing different sea level paleofloras and using different lapse rates. Most of the variation is from the different lapse rates, but most of the published analyses show relatively cool mean annual temperatures. Reanalysis of the paleotemperatures resulting in values warmer than the published values and using a regionally averaged lapse rate results in paleoelevations averaging 1350 m lower than the present Florissant elevation. As a result, paleotemperature analyses of the Florissant paleoflora can support any model of late Cenozoic uplift, stability, or downdropping for the Southern Rocky Mountains.