Floral diversity and climate change in central Colorado during the eocene and oligocene.

摘要

Earth's global climate cooled significantly through the Eocene and early Oligocene. Previous work using stable isotopes, fossil floras, and paleosols, suggests a sudden cooling event at ∼33.5 Ma or a more gradual cooling over a longer period of time. This study seeks to understand how well climate change in central Colorado followed regional and global trends by examining three fossil floras in detail---Florissant (34.07 Ma), Pitch-Pinnacle (29-33.7 Ma), and Creede (26.3-26.9 Ma)---and briefly discussing a promising less-known flora, the Antero (33.8 Ma). I examine two primary questions: (1) How did genus-level richness and floral composition change during this time? and (2) Can the timing of the cooling be further constrained? To address these questions, mean annual temperature (MAT), mean annual range of temperature (MART), mean annual precipitation (MAP), and paleoelevation were estimated using leaf physiognomy techniques and estimated paleo lapse rates. Bray-Curtis indices were calculated to compare the taxonomic similarities between the floras. Floral composition was also assessed. During the 7.2-7.8 million year period covered by this study, MAT declined ∼5-8°C, MART increased ∼8-10°C, and MAP decreased ∼30-90 cm. Florissant was by far the richest flora. Pitch-Pinnacle had the greatest taxonomic affinity with Florissant. Based on these results, it appears that climate change in Colorado followed the expected pattern of cooling, drying, and increased seasonality, and that richness was correlated with MAT (as modern studies suggest). The Pitch-Pinnacle flora probably grew before or during cooling, suggesting that cooling was gradual. Further work should focus on increased sample sizes and inclusion of additional sites in Colorado, such as the Antero flora, and assessment and comparison of palynoflora.