Pollen evidence of floristic turnover forced by cool aridity during the Oligocene in Colorado

Abstract

New pollen data from four Oligocene floras in volcanic landscapes of Colo\xad rado record important climatic shifts that reshaped the local flora and promoted the development of sub\xadarid vegetation types. We combined new pollen data with previous megafossil evidence to assess vegetation changes during the Eocene–Oligocene Transition (EOT). Pollen data are the basis for updating the list of flora identified at Creede. Local extinctions in response to lower sum\xad mer rainfall abruptly removed many of the exotic woody taxa of eastern North American and Asian affinity. This loss was followed by the appearance of xeric shrubland taxa of the Ponderosa pine\xadfir woodland and sagebrush flora that characterize the Colorado area today. Tell\xadtale genera appear and suggest an understory of shrubs such as Artemisia, Elaeagnus, Ribes, Ephedra, Jamesia, and Shepherdia. Poaceae are also present. Significantly, herbaceous taxa of the Asteraceae, Rosaceae, Cucurbitaceae, Euphorbiaceae, and Caryophyllaceae make their first appearances in the fossil record of Colorado here. The new Oligocene pollen data record a significant drop in summer rainfall and a climatic cooling at 33.8 Ma of several degrees that relates to the Oi\xad1 glaciation in Antarctica. The particular taxa that continued after the EOT were a basis for estimating changes in soil moisture during this time. The condi\xad tions in Colorado are reminiscent of Wolfe’s “terminal Eocene event.” This re\xad markable shift precipitated the development of a local pollen and megafossil flora more “modern” in aspect (e.g., a larger proportion of extant local genera are present). The impressive floristic turnover was probably a response to the increasingly continental climate that embraced the area during the Oligocene.