PALEOENVIRONMENTAL AND PALEOCLIMATIC RECONSTRUCTION OF THE MIDDLE MIOCENE CLARKIA FOSSIL SITE: AN UPDATE

Miocene is one of the analogs in geological time to the current global climate change, and thus has drawn considerable attention to the reconstruction of its environments and climate. The middle Miocene fossil deposit at Clarkia in northern Idaho of the United States is one of the best preserved Miocene deposits in the world and has yielded large amounts of exceptionally preserved fossils of plants, fishes, mollusks, diatoms, insects, fungi, dinoflagellate cysts, sponges, etc. A suite of diverse biomolecules and their isotope signatures has also been reported from plant fossils of this site. However, paleoenvironment and paleoclimate reconstruction of the site was preliminarily conducted by analyzing the geological setting and the composition of biotas, basically qualitatively. The past decades has witnessed the development of many new methodologies to infer paleoenvironment and paleoclimate, particularly in quantitative ways. An update of our understanding of all aspects of the environment and climate for this world renowned fossil site is needed. To achieve this goal we applied several technologies, including (1) stable hydrogen isotope analysis (to understand paleohydrological aspects), (2) plant leaf stomatal frequency analysis (to reveal paleo-CO₂ level), and particularly (3) quantitative paleobotanical methods using plant fossils. The latter methods include Leaf Margin Analysis (LMA), Leaf-Area Analysis (LAA), Climate Leaf Analysis Multivariate Program (CLAMP) analysis, Digital Leaf Physiognomy (DLP), and Coexistence Approach (CA), etc. They are widely applied for the reconstruction of various climatic parameters such as atmospheric temperature, humidity, and potential evaporation. The combination of various technologies help us obtain an updated and more precise understanding of the topography, vegetation, seasonality, and specific climatic parameters of this key Miocene fossil site.