THE MIDDLE MIOCENE CLARKIA LACUSTRINE DEPOSIT REVISITED: A QUANTITATIVE PALEOENVIRONMENTAL ASSESSMENT

The classic middle Miocene lacustrine deposit at Clarkia in northern Idaho of the United States is world renowned for its exceptionally-preserved terrestrial fossil biota and diverse biomolecules and biomarkers recovered both in situ from various fossils and from associated sediments. The past three decades have witnessed the applications of many cutting edge methodologies to Clarkia materials to (1) demonstrate the feasibility of these new methods and (2) better understand its paleoenvironmental conditions. New developments of quantitative organic geochemical proxies have made it possible to independently test previous paleoenvironmental reconstructions based upon traditional paleontological and sedimentological evidence for conditions both within the lake basin and its surroundings. Here we summarize the results from these recent quantitative analyses, including: (1) paleohydrological dynamics based upon molecular hydrogen isotope analysis of in situ lipids, (2) paleo-CO₂ levels reconstructed using plant leaf stomatal frequency analysis, (3) the ratio of leaf intercellular and atmospheric partial pressures of CO2 (c_i/c_a) changes and plant physiological adaptations revealed by bulk and molecular carbon isotope analysis on individual plant fossils, (4) various climatic parameters such as atmospheric temperature, humidity, and potential evaporation calculated from Leaf Margin Analysis (LMA), Leaf-Area Analysis (LAA), Climate Leaf Analysis Multivariate Program (CLAMP), Digital Leaf Physiognomy (DLP), and Coexistence Approach (CA), and (5) lake water pH and water depth changes as well as their impacts on fossil preservation based upon new glycerol dialkyl glycerol tetraether (GDGT) analysis of sequential samples along the type P-33 location section. As recent chronological data have pinpointed the time of Clarkia deposit within the Middle Miocene Climatic Optimum, these updated quantitative reconstructions not only provide new insights into the extraordinary preservation of megafossils and biomolecules from this lacustrine deposit but also offer valuable data for better understanding of global atmospheric conditions during this critical global warming period during Cenozoic.