9510078 Marshall Stable carbon isotope ratios and stomatal characters of Miocene leaves will be compared to those of modern leaves from nearest living relatives (NLR) Herbarium specimens of the NLR's will be analyzed to calibrate isotopic composition and stomatal characters against the 30% increase in atmospheric carbon dioxide concentration and the 1.2% decrease in isotopic composition since the industrial revolution. Using the calibrations, we will obtain from the fossil data ten to twenty independent, taxon-specific estimates of the isotopic composition and concentration of carbon dioxide in the Miocene atmosphere. The fossil leaves are of subtropical and temperate trees deposited ca. 15 million years ago in anoxic lake sediments near clarkia, Idaho. The dominant trees in the flora included species of Quercus, Castanea, Pseudofagus, Metasequoia , and Taxodium. These remarkably preserved fossils have previously yielded DNA, taxon-specific flavonoid profiles, and are sometimes still pigmented when first removed from the sediments. They are, to our knowledge, the best preserved specimens of this age in the world. Nonetheless to minimize the influence of diagenesis on isotopic composition all comparisons will rely on lignin extracted from the modern leaves and the fossils. Stomatal density (# stomata per m2 of leaf surface) and stomatal size will be measured on replicate samples of all leaves; stomatal index (#stomata/(#stomata + epidermal cells)) will be measured where possible. New collections of the nearest living relatives will also be made to quantify the effects of canopy position and altitude above sea level. The middle Miocene was a time of rapidly falling temperatures and CO2 concentrations. Published estimates of atmospheric composition rely on isotopic data from ocean sediments, which are uncertain because they are several steps removed from the atmosphere. We propose an alternative means of reconstructing atmospheric composition based on land plants, which are better coupled to the atmosphere. Because individual genera can be analyzed separately, the work will provide an independent estimate of atmospheric composition from each genus. If successful, this work may eventually lead to analyses of Miocene terrestrial photosynthesis, one of the major biogeochemical processes removing CO2 from the atmosphere, and may constrain our estimates of terrestrial ecosystem responses to the ongoing increase in atmospheric CO2.
