300 MILLION YEARS OF PEDOGENIC CARBONATES FROM WESTERN NORTH AMERICA: EVALUATING OXYGEN ISOTOPE RATIOS AS PROXIES FOR PALEOCLIMATE RECONSTRUCTIONS

Accumulations of terrestrial sediments and associated fossil soils dating back to the Late Paleozoic outcrop throughout much of western North America. Oxygen isotope ratios of fossil soil carbonates from these sediments demonstrate significant variability. Fossil soils from Montana, Wyoming, Utah, Colorado, Kansas, Arizona, New Mexico, Oklahoma and Texas are included in this study. While many new samples are presented, carbon isotope ratios from many of these samples have previously been published as proxies for paleoatmospheric carbon dioxide levels. The utility of these results to constrain paleoclimate reconstructions is evaluated with regard to consistency among related samples, temporal trends, tectonic activity in the region over this interval, paleogeographic considerations, maximum burial depths, secular trends and XRD mineralogy. Carbon and oxygen stable isotope ratios of associated cross cutting diagenetic veins are compared to the host materials to assess alteration after burial. Late Paleozoic and Late Jurassic time frames have both higher sampling densities and larger geographic ranges allowing more detailed examinations. The minimum and maximum range of ratios in this dataset differ by approximately 20 permil. Relative to oxygen 18, the most enriched values observed are from the Permian and the most depleted values are from the Late Cretaceous and Early Paleogene.