Ordovician oxygen isotopes and paleotemperatures

Valid marine δ18O proxies have been defined from three Ordovician units and with previously estimated sea water δ18O values we have calculated paleotemperatures for low latitude settings during a greenhouse climatic era. Valid marine proxies include: marine equant, bladed and translucent fibrous calcite. These cements are interpreted to be valid marine proxies based on cathodoluminescence, minor element content, abundance of microdolomite and stable isotopic (both altered marine and non-marine calcite phases) values. Valid marine proxies from the Chickamauga (Alabama), Holston (Tennessee) and Kullsberg (Sweden) formations have an overall range of δ18O values in each unit that is confined to within 1‰ unlike altered marine cements, which exhibit significantly wider ranges of δ18O values. We suggest a likely non-glacial Ordovician sea water value of −2±1‰ SMOW from which paleotemperatures can be calculated. Calculated paleotemperatures are broadly consistent with Crowley and Baumʹs (1995) Late Ordovician paleoclimatic model.