9315651 Walker The isotopic compositions of early diagenetic carbonate cements (marine and meteoric) will provide evidence to determine Ordovician paleoceanography and paleoclimate and the isotopic variation associated with pre-land plant, carbonate meteoric systems. Careful petrographic, cathodolumenescence, and trace element studies will allow delineation of the least altered marine cement phases. Least altered marine cements from two tropical, platform- marginal buildups located in paleogeographically divergent localities from the Llanvirn epoch will be compared with data from later epochs in the Ordovician to determine whether the variation in the 818O composition of Ordovician fibrous calcite reflects either local variations in sea water temperature or secular variation in the 818O composition of sea water. The goal of this part of the research is to determine the 818O composition of meteoric cement from temperate to polar paleolatitudes, in two paleogeographic divergent regions, which will be used to assess Ordovician climatic equability. Do periods of elevated pCO2, such as the Ordovician, correspond with periods of climatic equability as previously hypothesized? Early meteoric cements from both tropical and temperate platform carbonate deposits (four regions) will provide information with regard to the variation of stable isotopic composition (813C and 818O) associated with exposure surfaces on a landscape lacking vascular plants. The ultimate goal of this part of the research is to fully develop a model for the stable isotopic composition of pre-vascular plant meteoric carbonate from the Ordovician that can be broadly applied to early Paleozoic and older rocks.
