Oxygen isotopes of Pennsylvanian conodonts: surface water salinity in the US Midcontinent Sea
Final Report Abstract
Lower salinities in the Appalachian Basin and a significant salinity gradient in the Midcontinent sea were reconstructed for the Desmoinesian supporting the idea that freshwater discharge may have resulted in a salinity-stratified water column, at least in the eastern part of the epeiric sea. The salinity gradient is considerably reduced during the Missourian and Virgilian due to generally dryer climatic conditions in the late Pennsylvanian. In contrast to the Appalachian Basin (lower δ18O values in the Desmoinesian, higher δ18O values in the Missourian and Virgilian), no change in δ18O and inferred salinity is registered in the Midcontinent sea (Desmoinesian to Virgilian) suggesting that surface waters in the Midcontinent sea were not significantly influenced by freshwaters. The minor salinity contrast between the Appalachian and Midcontinent basins during the Missourian to Virgilian questions whether water column stratification was maintained by a halocline. Although difficult to estimate, we tentatively interpret the oxygen isotope ratios of conodonts from the Midcontinent Basin to represent near-normal marine salinities. Taking modern δ18O-salinity relationships, salinity would have been only slightly reduced. However, in case salinities were even only slightly lower in the Midcontinent Basin during the interglacial intervals, estimated glacioeustatic sea level changes of 120 m published by Joachimski et al. (2006) would have been overestimated.
