P???-dol?- carbon isotope trend and upper Silurian to lowermost Devonian chemostratigraphy based on sections in Podolia (Ukraine) and the East Baltic area

摘要

>Insufficient knowledge of carbon isotope cycling in the latest Silurian initiated the study of two regions at the western and southwestern margins of Baltica in order to obtain a more complete picture about the carbon isotope trend through the P???-dol?-. Shallow and open shelf carbonate rocks of the Dniester River outcrops and Kotuzhiny core in Podolia and deep shelf rocks of the East Baltic area, especially the Lithuanian cores, were studied for bulk-rock isotope analysis. The data sets of both regions begin >with the mid-Ludfordian excursion and include also some part of the lowermost Devonian. The data show a new minor twin positive ?′13C excursion (peak values 0.8-1.7a€°) in the upper Ludfordian. The P???-dol?- carbon isotope trend begins with a low of negative ?′13C values, succeeded by the lower to middle P???-dol?- a€?stabilitya€? interval (variable values below or close to 0a€° with a slight rising trend). The upper P???-dol?- begins with a medium to major excursion (peak values 2.3-4.5a€°), which reflects the pattern of the carbon isotope trend on the west of the Baltica palaeocontinent. Its wider significance awaits confirmation from observations elsewhere. The carbon isotope excursion at the Silurian-Devonian boundary, named here the SIDE excursion (its ?′13C values range from 1.6a€° in deep shelf settings to 3.8a€° in shallower ones and 4.5a€° in brachiopod shells), has been traced on several continents, and now also in Baltica. This excursion can serve as a well-dated global chemostratigraphic correlation tool. The shape of the excursion indicates the completeness of the studied section. We conclude that carbon isotope chemostratigraphy may contribute to subdividing the P???-dol?- Series into stages and that Baltica sensu lato seems to be the right place for such a development.