Three global carbon isotope shifts in the Silurian of Podolia (Ukraine): stratigraphical implications

Podolia is a classical Silurian area in the southwestern part of the Ukraine. Shallow to open shelf rocks cropping out there are usually, except for a few dolomitic horizons, highly fossiliferous and therefore the biostratigraphy of the region has a long successful history. The Ukrainian meeting of the Subcommission on Silurian Stratigraphy of the International Union of Geological Sciences, held in 1983, became an important milestone. Tsegelnyuk et al. summarized different results and views in a field guide published for the meeting (The Silurian of Podolia. The Guide to Excursion. Naukova Dumka, Kiev, 1983) and introduced a practically new stratigraphical terminology, and in some parts of the section, also new age interpretation of the beds. For our paper carbon isotopes were studied in eight sections on the banks of the Dniester River and its tributaries, covering ca 80% of the Silurian succession (Pridoli excluded). Three positive delta C-13 excursions were identified. The first excursion in the Kitaigorod 30 section reaches its peak value of 4.0 parts per thousand in the very bottom of the Demshin Subformation. The second shift reaches the peak value of 4.3 parts per thousand in the middle of the Muksha Subformation. The third shift is the most prominent one identified in the Silurian of Podolia - delta C-13 values increase steadily through the Isakovtsy Subformation, reaching 6.9 parts per thousand in the top, and a slightly lower value of 6.6 parts per thousand occurs in the bottom of the Prigorodok Formation. In the middle Silurian of the World three major positive excursions have been identified: in the early and latest Wenlock and in the late Ludlow. The general character of the carbon isotope trend and stratigraphical positions of the excursions established in this paper demonstrate that in Podolia there occurs the same set of global shifts, which can be used for the improvement of regional and global correlation of Wenlock and Ludlow sections of that area. Some refinements are suggested, but some details need additional study.