A bio- and chemostratigraphic search for the Mid-Ludfordian Carbon Isotope Excursion interval in the Ludlow of the Ohesaare core, Estonia

For years, the Ohesaare core section and its rich fossil assemblages have enticed researchers to suggest various ideas about Silurian stratigraphy in the East Baltic despite several sedimentary gaps occurring through the Ludlow interval in particular. One of the gaps has removed from the Ohesaare record the most important event in the Palaeozoic history of carbon isotopes – the MidLudfordian Carbon Isotope Excursion (MLCIE), which is partly accompanied by the Lau biotic and oceanic events. In our research, we have performed a detailed distribution analysis of chitinozoans, conodonts, ostracodes and vertebrate microremains with the aim of documenting the evidence regarding the levels of certain environmental events largely preceding the MLCIE. For this purpose, the Torgu Formation was subdivided into five working units. The first results indicate a gap within the uppermost part of the Torgu Formation as a possible level for the missing MLCIE. In order to verify this, we compared the fossil distribution pattern in Ohesaare with that on Gotland (Sweden) and in Kurzeme (Latvia), including δ13C data from the Uddvide (Gotland) and Ventspils (Latvia) core sections. Both Baltic and Bohemian data show rather unanimously that the MLCIE is located below the Ozarkodina crispa conodont Biozone. However, some reports from Gotland show peak values also in higher strata; occasional records of Oz. crispa from other places may cast some doubt on these findings and raise some ecostratigraphical concerns when discussing upper Ludfordian correlation.