δ13C chemostratigraphy of the Middle and Upper Ordovician succession in the Tartu-453 drillcore, southern Estonia, and the significance of the HICE

The δ13C isotope data from the Tartu-453 core section in southern Estonia enabled creation of a continuous Ordovician carbon isotope record, ranging from the Floian to the end of the Hirnantian. Five positive δ13C excursions and one negative δ13C excursion were recognized. Most of the positive excursions correspond to those defined on the Baltoscandian carbon isotope chemostratigraphic scheme, except for the Paroveja and Rakvere ICEs which are not well expressed in the Tartu-453 curve. Besides the positive δ13C excursions, a distinctive negative δ13C excursion, the Lower Sandbian NICE, was recorded in the Tartu-453 succession. The well-expressed HICE extends about 10 m higher than the traditional Ordovician-Silurian boundary (between the Porkuni and Juuru regional stages) into the Juuru Regional Stage, suggesting that the Ordovician-Silurian boundary is positioned significally higher than traditionally suggested for the East Baltic area.