Abstract
On the basis of bulk carbonate carbon isotope data from six drill cores and the type section of the Porkuni Stage of Estonia, δ 13C curves were compiled, which altogether cover nearly the whole succession of the latest Ordovician (Hirnantian) —well known as a time of short-lived glaciation and mass extinction. In pre-Porkuni and Silurian rocks, δ 13C values were low (from −0.2‰ to 1.7‰). For the Porkuni Stage three intervals of the curve were established: (1) lower — a positive shift up to 6.7‰, (2) middle part with variable values from 3.3‰ to 5.4‰, and (3) upper phase with slightly decreased values from 3.2‰ to 3.6‰. The correlation with the Dob's Linn (Scotland) kerogen δ 13C curve shows that the first two intervals coincide with the Normalograptus extraordinarius graptolite Biozone, the third one with the N. persculptus Biozone.
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