The Ohesaare borehole, in the northern part of the Silurian Baltic Gulf, penetrates shallow and deep shelf marlstones, limestones and dolostones. The Priekule borehole in the deep shelf belt of the basin was drilled through marlstones and graptolitic mudstones. Most of the successions (except the upper part) are characterized by a high Sr/Ca ratio, close to marine calcites, suggesting good preservation of carbonate material. Conodont alteration indices are low (1–2). Carbon isotopes were measured using whole-rock samples. In the Ohesaare core section two distinctive δ13C positive excursions were established — one in the early Wenlock riccartonensis and low rigidus-ellesae standard graptolite zones (+4.2‰) and the second in the late Wenlock nassa Zone (+4.6‰). Before and after the latter there are also intervals with low δ13C values, the first just above the lundgreni Zone (−1.8‰) and another in the ludensis (perhaps reaching into nilssoni) Zone (−3.8‰). In the Priekule section the early Wenlock δ13C peak is well represented (+3.1‰), but in the late Wenlock–early Ludlow δ13C values show little variation (from 2.0 to −1‰). In this section a high peak of δ13C values (+5.9‰) was determined in the Nova Beds (correlated with the topmost Gorstian and lowermost Ludfordian, Ludlow). The carbon isotope positive excursions show good correlation with lows of the global sea level curve and with most of the biotic and supposed climatic events (glaciation, ocean state).