SILURIAN CARBON ISOTOPE CHEMOSTRATIGRAPHY: NEW DATA FROM MID PALAEO-LATITUDES OF NORTHERN PERI-GONDWANA
On-going research of the stratigraphical interval from the uppermost Hirnantian to lower Telychian revealed a pronounced positive organic CIE at the level of the late Aeronian graptolite sedgwickii Bioevent. The latter bioevent is associated with high-frequency changes in oxygenation of sea water. Younger early Sheinwoodian CIE was recorded in an unnamed limestone unit within graptolite shales of lower part of the Motol Formation. Occurrence of shallow water limestone unit within deeper-water graptolite shales fits well with a strong shift into icehouse conditions during the early Sheinwoodian which was inferred from oxygen isotope data from the Eastern Baltic region. Sampling of upper part of the Motol Formation revealed the CIE in the both carbonate-carbon (Kozel Limestone Member) as well as organic carbon (graptolite shales) archives. New data show a distinct decrease in magnitude of the CIE with increasing water depth. Also, study of the mid-Ludfordian strata of the Kopanina Formation indicates a large positive CIE in both carbonate and organic carbon archives. The Silurian-Devonian (Klonk) CIE is at present, recorded only in carbonate carbon archive. All above-mentioned CIEs recorded in Perunica are predated by faunal crises and distinct lithological changes suggesting sea-level drop.