SILURIAN CARBON ISOTOPE CHEMOSTRATIGRAPHY: NEW DATA FROM MID PALAEO-LATITUDES OF NORTHERN PERI-GONDWANA

During the last three decades several pronounced carbon isotope excursions (CIEs) have been documented from Silurian tropical regions, including Baltica and Laurentia, but only a few studies document the CIEs in non-tropical settings. On-going chemostratigraphic research of Silurian strata of the Perunica microplate (mid palaeo-latitudes of northern peri–Gondwana) have provided evidence on all major Silurian CIEs previously recorded only in tropical areas. Here we present a review of all CIEs hitherto known from the Perunica microplate (Prague synform), together with new data on the late Aeronian, early Sheinwoodian, late Homerian, and mid-Ludfordian positive CIEs.

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.