Evidence for a widespread carbon isotopic event associated with late Middle Ordovician sedimentological and faunal changes in Estonia

An episode of remarkable biotic, climatic, sea-level and facies changes too k place during the late Viruan (Caradoc) epoch in the Baltoscandian area. W e studied the carbon isotopic composition of carbonate sediments from this period. Data on the stable carbon isotopic composition of whole-rock carbon ates from three south Estonian core sections, together with those on ostrac ode, distribution are presented. In two core sections, a positive delta(13) C shift of 2% was revealed in the upper part of the Keila Stage (mid-Carado c). The comparison of isotope and ostracode data in the sections suggested the occurrence of a gap of late Keilan age in the marginal area of the basi n equivalent to the North Estonian Confacies Belt. The beginning of the exc ursion approximately coincided with climatic and sea-level changes in the s hallow shelf area. The peak of the late Keilan excursion preceded the bioti c crises and maximum black shale accumulation in the Baltoscandian palaeoba sin. An approximately synchronous carbon isotopic event has been reported f rom North America, referring to a possible global oceanographic event in th e Caradoc epoch. Although the positive carbon isotopic excursion and relate d environmental events of the late Keilan age have some unique features, th ey show more similarities to the end-Ordovician and Silurian events, charac terized by oceanic change from stratified state to thermohaline circulation state, than to warm anoxic events related to eustatic sea-level rise.