ACTA UNlVERSlTATlS CAROLINAE - GEOLOGICA 1999,43 (112) "r- The mid-Caradocian biotic and isotopic event in the Ordovician of the East Baltic Tbnu MEIDLA I), Leho AINSAAR~', Linda HINTS~', Olle HINTS~', Tbnu MART MA^), Jaak NOLVAK~' " Institute of Geology, Universitj of Tartu, Vanemuise 46, 51014 Tartu, Estonia; e-mail: tmeidla@malh.ut.ee 2' Institute of Geology at Tallinn Technical Universitj, Eslonia Blvd. 7, 10143 Tallinn, Estonia; e-mail hintz@gi.ee INTRODUCTION In recent publications several aspects of the Ordovician biodiversity have beed discussed. This activity has been stimulated by increasing understanding of the general dynamics of biota during the Phanerozoic (Sepkoski 1997) and the importance of the Ordovician period in particular. Highly provincial and rapidly radiating Ordovician faunas had a key position in the formation of Palaeozoic marine faunas (sensu Sepkoski). In order to understand better the formation mechanisms of the regional faunal successions and their relationship to the global changes in biodiversity, the locaVregiona1 bioevents should be analyzed in the context of global change. An important tool for this purpose is the stable isotope record, which may shed light on the regional or possibly global nature of a particular bioevent in the local record. Our study deals with aspects of the mid-Caradocian event in the Baltoscandian Palaeobasin. The data on carbon stable isotopic composition allows us to present an enhanced correlation of sections along an onshore-offshore profile. Considering the data from North America, we suggest a possibly global reason of the rearrangement of the mid-Caradocian ecosystem. Fig. 1. Correlation of the midxaradocian sequence in Estonia along the facies gradient.