CARBON ISOTOPIC COMPOSITION OF LLANDOVERY ROCKS (EAST BALTIC SILURIAN) WITH ENVIRONMENTAL INTERPRETATION

Abstract

. Carbon isotope data fromsix borehole sections ofEstonia and West Latvia are presented and analysed against different geological and palaeontological phenomena in order to understand possible reasons for shifts in B'°C values. Four most important carbon isotopic events are established -in the Llandovery (from bottom): (1) a negative shift (-0.5 to —1.5%0) in the Coronograptus cyphus Zone (in the Pusku Beds), (2) a positive excursion (+3.0 to +4.0%0) in the Demirastrites triangulatus Zone (Ikla Member), (3) a negative excursion (—1.3 to —1.4%0) in the Stimulograptus sedgwickii Zone (Rumba Formation), (4) a small positive shift at the beginning of the Velise Formation in the Spirograptus guerichi-Streptograptus crispus Zone. The positive excursions seem to correspond to certain glacial events, negative ones to humid climatic episodes with rising sea level. In the case of the Rumba Formation the influence of carbon with low 5С released due to volcanic activity in the late Aeronian is suggested. The background carbon isotope values are relatively stable during the Llandovery, showing some changes in correlation with facies and climate controlled stratigraphical units.