HIGH RESOLUTION CARBON AND OXYGEN ISOTOPES OF THE EARLY ORDOVICIAN-LATE SILURIAN OF THE BALTICA: IMPLICATIONS FOR PALAEOENVIRONMENTAL CHANGES AND PALAEOTEMPERATURE TRENDS

Oxygen isotope paleotemperature studies of the Paleozoic are based mainly on brachiopod shell material which is resistant to diagenesis and generally precipitated in oxygen isotopic equilibrium with ambient sea water. Here we present high-resolution brachiopod and bulk rock C and O stable isotopic data from the Ordovician-Silurian, for evaluating the palaeotemperature and palaeoenvironmental variability in the Estonian Shelf of the Baltoscandian Palaeobasin. As the region has not been influenced significantly by tectonic events or deep burial diagenesis, the studied carbonate rocks and fossils are well preserved in most of the locations. δ¹⁸O and δ¹³C values for the Ordovician and Silurian range between ca -7‰ to 0‰ and -1.5‰ to +7.6‰ respectively. High δ¹⁸O values, sometimes accompanied also with higher δ¹³C values, correspond to cooling if the isotope signal reflects the original oxygen isotopic composition in seawater, and vice versa. Several δ¹³C_brac excursions identified in the Ordovician-Silurian in the Estonian Shelf reflect global chemostratigraphy and palaeoenvironmental history, being synchronous with the previously documented excursions in the bulk carbonate stable isotopic curves. Combining the published and new δ¹³C_brac and δ¹⁸O_brac data allow us to address chemostratigraphy in the interval from Lower Ordovician (Floian) up to the topmost Silurian (P). The δ¹⁸O_brac data suggest warmer temperatures during early Ordovician (Floian-Dapingian) and a cooling trend into the Mid-Ordovician. The Hirnantian glaciation HICE episode reveals the minimum temperature in this interval and the post-HICE data suggest a rising temperature trend. Another temperature minimum is evident in the strata reflecting the Ireviken Event (Sheinwoodian). Our study shows that δ¹⁸O values from the brachiopod Ordovician-Silurian carbonates could tentatively be interpreted as reflecting the major temperature trends.