Baltoscandian Ordovician and Silurian brachiopod carbon and oxygen stable isotope trends: Implications for palaeoenvironmental and palaeotemperature changes

Abstract

Oxygen isotope paleotemperature studies of the Palaeozoic carbonates are based mainly on brachiopod shell material which is resistant to diagenesis and generally precipitated in oxygen isotopic equilibrium with ambient seawater. Here we present brachiopod and bulk rock C and O stable isotopic data from the Baltoscandian Ordovician-Silurian succession, for evaluating the palaeotemperature and palaeoenvironmental variability in the Estonian Shelf facies. 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 carbonates and brachiopods range between ca -7‰ to 0‰ and -2.6‰ to +7.3‰ 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 Ordovician-Silurian δ13Cbrac excursions identified in the Estonian Shelf reflect global palaeoenvironmental history and events, being synchronous with the previously documented excursions in the bulk carbonate stable isotopic curves. Combining the published and new δ13Cbrac and δ18Obrac data allow us to address chemostratigraphic correlation of the interval from Lower Ordovician (Floian) up to the topmost Silurian (Přidoli). The δ18Obrac data confirm warmer temperatures during early Ordovician (Floian-Dapingian) and a cooling trend into the Mid-Ordovician documented by previous studies in different palaeobasins. 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.