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

Abstract

Oxygen isotope palaeotemperature studies of Paleozoic limestones 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 brachiopod C and O stable isotopic data from the Baltoscandian Ordovician-Silurian succession, and evaluate palaeotemperature and palaeoenvironmental variability during deposition of the Estonian Shelf facies. As the region has not been influenced significantly by tectonic events or deep burial diagenesis, the carbonate rocks and fossils are well-preserved in most of the locations studied. δ¹⁸O values for the Ordovician and Silurian carbonates and brachiopods range between ~–7 and 0‰. High δ¹⁸O values, locally accompanied by higher δ¹³O values, correspond to cooling if the isotope signal reflects the original oxygen isotopic composition in sea water and vice versa. Several Ordovician-Silurian δ¹³O_brac excursions identified on the Estonian Shelf reflect global palaeoenvironmental history and events, being synchronous with previously documented excursions in the bulk carbonate stable isotopic curves. Combining the published and new d13Cbrac and δ^13O_brac data allows us to address chemostratigraphic correlation of the interval from Lower Ordovician (Floian) up to the topmost Silurian (Přídolí). The δ^18O_brac data corroborate warmer temperatures during Early Ordovician (Floian-Dapingian) and a cooling trend into the Mid-Ordovician documented by previous studies in different palaeobasins. The Hirnatian isotopic carbon excursion (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 brachiopod d18O values from the Ordovician-Silurian carbonates may tentatively be interpreted as reflecting major temperature trends