δ13C and δ18O values of Triassic brachiopods and carbonate rocks as proxies for coeval seawater and palaeotemperature
Abstract A dataset of 160 isotope δ13C and δ18O values from Anisian, Ladinian, Carnian and Rhaetian articulate brachiopod shells, complemented by 158 carbon and oxygen isotope values from whole rock carbonates, define the first continuous stable isotope baseline trends for the Triassic seawater. The carbon isotope data suggest the existence of several short-term high amplitude excursions in the Early Triassic, followed by a predominance of values around 0.5 ± 1‰ during the Middle Triassic, a rise to ∼ 3.5‰ during the Carnian, plateau at this level during the Late Carnian to Early Norian, and a 1.5‰ decline in the Middle Norian to values around 2‰ during the Late Norian–Rhaetian interval. The causation scenarios for these rapid oscillations are at present equivocal, but may in part reflect a biological instability of the carbon cycle following the recovery from the end-Permian extinction event and/or an input of “mantle”-derived CO2 from enhanced volcanic activity. The δ18O values from well-preserved brachiopods from the Tethyan realm range from − 3.9 to − 0.6‰ V-PDB. These values require open marine Triassic seawater δ18O values close to 0‰ V-SMOW for the calculated temperatures to be within the range of tolerance of the coexisting reef-building corals. A 2‰ δ18O increase within the uppermost Cordevolian and early Julian suggests either a distinct temperature decline in the Southern Alps during that time interval, an increase in seawater salinity, or their combination. Oxygen isotope values for the Muschelkalk brachiopods range between − 6.2 and − 2.0‰ and likely reflect a strong influx of meteoric waters into the Germanic Basin that shifted the oxygen isotopes to more negative values.