Record of the end-Triassic mass extinction in shallow marine carbonates: the Lorüns section (Austria)

Sammanfattning: The end-Triassic mass extinction (ETE) was one of the five big mass extinctions of the Phanerozoic and occurred ~201 Ma ago. Research agrees that the main triggering factor for the ETE was the eruptions of the Central Atlantic Magmatic Volcanic Province (CAMP), linked to the break-up of the supercontinent Pangaea. The large amounts of CO2 and SO2 released from the volcanic eruptions had a series of consequences such as climatic changes and ocean acidification, all possibly contributing to an extinction of a large number of taxa in the marine and terrestrial realm. The Lorüns section constitutes a shallow-marine carbonate succession located in the Northern Calcareous Alps (NCA), Austria, and is a key section for studying the ETE because it records continuous sedimentation from the late Rhaetian to the Sinemurian. In this study, the Rhaetian Kössen Formation, the Schattwald Beds, and the Hettangian Lorüns oolite have been petrographically analysed. Geochemical analyses with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were also performed, mainly on the recovery facies from the Lorüns oolite. This study aims to offer a detailed description of the geological, geochemical and palaeoenvironmental conditions at Lorüns in the aftermath of the ETE and to compare the results with other Triassic-Jurassic boundary sections around the world. The lack of calcareous fossils and the deposition of the siliciclastic Schattwald Beds during the “Extinction interval” together with the formation of coated grains in the post-extinction Lorüns oolite indicate that ocean acidification occurred in this area, especially affecting acid-sensitive organisms. Seawater was oxygenated during the early Hettangian. However, dysoxia occurred in the sediments during this period and is ascribed to the loss of biogenic sediment mixing due to the extinction of infaunal taxa. Analysis of the weathering proxies Si, Al, and Ti shows that weathering decreased continuously during the Hettangian. This could be explained by the marine transgression in the Lorüns area, a greater distance to the siliciclastic input zone, or because of a shift towards a less humid and hot climate after the extreme greenhouse conditions caused by volcanism around the ETE. Contrarily to the models from Hardie (1996) and Stanley & Hardie (1998), who propose an “aragonite” sea in the Late Triassic and Early Jurassic, the sea was found to be low-Mg calcitic at the time of deposition of the Schattwald Beds and Lorüns oolite. Triassic-Jurassic boundary sections in the Arab Emirates also imply a low-Mg calcitic sea, suggesting that the signal recorded at Lorüns was global, or at least Tethys-wide. Hence, the shift from an “aragonite” to “calcite” sea might have occurred earlier than previously believed.