Perturbations of marine environments in the north-alpine Rhaetian

At the end of the Triassic one of the "big five" global biotic crises of the Phanerozoic occurred, which caused the extinction of about 50% of marine genera. This biotic event is still poorly understood, particularly with respect to its causes, duration and the degree of extinction among different biota. One of the unresolved problems is the question whether the end-Triassic crisis was a short and catastrophic event or whether it was the result of a long- term drop in biodiversity. Geologic and geochemical results suggest that the crisis occurred within a relatively short time interval in the uppermost Rhaetian and was caused by a rapid sea level fall and extensive volcanism in the "Central Atlantic Magmatic Province", which initiated severe environmental deterioration, particularly rapid and extreme climatic variations due to massive volcanic outgassing and changes in sea water chemistry. The stratigraphic record of several fossil groups however indicates a long-term drop in biodiversity in the Upper Triassic. Geochemical, palaeobotanic and palynologic results and recent micropalaeontologic and carbon isotope data from the Kössen Formation in the Northern Calcareous Alps are strongly suggestive of climatic change due to an increasing concentration of atmospheric CO 2 in the middle-late Rhaetian which could have affected the biodiversity. The present project is intended to detect palaeoenvironmental perturbations in marine palaeoenvironments which may have contributed to the end-Triassic biotic crisis. For this purpose Rhaetian successions of the intraplatform facies (Kössen Formation) and oceanic basin facies (Zlambach Formation) will be studied in detail by various micropalaeontologic, sedimentologic, geochemical and geophysical methods. The geochemical studies will be particularly important for the detection of volcanism and oceanic anoxia in the Rhaetian, and the stratigraphic correlation of Rhaetian shallow shelf and terrestrial deposits.

Previous research has shown that the diversity of marine organisms shows a strong decrease in the Late Triassic interval due to a number of extinction events. These extinction events were caused by different environmental catastrophes such as extensive volcanism and rapid climatic changes. The most dramatic and severe global extinction was identified at the end of the Triassic, close to the Triassic/Jurassic boundary. The present project was intended to identify changes of environmental conditions and marine communities in the Late Triassic (Rhaetian) at high-resolution and to reconstruct the end- Triassic environmental perturbations in great detail. The upper Triassic sedimentary rocks of the Northern Calcareous Alps (NCA) are one of the most important archives for marine environments and their fossil communities in the Late Triassic. For this reason the research was concentrated on marine sedimentary deposits of the latest Triassic interval (Rhaetian) in the NCA with regard to their depositional environments, chemistry (trace elements, stable isotopes) and microfossils. Changes in relative abundancies of stable carbon isotopes are indicative of significant perturbations of the carbon cycle and may therefore yield information about large-scale environmental changes and mass extinctions. One of the most important results is that a significant environmental change occurred long before the end-Triassic mass extinction the extinction of the Dachstein reef communities and stop of reef growth. Other data also show that reef debris was transported from the Dachstein reef to the deeper shelf in a cyclic pattern which is indicative of cyclic sea level changes. Another very important result is based on a detailed geochemical analysis of the Triassis/Jurassic boundary interval. The data clearly show that a number of volcanic events and a long-lasting period of oceanic acidification occurred after the end-Triassic event. This is in accordance with previous research results which suggest that extensive volcanism, rapid climatic change and oceanic acidification were the most important triggers of the end- Triassic mass extinction.