Stable Isotopes and environments in the East Alpine Miocene

Research project P 14366 Stable Isotopes and environments in the East Alpine Miocene Werner PILLER 09.10.2000 Pronounced changes in fossil communities - bio-events - occur during the Miocene in the East Alpine region. These changes are caused by different reasons. One of these are changing environmental parameters. These parameters can be inferred only secondarily for fossil material. The application of stable isotope analyses seems to be an appropriate way to gather proxy data for reconstructing so-me of these parameters. To get information on salinity, temperature, water mass distributions, concentration of organic material and productivity, oxygen (180/160) and carbon (13C/12C) isotope composition will be studied. The stable isotope analyses will be applied primarily to foraminiferal tests and, additionally, to echinoids. For forarninifera stable isotopy is well established. Forarninifera include planktic and benthic forms offering proxy data for conditions in the water column and for bottom water. For echinoids reliability of stable isotope data is poorly known, however, within this project the acquired data can be evaluated using foraminiferan data. To get a reliable data set for that part of the Miocene planned to be studied (approx. 9 million years), 8 time slices have been choosen: Early and Late Eggenburgian, Ottnangian, Karpatian, Early, Middle and Late Badenian, and Middle Sarmatian. In these time slices shallow marine and deeper water facies will be studied. The establishment of a widely continuous oxygen isotope curve for the studied Eastern Alpine Miocene is one of the main aims of the project. Within the proposed project cluster "Changes in Eastern Alpine Miocene Ecosystems and their Geodynamic Control" these stable isotope data can be compared with those environmental parameters inferred from palaeoecological studies of different fossil groups (foraminifera, ostracods, molluscs, bryozoans, echinoids, leafes, fruits and seeds, palynomorphs). These results can be compared with data on the geodynamic development of the East Alpine area resulting from palaeornagnetic studies and from simulations on synsedimentary tectonics. This will allow an estimation of the importance of these parameters on the palaeoenvironmental changes.

The Miocene epoch - c. 23 - 5 million years before present (My) - is characterized by pronounced global climatic changes and related environmental perturbations. The East Alpine area is, at the same time and in addition, affected by strong tectonic processes (e. g., uplift of the Eastern Alps, origin of basins along the East Alpine margin). The Alpine Foreland, the Vienna and Styrian Basins, both belonging to the Pannonian Basins System, were part of a large sea, the Central Paratethys. Around 11.5 Ma, this vast sea became a large lake - Lake Pannon - and dried out afterwards. The rocks and fossils of the Central Paratethys represent a huge archive for the changing environment in the Miocene. Environmental parameters of the past, however, cannot me measured directly. Their reconstruction is possible by comparing ecological demands of fossils with their living close relatives or by studying the chemical- mineralogical composition of shells (e.g., gastropods and bivalves). The stable Oxygen- and Carbon-Isotope composition of shells allows to reconstruct e.g. temperature and salinity of the water in which these creatures have lived or even their food requirements. Applying various methods and studying different organism groups, for the older time slice investigated (Eggenburgian; 20.5 - 18 Ma) a warm-temperate climate could be reconstructed. The next period reflects a slight cooling (Ottnangian; 18-17 Ma) followed by a climatic optimum with tropical/subtropical conditions (Karpatian - Badenian; 17 - 13 Ma). During the latter period coral reefs flourished in the Styrian and Vienna Basins reflecting a highly diverse tropical/subtropical ecosystem. The following period (Sarmatian; 13 - 11.5 Ma) was interpreted to reflect a continuous reduction in salinity in the Central Paratethys leading to the freshwater conditions of Lake Pannon. Our new results, however, clearly disprove this opinion: the Central Paratethys was a marine environment during the Sarmatian, in some parts even hypersaline conditions prevailed. Not until the Pannonian time marine conditions vanished and the sea switched to a freshwater system. Orbital parameters (precession, obliquity, eccentricity) can be held responsible as climate controlling factors also in the East Alpine region and can be reconstructed by the sedimentary rocks. Particularly for the Sarmatian and Pannonian a clear influence of these parameters can be shown. Their changes, however, did not only help for climate reconstruction but were also employed for the first time to refine the chronology of the study area.