Isotopic composition (O, C, Sr, and Nd) and trace element ratios (Sr / Ca, Mg / Ca) of Miocene marine and brackish ostracods from North Alpine Foreland deposits (Germany and Austria) as indicators for palaeoclimate

The Molasse basin, constituting the western part of the Paratethys, formed as a consequence of the Alpine orogeny during the Tertiary. To address the influence of the Alpine upliftment and associated geographic changes on the regional climate, the faunal assemblages and geochemistry of Miocene marine and brackish ostracods of the Molasse sediments were investigated. Eighteen sites from the Bavarian Molasse to the Vienna basin were studied, ranging in age from the Eggenburgian to the Pannonian (20 to 8.5 Ma). A total of 176 species of 71 genera were found, with the highest species diversity observed for Loxoconcha (12), Aurila (11), Cyprideis (9), and Cytheridea (7). On the basis of the different genera, the bathymetry of sedimentation can be characterized as infra- to circalittoral. From the Eggenburgian to the Early Sarmatian (20 to 13 Ma) open marine conditions are indicated by the faunal assemblages. For the Late Sarmatian and Pannonian (12 to 8.5 Ma), the occurrence of Cyprideis spp. and candonid species indicate brackish conditions. Chemical and isotopic analyses concentrated on valves of Cytheridea spp. and Cyprideis spp., but valves from other ostracod species were also analyzed to evaluate the effects of the environment of formation and vital effects. For the period from 20 to 13 Ma, the Sr isotope ratios (87Sr / 86Sr between 0.70850 and 0.70883) and δ18O values (range from 28.3‰ to 32.8‰; VSMOW) are compatible with open marine conditions. Nd isotope ratios for this period are quite different for different localities (between 0.51216–0.51225), indicating localized control on seawater 143Nd / 144Nd ratios. Mean δ13C values for this period range from − 5.2‰ to + 1.4‰ (VPDB), Sr / Ca ratios from 0.0039 to 0.0067, and Mg / Ca ratios from 0.0083 to 0.0287. For the period from 12 to 8.5 Ma the variations in 87Sr / 86Sr (0.70872–0.70883), δ18O (24.1‰ to 30.2 ‰) and δ13C values (− 7.8‰ to − 2.6‰), as well as Sr / Ca (0.0021 to 0.0059), and Mg / Ca (0.0116 to 0.0284) all increase. For this period the Nd isotope compositions are quite homogeneous (0.51218–0.51220), indicating a similar source lithological control on hydrochemistry which may allow the Sr / Ca and Mg / Ca ratios to be interpreted in terms of changes in water temperature and mineralogy of the precipitating phases. Geochemical changes for this period are compatible with alternating influences of seawater and brackish conditions as well as changes resulting from evaporation. For all periods δ18O and δ13C values as well as Sr/Ca and Mg/Ca of different species from the same samples generally have similar trends with small offsets. By comparison to records of climatic change from the global oceans, the δ18O values for marine fossils of the Molasse between 20 and 13 Ma are interpreted to reflect global changes in climate. This is also true for the increasingly brackish conditions up to 11 Ma, where combined geochemical records are still compatible with the global changes in temperatures. However, increasing complexity of the brackish system makes detailed comparisons with global changes difficult, particularly after 9.5 Ma (Late Pannonian stages D/E).