Middle Miocene seasonal temperature changes in the Styrian basin, Austria, as recorded by the isotopic composition of pectinid and brachiopod shells

Using the isotope record on pectinid and brachiopod shells, we reconstructed Middle Miocene palaeotemperatures for a shelf environment in the Styrian basin, Austria. Bivalve shells of Macrochlamys sp., collected from different horizons of the Lower Badenian Leitha limestone, show δ18O values in the range of −3 to 0‰ (PeeDee Belemnite). The large intrashell variability of 3‰ indicates significant seawater temperature fluctuation. The results suggest that despite the high geographical latitude (ca. 40°N), the climate in southern Austria was warm (subtropical) with a pronounced seasonality at that time. In contrast, the δ18O isotopic composition of a pectinid (Flabellipecten sp.) and an indeterminate terebratulid brachiopod, from the siliciclastic deposits overlying the Leitha limestone, ranges between 0 and −1‰. This indicates cooler mean annual temperatures and reduced seasonal variations. A 39Ar/40Ar age on fresh volcanic biotites shows a value of 14.2±0.1 Ma. The tuffs containing the biotites are located below the layer containing the terebratulid and the Flabellipecten sp. The stable isotope and radiogenic data suggest that the drop in temperature is in direct relationship with the East Antarctic Ice Sheet expansion, which started at ca. 14 Ma. Due to the variation in temperature, the subtropical fauna in the Retznei quarry disappeared during the Early Miocene and the biogenic limestones were replaced by clastic sediments. The δ18O isotopic profiles from three pectinids collected from the Leitha limestone indicate that these grew within ca. 1.5 years. Growth interruptions occurred during the warm season because during this period the pectinids very likely used their energy for the growth and maturation of gonads. Carbon isotopic compositions vary between 0 and 2‰. Statistical tests show that for some of the analyzed shells, the cyclicity of the δ13C profiles may be explained by the temperature-dependent fractionation between air CO2 and dissolved bicarbonate.