PALEOENVIRONMENTAL RECONSTRUCTION FROM STABLE ISOTOPIC COMPOSITIONS OF FOSSIL SKELETAL REMAINS OF THE MIDDLE MIOCENE FRESHWATER STEINHEIM BASIN, SW GERMANY

The Steinheim basin is a 3.5 km wide crater in Upper Jurassic limestone of the Swabian Alp formed by a meteorite impact during the middle Miocene (~14.3 Ma). The crater basin was successively filled by freshwater, forming a long-term-lake with strong lake-level fluctuations during its several 100 kyrs history within a period of large-scale global climate change. The up to 36 m thick, fossil-rich, calcareous lake sediments contain a diverse invertebrate and vertebrate fauna. According to the mammal remains Steinheim is dated to the mammal zone MN 7 (~13.5 to 14 Ma), for which it represents the international reference locality.

δ¹³C_VPDB and δ¹⁸O_VPDB values of tooth enamel-apatite of different sympatric large herbivorous mammal taxa (Gomphotherium, Anchitherium, suidae, cervidae, rhinocerotidae) were used as proxies for paleoenvironmental conditions in the Steinheim basin and for a paleoecologic reconstruction. Mean δ¹³C values of –12.4 to –10.2‰ are similar to those of other investigated Central European Miocene localities, indicating a C₃ plant diet. Mean δ¹⁸O_CO3 values vary from –6.3 to –7.7‰ except for the cervids, which have a δ¹⁸O_CO3 value of –4.1‰. Despite the small overall isotopic variability the taxa display systematic intra-specific differences with rhinoceroses having the lowest and Gomphotherium the highest δ¹³C values. Several cervids, a Palaeomeryx, and a peccary have δ¹⁸O_CO3 values between –3.9 to –1.8‰, possibly due to consumption of plant tissues with ¹⁸O-enriched water. The δ¹⁸O_H2O value of the lake water, probably being the only drinking-water source on the Karst plateau, will be calculated from enamel δ¹⁸O_PO4 values using existing species-specific calibrations for large mammals.

The fossil freshwater snail Gyraulus of the Steinheim basin displays a gradual evolution of the shell morphology triggered by lake-level fluctuations and changes in lake water chemistry. A similar evolution is observed for the sculpture of ostracod carapaces and the changes in abundance of surface and deep-water dwellers. δ¹⁸O_CO3 values of aragonitic Gyraulus shells and calcitic ostracod carapaces from the seven different planorbid beds will be used to calculate the water temperatures using the δ¹⁸O_H2O values derived from the mammal teeth and to infer the development of the lake water composition.