Changing seasonality patterns from Miocene Climate Optimum to Miocene Climate Transition deduced from the Crassostrea isotope archive

The Western Tethyan estuarine oyster Crassostrea gryphoides (Schlotheim, 1813) is geologically long lived. Appearing in the Oligocene it persists up to the Pliocene in the entire Western Tethys. With sizes of over 80 cm length, it is the largest Miocene bivalve in the Western Tethys Region. Its modern congeners are economically important in shellfish farming. Therefore, numerous studies focused on the biology and ecology of Crassostrea including several sclerochronological studies Herein we measured 5 shells from the Miocene Climate Optimum (MCO) and the subsequent Miocene Climate Transition (MCT) to evaluate changes of seasonality patterns.
   MCO shells exhibit highly regular seasonal rhythms of warm-wet and dry-cool seasons. Optimal conditions resulted in extraordinary growth rates. Estuarine waters during the MCO in Central Europe display a seasonal temperature range of c. 9-10°C. Absolute water temperatures have ranged from 17-19°C during cool seasons and up to 28°C in warm seasons. Already during the early phase of the MCO, the growth rates are declining. Still, a very regular and well expressed seasonality is dominating, but extreme climate events did occur. The seasonal temperature range is still c. 9°C but the cool season temperature is slightly lower (16°C) and the warm season water temperature does not exceed c. 25°C. At 12.5-12.0 Ma. The seasonality pattern is breaking down and is replaced by successions of dry years with irregular precipitation events. The amplitude of seasonal temperature range is decreasing to 5- 8°C. No clear cooling trend can be postulated for that time as the winter season water temperatures range from in Central Europe instead of a simple temperature decline scenario.