Paleo-oxygenation in Tertiary epeiric seas: evidence from dinoflagellate cysts

The dinoflagellate cyst record from an epeiric setting in the Oligocene of Southwest Germany exhibits major changes associated with variations in paleo-oxygenation inferred from benthic foraminifera. Dinoflagellate cyst diversity and equitability strongly decline in the case of extreme oxygen depletion. The relative abundance of the dinoflagellate cyst Thalassiphora pelagica inversely correlates with the availability of benthic oxygen. The T. pelagica spectrum consists of different morphotypes which are considered to represent different stages within the cystal part of the T. pelagica life cycle following the concept of Benedek and Gocht. The distribution of these morphotypes is also linked to oxygen availability. Horizons with highest T. pelagica abundances are characterized by the occurrence of T. pelagica specimens representing an early stage in cyst formation. A model is proposed that uses T. pelagica as a proxy to assess oxygenation both at the sediment surface and in the water column. The occurrence of T. pelagica specimens representing an early stage in cyst formation is interpreted to reflect the extension of oxygen-depletion upwards into the water column. Because T. pelagica is an easily recognized, globally distributed element of Paleogene and Early Neogene dinoflagellate cyst assemblages, the application of this model may be of importance in the reconstruction of paleo-oxygenation in epeiric settings from these times. Its sensitivity to changes in benthic oxygenation can be as high as (if not higher than) that of the approach based on foraminifera. Moreover, it is applicable to decalcified sediments.