The Central Paratethys during Oligocene as an ancient counterpart of the present-day Black Sea: Unique records from the coccolith limestones

Abstract

Four isochronous Oligocene coccolith limestone horizons from the Carpathians were examined in order to reconstruct paleoceanographic conditions in the Central Paratethys. The dominance of small and size-uniform pyrite framboids, the occurrence of low-diversity dinoflagellate cysts and coccolithophorids and the presence of biomarker molecule 28,30-dinorhopane indicate that the water column was stratified with the upper water column being relatively well‑oxygenated, but the bottom water being anoxic. The latter is confirmed by the fine and consistent horizontal laminations in various parts of the basin, scarcity of benthic organisms and their trace fossils. The limestones exhibit typical marine δ13C values, but are significantly depleted in 18O and enriched in 87Sr relative to contemporaneous ocean water. These isotopic compositions result from a decreased salinity of the surface waters caused by an increased riverine input. This is confirmed by the lack or impoverishment of planktonic foraminifers, presence to abundance of goniodomid dinoflagellate cysts and massive occurrence of low-diversity nannoplankton assemblages, which indicates decreased salinities as low as 17‰ and high productivity in the upper water column. These observations indicate that the limestones were formed during periods when connection of the Central Paratethys with the global ocean was limited, which impeded water exchange causing the development of low-salinity conditions of surface water and bottom-water anoxia. During the deposition of the oldest Tylawa horizon, primary productivity was enhanced and chemocline was positioned exceptionally high in the water column. Moreover, decreased δ13C values in both carbonates and organic matter of this horizon suggest that widespread methane venting took place in the basin during NP23. All these data show that during the Oligocene the Central Paratethys experienced similar conditions to those of the current Black Sea, which can be used as a modern analogue, especially for the Tylawa horizon. Therefore, the Tylawa horizon can be perceived as a potential effect of future post-depositional processes of coccolith marls analogous to those having been deposited in the Black Sea for 2.7 kyr.