The Aktulagay section in Kazakhstan provides an expanded northern Tethyan record of the middle Ypresian (calcareous nannoplankton zones NP11–13, ~ 54–50 Ma), including the Early Eocene Climatic Optimum (EECO). The marl sequence features a series of sapropel beds, observed throughout the Peri-Tethys, indicative of the basin-wide occurrence of episodic hypoxic events. In order to unravel the paleoenvironmental evolution at Aktulagay during this period of global warming, we investigated the benthic foraminiferal record by means of a detailed multivariate analysis of the > 63 μm fraction, as well as through stable isotopic (C, O) investigations on excellently preserved benthic foraminiferal specimens.
The Alashen Formation (NP11 to lower NP12; ~ 54 to 52.5 Ma), in the lower part of the sequence, contains a diverse assemblage of deep outer neritic (~ 200–250 m) benthic foraminifera, with common Pulsiphonina prima and Paralabamina lunata. The seafloor conditions are interpreted as initially (54 Ma) well-ventilated and oligo- to mesotrophic, gradually changing to more eutrophic and oxygen-limited, culminating in more permanent low oxygen conditions and eutrophy in the sapropel-bearing Aktulagay B1 unit (middle NP12; ~ 52.5-52 Ma). The latter conclusion is corroborated by the dominance of Anomalinoides acutus and Bulimina aksuatica and the lower diversity. Also the upward migration of endobenthic species to the sediment–water interface, as suggested by rising δ13Cendobenthic values, supports this interpretation. A transgression, which flooded lowlands, might have caused this development. In the Aktulagay B2 unit (top NP12–NP13; ~ 52–50 Ma), benthic foraminiferal assemblages dominated by Epistominella minuta suggest an oligotrophic environment, with transient pulses of phytodetritus and moderate ventilation.
The Aktulagay B2 unit coincides with the peak temperature interval of the EECO, as indicated by its position close to the base of NP13 and rising δ13Cepibenthic values. Large river plumes, episodically reaching the area, in a monsoonal climatic context, might explain this basin development. Although it is not unlikely that some of the observed patterns are related to long-term climate change, it can currently not be excluded that changing paleogeography and variable connections to the Tethys, Atlantic and the Arctic Ocean were responsible for the long-term period with dysoxia and anoxia during deposition of the sapropel beds at the Peri-Tethyan seafloor.
The evolution of the basin as observed in Aktulagay shows similarities to the evolution of the North Sea Basin as observed in Denmark, suggesting that these basins were connected during the Early Eocene.