| Summary: | The early Eocene is characterized by long-term global warming culminating in the Early Eocene Climatic Optimum (EECO). During this time interval, the Peri-Tethys was connected to the Arctic and Atlantic Oceans by seaways. The Aktulagay section in Kazakhstan provides an expanded record of the middle Ypresian (NP11-13, ~54-50 Ma), including the EECO. It features a series of sapropel beds, observed throughout the Peri-Tethys. In order to unravel paleoenvironmental changes, we carried out quantitative faunal studies and stable isotopic (O, C) investigations on excellently preserved foraminiferal assemblages. The period from 54 to 52.5 Ma (NP11 to lower NP12) is characterized by a diverse assemblage of deep outer neritic (~200-250 m) benthic foraminifera, with common Pulsiphonina prima and Paralabamina lunata. The initially well-ventilated oligo- to mesotrophic seafloor conditions gradually changed to more eutrophic and oxygen-limited. These conditions were more permanent in the sapropel-bearing unit at 52.5-52 Ma (middle NP12), where Anomalinoides acutus and Bulimina aksuatica dominate the record. Benthic foraminiferal assemblages dominated by Epistominella minuta at ~52-50 Ma (top NP12-NP13) suggest an oligotrophic environment with transient pulses of phytodetritus. It is tempting to link the long-term period of dysoxia to climate change, but also variable connections to the Peri-Tethys could be responsible. status: published
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