| Summary: | The early Eocene is characterized by long-term global warming culminating in the Early Eocene Climatic Optimum (EECO). During this time interval, of which previously only the PETM was intensively studied, the Peri-Tethys was characterized by its position between north-south and east-west trending seaways. The Aktulagay section in Kazakhstan provides an expanded record of the middle Ypresian (NP11-13, ~54-50 Ma), covering the lower part of 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 isotopic investigations on excellently preserved foraminiferal assemblages. The period from 54 to 52 Ma is characterized by a diverse assemblage of deep outer neritic benthic foraminifera, with common Pulsiphonina prima and Paralabamina lunata. The initially (54 Ma) well-ventilated oligo- to mesotrophic sea floor conditions gradually change to more eutrophic and oxygen-limited. This gradual onset culminates in permanent stratification in the sapropel-bearing unit around 52 Ma, with the dominance of Anomalinoides acutus and Bulimina aksuatica. Benthic foraminiferal assemblages dominated by Epistominella minuta at ~51-50 Ma indicate a highly seasonal food supply, possibly riverine input and episodic disruption of stratification. This coincides with the peak of the EECO, as indicated by its position close to the base of NP13 and rising δ13Cforam values. Although it is tempting to link the observed patterns to climate change, we can currently not exclude that changing paleogeography and variable connections to the Tethys, Atlantic and the Arctic ocean largely determined the long-term period of dysoxia and anoxia during deposition of the sapropel beds at the Peri-Tethyan seafloor. These rapid, small biofacies shifts observed in the Aktulagay section may reflect a stepwise transition in long-term re-shaping of benthic foraminiferal communities. status: published
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