Magneto-biostratigraphic age constraints on the palaeoenvironmental evolution of the South Caspian basin during the Early-Middle Pleistocene (Kura basin, Azerbaijan)

The sedimentary record of the Caspian Basin is an exceptional archive for the palaeoenvironmental, palaeoclimatic and biodiversity changes of continental Eurasia. During the Pliocene-Pleistocene, the Caspian Basin was mostly isolated but experienced large lake level fluctuations and short episodes of connection with the open ocean as well as the Black Sea Basin. A series of turnover events shaped a faunal record that forms the backbone of the Caspian geological time scale. The precise ages of these events are still highly debated, mostly due to the lack of well-dated sections. Here, we provide an integrated magneto-biostratigraphic age model for two sections from the Kura Basin – Goychay and Hajigabul. Our results reveal several major intervals with elevated salinity, associated with mesohaline faunas, and propose the following age constraints: 1) The Productive Series-Akchagylian boundary is marked by a marine transgression from the open ocean that occurred around the Gauss-Matuyama reversal (∼2.7–2.6 Ma); 2) The Akchagylian-Apsheronian transition is characterized by a regression event and introduction of a new, “Pontocaspian” mollusc assemblage, dated within the Reunion subchron (∼2.13 Ma). The ostracod assemblages of the two sections do not show a major faunal turnover here; 3) The early Bakunian transgression occurs after the upper Apsheronian lowstand (0.85–0.83 Ma). We conclude that major transitions in the age-indicative mollusc groups sometimes occur at different time intervals (i.e. diachronic) and are highly depended on the local depositional environments. A high-resolution interdisciplinary approach on sections outside the Kura Basin is required to better understand the potential diachroneity of these turnover events in the entire Caspian Basin. •Akchagylian flooding of the Caspian Basin with mesohaline fauna is dated around 2.7–2.6 Ma.•Regression in the Caspian Sea from 2.2 to 2.1 Ma is associated with new Apsheronian mollusc groups.•FO of Apsheronian indicative molluscs is within the Reunion subchron at an age of ∼2.13 Ma.•Faunal indications of periodical reconnections between the Caspian Sea and the Black Sea in the Apsheronian.•The Apsheronian – Bakunian boundary is dated at 0.83–0.85 Ma.