LAUSR

Abstract Marine influence on low-salinity environments can trigger aquatic ecosystem shifts, including biodiversity turnovers. High-resolution palaeoenvironmental records of marine connection events are particularly valuable, as they provide natural laboratories to understand analogous oceanographic and biodiversity turnover events in present-day climate- and anthropogenically-induced incursions. One such incursion event occurred across the Plio-Pleistocene transition when water from the open ocean spilled into the Eurasian continental interior, inundating the Caspian area. Here we record the so-called Akchagylian marine incursion using well-dated palynological and geochemical records of the Lokbatan section (Azerbaijan). Immediately prior to the intensification of northern hemisphere glaciations (~2.75 Ma), fresh-brackish peri-Paratethyan dinocyst assemblages were replaced by monospecific assemblages of the marine dinocyst, Operculodinium centrocarpum sensu Wall and Dale (1966) . This indicates that the Caspian Sea experienced a marine incursion during a period of global high sea level. The marine incursion also registered in the geochemical record as a peak in excess‑strontium and carbonate content. Marine influence on the Caspian ceased after ~2.46 Ma and a second biological turnover took place, with low-salinity tolerant peri-Paratethyan dinoflagellate communities replacing the marine assemblages. The large-scale Akchagylian marine incursion episode shows the extreme degree of biodiversity change that marine influence on fresh-brackish water basins could trigger. Similar processes are increasingly relevant to present-day marginal and landlocked basins, which face ever-greater incursions from marine species and water due to both climate-mediated sea-level rise and human-made infrastructure projects.