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.

海洋对低盐度环境的影响会引发水生生态系统的变化，包括生物多样性的转变。海洋连接事件的高分辨率古环境记录特别有价值，因为它们提供了自然实验室来了解当今气候和人为引起的入侵中类似的海洋学和生物多样性更替事件。当来自公海的水溢出到欧亚大陆内部，淹没里海地区时，发生了一次这样的入侵事件，跨越了上新世-更新世过渡期。在这里，我们记录了所谓的Akchagylian 海洋入侵使用 Lokbatan 部分（阿塞拜疆）的日期很好的孢粉学和地球化学记录。紧接在北半球冰河期（~2.75 Ma）加剧之前，新鲜的半咸味peri-Paratethyan dinocyst 组合被海洋双胞胎Operculodinium centrocarpum 的单种组合所取代Sensu Wall 和 Dale (1966)。这表明里海在全球高海平面期间经历了海洋入侵。海洋入侵还在地球化学记录中记录为过量锶和碳酸盐含量的峰值。海洋对里海的影响在约 2.46 Ma 后停止，发生了第二次生物周转，低盐度耐受的近副甲藻群落取代了海洋组合。大规模的 Akchagylian 海洋入侵事件表明，海洋对咸淡水盆地的影响可能引发极端程度的生物多样性变化。类似的过程与当今的边缘和内陆盆地越来越相关，