Biomarkers reveal two paramount Pliocene-Pleistocene connectivity events in the Caspian Sea Basin

Landlocked basins like the Caspian Sea are highly sensitive to changes in their hydrological budget, especially at times of disconnection from the global ocean. Here, we reconstruct the Pliocene to Pleistocene palaeohydrological and palaeoenvironmental changes occurring in the South Caspian Basin between ~3.6 and ~ 1.9 Ma, using compound-specific hydrogen isotope (δ²H) data on long chain n-alkanes and alkenones. Additionally, we established a record of mean annual air temperature (MAT) and the source of organic matter, based on the relative distribution of branched and isoprenoid glycerol dialkyl glycerol tetraethers (BIT). The ~55‰ variation in the δ²H measured on the terrestrial plant long chain n-alkanes indicates significant continental hydrological changes in the region surrounding the Caspian Sea over the investigated 1.7 Myr interval. The MAT and BIT data show that the so-called Akchagylian marine incursion at around 2.75 Ma, marked by influx of marine biota into the Caspian Basin, originated from a cold region of the open ocean, endorsing a hydrological connection with the Arctic domain. The onset of the regional Apsheronian stage at ~2.13 Ma, identified by the invasion of Tyrrhenocythere sp. ostracods, coincided with a change towards constant δ²Hn-alkane and is shortly followed by the occurrence of alkenones in the Caspian Basin. The relative distributions of alkenones and their δ²H values indicate that a connection with a saline basin, most likely the Black Sea, was established at the Akchagylian–Apsheronian transition.