Basin connection affects Caspian Sea environments and biota at 2.73 Ma.

The Caspian Sea is the largest isolated water body in the world and both its hydrology and biota are highly sensitive to changes in water conditions driven by climate and basin connectivity. During most of the Pliocene, the Caspian Sea was disconnected from the global oceans and a thick sequence of fluvio-deltaic and lacustrine sediments was deposited in the South Caspian Basin (the Productive Series). Overlying this sequence, the Akchagylian mudrocks were formed in relatively deeper water settings following a large-scale transgressive event that caused the Caspian Lake to expand over vast areas that had previously been sub-aerial; including parts of Azerbaijan, Georgia, Iran, Turkmenistan, Kazakhstan and South-western Russia. Although the occurrence of this transgressive event is well noted, the exact nature and timing are not completely constrained. Here, we present new radiometric dating (⁴⁰Ar/³⁹Ar on volcanic ashes) as well as palynological, sedimentological and geochemical evidence for the connection at the Lokbatan section, Azerbaijan. Proxy data demonstrate clear effects of the basin connection on biota and chemistry in the South Caspian Basin, including evidence of complete overturn in phytoplankton communities as well as changes in sediment chemistry across the transition interval. The new age data place the transgression at the Lokbatan section at ~2.73 Ma, which is broadly concordant with other new data from elsewhere in Azerbaijan (Van Baak et al., in prep). This date suggests that the event was more or less contemporaneous with the intensification of northern hemisphere glaciations at, or immediately preceding the Pliocene-Pleistocene boundary at 2.58 Ma. Paradoxically, the base of the Akchagylian as encountered north of the Caucasus appears to be somewhat older than in the south (time equivalent of the Surakhany Suite in Azerbaijan). New dating of the Plio-Pleistocene deposits in the South Caspian Basin may warrant a renewed look at the north-south correlation of the Akchagylian, which is far from straightforward and has often relied on magnetostratigraphic zonations. Reassessment of the available data may help to reconcile apparent differences in timing and to improve understanding of the transgression at a basin scale.