The dire straits of Paratethys: gateways to the anoxic giant of Eurasia

A complex interplay of palaeoclimatic, eustatic and tectonic processes led to fragmentation and dis-sipation of the vast Tethys Ocean in Eocene–Oligocene times. The resulting Paratethys Sea occupied the northern Tethys region on Eurasia, grouping water masses of various sub-basins, separated from each other and from the open ocean through narrow and shallow gateways and land bridges. Changes in marine gateway configu-ration and internal connectivity affected regional hydrology, shifting most Paratethyan basins to extreme car-bon-sink anoxic environments, anomalohaline evaporitic or brackish conditions, or even endorheic lakes. Paratethys gateway restriction triggered the onset of a long-lasting (c. 20 myr) giant anoxic sea, characterized by stratified water masses and anoxic bottom-water conditions, resulting in thick hydrocarbon source rocks. Here, we review the geological evolution of the ‘dire straits’ of Paratethys that played a crucial role in the Eocene–Oligocene connectivity history of the Central Eurasian seas and we show that the main anoxic phases (Kuma and Maikop) correspond to restricted connectivity with the global ocean and a period of CO₂ depletion in the atmosphere. Paratethys represents one of the largest carbon sinks in Earth’s history and may thus have played a prominent role in global climate change.