What triggered 20 million years of anoxia in central Eurasia?

Organic-rich sediments may form in response to (1) increased preservation during periods of stratification of the water column, and/or (2) enhanced biological productivity. In marginal basins like the Paratethys, both of these effects are typically linked to drivers such as increased river runoff, for example due to short-term increases in precipitation across drainage basins, or a change in the ocean gateway dynamics caused by either sea-level fluctuations or tectonics.

During the Oligocene and Early Miocene, the Paratethys Sea experienced long-term anoxic conditions lasting for 20 million years. The sediments deposited at this time, typically referred to as the Maykop Formation, provide an important hydrocarbon source for both the Black Sea and Caspian Sea areas. Due to its substantial time span the Maykop Formation cannot be interpreted as the effect of a short-term disturbance in water circulation and/or biological productivity. Instead, a mechanism needs to be found which may explain the establishment of long-term anoxia in the basin.

A link between the hydrological budget of an enclosed basin, the sea surface area, and water stratification has previously been established for several other geological events of regional importance; for example: (1) the Caspian Sea desiccation during the Pliocene Productive Series, (2) the Messinian Salinity Crisis in the Mediterranean Sea, and (3) the Middle Miocene Badenian-Sarmatian Extinction Event. Here we investigate what role the Paratethys hydrological budget may play in causing long-term anoxic conditions in the Oligocene and Early Miocene. For this, we focus on one of the obvious changes over the past 40 million years in the Paratethys region, the progressive decrease in sea surface area. Based on paleogeographic reconstructions and paleoclimate models we test the sensitivity of the Maykop basin’s hydrologic budget to this change in sea surface area. Our calculations of the hydrologic budget include past and present values for river runoff in various parts of the Paratethys region, and precipitation and evaporation over the Paratethys surface area. We use this quantitative analysis as a basis to explore the potential forcing mechanisms for this long-lasting period of oceanic anoxia in the middle of the Eurasian continent. Moreover, from a general point of view the mechanism we will propose may play an important role in establishing and maintaining anoxia in other epicontinental and/or semi-restricted basin settings.