Could the Oceans Become Anoxic?
Anoxia it the chemical state of bodies of water losing its free oxygen. This is largely due to density stratification between less dense surface waters and colder, saltier waters at depth. This stratification can cut of upwelling and downwelling, preventing the movement of oxygen into deeper waters. Large portions of the world’s ocean basins have become anoxic in the geologic past. During a million year interval of the Late Cretaceous Period the world’s ocean basins became density stratified. This period is called the Cenomanian-Turonian Oceanic Anoxic Event (OAE). This happened between about 90.5 and 91.5 million years ago (the Cenomanian and Turonian are named epochs of the Cretaceous Period). The world was much warmer in the Cretaceous Period, and there were no continental glaciers. The oceans were warmer, and a thick thermocline and intense pycnocline blocked oxygen-rich surface waters from penetrating deep water. Organic-rich deposits preserved in ocean sediments of the OAE show that there is no bioturbation, suggesting that plankton in the grew in the shallow mixing zone was not consumed if their remains sank into the anoxic condition that existed at the seabed.
Density stratification can cut of oxygen supply to deep water in restricted basins (including isolated lake basins and inland sea basins). The Black Sea is an inland sea that has anoxic conditions. Marine surface waters flow into the Black Sea from the Aegean Sea through the shallow Bosphorus Straight (Figure 9.40). Denser saline water trapped in the basin are unable to circulate out of the basin. A strong pycnocline prevents oxygen from reaching depths below about 100 meters.
Figure 9.40. Thermohaline density stratification cuts off oxygen supply to deep water in the Black Sea, causing anoxic conditions below ~100 meters. Normal seawater exists above a halocline in the basin.