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Ocean anoxia – widespread oxygen-starved dead zones in oceans – did the killing of ocean life in several mass extinctions of Earth’s past.  Anoxia went hand-in-hand with CO2emissions, rising global temperatures, and (often) ocean acidification, a situation which today’s climate change is recreating with uncanny likeness.

Even in normal, healthy oceans, dissolved oxygen levels in middle-depth waters (between about 500 to 1,500 meters) are low enough to discourage most higher animals. This makes those depths an important refuge for krill and other prey species to hang out during the day, safe from visual predators. In the dark of night, these creatures venture nearer the surface to graze on plankton, an impressive commute given their small size.

Depth slices through the oceans showing how dissolved oxygen declines from the surface to middle depths and then rises again in deep water. Constructed from World Ocean Atlas 2013.

There are places around the world where these oxygen minimum zones are much shallower than elsewhere, and there are also coasts where polluted river water delivers excess nutrients into the sea, causing coastal dead zones, for example in the Gulf of Mexico. If we look at conditions that led to past “Ocean Anoxic Events” (OAEs), and compare them to our altered climate in coming decades, parallels are sobering.

The complex biogeochemical connections in ocean anoxia. The 3 connected drivers of anoxia include warming (left), acidification (center), and nutrient supply (right). This diagram is schematic, based partially on Gehlen et al in “Ocean Acidification”, OUP 2011, and on others referenced below.

Note – don’t humans like to think they know pretty much all we need to know, yet small warming change in ocean water density can prevent down flow at poles, disrupting deep ocean currents and up-welling at critical marine life nurseries, so many elements to knock our world out of “unstable equilibrium”.

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