Anoxic Event - Background

Background

Analysis of the geologic records occurring before and after the affected ages suggest that onsets are rapid and so are recoveries. Both sets of data suggest that a sudden climate threshold or tipping point occurs at about four times the Earth's mean carbon dioxide levels relative to the baseline concentrations of about 280 ppmv that existed circa 1750. This date is significant in that it is regarded as the beginning of the Industrial Age.

Analysis of strata data suggests that in the era when Earth had a predominantly overheated climate, with heavy daily rains and violent storms, the global climate of the time resulted in far heavier erosion, which in turn fed more nutrients into the world's waters. At the same time it caused deep water circulation between the poles and the equator to stop in a cataclysmic fashion. This obstruction in oceanic circulation led to 'death in the depths' from oxygen deprivation. The stagnation caused by this lack of circulation could not be offset by natural processes and became a source of mildly poisonous hydrogen sulfides. The stratified waters would support life in the oxygenated surface layer but the deeper layers became a lethal mixture where life was impossible. The toxic lower layers halted scavenger activity along the organically rich ooze, or sapropel, and all creatures that died in it drifted down and accumulated on the abyssal basins and bottoms. All these life forms unwarily drifting into the anoxic or toxic layers would have died and contributed to the continual accumulation of unicellular microorganisms. The surface layer benefited from an explosion in life, spurred by the increased nutrients from the super-greenhouse conditions, which was then killing itself in waste products. Ironically these deposits of sedimentary organic materials may have accumulated into lipid rich deposits. It is now widely believed that most of today's fossil oil reserves were formed in several distinct anoxic events in earth's geologic history.

There are currently several places on earth that are exhibiting the features of anoxic events on a localized level such as algae blooms and localized "dead zones". Dead zones exist off the East Coast of the United States in the Chesapeake Bay, in the Scandinavian strait Kattegat, the Black Sea (which may have been anoxic in its deepest levels for millennia, however), in the northern Adriatic as well as a dead zone off the coast of Louisiana. The current surge of jellyfish worldwide is sometimes regarded as the first stirrings of an anoxic event. Other marine dead zones have appeared in coastal waters of South America, China, Japan, and New Zealand. A 2008 study counted 405 dead zones worldwide.

This is a recent understanding. This picture was only pieced together during the last three decades. The handful of known and suspected anoxic events have been tied geologically to large-scale production of the world's oil reserves in worldwide bands of black shale in the geologic record. Likewise the high relative temperatures believed linked to so called "super-greenhouse events" Oceanic anoxic events were in all likelihood caused or stimulated by extreme episodes of volcanic outgassing. These events contributed to the characteristic elevated carbon dioxide levels four to six times current levels that are attributed to these periods. At even a few degrees warmer, rain forests are extremely vulnerable to fire hazards. These forests have little natural resistance to fires, and some conjecture a critical tipping point. Practically overnight the increase of temperature might have been reached and triggered a huge burn-off of planetary forests. This would have released unprecedented amounts of carbon dioxide into the atmosphere. With a change of mean temperatures of three degrees Celsius, the ice caps melted. This triggered a runaway effect. In the super-greenhouse ecologies—the term meaning average temperature rose to or beyond six degrees above today—the seas were so warm, it is believed the water temperatures at the two poles were in the lower 80s°F (i.e. above 27 °C). The Cretaceous and Jurassic periods' world ecologies were essentially ice free, had massive storms driven by warm oceans, and were dying from the double hit of lack of oxygen and toxic hydrogen sulfide accumulations at lower layers because of a shut down in the ocean conveyor belts. In this time, most of the world would experience the highly noxious scent of rotten eggs and the seas would have slowly acquired a deep green hue from the high amounts of algae.