The Great Oxygenation Event, also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen
Revolution, or Great Oxidation, was the biologically induced appearance of dioxygen in Earth's
atmosphere. Geological, isotopic, and chemical evidence suggest that this major environmental
change happened around 2.3 billion years ago. Cyanobacteria, which appeared about 200 million
years before the GOE, began producing oxygen by photosynthesis. Before the GOE, any free
oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE
was the point when these oxygen sinks became saturated and could not capture all of the
oxygen that was produced by cyanobacterial photosynthesis. After the GOE, the excess
free oxygen started to accumulate in the atmosphere. Free oxygen is toxic to obligate anaerobic
organisms, and the rising concentrations may have wiped out most of the Earth's anaerobic
inhabitants at the time. Cyanobacteria were therefore responsible for one of the most
significant extinction events in Earth's history. Additionally, the free oxygen reacted with
atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering
the Huronian glaciation, possibly the longest snowball Earth episode in the Earth's history.
Eventually, aerobic organisms began to evolve, consuming oxygen and bringing about an equilibrium
in its availability. Free oxygen has been an important constituent of the atmosphere
ever since.
Timing
The most widely accepted chronology of the Great Oxygenation Event suggests that free
oxygen was first produced by prokaryotic and then later eukaryotic organisms that carried
out oxygenic photosynthesis, producing oxygen as a waste product. These organisms lived
long before the GOE, perhaps as early as 3,500 million years ago.
The oxygen they produced would have quickly been removed from the atmosphere by the weathering
of reduced minerals, most notably iron. This 'mass rusting' led to the deposition of iron(III)
oxide to form banded-iron formations such as those sediments in Minnesota and Pilbara,
Western Australia. Oxygen only began to persist in the atmosphere
in small quantities shortly before the start of the GOE. Without a draw-down, oxygen could
accumulate very rapidly. For example, at today's rates of photosynthesis,
modern atmospheric O2 levels could be produced in around 2,000 years. See oxygen cycle capacities
and fluxes. Another hypothesis is an interpretation of
the supposed oxygen indicator, mass-independent fractionation of sulfur isotopes, used in
previous studies, and that oxygen producers did not evolve until right before the major
rise in atmospheric oxygen concentration. This hypothesis would eliminate the need to
explain a lag in time between the evolution of oxyphotosynthetic microbes and the rise
in free oxygen. Either way, the oxygen did eventually accumulate
in the atmosphere, with two major consequences. First, it oxidized atmospheric methane to
carbon dioxide and water, triggering the Huronian glaciation.
The latter may have been a full-blown, and possibly the longest ever, snowball Earth
episode, lasting 300–400 million years. Second, the increased oxygen concentrations
provided a new opportunity for biological diversification, as well as tremendous changes
in the nature of chemical interactions between rocks, sand, clay, and other geological substrates
and the Earth's air, oceans, and other surface waters.
Despite the natural recycling of organic matter, life had remained energetically limited until
the widespread availability of oxygen. This breakthrough in metabolic evolution greatly
increased the free energy supply to living organisms, having a truly global environmental
impact; mitochondria evolved after the GOE. With more energy available from oxygen, organisms
had the means for new, more complex morphologies. These new morphologies in turn helped drive
evolution through interaction between organisms. Time lag theory
The gap between the start of oxygen production from photosynthetic organisms and the geologically
rapid increase in atmospheric oxygen may have been as long as 900 million years. Several
hypotheses might explain the time lag: Tectonic trigger
The oxygen increase had to await tectonically driven changes in the Earth, including the
appearance of shelf seas, where reduced organic carbon could reach the sediments and be buried.
The newly produced oxygen was first consumed in various chemical reactions in the oceans,
primarily with iron. Evidence is found in older rocks that contain massive banded iron
formations that were apparently laid down as this iron and oxygen first combined; most
of the planet's commercial iron ore is in these deposits.
Nickel famine Chemosynthetic organisms were a source of
methane, which was an important trap for molecular oxygen, because oxygen readily oxidizes methane
to carbon dioxide and water in the presence of UV radiation. Modern methanogens require
nickel as an enzyme cofactor. As the Earth's crust cooled, the supply of nickel from volcanoes
was reduced and less methane was produced. This allowed the oxygen concentration in the
atmosphere to increase. From 2.7 to 2.4 billion years ago, the levels of nickel deposited
declined steadily; it was originally 400 times today's levels.
Bistability A 2006 theory, called bistability, comes from
a mathematical model of the atmosphere. In this model, UV shielding decreases the rate
of methane oxidation once oxygen levels are sufficient to support the formation of an
ozone layer. This explanation proposes an atmospheric system with two steady states,
one with lower atmospheric oxygen content, and the other with higher oxygen content.
The Great Oxidation can then be understood as a switch between lower and upper stable
steady states. Late evolution of oxy-photosynthesis theory
There is a possibility that the oxygen indicator was misinterpreted. During the proposed time
of the lag in the previous theory, there was change from mass-independently fractionated
sulfur to mass-dependently fractionated sulfur in sediments. This was assumed to be a result
of the appearance of oxygen in the atmosphere. This change from MIF to MDF of sulfur isotopes
also may have been caused by an increase in glacial weathering, or the homogenization
of the marine sulfur pool as a result of an increased thermal gradient during the Huronian
glaciation period. Role in mineral diversification
The Great Oxygenation Event triggered an explosive growth in the diversity of minerals on Earth.
It is estimated that this event alone was directly responsible for more than 2,500 new
minerals of the total of about 4,500 minerals found on Earth. Most of these new minerals
were hydrated, oxidized forms of minerals formed due to dynamic mantle and crust processes
after the Great Oxygenation event.
See also Banded iron formation
Evolution of dietary antioxidants Geological history of oxygen
Huronian glaciation Iodide
Medea hypothesis Pasteur point
Rare Earth hypothesis References
External links First breath: Earth's billion-year struggle
for oxygen New Scientist, #2746, 5 February 2010 by Nick Lane. [2]