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]
