Große Sauerstoffkatastrophe

O₂ Bildung in der Erdatmosphäre. Die roten und grünen Linien repräsentieren den Bereich der Schätzuungen, wobei die Zeit in Milliarden Jahren gemessen wird (Ga).
Stufe 1 (3.85–2.45 Ga): Praktisch kein O₂ in der Atmosphäre
Stufe 2 (2.45–1.85 Ga): O₂ wurde produziert, aber in den Ozeanen und vom Fels des Meeresgrundes absorbiert.
Stufe 3 (1.85–0.85 Ga): O₂ beginnt aus den Ozeanen auszugasen, wird aber von den Landflächen und der sich bildenden Ozonschit absorbiert.
Stufe 4 & 5 (0.85–0.54 Ga) & (0.54 Ga–present): O₂ Sie Sauerstoffsenken sind gefüllt und das Gas reichert sich an.

Die große Sauerstoffkatastrope', Sauerstoff-Krise oder Great Oxigenation event (GOE) war das Erscheinen freien Sauerstoffs in der Erdatmosphäre. Diese wesentliche, von Lebewesen verursachte Umweltveränderung fand vor ca. 2.4 Milliarden Jahren statt.

Durch Photosyntese wurde Sauerstoff sowohl vor wie auch nach dem GOE produziert. Es gab jedoch einen wesentlichen Unterschied: Vor dem GOE fing organisches Material und gelöstes Eisen jedweden freien Sauerstoff. Der GOE war der Punkt, an dem diese Mineralien gesättigt waren und keinen weiteren Sauerstoff mehr aufnehmen konnten. Der überschüssige freie Sauerstoff begann sich in der Atmosphäre anzureichern.

Der steigende atmosphärische Sauerstoffgehalt hat möglicherweise einen großen Teil der anaeroben Organismen ausgelöscht, die zu dieser Zeit die Erde bevölkerten. Der von Cyanobakterien produzierte Sauerstoff war für anaerob lebende Organismen tödlich und wesentlich für das wahrscheinlich größte Massenaussterben auf der Erde verantwortlich. Darüber hinaus reagierte der freie Sauerstoff mit dem Methan der Atmosphäre - einem Treibhausgas. Die dadurch sinkende Methankonzentration lösten die Huronische Eiszheit aus, die möglicherweise das längste Schneeball-Erde-Ereignis in der gesamten Erdgeschichte darstellt. Seit dieser Zeit ist freier Sauerstoff ein wichtiger Bestandteil der Erdatmosphäre.

Timing

The most widely accepted chronology of the Great Oxygenation Event suggests that oxygen was first produced by photosynthetic organisms (prokaryotic, then eukaryotic) that emitted oxygen as a waste product. These organisms lived long before the GOE, perhaps as early as Vorlage:Ma. 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 banded-iron formations, shown for example in sediments in Minnesota. Oxygen only began to persist in the atmosphere in small quantities shortly (~50 million years) before the start of the GOE. Without a draw-down, oxygen could accumulate very rapidly: for example, at today's rates of photosynthesis (which are admittedly much greater than those in the plant-free Precambrian), modern atmospheric O₂ levels could be produced in around 2,000 years.

Another theory 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 theory 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 (a strong greenhouse gas) to carbon dioxide (a weaker one) 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 levels 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 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.

Time lag theory

The lag (which may have been as long as 900 million years) was between the time oxygen production from photosynthetic organisms started and the time of the oxygen catastrophe's geologically rapid increase in atmospheric oxygen (about 2.5–2.4 billion years ago). There are a number of hypotheses to explain this time lag:

Tectonic trigger

One phenomenon that explains this lag is that the oxygen increase had to await tectonically driven changes in the Earth's 'anatomy', including the appearance of shelf seas where reduced organic carbon could reach the sediments and be buried. Also, the newly produced oxygen was first consumed in various chemical reactions in the oceans, primarily with iron. Evidence for this phenomenon 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. But these chemical phenomena do not seem to account for the lag completely.Vorlage:Citation needed

Nickel famine

Chemosynthetic organisms were a source of methane, which was also a big trap for molecular oxygen, because oxygen readily oxidizes methane to carbon dioxide (CO₂) 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 allowing oxygen to dominate the atmosphere. 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, to explain the 300-million-year lag comes from a mathematical model of the atmosphere which recognizes that UV shielding decreases the rate of methane oxidation once oxygen levels are sufficient to support the formation of an ozone layer. This explanation proposes a system with two steady states, one with lower (0.02%) atmospheric oxygen content, and the other with higher (21% or more) 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 (MIF) sulfur to mass-dependently (MDF) fractionated sulfur in sediments. This was assumed to be a result of the appearance of oxygen in the atmosphere (since oxygen would have prevented the photolysis of sulfur dioxide, which causes MIF). 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

Recent research has shown that 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.

Vorlage:Horizontal timeline

External links

• First breath: Earth's billion-year struggle for oxygen New Scientist, #2746, 5 February 2010 by Nick Lane. [2]