Geological History Of Oxygen

Although oxygen is the most abundant element in Earth's crust, due to its high reactivity it mostly exists in compound (oxide) forms such as water, carbon dioxide, iron oxides and silicates. Before photosynthesis evolved, Earth's atmosphere had no free diatomic elemental oxygen (O₂). Small quantities of oxygen were released by geological and biological processes, but did not build up in the reducing atmosphere due to reactions with then-abundant reducing gases such as atmospheric methane and hydrogen sulfide and surface reductants such as ferrous iron.

Oxygen began building up in the prebiotic atmosphere at approximately 1.85 Ga during the Neoarchean-Paleoproterozoic boundary, a paleogeological event known as the Great Oxygenation Event (GOE). At current rates of primary production, today's concentration of oxygen could be produced by photosynthetic organisms in 2,000 years. In the absence of plants, the rate of oxygen production by photosynthesis was slower in the Precambrian, and the concentrations of O₂ attained were less than 10% of today's and probably fluctuated greatly.

The increase in oxygen concentrations had wide ranging and significant impacts on Earth's biosphere. Most significantly, the rise of oxygen and the oxidative depletion of greenhouse gases (especially atmospheric methane) due to the GOE led to an icehouse Earth that caused a mass extinction of anaerobic microbes, but paved the way for the evolution of eukaryotes and later the rise of complex lifeforms.

Before the Great Oxidation Event

Photosynthetic prokaryotic organisms that produced O₂ as a byproduct lived long before the first build-up of free oxygen in the atmosphere, perhaps as early as 3.5 billion years ago. The oxygen cyanobacteria produced would have been rapidly removed from the oceans by weathering of reducing minerals, most notably ferrous iron. This rusting led to the deposition of the oxidized ferric iron oxide on the ocean floor, forming banded iron formations. Thus, the oceans rusted and turned red. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event.

Effects on life

Early fluctuations in oxygen concentration had little direct effect on life, with mass extinctions not observed until around the start of the Cambrian period, . The presence of provided life with new opportunities. Aerobic metabolism is more efficient than anaerobic pathways, and the presence of oxygen created new possibilities for life to explore. Since the start of the Cambrian period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume. 430-million-year-old fossilized charcoal produced by wildfires show that the atmospheric oxygen levels in the Silurian must have been equivalent to, or possibly above, present day levels. The maximum of 35% was reached towards the end of the Carboniferous period (about 300 million years ago), a peak which may have contributed to the large size of various arthropods, including insects, millipedes and scorpions. Whilst human activities, such as the burning of fossil fuels, affect relative carbon dioxide concentrations, their effect on the much larger concentration of oxygen is less significant.

The Great Oxygenation Event had the first major effect on the course of evolution. Due to the rapid buildup of oxygen in the atmosphere, the mostly anaerobic microbial biosphere that existed during the Archean eon was devastated, and only the aerobes that had antioxidant capabilities to neutralize oxygen thrived out in the open. This then led to symbiosis of anaerobic and aerobic organisms, who metabolically complemented each other, and eventually led to endosymbiosis and the evolution of eukaryotes during the Proterozoic eon, who were now actually reliant on aerobic respiration to survive. After the Huronian glaciation came to an end, the Earth entered a long period of geological and climatic stability known as the Boring Billion. However, this long period was noticeably euxinic, meaning oxygen was scarce and the ocean and atmosphere were significantly sulfidic, and that evolution then was likely comparatively slow and quite conservative.

The Boring Billion ended during the Neoproterozoic period with a significant increase in photosynthetic activities, causing oxygen levels to rise 10- to 20-fold to about one-tenth of the modern level. This rise in oxygen concentration, known as the Neoproterozoic oxygenation event or "Second Great Oxygenation Event", was likely caused by the evolution of nitrogen fixation in cyanobacteria and the rise of eukaryotic photoautotrophs (green and red algae), and often cited as a possible contributor to later large-scale evolutionary radiations such as the Avalon explosion and the Cambrian explosion, which not only trended in largerPayne, J. L.; McClain, C. R.; Boyer, A. G; Brown, J. H.; Finnegan, S.; et al. (2011). "The evolutionary consequences of oxygenic photosynthesis: a body size perspective". ''Photosynth. Res.'' 1007: 37-57. DOI 10.1007/s11120-010-9593-1 but also more robust and motile multicellular organisms. The climatic changes associated with rising oxygen also produced cycles of glaciation and extinction events, each of which created disturbances that sped up ecological turnovers. During the Silurian and Devonian periods, the colonization and proliferation on land by early plants (which evolved from freshwater green algae) further increased the atmospheric oxygen concentration, leading to the historic peak during the Carboniferous period.

Data show an increase in biovolume soon after oxygenation events by more than 100-fold and a moderate correlation between atmospheric oxygen and maximum body size later in the geological record. The large size of many arthropods in the Carboniferous period, when the oxygen concentration in the atmosphere reached 35%, has been attributed to the limiting role of diffusion in these organisms' metabolism. But J.B.S. Haldane's essay On Being the Right Size points out that it would only apply to insects. However, the biological basis for this correlation is not firm, and many lines of evidence show that oxygen concentration is not size-limiting in modern insects. Ecological constraints can better explain the diminutive size of post-Carboniferous dragonflies – for instance, the appearance of flying competitors such as pterosaurs, birds, and bats.

Rising oxygen concentrations have been cited as one of several drivers for evolutionary diversification, although the physiological arguments behind such arguments are questionable, and a consistent pattern between oxygen concentrations and the rate of evolution is not clearly evident. The most celebrated link between oxygen and evolution occurred at the end of the last of the Snowball Earth glaciations, where complex multicellular life is first found in the fossil record. Under low oxygen concentrations and before the evolution of nitrogen fixation, biologically-available nitrogen compounds were in limited supply, and periodic "nitrogen crises" could render the ocean inhospitable to life. Significant concentrations of oxygen were just one of the prerequisites for the evolution of complex life. Models based on uniformitarian principles (i.e. extrapolating present-day ocean dynamics into deep time) suggest that such a concentration was only reached immediately before metazoa first appeared in the fossil record. Further, anoxic or otherwise chemically "inhospitable" oceanic conditions that resemble those supposed to inhibit macroscopic life re-occurred at intervals through the early Cambrian, and also in the late Cretaceous – with no apparent effect on lifeforms at these times. This might suggest that the geochemical signatures found in ocean sediments reflect the atmosphere in a different way before the Cambrian – perhaps as a result of the fundamentally different mode of nutrient cycling in the absence of planktivory.

An oxygen-rich atmosphere can release phosphorus and iron from rock, by weathering, and these elements then become available for sustenance of new species whose metabolisms require these elements as oxides.

Nutrient Cycling Changes as Oxygen Levels Rose

The coupling between oxygen levels and nutrient cycling has been a critical factor in shaping Earth's biosphere. Oxygen plays a central role in the oxidation of minerals during weathering. Weathering of the contienents which releases nutrients such as phosphorus and iron into rivers and oceans. Phosphorus, is an essential element for ATP, DNA, and membranes. This makes its bioavailability a limiting factor for primary productivity of ecosystems. When oxygen levels increase, weathering rates are also seen to rise. Which likely expand the nutrient base of marine ecosystems. As a result, this enables higher production and supports more complex food webs. In marine environments, oxygen also regulates the redox cycling of nitrogen. In anoxic conditions, denitrification processes dominate, which convert bioavailable nitrogen (nitrate and ammonium) back into inert N₂ gas. This removes it from the biosphere. As oxygen concentrations rose, nitrification became more prevalent, converting ammonia into nitrate and nitrite, more stable and accessible forms for primary producers. This shift helped stabilize nitrogen availability, which can limit biological expansion if depleted.

The emergence of planktivory, microorganisms feeding on plankton, was another historic turning point in nutrient cycling. Prior to the evolution of complex grazing organisms, nutrient regeneration occurred mainly through microbial remineralization, which was a much slower process. With the rise of planktivores and active predation in the water column during the Cambrian, nutrient cycling accelerated and became more efficient. The led to tighter biological feedback loops and higher rates of primary production. In addition, the shift toward higher rates of primariy production is inferred to have influenced isotopic signatures in marines sediments.The co-evolution of oxygenation and nutrient cycling created a feedback system that enabled both the expansion of biomass and the diversification of life. Rising oxygen not only enabled more complex metabolisms but also transformed the nutrient landscape. The relationship was not coalescent, however. Many disruptions, such as climate shifts, caused a temporary deconstruction of the nutrient-oxygen dynamic in marine systems.

Disruptions led to eriods of environmental instability, such as during Snowball Earth events. The Late Devonian, or Ocean Anoxic Events demonstrate how fragile the oxygen-nutrient balance can be. During these intervals, rapid climate change or massive volcanic activity altered ocean circulation and chemistry. This caused widespread deoxygenation and led to declines in biodiversity and primary productivity. Widespread oxygention from sudden shifts in global climate caused key nutrients to become locked in sediments or lost within inefficient nutrient recycling. These periods demonstrate that while oxygenation was essential to biological expansion, it also made the biosphere more sensitive to disruption. The resilience of nutrient cycling systems, particularly through microbial buffering and evolving trophic interactions, helped restore the oxygentation of the global climate. Over time, this restored nutrient cycling and was able to increase levels of biodiversity again.

See also

* Great Oxidation Event
* Neoproterozoic oxygenation event
* Silurian-Devonian Terrestrial Revolution

References

External links

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* ; {{cite magazine , title=Review of ''Out of Thin Air'' by Peter Ward , url=https://www.newscientist.com/article/mg19225780-124-review-out-of-thin-air-by-peter-ward/ , magazine=New Scientist

Biogeochemistry
Oxygen
Oxygen