Keep Oxygen, Dump Carbon

 

Take a breath! Get your regular intake of oxygen and smoothly expel a whiff of carbon dioxide. The concentration of carbon dioxide in Earth's atmosphere is rising. In turn, the concentration of oxygen drops a tiny bit.

 

Back: "Two Billion Years Apart"

Oxygen, flat and stable...

Combustion of oxygen by fossil fuels makes up for about 3% of the total amount of oxygen that annually is drawn from the atmosphere. Respiration of plants or animals and microbial oxidation consume much more oxygen than the burning of fossil fuels. The atmospheric reservoir of oxygen would sustain respiration for about 5500 years. That is a very long time on a human time-scale but on a geological timescale that is a very short time. Thus taking a geological view, oxygen in the atmosphere is sustained by the biosphere in the same manner as the biosphere sustains the concentration of carbon dioxide. Photosynthesis steadily replenishes oxygen, and it consumes carbon-dioxide and balances oxygen consumption and carbon-dioxide replenishment. A little less than half of the total oxygen production occurs in the surface waters of the ocean. These waters frequently are over-saturated with oxygen, be it by photosynthesis or air bubbles due to wave breaking.

Terraced fields in Yunnan Provice (China) 
Credit: Hongkai Gao (imaggeo.egu.eu)

Since half a Billion years, the marine and terrestrial biosphere keeps the Earth vigorously oxygenated. Contrary to the amount of atmospheric carbon dioxide, the amount of oxygen in the modern atmosphere is far too high that burning of coal, oil and gas since the start of the industrial revolution may have had a noticeable effect on it. The biosphere is exchanging oxygen with the atmosphere and the ocean. The amount of oxygen in the ocean is about a hundred times smaller than the amount of oxygen in the atmosphere. Most of the oxygen on Earth is bound in the rocks of the lithosphere. The lithosphere contains hundred times more oxygen than the atmosphere. Just as for carbon, oxygen mainly is stored in the lithosphere.

Weathering of rocks on the continent would consume the oxygen in the atmosphere in about 20 Million years. That indeed is a very long time on a human time-scale, but on a geological time-scale it is a short time. The weathering of rocks leads to oxygen-enriched minerals that are washed into the sea. The minerals get buried in marine sediments for very long periods. Finally, they are incorporated into the crust of the Earth, which slowly gets richer in oxides. Some of these oxides will be recycled through plate-tectonics, subduction and volcanism into the atmosphere.

It took three Billion years to reach the current level of oxygen concentration in the atmosphere and the ocean. Eventually, a vast reservoir of free molecular oxygen had formed. In essence, on Earth only the dead matter in the biosphere is left to consume the oxygen that is produced by photosynthesis.

 

Carbon, down...

To accumulate oxygen in the atmosphere, vast amounts of carbon have been deposited into the Earth's crust, be it a bit of coal, carbon rich shale or limestone. That in turn leaves the reservoir of carbon in the atmosphere and the ocean sensitive to a vigorous biosphere or active humans.

Taïga burning near Krasnoiarsk

Credit: Jean-Daniel Paris (imaggeo.egu.eu)

The ocean buffers fluctuations of carbon concentrations in the atmosphere, also the anthropogenic increase of carbon dioxide. So far the ocean has taken up a substantial share of the carbon dioxide that was added to the atmosphere by human economic activity. The anthropogenic increase of carbon-dioxide concentration in the atmosphere is only half of what should have happened because of the amount of fossil fuels burned since the beginning of the industrial revolution. The ocean absorbed the other half of the carbon exhausted by burning fossil fuels. The carbon dioxide capture in the sea-water currently shifts acidity of the ocean.

The absorption of carbon-dioxide in the ocean leads to the formation of carbonic acid in the surface waters of the ocean and to increased dissolution of carbonates. That in turn is changing the life for species with an external carbonate skeleton, be it coral, algae or shellfish. Some worry that this change will impinge negatively on the amount of photosynthesis in the ocean.

Ukko El'Hob

 

This text together with the two related texts were inspired by the article “The rise of oxygen in Earth's early ocean and atmosphere” by Timothy W. Lyons, Christopher T. Reinhard and Noah J. Planavsky, which was published in February 2014 in Nature. Many insights are taken from “The global oxygen cycle “ by S.T. Petsch, which was published 2003 by Elsevier in volume 8 of in the “Treatise on Geochemistry” (Editor: William H. Schlesinger. Executive Editors: Heinrich D. Holland and Karl K. Turekian). Any inconsistency, error or slanted statement is responsibility of the author.

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Oxygen and the Culprit

 

Take a breath! Get your regular intake of oxygen and smoothly expel a whiff of carbon dioxide. The concentration of carbon dioxide in Earth's atmosphere is rising. In turn, the concentration of oxygen drops a tiny bit 

 

Back: "Two Billion Years Apart"

Oxygen, high and stable.

Nowadays we live in an oxygen rich world, and we take that for granted. A fifth of Earth's atmosphere is free molecular oxygen. This is a large reservoir of oxygen, which has not been altered much by the burning of fossil fuels. The waters of the world ocean are oxygenated down to the greatest depths. Oxygen concentration in the deep sea is sufficient to sustain animal life at the bottom of the deep sea.

Oxygen in the sea water is consumed by organic matter that is precipitating from the surface waters of the ocean. The oxygen minimum in the ocean is located at mid-depth. Oxygen is transported into the depth of the sea by lateral advection of oxygen rich waters. Both processes, the vertical precipitation of oxygen-consuming matter and the lateral advection of oxygen balance at mid-depth.

The sunrise - Romanian Black Sea Coast
Credit: Gerrit de Rooij (imaggeo.egu.eu)

The current global configuration of continents favours the oxygenation of the deep sea through slowly global overturning of water masses. Oxygen rich waters form at the surface of sub-polar seas. These waters sink to the bottom and spread into the world ocean. Finally, the waters well up at a place far away from their origin. Upwelling waters at mid-depth along the American east-coast are among the waters with the lowest concentration of oxygen. The ventilation shafts of the modern ocean are located in the sup-polar North Atlantic Ocean and the circumpolar Antarctic Ocean.

Apart from the general global pattern, today oxygen is absent in some parts of the global ocean only. Currently, the deeper layers of the Black Sea are free of oxygen because of the large inflow of terrestrial organic matter. Likewise, the bottom waters of some highly eutrophic coastal zones and waters close to hydrothermal vents are free of oxygen. In these waters, hydrogen-sulphur molecules (mainly hydrogen sulphide) are found instead of oxygen. Hydrogen-sulphide is toxic for oxygen-breathing life-forms. Ancient life-forms exist that dwell on hydrogen-sulphide. These life-forms are survivor of life-forms that populated Earth before photosynthesis started. Oxygen is toxic for these life-forms.

 

Oxygen, getting up and rise…

Since two Billion years, a substantial amount of free oxygen is found in the atmosphere and the ocean. Oxygen level increased first in the atmosphere and much later in the global ocean. However, free oxygen was very rare in the first half of Earth's history. The lasting switch from an oxygen-poor environment to the oxygenated environment happened two billion years ago. That event got named the "Great Oxidation Event".

...somewhere in Iran
Credit: Amirhossein Mojtahedzadeh (imaggeo.egu.eu)

Its geological marker are the first occurrence of reddish soils and disappearance of easily oxidized minerals in ancient stream beds. However the naming convention "Great Oxidation Event" seems misleading: the switch from an oxygen poor Earth to an oxygen rich Earth was more like a very long take-over battle than a rapid move.

Three Billion years before present, change had started. The atmosphere contained at least a very tiny amount free oxygen. However no oxygenation of the Earth had occurred yet. It possibly has taken one Billion years more to establish in the atmosphere a stable, albeit low level of free oxygen (< 1%). Then it took another Billion years to oxygenate the ocean and to push the concentration of oxygen in the atmosphere up to contemporary values of 21%.

 

The culprit: New life...

Colonization by lichens
Credit: Antonio Jordán (imaggeo.egu.eu)

There are forms of photosynthesis that do not produce oxygen as by-product. Only oxygenic photosynthesis is the sources of free oxygen. Oxygenation of the Earth developed with the evolution of the photosynthetic metabolism. Photosynthetic life-forms were the driver turning the switch to an oxygenated Earth. Algae and bacteria (prokaryote and eukaryote) living in the sea or at its shores were the culprits providing whiffs of oxygen. These algae and bacteria evolved from earlier life-forms using non-photosynthetic metabolisms and that were dwelling, for example, on hydrogen-sulphide. Today such life-forms still exist. They stay shut-in in peculiar environments because oxygen is toxic for them in the same manner as hydrogen-sulphide is toxic for photosynthetic life-forms.

For about two billion years the early life-forms of photosynthetic algae and bacteria survived in a hostile marine environment rich of hydrogen-sulphide. That long time span was needed that effective photosynthesis metabolisms evolved and oxygenation of the global ocean occurred. In that long period, sometimes called by earth scientists the "boring Billion", profound changes of Earth's geology and geochemistry occurred also.

Likely these changes helped establishing the geochemical cycles that keep free molecular oxygen in the atmosphere and the ocean at high levels: Hydrogen escaped into space. Volcanism occurred on land, easing that freshly vented hydrogen escaped into space. Tectonic reorganization of the continental plates modified the layout of the sea. Consequently circulation of water masses in the global ocean changed. Sedimentation basins opened and closed. Burial of organic-carbon in limestone and shale prevented rapid recycling of oxygen. Methane and iron pyrites got oxidized in the atmosphere and ocean, and the Earth's crust was enriched with oxidized minerals. Redox-sensitive trace-elements (chromium, molybdenum, manganese) got buried in the sediments.

Ukko El'Hob

This text together with the two related texts were inspired by the article “The rise of oxygen in Earth's early ocean and atmosphere” by Timothy W. Lyons, Christopher T. Reinhard and Noah J. Planavsky, which was published in February 2014 in Nature. Many insights are taken from “The global oxygen cycle “ by S.T. Petsch, which was published 2003 by Elsevier in volume 8 of in the “Treatise on Geochemistry” (Editor: William H. Schlesinger. Executive Editors: Heinrich D. Holland and Karl K. Turekian). Any inconsistency, error or slanted statement is responsibility of the author.

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Two Plays a Billion Years apart

Take a breath! Get your regular intake of oxygen and smoothly expel a whiff of carbon dioxide: The concentration of carbon dioxide in Earth's atmosphere is rising. In turn, the concentration of oxygen drops a tiny bit.

Currently, each year human economic activity converts fossil carbon into 7 Gigatons of carbon dioxide. Burning coal, oil and gas consumes oxygen and exhausts carbon dioxide. The amount of carbon dioxide exhausted each year adds up to about 1% of the carbon stockpiled in the atmosphere. That steady input of carbon keeps anthropogenic global change going.

Peaty soil in the Andean highland (Ecuador)
Credit: Martin Mergili (imaggeo.egu.eu)

How dramatically the current play of anthropogenic global change enfolds, that play is nothing compared to the play that had enfolded long time ago when oxygen got released into the environment. Some Billion years ago, the oxygen-producing photosynthesising life battle to master the world. 

Let's look at both plays, the ancient and the modern!  

 

Carbon, thumps up!

Most remarkably, since the industrial revolution set off two centuries ago the concentration of carbon dioxide in the atmosphere increased by 40%. Much of the carbon exhausted by human economic activity got neither absorbed in the biosphere or got stored in the deep ocean. The deep ocean is the main reservoir of carbon on Earth that could be recycled rapidly.

A black smoker in 3,000 meters depth
at the Mid-Atlantic Ridge.
Credit: MARUM - Center for Marine Environmental
Sciences (imaggeo.egu.eu)

In well less than a decade the biosphere would consume the total carbon-dioxide in the atmosphere through photosynthesis. A further decade would be needed to consume the carbon in the surface layer of the ocean. About three-hundred years would be needed to consume the carbon stored in the deep ocean. For humans, that is an elapse of time but for geological processes this is an extremely short period.

Respiration and microbial oxidation together are of a very similar strength than photosynthesis. Jointly these processes recycle rapidly the carbon that is available in the atmosphere. Recycling of the carbon stored in the ocean is a bit slower.

The annual anthropogenic input of carbon-dioxide into the atmosphere is several times bigger than the annual imbalance between photosynthesis, respiration and microbial oxidation. This makes the anthropogenic input of carbon into the atmosphere such a strong driver of change.

Oxygen, flat and stable... 

Carbon, down...

Oxygen, thumps up !

Redox processes causing a pattern of
red to greyish green mottles in a
Regosol, Ria Formosa, Portugal
Credit: Antonio Jordán (imaggeo.egu.eu)

Roughly two-and-half billion years ago, a different gas than carbon dioxide was driving global change. Free oxygen - two oxygen atoms binding in an oxygen molecule – caused troubles. Oxygen was exhausted into the environment by the first photosynthetic life-forms that had evolved on Earth because oxygen is a dangerous waste for their metabolism. Photosynthesis harvests ample energy from sunlight instead of using chemical processes, which are less efficient than photosynthesis and rely on more limited resources.

It is a fascinating play enfolded once photosynthesis evolved. Initially, oxygen increased in the atmosphere. The oxygen build-up in the ocean was delayed. Oxygen concentration increased to about-modern levels only two billion years after oxygen appeared for the first time. Finally, a network of multiple geochemical feedback-loops relate biosphere, oceans, atmosphere and the lithosphere.

 

Oxygen, high and stable…

Oxygen, getting up and rise…

The culprit: New life...

 

Finally, all oxygenated...

Finally, the photosynthesising algae and bacteria ruled the seas. Nearly 4 Billion years in its history, Earth switched into its present oxygenated stage. The play had been settled in favour for life as we know it. The Ediacaran [*] started 600 Million years ago; first complex animals and plants lived in a widely oxygenated ocean. To grow and store energy in their tissues, the plants tap into the ample energy that sunlight provides. Animals get sufficient oxygen supply to consume plant tissue vigorously. The continents were barren land, but that would change rapidly. A long-lasting phase of Earth history came to its end, oxygen and carbon-dioxide are taking over from sulphur, iron, hydrogen and methane.

Since the Ediacaran, the oxygen concentration in the atmosphere varied around present values. Feedback between geochemical cycles and the biosphere cause variations of the oxygen concentration, but keep its excursions limited. Up to recent times the same could be said for the carbon-dioxide concentrations; feedback between geochemical cycles and the biosphere kept excursions of carbon-dioxide concentration limited. Now humans have changed that balance and put on stage: anthropogenic global change. Human economic activity, burning fossil fuels to drive economy of billions of people, kick the feedbacks of carbon fluxes out of current balances. 

Ukko El’Hob

[*] from Wikipedia: “The Ediacaran Period (ca. 635-542 Mya) represents the time from the end of global Marinoan glaciation to the first appearance worldwide of somewhat complicated trace fossils (Treptichnus pedum). Although the Ediacaran Period does contain soft-bodied fossils, it is unusual in comparison to later periods because its beginning is not defined by a change in the fossil record. Rather, the beginning is defined at the base of a chemically distinctive carbonate layer that is referred to as a "cap carbonate", because it caps glacial deposits. This bed is characterized by an unusual depletion of 13C that indicates a sudden climatic change at the end of the Marinoan ice age.”

This text was inspired by the article “The rise of oxygen in Earth's early ocean and atmosphere” by Timothy W. Lyons, Christopher T. Reinhard and Noah J. Planavsky, which was published in February 2014 in Nature. Many insights are taken from “The global oxygen cycle “ by S.T. Petsch, which was published 2003 by Elsevier in volume 8 of in the “Treatise on Geochemistry” (Editor: William H. Schlesinger. Executive Editors: Heinrich D. Holland and Karl K. Turekian). Any inconsistency, error or slanted statement is responsibility of the author.