Tuesday, 18 March 2014

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.

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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