Tuesday, 18 March 2014

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 

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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