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