Missing Oxygen
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Missing Oxygen

Why Did Earth’s Oxygenation Take So Long?

Published in the EarthSphere Blog. Cover Image: Before the Oxygen by WM House; ArcheanArt)

Prologue

Previously in the Forgotten Origins series of articles, we discussed banded iron formations. The history of these formations tracks the development of Earths’ oxygen and raises questions about the 1.4 billion years between the first hints of free oxygen and the Great Oxidation Event between 2.4 and 2.3 billion years ago.


Science and Sulfur Isotopes

Science is not always big and bold. Sometimes discoveries lie in the small and unseen. When volcanic gases react with the atmosphere, the reactions produce certain sulfur isotopes, which eventually become incorporated into rocks. Atmospheric composition is important in determining the final form of these sulfur isotopes, and the presence of free oxygen causes a variation in the isotope’s final chemical structure. So, in the geological record, an atmosphere with oxygen is distinguished from an anoxic (oxygen-free) atmosphere by analyzing sulfur isotopes.

Genming Luo and his associates at MIT used this technology to analyze rock cores from South Africa. Their investigations let them determine when the transition from an anoxic atmosphere to an oxygenated one occurred. These MIT scientists placed the timing of the Great Oxygenation Event at 2.33 billion years. They also believed that full oxygenation of the atmosphere took about ten million years.

Wait a minute! If it only took ten million years for the transition to occur, what happened to the oxygen produced over the previous billion years?

Concentrations of chemicals in the atmosphere, including oxygen, represent a balance between sources and sinks. An example of sources and sinks occurs when fossil fuels burn and become a source of carbon dioxide (CO2). However, trees use CO2 to grow through photosynthesis; thus, trees are a sink. Blue-green algae was an active oxygen source in the early Archean, excreting lots of oxygen. If this oxygen wasn’t collecting in the atmosphere, there must have been a chemical sink removing it.

Volcanic Gases and Earth’s Mantle

A 2020 paper published in Nature (Shintaro Kadoya, et al.) proposes a solution to the missing oxygen mystery. The work by Kadoya indicates volcanic gases created the sink responsible for the missing oxygen. But that theory is just the tip of an iceberg whose roots extend deep below Earth’s surface. The mantle below Earth’s crust is the source of magma for volcanic hotspots like Iceland and Hawaii. Mantle plumes transport magma to Earth’s surface, where the lava flows from volcanos, spewing a broad range of noxious gases into the atmosphere.

Kadoya’s work focuses on research suggesting the mantle’s composition in the Archean Eon was different from today, and ancient Earth’s mantle contained less oxygen. A less-oxidized mantle produces higher volumes of gases prone to react with oxygen. Gases, like hydrogen, naturally combine with free oxygen, thus removing it from the atmosphere. He suggests that oxygenating the mantle took almost a quarter of Earth’s history — a time during which a fierce tug-of-war between biology and geology took place. For a billion years, gases from active volcanoes gobbled up all the oxygen the biosphere could produce. Only after the mantle was fully oxygenated did the oxygen-reactive gases stop flooding our atmosphere. This change allowed oxygen to accumulate, opening an evolutionary pathway for oxygen-breathing species to develop and thrive.

But the mantle oxygenation scenario is not a fact, only a theory. Another line of thought views the formation of supercontinents as the key for understanding the Great Oxygenation Event. This theory builds on the biological needs of single-celled organisms and pinpoints nutrient supplies as the critical link for understanding why Earth’s oxygenation took so long.

Plate Tectonics, Mountain Building, and Nutrients

One of the modern environmental problems we face is the ecologically destructive effects of algal blooms. Often, these algal blooms are initiated by excess pollution. The two usual culprits are agricultural runoff and waste treatment leakage from urban areas. Both sources deliver excess nutrients into our waterways. Under normal circumstances, algae populations are self-limiting because when they grow too large, the algae use all available nutrients. Once critical nutrients become scarce, the population declines.

Excess nutrients allow an algae population to keep growing. This growth sets up a chain reaction where so many algae are dying that the bacteria trying to digest the dead cells use all available oxygen in the water leading to a die-off of fish and other aquatic life. There is also a problem with the toxins excreted by the algae — surrounding aquatic life is poisoned and killed as algae levels increase.

But we can flip this situation around and examine what happens when nutrients are always scarce. This scenario leads to perpetually small populations of algae. There are never enough nutrients for a population explosion. Suppose these algae are the ones responsible for supplying oxygen. In that case, oxygen production is perpetually low, leading to alternating aerobic and anaerobic conditions and the deposition of banded iron formations, as discussed in the last article.

The collision of continental plates is one way to create a larger nutrient supply. These collisions create mountains, and as soon as mountains rise up, erosion starts wearing them down. Weathering and erosion of the mountain rocks create a supply of fresh nutrients for streams and rivers to carry to the ocean. The theory proposes a geologically quick release of nutrients, which sparked the growth of massive oxygen producing algae populations, eventually led to the Great Oxygenation Event.

(Excerpts from Vanishing Origins, read the book on Wattpad as it unfolds)

Or, see my current set of Medium articles as I chronologically trace the development of life on our planet: EarthSphere Page — Forgotten Origins


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Sources:

A billion years of missing oxygen (by WM House; ArcheanWeb)

Formation of supercontinents linked to increases in atmospheric oxygen (by Ian Campbell & Charlotte Allen; Nature Geoscience)

Banded Iron Formation (Source: American Museum of Natural History)

The Great Oxygenation Event — when Earth took its first breath (Source: Scientific Scribbles)

Earth as an Evolving Planetary System (Third Edition), 2016 — Banded Iron Formation (by Kent C. Condie, 2016)

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.