Privileged Planet: new oxygen and subduction relationship (Introduction)

by David Turell , Friday, May 10, 2019, 21:44 (1806 days ago) @ David Turell

Further evidence about how newly arrived oxygen levels and subduction work together:

https://phys.org/news/2019-05-life-earth-affected.html

"It is well known that life on Earth and the geology of the planet are intertwined, but a new study provides fresh evidence for just how deep—literally—that connection goes. Geoscientists at Caltech and UC Berkeley have identified a chemical signature in igneous rocks recording the onset of oxygenation of Earth's deep oceans—a signal that managed to survive the furnace of the mantle. This oxygenation is of great interest, as it ushered in the modern era of high atmospheric and oceanic oxygen levels, and is believed to have allowed the diversification of life in the sea.

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"Earth is not thought to have always had an oxygenated atmosphere and deep ocean. Rather, scientists believe, the emergence of oxygen—and with it the ability for the planet to sustain aerobic life—occurred in two steps. The first event, which took place between about 2.3 and 2.4 billion years ago, resulted in a greater than 100,000-fold increase in atmospheric O2 in the atmosphere, to about 1 percent of modern levels.

"Although it was dramatically higher than it had previously been, the atmospheric O2 concentration at this time still was too low to oxygenate the deep ocean, which is thought to have remained anoxic until around 400 to 800 million years ago. Around that time, atmospheric O2 concentrations are thought to have increased to 10 to 50 percent of modern levels. That second jump has been proposed to have allowed oxygen to circulate into the deep ocean.

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"Their analysis revealed a distinct signature: a detectable increase in oxidized iron in bulk-rock samples between 800 and 400 million years ago, the same time interval that independent studies proposed the oxygenation of the deep ocean occurred.

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"Stolper and Bucholz additionally compiled another proxy also thought to reflect the oxidation state of the mantle source of arc magmas. Reassuringly, this independent record yielded similar results to the iron-oxidation-state record. Based on this, the researchers propose that the oxygenation of the deep ocean impacted not only on the earth's surface and oceans but also changed the geochemistry of a major class of igneous rocks.

"This work complements earlier research by Bucholz that examines changes in the oxidation signatures of minerals in igneous rocks associated with the first oxygenation event 2.3 billion years ago. She collected sedimentary-type, or S-type, granites, which are formed during the burial and heating of sediments during the collision of two landmasses—for example, in the Himalayas, where the Indian subcontinent is colliding with Asia.

"'The granites represent melted sediments that were deposited at the surface of Earth. I wanted to test the idea that sediments might still record the first rise of oxygen on Earth, despite having been heated up and melted to create granite," she says. "And indeed, it does."

"Both studies speak to the strong connection between the geology of Earth and the life that flourishes on it, she says. "The evolution of the planet and of the life on it are intertwined. We can't understand one without understanding the other," says Bucholz."

Comment: To support life a planet must have floating plates that allow the process of subduction. Just more evidence about how special the Earth is as planet that allows life to appear.