How life on Earth has affected its inner workings

It is well known that life on Earth and the geology of the planet are closely related, but a new study provides new evidence of the depth, literally, of the connection. Geoscientists at Caltech and the University of California at Berkeley have identified a chemical signature in igneous rocks indicating the onset of oxygenation of the Earth's deep oceans – a signal that has managed to survive in the furnace of the coat. This oxygenation is of great interest because it has marked the modern era of high atmospheric and ocean oxygen levels and would have allowed the diversification of life in the oceans.

Their conclusions, which were published in Proceedings of the National Academy of Sciences On April 11, support a fundamental theory of island magma arc geochemistry and offer a rare example of biological processes on the planet's surface affecting the inner Earth.

Island arches form when one oceanic tectonic plate slides under another during a process called subduction. The subduction plate descends and releases water-rich fluids into the underlying mantle, melting it and producing magmas that eventually reach the surface of the earth. This process builds island arctic volcanoes like those found today in the Japanese islands and elsewhere in the Pacific Ring of Fire. Finally, through plate tectonics, island arches collide with and join continents, preserving them in the rock archives over time.

The most abundant magmatic or igneous rocks are basalts, dark-colored and fine-grained rocks commonly found in lava flows. Today, most basalts on earth are not formed at the level of the island arcs, but rather at the level of the oceanic ridges located at depth. A well-known difference between the two is that island arc basalts are more oxidized than those found in oceanic ridges.

A fundamental but controversial hypothesis for this difference is that the oceanic crust is oxidized by oxygen and sulphate in the depths of the ocean before it is subducted in the mantle, delivering an oxidized material to the source of the mantle located above the subduction.

But we do not think that the Earth has always had an oxygenated atmosphere and a deep ocean. Instead, scientists believe that the emergence of oxygen, and hence the planet's ability to maintain aerobic life, has occurred in two stages. The first event, which occurred about 2.3 to 2.4 billion years ago, has resulted in an increase of atmospheric O2 in the atmosphere over 100,000 times, to reach about 1% of current levels.

Although it was considerably higher than it was before, the atmospheric concentration of O2 was still too low to allow oxygenation of the deep ocean, which would have remained anoxic up to about 400 to 800 million years. At about the same time, atmospheric concentrations of O2 are thought to have reached 10-50% of current levels. This second jump has been proposed to allow oxygen to circulate in the depths of the ocean.

"If the reason why modern island arcs are relatively oxidized is due to the presence of dissolved oxygen and sulphate in the deep oceans, it is an interesting predictive potential," says Daniel Stolper (Caltech Ph.D. & # 39; 14). the authors of the article and an assistant professor of earth sciences and planets at the University of Berkeley. "We know pretty well when the deep oceans have been oxygenated and so, if that idea is correct, we could see a change in how ancient island arc rocks oxidized before or after this oxygenation."

To investigate the signal of this oxygenation event in the igneous rocks of the island arc, Stolper is associated with Claire Bucholz, assistant professor of geology at Caltech, who studies ancient and modern magmatic rocks. . Stolper and Bucholz have screened the island's ancient arch archives and compiled geochemical measurements revealing the state of oxidation of the arc rocks appeared tens of millions or billions of years ago. billion years. Their idea was simple: if the oxidized materials on the surface are subducted and oxidize the mantle regions that are originally island arcs, the old island arcs should be less oxidized (and therefore "reduced") than their modern counterparts.

"This is no longer as common, but scientists routinely measured the state of iron oxidation in their rock samples," Bucholz says. "So there was a wealth of data that only needed to be reviewed."

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 as that according to independent studies have proposed. oxygenation of the deep ocean. To be deeper, the researchers also explored other possible explanations of the signal. For example, it is commonly accepted that the oxidation state of iron in loose rocks may be compromised by metamorphic processes – heating and compaction of rocks – or by processes modifying them at the surface of the earth. or near it. Bucholz and Stolper have developed various tests to determine if such processes have affected the disk. According to Bucholz, modifications have certainly taken place, but the changes are consistent wherever samples have been taken. "The amount of oxidized iron in the samples may have been displaced after cooling and solidification, but it seems to have been shifted similarly in all the samples," she says.

Stolper and Bucholz have also compiled another proxy, supposed to reflect the oxidation state of the mantle source of the magma arc. In a reassuring manner, this independent record has given results similar to those of the state of iron oxidation. On this basis, the researchers suggested that oxygenation of the deep seabed not only affected the Earth's surface and the oceans, but also altered the geochemistry of a large class of igneous rocks.

This work complements Bucholz 's previous research that examined changes in the mineral oxidation signatures in igneous rocks associated with the first oxygenation event 2.3 billion years ago. It collected sedimentary type or S-type granites, formed during sediment burial and sedimentation during the collision of two landmasses, for example in the Himalayas, where the Indian subcontinent is in collision with Asia.

"Granites represent molten sediments that have deposited on the surface of the Earth, and I wanted to test the idea that sediments could still record the first increase of oxygen on Earth, although they have been heated and melted to create granite, "she explains. "And indeed, that's the case."

Both studies speak of the close connection between the geology of the Earth and the life that develops there, she says. "The evolution of the planet and the life that lies there are intertwined – we can not understand one without understanding the other," Bucholz said.

the PNAS The study called "Early Proterozoic Early Phanerozoic Elevation of the Redox State of the Insular Archipelago because of Deep Ocean Oxygenation and Increased Levels of Marine Sulfates".