Buried oxygen rose to the occasion as Earth's early atmosphere formed

August 2, 2016 by Jim Shelton, Yale University
Credit: Yale University

Oxygen buried deep underground in minerals may have prompted the churning of Earth's rocky mantle billions of years ago and helped transform the planet's early atmosphere, according to a new study.

Research by geoscientists at Yale, Arizona State University, and Bayerisches Geoinstitut in Germany suggests that convection in Earth's —the slow movement of rocks circulating beneath the surface, caused by heat from inside the Earth—is affected by the distribution of in those minerals. The findings appear online Aug. 1 in the journal Nature Geoscience.

"When there's less oxygen present in the rock, it's denser than when there is more oxygen present, even though the rest of the elements are the same. The more oxidized rock preferentially rises over the reduced rock," said Kanani Lee, the study's principal investigator and an associate professor of geology and geophysics at Yale.

This process had consequences both above and below the surface. Deep below the surface, the more oxygen-depleted rocks sank to the bottom of the rocky mantle, leading to the creation of massive, dense piles just above the Earth's core such as those found deep beneath the Pacific and Atlantic oceans.

"This is the first time anyone has shown that the relative amount of oxygen deep in the Earth influences the minerals that rocks are made of and how it changes their densities," Lee said.

Tingting Gu, a former Yale postdoctoral associate and the paper's lead author, added, "The mantle is not entirely isolated from the . For example, gases from volcanic eruptions connect the mantle with the atmosphere. Our model predicts that early in Earth's history, the shallow mantle was less oxidized and thus released gases such as methane that would consume oxygen produced by photosynthesis. But as time progressed and the less dense oxidized material rose in the mantle, biotic oxygen could be preserved and accumulate in the atmosphere. This process could be unique among the terrestrial planets because of their different compositions."

Explore further: Geologist's discoveries resolve debate about oxygen in Earth's mantle

More information: [18] Redox-induced lower mantle density contrast and effect on mantle structure and primitive oxygen, Nature Geoscience, nature.com/articles/doi:10.1038/ngeo2772

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torbjorn_b_g_larsson
4.2 / 5 (5) Aug 02, 2016
So essentially you need an atmosphere where initial hydrogen can leak away and make oxygen from upper atmosphere water dissociation dominant (Earth), but not so leaky that the water supply is removed and the carbon dioxide atmosphere makes it a runaway hothouse (Venus).

If you have that, you will have good conditions for plate tectonics and continents (less dense upper mantle, continental crust).

So a geodynamo and/or a superEarth are good requisites for long term habitability.
Vidyaguy
1.8 / 5 (5) Aug 02, 2016
I think you missed the point...This has nothing to do with global warming, but may play into the scenario of the great Oxygen Event a couple of million years ago.
jonesdave
3.3 / 5 (7) Aug 02, 2016
I think you missed the point...This has nothing to do with global warming, but may play into the scenario of the great Oxygen Event a couple of million years ago.


???? I think you'll find that Torbjorn was not talking about current global warming. And the Great Oxygenation Event was ~ 2.5 billion years ago.
Vidyaguy
3 / 5 (2) Aug 02, 2016
Quite correct, and the oxygen balance, including both sources and absorption, was critical. I specifically stated that this has nothing to do with global warming; yet Torbjorn refers to it in the last sentence of his first paragraph, which is why I pointed that out. As to the oxygenation event being 2.3 billion years ago, I am quite familiar with it, but apologize for the typo (millions rather than billions). Torbjorn's observations - while accurate - still have little to do with this particular paper.
torbjorn_b_g_larsson
1 / 5 (1) Aug 03, 2016
jones, Vidya: Vidya's observations has little to do with my comment. I was commenting on the astrobiology potential for habitability on terrestrial planets. something this work seem to have implications for.

The "global warming" that will eventually sterilize Earth has little to do with the then CO2 free but still H2O filled N2 hothouse atmosphere. It will be forced by the Sun maturing towards the red giant stage with concomitant increased radiation flux. (That is why the CO2 will be gone, increased heat will mean increased weathering and locking up of the CO2 in minerals.)

Climate models are necessary to predict when that heat death happens. But it is tenuous to connect it to the greenhouse effect, and not at all applicable to the current global warming forcing, which is due to increased, not decreased, CO2 (and CH4) due to our social practices.

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