Changes in Earth's crust caused oxygen to fill the atmosphere

September 18, 2017
Matthijs Smit of the University of British Columbia examines ancient rocks from the deep crust in Norway during the summer of 2017. Credit: Matthijs Smit

Scientists have long wondered how Earth's atmosphere filled with oxygen. UBC geologist Matthijs Smit and research partner Klaus Mezger may have found the answer in continental rocks that are billions of years old.

"Oxygenation was waiting to happen," said Smit. "All it may have needed was for the continents to mature."

Earth's early atmosphere and oceans were devoid of free , even though tiny cyanobacteria were producing the gas as a byproduct of photosynthesis. Free oxygen is oxygen that isn't combined with other elements such as carbon or nitrogen, and aerobic organisms need it to live. A change occurred about three billion years ago, when small regions containing free oxygen began to appear in the oceans. Then, about 2.4 billion years ago, oxygen in the atmosphere suddenly increased by about 10,000 times in just 200 million years. This period, known as the Great Oxidation Event, changed on the surface of the Earth completely.

Smit, a professor in UBC's department of , & atmospheric sciences, and colleague, professor Klaus Mezger of the University of Bern, were aware that the composition of continents also changed during this period. They set out to find a link, looking closely at records detailing the geochemistry of shales and igneous types from around the world—more than 48,000 rocks dating back billions of years.

"It turned out that a staggering change occurred in the composition of continents at the same time free oxygen was starting to accumulate in the oceans," Smit said.

Before oxygenation, continents were composed of rocks rich in magnesium and low in silica - similar to what can be found today in places like Iceland and the Faroe Islands. But more importantly, those rocks contained a mineral called olivine. When olivine comes into contact with water, it initiates chemical reactions that consume oxygen and lock it up. That is likely what happened to the oxygen produced by cyanobacteria early in Earth's history.

However, as the continental crust evolved to a composition more like today's, olivine virtually disappeared. Without that mineral to react with water and consume oxygen, the gas was finally allowed to accumulate. Oceans eventually became saturated, and oxygen crossed into the atmosphere.

"It really appears to have been the starting point for life diversification as we know it," Smit said. "After that change, the Earth became much more habitable and suitable for the evolution of complex life, but that needed some trigger mechanism, and that's what we may have found."

As for what caused the composition of continents to change, that is the subject of ongoing study. Smit notes that modern plate tectonics began at around the same time, and many scientists theorize that there is a connection.

The study is published in Nature Geoscience.

Explore further: How continents were recycled

More information: Primitive continents suppressed Earth's early O2 cycle, Nature Geoscience, DOI: 10.1038/ngeo3030

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1365logan
3.5 / 5 (2) Sep 18, 2017
So now we are supposed to ignore Iron in the Oceans bonding with Oxygen produced by Stromatolite.

I don't think that a hypothesis such as this one presented scales as well as the Stromatolite theory. Nor will it have such wide acceptance. There is just too much red earth and sandstone on the planet supporting Stromatolite oxygenation of the oceans.

My Grandfather Dr. Francis Morris Van Tuyl postulated the oxygenation of Iron back in the 1950's as the head of Geology at the Colorado School of Mines in Golden Colorado. The Stromatolite theory came much later after his death in 1974.

Once the Oceans were completely enriched with oxygen from the Stromatolites the O2 then filled the earths atmosphere. There may be some room for the argument Professor Smit presents after the oceans were depleted of Iron. As evidenced by the fact the subduction has erased a large volume of evidence of Stromatolite fossils. Which are in fact the oldest fossil records on the planet.
shoshonite
5 / 5 (1) Sep 18, 2017
Olivine is full of oxygen. The oxygen in olivine is coordinated with silicon in a high ratio. As the degree of coordination between oxygen and silicon decreases oxygen goes to excess. The ratio of 4:1 oxygen to Silicon in "primordial" rock containing olivine, pyroxene etc. goes to 2:1 in a stepwise fashion. Over time, igneous differentiation leads to more siliceous end member mineral aggregations....such as the feldspars and quartz...(SiO2. Where does the oxygen go as it is mineralogically excluded from coordinated silicon-oxygen sites? It moves to the hydrosphere, the atmosphere and finally to the biosphere. (I was at Mines too..but I missed your Grandfather...Finny was head when I finished my MS there.) I came up with this coordination chemistry idea from my classes with Slaughter in Silicates)...I've never seen it in print but it makes sense...given the mineral suites associated with magmatic evolution.
Jonseer
5 / 5 (4) Sep 18, 2017
So now we are supposed to ignore Iron in the Oceans bonding with Oxygen produced by Stromatolite.....

Your problem is you immediately and without any need adopt an absolutist perspective and wrongly assume the conclusions made by this study present an either/or choice for the increase in oxygen.

They did no such thing.

All they did was say the evidence points to certain circumstances occurring that helped to dramatically increase the amount of oxygen in the air.

Disproving the role of iron in that was not the point or part of their study. So it's not mentioned.

The only place this study does that is in your mind.
KBK
not rated yet Sep 22, 2017
It's always interesting to consider that massive levels of ubiquitous oxidation are necessary for life on earth as we know it.

Yet we think of this place as stable and neutral. Nothing could be further from the truth.

All a matter of perspective.

So, yes, we are upsetting the earth's balance. A balance of oxidation states, balanced on the head of a pin, in some ways. A balance that is more fragile and active than most think.
katesisco
not rated yet Sep 25, 2017
Iron may have been extraterrestrial. A shellacking could have cut off olivine and stopped O2 consumption. Now we need to decide where the iron came from.

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