Earth's breathable atmosphere tied to plate tectonics?

Jun 20, 2014 by Johnny Bontemps,
A new study links continents and plate tectonics to the rise of oxygen on Earth. Credit: The International Space Station.

The rise of oxygen is one of the biggest puzzle in Earth's history. Our planet's atmosphere started out oxygen-free. Then, around 3.5 billion years ago, tiny microbes called cyanobacteria (or blue-green algae) learned out to carry out photosynthesis. They began using energy from sunlight to make their food from carbon dioxide and water, giving off oxygen as waste.

But it took another 3 billion years for to climb from trace amounts to at least 20 percent of the atmosphere, or high enough to support the emergence of complex life. And so far the mechanism behind that rise has remained unclear.

Now a new study by University of Exeter biochemist Benjamin Mills and his colleagues offers a new potential clue.

Using a computer model, they showed that may have indirectly fueled the sharp increase in oxygen between 1.5 billion and half a billion years ago. In particular, a process tied to the way continents remove carbon dioxide from the atmosphere may have increased the supply of phosphorus, a key nutrient for photosynthetic microbes in the ocean. The paper was published this month in the Proceedings of the National Academy of Science.

"This is a novel perspective for the late Proterozoic—a critical time of dramatic climate change, rising oxygen in the ocean and atmosphere, and origins and diversification of complex life," says Timothy Lyons, a biogeochemist not involved in the study.

From Seafloor to Terrestrial "Weathering'

Continents play a crucial role in the carbon cycle by removing from the atmosphere. Carbon dioxide mixes with rain water, forming a weak acid (carbonic acid) which slowly wears down or "weathers" rocks on land.

The process releases minerals such as calcium and magnesium from the rocks. These minerals then combine with carbonate and settle at the bottom of the ocean forming layers of calcium carbonate, or limestone.

In other words, the weathering process simply pulls carbon from the atmosphere and turns it into a layer of sediment on the seafloor.

However, continental rocks aren't the only route by which carbon is removed from the atmosphere. Ocean ridges, the places where fresh crust is made on the seafloor, can undergo a similar "weathering" process. In fact, seafloor weathering was the main route of carbon removal in the early chapter of Earth's history, before the formation of continents.

Earth’s breathable atmosphere tied to plate tectonics?
The chemical weathering of rocks releases calcium ions in rivers and oceans, where they react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes limestone. Credit: Greg Carley

According to the new study, the rise of oxygen may have been due to a shift in balance between the two processes—between seafloor and continental weathering.

Potential Culprits

What caused that shift? The model looked at two factors: a brighter sun, and a slowdown in fresh crust production.

Our sun has slowly been getting brighter. It's now 20 to 30 percent brighter than when our Earth first formed. Because the weathering process depends on temperature, a brighter Sun may have sped up the process on land. What's more, the amount of fresh ocean floor formed has slowed down over time. And the weathering process generally happens with the newer crust. Taken together, these two factor may have shifted the balance between the seafloor and the terrestrial process.

The Phosphorus Boost

But why does that shift matter? "Rocks on continents contain phosphorus, which is a key limiting nutrient for photosynthetic microbes," Mills says.

The terrestrial weathering increases the amount of phosphorus in streams and rivers, and ultimately in the ocean. The amount of phosphorus then dictates how much photosynthesis, hence how much oxygen is produced.

"The paper a great step forward," says Lyons. "The fundamental mechanistic perspective, particularly the co-consideration of seafloor and continental processes, is broadly relevant and clever."

One drawback, though, Lyons says, is that the model doesn't account for the shorter-term variations of oxygen's up and down. "The model, as proposed, isn't able to explain the details of the transition," Lyons adds. "But overall it still support the long term increase in oxygen."

Earth’s breathable atmosphere tied to plate tectonics?
A close-up of the cyanobacteria producing oxygen. Credit: Tanja Bosak, MIT

Implications for Astrobiology

The study provides an indirect link between plate tectonic and continents on one hand and the evolution of on the other, an idea worth keeping in mind in the search for life beyond our world.

"This is not the only reason oxygen rose to high levels, but it seems to be an important piece of the puzzle. Whilst the can function without large continents, it seems that their emergence was critical to our own evolution," Mills says in a news release.

Mills later adds in a phone interview:

Earth’s breathable atmosphere tied to plate tectonics?
A shift from seafloor to continent weathering may have indirectly caused the rise of oxygen, which then led to the emergence of complex life on Earth.

"A large number of key limiting nutrients, and not just phosphate, come from the continents. It seems that to develop a biosphere like we have on this planet, you're going to need significant continental area."

In fact, the recycling of continents via plate tectonics has become of major interest for many astrobiologists. Several have argued that, along with water, plate tectonics could be an essential requirement for life.

"What I like in particular is the rigorous links between tectonic drivers and (and life by association), which must be a considered in any view of extrasolar planets and their ability to sustain life through nutrient balances—with oxygenation as a possible consequence," Lyons adds. "Plate tectonics and relationships to nutrient cycling, phosphorus in particular, should be an essential part in any exploration for life—on the early Earth and farther from home."

Explore further: Earth's breathable atmosphere a result of continents taking control of the carbon cycle

More information: Benjamin Mills, Timothy M. Lenton, and Andrew J. Watson. "Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering." PNAS 2014 ; published ahead of print June 9, 2014, DOI: 10.1073/pnas.1321679111

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4.3 / 5 (4) Jun 20, 2014
"But it took another 3 billion years for oxygen levels to climb from trace amounts to at least 20 percent of the atmosphere,".

We have no evidence that cyanobacteria existed 3.5 billion years ago, and I think the geological record constrains any large oxygen production before 2.5 billion years ago as only localized pockets can be found. Dated phylogenies prefer to put the cyanobacteria stem at 3 billion, and the oxygenating photosynthetic among them right before the oxygenation of the atmosphere.

The 3.5 Ga bp bacterial mats (MISS) and stromatolites are certainly signs of bacteria communities, but I think even today some of the deep lake ones are devoid of cyanobacteria. It is an ecological nich, and specifically then not a clade trait.

That doesn't tell against the suggested mechanism as such, or the putative role for plate tectonics. But habitability isn't just about having enough energy for heterotrophs (and so perhaps complex multicellular life). Autotrophs are life too.
4 / 5 (5) Jun 20, 2014
We have no evidence that cyanobacteria existed 3.5 billion years ago
Um, yes actually we do: http://www.ucmp.b...ofr.html and;f=false for example.

and I think the geological record constrains any large oxygen production before 2.5 billion years ago as only localized pockets can be found
5 / 5 (2) Jun 20, 2014
@Maggnus: Um, no actually we don't: Schopf's fossils were rejected by Brasier et al already -06.

Everybody knows that! =D Or at least should know that they are controversial. ["A fresh look at the fossil evidence for early Archean cellular life", Brasier et al, Phil. Trans. R. Soc. B. A key paper, setting the bar.]

The accepted such old fossils after that were the stromatolites themselves, and lately Hazen et al MISS AFAIK. Besides, even if those fossils would have the same traits as cyanobacteria, they wouldn't test for them.

The first complete cyanobacteria phylogeny is dated, and is able to robustly push their origin as far back as 2.45 Ga bp for the first time. They sum up the record: "the earliest unequivocal cyanobacteria fossils date back 2.0 Bya and come from two localities". ["Evolution of multicellularity coincided with increased diversification of cyanobacteria and teh Great Oxidation Event", Schirrmeister et al, PNAS EE 2013]

5 / 5 (2) Jun 20, 2014

The rise of cyanobacteria coincides with manganese use and ultimately fixation to instead oxidize secondary products, ultimately water as in cyanobacteria. That follows after iron use for early photosynthesis. (First likely as independent oxidizing and photoreaction sites, still seen in Fe using prokaryotes of some deep and nutrient scarce fresh water lakes.)

Soluble Fe2+became scarce as it was used up to become insoluble Fe3+ (Fe3O4), but Mn2+ could stay in solution longer. Notably the first Mn-oxidizers are believed not to be cyanobacteria, since the photosystem is different. [Manganese-oxidizing photosynthesis before the rise of cyanobacteria", Johnson et al, PNAS EE 2013]

5 / 5 (2) Jun 20, 2014

They look at the oxygen record: "Some have postulated the existence of "oxygen oases" adjacent to cyanobacterial mats before the rise of oxygen, in which oxygen production could exceed the vanishingly low global O2 average (56). The observation of Mn, redox-sensitive detrital grains, and anomalous multiple sulfur isotope fractionations within Koegas Subgroup facies model do not support the existence of such oases in this sedimentary basin." [Goes on to describe how this evidence is extensive in space, tens of km, and in time, hundreds of m of sediments, including deep sea stratigraphy.]

So maybe not even pockets of oxygen. (The anomalous sulfur record seems iffy to use, btw, I wouldn't trust that.)

This is 2.415 Ga bp. Either one of these papers is erroneous. Or the cyanobacteria started to produce oxygen right before the global oxygenation, after other oxidizers emptied out the Fe and had started in on the remaining Mn.
5 / 5 (2) Jun 20, 2014
I can not not return to point out Brasier et al key paper. Schopf et al were early astrobiology enthusiasts under NASA, and they didn't use testing but pattern recognition.

The replacement with science cleared up the field and is why for example the recent "tracks in martian meteorites" is controversial: the only accepted bacterial tracks in Earth samples are _volcanic glass_, where contenders can't be found. Other tracks can be corrosion related, for example.

The methodology was also instrumental in rejecting the Viking experiments. And is now used in Curiosity. E.g. the found organic carbon isn't pattern match accepted, the null hypothesis that it is volcanic/impact generated can't be excluded at the currently found low levels.

5 / 5 (1) Jun 20, 2014
Well known for a long time to people who are aware of the Rare Earth Hypothesis.

Not new stuff. There are actually a huge number of variables that contribute to or limit any planet's habitability. It's just recently that "mainstream" science has been dragged kicking and screaming to face some sobering facts about the subject.
4.2 / 5 (5) Jun 20, 2014
Torbjorn, I think you might be referring to this? file:///C:/Users/owner/Downloads/32bfe51127a43b0d5a.pdf

It's an interesting debate they have going, and whether or not you agree with Schopf et al, to out of hand dismiss them as having been proven wrong is disingenuous. Even Pinti et al and Marshall et al give some additional credence to their initial findings, and Schopf has published as recently as 2013 in support of his initial findings.

But yes, certainly controversial and I have obviously fallen behind on the subject! :)
5 / 5 (1) Jun 20, 2014

Regarding Bill Schopf's mistaken micro fossils in the Apex Chert, Robert Hazen goes over this even handedly and in detail in Ch 5 of his excellent lecture series, The Origins Of Life, with the advent of new 3D scanning. The new shapes we given names like "the wrong trousers". The final conclusion is that 'we really don't know the age of the Apex Chert.
5 / 5 (2) Jun 20, 2014
@Maggnus, can you provide us with a better URL?

It's not my area, but it's interesting none the less. From the article, the sharp rise in Oxygen came about 1.5 billion years ago. And that was some how linked to the an uplifting of Phosphorous at the boundaries of the continental plates? Ok. I was always taught that the rise of Life caused the dramatic increase in Oxygen. How can this plate tectonics cause the rise in O2.
5 / 5 (1) Jun 20, 2014
In March 7th 2002 issue Nature ran both arguments. Prof Robert Hazen brings the lively public debate to life with all the human drama that makes science such fun.
3 / 5 (2) Jun 21, 2014
@Maggnus, can you provide us with a better URL?
I just copied and pasted it in the search bar and it worked, but try here: http://www.ncbi.n...14979661 or here: http://link.sprin...62244.d3
1.5 / 5 (2) Jun 22, 2014
Viewing the lithospheric evolutionary interaction with the biosphere leads to a yet higher order sequence of evolution that V.I. Vernadsky pioneered wherein the Noosphere or the age of human creative intelligence uniquely subsumes evolution itself.
5 / 5 (1) Jun 22, 2014
Viewing the lithospheric evolutionary interaction with the biosphere leads to a yet higher order sequence of evolution that V.I. Vernadsky pioneered wherein the Noosphere or the age of human creative intelligence uniquely subsumes evolution itself.

And that's fine, but what does that have to do with atmospheric alteration, whether by geophysical process or biological ones 3.5 to 2.7 BYA? Staying on topic is considered polite.
not rated yet Jun 23, 2014
If it is not cyanobacteria, then it must be diatoms. Look at the massive amount of diatomaceous earth around the world and sea floor, you will know how much of these algae contributed to the earth's atmosphere (considering every 1kg of CO2 produces 0.55 kg of algae biomass).