Ancient Zircons Help Reveal Early Earth Atmosphere


Roughly 2.4 billion years ago, Earth’s atmosphere underwent a huge change known as the “Great Oxidation Event”. This switch from an oxygen-poor to an oxygen-rich environment may be accountable for giving rise to life. However, scientists are extremely curious about what our atmosphere may have been like not long after our planet formed. Now researchers from the New York Center for Astrobiology at Rensselaer Polytechnic Institute are using some of the oldest minerals known to exist to help understand what may have occurred some five million years after Earth arose.

For the most part, scientists have theorized that early-Earth atmosphere was dominated by noxious methane, carbon monoxide, hydrogen sulfide, and ammonia. This highly reduced mixture results in a limited amount of oxygen and has led to a wide variety of theories about how life may have started in such a hostile environment. However, by taking a closer look at ancient minerals for oxidation levels, scientists at Rensselaer have proved the early-Earth atmosphere wasn’t like that at all… but held copious amounts of water, carbon dioxide, and sulfur dioxide.

“We can now say with some certainty that many scientists studying the origins of life on Earth simply picked the wrong atmosphere,” said Bruce Watson, Institute Professor of Science at Rensselaer.

How can they be so sure? Their findings depend on the theory that Earth’s atmosphere was formed volcanically. Each time magma flows to the surface, it releases gases. If it doesn’t come to the top, then it interacts with the surrounding rocks where it cools and becomes a rocky deposit in its own right. These deposits – and their elemental construction – allows science to paint an accurate portrait of the conditions at the time of their formation.

“Most scientists would argue that this outgassing from magma was the main input to the atmosphere,” Watson said. “To understand the nature of the atmosphere ‘in the beginning,’ we needed to determine what gas species were in the magmas supplying the atmosphere.”

One of the most important of all magma components is zircon – a mineral nearly as old as Earth itself. By determining the oxidation levels of the magmas that formed these ancient zircons, scientists are able to deduce how much oxygen was being released into the atmosphere.

“By determining the oxidation state of the magmas that created zircon, we could then determine the types of gases that would eventually make their way into the atmosphere,” said study lead author Dustin Trail, a postdoctoral researcher in the Center for Astrobiology.

To enable their work, the team set about cooking up magma in a laboratory setting – which led to the creation of an oxidation gauge to assist them in comparing their artificial specimens against natural zircons. Their study also included a watchful eye for a rare Earth metal called cerium that can exist in two oxidation states. By exposing cerium in zircon, the team can be confident the atmosphere was more oxidized after their creation. These new findings point to an atmospheric state more like our present day conditions… setting the stage for a new starting point on which to base life’s beginnings on Earth.

“Our planet is the stage on which all of life has played out,” Watson said. “We can’t even begin to talk about life on Earth until we know what that stage is. And oxygen conditions were vitally important because of how they affect the types of organic molecules that can be formed.”

While “life as we know it” is highly dependent on oxygen, our current atmosphere probably isn’t the ideal model for spawning primordial life. It’s more likely a methane-rich atmosphere might “have much more biologic potential to jump from inorganic compounds to life-supporting amino acids and DNA.” This leaves the door wide open to alternate theories, such as panspermia. But don’t sell the team’s results short. They still reveal the beginning nature of gases here on Earth, even if they don’t solve the riddle of the Great Oxidation Event.

Original Story Source: Rensselaer Polytechnic Institute News Release.

7 Replies to “Ancient Zircons Help Reveal Early Earth Atmosphere”

  1. I gently, with my Zircon encrusted tweezers, extract molecular evidence of solar evolution and it is good to know that sometimes it rains diamonds… sometimes stone… but mostly water. Thank you Mr. Sol! ~*~

  2. What really happened was that the rise of life accounted for the great oxygenation event and not vice-versa. However, the great oxygenation event did allow life to become more energetic and that, in turn, allowed the evolution complex multi-cellular life to occur.

    1. Good catch from you and Aerandir90 both! It could be that “complex” dropped out of the text.

  3. Big thanks for this! Having some sort of handle on testing early atmosphere composition after the Earth-Moon impactor is valuable.

    But, huge typ’Oh here:

    Now researchers from the New York Center for Astrobiology at Rensselaer Polytechnic Institute are using some of the oldest minerals known to exist to help understand what may have occurred some five million years after Earth arose.

    For a moment there I thought they had a take on early crust formation. Alas, it should be “some five hundred million years after Earth arose”.

    Back to the science:

    For the most part, scientists have theorized that early-Earth atmosphere was dominated by noxious methane, carbon monoxide, hydrogen sulfide, and ammonia.

    I think the linked news release put it better. An early reducing atmosphere was the original prediction after Millers’ experiments, since the organic productivity and species diversity is orders of magnitude larger.

    However observations of Venus and Mars atmospheres, observations of productivity under neutral conditions, and the discovery that local environments such as hydrothermal vents makes for reducing environments changed that. Background near neutrality is even beneficial as far as I know, since redox cycling would help set up drivers for pre- and protometabolism.

    So astrobiology texts typically cover all possible early atmospheres.

    What the news release put the finger on is the tendency to keep and extend reductive models, mainly because it makes for easier model work.

    The result ties really well into recent work on early gene families and the metabolic networks they solidified (from an RNA world take over) or evolved later. Enzymes handling _both_ hydrogen sulfide, locally available around volcanoes and hydrothermal vents, and sulfuric acid, producing or produced by sulfur dioxide, were early redox source utilizers. [“Rapid early innovation during an Archaean genetic expansion”, David & Alm, Nature 2010.]

    Invention using hydrogen sulfide trickled out early, while the oxidized sulfur compounds continued to be an important source for evolution between 4 to 3 Ga bp.

    [/goes away to look for Watson’s et al paper in Nature, a must read]

  4. “This switch from an oxygen-poor to an oxygen-rich environment may be accountable for giving rise to life.”

    What? I always used to think that the advent of plants caused the amount of oxygen in the atmosphere to proliferate, and that life existed before the atmosphere had all that oxygen.

    Also there’s a typo in this sentence:
    “To enable their work, the team set about cookinf up magma in a laboratory setting…”

    1. Plants did not evolve on Earth until the Silurian ~440 million years ago. Oxygen began to accumulate in our atmosphere ~2.8 billion years ago after the oxygen produced by phytoplankton (photosynthetic bacteria and protists) oxidized the Earth’s crust. Once the rocks on the surface were oxidized the oxygen concentration in the atmosphere and oceans could begin to increase.

  5. This is even more exciting than I originally thought.

    Their data stretches back to ~ 4.35 Ga bp [billion years before present] so includes earlier similar data implying a then available large water source. This again snuggles up against the new latest date of the Earth-Moon impactor, ~ 4.36 Ga bp. It may be that we have evidence of crust (re)formation in ~ 10 Ma.

    Which questions the current consensus of several tens or hundreds of million years for the original crust formation. There are competing hypotheses that crust solidified in ~ 10 Ma and explain the KREEP like nature of the initial crust. Unfortunately the balance of evidence seems to go against that hypothesis among the specialists.

    One can still relax this tentative problem by suggesting the Moon originated earlier. We need a more thorough redating of Moon samples. This would serve to test the new suggested Moon dating and seems to be necessary to resolve early Earth history by these atmosphere composition results.

    To get back to the question of Earth’s early oxidation state, it is a complex one.

    This can be illustrated by Mercury vs Venus. Findings of Mercury from Messenger and impact ejected meteorites is that it is and probably formed as highly reduced. Not surprising seeing that Mercury is situated close to the enormous hydrogen/proton source of the Sun.

    Venus is instead today mainly neutral by its massive CO2 atmosphere. One reason for that can be its leakage of hydrogen. That originates in UV hydrolysis of residual water vapor and is enabled by a stronger solar wind closer to the Sun and a weak planetary magnetic field.

    The further away you get from the Sun, the closer you get to have a supply of volatiles originating beyond the “snow line” whether originally or from later impactor delivery, the more of a neutral oxidation state you get. It may well be that Earth started out near neutral as one can expect.

    But also this: one way of getting rid of excess hydrogen, while keeping a hefty supply of water later on, is an original hydrogen escape atmosphere. Some controversial hypotheses combines a huge initial hydrogen pressure (on the order of ~ 30 %) with a hot atmosphere exobase to set off a runaway hydrodynamical non-thermal escape of hydrogen. (Which also carries other gases out and explain some of the lower pressure of Earth vs Venus.)

    Moreover: we have the Earth-Moon impactor. It was contributing its own volatiles. Then effectively throwing up much of the combined volatile supply and parceling some away in the Moon. Even if the bulk of that volatile mass was likely recaptured by Earth very soon.

    However I would think that a late Earth-Moon impactor and these results pushing up against that has interesting consequences. It implies the original state of the Earth and its atmosphere could become pretty much uninteresting as regards abiogenesis. If this will turn out to be the correct timeline, presumably Earth was near enough neutral at the time life could form.*

    Then locally reduced environments would be relatively massive contributors of organics and redox energy sources for chemical evolution and early metabolism both. There would be many of those associated with a recently reformed thin crust.

    * Likely then for the 2nd time, making Theia a biosphere killer impact.

Comments are closed.