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    geomancer's Avatar
    geomancer
     

    Did volcanoes cause the Great Oxidation Event?

    https://www.cosmosmagazine.com/node/4852/full

    [The Great Oxidation Event marks a relatively short episode when significant amounts a oxygen first appeared in Earth's atmosphere sometime around 2.7 to 2.5 billion years ago]

    Did volcanoes cause the Great Oxidation Event?
    Monday, 17 October 2011
    by Becky Crew

    Cosmos Online

    SYDNEY: The Great Oxidation Event, a milestone in the history of life that saw the first appearance of oxygen (O2) in the Earth's atmosphere, was the result of geological processes that could one day be mimicked on Mars.

    The question of what caused the Great Oxidation Event 2.45 billion years ago, which paved the way for the evolution of the first complex animals such as the earliest fish, snakes and dinosaurs, has remained one of the biggest puzzles of the early Earth.

    For years it's been suggested that the oxygenation of the atmosphere was caused mainly by biological processes such as the production of oxygen through photosynthesis by sulphur-breathing marine organisms called cyanobacteria. But new research has suggested that geological processes were the main factor, with falling sea levels and a radically changing landscape where continents were forming and scores of volcanoes were rising out of the oceans.

    "The first appearance of oxygen in the atmosphere at the end of the Archean (2.5 million years ago) has profoundly changed biogeochemical processes on the Earth. It is probably the biggest change that occurred on the Earth's surface," said lead author Fabrice Gaillard from the French National Centre for Scientific Research of the paper published in the current issue of Nature.

    "We show that oxygenation of the atmosphere is mainly a geological process that can be explained by the appearance of subaerial volcanism, which, in turn is due to continent emergence."

    How did oxygen accumulate?

    While atmospheric oxygen (O2) constitutes 21% of the Earth's current atmosphere, for half of the planet's history its atmosphere contained no O2 at all, and was instead dominated by CO2. Scientists have increasingly attributed the first significant build-up of oxygen in the atmosphere to the appearance of cyanobacteria, which produced oxygen through photosynthesis around 200 million to 300 million years before the Great Oxidation Event (GOE).

    But this oxygen failed to accumulate in the atmosphere until suddenly during the GOE, so some other process must have triggered this accumulation. Given the available evidence, this conundrum has proved difficult for scientists to solve, but now Gaillard and his team have proposed a new model, based on previous thermal and chemical models, which suggests that a rise in the Earth's crust formation could be the key.

    "Debates have been going on for years, swinging back and forth from biological to geological factors, but this [study] points to a real mechanism, that seems to be consistent with the geological record," commented Malcolm Walter from the Australian Centre for Astrobiology at the University of New South Wales in Sydney.

    Time of the rising volcanoes

    During the Archean eon, which spanned from 3.8 billion to 2.5 billion years ago, oceans covered most of the Earth, and only a few continents had emerged from the sea. According to Gaillard, the absence of modern plate tectonic processes in this early Waterworld was due to the fact that the Earth's crust was still quite hot and soft. "[it was] unable to amalgamate as thick continent - like a sort of creamy camembert," he said"

    In the late-Archaen, 2.7 billion years ago, the biggest event of continental crust formation occurred. More continents were able to form and built up to around 30% of the Earth's surface, which is similar to the current percentage. This tectonic shift caused underwater volcanoes to rise up above the surface - known as subaerial volcanism - which occurred increasingly between 2.7 billion and 2.5 billion years ago.

    Not just the bugs

    The subaerial volcanoes were now releasing their magmatic volatiles - the gases present in magma, such as carbon dioxide, sulphur and chlorine - into the atmosphere instead of the oceans, at a significantly lower pressure.

    The change in pressure caused a change in the oxidation state of the sulphur in these gases, which saw a decrease in hydrogen sulphide and increase in sulphur dioxide (SO2). In this form, there could have been an increased level of dissolution of sulphate in the oceans, which fed sulphate-reducing cyanobacteria, resulting in an increase in atmospheric oxygen.

    "So it may not be the 'bugs' (cyanobacteria) that were so important, in the oxygenation of the atmosphere during the GOE, but terrestrial volcanism. I'm sure the topic will be debated, but this is something well worth taking on board," commented Patricia Rich from Monash University in Melbourne.

    Life on Mars?

    Rich added that perhaps both the rise of cyanobacteria and terrestrial volcanism worked in combination to create an atmosphere that allowed complex life forms to gradually develop hard shells and skeletons to appear as the 'first animals' - Ediacarans and Vendians - 600 million years ago.

    "When shells and skeletons began to appear ... our record of multicellular organisms literally did get magnificently enhanced for the first time in the history of life," she said.

    "Because of our more quantitative approach, our work better defines the sequence of events conducive to oxygenation of the atmosphere," added Gaillard. "Oxygenation of the atmosphere might then occur on other planets without requirement of biology. Mars for example has an oxygenated atmosphere, Venus, not."
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