Deep Burial of Organic Carbon into the Mantle Enabled “Great Oxidation Event”

The “Great Oxidation Event” (GOE) occurred about 2.5–2.2 billion years ago, when oxygen levels in the atmosphere began to rise up to 10,000 times, reaching up to 1% of the present level.

The rise across GOE and the subsequent increases in Earth’s atmospheric oxygen enabled the emergence of multicellular organisms, finally leading to plants, dinosaurs, and primates. A combination of factors led to the GOE, including the actions of oxygen-producing cyanobacteria in the oceans and the burial of dead organic matter.

A new paper by DCO Extreme Physics and Chemistry and Reservoirs and Fluxes Community member Rajdeep Dasgupta (Rice University, USA) and Megan Duncan (now at the Carnegie Institution for Science, USA) proposes that subduction, the descent of the tectonic plates into the mantle, was a key process in the efficient, long-term storage of reduced carbon, making the GOE possible. Using high-temperature and high-pressure experiments combined with thermodynamic models, the researchers looked back in time to model how carbon ‘burial’ would impact atmospheric oxygen levels. They describe their findings in a new paper in the journal Nature Geoscience [1].

“If organic carbon stays only in near surface environments, there are a host of processes that can decompose that organic matter and release carbon back into the atmosphere,” said Dasgupta. “We thought, if there is a way to subduct organic carbon deep into the mantle, then it is sequestered out of the surface pool for a long period of time, and that can explain not only the rise of oxygen but also the stability and higher level of oxygen from that point onward.”

One way carbon is brought back to the surface is through the outgassing at volcanoes. To estimate the impact of volcanic eruptions, especially at subduction zones, Duncan and Dasgupta measured the ability of water-bearing, silica-rich magma that forms from the downgoing plate to dissolve graphite, the remnants of organic carbon. Even under the hotter conditions of early subduction zones, the magma did not dissolve enough of the carbon to prevent it from becoming buried deep in the mantle.

Duncan and Dasgupta found they could model the rise of oxygen by taking into account the efficiency of deep carbon burial into the mantle and the plausible rates of subduction through time.

“Subduction is a key way to add carbon to the mantle fairly early in Earth’s history,” said first author Megan Duncan. Knowledge of this process can reveal details of what occurred in the mantle through time, including the formation of diamonds.

People have discovered diamonds dated to 2–3 billion years ago with subduction signatures associated with light carbon isotope signatures, a potential indication that the diamond formed from once-living cells. Early subduction carried the organic matter of single-celled organisms into the mantle, the carbon turned into graphite and then became diamonds at depths greater than 120 km.

The study is one of the first to link subduction and deep storage of reduced carbon with the rise of atmospheric oxygen, which made Earth habitable for larger organisms, including humans. “This whole mechanism doesn’t work without the onset of subduction,” said Duncan. “The study indicates that the rise in oxygen levels and the ability of the atmosphere to support life as we know it is strongly tied to the plate tectonic cycle.”

This diagram shows how organic carbon can be stored through deep subduction, a process that may have locked up large amounts of reduced carbon in Earth's mantle, enabling the build up of oxygen in the atmosphere and giving rise to the "Great Oxidation Event," about 2.4 billion years ago. Credit: Image by Raj Dasgupta, courtesy of Rice University
 
Rajdeep Dasgupta. Credit: Jeff Fitlow/Rice University
 
Megan Duncan. Credit: Jeff Fitlow/Rice University

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