One reason Earth is a habitable planet with a breathable atmosphere is thanks to the evolution of microbes that take in carbon dioxide and pump out oxygen. Some people credit these microbes with causing the Great Oxidation Event (GOE), a rapid spike in oxygen levels that occurred about 2.4 billion years ago. But these organisms evolved about 500 million years before the GOE, suggesting that some other factor likely triggered the sharp increase. Now, a new paper in Nature Geoscience, finds that deep Earth processes were behind this formative event in Earth’s history.
DCO members James Eguchi (previously at Rice University but now at the University of California, Riverside, USA) and Rajdeep Dasgupta, with Johnny Seales (both at Rice University, USA) propose that tectonic activity can explain not only the GOE but also a subsequent odd event called the Lomagundi carbon isotope excursion, when a global shift in carbon isotopes occurred. The researchers find that an increase in volcanic carbon emissions created the GOE, and that the different ways that the resulting glut of carbon was recycled through the deep Earth can explain the Lomagundi event.
“Deep processes occurring in the mantle specifically related to carbon can actually play a big role in the evolution of surface habitability, [which is recorded in] the geologic records,” said Eguchi. “It’s important to try to tie the deep processes to the surface [ones].”
The less well-known Lomagundi event occurred about 100 million years after the GOE and persisted for a few hundred million years. During this time, carbonates that formed at the surface had unusually high concentrations of 13C, an isotope of carbon that has an extra neutron in the nucleus and accounts for about one percent of Earth’s carbon. Carbonates typically have a higher concentration of 13C compared to organic carbon, because microbes and other life forms that generate organic carbon prefer to consume 12C, the most common isotope. During the Lomagundi event, however, carbonates had about 10 percent more 13C than carbonates that formed earlier.
Using previously collected isotope data, the researchers modeled carbon fluxes in and out of the subsurface at that time. Scientists think that changes in tectonic activity caused an increase in the release of carbon dioxide from volcanoes just before the GOE. Based on their modeling, the researchers present a scenario where the excess carbon dioxide caused the climate to warm, leading to greater amounts of rainfall and weathering of the rocky continents. As the eroded minerals washed into the oceans, they fertilized the growth of microbes. It was likely this microbial boom that pumped out enough oxygen to cause the GOE.
The runoff from the erosion also likely caused the buildup of carbonates in sediments at the bottom of the ocean. Along with organic carbon from dead microbes, the inorganic carbon in the carbonates would have been carried into the subsurface at subduction zones, where oceanic plates sink between continental plates.
At subduction zones is where the paths of the two types of carbon diverge. Studies suggest that carbonate minerals melt when they enter the mantle and escape back to the surface through arc volcanoes, which line the subduction zone. Organic carbon from dead organisms however, turns to graphite, which is more stable. Graphite can sit in the mantle for hundreds of millions of years before it returns to the surface, possibly carried by a hot plume from the deep mantle that erupts to form island hotspot volcanoes, such as in the Hawaiian Islands.
The timing of the two different recycling processes may explain the Lomagundi event. Since carbonates are rich in 13C and emerged first, they could have created the shift in isotope concentrations that geologists have observed. When the 12C-rich carbon from graphite returned to the surface later on, isotope levels came back down to normal.
“I’m interested to see if we can apply this association between oxidation events and carbon isotope excursions at other times in Earth’s history,” said Eguchi. The GOE wasn’t Earth’s only oxidation event, just the most dramatic. Another example is the Neoproterozic oxygenation event, which occurred between 850 and 540 million years ago. It was this event that oxygenated the deep oceans and allowed oxygen to accumulate to levels that support more complex animal life.
Eguchi is continuing his research into the connections between mantle and surface processes as a postdoctoral research at the University of California, Riverside. Currently he is exploring carbon isotopes in different volcanic settings to see if more organic carbon erupts at hotspot volcanoes and if more carbonates erupt at arc volcanoes.