Earth’s breathable atmosphere is key for life, and a new study in Nature Geoscience by CLEVER Planets scientists James Eguchi, Rajdeep Dasgupta, and Johnny Seales suggests that the first burst of oxygen was added by a spate of volcanic eruptions brought about by tectonics, tying together the deep Earth and surface habitability.
The study offers a new theory to help explain the appearance of significant concentrations of oxygen in Earth’s atmosphere about 2.5 billion years ago, something scientists call the Great Oxidation Event (GOE).
Scientists have long pointed to photosynthesis as a likely source for increased oxygen during the GOE, but cyanobacteria were alive on Earth as much as 500 million years before the GOE. Then 100 million years later, a marked change in the ratio of carbon isotopes in carbonate minerals began. Geologists refer to this as the Lomagundi Event, and it lasted several hundred million years.
One in a hundred carbon atoms are the isotope carbon-13, and the other 99 are carbon-12. This 1-to-99 ratio is well documented in carbonates that formed before and after Lomagundi, but those formed during the event have about 10% more carbon-13.
Eguchi said the explosion in cyanobacteria associated with the GOE has long been viewed as playing a role in Lomagundi, because cyanobacteria’s preference for carbon-12 over carbon-13 causes a depletion of carbon-12 in the reservoir from which carbonates are being produced.
However, according to the geologic record, this increase in the carbon-13-to-carbon-12 ratio actually occurs up to 10s of millions of years after the GOE.
The scenario Eguchi, Dasgupta and Seales arrived at to explain all of these factors is:
- A dramatic increase in tectonic activity led to the formation of hundreds of volcanoes that spewed carbon dioxide into the atmosphere.
- The climate warmed, increasing rainfall, which in turn increased “weathering,” the chemical breakdown of rocky minerals on Earth’s barren continents.
- Weathering produced a mineral-rich runoff that poured into the oceans, supporting a boom in both cyanobacteria and carbonates.
- The organic and inorganic carbon from these wound up on the seafloor and was eventually recycled back into Earth’s mantle at subduction zones, where oceanic plates are dragged beneath continents.
- When sediments remelted into the mantle, inorganic carbon, hosted in carbonates, tended to be released early, re-entering the atmosphere through arc volcanoes directly above subduction zones.
- Organic carbon, which contained very little carbon-13, was drawn deep into the mantle and emerged hundreds of millions of years later as carbon dioxide from island hotspot volcanoes like Hawaii.
Eguchi said the study emphasizes the importance of the role that deep Earth processes can play in the evolution of life at the surface.
“We’re proposing that carbon dioxide emissions were very important to this proliferation of life,” he said. “It’s really trying to tie in how these deeper processes have affected surface life on our planet in the past.”
Read more about the study in the official Rice University press release, and check out the full paper in Nature Geoscience.