Natural processes that move carbon through Earth, like plate tectonics, subduction, and volcanic activity, seem old and stable today, but the planet wasn’t always so calm. During the Archaean eon (4 to 2.5 billion years ago) – Earth’s teenage years – the planet was hotter and had more volcanic eruptions. This eon ended with a rise in atmospheric oxygen called the Great Oxidation Event, which was the very beginning of the more mature plate tectonics that we recognize today.
In a new paper in Nature, a team of researchers, including DCO Reservoirs and Fluxes Community members Bernard Marty and Michael Broadley (CRPG-CNRS, France) and Executive Committee member Claude Jaupart (Institut de Physique du Globe de Paris, France), detected a sharp rise in volcanic activity between 2.6 to 2.2. billion years ago that likely released a burst of water, carbon dioxide, and other volatile compounds into the atmosphere . The researchers analyzed traces of Archaean atmosphere trapped within fluid-filled bubbles called inclusions inside rocks that formed at that time. They propose that this massive flux of volatiles may have started the Great Oxidation Event.
“We don’t know much about the Archaean eon, 3 billion years or so ago,” said Marty, co-Chair of the Reservoirs and Fluxes Community Science Steering Committee. Few rocks still exist from that time period, and as he points out, we know little about the Archaean atmosphere and volcanic fluxes “because there were no volcanologists at the time.”
In the new study, the researchers examined isotope data from rocks that formed 3.5 to 2.7 billion years ago. “Isotopes are like the DNA of matter,” said Marty. “Each element has a specific signature that is a record of what has happened before.”
Specifically, they looked at the isotopic composition of the noble gas xenon, which can be used to estimate volcanic activity over time. There are nine different isotopes of xenon that differ only in the number of neutrons in the nucleus. The 129Xe isotope comes from the radioactive decay of iodine in the mantle and escapes to the surface through volcanic eruptions, so concentrations of the isotope in the atmosphere are directly related to volcanic emissions. 129Xe has gradually built up in Earth’s atmosphere over time.
As expected, the researchers saw that the Archaean atmosphere had little xenon compared to today’s atmosphere, which has had far longer to accumulate the gas. But they were surprised to see a rapid increase in 129Xe from 2.6 to 2 billion years ago. This signifies that Earth experienced massive volcanic activity around this time period, and released huge quantities of carbon and other volatiles into the atmosphere, at rates 10 to 100 times faster than today.
The volcanic activity associated with today’s plate tectonics can’t explain the rapid rise, so Marty and his CRPG-CNRS colleagues collaborated with Jaupart, an expert on heat release from Earth, to create a model of Earth’s temperature and volcanic activity over time. According to an independent record left by volcanic rocks, Earth’s internal temperature peaked between 3 and 2.5 billion years ago, warmed by the breakdown of radioactive elements. Since primitive plate tectonics were not very efficient at releasing this heat, scientists think that Earth overheated and created huge volumes of foamy mush, containing melted magma, gases, and crystals. The 129Xe signature suggest that the mush erupted for 200 or 300 million years, releasing gases at the surface.
Like a teenage tantrum, the rise in volcanic activity let Earth blow off a lot of steam and other volatiles, before it settled into a more mature pattern of plate tectonics.
The researchers speculate that this burst of volatiles at the end of the Archean epoch may be responsible for the Great Oxidation Event that resulted in oxygen gas in the atmosphere. “We propose that the sudden release of carbon dioxide and nitrogen into the atmosphere supplied a lot of nutrients for bacteria, so there could have been a boost of bacterial activity, meaning that they took carbon dioxide and nitrogen and released oxygen,” said Marty.
This rapid rise in carbon dioxide at the end of the Archaean occurred on a similar scale to human carbon dioxide emissions today. Understanding how catastrophic changes to the global carbon cycle impacted the ancient Earth may help us to predict the changes that will results from our current rates of emissions.
Next, the researchers plan to analyze additional rock samples from this time period, to achieve better resolution on the timing of Earth’s mantle “tantrum” and how it may have affected the evolution of the planet. They also plan to look at the role of the Sun and how changes in solar output may have impacted Earth’s temperature in the past.
“We really need to better characterize ancient Earth,” said Marty. “In the Archaean is certainly where bacterial activity developed and life blossomed. We need to document this period of time better and know much more about the cycle of volatiles in the distant past.”
Main image: Archean rocks, such as the 3.45 Ga-old Dresser formation, in Northwest Australia are in short on Earth today, making it difficult to learn about conditions on Earth during the Archaean eon. Courtesy of B. Marty