About 2.4 billion years ago, the Earth was unimaginably different from the planet we know today. There was a negligible amount of oxygen in the atmosphere. In fact, the gas reacted with abundant methane as soon as it was produced by newly arrived photosynthetic microbes. Clouds of methane produced by other sea-dwelling microbes called methanogens kept the planet warm through the greenhouse effect. Unlike today, the temperature-rising gases were a good thing, because the young Sun was faint and the trapped heat kept all of Earth’s water from freezing.
But around 2.4 billion years ago conditions changed dramatically. Methane levels dropped, large amounts of oxygen were unleashed, and the planet entered an ice age. Thankfully, this ice age wasn’t severe enough to prevent complex life from forming. Geochemists are still trying to figure out what happened during this time, called the Great Oxidation Event, that spared Earth from becoming a lifeless ice ball. Now, using nickel-isotope analysis, chemists think they have an explanation. New kinds of rock weathering during this time could have continued to seed the oceans with metals needed by methanogens, allowing the microbes to continue to produce their planet-warming gases (Nat. Geosci. 2019, DOI: 10.1038/s41561-019-0320-z).
“We could have been a planet with primitive life forever, but something happened,” says Laura Wasylenki, a geochemist at Northern Arizona University who led the research.
On ancient Earth, oxygen had no chance to accumulate because there was so much methane in the atmosphere, Wasylenki says. Methane and oxygen react to form carbon dioxide and water. For oxygen to build up to today’s levels, “something had to really knock methanogen productivity back,” Wasylenki says. During the Great Oxidation Event, atmospheric methane dropped to one-sixth its former level.
Previous research has suggested that methanogens declined due to what University of Alberta geochemist Kurt Konhauser termed “the great nickel famine.” Methanogen metabolism relies on enzymes with a nickel complex at their heart. As Earth’s core cooled, magma became poor in the metal, and less of it made its way from rocks and into the oceans.
But Wasylenki suspected that something must have still been feeding the methanogens their crucial micronutrient—after all, they didn’t disappear completely. Shui-Jiong Wang of the China University of Geosciences, then a post-doc in Wasylenki’s lab, suggested they measure nickel isotopes in ancient glacial sediments from the Great Oxidation Event to look for clues.
Compared with levels of heavy nickel isotopes, levels of light isotopes were low in these samples. Sulfide minerals tend to be enriched in light nickel isotopes, and so this evidence suggested to Wasylenki and Wang that during the Great Oxidation Event weathering of sulfide minerals washed a modest amount of nickel into the seas. This weathering could have been triggered by the “whiffs” of oxygen produced by photosynthetic microbes at the time.
Konhauser says these findings add “an additional source of nickel supply we hadn’t considered.” But he says the story of the Great Oxidation Event and the role microbes played in ancient Earth’s climate evolution is far from settled. It’s not clear whether methanogens would have been able to use all the nickel that flushed into shallow waters as a consequence of this proposed weathering, he says. Methanogens likely faced tough competition from photosynthetic microbes, and oxygen in shallow waters would have been toxic to them.
Timothy Lyons, a geochemist at the University of California, Riverside, further complicates the picture. Sulfide mineral weathering would have flushed sulfates into the seas. Some microbes use sulfates to oxidize methane, making it hard for the gas to stick around. But Lyons is intrigued by the nickel findings.
“This launches a new field of inquiry,” Lyons says. Researchers now need to ask how much methane could have been produced and sustained in the atmosphere under these conditions, and how this would have affected the habitability of the planet, he says.