A decrease in the amount of dissolved nickel in ocean waters beginning 2.7 billion years ago could have stifled methane-producing bacteria and set the scene for oxidation of the Earth’s atmosphere, researchers report in the April 9 Nature.
Billions of years ago, methane-producing bacteria called methanogens thrived in nickel-rich seas. The high amounts of methane that this early life pumped into the environment prevented oxygen accumulation in the atmosphere because the methane reacted with any oxygen, creating carbon dioxide and water, the researchers suggest.
But 2.4 billion years ago, the Earth’s atmosphere changed. Atmospheric methane levels decreased, allowing oxygen levels to increase in what scientists call “the Great Oxidation Event.” Researchers debate how these atmospheric changes occurred.
In the new research, earth scientist Kurt Konhauser of the University of Alberta in Edmonton, Canada, and his colleagues measured nickel-to-iron ratios in banded iron formations, rocks consisting of layers of iron, silica and trace metals that formed from sediments in ocean water billions of years ago. The composition of the rocks provides a record of how much trace metals were in the oceans when the rocks formed, Konhauser says.
By measuring more than 30 different banded iron formations of different ages, Konhauser says, the team found that the amount of nickel in the rocks began to drop about 2.7 billion years ago, and that levels had halved by 2.5 billion years ago. Konhauser and his colleagues used the rock data to calculate decreases in the amount of dissolved nickel in seawater.
“This is a really interesting data set,” says earth scientist David Catling of the University of Washington in Seattle. “As far as I’m aware, no one else has deduced nickel concentrations in the ocean over time.”
The lower nickel content of the ocean waters reduced the activity of the methanogens, so the supply of methane gas diffusing from seawater into the atmosphere decreased over time. “Methanogens use nickel-based enzymes to power their many important metabolic reactions,” Konhauser says. The researchers speculate that this decrease in methane production in the ocean set the stage for the increase of atmospheric oxygen levels during the Great Oxidation Event.
The decrease in nickel levels was probably caused by geological changes that occurred as the Earth’s upper mantle cooled, the researchers say. Early in Earth’s history, the mantle was much hotter and volcanic eruptions produced rocks very rich in nickel. . The weathering of these rocks provided a source of dissolved nickel in the ocean. But as the mantle cooled, the supply of nickel-rich rocks, and thus of nickel, decreased, and the ocean nickel levels dwindled.
Catling notes that nickel might be one of several factors that contributed to the changing atmosphere. “You’ve got to take the full balance sheet of oxidation and reduction reactions into account,” he says. “Changes in nickel may be only one part of the answer.”
