Ames scientist's theory on ancient atmosphere discussed in Nature magazine.
NASA Scientist Proposes Alternate Theory of Earth's Ancient Atmosphere
Scientists agree that the rise of oxygen in Earth's early atmosphere occurred a little less than 2.5 billion years ago, but explanations of how and why oxygen built up in the atmosphere still remains controversial. Was it due to a feast or famine for microbes in the oceans?
A new theory suggests that a nickel famine in the oceans caused the collapse of atmospheric methane by inhibiting the enzymatic activity needed to produce methane in microbes, called methanogens. The theory appears in "Oceanic Nickel Depletion and a Methanogen Famine before the Great Oxidation Event," by Kurt Konhauser, et al, published in the April 9 issue of Nature.
The second theory suggests that newly abundant sulfate salts in seawater provided a feast for microbes, which are competitors with the methanogens. As sulfates became more abundant, the methanogens retreated, and the atmosphere favored more strongly oxygen. The result is a positive feedback loop in which more oxygen begets more sulfates, and more sulfates beget more oxygen. The second theory appears in an earlier paper called "The Loss of Mass-Independent Fractionation of Sulfur Due to a Paleoproterozoic Collapse of Atmospheric Methane," by Kevin Zahnle, et al, published in Geobiology 2006.
"I still think there is a good chance that sulfates may have triggered the oxidation of our atmosphere'" said Zahnle, a space scientist at NASA Ames Research Center, Moffett Field, Calif. and a contributing author to the Nature paper. "The weathering of sulfides such as iron pyrites, or fool's gold, would have raised the amount of sulfates in the oceans, and given the sulfate-eating bacteria an advantage over the nickel-dependent methanogens."
So what caused the Great Oxidation Event, 2.4 billion years ago?
At the heart of the nickel hypothesis are distinctive "nickel to iron" ratios preserved in banded iron formations. These rocks formed in seawater and their composition therefore indicates the composition of the seawater in which they were made. The new data show a trend in the "nickel-to-iron" ratios from high values 3.8 to 2.7 billion years ago to strikingly smaller "nickel to iron" ratios in rocks younger than 2.5 billion years old. The higher older values are never reached again. This declining trend persists for the remainder of the Palaeoproterozoic BIF record.
According to Konhauser, the decline in nickel is unlikely to have been driven by changes in nickel sinks (deposits), which appear to be insensitive to the evolving element concentrations and conditions of the seas. Instead, the decline in nickel was caused by cooling of the Earth’s mantle and a change in composition of the volcanic rocks erupted. A special nickel-rich volcanic rock called “komatiites” stopped being erupted after about 2.7 billion years ago.
"Although methanogens continue to grow in the absence of nickel, the actual methane production depends strongly on dissolved nickel availability. It has been demonstrated in numerous laboratories and natural settings that methane production decreases progressively when nickel concentrations are reduced," said Konhauser.
In this new work, the scientists propose that the decline in biogenic methane production occurred before the rise of abundant environmental oxygen, which could mean that the nickel story and the sulfate story are not related. The reason the nickel goes away is that nickel is associated with high temperature lavas that make komatiites. As the Earth cooled, nickel became rarer in volcanic rocks, and thus rarer in the seas. Since methanogens depend on nickel to flourish, a diminishing supply of volcanic nickel would link mantle evolution to the rise of atmospheric oxygen.
"Oxygen in the air is what makes animals possible. Earth went through a lot of changes 2.5 billion years ago. If we could figure out which of these gave us oxygen, we’d have a much better understanding of the kinds of planets that could evolve intelligent life." said Zahnle.
Partial funding was provided by NASA Exobiology and Evolutionary Biology Program.
Ruth Dasso Marlaire
Ames Research Center, Moffett Field, Calif.