The Rise of Oxygen

 

Cyanobacteria (above) became the first microbes to produce oxygen by photosynthesis. Credit: UC Berkeley

Animals need oxygen. “You cannot evolve animals like us without having a significant amount of oxygen,” says geochemist Dick Holland of Harvard University. “Without the Great Oxidation Event [a dramatic rise of oxygen in Earth’s atmosphere some 2.3 billion years ago], we would not be here. No dinosaurs, no fish, no snakes – just a lot of microorganisms.”

Oxygen has not always been as abundant as it is today. Most scientists believe that for half of Earth’s 4.6-billion-year history, the atmosphere contained almost no oxygen.

Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago. But, mysteriously, there was a long lag time – hundreds of millions of years – before Earth’s atmosphere first gained significant amounts oxygen, some 2.4 billion to 2.3 billion years ago.

Burial at Sea

The conventional theory of how oxygen accumulated in the atmosphere focused on the burial of organic matter in seafloor sediments that later hardened into rock.

Cyanobacteria are microbes that live primarily in seawater. They are believed to have been the first organisms on Earth to perform oxygenic photosynthesis. In this process, they produce organic carbon, the building blocks of life’s molecules, and release oxygen gas (O₂). The O₂ enters into the seawater, and from there some of it escapes into the atmosphere.

When these microbes die, their remains become buried in seafloor sediment. Their decomposition removes oxygen from seawater, and in turn, from the atmosphere.

As the carbon-burial theory goes, when organic material is buried, oxygen becomes available to build up in the atmosphere. So perhaps there was a sudden increase 2.3 billion years ago in the amount of organic carbon that was buried, leaving more free oxygen.

But there’s a glitch: Studies have shown that the amount of buried carbon found in sedimentary rocks remained constant during the early stages of the Great Oxidation Event. So a change in the carbon-burial rate can’t explain the buildup of oxygen in the atmosphere.

Hydrogen to the Rescue

Jim Kasting (above) proposed a decade ago that Earth gained an oxygen-rich atmosphere because molecular hydrogen belched out by volcanoes diffused into space. Credit: Penn State University

An alternative explanation is that oxygen built up because there was a reduction in gases – hydrogen, for example – that react with and thus “soak up” oxygen. Pennsylvania State University atmospheric scientist Jim Kasting proposed a decade ago that Earth gained an oxygen-rich atmosphere because molecular hydrogen belched out by volcanoes diffused into space.

At first, that doesn’t seem to make sense. If volcanoes were putting out hydrogen, and cyanobacteria were pumping out oxygen, why wouldn’t they just combine to form water and be done with it?

Actually, that did occur to some extent. But Kasting believes more of the oxygen produced by photosynthesis ended up buried within Earth’s mantle, the layer beneath the crust, before the hydrogen could get to it. He is not sure how, but cites three theories:

(1) oxygen reacted with iron in seawater, and the resulting iron oxide precipitated onto the seafloor, then was buried deep within the Earth;
(2) oxygen-rich water in seafloor sediments was buried within the Earth, leaving oxygen in the mantle when the water’s hydrogen was belched out by volcanoes; and
(3) oxygen-rich sulfates in undersea hot springs reacted with iron in seafloor sediments, which were buried to put oxygen into the mantle.

Burial of oxygen, in the form of oxides, in Earth’s mantle had two effects. First, it allowed hydrogen to continue escaping into space. And second, the buried oxides reacted with hydrogen and other “reduced” gases such as carbon monoxide and hydrogen sulfide that also were present within Earth’s mantle. That pulled the hydrogen out of circulation. The result: When these buried oxygen-rich sediments got recycled by the Earth, and their gases got burped out again by volcanoes, the gases contained less free hydrogen than previously.

So oxygen was able to build up in the atmosphere, causing perhaps the most dramatic shift in the history of life on the planet.

Before that happened, the amount of oxygen in Earth’s atmosphere was about one ten-quadrillionth of the amount present today, Kasting says. Oxygen now makes up nearly 21 percent of Earth’s atmosphere; most of the rest is nitrogen.

From Hydrogen to Methane

Catling believes that oxygen-rich sediments were buried both in the Earth’s crust and in the mantle. Credit: Oxford University

Now, an emerging theory says that hydrogen hitchhiked into the upper atmosphere as a component of methane, or natural gas, which was broken down by ultraviolet sunlight, freeing hydrogen to escape into space.

The theory was outlined in a paper published in the journal Science on Aug. 3, 2001, by David Catling, Kevin Zahnle and Christopher McKay of NASA’s Ames Research Center. (Catling, a planetary scientist, since has moved to the University of Washington.) The paper used calculations of atmospheric reactions and processes to explain why methane would have built up in the atmosphere between 2.7 billion and 2.3 billion years ago, allowing hydrogen to escape and, eventually, oxygen to accumulate.

Much hydrogen and oxygen both originated from water broken down by cyanobacteria. As mentioned earlier, the cyanobacteria consumed water and carbon dioxide during photosynthesis, making organic matter and releasing oxygen. Other bacteria consumed the organic matter, yielding molecular hydrogen and acetate. These, in turn, were consumed by microbes that produced methane, according to Catling.

When cyanobacteria first began producing oxygen, much of the oxygen reacted with iron, sulfur and other chemicals, in the oceans, and in Earth’s surface rocks, when raindrops containing dissolved oxygen weathered and eroded the rocks. These processes carried the oxygen into seafloor sediments and eventually into Earth’s interior. Catling believes that oxygen-rich sediments were buried both in the Earth’s crust and in the mantle. However, he thinks that oxidized sediments recycled through the crust would have been the critical factor for the stability of oxygen in the atmosphere. These sediments reacted with hydrogen and other “reduced” gases, diminishing the flow of such gases to the atmosphere – not from volcanoes, but from hot, compressed rocks known as metamorphic rocks.

The burial of oxides in the crust allowed methane to build in the atmosphere, Catling says. Some methane reacted with oxygen, but most did not. This excess methane accumulated in the atmosphere to concentrations a few hundred to a few thousand times greater than modern levels, Catling calculates.

Ultraviolet sunlight in the upper atmosphere broke methane into its components, carbon and hydrogen. The hydrogen diffused into space, leaving oxygen to start accumulating.

Microbial mat producing oxygen through photosynthesis. Credit: UTA Department of Geology

Catling says that because methane is a “greenhouse gas,” his theory also explains how Earth was warmed billions of years ago, when the Sun was fainter.

Two weeks before Catling’s theory hit print, the journal Nature published another study with implications for how Earth’s oxygen-rich atmosphere arose. NASA Ames Research Center biogeochemist David Des Marais, biogeochemist-astrobiologist Tori Hoehler and Brad Bebout reported they measured massive hydrogen production by microbial mats on the coast of Mexico’s Baja Peninsula.

“If the Earth’s early microbial mats acted similarly to modern ones we studied, they may have pumped a thousand times more hydrogen into the atmosphere than did volcanoes and hydrothermal vents, the other main sources,” Hoehler said at the time.

Des Marais contends much hydrogen could have escaped to space without reacting with oxygen because the mats keep producing hydrogen at night when photosynthetic oxygen production turns off. Catling and Kasting disagree, saying that any such hydrogen would have reacted with oxygen in the atmosphere and that most of the rest would have been consumed by archea that made methane.

What’s Next?

Scientists are making progress on understanding the Great Oxidation Event, but still greater mysteries remain to be unraveled in the saga of oxygen on Earth.

“Although we think we know when oxygen first appeared and rose, we know very little about its rise to the present level, especially about the relationship between atmospheric oxygen and the development of animals,” says Catling

Some believe that after the initial rise of atmospheric oxygen more than 2 billion years ago, oxygen was only 2 to 4 percent of the atmosphere. Today it comprises more than 20 percent.

There is evidence that oxygen levels also rose 1.3 billion years ago and again before the Cambrian Explosion, a rapid proliferation of animal life that began 540 million years ago. Some researchers believe increasing levels of atmospheric oxygen helped trigger the Cambrian Explosion.

Catling says the reason for those rises in atmospheric oxygen “is even more of a mystery than the first one.”

“There were huge ice ages [Snowball Earth events] just before the Cambrian Explosion, but also associated with the Great Oxidation Event,” Holland says. “It is important to have a much better understanding of those events and the history of life.”

Kasting, Catling, Des Marais, Hoehler and Holland are members of the NASA Astrobiology Institute so those issues have special relevance for them.

“We want to understand what controls the rise of oxygen on the Earth and maybe other planets,” says Kasting. “Oxygen is our best biomarker for looking for life on extrasolar planets.”