Rise of oxygen responsible for first ice age?


Climate change

Snow-covered mountain

Credit: Tracy Szela/National Science Foundation

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An international team of geologists have uncovered the answer to an ice-age-old question, discovering that the earliest ice age may have been due to the rise of oxygen in Earth's atmosphere, which consumed atmospheric greenhouse gases and chilled the globe.

The scientists uncovered evidence that the addition of oxygen to Earth's atmosphere - generally known as the 'Great Oxygenation Event' - coincided with the first widespread ice age on the planet.

"We can now put our hands on the rock library that preserves evidence of irreversible atmospheric change," said researcher Alan Kaufman, from the University of Maryland in the US.

"This singular event had a profound effect on the climate, and also on life."

Using sulfur isotopes to determine the oxygen content of 2.3 billion year-old rocks found in South Africa, they discovered evidence of a sudden increase in atmospheric oxygen that broadly coincided with physical evidence of glacial debris, and geochemical evidence of a new world-order for the carbon cycle.

"The sulfur isotope change we recorded coincided with the first known anomaly in the carbon cycle. This may have resulted from the diversification of photosynthetic life that produced the oxygen that changed the atmosphere," Kaufman said.

Two and a half billion years ago, before the Earth's atmosphere contained appreciable oxygen, photosynthetic bacteria were using the sun's power to to create usable energy, releasing oxygen as a byproduct. This oxygen first likely oxygenated the surface of the ocean, and only later the atmosphere.

The first formed oxygen reacted with iron in the oceans, creating iron oxides that settled to the ocean floor in sediments called banded iron-formations - layered deposits of red-brown rock that accumulated in ocean basins worldwide.

Later, once the iron was used up, oxygen escaped from the oceans and started filling up the atmosphere.

Once oxygen made it into the atmosphere, the scientists suggest that it reacted with methane, a powerful greenhouse gas, to form carbon dioxide, which is 62 times less effective at warming the surface of the planet.

"With less warming potential, surface temperatures may have plummeted, resulting in globe-encompassing glaciers and sea ice," said Kaufman.

In addition to its affect on climate, the rise in oxygen stimulated the rise in ozone in the upper atmosphere called the stratosphere - our global sunscreen.

This gas layer, which lies between 10 and 50 kilometres above the Earth's surface, decreased the amount of damaging ultraviolet solar rays reaching the oceans, allowing photosynthetic organisms that previously lived deeper down to move up to the surface, and hence increase their output of oxygen - further building up stratospheric ozone.

"New oxygen in the atmosphere would also have stimulated weathering processes, delivering more nutrients to the seas, and may have also pushed biological evolution towards [more complex lifeforms], which require free oxygen for important biosynthetic pathways," said Kaufman.

The result of the Great Oxidation Event, according to the researchers, was a complete transformation of Earth's atmosphere, of its climate, and of the life that populated its surface.

The study is currently published in the May issue of the journal Geology.