Aussie bedrock indicates origin of earth's atmosphere

Core samples from the Australian outback have helped American scientists piece together the events that spawned our planet's breathable atmosphere.

Samples from bedrock near Mount Isa in Queensland were some of the last measurements taken by Ohio State University geologists in a study that revealed new evidence of a geologic event, called the Steptoean Positive Carbon Isotope Excursion (SPICE).

The study suggests that upheavals in the earth's crust initiated a kind of reverse-greenhouse effect 500 million years ago that cooled the world's oceans, spawned giant plankton blooms and sent a burst of oxygen into the atmosphere.

That oxygen may have helped trigger one of the largest growths of biodiversity in earth's history.

Matthew Saltzman, associate professor of earth sciences at Ohio State, reported the findings at the meeting of the Geological Society of America in Denver.

For a decade, he and his team have been assembling evidence of climate change that occurred 500 million years ago, during the late Cambrian period. They measured the amounts of different chemicals in rock cores taken from around the world, piecing together a complex chain of events from the period.

Amounts of carbon and sulfur in the rocks suggest that the event dramatically cooled earth's climate over two million years - a very short time by geologic standards. Before the event, the earth was a hothouse, with up to 20 times more carbon dioxide in the atmosphere compared to present day.

After the SPICE event, the planet had cooled and the carbon dioxide had been replaced with oxygen, providing an atmospheric composition similar to the current mix.

"If we could go back in time and walk around in the late Cambrian, this seems to be the first time we would have felt at home," Saltzman said.

"Of course, there was no life on land at the time, so it wouldn't have been all that comfortable."

The land was devoid of plants and animals, but there was life in the ocean (mainly in the form of plankton, sea sponges, and trilobites). Most of the early ancestors of the plants and animals we know today existed during the Cambrian, but life wasn't very diverse.

Eight million years later (around 490 million years ago) came the time called the Ordovician period, in which many new species sprang into being. The first coral reefs formed during that time, accompanied by the first true fish. New plants evolved and began colonising land.

"If you picture the evolutionary 'tree of life', most of the main branches existed during the Cambrian, but most of the smaller branches didn't get filled in until the Ordovician," said Saltzman.

"That's when animal life really began to develop at the family and genus level."

The composition of the atmosphere has changed many times over the earth's history, but the pace of change during the Cambrian is exceptional.

"After this pulse of oxygen, the world remained in an essentially stable, warm climate, until late in the Ordovician," he said.

"We know that oxygen was released during the SPICE event, and we know that it persisted in the atmosphere for millions of years - during the time of the Ordovician radiation - so the timelines appear to match up. But to say that the SPICE event triggered the diversification is tricky, because it's hard to tell exactly when the diversification started."

While not all the details are known for sure as yet, Saltzman's general hypothesis goes like this:

During the Cambrian period, most of the continents as we know them today were either underwater or part of the Gondwana supercontinent. Tectonic activity was pushing new rock to the surface, where it was immediately eaten away by acid rain. This sort of chemical weathering pulls carbon dioxide from the air, traps the carbon in sediments and releases oxygen - similar to the greenhouse effect in reverse.

The changing atmospheric gases cooled the climate and oceans, allowing plankton to prosper in the more hospitable temperatures. This in turn created more oxygen through photosynthesis.

"It was a double whammy," he said. "There's really no way around it when we combine the carbon and sulfur isotope data - oxygen levels dramatically rose during that time."

So what does Saltzman think this event can tell us about climate change today?

"Oxygen levels have been stable for the last 50 million years, but they have fluctuated over the last 500 million," he said.

"We showed that the oxygen burst in the late Cambrian happened over only two million years, so that is an indication of the sensitivity of the carbon cycle and how fast things can change."