Researchers find origin of 'breathable' atmosphere half a billion...(tory. Researchers have been trying t...)

Researchers have been trying to understand the sudden climate change during the Cambrian period ever since Saltzman found the first evidence of the SPICE event in rock in the American west in 1998. Later, rock from a site in Europe bolstered his hypothesis, but these latest finds in central Iowa and Queensland, Australia, prove that the SPICE event occurred worldwide.

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

From our previous work, we knew that carbon was captured and oxygen was released during the SPICE event, but we didn't know for sure that the oxygen stayed in the atmosphere, Saltzman said.

They compared measurements of inorganic carbon -- captured during weathering -- with organic carbon -- produced by plankton during photosynthesis. And because plankton contain different ratios of the isotopes of carbon depending on the amount of oxygen in the air, the geologists were able to double-check their estimates of how much oxygen was released during the period, and how long it stayed in the atmosphere.

They also studied isotopes of sulfur, to determine whether much of the oxygen being produced was re-captured by sediments.

It wasn't.

Saltzman explained the chain of events this way: Tectonic activity led to increased weathering, which pulled carbon dioxide from the air and cooled the climate. Then, as the oceans cooled to more hospitable temperatures, the plankton prospered -- and 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 an
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