Carbon dioxide mysteriously dropped during the time of the ancient supercontinent Pangea despite conditions that seemingly would have caused CO2 to spike. Instead, the biggest drop in the last 500 million years occurred. The cause of the drop is uncertain, but a new study argues a massive mountain range was the cause.

Normally rain is slightly acidic because CO2 dissolves into it forming weak carbonic acid. This in turn causes rock weathering, which acts as an important carbon sink. But much of the super-sized Pangea was far from the ocean and arid. The rock weathering effect was lessened, so CO2 should have risen, right? It didn't, the study reports, due to the imposing Hercynian mountain range.

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"The steep slopes of these Hercynian mountains produced physical erosion," team leader Yves Godderis of the French National Centre for Scientific Research said in a release. "In a humid equatorial environment, this physical erosion promoted rock weathering and removing CO2 from the atmosphere."

The mountains ran from the Appalachians to Ireland to southwestern England through Paris, into Germany and continued east.

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The researchers suggest if the mountains hadn't formed, atmospheric CO2 would have spiked sharply for a long period of time. Eventually, increased temps could have lead to more rock weathering, and a self-correction, the scientists suggest.

"There's no doubt that this would have stalled Earth's temperature at a high level for a long, long time," Godderis said. "The world would look very different today if these mountains had not developed when they did."