The weathering Himalayas might not be behind Earth's cooling over the past 15 million years

• A new study indicates that the Himalayas might not be behind global cooling that started 15 million years ago.
  
• The weathering of rocks should have increased the carbon deposit in deep sea sediments but instead, it has decreased.
  
• This is not the first time that reasons behind Earth’s cooling have been called into question.
  

Fifteen million years ago, the planet entered a phase of slow and continuous cooling. The most popular theory for the cooling was the formation of major mountain ranges — like the Himalayas.

But, it’s possible that the Himalayas weren’t responsible at all according to a new study published in Nature Geoscience.

The current theory states that when the Indian and Asian continents collided, it brought fresh rocks up to the surface. The new rocks were more vulnerable to weathering — capturing and storing carbon dioxide — and the Earth started to cool as a result.

If that was the case, the carbon deposit in the deep sea sediments should have increased. But, researchers from Rutgers found that they had decreased instead, paving the way for a more mysterious reason.

The mystery of global cooling

When the Himalayas and other major mountain ranges formed, they accelerated erosion and the weathering of rocks.

The rocks captured the carbon dioxide and rivers carried it out to sea. In the oceans, the algae used the dissolved inorganic carbon to build their calcium carbonate shells.

Once the algae died, their skeletons fell to the ocean floor and got buried — trapping the carbon in deep-sea sediments, far from the atmosphere.

So, if weathering increases — as assumed after the formation of the Himalayas — calcium carbonate in the deep sea should increase as well.

Instead, the new study finds that calcium carbonate in shells that decreased significantly over the past 15 million years.

This means that rock weathering in the Himalayas, on any other major mountain range, might not be what brought on global cooling.

Another discovery made during the study was that coccolithophores — a type of algae — adapted to the decline in carbon dioxide 15 million years ago, by reducing its production of calcium carbonate.

More questions

This is not the first time that the causes behind Earth cooling have been called into question.

Earlier this year, another study published in Nature stated that there was no change in weathering from the start of the ice age 2.5 million years go to the cooling period that kicked in 15 million years ago.

The reduction of carbon dioxide from the atmosphere can be explained without an increase in weathering.

In fact, the study states that if the rate of weathering is taken into account, it was possible that Earth would have been left without carbon dioxide altogether. "That would have turned our planet into an icy wasteland hostile to life," said Caves Rugenstein, one of the co-authors of the study.


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