Tropical tectonic collisions may have triggered ancient ice ages


Massive tectonic collisions in the tropics may have been at the origin of the last three great glaciations of the Earth.

Prior to each of these ice ages, new research has revealed continental collisions and island arcs building long mountain ranges in tropical latitudes. These mountains may have paved the way for a cooling climate: when they have eroded into the seas, they have changed the chemistry of the ocean so that it can absorb more carbon from the ocean. l & # 39; atmosphere. As atmospheric carbon retains heat, less carbon in the sky translates into colder temperatures, which allows for the formation of ice patches and glaciers.

"This could provide a simple tectonic process that explains how Earth enters and exits glacial times," said study co-author Oliver Jagoutz, a professor of geology at the Massachusetts Institute of Technology. [Earth’s 8 Biggest Mysteries]

A brief history of ice

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During the Phanerozoic era, which extends over 540 million years, the Earth was released from ice 75% of the time, even at the North and South poles. But the planet has also known three glacial periods, or ice ages, at least where there were at least some permanent ice patches. The first took place at the end of the Ordovician period, 455 to 440 million years ago, when the first jawed fish were in full evolution. The second was in the Permo-Carboniferous, 335 to 280 million years ago, the age of amphibians and strange reptiles resembling mammals such as dimetrodon. The last ice age is in progress. It started about 35 million years ago, when the modern Antarctic ice caps were formed.

This study does not focus on small glacial advances, such as the ice age that ended around 11,700 years ago. Jagoutz told Live Science that changes in Earth's orbit that redistribute the heat of the sun have caused brief advances and retreats from glaciers. The question is why the Earth has icy periods.

"It seems that the state of the climate in which Earth loves to be is hotter than today, and these periods of glaciation are unusual," said Jagutz. "And if they're unusual, there must be something specific going on."

Collision and erosion

Jagoutz and his colleagues believe that "the specificity" lies in the formation of mountain ranges in the tropics.

At first glance, it may seem strange that the tropical mountains can create an ice age. But the atmosphere, the oceans and the earth are all linked. The continental crust is rich in silicate minerals. When these silicate-rich rocks erode and dissolve in the oceans, they make the seawater more alkaline or more basic. The carbon dioxide of the atmosphere dissolves easily in this alkaline seawater. The more alkaline it is, the more ocean can hold carbon.

Currently, the use of fossil fuels by humans exceeds the ability of the ocean to retain carbon. Over the past 200 years, seawater has become 30% more acidic. Millions of years ago, huge mountain building events could have done exactly the opposite, making the ocean more alkaline. The tropical mountains, in particular, would have done the job effectively. The tropics are wet, so erosion is fast and rocks raised by tropical tectonics are rich in easily soluble magnesium and calcium.

The idea that tropical erosion could have influenced the climate was not new, but Jagoutz and his team were the first to gather a database of all the geological recordings of these major tectonic collisions and to compare them at the beginning of the ice ages. They found that during the Phanerozoic, the length of active collision zones between oceanic and continental plates – called "sutures" – ranged from zero to 30,000 kilometers (18,640 miles). Each of the major ice ages was preceded by a peak in the length of these active collisions in the tropics, when the sutures ranged between 6,214 miles and 18,640 miles (10,000 to 30,000 km).

"Whenever you have had an ice age, you have had a greater length of suture zone in the tropics," said Jagoutz.

Geological time

The geological traces left by the old collisions are called ophiolites, which are oceanic volcanic rocks pushed over the continental crust. The researchers found none of these extreme ophiolites at a time when the Earth was not frozen. And it is the ophiolites in the tropics, or regions within 20 degrees of latitude, that seem to be of importance for the cooling of the planet.

According to Jagoutz, there are other theories explaining the reasons why the Earth has periods of ice, namely that volcanic activity varies and that carbon is pumped more or less into the atmosphere. But the data on the history of volcanism do not always correspond to glacial periods, he said, and the theory of volcanoes does not really explain why the ice ages should stop as well. The tectonic explanation gives good results: once mountain ranges rich in calcium and magnesium are completely eroded or leave the tropics by continental drift, their effect on the climate fades and the Earth returns to its state typical and embalmed.

Jagoutz and his colleagues applied for a grant from the National Science Foundation to deepen their theory. Whether they are true or false, the tropical mountains will not save mankind from climate change caused by humans anytime soon. This mountain building process takes place over millions of years, said Jagoutz, and has little to do with the type of variations that determine whether, for example, Miami is habitable or flooded by the rise of the seas. Some researchers, however, have been thinking of geoengineering projects that would grind rocks rich in calcium or magnesium and spread them into the tropical oceans, or inject carbon dioxide into similar rocks.

"People want to use this natural process to help with the artificial climate change,[but[Ilyabeaucoupdeproblèmesaveclesiencommecommentfaireensortequeceprocessussedéroulesuruneéchelledetempsquisoitpertinentepourleshumains"adéclaréJagoutz"C'?Esttrèsdifficile"[goal[Therearealotsofissuewithhislikehowdoyougetthisprocessgoingonatimescalethatisrelevanttohumans?"Jagoutzsaid"Thatisverydifficult"[mais[Ilyabeaucoupdeproblèmesaveclesiencommecommentfaireensortequeceprocessussedéroulesuruneéchelledetempsquisoitpertinentepourleshumains?”adéclaréJagoutz”C'esttrèsdifficile”[but[therearealotsofissuewithhislikehowdoyougetthisprocessgoingonatimescalethatisrelevanttohumans?”Jagoutzsaid”Thatisverydifficult”

The research appears today (March 14) in the journal Science.

Originally published on Science live.