Volcanoes had fewer greenhouse gas emissions in Earth’s past

The ability of volcanoes to change Earth’s climate may not be as ancient as previously thought.

Throughout our planet’s history, the climate has oscillated between “ice house” and “greenhouse” conditions, largely determined by the levels of greenhouse gases, such as carbon dioxide, in the atmosphere.

Volcanic arcs, giant chains of erupting peaks in places like Japan, can play a role in this by releasing carbon dioxide from the Earth’s interior. But modeling research led Ben Mather at the University of Melbourne, Australia, suggest that it became the dominant source of carbon emissions only at the end of the age of the dinosaurs, about 100 million years ago.

This is because about 150 million years ago, phytoplankton with calcium carbonate scales appeared in the oceans. When these plankton die, they leave enormous deposits of calcium carbonate on the deep sea floor, Mather says.

When tectonic plates move and are recycled into the Earth’s molten interior by sliding under each other, a process known as subduction, these huge reservoirs of stored carbon end up being pushed into the mantle.

“Most of the carbon from plankton leaving the subducting ocean plate will mix into the molten interior, but some of it will be emitted via volcanic arc volcanoes,” Mather says.

However, before 150 million years ago, material released by volcanic arcs was relatively low in carbon dioxide due to the absence of these crusty plankton, Mather says.

He and his colleagues have modeled plate tectonics over the past half a billion years and its role in the carbon cycle. They found that for most of Earth’s history, much of the carbon trapped inside the planet was released through cracks in the Earth’s crust in a process called rifting, rather than by volcanic arcs.

Rifting is the process by which continents are torn apart over geological timescales and can occur on land, as in the East African Rift, or along mid-ocean ridges.

“When tectonic plates move apart, what you’re essentially doing is ‘reroofing’ some of the molten interior of the Earth,” Mather says. “When that happens, new crust forms at the mid-ocean margins and carbon is released.”

The amount of carbon released into the atmosphere from continental rifts and mid-ocean ridges is a product of the length of the rift and how quickly it breaks up, but the proportion of carbon previously emitted has remained relatively constant, Mather says. “But emissions from volcanic arcs have increased dramatically in the past 100 million years thanks to this new stock of carbon on the sea floor from suppliers of calcium carbonate from plankton,” he says. “Compared to 150 million years ago, volcanic arcs now emit two-thirds more carbon.”

The Earth is currently going through a short warm period known as an ice age within a much longer ice age that began 34 million years ago. One factor contributing to the ongoing cold snap is that these phytoplankton take a lot of carbon from the ocean and trap it on the sea floor. While the amount of carbon in volcanic arc eruptions has increased, it is still less than what is stored by phytoplankton on the seafloor and what is pulled into the Earth’s interior by tectonic movement.

Alan Collins Modeling work like this study is vital to understanding how the impact of volcanic and tectonic activity on climate has changed over time, say his colleagues, from the University of Adelaide in Australia.

“The composition of ocean sediments has changed with the evolution of different organisms that use different elements in their formation, such as the evolution and gradual dominance of calcium carbonate zooplankton,” Collins says.

Journal reference: Nature Communications Earth and Environment, DOI TK

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