Ancient Volcanoes: Understanding Low Greenhouse Gas Emissions in Earth’s History

New research suggests that the impact of volcanoes on Earth’s climate may not be as ancient as previously believed.

The Earth’s climate has experienced shifts between “icehouse” and “greenhouse” conditions, largely dictated by greenhouse gas levels like carbon dioxide.

Volcanic arcs, including significant eruptions from mountain ranges such as Japan’s, release CO2 from deep within the Earth. Recent findings indicate that dinosaurs became a substantial source of carbon emissions only towards the end of their reign, approximately 100 million years ago, according to Ben Mather and his team from the University of Melbourne.

This correlates with the emergence of phytoplankton featuring calcium carbonate scales in the oceans approximately 150 million years ago. When these organisms perish, they deposit large amounts of calcium carbonate on the ocean floor.

As tectonic plates shift, these significant reservoirs of carbon are pushed into the mantle and recycled into the Earth’s molten core via a process known as subduction.

“Most of the carbon derived from plankton on the subducting oceanic plate mixes into the melt interior, but a portion is released through volcanic arcs,” explains Mather.

Before the emergence of scaly plankton, volcanic arc emissions contained relatively lower levels of CO2, according to Mather.

Through modeling, Mather and colleagues examined tectonics’ long-term impact on the carbon cycle over the past 500 million years. They discovered that much of the carbon stored within Earth throughout its history was released through crustal fractures in a process termed rifting, not primarily through volcanic arcs.

Rifting, a geological process where continents separate, can occur on land (as in the East African Rift) or along mid-ocean ridges.

“As tectonic plates separate, they effectively ‘roof off’ parts of the molten Earth,” Mather states. “This process generates new crust at mid-ocean ridges, releasing carbon.” The amount of carbon entering the atmosphere from continental fractures and mid-ocean ridges relies on the cracks’ length and the rate at which they separate, a process that has remained relatively stable. However, emissions from volcanic arcs have surged in the last 100 million years due to new carbon reservoirs formed by plankton.

Currently, Earth is in a temporary warm phase called an interglacial period, nested within a larger ice age that began 34 million years ago. One reason for the persistent cold phases is that phytoplankton sequester substantial amounts of carbon from the ocean, depositing it on the sea floor. Although volcanic emissions are rising, they still pale in comparison to the carbon stored by phytoplankton and that sequestered through tectonic movements.

According to Alan Collins and his team from the University of Adelaide, modeling studies like this are crucial for comprehending how volcanic and tectonic activities have influenced climate patterns over geological timescales.

“The composition of marine sediments has shifted as new organisms evolved, utilizing diverse elements, including the rise of calcium carbonate-based zooplankton,” Collins emphasizes.

Reference journal: Nature Communications Earth and Environment, DOI TK

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