Earth's climate is influenced by various factors, both natural and human-induced. One of these factors is the carbon cycle, the exchange of carbon between different reservoirs, such as the atmosphere, the oceans, the biosphere, and the geosphere.
Carbon dioxide (CO2) is a major greenhouse gas that traps heat in the atmosphere and affects the global temperature.
However, not all the carbon in the Earth system stays in the atmosphere. Some of it is stored in rocks, especially in the oceanic crust, which covers most of the seafloor.
A new study by researchers from the University of California, Berkeley, and the University of Hawaii at Manoa reveals that a mineral called smectite, which is produced by plate tectonics in the oceanic crust, has a global cooling effect, counteracting the greenhouse effect and stabilizing the climate.
The formation of smectite in the oceanic crust
David McNew/Getty Images
Smectite is a class of phyllosilicate minerals that have a layered structure and can swell when exposed to water.
Smectite minerals include montmorillonite, nontronite, saponite, and hectorite. It forms when seawater reacts with the basaltic rocks that make up the oceanic crust, which is the outer layer of the Earth's lithosphere that underlies the oceans.
This process is called alteration, and it occurs mainly along cracks and veins in the rocks. Alteration also produces other secondary minerals, such as celadonite, analcite, and carbonates.
The oceanic crust is constantly moving due to plate tectonics, the movement of Earth's crustal plates. When two plates converge, one plate slides under another and sinks into the mantle, the layer of hot, molten rock beneath the crust.
This process is called subduction, and it consumes oceanic crust, which contains carbon in the form of carbonate minerals, such as calcite and aragonite.
These minerals are unstable at high temperatures and pressures, and they release CO2 into the mantle, which can eventually return to the atmosphere through volcanism.
However, not all the carbon in the subducted oceanic crust is lost to the mantle. Some of it is transformed into smectite, which is stable at deeper depths and can store carbon for millions of years.
Smectite is a magnesium-rich mineral, and it forms when seawater reacts with the peridotite rocks that make up the upper mantle. Peridotite is a dry and dense rock that consists mainly of olivine and pyroxene minerals.
When peridotite is hydrated by seawater, it becomes serpentine, a water-rich and less dense mineral.
Serpentine can also react with seawater and form smectite, which can incorporate carbon into its structure.
The existence of smectite in subducted oceanic crust has been hypothesized for decades, but it has never been directly observed, until now.
The researchers used a technique called synchrotron X-ray diffraction to analyze samples of oceanic crust that were recovered from a subduction zone in the Philippines.
They found that smectite was present in the samples, confirming that it is indeed produced by plate tectonics.
The discovery of smectite in subducted oceanic crust has important implications for the global climate.
The researchers estimated that smectite sequesters about 0.3 gigatons of carbon per year, which is equivalent to about 10% of the current annual CO2 emissions from human activities.
This means that plate tectonics acts as a natural carbon sink, removing CO2 from the atmosphere and storing it in the deep Earth.
The researchers also calculated that smectite formation has a global cooling effect of about 0.01 degrees Celsius per million years, which is significant over geological timescales.
This effect counteracts the greenhouse effect and helps to stabilize the climate, preventing it from becoming too hot or too cold.
The researchers suggested that smectite formation is part of Earth's hidden thermostat, a feedback mechanism that regulates the temperature of the planet.
The study, published in the journal Nature Geoscience, sheds new light on the role of smectite in the carbon cycle and the climate system.
It also highlights the importance of studying the deep Earth, which is largely inaccessible and poorly understood, but has a profound impact on the surface environment and life.
The researchers hope that their findings will inspire further research on the interactions between carbon, rocks, and water in the subduction zones, and their implications for the evolution of Earth and its climate.
