A mineral produced by plate tectonics has a global cooling effect, study finds

Geologists at MIT have discovered that smectite, a seabed clay mineral, has the unexpected capacity to trap carbon over millions of years.

A single clay grain looks like the folds of an accordion under a microscope. It is well known that these folds serve as efficient organic carbon traps.

The MIT group has now demonstrated that plate tectonics is the source of the carbon-trapping clays: Rocks that eventually weather into minerals like smectite can be brought to the surface when oceanic crust collides with a continental plate. The deceased creature fragments are eventually trapped in the minute folds of the minerals in the clay silt when it returns to the ocean. By doing this, the organic carbon is prevented from being ingested by microorganisms and released as carbon dioxide back into the atmosphere.

Smectite can have a worldwide influence over millions of years, contributing to global cooling. The scientists demonstrated through a number of investigations that smectite was most likely formed following a number of significant tectonic events throughout the previous 500 million years. The clays stored enough carbon during each tectonic event to cause Earth to cool and trigger the ice age that followed.

These results are the first to demonstrate how the formation of carbon-trapping smectite can initiate ice ages due to plate tectonics.

These clays are still present in some tectonically active areas, and scientists think that smectite is still storing carbon, functioning as a slow-acting natural barrier against human activity that warms the planet.

Joshua Murray, a PhD student in the Department of Earth, Atmospheric, and Planetary Sciences at MIT, states that "the influence of these unassuming clay minerals has wide-ranging implications for the habitability of planets." "There may even be a modern application for these clays in offsetting some of the carbon that humanity has placed into the atmosphere."

Murray and MIT geology professor Oliver Jagoutz published their research results in Nature Geoscience.

A visible and distinct clay

The team's earlier research, which demonstrated that every major ice age on Earth was probably brought on by a tectonic catastrophe in the tropics, is being followed up on by this current study. The scientists discovered that the atmosphere was exposed to ocean rocks known as ophiolites during each of these tectonic episodes.

They proposed that specific weathering processes, such exposure to wind, rain, and chemical interactions, might cause ophiolites to undergo weathering in a tropical location following a tectonic collision, converting the rocks into a variety of minerals, including clays.

"Those clay minerals, depending on the kinds you create, influence the climate in different ways," says Murray.

The minerals that may emerge from this weathering action, as well as whether and how these minerals could directly contribute to cooling the planet, were unknown at the time. Therefore, even though it seemed that plate tectonics and ice ages were related, it was still unclear how precisely one might cause the other.

The goal of the new study was to determine if the carbon-trapping minerals that the team's projected tectonic tropical weathering mechanism would create in sufficient numbers to start a global ice age.

The group initially perused the geologic literature to gather information on the many ways that main magmatic minerals weather over time, as well as the kinds of clay minerals that can result from this weathering. A weathering simulation of many rock types that are known to be exposed in tectonic collisions was then created using these observations.

"Then we look at what happens to these rock types when they break down due to weathering and the influence of a tropical environment, and what minerals form as a result," Jagoutz explains.

Subsequently, they inserted every worn-out "end-product" mineral into a model of the Earth's carbon cycle to see the potential impact of each mineral on the interaction of inorganic carbon, which is found in the atmosphere as carbon dioxide, or organic carbon, which includes remnants of extinct creatures.

Smectite was the mineral whose existence and impact were most evident based on these investigations. The clay was an exceptionally good organic carbon trap in addition to being a naturally weathered result of tropical tectonics. Smectite seems to provide a strong theoretical link between tectonics and ice periods.

However, were there genuinely enough of the clays to start the preceding four ice ages? The best way for scientists to verify this would be to discover smectite in prehistoric rock strata that go back to each global cooling epoch.

"Unfortunately, as clays are buried by other sediments, they get cooked a bit, so we can't measure them directly," Murray explains. "But we can look for their fingerprints."

a gradual increase

The scientists reasoned that because smectites are a byproduct of ophiolites, distinctive elements like nickel and chromium, which would be retained in ancient sediments, are also present in these ocean rocks. In the event that smectites were formerly present, nickel and chromium ought to be as well.

The group examined hundreds of oceanic sedimentary rocks that have been deposited over the last 500 million years in a database to evaluate this theory. Four distinct ice ages occurred on Earth during this time. The presence of significant nickel and chromium spikes in the rocks at each of these eras led the researchers to conclude that smectite must have also been present.

Their calculations showed that the clay mineral might have contributed less than a tenth of a percent to the preservation of organic carbon. This is a very little sum in absolute terms. However, scientists determined that the accumulated, trapped carbon in the clay over millions of years was sufficient to start each of the four major ice eras.

"We found that you really don't need much of this material to have a huge effect on the climate," Jagoutz explains.

"These clays also have probably contributed some of the Earth's cooling in the last 3 to 5 million years, before humans got involved," says Murray. "These clays are probably influencing the climate even in the absence of people. It simply moves so slowly."

"Jagoutz and Murray's work is a nice demonstration of how important it is to consider all biotic and physical components of the global carbon cycle," says Penn State University geosciences professor Lee Kump, who was not involved in the research. "Feedbacks among all these components control atmospheric greenhouse gas concentrations on all time scales, from the annual rise and fall of atmospheric carbon dioxide levels to the swings from icehouse to greenhouse over millions of years."

Is it possible to purposely use smectites to reduce global carbon emissions even more? Murray sees some opportunity, such protecting carbon stores like permafrost zones. Permafrost is expected to melt as a result of global warming, exposing long-buried organic carbon. The clays in these areas may stop this exposed carbon from escaping and warming the atmosphere even more if smectites were put there.

"If you want to understand how nature works, you have to understand it on the mineral and grain scale," explains Jagoutz. Additionally, this is how we will go ahead in order to address this climate disaster. There's a considerable possibility you may discover something truly helpful if you examine these natural processes."