Geologists at the Massachusetts Institute of Technology have identified a clay mineral on the seafloor known as smectite that appears capable of storing carbon for millions of years. This finding opens a door to understanding a previously unknown mechanism for removing CO2 from the atmosphere and suggests that this natural process could contribute to mitigating global warming over long timescales.
Under the microscope, a small grain of this clay reveals accordion-like folds that act as efficient traps for organic carbon. The researchers show that the carbon-capturing properties of smectite result from geological processes associated with plate tectonics: when oceanic crust collides with a continental plate, rocks rise to the surface and, over long periods, transform into minerals including smectite.
Over time, clay sediment settles back into the ocean, and minerals trap portions of dead organisms within their microscopic folds. This sequestration hinders microbes from consuming organic carbon and releasing CO2 back into the atmosphere.
Help cool the planet
For millions of years, smectite beneath oceans could influence global climate by acting as a carbon reservoir. Analyses indicate that smectite likely formed after several major tectonic events across the last 500 million years. During each event, the clay trapped enough carbon to contribute to planetary cooling and, in some cases, the onset of ice ages.
In fact, this clay has helped shape climatic cycles by acting as a long-term sink for carbon, a process that may have contributed to the initiation of ice ages in the geological record.
In a broader view, the discovery shows for the first time that plate tectonics can influence climate patterns through the production of carbon-sequestering smectite, a mineral that remains present in some tectonically active regions today and continues to play a role in moderating atmospheric carbon levels, even as surface activities slowly rise.
Scientists believe smectite continues to sequester carbon at present, offering a natural buffer against warming from human activities that are slow-acting but persistent.
The potential of smectite as a climate buffer
The potential impact of these modest clay minerals extends beyond climate alone. Joshua Murray, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences, notes that there may even be modern applications for this clay to offset some of the carbon emitted by humanity. His team, along with Oliver Jagoutz, professors of geology at MIT, published their findings in the journal Natural Geology.
The new work builds on the team’s prior research suggesting that each of Earth’s major ice ages was likely triggered by tropical tectonic events. These events exposed oceanic rocks called ophiolites to weathering, which, under wind and rain and chemical interactions, transform rocks into minerals including clay.
“These clay minerals influence climate in different ways depending on the species present,” explains Murray. At the time, it remained uncertain which minerals could emerge from tropical weathering and whether they might directly contribute to planetary cooling. While a link between plate tectonics and ice ages was suspected, the exact mechanism was not clear.
With the current study, the team explored whether the tropical erosion process could produce carbon-sequestering minerals in quantities sufficient to influence global climate and potentially trigger a world-scale cooling event.
Checking cooling potential
The researchers began by surveying existing geology studies and data on the weathering of large igneous minerals over time, focusing on which clay minerals might form from such weathering. They then integrated these measurements into simulations of erosion from rock types involved in tectonic collisions.
“Next, they examined what happens to eroded rocks as they break down in tropical environments and which minerals emerge,” Jagoutz explains. They incorporated each end-product mineral into simulations of the Earth’s carbon cycle to gauge how a specific mineral could influence atmospheric carbon dioxide through interactions with organic or inorganic carbon from dead organisms.
Among the minerals tested, smectite stood out for its clear cooling potential. As a naturally produced product of tropical tectonics and an effective trap for organic carbon, smectite appeared to provide a plausible link between tectonics and the cooling of the planet.
Yet a key question remained: was the amount of smectite sufficient to drive past ice ages? Direct measurements are not possible because the clay is buried under sediments, but researchers can search for its fingerprints in ancient rocks.
“We can’t measure them directly, but we can look for traces,” says Murray. The team reasoned that since smectite derives from ophiolites, oceanic rocks, these rocks carry distinctive elements such as nickel and chromium that can be preserved in ancient sediments. If smectite existed in the past, those elements would appear in the same state in sedimentary records.
It’s a slow process
To test this, the researchers analyzed a database of thousands of ocean sediments formed over the last 500 million years, a span that includes four major ice ages. Looking at rocks from each interval, they observed notable peaks of nickel and chromium and concluded that smectite was likely present during those times.
The study concludes that the carbon trapped within clay over millions of years was enough to contribute to each of the planet’s four major ice ages. Although the amount of clay involved is small, the cumulative effect over vast timescales could be significant, influencing atmospheric carbon levels and climate trends.
“It doesn’t take much of this material to have a noticeable impact on the climate,” Jagoutz notes.
“This clay may also have contributed to the cooling of the Earth over the last 3 to 5 million years,” Murray adds. “In the absence of human activity, this clay probably makes a difference to the climate, though it is a very slow process.”
Possible practical application against climate change
Experts outside the study view the findings as a reminder of how all components of the carbon cycle interact. The work highlights the importance of considering biotic and physical factors when thinking about climate dynamics and greenhouse gas concentrations across time scales, from yearly fluctuations to millions of years of climate transitions.
Could smectites be used deliberately to aid in reducing global carbon emissions? Murray sees potential for supporting carbon stocks in permafrost regions where warming threatens to release long-buried organic carbon. If smectite could be applied to these areas, it might help prevent release and further warming, though practical deployment would require careful study.
“If you want to understand how nature works, you have to study it at the scale of minerals and grains, and that’s how you find solutions to climate challenges,” says Jagoutz. “This line of research could lead to useful, real-world applications.”
Reference work: DOI: 10.1038/s41561-023-01342-9
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