Mixing Rock Into Soil Could Help Curb Rising Temperatures, Study Suggests

Adding finely crushed rocks into our fields might help to drive down the rising temperatures caused by carbon dioxide emissions, according to a new Nature Geoscience study.

Looking at oceanic carbonates and clay minerals dating back to two major global warming periods approximately 40 million and 56 million years ago, researchers have found that differences in exposed rock volumes on the planet's surface could have contributed to the differing lengths of time it took for the climate to recover from each event. Rock exposure has an impact, they say, as rock weathering initiates a process that can help to capture carbon dioxide in the oceans.

To deal with today’s rising carbon dioxide emissions, the researchers suggest that finding novel ways to boost levels of exposed rock could be a long-term strategy to promote natural carbon capture and storage.

Rock weathering helps the climate recover after major events

Around 56 million years ago, the Earth went through a period known as the Paleocene–Eocene Thermal Maximum (PETM), where temperatures rose by an average of 5 to 8 degrees Celsius. Persisting for an estimated 200,000 years, the unusually high temperatures are believed to have been caused by an uptick in volcanism and the release of massive amounts of carbon dioxide into the atmosphere.

Scientists have studied the mechanisms that allowed the climate to recover and the Earth to cool back down again after the PETM. In 2021, Professor Philip Pogge von Strandmann of Johannes Gutenberg University Mainz (JGU) published a paper in Science Advances that showed that rainwater combined with atmospheric carbon dioxide would have resulted in increased levels of carbonic acid in the rain cycle at this time.

This acid would begin to weather rocks more heavily, releasing minerals like calcium and magnesium into the waterways. From there, these minerals were transported into the ocean where they could react with the carbon dioxide absorbed by the oceans – trapping it in insoluble limestone deposits and effectively removing it from the atmosphere. 

"In other words, there is a feedback effect that helps control the climate. High temperatures accelerate the chemical rock weathering process, reducing the levels of carbon dioxide in the atmosphere, allowing the climate to recover," says Pogge von Strandmann.

Approximately 16 million years after the PETM, the Earth experienced another significant warming event – the Middle Eocene Climatic Optimum (MECO). But while the MECO released similar amounts of carbon dioxide into the atmosphere as the PETM, the MECO warming period lasted for nearly twice as long.

Why did the Earth take so long to recover? Geochemistry might hold the answer, once again.

“Soil shielding” prevents weathering, stops carbon trapping

In their latest study, Pogge von Strandmann and colleagues examined oceanic carbonates and clay minerals dating back to the PETM and compared these against similar samples from the MECO. By measuring lithium isotope ratios, the researchers were able to build up a better picture of the silicate rock weathering happening during these periods.

"Just as during the PETM, there was also intensified weathering and erosion in the MECO,” Pogge von Strandmann says. “However, there was far less exposed rock on the Earth's surface 40 million years ago. Instead, the Earth was extensively covered by a global rainforest, the soil of which largely consisted of clay minerals.”

Unlike rock, clay does not contribute to the carbonate–silicate cycle that helps to trap carbon dioxide in the seafloor. In fact, clay minerals are one of the main products of the weathering process. The researchers believe that the clay mineral dynamics during these periods – specifically the increase in clay formation suggested by the lithium isotope data – could have had a significant impact on climate.

"So despite the high temperatures, the widespread clay soil prevented rocks from being effectively weathered, a process known as soil shielding,” explains Pogge von Strandmann. Calcium and magnesium ions are attracted to the negatively charged surface layers of some clays, which leads to more of these minerals being retained in soils and thus can impact the availability of these ions to trap carbon dioxide.

What does this mean for today’s climate?

By studying historic weathering and clay mineral dynamics, the researchers are hoping to uncover new ways to mitigate rising carbon dioxide levels in the present day.

"We study paleoclimates to determine whether and how we can positively influence our present climate. One option might be to boost the chemical weathering of rock. To help achieve this, we could plow finely crushed rock into our fields," says Pogge von Strandmann.

In such environments where weathering forms clay, this negates some of the positive climate impacts of the weathering. But by distributing fine-grain rock through soils, the researchers hope that the weathering process could proceed extremely rapidly, helping to support the natural carbon–silicate cycle in trapping additional carbon dioxide.

The extent to which crushed rock would erode in soil and whether it might also encourage clay formation has yet to be thoroughly studied. It is possible that local factors in the environment, such as pre-existing levels of clay and rock, could affect this significantly. The researchers say that further investigations of historic clay dynamics, plus studies examining the behavior of fine rock in soil, should help to confirm the feasibility of crushed rock as a negative emissions technology.

Reference: Krause AJ, Sluijs A, Van Der Ploeg R, Lenton TM, Pogge Von Strandmann PAE. Enhanced clay formation key in sustaining the Middle Eocene Climatic Optimum. Nat Geosci. 2023;16(8):730-738. doi: 10.1038/s41561-023-01234-y

This article is a rework of a press release issued by Johannes Gutenberg University Mainz. Material has been edited for length and content.