Hydrology, Cryosphere & Earth Surface News

Landslides Send Carbon-Rich Soils into Long-Term Storage

Earthquake-triggered landslides move soils down steep slopes and deposit the sediments near rivers, sequestering the carbon contained within them for millions of years.

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Earth’s carbon cycle—the continual movement of this life-critical element through the environment—is an intricate process in which soil, rocks, water, and plate tectonics store, release, and move carbon over varying timescales.

Now researchers have shown that earthquake-triggered landslides can play a role in the carbon cycle by sequestering carbon, albeit in relatively small quantities. Landslides can transport carbon-rich soils down steep slopes toward rivers and lakes, where the soils are incorporated into sediments and trapped for millions of years, the researchers reported month week in Nature Geoscience.

This new result reveals an added link between plate tectonics and the carbon cycle and demonstrates a natural method of carbon sequestration, the researchers said.

Violent Weathering

On grand scales of geologic time, soil-bound carbon is ephemeral; soils house carbon for hundreds or perhaps thousands of years. Rainwater pushing through soil can mobilize this carbon into rivers, which eventually flow to the ocean. While in these waters and in soil, carbon can escape into the atmosphere and become carbon dioxide (CO2). This molecule is consumed by plants as they photosynthesize. At the same time, carbon dissolved in the ocean is incorporated into the bodies of marine creatures such as corals and plankton, which in turn carry carbon to the seafloor when they die. Over time, the shells of these animals, along with dead plants, are compacted into carbon-rich sediments and turned into rocks like limestone.

Tectonics has a role to play here too: The movement of plates pushes up mountains that attract rain clouds, exposing rocks to weathering. Plants and animals rework these weathered rocks, converting them into soils, and the carbon cycle begins anew.

But nature, it seems, can take shortcuts through this cycle in ways that cut out the atmosphere entirely. The mechanism is, again, weathering and tectonics, although this time in more violent bursts through earthquake-triggered landslides, explained Robert Hilton, an Earth scientist at Durham University in the United Kingdom.

Between 2014 and 2017, scientists working with Hilton traveled to Lake Paringa, a U-shaped body of water on New Zealand’s South Island. The researchers had previously pinpointed this area as being a hotbed of both tectonic and landslide activity: Four earthquakes, all of whose estimated magnitudes topped 7.6, had rocked the region within the past 1,100 years and had triggered massive landslides.

Lake Paringa in New Zealand sits atop carbon-rich sediments deposited by four landslides over the past 1,100 years.
New Zealand’s Lake Paringa sits atop carbon-rich sediments deposited by four landslides over the past 1,100 years. Credit: Michal Klajban, CC BY-SA 4.0

The scientists extracted a 6-meter sediment core from beneath the lake, opening a window to the past. “We could look at the sediments that got washed in after the earthquakes,” said Hilton. Specifically, they examined the carbon isotopes 12C and 13C contained in landslide deposits sourced from slumps that were known from past research to be triggered by earthquakes. Their analysis showed an uptick in the amount of carbon buried after each earthquake-induced landslide eroded large amounts of carbon-rich material.

In fact, the amount of carbon delivered to Lake Paringa after each earthquake was 2–3 times greater than the amount delivered between the earthquakes, Hilton said. The overall increase in carbon delivery was substantial: More than 40% of the carbon in the core was associated with the cumulative effect of these landslides, the team noted.

The observations led to a curious conclusion: Although most earthquakes create elevation changes that trigger weathering and carbon release, landslides from large earthquakes can “significantly contribute to carbon export from mountain forests over millennia,” the researchers wrote in their study.

This study highlights the “previously under-appreciated role that large earthquake events may play in CO2 sequestration and global climate on geologic time scales,” said Lonnie Leithold, a sedimentologist at North Carolina State University in Rayleigh who was not involved in the research.

Not Going to Save the Day

The new results show an unexpected way that nature sequesters carbon. And carbon sequestration—the idea that carbon can be locked up in a form inaccessible to the atmosphere—is a hot topic in climate change science as a possible way of mitigating global warming.

But despite earthquake-driven sequestration likely occurring worldwide, particularly in the Himalaya and Andes mountain ranges, it won’t solve the immediate problem of global warming, the researchers note. “The amounts of carbon involved in these natural carbon-removal processes are hundreds to thousands of times lower than anthropogenic CO2 emissions,” said Hilton. “These kinds of processes aren’t going to save the day.”

—Katherine Kornei (email: [email protected]; @katherinekornei), Freelance Science Journalist

Citation: Kornei, K. (2018), Landslides send carbon-rich soils into long-term storage, Eos, 99, https://doi.org/10.1029/2018EO105705. Published on 10 September 2018.
Text © 2018. The authors. CC BY-NC-ND 3.0
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