Can the seabed assist the ocean in absorbing more CO2?

The ocean plays an essential part in safeguarding the planet, absorbing approximately 31% of the carbon dioxide (CO2) released from human activities, retaining it in its waters and neutralizing some by interacting with sediments. This natural mechanism has maintained Earth’s climate in equilibrium for eons, though it operates slowly, unfolding over millennia.

Currently, with carbon outputs increasing much quicker than the ocean can assimilate them, scientists are seeking methods to gain a better understanding of — and potentially enhance — the ocean’s inherent capacity to capture and store carbon.

Just beyond the surf at Huntington Beach, a group of USC oceanographers is probing the possible contribution of the sandy ocean floor in this mechanism.

“The seafloor plays an active role in carbon cycling,” stated Will Berelson, the Paxson H. Offield Professor in Coastal and Marine Systems at the USC Dornsife College of Letters, Arts and Sciences and the principal investigator of the study.

“In this shallow region just offshore, where waves and currents continually disturb the sandy substrate, we’re observing chemical interactions occurring in the small pockets of water between sand grains,” he noted. “These interactions might assist the ocean in storing more carbon, and we’re attempting to determine how common and effective this natural process is.”

CO2’s signature

That concealed chemistry initiates with porewater — water trapped in the gaps between grains of sand — which contains dissolved carbon along with other chemical compounds. As porewater filters through layers of sediment on the seabed, it engages with minerals, organic materials, and tiny microbes, triggering a series of chemical interactions.

Some of these interactions, USC scientists propose, might contribute to neutralizing CO2 and elevating the ocean’s alkalinity — effectively making the water less acidic.

“Ocean acidification is a well-established outcome of increasing atmospheric CO2 concentrations,” Berelson remarked. “When carbon dioxide dissolves in seawater, it creates a mild acid that reduces the pH. This alteration in chemistry makes it more difficult for marine organisms such as corals, oysters, and specific plankton…

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to construct their shells and frameworks — interfering with food networks, fisheries, and potentially jeopardizing entire ecosystems.”

The study, carried out with graduate student Matt Quinan and partly funded by USC Sea Grant, examines the continental shelf near Huntington Beach in Orange County — a shallow section of ocean floor that may serve as a natural carbon repository. The team is particularly focused on the porewater, which contains chemical indicators that show how carbon shifts and transforms within the sediment.

Collecting samples from the seabed

To investigate the seabed without disturbing it, the team engineered a custom device known as an in situ porewater sampler. Deployed from a boat, this apparatus gently extracts water from beneath the sand, preserving it for laboratory examination.

These specimens offer insight into the physical, chemical, and biological activities occurring just beneath the surface.

“By examining the levels of dissolved carbon and other substances in the pore water, we can deduce the types of reactions taking place within the sediment,” Berelson stated.

This encompasses microbial respiration — where microscopic organisms decompose organic material and emit CO2 — as well as mineral reactions that may either immobilize carbon within the seabed or release it back into the ocean. These nuanced exchanges may significantly influence the regulation of the ocean’s carbon sequestration capacity, the researchers noted.

“We gauge the total concentration of CO2 in each sample by converting all dissolved carbon into gas and employing a laser-based device to assess it,” Berelson clarified. “That device not only indicates the amount of CO2 present — it also reveals the type of CO2. It detects the carbon’s isotopic signature, akin to a fingerprint, assisting us in understanding how the carbon arrived and its prior experiences.”

By enhancing our comprehension of the carbon interactions occurring beneath the surface, researchers aim to develop more precise models of how the ocean reacts to escalating carbon dioxide levels and how it may continue to do so moving forward.