The ocean plays a vital role in protecting the planet, absorbing about 31% of the carbon dioxide (CO2) released by human activity, storing it in its waters and neutralizing some of it by reacting with sediments. This natural process has helped keep Earth's climate in balance for millennia, but it moves slowly, unfolding over thousands of years.
Today, with carbon emissions rising far faster than the ocean can absorb them, scientists are looking for ways to better understand - and potentially enhance - the ocean's natural ability to capture and store carbon.
Just beyond the surf at Huntington Beach, a team of USC oceanographers is investigating the potential role of the sandy ocean floor in this process.
"The seafloor is actively involved in carbon cycling," said Will Berelson, the Paxson H. Offield Professor in Coastal and Marine Systems at the USC Dornsife College of Letters, Arts and Sciences and the study's lead researcher.
"In this shallow area just offshore, where waves and currents are constantly stirring up the sandy bottom, we're seeing chemical reactions take place in the tiny pockets of water between grains of sand," he said. "Those reactions might help the ocean store more carbon, and we're trying to find out how prevalent and effective is this natural process."
CO2's fingerprint
That hidden chemistry starts with porewater - water trapped in the spaces between grains of sand - which contains dissolved carbon and other chemical compounds. As porewater seeps through the layers of sediment on the seafloor, it interacts with minerals, organic matter and tiny microbes, setting off a cascade of chemical reactions.
Some of these reactions, USC scientists believe, may help neutralize CO2 and increase the ocean's alkalinity - essentially making the water less acidic.
"Ocean acidification is a well-documented consequence of rising atmospheric CO2 levels," Berelson said. "When carbon dioxide dissolves in seawater, it forms a weak acid that lowers the pH. That shift in chemistry makes it harder for marine organisms like corals, oysters and certain plankton to build their shells and skeletons - disrupting food webs, fisheries and potentially threatening entire ecosystems."
The research, conducted with graduate student Matt Quinan and supported in part by USC Sea Grant, focuses on the continental shelf off Huntington Beach in Orange County - a shallow stretch of ocean floor that may act as a natural carbon sink. The team is especially focused on the porewater, which carries chemical fingerprints that reveal how carbon moves and transforms in the sediment.
Sampling the seafloor
To study the seafloor without disturbing it, the team developed a custom-built device called an in situ porewater sampler. Lowered from a boat, the instrument gently draws up water from beneath the sand, capturing it intact for lab analysis.
These samples provide a window into the physical, chemical and biological processes happening just beneath the surface.
"By analyzing the concentrations of dissolved carbon and other compounds in the pore water, we can infer what kinds of reactions are occurring within the sediment," Berelson said.
These include microbial respiration - where tiny organisms break down organic material and release CO2 - and mineral reactions that can either lock carbon into the seafloor or return it to the ocean. These subtle exchanges may play a significant role in regulating the ocean's carbon storage capacity, the researchers said.
"We measure the total amount of CO2 in each sample by turning all the dissolved carbon into gas and using a laser-based instrument to analyze it," Berelson explained. "That instrument doesn't just tell us how much CO2 is there - it also tells us what kind of CO2. It reads the carbon's isotopic signature, like a fingerprint, and that helps us figure out how the carbon got there and what it's been through."
By improving our understanding of the carbon reactions taking place beneath the surf, researchers hope to build more accurate models of how the ocean responds to rising carbon dioxide levels and how it might continue to do so in the future.
