The movement of carbon between the atmosphere, oceans, and continents, known as the carbon cycle, is essential for regulating Earth’s climate. Volcanic eruptions and human activities release carbon dioxide (CO2) into the atmosphere, while forests and oceans absorb it. In a balanced system, the right amount of CO2 is emitted and absorbed to maintain a healthy climate. Carbon sequestration is a key strategy in combating climate change.
A new study reveals that the shape and depth of the ocean floor explain up to 50% of the changes in the depth at which carbon has been sequestered in the ocean over the past 80 million years. Previously, these changes were attributed to other factors. Although scientists have long known that the ocean, the largest carbon absorber on Earth, directly controls atmospheric CO2 levels, the impact of seafloor topography changes on oceanic carbon sequestration was not well understood until now.
This research, published in the journal Proceedings of the National Academy of Sciences, demonstrates that the shape and depth of the ocean floor play significant roles in the long-term carbon cycle. “We were able to show, for the first time, that the shape and depth of the ocean floor play major roles in the long-term carbon cycle,” said Matthew Bogumil, the study’s lead author and a UCLA doctoral student in Earth, planetary, and space sciences.
The long-term carbon cycle involves many elements operating on different time scales, including seafloor bathymetry—the mean depth and shape of the ocean floor. Bathymetry is influenced by the positions of continents and oceans, sea level, and mantle flow. Scientists use carbon cycle models, calibrated with paleoclimate datasets, to understand the global marine carbon cycle and its response to natural changes.
The researchers reconstructed ocean bathymetry over the last 80 million years and integrated this data into a computer model to measure marine carbon sequestration. The findings showed that ocean alkalinity, calcite saturation state, and the carbonate compensation depth were heavily influenced by changes to shallow ocean areas (about 600 meters or less) and the distribution of deeper regions (greater than 1,000 meters). These factors are critical for understanding carbon storage in the ocean floor.
For the current geologic era, the Cenozoic, bathymetry alone accounted for 33%–50% of the observed variation in carbon sequestration. The researchers concluded that overlooking bathymetric changes leads to misattributing changes in carbon sequestration to less certain factors, such as atmospheric CO2 levels, water column temperature, and riverine silicates and carbonates.
This new understanding of the significant role of ocean floor shape and depth in carbon sequestration can also aid in the search for habitable planets. “When looking at faraway planets, we currently have a limited set of tools to give us a hint about their potential for habitability,” said co-author Carolina Lithgow-Bertelloni, a UCLA professor and department chair of Earth, planetary, and space sciences. “Now that we understand the important role bathymetry plays in the carbon cycle, we can directly connect the planet’s interior evolution to its surface environment when making inferences from JWST observations and understanding planetary habitability in general.”
This breakthrough is just the beginning. “Now that we know how important bathymetry is in general, we plan to use new simulations and models to better understand how differently shaped ocean floors will specifically affect the carbon cycle and how this has changed over Earth’s history, especially the early Earth, when most of the land was underwater,” Bogumil said.
