Researchers have identified vast deposits of lava rubble beneath the South Atlantic Ocean that trap significant amounts of carbon dioxide over tens of millions of years. These breccia formations, formed from eroded underwater mountains, store far more CO2 than typical ocean crust samples. The discovery highlights a previously unrecognized mechanism in Earth's long-term carbon cycle.
Rock samples dating back about 60 million years, drilled from deep below the South Atlantic Ocean surface, reveal how carbon dioxide can remain sequestered for extended geological periods. Led by Dr. Rosalind Coggon, a Royal Society Research Fellow at the University of Southampton, the study examined lava material that interacts with seawater as it cools.
These interactions allow CO2 to become incorporated into the rocks. As underwater mountains erode, they produce large volumes of volcanic rubble known as breccia, similar to scree slopes on land. Dr. Coggon explained: "We've known for a long time that erosion on the slopes of underwater mountains produces large volumes of volcanic rubble, known as breccia -- much like scree slopes on continental mountains. However, our drilling efforts recovered the first cores of this material after it has spent tens of millions of years being rafted across the seafloor as Earth's tectonic plates spread apart."
The porous and permeable nature of these deposits enables them to hold substantial seawater CO2, which is gradually cemented by calcium carbonate minerals formed as seawater flows through. This process is part of the broader carbon cycle, where volcanic activity at mid-ocean ridges releases CO2 from Earth's interior into the oceans and atmosphere. Seawater then reacts with the cooling lavas, transferring elements and storing CO2 in mineral form.
Dr. Coggon noted: "While drilling deep into the seafloor of the South Atlantic, we discovered lava rubble that contained between two and 40 times more CO2 than previously sampled lavas." This finding, from Expedition 390/393 of the International Ocean Discovery Program, underscores the role of breccia in stabilizing Earth's climate over millions of years by acting as a natural carbon reservoir.
The research, published in Nature Geoscience, provides new insights into how carbon exchanges between the planet's interior, oceans, and air, reshaping understandings of long-term geological carbon storage.