Samples of basaltic breccia, cemented by white calcium carbonate minerals, were obtained from IODP Site U1557. These rocks, which formed about 60 million years ago and were extracted from deep beneath the ocean floor, have helped scientists understand how significant amounts of carbon dioxide can remain trapped for extremely long periods of time. Research has shown that CO2 becomes sequestered within layers of basaltic gravel accumulating on the seafloor.

The Role of Basaltic Gravel in Carbon Dioxide Storage

Scientists studied basaltic materials recovered from great depths beneath the South Atlantic to determine how much CO2 is incorporated into these rocks as a result of interactions between seawater and cooling volcanic material. A study conducted by the University of Southampton revealed that accumulations of fragmented lava, formed by the erosion of underwater mountains, act as natural reservoirs for CO2. For the first time, their role as extensive carbon-holding structures has been clearly identified, offering new insights into how Earth manages carbon over millions of years.

Basaltic Gravel as a Long-Term Geological “Sponge”

Lead author Dr. Rosalind Coggon, a Royal Society Research Fellow at the University of Southampton, explained: “It has long been known that the erosion of underwater mountain slopes leads to the formation of large volumes of volcanic gravel, known as breccia, similar to scree found on continental mountains. However, our drilling operations have, for the first time, allowed us to obtain cores of this material after it has spent tens of millions of years moving across the seafloor as Earth’s tectonic plates shift. Importantly, the cores showed that these porous, permeable deposits can accumulate large amounts of marine CO2, as they gradually become cemented by calcium carbonate minerals formed from seawater passing through them.”

Carbon Movement Over Geological Time

The amount of carbon dioxide in the atmosphere is determined by the slow exchange of carbon between Earth’s interior, the oceans, and the atmosphere over millions of years. To understand this long-term carbon cycle, it is essential to study where and how carbon is added to or removed from different parts of the planet. Dr. Coggon noted: “The oceans are covered with volcanic rocks that form at mid-ocean ridges as tectonic plates diverge, creating new oceanic crust. This volcanic activity releases CO2 from deep within the Earth into the ocean and atmosphere. However, ocean basins are not just reservoirs for seawater. Seawater flows through cracks in the cooling lava for millions of years, reacting with the rocks and transferring elements between the ocean and the crust. This process removes CO2 from the water and stores it in minerals such as calcium carbonate within the rock.”

Quantitative Assessment of CO2 Storage

As part of the project, the research team quantitatively assessed how much CO2 is incorporated into the oceanic crust as a result of these chemical reactions.

Discovery of Significant CO2 Storage in Breccia

“During drilling of the South Atlantic seafloor, we discovered basaltic gravel that contained between two and forty times more CO2 than previously studied lavas,” said Dr. Coggon. “This research highlights the importance of such breccia, formed by the erosion of seamounts along mid-ocean ridges, as a sponge for carbon in the long-term carbon cycle.” These findings were obtained during Expedition 390/393 of the International Ocean Discovery Program.