The living architectures of corals harbor rich biodiversity, but they may also have played, over geological time, a central role in global climate balance. A recent study indicates that their presence or disappearance has regulated Earth's capacity to absorb excess atmospheric carbon. This mechanism, highlighted by an international team, sheds light on the interactions between life, rocks, and climate across eras.
Published in
Proceedings of the National Academy of Sciences, this research uses paleoclimatic and geological simulations to trace the history of reefs over 250 million years. It reveals that our planet has alternated between two functional states, dictated by the vitality of shallow-water coral reefs. These states directly influenced the rate of carbon sequestration and the speed of climate regulation after major disturbances.
The two faces of the marine carbon cycle
When reefs are thriving, calcium carbonate accumulates in coastal sediments. This process traps carbon in a very stable manner over the long term, but it is relatively slow and limits the availability of carbonate ions in the open ocean. This can restrict the development of calcifying plankton in the open ocean, which is nevertheless a key player in the rapid biological pump of carbon to the seafloor.
Thus, when Earth must absorb a sudden excess of atmospheric CO₂ (after a major volcanic eruption, for example), this "reef-dominated" configuration could slow the initial recovery speed. The climate system would then take several hundred thousand years to regain its balance, as it primarily depends on a slower coastal sequestration process.
The second state appears during massive reef collapses, caused by tectonic changes, sea level variations, or climate crises. Without this main coastal sink, carbonate dissolves in the water column. Calcium and alkalinity ions then accumulate in the open ocean, which increases its alkalinity and its capacity to absorb carbon.
This chemical change stimulates the productivity of calcifying nanoplankton in the open ocean. The biological pump is then reactivated, accelerating the transfer of carbon from the surface to deep sediments. Climate recovery after a massive carbon input can thus be faster, shifting to a scale of tens of thousands of years.
A deep biological and chemical legacy
This alternation of states not only influenced climate but also shaped the evolution of marine life and ocean chemistry. Reef collapse phases, by promoting the export of nutrients and carbonates offshore, created favorable conditions for the rise of plankton. The major expansion of these organisms in the fossil record precisely coincides with periods when reefs were in decline.
The study's authors emphasize that these shifts altered the global biogeochemical balance. The decline of shallow-water reefs directly favored the diversification and efficiency of open-ocean plankton communities, thereby redrawing the pathways of the carbon cycle.
Reefs thus appear to be much more than simple environmental indicators. They are full-fledged geophysical actors, capable of influencing the planet's response to disturbances. Their state determines whether Earth operates in "slow" or "fast" recovery mode, an important factor for understanding past climates. This discovery reinforces the need to fully integrate them into models that reconstruct climate history.
Article author: Cédric DEPOND