When we think of coral reefs, we picture bright fish, clear water and colourful corals. But reefs have also shaped the planet in deeper ways.
Authors
- Tristan Salles
Associate Professor, School of Geosciences, University of Sydney
- Laurent Husson
Earth sciences researcher, Centre national de la recherche scientifique (CNRS)
Our new study , published in Proceedings of the National Academy of Sciences, shows reefs have helped regulate Earth's climate and life for more than 250 million years.
They link geology, chemistry and biology into one grand planetary feedback loop. And their rise and fall over hundreds of millions of years set the pace of recovery from past carbon dioxide shocks, holding vital lessons for today.
From hot to cold
Earth's climate has swung between hot and cold periods over its long history.
These shifts reflect how carbon dioxide enters and leaves the atmosphere - since more carbon in the air means higher temperatures. Much of this happens through chemical reactions on land and the burial of carbonate minerals in the ocean.
A key part of this balance is ocean alkalinity. This describes the ocean's ability to neutralise acids and absorb carbon dioxide.
To investigate how reefs have influenced this process, we used reconstructions of ancient geography, river systems and climate, and then ran computer models back to the Triassic Period - about 250-200 million years ago. This was when the first dinosaurs appeared.
These tools revealed that reefs influenced how fast Earth recovered from large releases of carbon dioxide.
Two major modes
We found Earth switches between two major modes depending on the state of corals reefs.
The first mode occurs when tropical shelves (shallow, submerged continental areas in tropical latitudes) are broad and reefs thrive. This causes calcium carbonate - the chemical compound that builds corals - to accumulate in shallow seas. Calcium makes water more alkaline, so when it's locked up in coral the ocean becomes less alkaline.
With less alkalinity, the ocean loses some of its ability to soak up carbon dioxide. As a result, when carbon levels increase due to things like volcanic eruptions, the atmosphere can take hundreds of thousands of years to recover.
The second state happens when climate shifts, sea level falls, or tectonics restrict shallow habitats, and reefs shrink or disappear. Calcium then builds up in the deep ocean, making it more alkaline.
This means the ocean can absorb carbon dioxide more quickly.
A shift in recovery time
Depending on which mode it's in, Earth will respond very differently to the same increase in atmospheric carbon levels.
In phases when reefs dominate, recovery slows because shallow seas trap the dissolved minerals, known as ions, that would help the ocean absorb carbon.
In phases when reefs collapse, recovery speeds up because the ocean's buffering system is stronger and it is better able to absorb carbon dioxide.
These alternating periods have operated for more than 250 million years. They shaped climate rhythms and influenced how marine life evolved.
The plankton connection
That's not all that happens when reefs collapse.
When calcium and carbonate ions shift from coastal seas to the open ocean, nutrients follow. This fuels plankton growth.
These tiny algae absorb carbon from near the surface and take it to the bottom the ocean when they die, where it is trapped in deep-sea sediment.
The fossil record shows more new kinds of plankton evolved in periods when reefs collapsed. In contrast, in phases when reefs dominated, evolutionary change was slower because there were less nutrients for plankton in the open ocean.
In essence, the rise and fall of reefs helped set the tempo of ocean biological evolution. And this biological impact made the reefs' impact on the carbon cycle and global climate even more pronounced.
A message from the deep past
Today, humanity is adding carbon dioxide to the atmosphere at a rate comparable to some of the greatest carbon disruptions in Earth's history. At the same time, coral reefs are declining due to warming, acidification and pollution.
If the current reef loss mirrors ancient reef-collapse events, calcium and carbonates may again shift to the deep ocean. In theory, it could strengthen the absorption of carbon dioxide over the long term. But this would come only after catastrophic ecological loss.
The key lesson is that Earth will recover - but not on human timescales. Geological recovery takes thousands to hundreds of thousands of years.
Tristan Salles receives funding from Australian Research Council.
Laurent Husson does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
