Corals build their skeletons from calcium carbonate, releasing carbon dioxide as a byproduct.
Reinhard Dirscherl/Alamy
The size of coral reefs around the world has played a key role in Earth's climate over the past 250 million years – but not in the way you might expect.
Coral reefs are net producers of carbon dioxide. because the greenhouse gas is a byproduct of the formation of calcium carbonate, which is what coral skeletons are made of.
Some species of plankton also build shells from calcium carbonate, and when they die, this mineral is buried on the seafloor. When large areas of shallow marine environments are covered with corals, the calcium and carbonate ions that are normally absorbed by deep-sea plankton are no longer available.
Tristan Salles from the University of Sydney (Australia) and his colleagues modeled the interactions between shallow-water corals and deep-sea ocean plankton over the past 250 million years, combining reconstructions of plate tectonics, climate modeling and changes in sediment flowing into the sea.
They found that the balance between corals and plankton is disrupted when plate tectonics and geomorphology create vast areas of shallow continental shelves that provide ideal habitat for reef-building corals.
When coral reefs are less extensive, calcium and alkalinity build up in the ocean, plankton become more productive, and more carbonates accumulate in the deep sea, which helps lower CO2 levels and lower temperatures.
The researchers found that over the past 250 million years, there were three major periods when the carbon cycle was seriously disrupted. These events, which occurred in the mid-Triassic, mid-Jurassic and late Cretaceous, affected vast coral reefs using enormous amounts of calcium carbonate, resulting in a significant increase in sea temperatures.
When the balance between shallow coral reefs and deep-sea plankton is disrupted, it can take hundreds of thousands to millions of years to restore balance, Salles said.
“So even if the system manages to recover from a huge crisis, the rebalancing will take place over a very long period of time, far beyond human timescales,” says Salles.
The silver lining, Salles says, is that if planktonic nutrient blooms ever get out of control, corals are perfectly positioned to absorb the excess nutrients to build coral reefs.
Now, human emissions of carbon dioxide are causing global warming and ocean acidification at an unprecedented rate, killing both corals and plankton, Salles says. The consequences are unknown but would likely be environmentally catastrophic, he says.
“The deep feedbacks we modeled are not applicable today—the rate of modern change is too fast for carbonate platform feedbacks to have any comparable significance.”
Alexander Skiles The Australian National University in Canberra says the research shows there is a “deeply intertwined feedback loop between life and climate.”
He says people often think that species evolve and adapt in response to Earth's climate, which is governed by “inevitable physical and chemical processes.”
“More and more, however, we are seeing examples where species directly influence climate itself, creating a feedback loop that evolves at the same time,” Skeels says. “Not only corals, but also older microbial colonies such as stromatolites played a key role in modulating atmospheric carbon.
“We know that carbon is warming our climate, and it's happening so quickly today, and reefs may have contributed to this process on very long time scales, explaining fluctuations in warm and cold intervals.”
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