Reefs grow by crawling along the seafloor
In November 1835, the HMS Beagle visited Tahiti, in the South Pacific. Climbing up the island’s slopes, the young Charles Darwin looked across the sea to nearby Moorea, and saw an island surrounded by a barrier reef. During and after his voyage, Darwin constructed a theory for reef formation that explained how fringing reefs grow into barrier reefs, which then convert into atolls. “The close similarity in form, dimensions, structure, and relative position between fringing and encircling barrier-reefs, and between these latter and atolls,” he wrote, “is the necessary result of the transformation, during subsidence, of the one class into the other. On this view, the three classes of reefs ought to graduate into each other.”
For centuries, Darwin’s solution to his first scientific problem has been accepted as fact by the textbooks. As islands sink below sea level due to subsidence, coral reefs grow vertically to maintain their position at sea level. In that way, a fringing reef grows to become a barrier reef, and a barrier reef becomes an atoll when the island sinks below sea level, leaving a lagoon in its place. The theory is clever, elegant, and simple. And new research published last week in Nature Scientific Reports shows that it’s probably wrong.
There is a big problem with Darwin’s theory of reef formation: the earth has experienced significant changes in sea level over the past two million years. If Darwin’s theory was correct, then new reefs would form adjacent to olds ones as sea level changed, forming multiple ringed reefs around a central island. Rather than the slow rise in sea level that Darwin assumed, reefs have been subjected to a repeated cycle of sea level rise and fall, governed by the growth and melting of ice at the planet’s poles. Given what we now know about our planet’s geological and oceanographic history, vertical growth can’t explain the transition between reef types.
If that’s so, then maybe each reef type forms in its own way, and they don’t transition from one to the other. Several decades ago, scientists tried to pursue that line of thinking. But researchers have since discovered genetic relatedness among reef types, which suggests that fringing reefs can become barrier reefs, which can later form atolls. But how? Once again, scientists were faced with the same problem that Darwin had tried to solve.
To try to answer the question, Paul Blanchon returned to Tahiti, armed with almost two hundred additional years of scientific knowledge compared to when Darwin visited the island, along with considerably more advanced technology and tools at his disposal.
By studying the geology and genetics from cores that had been drilled out of the reefs, Blanchon and his colleagues discovered that reefs don’t maintain themselves at sea level by growing vertically. Instead, they walk along the sea floor, up the slope of the island.
Here’s how it works: as sea level rises, reefs follow the shorelines, growing along the seafloor itself as it rises towards the island. The increase in sediment that results from the upslope movement prevents a type of fast-growing corals called acroporids from dominating the reef, being replaced by other slower-growing corals.
Sometimes, as the reef creeps towards the shoreline, it encounters a flat, horizontal surface. But it’s not just any old horizontal bit of seafloor. It’s the top of the skeleton of an ancient fringing reef.
When the older reef was alive, sea level was relatively stable, allowing the reef to form a horizontal flat surface just beneath the water’s surface. Then sea level dropped as ice formed at the poles, and the ancient reef died.
Thousands of years later, as sea level began to rise again, a new reef could once again march up the seafloor towards the island. On that march, it eventually found itself atop the old reef. At Tahiti, that happened around 14 thousand years ago.
Deprived of its ability to crawl further along the slope, the reef created a lagoon that trapped the island’s sediment, preventing it from washing out to sea. The reduction in sediment flow suddenly allowed the fast-growing acroporids to recolonize the reef. The presence of the acroporids meant that the reef could grow more swiftly, rising vertically in lockstep with the rising seas. Suddenly, a fringing reef became a barrier reef.
While Darwin was right that fringing reefs transition into barrier reefs, his explanation for the transformation was wrong. He was also right about how barrier reefs become atolls: that happens when the island sinks below sea level, leaving behind a shallow sediment-filled lagoon.
“Reefs are not the static features we take for granted, but are dynamic and change form dramatically over time,” says Blanchon. Some 20,000 years ago, in fact, barrier reefs and atolls didn’t even exist. The only reefs a sailor at that time would have seen are small fringing reefs clinging to the shorelines. It’s has only been during the most recent rise in sea level, following the last “ice age,” that conditions existed for barrier reefs and atolls to form in the first place. “In other words, there were times in the past when reefs as we know them hardly existed,” he adds.
That leaves open an important question as we ponder the future of our oceans: could reef structures change dramatically again, due to future sea level rise? If climate change drives sea level rise so rapidly that the reefs are unable to keep up, will we lose them entirely? “That’s a big question, too big in fact to answer with our present predictions and time frames,” Blanchon told me. “But it could certainly be one future scenario, especially if we keep developing every last fossil carbon deposit on the planet, as we seem to be doing.” – Jason G. Goldman | 28 May 2014
Source: Blanchon P., Granados-Corea M., Abbey E., Braga J.C., Braithwaite C., Kennedy D.M., Spencer T., Webster J.M. & Woodroffe C.D. (2014). Postglacial Fringing-Reef to Barrier-Reef conversion on Tahiti links Darwin’s reef types., Scientific Reports, PMID: 24845540
Header image: shutterstock.com
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