April Flowers for redOrbit.com – Your Universe Online
In the South Pacific, three types of coral reef island formations have fascinated geologists for ages. The coral of Tahiti forms a “fringing” reef, with a shelf growing close to the island´s shore. In Bora Bora, the “barrier” reefs are separated from the main island by a calm lagoon. Manuae represents the last type, an “atoll,” which appears as a ring of coral enclosing a lagoon with no island at the center.
The mechanism underlying these reef shapes and how they developed over evolutionary time has produced an enduring debate between two hypotheses — the first from English naturalist Charles Darwin and the second from geologist Reginald Daly. A new study from researchers at MIT and Woods Hole Oceanographic Institution (WHOI) uses modern measurements and computer modeling to put this very old conundrum to rest. The findings of this study were published in the journal Geology.
Fringing reefs, barrier reefs and atolls reflect different stages in a dramatic process, according to Darwin´s theory. This process occurs as an island sinks into the ocean floor — the ultimate fate of all volcanic ocean islands. As the volcanic rock cools and is carried away from the “hot spot” of the undersea volcano by tectonic plate movement, the island begins to sink as much as a few millimeters per year.
Coral reefs on the island´s flanks grow upward toward the sea surface as the island sinks — getting enough sunlight that the living coral organisms on top, and their symbiotic algae, get enough sunlight to keep pace with the sinking. A fringing reef progresses to a barrier reef and finally to a signature atoll as the coral grows and the island sinks.
MIT/WHOI graduate student Michael Toomey, along with collaborators Taylor Perron, the Cecil and Ida Green Assistant Professor of Geology at MIT, and Andrew Ashton, a coastal geomorphologist at WHOI, discovered Darwin´s reef theory can´t explain the trajectories of all volcanic ocean-island systems.
The team realized the Hawaiian Islands are following a different kind of progression — finding fringing reefs where they expected to find no reef at all, and drowned barrier reefs where they expected to find living barrier reefs. “Those islands are just not sinking into atolls like the Society Islands,” Toomey says, “so we wanted to develop a model to explain these differences.”
On the other hand, Reginald Daly´s theory argues that sea-level cycles, not island subsidence, are the key to understanding coral formations. During ice ages, when the water becomes locked in ice sheets on land, sea-level drops. It rises again between glaciations as the ice melts, suggesting to Daly that exposure to increased wave energy during sea-level drops would erode an island away. As the sea level rises again, the coral would regrow on submerged island platforms.
The research team decided to create a computer model focusing on the relationships among coral growth, sunlight availability, water depth and erosion to consider both Darwin’s and Daly´s theories. The model also calculates how a coral reef develops as sea level varies over hundreds of thousands of years in combination with island subsidence rates.
There is a delicate balance between island sinking and subsidence. The reef will drown if the combination of sea-level change and island sinking deepens the water faster than the coral can grow. Likewise, if the coral grows faster than the water deepens, the coral growth will catch up with the sea surface, then slow down again as the reef is exposed to eroding waves at sea level.
If the team includes island subsidence without glacial sea-level cycles, they can model Darwin´s scenario. The configuration that emerges, however, does not resemble current reef formations. When they added in a sea-level history based on geological evidence and paleoclimate data that allows the computer model to account for sea-level oscillation between the present level and approximately 393 feet below present levels every 100,000 years, the results were closer to real-world observations.
The model ran a course of four glacial cycles, approximately 400,000 years, into the past. This yielded a coral-reef distribution that matched closely with the real-world observations — with the barrier reefs, drowned barrier reefs, and other forms all in the correct places on the map. “What this shows,” Perron says, “is that while island subsidence is important, as Darwin suggested, sea-level oscillations are also important for determining the distribution of reef types around the world.”
The model is sophisticated enough to explain why the Society Islands follow Darwinian progression, but others do not. Without sea-level oscillations, most of the environment would likely progress as Darwin theorized, according to the simulations. Ashton says, when the oscillations are included, “It turns out there is only a little ℠Goldilocks´ zone, a narrow range of subsidence and reef-accretion rates, in which you can get that progression.”
The researchers found it interesting the parameters needed by the model to create that tiny zone of Darwin progression match up with the actual growth and subsidence rates of Tahiti. Apparently, Tahiti has subsided just slowly enough over the last few glacial cycles for the deep lagoon to develop without drowning permanently.
Hawaii, on the other hand, is sinking so quickly — at more than 2 millimeters (0.07 inches per year) — that it will never see Darwinian configuration. A little reef terrace is formed every time the sea level falls to its lowest point. At the end of each glacial period, as the sea-level rises, the reef drowns and remains drowned.
The biggest revelation gleaned from this model is that coral reefs are very sensitive to sea-level changes. Peter Burgess, professor and chair of earth sciences at Royal Holloway, University of London (RHUL), says this is useful information because exploring different scenarios of real-world reef formation can help produce a record of how sea-level oscillated over long time scales. “It´s helpful to know how fast the sea level has changed in the past,” he says, “because there is a high probability it will change rapidly in the next couple hundred years, and we´d like to understand how that change might happen.”
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