A sprawling chain of volcanic islands and atolls in the central Pacific Ocean has grown in size over the past 70 years, despite sea level rises, a new study shows.
It confirms that atolls are built from sediment off modern coral reefs – and the health of the reefs is vital to keep the process going.
New Zealand and Canadian researchers studied an island on Ailinglaplap Atoll in the Marshall Islands using aerial photographs, satellite imagery and radiocarbon dating of sediment deposits.
Sediment build-up appeared to have caused the merging of what had been two separate islands, and a spit at the western end of the island was continuing to extend, adding to the land area above sea level, the study found.
Radiocarbon dating was used to calculate the ages of coral fragments, shells and microscopic marine organisms at the island’s western end. The sediment deposits were from more recent material, generally after 1950.
Atolls are rings of coral, with islands on top of the coral and a lagoon inside the ring.
University of Auckland senior lecturer Dr Murray Ford, a coastal geomorphologist who worked on the study, said the research showed that the atolls essentially depended on the health of the coral reef they were built on.
“It’s all about the reef health, being able to produce sand and gravel to help make these islands and maintain them,” he said.
Threats to the health of coral reefs included ocean acidification, reef bleaching and pollution. There was a risk the engine room of island growth could be turned off, which would eventually have an effect on the island.
The study confirmed the modern reef was the source of material that built up the island. Previously it was considered possible that older material being recycled around was causing changes to atolls, Ford said.
“The big picture with this is the modern day coral reef can build an island even though the sea level is rising,” he said.
“The nice thing about these islands is everything that builds the island comes from the reef.”