Tropical corals to help unlock ocean's secret history

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Kiwi scientists are set to unlock a secret history of the Pacific Ocean using tropical corals thousands of years old, in an $800,000, world-first study.

The research team, led by the head of Auckland University's School of Environment, Professor Paul Kench, will ultimately provide a detailed record that will help scientists to better predict what future climate change could mean for the planet's oceans.

Professor Kench believed having a more comprehensive record -- in which huge changes to the ocean over the past few thousand years could be marked and explained -- would give better context to the rapid changes documented over the past three decades.

These have been largely attributed to more carbon dioxide being pumped into the atmosphere by the burning of fossil fuels, and increasingly being dissolved in the oceans, causing them to acidify.

"But to put that into context, you've got to know what the natural variations in ocean pH may have been over the last hundreds to thousands of years -- and that allows us to either say that what we are seeing is really unusual or it's a part of longer-term cycles which the ocean basins are accustomed to," Professor Kench said.

His team will reconstruct this lengthy period using boron and oxygen isotopes in living and fossil corals.

These were unique because they could live for centuries, retaining information on past ocean and climate conditions in the chemistry of their calcium carbonate skeletons.

To gain a wide-ranging amount of data, the team would be collecting samples from places as far flung as Kiribati, the Marshall Islands and Great Barrier Reef.

"In the northern Great Barrier Reef, we'll be slicing through some very large corals, which are called microatolls -- these grow on reefs and become these very circular, tabular features," Professor Kench said.

"When you cut through them you see that they grow a bit like tree rings -- and every year, they lay down two bands of calcium carbonate, which allows us one way to date them."

More importantly, he said, the calcium carbonate retained chemical signatures.

"So we're going to first develop a chronology of coral growth that goes back as far as we can get, then we're going to sample it and use the isotope of boron, which has been shown to faithfully record ocean pH levels."

The research would also reveal whether high ocean acidification caused coral skeletons to grow more slowly.

If this was the case, then a more acidic future ocean -- something climate scientists predict -- would affect not only corals but also other organisms at the base of the ocean food chain that used calcium carbonate as their building blocks.

The work, supported by the Marsden Fund, would also give scientists across the globe a more accurate baseline from which to model future climate change.

"There are quite a number of scientists around the world who are using coral skeletons to reconstruct climate scenarios ... but this is the first real detailed study looking at ocean water dynamics and the chemistry behind it," Professor Kench said.

SECRET HISTORY

• A team of scientists led by Auckland University professor Paul Kench will this year begin collecting coral samples from across the Pacific Ocean.

• Chemical signatures retained by the calcium carbonate in these long-lived corals will allow them to reconstruct pH levels and other factors in the ocean, and build a comprehensive record stretching back thousands of years.

• By gaining a deeper knowledge of ocean dynamics like acidity, scientists will better understand natural variations in and between ocean basins.

• These could give better context to climate models that have shown worryingly rapid changes over recent decades.