New findings: How the Great Barrier Reef was formed

Great Barrier Reef
New findings: How the Great Barrier Reef was formed

TT

Updated 12.25 | Published 06.10

           

           

           

           

           

         

          The barrier reef is located on the continental shelf off the northeast coast of Australia.

          1 / 6 Photo:   Anders Humlebo/TT

It is 300 times larger than any other coral reef on our planet. But how and when was the Great Barrier Reef formed?

New measurement methods have now given access to the answers - formed deep in the ocean floor hundreds of thousands of years ago - and at the same time gossip about today's threats to the world's largest living organism.

It stretches over 345,000 square kilometers, corresponding to three quarters of Sweden. In addition to the corals themselves, the Great Barrier Reef is home to thousands of different animals and plants, including several hundred species of birds.

Other barrier reefs around the world, including Belize and New Caledonia, are also stunningly beautiful and impressive – but extremely much smaller. Why did such a megareef form just along the northeast coast of Australia? How did it happen, and when did it happen?

Some of the answers have now been found by researchers at the Christian Albrechts University (CAU) in Kiel, Germany, together with colleagues in Graz, Austria and in Southampton, England. With a new measurement method, called tex86, they have been able to map temperatures and other conditions several hundred thousand years back via drill samples from the barrier reef area.

Millions of years

Australia is close to the seismic "Ring of Fire" in the Pacific Ocean, but is geologically much more stable. The same applies to the approximately 60 kilometer wide "shelf" that juts out from the current Queensland coast to the border where the seabed drops from only a few tens of meters deep to 1,000 meters or more.

Water levels have varied greatly, but occasionally corals and fish have thrived here in a relatively protected environment for millions of years. The CAU researchers are now publishing measurements going back about a million years, and they show that water temperatures were quite low for a long time, often 20-25 degrees.

"The corals grew more slowly at those temperatures, which prevented the small coral reefs that existed at the time from expanding into a larger barrier reef," CAU writes in a press release.

Was built on

But then, about 700,000 years ago, the heat in the ocean reached a kind of magical limit. The summer temperatures settled fairly steadily in the range of 26 to 29 degrees, which turned out to be perfect. This is quite close to the maximum heat level corals thrive in, but because the values ​​remained stable for long periods, the "framework" of a giant barrier reef could be formed in peace and quiet. Even in the eras when the water levels fell, the foundations of the reef remained, and could then be built on when the water rose again.

In the past, it has been speculated that the sea levels or sediment washed out from land were more controlling in the development, but the Kiel-led study changes the picture.

"For the first time, we have been able to prove that higher summer temperatures along the coast of Australia were decisive for the formation of the Great Barrier Reef," says CAU researcher Benjamin Petrick, the study's first author, according to the press release.

25-30 perfect

But higher temperatures and thriving reefs are not equated - unfortunately for the Barrier Reef, now that man-made climate change is also wreaking havoc in the oceans.

The other month, a research report showed that water temperatures at the Great Barrier Reef over the past decade have been higher than at any other time since at least the beginning of the 17th century.

So relatively rapid temperature changes, whether up or down, are bad for reefs, is the conclusion. There is an ideal level somewhere between 25 and 30 degrees, and "our data suggest that long periods of suitable temperatures are a requirement for the Great Barrier Reef to develop and maintain," the researchers write in a report now published in the journal Science Advances.
 

FACTS

Great Barrier Reef

A barrier reef is so called because it lies along a coast, and precisely forms a kind of barrier between the shoreline and the ocean depths further out.

In the case of Australia, there are around 3,000 smaller reefs and 900 islands that form a "string of pearls" along the edge of the Coral Sea outside the state of Queensland. In some places the water is so shallow that it is hardly possible to snorkel there, the deepest being around 60 meters to the seabed at the Barrier Reef.

Tropical coral reefs cannot go much deeper, as they need light to thrive. There are also cold water reefs that may lie further down in the oceans, but they are not as spectacular. And often not that big either – the world's largest, Røstrevet in Norway, spreads over approximately 100 square kilometers.

The base of a reef consists of lime from the skeleton