The Great Barrier Reef off the coast of Australia is the largest living coral reef system on Earth, about 300 times larger than the next largest reef, found off the coast of Belize. This unique UNESCO World Heritage Site is home to a wide variety of species and plays a vital role in global ecosystems. The environmental conditions under which this immense, continuous reef system formed have been largely unexplored.
An international team of researchers led by Kiel University has now answered this question for the first time using the TEX86 method, a new method for reconstructing past temperatures. They showed that summer temperatures off the coast of Australia rose from 26 to 29 degrees Celsius about 700,000 years ago and hardly fell below this limit even in the following warm and cold periods.
This warming phase and the relative stability of temperatures in the following periods played a vital role in the rapid expansion of today’s Great Barrier Reef. Their findings have now been published in the journal Science Advances.
Cold seawater temperatures once prevented rapid coral growth
The environmental conditions that allowed the Great Barrier Reef to grow to its current size have been a scientific puzzle. Numerous causes have been suggested, from changing sea levels to changes in the amount of sediment reaching the ocean. In addition to these explanations, there have also been attempts to reconstruct the temperatures at the time of the Great Barrier Reef’s formation. So far, however, these did not yield definitive answers.
The Kiel researchers led by Dr. Benjamin Petrick, as well as Dr. Lars Reuning, Professor Lorenz Schwark, and Professor Miriam Pfeiffer, all from the Institute of Geosciences at the Kiel University, together with colleagues from the Universities of Graz and Southampton, have now succeeded in reliably reconstructing past summer sea surface temperatures.
“For the first time, we were able to prove that warming of summer temperatures off the coast of Australia was crucial for the formation of the Great Barrier Reef. About 700,000 years ago, temperatures rose from 26 to 29 degrees Celsius. These temperatures, which are ideal for corals, favored the rapid growth of the reef,” says the first author of the study, Dr. Petrick of Kiel University.
Researchers analyze temperature differences with the help of chemical fossils
For their study, the researchers used sediment samples taken by the Ocean Drilling Program (ODP) from a well near the Great Barrier Reef. From these samples, they reconstructed seawater temperatures using the TEX86 method. This modern method uses molecular fossils as a so-called paleo-thermometer. This allowed the researchers to place the development of the Great Barrier Reef around 700,000 years ago in the context of regional climate changes. This period in the Earth’s history was crucial for the formation of the reef as it exists today.
Prior to this, summer temperatures often remained at 26 degrees Celsius or below. Corals grew more slowly at these temperatures, preventing the smaller coral reefs that existed at the time from expanding into a large barrier reef. After 700,000, summer temperatures rose to 29 degrees Celsius, an ideal temperature for fast-growing corals.
Even during the subsequent cold and warm periods, temperatures rarely dropped below 26 degrees Celsius—allowing the reef to expand into the largest living barrier reef on Earth.
“In our new study, we were also able to show that the low temperature variability between 26 and 29 degrees over a long period is another important prerequisite for the stability and healthy growth of the Great Barrier Reef,” explains Professor Pfeiffer from Kiel University, co-head of the German Research Foundation’s (DFG) Geoscience Priority Program 2299 “Tropical Climate Variability and Coral Reefs” and one of the co-authors of the study.
Climate change and coral bleaching threaten the Great Barrier Reef
In recent years, the Great Barrier Reef has warmed very rapidly beyond this ideal window due to climate change and its consequences, such as ocean warming. As well as cold temperatures, high temperatures and associated heat waves also contribute to hindering coral growth.
As temperatures continue to rise, the corals will become more vulnerable to environmental impacts such as sea level rise and pollution. Just as regional warming created the conditions for the formation of the Great Barrier Reef 700,000 years ago, human-induced global warming poses the greatest threat to the survival of coral reefs today.
More information:
Benjamin Petrick et al, High sea surface temperatures were a prerequisite for the development and expansion of the Great Barrier Reef, Science Advances (2024). DOI: 10.1126/sciadv.ado2058
Citation:
Not too warm and not too cold—finding the Goldilocks Zone of the Great Barrier Reef (2024, December 5)
