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Paper: Re-evaluating mid-Holocene reef “turn-off” on the inshore Southern Great Barrier Reef
Authors: Leonard, N.D., Lepore, M.L., Zhao, J.X., Rodriguez-Ramirez, A., Butler, I.R., Clark, T.R., Roff, G., McCook, L., Nguyen, A.D., Feng, Y. and Pandolfi, J.M.
A new study has reconstructed the complex growth history of coral communities in the Keppel Islands, southern Great Barrier Reef, revealing that the area might provide a safe-haven for coral under climate change.
In the face of global climate change, the future of our coral reefs looks increasingly uncertain. The Great Barrier Reef, which lies off the coast of Australia and spans 15 degrees of latitude, is under particular threat. The reef is home to more than 2800 individual reefs of various shapes, sizes and evolutionary states. Following continued coral bleaching, as a result of increasing ocean water temperatures, scientists are now in a race against time to protect this unique and valuable ecosystem.
Scientists have been looking back in time to understand how the coral of the Great Barrier Reef responded when the climate changed in the past, and predict how patterns of coral growth may change in the future.
Studies have shown that coral growth across the Great Barrier Reef slowed dramatically between 2500 and 5500 years ago. A roughly coincident hiatus in coral growth was also measured in corals from China, Hawaii, Japan and the tropical Atlantic – leading to speculation that coral reefs were experiencing stress due to a wider climate phenomenon at that time.
In a paper recently published in Quaternary Science Reviews, Dr Nicole Leonard and co-authors revisited the corals of the southern Great Barrier Reef with the aim of identifying what stalled coral growth and whether it affected all coral communities equally. The team collected 29 core sections from coral reefs fringing the Keppel Islands, in the southern Great Barrier Reef. These slices through the coral revealed concentric growth layers – allowing them to assess the rate of coral growth, both vertically and laterally, across five individual reefs. The team dated the cores at intervals using the uranium-thorium dating technique to determine changes in the rate of growth.
The detailed growth and age data told Leonard and the team that the Keppel Islands corals experienced no clear growth hiatus between 2500 and 5500 years ago, as had previously been observed in corals from other tropical locations. In fact, the coral growth appeared to be far more locally complex than scientists had previously realized. Sometimes the Keppel Islands corals grew at a similar pace across the islands, but often they had completely different growth histories from each other – with observed growing patterns at one of the sites lagging thousands of years behind others.
Spells of synchronous growth were, however, observed in many corals across the Keppel Islands 5500 years ago, and again 4600, 2800 and 1200 years ago. Leonard and co-authors suggest that these coincident growth phases, which have also been observed in other areas of the Great Barrier Reef, are associated with periods of higher sea surface temperature and reduced El Niño/La Niña strength. Leonard and colleagues report that higher relative sea level at these times would likely have created more vertical space for coral growth.
With increasing global temperatures it is believed that mid to higher latitude reefs, including the southern Great Barrier Reef, may become a refugia for coral communities. The growth history of the Keppel Islands suggests this may well be true; coral in this region appear to have thrived when sea level and temperatures were higher in the past.
And yet, a recent rise in extreme weather events has exposed the Keppel Islands region to cyclone damage, floods, and bleaching events, all of which are exacerbated by changing human activities. The Keppel Islands may well thrive under a warmer climate, but extreme events are projected to become more frequent with climate change, and could stress the corals in ways we are only just beginning to understand.
Warmer climate could mean corals thrive in the southern Great Barrier Reef by Erin Martin-Jones is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
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