—The Keppel Islands—

Keppel Bay’s 7700-year-old reefs

 By Dr Alison Jones  |    23 Aug 2020

Unlike other reefs of the Great Barrier Reef, Keppel Bay reefs have been able to alter their pattern of growth and survive changes in sea level and climate about every 1000–2000 years, a new study has revealed [1].

Approximately 5500 years ago, when sea levels fell, all the reefs of the 8000 year old Great Barrier Reef slowed down to almost zero vertical growth, but the study found, the reefs of Halfway Island and North Keppel Island started growing sideways at an astonishing rate.

At Mazie Bay, off North Keppel Island, reefs grew sideways at about 70 cm per year in the next thousand years: up to 6 times faster than previously recorded growth rates for the Great Barrier Reef [2].

Dating the Keppel reefs

The study used uranium-thorium isotope ratio dating to look at the entire life history of five Keppel Bay reefs.

Uranium (U) is soluble in seawater whereas Thorium (Th) is not soluble; so the age of a coral fossil can be determined by the ratio of the parent isotope of Uranium (U-234) to its daughter isotope of Thorium (Th-230) because the Thorium is only there because of the decay of the Uranium taken up by the coral as it grows.

It found that the first reefs to form in Keppel Bay were at Halfway and Wedge Islands, the oldest in the group of 19 continental islands, about 7700–7500 years ago.

Next to appear, about 6900 years ago, were the Mazie Bay reefs at North Keppel Island, followed by, about 6000 years ago, the Great Keppel Island, Humpy Island and other North Keppel Island reefs. Divided Island reef appeared much later, about 2000 years ago.

A timeline showing the approximate initiation of reefs in Keppel Bay in relation to sea level fall and the present (1950 in carbon dating terms).

Mud may have protected the coral structures from erosion

The scientists think such high growth rates and the survival of Keppel Bay reefs in a muddy bay were made possible because:

  • sediment transported by the mighty Fitzroy River into Keppel Bay quickly buries and preserves the coral framework, reducing coral erosion and bioerosion (by marine creatures), and providing a substrate for new coral to grow on and
  • the Keppel reefs have lots of Acropora (mainly Acropora formosa) which grows fast and can quickly develop large, three-dimensional reef structures [3].

Cuba’s mud-adapted reef system – Gran Banco de Buena Esperanza 

Other reef systems, such as the 10,000-year-old reefs of Gran Banco de Buena Esperanza in Cuba (below), have grown in muddy conditions just like Keppel Bay reefs [4].

Located at the mouth of the Río Cauto—Cuba’s longest river—Gran Banco de Buena Esperanza spans 25 by 40 kilometres of the central part of the bay. The corals here have grown into a maze-like network of ridges interspersed with ponds and channels, and reaching 25 metres above the seafloor.

The reefs actually extend another 50 metres below, buried in mud, sand and compacted red clay. The reef system sustains coral species that have adapted to the Gulf of Guacanayabo’s turbid, muddy environment [5, 6].

Gran Banco de Buena Esperanza and Keppel Bay reefs are an example of the opportunistic nature of reef systems and shows us that they can survive and even thrive under the most unlikely of conditions.

Is mud less of a problem to reef survival than we humans (a relatively young species) like to think? After all, it started piling up 458 million years ago [7]. Perhaps we need to expand the timescales over which we predict reef futures, at least in the Keppels.

Author

Dr Alison Jones

References

  1. Leonard, N.D., M.L. Lepore, J.-x. Zhao, A. Rodriguez-Ramirez, I.R. Butler, T.R. Clark, G. Roff, L. McCook, A.D. Nguyen, Y. Feng, and J.M. Pandolfi, Re-evaluating mid-Holocene reef “turn-off” on the inshore Southern Great Barrier Reef. Quaternary Science Reviews, 2020. 244: p. 106518.
  2. Leonard, N.D., M.L. Lepore, J.-x. Zhao, A. Rodriguez-Ramirez, I. Butler, T.R. Clark, G. Roff, L. McCook, A.D. Nguyen, Y. Feng, and J.M. Pandolfi, A U-Th Dating Approach to Understanding Past Coral Reef Dynamics and Geomorphological Constraints on Future Reef Growth Potential; Mazie Bay, Southern Great Barrier Reef. Paleoceanography and Paleoclimatology, 2020. 35 (2): p. e2019PA003768.
  3. Harriott, V.J., Growth of the staghorn coral Acropora formosa at Houtman Abrolhos, Western Australia. Marine Biology, 1998. 132 (2): p. 319-325.
  4. Hansen, K., Cuba’s Gulf of Guacanayabo. NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey. 2020.
  5. Zlatarski, V.N. and S. González Ferrer, Gran Banco de Buena Esperanza: unique Caribbean coral reef system. Reef Encounter. The News Journal of the International Society for Reef Studies, 2017. 32 (1): p. 60.
  6. Zlatarski, V.N. and B.J. Greenstein, The reticulate coral reef system in Golfo de Guacanayabo, SE Cuba. Coral Reefs, 2020. 39 (3): p. 509-513.
  7. Poppick, L., The origins of mud, in Knowable Magazine. 2020.