Corals are careful historians. Day in and day out, as they build their skeletons larger and larger, they diligently record the water temperature, their own growth rates, and whether they are healthy or stressed. These coral history books are being accessed to explore possible acclimation and provide historical context for the severe bleaching events in 2016 and 2017.
Dr. Thomas DeCarlo, Professor Malcolm McCulloch, Anton Kuret and Dr. Laura Gajdzik are drilling skeletal cores from massive Porites colonies around Lizard Island. The team uses underwater pneumatic drills to extract the cores, which can be close to 3 meters in length. Cement plugs are carefully placed in the holes so that the coral grows over the hole and recovers completely within one or two years.
Back at the University of Western Australia, the cores are placed in a computed tomography (CT) scanner, like the ones you might see in a hospital. The 3-dimensional CT images reveal alternating low- and high-density bands which the corals form each year, similar to tree rings. By counting the bands and measuring the distance between them, Dr. DeCarlo can determine historic growth rates. The longest core the team collected could span 250 years or so, beginning around the time Captain James Cook visited and named Lizard Island in 1770.
In a recent publication, Dr. DeCarlo showed how thin layers in the skeleton are formed every full moon, allowing calculation of monthly growth rates. These high-resolution growth histories also show anomalies in density (called “stress bands”). The chemical composition of the layers provides information on past water temperatures.
This research will provide information on past ocean conditions, coral growth and bleaching events. It may indicate whether thermal stress thresholds have changed over time, whether exposure to past warm events has helped these corals to acclimatise and become more resilient, and whether they have previously suffered thermal stress comparable to the bleaching conditions of 2016 and 2017.
Thomas DeCarlo is the 2017 John & Laurine Proud Postdoctoral Fellow at the Australian Museum’s Lizard Island Research Station.