As rising ocean temperatures bleach corals in the Great Barrier Reef, scientists seek better insight into climate trends in the region. However, climate models do not yet adequately reflect trends in southern tropical and subtropical oceans. To collect new data that could improve these models, Sadler et al. turned to records captured in ancient coral beds.

As stony corals grow, they build their skeletons from calcium (Ca) in the surrounding seawater. Strontium (Sr) atoms are sized similarly to calcium and have the same ionic charge, so they also get incorporated into coral skeletons. The abundance of strontium in newly forming coral depends on the temperature of the seawater that bathes it—the colder the temperatures are, the more strontium infiltrates coral skeletons as they form. In this way, corals can serve as archives of past climate conditions.

To build this archive, scientists first had to examine how Sr/Ca ratios behaved during modern times, when sea temperatures were known. In February 2014, the researchers visited massive Porites coral colonies at Heron Reef in the southern Great Barrier Reef, located off the northeastern coast of Australia. The authors used scuba and pneumatic drills to penetrate the growing coral colonies and retrieve core samples.

Porites coral grows by building new layers of skeleton on its surface. Therefore, over time, the coral skeletal layers served as a timeline, with deeper layers corresponding to earlier periods, similar to tree rings. At various points along the cores, the researchers measured Ca and Sr levels and matched them to known ocean temperature data for the corresponding time periods. This gave an equation that related Sr/Ca ratios to ocean temperature.

The scientists then applied the equation to ancient corals from the mid-Holocene, which they later collected using a small, portable drilling platform at a site about 2 kilometers away from the modern coral site. Radioactive dating revealed the ages of coral tissue along the length of each core. After determining Sr/Ca ratios, the team used the equation derived from the modern coral to determine prehistoric ocean temperatures.

The ancient corals revealed that about 5200 years ago, ocean temperatures at Heron Reef were 1.3°C–2.8°C cooler than present, and about 7000 years ago, they were 1.3°C cooler than present. The results also suggest that a warm period known as the mid-Holocene Thermal Maximum occurred between 6000 and 6800 years ago, earlier than previous estimates that placed it between 5350 and 4480 years ago.

The authors say that more research is needed to confirm the earlier dates for the mid-Holocene Thermal Maximum in the subtropical western Pacific, but their findings demonstrate that ancient Porites corals could provide the necessary data for these ongoing efforts. (Paleoceanography, doi:10.1002/2016PA002943, 2016)

—Sarah Stanley, Freelance Writer