REPOSITORY ON COASTAL ECOSYSTEMS:
BRIDGING ARTISTIC, SCIENTIFIC AND COMMUNITY-LED PRACTICES I N THE RIAU ARCHIPELAGO

Coral reefs are often admired for their vibrant colours and the rich marine life they support. But beneath their surface lies something less visible: a long environmental history preserved within the skeletons of corals themselves. Layer by layer, year after year, corals build skeletons that preserve a memory of the ocean’s past, and clues about the future of reefs in a changing climate, from changes in sea level to fluctuations in temperature and rainfall.

For Dr. Riovie Ramos, marine scientist and geochemist, and Research Fellow at the Earth Observatory Singapore, Nanyang Technological University, these skeletons act as natural archives of the ocean. By studying corals and reefs, scientists can reconstruct past climate records and trace how reefs formed, grew, and responded to sea-level change over thousands of years. Riovie’s current research uses Indo-Pacific coral reef data from the Holocene period to assess how reefs might respond to accelerating sea-level rise.

‘’Hard corals build a calcium carbonate skeleton beneath their living tissue,” Riovie explains. ‘’They grow upward and outward, layer by layer, year by year, and those layers store information about the ocean conditions when the coral was alive.’’

Some coral species, such as Porites, are especially useful for this research because they form massive, long-lived colonies that can grow continuously for decades to centuries. Only the outer coloured surface is alive; the interior consists of a hard skeleton made of calcium carbonate. As the coral grows, it lays down annual density bands, much like tree rings, which allow scientists to estimate the coral’s age. At the same time, the skeleton incorporates chemical traces from the surrounding seawater, preserving proxies for past temperature, salinity, rainfall, and other environmental conditions.

A three-metre coral colony can contain roughly three hundred years of environmental history.

To access this record, scientists use underwater drills to extract narrow cylindrical samples known as coral cores from living or dead massive coral colonies. These cores are drilled vertically through the colony’s growth axis, often from the highest central point, to recover the clearest sequence of layers through time. In the lab, the cores are cut into slabs and imaged using X-rays or CT scans to reveal their density bands. Scientists then micro-drill along the coral’s main growth axis, collecting fine powder samples for chemical analysis. By analysing the chemistry preserved in these layers, including oxygen isotopes and strontium-calcium ratios, and calibrating these signals against modern records such as satellite sea-surface temperatures, scientists can reconstruct past rainfall, salinity, pollution levels, sea surface temperature, and even larger climate events such as El Niño.

As natural archives, corals help scientists look back beyond the period covered by modern scientific measurements. In regions where long instrumental records are scarce, these calibrated coral records provide crucial evidence of how oceans have changed over decades and centuries.

For Riovie and her colleagues, understanding these past patterns is key to anticipating the future. By combining coral core records with modern observations and climate projections, they can examine how oceans have varied over time and use these baselines to better contextualize the pace and magnitude of current climate change, ultimately improving future climate models.