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During fieldwork in July 2020, a living microatoll (foreground) near Siloso Point, Sentosa, was found to be partially dead from prolonged exposure to the air in extreme low tides. The flat-topped microatoll's dead upper half is now covered in mud.

PHOTO: ARON MELTZNER/NTU

SummaryNTU researchers developed a method that can use fossil corals alone to reconstruct sea level histories in previously inaccessible tropical regions.By determining live microatolls' air exposure limit (3.5 hours), they can estimate past sea levels by comparing fossil elevations.This method improves sea level projections and assesses vertical land motion; other studies found past higher sea levels and recent island subsidence.AI generated

Published Feb 09, 2026, 05:00 AM

Updated Feb 09, 2026, 05:00 AM

SINGAPORE – The sea-level histories of less-studied tropical regions can now be traced as Nanyang Technological University (NTU) researchers have developed a way to unlock the “memories” of ancient corals.

They can do this by imagining a live microatoll – a flat-topped colony of corals – next to the long-dead corals, visualising how it would grow by using data from observing living microatolls elsewhere as well as tide data. The difference in elevation between the dead corals and the visualised corals would indicate how much higher or lower the sea level of an area was thousands of years ago.

“(This method) now allows us to go around South-east Asia and the Caribbean, and other places we can find only fossil microatolls, and reconstruct sea-level histories in those locations,” said geologist Aron Meltzner, an assistant professor at NTU’s Asian School of the Environment.

“There were a lot of places where researchers couldn’t use the fossils to reconstruct sea levels when we don’t have living microatolls beside them… a lot of geological information out there that researchers in the past ignored.”

Such sea-level archives are critical in improving sea-level projections.

“We want to work out what had happened in the past – both in terms of sea-level change and vertical land motion – so that we can improve the models that we use to feed into sea-level projections,” said Prof Meltzner.

Vertical land motion refers to whether a specific land area is sinking or rising, which can have implications for sea-level rise on a local and regional scale.

A separate 2025 study by the NTU Earth Observatory of Singapore and the Singapore Land Authority found that between 2004 and 2012, islandwide land subsidence of up to 2.2mm a year was observed in the Republic owing to three powerful earthquakes in Sumatra in that period.

A fossil coral from Pulau Natuna, Indonesia, that grew about 1,800 years ago. Researchers can now use such fossil corals to reconstruct past sea levels.

Except for that period, Singapore’s main island is generally stable, with sinking of land close to zero, the study added.

The intertidal shores of the tropics are often dotted with microatolls that store the memories of the ocean. But many of these flat-topped coral colonies are dead and fossilised, which makes it hard to reconstruct an area’s sea-level history without living corals next to them.

The difference in elevation between the fossil and living corals shows the difference in sea level over time. A living microatoll has living coral tissues encrusting its top rim and perimeters.

Microatolls are a record of sea-level rise each year because their tissues grow layer by layer upwards initially, and later sideways, similar to how trees form rings. Since their height is controlled by exposure to air, which kills them, these corals can grow only to the height of the lowest sea level.

Researchers from the Earth Observatory of Singapore (EOS) found a way to make the fossils “readable”, even without living corals beside them – by obtaining clues through observation of living microatolls at Sentosa and Pulau Biola, an islet within the Southern Islands.

The clues come from pinning down how long live microatolls can survive out of water, in extreme low tides.

The researchers learnt that the top rim of live corals starts to die off and turn white after more than three hours of exposure to the air. This told the researchers what the greatest height is that a living microatoll today can reach.

With this information, the scientists can install a portable tide gauge next to a several thousand-year-old fossil coral, and use hourly tide data to visualise how high a living microatoll would grow, as if the fossil were alive today.

Most existing records are from satellite data and tide gauges that go back a couple of centuries. In Singapore, tide gauge data dates back only to the 1960s or 1970s.

Microatolls are also found on Pulau Semakau, St John’s Island, Pulau Tekukor and Lazarus Island. The oldest fossils in Singapore are about 7,800 years old.

Previous microatoll studies here have found that the sea level was slightly higher in the past, around 3,000 years ago, and that it fell over time. Fossil corals were found at a higher elevation than living ones.

But the living corals, a few decades old, indicate modern sea level rise caused by global warming.

The research team derived the 3½-hour air exposure limit that causes the upper rim to partially die by monitoring live microatolls at Pulau Biola and the Siloso Point intertidal reef on the western edge of Sentosa.

The monitoring at Sentosa started in 2020, and the team also relied on tide data to find out how water levels changed over time and affected the corals’ health, said Dr Jennifer Quye-Sawyer, a former research fellow at EOS and first author of the study.

Low spring tides occur from pre-dawn to mid-morning between April and September and from late afternoon between October and March.

The researchers also observed that the upper rims of the microatolls were more likely to die during the south-west monsoon period between June and September, when tides are lower.

“There was a strong link between die-down timing and very low water events. At Sentosa, we observed new coral die-downs only shortly after the lowest water levels of the year,” said Dr Quye-Sawyer.

Before this, little was known about the exact environmental conditions and amount of exposure to air that causes a microatoll to experience these partial deaths. With the study’s finding, sea level reconstructions can be more accurate, said Prof Meltzner.

The study was published in the journal Scientific Reports in November 2025.

Prof Meltzner’s team and collaborators abroad will be using the new method to trace the histories of fossil corals in places such as Indonesia and the Philippines.

Commenting on the research, geographer Colin Woodroffe from the University of Wollongong, Australia, said: “This paper appears to provide additional insight into the way in which the coral tissue dies back on the outer lip of microatolls.

“This will be of value in interpreting adjustments, such as when there has been earthquake-related uplift, or subsidence, as well as tracking short-term sea-level (shifts).”

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