Led by Associate Professor James Flexner from the University of Sydney, the study marks the first time U-Th dating has been applied to historical coral architecture. Unlike radiocarbon dating, which is unreliable for materials less than 400 years old, U-Th dating narrows down the date of a coral's death to within just a few years, making it a powerful tool for understanding the recent past. The method produces precise age estimates without the need for extensive excavation, allowing archaeologists to build a clearer picture of how European colonizers impacted local cultures across diverse landscapes worldwide.
Photograph of exposed beach rock and near-shore corals in the motu of Kouaku where labourers gathered building materials for the Catholic mission. (Flexner/Antiquity Publications Ltd)
The Missionary Influence on Mangarevan Life
The Mangareva Islands, located approximately 1,600 kilometers southeast of Tahiti in the South Pacific, were transformed dramatically after French Catholic missionaries established an outpost there in 1834. These missionaries, known as the frères bâtisseurs (lay builder-brothers), trained Polynesian converts in European construction techniques and commenced a large building program.
"They built cathedrals, churches, schools, communal bread ovens, watch towers and small stone cottages out of locally sourced coral from nearby shore reefs, as well as beach rock corals from exposed formations on land," Flexner explained in the Antiquity release.
A typical cottage type coral building found on the Mangareva Islands. (Flexner/Antiquity Publications Ltd)
Coral was the dominant building material for houses in Mangareva until timber became more widely available in the 1870s. The missionaries kept detailed records of their own monumental constructions, including the impressive St. Michael's Cathedral in Rikitea, but wrote almost nothing about the everyday homes built by local Mangarevan families. This gap in the historical record is precisely what the new dating method helps to fill, giving researchers a window into the domestic lives of ordinary Polynesian people during a period of profound colonial upheaval.
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Surprising Discoveries Within the Coral Walls
The team sampled minimally weathered branch corals from Acropora sp. coral blocks in nine ruined buildings across the islands, including a control sample from the 1850s boys' school on Aukena, whose construction dates were already known. The results were largely consistent with the historical record, but one unexpected finding stood out. "What surprised us was that several coral blocks returned dates earlier than expected," Flexner noted. "A few even pre-dated European arrival, suggesting the builders may have reused older coral taken from nearby sites. But none of the examples showed centuries-long age differences, challenging earlier theories that coral from ancient structures was widely repurposed for 19th century buildings."
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Left) example of unweathered in situ branch corals within a coral limestone block from the north exterior wall of structure AKH-10 on Akamaru Island; right) the north exterior wall of AKH-10 and building plan showing the sampling location. The sample returned a date of 1840±3 CE. (Antiquity / CC BY 4.0)
There are two possible explanations for this "old coral" problem. An archaeologist visiting Mangareva in the 1930s noted piles of coral rubble believed to be the remains of marae — once sacred structures that were overthrown during the missionary period — raising the possibility that ancient coral was repurposed for new buildings. Alternatively, some branches of the Acropora genus die off away from the area of active reef growth over a period of years or decades, while retaining their "fresh" appearance. The evidence found within the walls of these coral structures, including glassware, cooking pots, and ceramics, also indicated a rich variety of activities, from feasting events to changes in everyday domestic habits.
"These coral cottages are a microcosm of life itself," Flexner said. "They are a huge potential source of information on the social, cultural and cosmological ways of being for Pacific peoples."
St. Michael's Cathedral in Rikitea, Mangareva, built by French Catholic missionaries in the 19th century. (FRED/CC BY-SA 4.0)
Beyond Architecture: An Archive of Ecological Change
The U-Th dating method was originally used in Polynesia to date prehistoric coral and cave formations, including the initial discovery of the Tonga archipelago and the Mangareva Islands, Hawaiian sacred sites, and coral blocks from marae (ancient temples) in Mo'orea. Expanding this technique to colonial-era buildings opens up new possibilities for understanding undocumented Pacific archaeology and, potentially, similar contexts in Africa and the Caribbean.
The implications of this research also extend into environmental science. "People think of coral mainly in the context of bleaching and climate change today, but each coral block used for the construction of these houses retains a chemical record of the environment in which the coral grew, offering a historical archive of coral reefs and past ecological change," Flexner stated. This archive could prove invaluable for understanding changes to reef systems over time, particularly those resulting from human impact. Associate Professor Flexner hopes future collaborations will extend the work to neighboring archipelagos where coral construction also flourished, strengthening conservation and heritage protection across the region.
Top image: Coral watch tower on Mata Kuiti Point, Aukena Island, French Polynesia. Source: James Flexner, University of Sydney/Antiquity Publications Ltd
By Gary Manners
References
Flexner, J. 2026. 'Coral houses' are dotted throughout the Pacific. Now scientists know exactly when they were built. The Conversation. Available at: https://theconversation.com/coral-houses-are-dotted-throughout-the-pacific-now-scientists-know-exactly-when-they-were-built-278893
Flexner, J. et al. 2026. Direct dating of colonial-era coral building materials using the U-Th method in the Mangareva Islands, French Polynesia. Antiquity. Available at: https://doi.org/10.15184/aqy.2026.10325
University of Sydney. 2026. Advanced dating method reveals age of Pacific coral architecture. Phys.org. Available at: https://phys.org/news/2026-03-advanced-dating-method-reveals-age.html
