A team of archaeologists led by scientists from the University of Sydney has used modern, advanced dating techniques to construct a precise timeline of historic coral structures found along the coast of the Mangareva Islands in French Polynesia.
The research team behind the historic discovery said their work revealed previously hidden patterns of architectural development and cultural life in these ancient Pacific societies. They also note that the method they employed marks the first time uranium–thorium dating (U-Th dating) that takes advantage of the material’s radioactive properties has been applied to date historical coral architecture.
“This method produces precise age estimates without the need for extensive excavation, enabling archaeologists to better understand how European colonisers impacted local cultures across diverse landscapes worldwide,” they explained.
Few Records Exist Detailing Historic Coral Polynesian Structures
According to a statement announcing the research, Mangareva sits within an archipelago of ancient volcanic peaks, located within a lagoon. This remote location is surrounded by a fringing reef that includes several long, narrow coral islets, called motu.
The project’s leader, Associate Professor James Flexner, noted that the local Mangarevan people first learned how to make coral structures from newly arriving French Catholic missionaries who commenced a “large construction program” during the 1830s.
“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,” the professor explained.
This construction technique dominated the area for a few decades before lumber became the leading construction material in the 1870s. During this brief phase, the European settlers kept detailed records of their own coral buildings. However, the research team noted that they recorded “almost nothing” about the Magarevan people’s everyday structures built using the same technique.
While radiocarbon dating techniques are valuable for older organic materials, the research team noted that they would need to adopt a different approach to accurately date these historic yet more recent coral structures. Fortunately, Professor Flexner said he and his colleagues could employ a technique called radium-thorium dating to provide a timeline for organic materials from this period.
“Using the uranium-thorium dating method, we can date the construction materials used in the buildings with remarkable accuracy, giving us more clues to cultural and domestic life in the Pacific and deepening our understanding of colonial heritage,” the researcher explained, adding that “smaller timescales can make a big difference for interpreting the past.”
Unraveling the Social, Cultural, and ‘Cosmological’ Ways of Pacific Peoples
After selecting various structures made by the local native Mangarevan families, the researchers delivered them to the University of Queensland’s Radiogenic Isotope Facility, where they were tested using the U-Th dating method. Professor Flexner said the results confirmed some of their earlier suspicions and included a few surprises.
Among the most surprising were the U-Th test results suggesting some of the structures predated European arrival. Professor Flexner said some researchers have suggested that these original coral structure buildings were made with coral taken from “nearby sites,” rather than from scratch.
However, he adds, none of the older structures showed age differences of centuries, “challenging earlier theories that coral from ancient structures was widely repurposed for 19th-century buildings.”
The researchers also collected evidence from inside the structures, including ceramics, cooking pots, and glassware. Professor Flexner said these artefacts suggest the locals were participating in “feasting events.”
The team also found other objects that they said pointed to changes in the daily habits of the local Mangerevan people. Professor Flexner said that these noticeable changes included changes in “how a family prepares and eats meals together, to how people move throughout the home, how they might pray and worship, or how they sleep.”
“These coral cottages are a microcosm of life itself,” he explained. “They are a huge potential source of information on the social, cultural, and cosmological ways of being for Pacific peoples.”
Mangerevan Houses Contain a Valuable ‘Chemical Record’ of the Area
When discussing the implications of their findings, the University of Sydney researchers said a more accurate timeline can “open new avenues for understanding” how these local Pacific islanders adapted coral structure-building technologies they learned from European missionaries. Professor Flener said their work also demonstrates the value of the u-Th method, a technique he believes could “revolutionise the study of undocumented architecture and people in other pre-European as well as colonial contexts beyond Oceania, including Africa and the Caribbean.”
Beyond the anthropological value of an accurate timeline of coral structure construction, the research team noted the potential for studying historical reef conditions. This includes the potential creation of more accurate timelines for the region’s coral bleaching, which could affect modern-day conservation efforts.
“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,” the professor explained.
Flexner said that accessing this stored chemical archive could help scientists understand the evolution of reef systems over time, including those caused by human activities. However, the research team noted that, before further studies are conducted, they are planning in advance to help local communities preserve the invaluable history contained within these Mangrevan-built coral structures.
“We are actively partnering with communities and local authorities to strengthen conservation and heritage protection in these regions so we can continue to piece together the stories of the past and build a more informed and sustainable future,” he explained.
The study “Direct dating of colonial-era building materials using the U-Th method in the Mangareva Islands, French Polynesia” was published in Antiquity.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.