A TEAM of archaeologists has used an advanced dating technique to establish the first precise construction timeline for houses built out of coral in French Polynesia. The findings reveal previously hidden patterns of architectural development and cultural life in Pacific societies.
The University of Sydney-led study, published in the journal Antiquity, marks the first time uranium–thorium dating (U-Th dating) 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 colonizers impacted local cultures across diverse landscapes worldwide.
Associate Professor James Flexner, an ARC Future Fellow in the Faculty of Arts and Social Sciences, hopes that future collaborations in this area will extend the work to neighboring archipelagos where coral construction also flourished. "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 said.
Flexner led the research on Mangareva, a group of islands in French Polynesia in the South Pacific, where coral was the main material for houses before timber became dominant in the 1870s.
“Mangarevan people learned the building technique from French Catholic missionaries who arrived on the island in the 1830s and commenced a large construction program,” Flexner said. “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.”
U-Th dating allows the dating of “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,” he said. “Smaller timescales can make a big difference for interpreting the past.”
Mangareva sits within an archipelago of ancient volcanic peaks, located within a lagoon surrounded by a fringing reef that includes several long and narrow motu, or coral islets.
Ten coral samples from the Mangarevan structures were dated by the University of Queensland’s Radiogenic Isotope Facility. The results open new avenues for understanding how Pacific people adapted building technologies introduced by the Europeans.
Before the Europeans
“What surprised us was that several coral blocks returned dates earlier than expected,” Flexner said. “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.”
Dating the coral helped the researchers track how everyday life in the Pacific evolved following European contact and continues to be shaped by ongoing colonial influences.
“Some of the evidence we found within the walls of the coral structures, including glassware, cooking pots and ceramics, indicated activities such as feasting events, whereas others pointed to changes in habits of everyday domestic life, from 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,” Flexner said.
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 Mangareva Islands, Hawaiian sacred sites, and coral blocks from marae, ancient temples in Mo’orea.
Unlike radiocarbon dating, which is unreliable for materials less than 500 years old, uranium–thorium dating yields results accurate to within a few years.
“Expanding the U-Th method to date coral houses as we have done in Mangareva could revolutionize the study of undocumented architecture and people in other pre-European as well as colonial contexts beyond Oceania, including Africa and the Caribbean,” Flexner said.
Precisely dating coral buildings may also eventually help researchers understand historical reef conditions. “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,” he said.
He added that this archive could help in “uncovering the cultural histories of colonial landscapes,” which can “prove invaluable for understanding changes to reef systems over time, particularly those resulting from human impact.”