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The Mangareva Islands, today part of French Polynesia, are an archipelago in the South Pacific, situated in the middle of a lagoon surrounded by a coral reef. The islands are believed to have been discovered by Polynesian navigators c.AD 850, and firm evidence for their presence dates from the AD 1100s onwards.
The first Europeans reached Mangareva in 1797, followed several decades later by a group of French Catholic missionaries in 1834. Having set up an outpost on the islands, these missionaries began a building programme, led by a team of lay builder-brothers who also trained Mangarevan converts in European construction techniques. Using local materials, they built a series of coral constructions across the islands, including a large cathedral, multiple churches, schools, watchtowers, communal bread ovens, and even a royal palace for the chief’s family after his conversion to Catholicism. Their arrival also brought about a complete transformation of Mangarevan domestic spaces. Within just a few decades, the islanders’ traditional wood-and-thatch buildings had been replaced by a new type of small stone cottage. These houses, made of coral from shore reefs and beach rock-coral formations became the most popular type of home for several decades, until timber took over in the 1870s. While construction projects like the churches and schools were carefully documented by the missionaries, details about these domestic structures are entirely absent from the written record.
To remedy this, a recent study led by the University of Sydney, exploring the 19th-century architecture of the islands, carried out excavations at 12 of these coral cottages. They uncovered a variety of ceramics, glassware, and other items, revealing that the houses are microcosms of the social, cultural, and religious world of a Pacific community in a time of massive change. It is, therefore, particularly important to understand more about their construction.
The researchers employed a technique known as U-series or uranium-thorium (U-Th) dating, a type of analysis used on materials with high calcium-carbonate content such as stalactites or marine coral. U-Th has several benefits: it can produce precise date estimates from small samples, making it relatively non-destructive, and unlike radiocarbon dating the error ranges are a matter of years or decades, rather than centuries or even millennia. Also unlike radiocarbon, which is not accurate for dates within the last 400 years, U-Th can be applied right up to the present. In the past, it has been used to work out the age of ancient cave paintings by dating calcite deposits overlying the art, as well as to analyse Polynesian pre-colonial archaeology such as sacred sites in Hawaii and marae (ancient temples) in Mo’orea, but this is the first time it has been tested on historical coral architecture.
Samples of coral were taken from seven stone cottages, as well as a watchtower at Mata-Kuiti Point. A boys’ school, known to have been built between 1853 and 1858, was used as a control. The results reveal previously unrecognised patterns of architectural development and cultural life on Mangareva. Several samples date to within the first decades of the missionary presence on the islands, as expected. Of particular interest was a piece of coral found in a pit in one of the houses alongside various objects related to eating and drinking, as well as construction material, which excavators hypothesised represented waste from a feasting event connected to the building of the house. The dates from the coral in the pit and the house wall support this conclusion.
Interestingly, some samples produced dates earlier than the 1830s, and some even pre-dated European contact in the 1790s. The researchers offer two possible explanations for this. An archaeological study in the 1930s suggested that material from pre-colonial marae across Mangareva, which were dismantled after the missionaries’ arrival, may have been reused in the coral cottages. However, the U-Th dates are at most a few decades earlier than the 19th-century construction campaign, making it unlikely – although not impossible – that much of the coral was originally used in ancient structures. The other explanation, favoured by the researchers, is that the coral blocks in question included branches that had died off over a period of years or decades before they were harvested for the construction of the cottages.
Even taking into account these discrepancies, the U-Th dates provided are only a few years off from what would be expected – still well within the window that the best radiocarbon results would provide – and demonstrate that U-Th could provide useful terminus post quem dates for buildings of completely unknown age in the future.
The study, recently published in Antiquity (https://doi.org/10.15184/aqy.2026.10325), is focused on a relatively small sample, but has unearthed valuable information about an element of history that is missing from the written record. By enabling a better understanding of the chronology of these structures, the research helps to track how daily life on the islands evolved under European influence.
Text: Amy Brunskill / Images: James Flexner, University of Sydney
