Mangareva historical archaeology

The Mangareva Islands are an archipelago of small, ancient extinct volcanic peaks, located within a lagoon surrounded by a fringing reef. The reef includes several long, narrow motu (coral islets), many of which feature beach rock terraces on the surface and corals in the intertidal zone. Evidence of initial discovery by Polynesian navigators is estimated at approximately 850 CE based on direct U-Th dating of a coral abrader. More concrete evidence for human occupation and environmental disturbance appears by 1100 CE (Kirch et al. Reference Kirch2021).

European encounters with the Mangareva Islands began with the British missionary ship Duff in 1797 (Wilson Reference Wilson1799). This was followed several decades later by the arrival of French Catholic missionaries from the Pères des Sacrés Coeurs de Jésus et de Marie in 1834 (Laval Reference Laval1968). Upon establishing a foothold in the islands, the missionaries began a programme of construction, led by a cadre of frères bâtisseurs, lay builder-brothers, who also trained Polynesian converts in European construction techniques (Delbos Reference Delbos2011).

In addition to a massive cathedral in Rikitea, then and now the main village in Mangareva, the missionaries and converts built multiple churches in the neighbouring islands of Aukena, Akamaru and Taravai. Missionary infrastructure included separate boys’ and girls’ schools in Aukena and Mangareva, a dormitory and workshop for frères tisserands (weaver-brothers), a massive blast furnace near the boys’ school, and communal bread ovens throughout the mission villages. The missionaries also built a royal palace for the family of Maputeoa, the ranking chief and regent of the Mangareva Islands, after his conversion to Catholicism in 1836 (Flexner Reference Flexner2024).

This building activity impacted the Polynesian landscape, particularly through the desecration and destruction of once sacred sites called marae. Previous archaeological research interpreted piles of stone used for building materials in the nineteenth century as likely derived partially from blocks once incorporated into marae (Emory Reference Emory1939: 8). In addition to locally sourcing coral building materials from near-shore reefs, the missionary builders and converts cut blocks of beach rock and corals from exposed formations in the motu of Kouaku, approximately 14km to the south-east of Rikitea (Flexner et al. Reference Flexner2025: 33–34).

The most common building type recorded during archaeological survey is the small stone cottage that was the typical dwelling for missionaries and Mangarevans alike, accounting for more than 50 per cent (69/123) of the recorded structures across the four main islands where missionary construction occurred during the nineteenth century. The houses are constructed of rectangular blocks of beach rock or volcanic rock for the corners, window and door apertures, infilled with more uneven coral blocks that comprise the walls. Stone house construction started by the late 1830s, transitioning to wooden houses by the beginning of the 1870s (Laval Reference Laval1968: LXXXI–LXXXIV).

As part of a survey of nineteenth-century constructions in the Mangareva Islands, 1 × 1m test pits were opened in the interior and exterior of 12 houses. The goal was to uncover information about construction techniques, for example how the building foundations were laid, and material culture from the occupation of the dwellings. In addition to missionary houses associated with the early churches in Aukena and Akamaru, excavations sampled undated houses previously occupied by Polynesian families. This selection comprised one house in Aukena, one in Mangareva and eight in Akamaru, including two in Tokani Bay, which was the location of the earliest missionary settlement (a full survey and excavation report, Flexner et al. Reference Flexner2025, is available upon request to the authors).

During the excavations, the presence of unweathered branch corals in several in situ coral limestone blocks was noted (Figure 2), as well as in blocks that were clearly architectural but had fallen from the adjacent ruins. Samples of branch corals were removed using hand tools, being careful not to damage the building fabric more than necessary. Seven houses were dated using the U-Th method, as well as the boys’ school (which had known dates based on missionary documents), the watch tower at Mata-Kuiti point and a branch coral recovered from a pit feature within one of the houses.

Figure 2. 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; top right) the north exterior wall of AKH-10; bottom right) building plan of AKH-10 showing the sampling location. The sample returned a date of 1840±3 CE (figure by author).

U-Th dates from colonial buildings in the Mangareva Islands

All 10 coral samples used in this study were dated by the University of Queensland Radiogenic Isotope Facility (UQRIF) under ultra-clean conditions using the U-Th method. Complete information on chemistry and standards provided to the author by UQRIF is available in Table S1 in the online supplementary material. The resultant dates, including a conversion to calendar years CE, are presented in Table 1. The boys’ school on Aukena (AUH-11, the Collège d’Anaotiki) acted as a ‘control’ site with a known period of construction. The coral sample from that site was more weathered compared with other samples, as limited branch coral was within reach for sampling, but still returned a reasonably close date of 1831±2 CE. The nearby watch tower AUH-7, which is likely of similar age (see Laval Reference Laval1968: CXXVI), returned a date that was far too early, pre-dating European contact with the Mangareva Islands. This sample was also notably weathered.

Table 1. U-Th dating results presented in chronological order, adjusted to CE calendar years (‘present’ in BP is 1950 for compatibility with radiocarbon dates).

The samples from Akamaru were all visibly less weathered (Figure 3). Even so, one sample returned a pre-contact date (AKH-11). Of the two houses in Tokani Bay, AKH-32 yielded a pre-missionary date of 1820±24 CE, which is also the largest error range among the U-Th dates. This may be due to the introduction of non-radiogenic thorium via terrigenous dirt incorporated into the coral skeleton either during life or postmortem (N. Leonard pers. comm. June 2025). The remaining samples fall within the first decade or so of missionary presence in the islands, from AKH-7 (1834±3) to AKH-35 (1844±2). This latter sample is from the second house in Tokani bay, located approximately 200m from the first; it is possible they were roughly contemporaneous, but that the sample from AKH-32 had more inbuilt age.

Figure 3. The branch coral samples: top row from left) AKH-1, AKH-7, AKH-10, AKH-11, AKH-20; bottom row from left) AKH-20 PN-318, AKH-32, AKH-35, AUH-7, AUH-11 (figure by author, each square in the scale bar is 1cm).

The remaining dates are from a fallen coral block in the south-west corner of the house AKH-20 and a branch coral recovered from an excavated context (PN-318) in a 1 × 1m test pit (AKH-20 TU1) located in the south-west corner of the same house (Figure 4). The fill of PN-318 consisted of an extremely loose brown sediment with a high concentration of relatively large fragments of bone, shell, glass and iron artefacts compared with the other houses excavated in Akamaru. Excavation exposed the southern and eastern limits of the pit feature, which extends beyond the northern and western boundaries of TU1, and removed sediment from approximately 0.50–0.90m below the current ground surface (Flexner et al. Reference Flexner2025: 112–15). While analysis is ongoing, a high proportion of objects relating to food and drink (including alcohol) were noted, as well as construction materials.

Figure 4. Plan of AKH-20 (below), showing location of test pit and coral sample, along with photographs of the building (top right) and pit feature PN-318 (top left) (figure by author).

At the time of excavation, it was hypothesised that the pit might represent the waste from a single feasting event, possibly coinciding with the construction of the house as the pit was sealed by the deposits adjacent to the house foundation. Having a garbage pit inside a tropical household would have been both unsanitary and unpleasant, and therefore unlikely. The two U-Th dates overlap (1846±2 for the coral block and 1848±4 for the sample recovered from PN-318), reinforcing the hypothesis that the walls of the house were constructed shortly after the feast was held. As with other samples there is the possibility of some inbuilt age, but the overlap makes sense if the coral sample from the pit fell off during the trimming of coral blocks ahead of building the house.

Logically, coral blocks cut from the same part of the reef should be more likely to contain branch corals of similar age, however this has not been empirically tested. The objective in this initial run of U-Th dates was simply to test the reliability of the method for dating nineteenth-century buildings, so a single sample from multiple houses was sufficient. The collection of additional samples from the same houses, as well as sampling directly from areas where there is evidence for historical cutting of coral blocks, such as Kouaku, would be beneficial for future studies.

The sample of 10 dates presented here represents a small but significant portion of the 69 stone cottages that exist in varying states of ruination around the islands of Aukena, Akamaru, Taravai and Mangareva. The fact that four of the dates range from a few years to over 50 years prior to missionary arrival in the Mangareva Islands should not be taken as discouragement or a problem with the method. Rather, while the precision of each individual date is high (even in the case of AKH-32, which still has an error margin smaller than most radiocarbon determinations at 95.4% confidence), the accuracy of what is being dated does not necessarily directly correlate with the construction of the building.

What these dates represent is the death of the corals cut off the reefs of the Mangareva Islands. In some cases, blocks may have been reused from pre-European structures, as suggested by Emory (Reference Emory1939: 8), or may have been cut from locations where the coral was already dead but minimally weathered, such as the near-shore reef limestones alongside beach rock deposits in the motu (Figure 5). Coral exoskeletons can be quite hard. Living seaward corals in the reef would have had less inbuilt age, while less-weathered branch corals with several decades of inbuilt age might still have been present on blocks from the shoreward side. This would be akin to the ‘old wood’ problem in radiocarbon dating (Schiffer Reference Schiffer1986) and time-lag in dendrochronology (Boswijk et al. Reference Boswijk2014).

Figure 5. Photograph of exposed beach rock and near-shore corals in the motu of Kouaku where labourers gathered building materials for the Catholic mission (figure by author).

For buildings of completely unknown age, it might be appropriate to treat U-Th dates as a kind of terminus post quem relative to the actual construction date(s). Reused blocks could have been cut years or decades before missionary arrival. Corals might have died years or decades before a particular block was cut from the reef limestones, and there might be an interval of years again before the block is trimmed and placed in a wall. A similar chronology problem is understood in dating structures using associated artefacts such as coins (Deetz Reference Deetz1996: 24). As always, triangulating chronology from all available information, including artefacts and the documentary record, rather than relying on a single radiometric dating technique, provides the most robust means of building site chronologies in historical archaeology.