Main Objectives:
The mission conducted in January 2025 aimed to identify the materials used in the construction of marae, traditional Polynesian structures with religious and social functions. This work contributes to a better understanding of the history of motu formation, as well as past interactions between human societies and their environment.
Field Actions and Preliminary Results:
Investigations were carried out on the motus of Onetahi and Tiaraunu to characterize the materials used in the construction of the marae and the local sediments. Analyses show that these structures are primarily composed of limestone slabs and coral fragments derived from corals, red algae, and mollusks, naturally cemented to form what is commonly known as beachrock.
However, the composition varies across sites. Some marae are entirely formed from coral colonies dominated by the genus Porites, while pebbles, coral blocks, and occasional volcanic fragments were also observed. Several sediment cores were collected to examine the stratigraphy of the motus. The deposits consist mainly of coral sands and gravels and extend down to the freshwater lens located between 1.60 m and 1.70 m beneath the upper surface of the islets.
These observations suggest that the selection of materials resulted from deliberate choices made by past populations, adapted to available resources and local constraints.
Perspectives:
New observations have revealed extensive areas covered with coral and reef blocks, likely deposited by major marine events such as ancient storms or cyclones. Dating these deposits will help reconstruct the timeline of such events over the past millennia.
In the long term, this research could support the creation of a regional database on past extreme events, providing a valuable tool for anticipating climate-related risks in low-lying Pacific islands.