Redmond Stein, Lorelei Curtin, Nicholas Balascio, Raymond Bradley, Dorothy Peteet, Rafael Rapu, Valenti Rull, Andrea Seelenfreund, William J D'Andrea

From approximately 1200-1600 CE, Polynesian settlers on the island of Rapa Nui engaged in megalithic monument construction, crafting hundreds of Ahu platforms and Moai statues from volcanic bedrock. The decline of this tradition has intrigued archaeologists for decades. The most widely disseminated hypothesis surrounding the demise of the Ahu Moai culture suggests that the Rapanui overexploited the island’s resources via slash-and-burn agriculture, leading to demographic collapse and warfare [1,2]. However, there is little evidence to support this claim, and genomic evidence refutes the idea of a population crash prior to European arrival [3]. Here, we present new evidence for a transition to drought conditions on Rapa Nui coincident with the end of the Ahu Moai culture, based on two independent reconstructions of hydrogen isotopes in rainfall (∂2Hprecip) inferred from hydrogen isotopes of sedimentary leaf waxes (∂2Hwax). In accordance with observational data and model simulations, we interpret more negative ∂2Hprecip values to reflect an increase in the frequency of large storms and total rainfall amount over Rapa Nui. We show that the 16th-17th century decline in Ahu and Moai construction coincided with a sustained, multi-century decrease in precipitation of ~700-900 mm/year, which likely spurred territorialism over freshwater resources and instigated societal reorganization.