Developing 230Th/U Dating of Coral Artifacts for High-Precision Cultural Chronologies in Eastern Polynesia

Berkeley Geochronology Center, Berkeley CA

Investigators

Abstract

Researchers wish to understand the tempo of the rise of complex, populous societies from small founder groups and the pace of accompanying environmental change. Archaeological study of Polynesian cultural sequences created on island ecosystems is well suited to such investigations. Beginning with small bands of pioneering settlers, the first humans on the landscape of each newly discovered island, Polynesian societies of increasing size and hierarchical complexity developed over time. This development was accompanied by environmental changes including profound modifications to native flora and fauna, landscapes, and marine resources. However, the timing of initial settlement and the tempo of ensuing cultural and environmental change remain highly uncertain despite nearly 70 years of research. In the current project, Drs. W. Sharp and P. Kirch and their collaborators Drs. M. Allen and G. Molle will further develop a novel approach to dating Polynesian cultural sequences. The expected improvements in dating will, for the first time, provide a clear picture of the speed of cultural and environmental change in these Polynesian societies. Better dates in key Eastern Polynesian archipelagoes will also help resolve a longstanding debate over the timing of the extraordinary trans-Pacific dispersal of the Polynesians. Developing the capability to precisely date coral artifacts will help Pacific archaeologists erect refined chronologies for human occupation and human-ecosystem interaction elsewhere in Oceania. Polynesian archaeologists have traditionally depended almost solely upon radiocarbon dating (i.e., 14C dating) to provide a quantitative temporal framework for colonization of islands and subsequent evolution of island cultures and environments. Despite significant improvements in the radiocarbon technique through the years, radiocarbon dating has severe inherent limitations when applied to the past ~1000 years, the time frame of cultural development in eastern Polynesia. An alternative dating technique that has already demonstated considerable success in applications in Polynesia is 238U-234U-230Th dating of corals, herein referred to as 230Th coral dating. The technique is based on decay of naturally occurring uranium in corals, and reliable 230Th coral ages with precisions of 1-2% are readily attainable. For example, for 500-year old corals this translates into dates with uncertainties of only five to ten years. In the current project, Sharp, Kirch and their collaborators will apply 230Th coral dating to coral abraders made from pieces of Acropora or other corals used by Polynesians to shape fish hooks made from shell. Such coral abraders are common in Polynesian archaeological sequences and have been shown to be highly suitable for 230Th dating. In sum, the project will provide chronologies for cultural sequences in key archipelagoes of Eastern Polynesia of unprecedented accuracy and precision, thereby substantively advancing our understanding of Polynesian prehistory.