Coral yields precise Polynesian arrival date
Accurate method Researchers have dated ancient coral tools in Tonga to pin down the most precise date for human settlement of Polynesia yet.
The research is published today in PLoS ONE.
"We've been able to date the founding population of Polynesia to within a very narrow window of 16 years," says archaeologist Professor Marshall Weisler of the University of Queensland.
Polynesia consists of all the islands in the triangular area defined by Hawaii in the north, New Zealand in the south and Easter Island in the east.
It is believed to be the last place on Earth to become inhabited by humans.
People known as Lapita, the creators of distinctive pottery, have previously been estimated to have arrived in Tonga sometime around 3000 years ago.
The most precise arrival date so far has come from radiocarbon dating of charcoal excavated from the village of Nukuleka, but this gives a 178-year window between 2769 and 2947 years ago.
Weisler proposed using another dating method to get a more precise date on first settlement.
During excavations at Nukuleka, lead author, Professor David Burley of Simon Fraser University in Canada found numerous fingers of acropora coral, which had been used by Lapita people as files and rasps.
"They'd break living coral fingers off the reef and use these for abrading shell and wood for shaping and forming these raw materials into various tools and artefacts," says Weisler.
The researchers dated the coral using a method known as uranium/thorium dating, often used to study long-term changes in climate.
The method relies on the decay products of uranium to calculate the age of the coral.
"It's very very much more precise," says Weisler.
The result suggested Lapita people arrived within a 16-year window between 2830 and 2846 years ago.
It's great to have a precise date but how confident are the researchers that their date is accurate?
After all, these corals have been sitting in the ground for almost 3000 years and dirt could easily have contaminated the pores of the coral, says Weisler.
To make sure they didn't get a bogus date, they developed a special protocol that involved washing the coral with hydrogen peroxide and then only selecting the purist samples.
"When we cut off a centimetre-long sample, we'd crush it and process it and look at it under high magnification and only pick out those very small pieces that were pristine, that hadn't been affected by any erosion processes," says Weisler.
Weisler says the research lays the way for more precise dating of human colonisation of the Pacific.
"This is a great opportunity to date the speed of Lapita colonisation which is the broadest migration of people anywhere in the world - across the widest expanse of ocean over the fastest period of time ever in human history," he says.
The research was funded by the Australian Research Council and also involved Professor Jian-xin Zhao from the University of Queensland's Radiogenic Isotope Facility.