Researchers say a prolonged drought in the southwest tropical Pacific may help explain why ancestral Polynesian voyagers began pushing east across thousands of kilometers of ocean after roughly 1,700 years of limited expansion.
David Sear of the University of Southampton, Manoj Joshi of the University of East Anglia, and Mark Peaple of the University of Southampton report in the Journal of Pacific Archaeology that sediment records from swamps and lakes show an unusually severe dry period between 850 and 1200 AD. According to the researchers, it was the driest interval the region had seen in the past 2,000 years.
The timing lines up awkwardly well with one of Pacific archaeology’s long-running puzzles. The Lapita people, ancestors of Polynesians, reached Samoa and Tonga about 3,000 years ago, bringing distinctive pottery and an island-based culture. Then expansion farther east appears to have stalled for about 1,700 years, while communities in Samoa and Tonga grew and developed post-Lapita cultures.
Between 900 and 1100 AD, that pause ended. Archaeological evidence cited by the researchers indicates ancestral Polynesians began a rapid phase of eastward migration. Within about a century, voyagers using large double-hulled sailing canoes had reached Hawaii, Aotearoa, also known as New Zealand, and Rapa Nui, also known as Easter Island. The distribution of sweet potatoes across Pacific islands suggests contact with the continental Americas as well.
How mud records old rain
The new evidence comes from hydrogen isotopes preserved in ancient organic material. Isotopes are versions of the same element with different atomic weights. In the tropics, the hydrogen isotope mix in rainfall changes with the amount of rain. Plants and algae take up that water as they grow, locking the rainfall signal into molecules that can persist in sediment for thousands of years.
By reading those chemical traces in lake and swamp mud, the researchers reconstructed past rainfall in the Tonga and Samoa region. Their reconstruction points to sustained drought during the same broad period when larger island populations would have been placing more pressure on food and freshwater supplies.
That does not reduce Polynesian expansion to a climate event with canoes attached. The researchers describe several possible factors acting together: harsher environmental stress, population growth, and improved voyaging technology. Genetic evidence cited by the team indicates Samoa’s population rose quickly around 1000 AD, possibly because new people arrived.
The rain belt problem
The climate mechanism is the South Pacific Convergence Zone, a broad band of clouds and rainfall that shifts across the tropical Pacific. Its position is affected by sea surface temperatures. Short-term movement is linked to El Niño and La Niña, while longer shifts can leave some islands unusually dry or wet for decades or centuries.
On small islands, that matters more than a tidy climate graph suggests. Freshwater and food production depend heavily on rainfall. The researchers argue that prolonged drought, arriving when populations were larger, could have made established islands harder to sustain and raised the stakes for finding new ones.
The finding also gives a more grounded backdrop to the seafaring traditions that films such as Moana borrow from. The movies are fiction, but the navigation problem was not. Polynesian voyagers crossed enormous distances in the Pacific, and the new study suggests they did so during a period when staying put may have become materially harder.
This story draws on original reporting from Ars Technica.
