A pā in the Waikato has been dated back to 1768 AD—younger than previously thought—using a precise form of radiocarbon dating never before applied to pā.
About 7000 pā have been identified across New Zealand. Some are the area of a modern-day house, others span hectares, and many are in the lush areas of the northern half of the North Island now used for farming.
Many pā in the Waikato were swamp pā, which took advantage of navigable waterways and rich natural resources. In the case of the 6000-square-metre Otāhau Pā, built on a strip of land between the Komakorau and Mangamotu streams near the Waikato River, the boggy soil and stream has preserved the palisade posts, 100 of which are still visible above water.
Some of the posts were the trunks of mīro, a winner in terms of carbon dating because of its sharp, clear growth rings that can be cut out and measured for radiocarbon.
Radiocarbon, along with regular carbon, is incorporated from the atmosphere into living creatures and plants. When the creatures and plants die, the radiocarbon will slowly decay to nitrogen—so the ratio of remaining radiocarbon to regular carbon tells researchers how old the artefact is.
But regular radiocarbon dating often isn’t precise enough because there have been natural fluctuations of radiocarbon in the atmosphere throughout history. In a technique called ‘radiocarbon wiggle-matching’, the radiocarbon amounts in individual tree rings of the pā palisade post are compared with the radiocarbon amounts in tree rings of a kauri of known calendar age, and so a more precise date is reached.
With this most recent study, by Alan Hogg and his team from the University of Waikato, wiggle-matching could date the palisade post construction to within four years.
The authors write in the Journal of Archeological Science Reports that the pā is much younger than was expected by kaumātua from Taupiri Marae, 1.5 kilometres from Otāhau Pā. There is not much oral history on the pā from that time period, and as the area was in a state of warfare, they suggest that it may have been settled by another hapū.