Archaeomagnetic studies of Maori Hangi Stones from New Zealand

Abstract:

Global palaeosecular variation models still suffer from a paucity of high quality data from the SW Pacific region. Over the past two years we have worked to fill this gap with archaeomagnetic data – directions and palaeointensities – by studying the thermoremanent magnetization of Maori hangi cooking stones. Used as heat retainers, these stones are heated, frequently above the Curie temperatures of constituent magnetic minerals, before being buried in earth ovens. After removal of the food, hangi sites are often abandoned with the stones still in situ, carrying a record of the magnetic field in which they were last cooled.

We have sampled a range of archaeological hangi sites throughout New Zealand, dating to early prehistoric times (ca 700 BP). The stones vary in lithology from andesites, originating from the central North Island volcanoes, favoured by Maori for their durability and with NRM intensities up to 30 A/m, to greywackes and schists from the main axial ranges, with NRMs as weak as 10^-4A/m.

In all cases, we have independently oriented and retrieved several stones, and we have made several specimens from each stone, either by drilling (standard cylindrical specimens) or sawing (pseudo-cubes) in the laboratory. We have calculated site mean palaeomagnetic directions from principal component analysis of thermal demagnetization data, discarding the data of stones that show evidence of disturbance. We have carried out palaeointensity experiments using a Coe/Thellier method with pTRM and tail checks, and with selection criteria modified to the situation. Rock magnetic experiments contribute to our understanding of the mineralogy, domain state and blocking temperature spectra.

The palaeodirections fall between declinations of 348^o and 24.5^o, and inclinations of -46.4^o and -72.4^o, with palaeointensities between 43.7±1.4 and 81.3±6.1 mT. Most fall within the expected range of secular variation for New Zealand. However the palaeointensity of 81.34±6.08mT, from an archaeological site at Opihi in the south of the South Island, dated at 382±20 BP, is unexpectedly high.

We present our data against model records for New Zealand calculated using the ARCH3k model of Korte et al (2009), and suggest that the addition of our new data will improve the model for the most recent period of time.