High-precision dating and ancient DNA profiling of moa (Aves: Dinornithiformes) eggshell documents a complex feature at Wairau Bar and refines the chronology of New Zealand settlement by Polynesians

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Highlights

  • A complex archaeological event is analysed using multiproxy data.

  • Ancient DNA identifications of moa eggshell fragments allow multiple dates on this short-life substrate.

  • Bayesian inference of AMS dates provides first high-precision chronology for Wairau Bar.

  • The results provide new insights into Polynesian colonisation of East Polynesia.

Abstract

Wairau Bar, New Zealand, is one of the few prehistoric sites in the world that could lay claim to being a site of first human intrusion into a pristine environment. It is certainly one of the best places to study human impact on a hitherto unoccupied land. Its potential status as a colonization phase settlement for New Zealand's Maori population raises questions that require fine-grained chronological resolution. Unfortunately, the simple stratigraphy of the Wairau Bar site has offered little opportunity for the development of high-resolution chronologies. This situation changed recently when new excavations exposed a complex, midden-rich feature which contained a wide range of dateable material, including hundreds of fragments of eggshell of the extinct megaherbivorous moa (Aves: Dinornithiformes). The thick eggshell, with its minimal inbuilt age and high resistance to contamination, is an ideal material for radiocarbon dating. Its refractory properties also allow high-quality preservation of DNA. The moa eggshell yielded radiocarbon that facilitated reconstruction of the chronology of deposition at a fine resolution. Ancient DNA profiling of eggshell fragments was used to ensure that dated fragments were from different individuals. Bayesian analysis of the dated fragments showed that the midden was laid down over a brief period in the early decades of the 14th century CE. This improved chronology provides a benchmark for understanding the duration of site occupation and revises current interpretations of the timing of Polynesian settlement of New Zealand.

Introduction

Wairau Bar is one of the earliest and most significant archaeological sites in New Zealand. It has the widest range and largest number of artefacts typical of the Archaic Phase of East Polynesian culture of any site in the Pacific (Higham et al., 1999) and is the source of much of our knowledge about the earliest settlement phase in New Zealand and of the historical connections between New Zealand and the tropical East Polynesian homelands (Duff, 1956, GOLSON, 1959, Davidson, 1984). Following the discovery of two “moa-hunter” burials in 1939 and 1942 by J.R. Eyles, the site was investigated over several field seasons between 1942 and 1964 by teams from the Canterbury Museum as well as by Eyles and others (Brooks et al., 2011). Most of this work focused on the many burials discovered at the site and their associated grave goods although, later, the stratigraphy and other aspects of the site were also investigated (Trotter, 1977). This work was conducted either before the invention of radiocarbon dating or during its infancy.

The importance of understanding the chronology of the site has long been recognized (Duff, 1956, Trotter, 1977, Anderson, 1989, Anderson, 1991, Challis, 1991, Higham et al., 1999); indeed, the first radiocarbon determinations from a New Zealand archaeological site were on samples from Wairau Bar (Duff, 1956: xii). However, access to the site was restricted from the mid-1960s at the very time when radiocarbon dating was becoming a routine tool for archaeologists. As a result, modern attempts at dating the site have been reliant on the use of curated material from the early excavations which was of questionable provenance. In fact, the additional problems of potential inbuilt age (unidentified charcoal samples) and the applicability and efficacy of methods of pretreatment (bone samples) mean that nearly all of the dates produced prior to 1999 are considered unreliable (Higham et al., 1999). To further complicate matters, a major fluctuation in the southern hemisphere terrestrial radiocarbon calibration curve is centred on the 14th century CE, making it difficult to obtain high-resolution chronologies for the Polynesian colonization phase (McFadgen et al., 1994). Finally, the site itself presents a particular problem for resolving chronology in its shallow, unbroken stratigraphy. The site is essentially one layer so it is difficult to improve the precision of dates using statistical tools which rely on knowledge of stratigraphic position to constrain date ranges.

In 1999, Higham et al. (1999) set out to refine the site chronology by dating moa eggshell (using pieces from 11 more-or-less complete but broken eggs) from ten of the burials, as well as marine shell samples (n = 2) from two midden contexts. The resulting dates, calibrated using the Intcal98 (Northern Hemisphere) calibration curve (Stuiver et al., 1998), indicated that the site had been occupied from the late 13th century. These dates are now considered the most reliable suite for dating the burial complex at Wairau Bar, but because they were from a number of different features of unknown stratigraphic relationship they cannot be combined to provide a high precision date of a single event.

Recent developments have provided the opportunity to generate a high precision age estimate for a single feature at Wairau Bar. First, a new program of excavations in 2009 produced multiple samples of moa eggshell from a single feature. Moa eggshell has negligible inbuilt age and is resistant to contamination (Higham, 1994). Deriving from a single feature Bayesian methods could be used to improve the dating precision. Underpinning this work, ancient DNA (aDNA) analysis was used to eliminate the possibility of dating multiple fragments derived from the same egg.

Section snippets

Excavations in 2009

Excavations at Wairau Bar in January 2009 were carried out under the direction of RW and CJ (Brooks et al., 2009). They were carried out as part of a program of repatriation of human remains that had been excavated in the mid twentieth century. Several parts of the site were investigated, including middens, living floors, and food preparation areas. One of the food preparation areas contained a circular array of five very large cooking pits which were individually up to 6 m in diameter.

Research aims and methods

This work was aimed at refining the chronology of the Wairau Bar site by generating the first high precision age estimate from a single, short-duration feature.

We adopted a three-stage methodology as follows:

Results

The results of sample selection and dating are described below.

Bayesian analysis

The results of the Bayesian recalibration are shown in Fig. 3. We generated HPDs (Highest Posterior Densities) for the start and end of the sequence (Fig. 3). The 68% HPD for the start was very sharp and had its maximum probability at about 1320. Similarly the peak of the 68% HPD for the end of the sequence was very sharp and firmly in the first half of the 14th century. We also generated an HPD for the duration of the sequence (Fig. 3 inset) which indicates (95% confidence) that the duration

Discussion

The application of new aDNA techniques to archaeological dating methods has allowed us, for the first time, to obtain multiple independent radiocarbon dates on an “ideal” dating substrate from a single, rapidly accumulated deposit. The contemporaneity of materials in the deposit meant that Bayesian methods could be employed to develop a high-precision chronology for the deposition event. The results placed that event in the first half of the 14th century CE. This is some 50 years later than a

Acknowledgements

We would like to express our gratitude to Rangitane ki Wairau for their consent to analyse moa eggshells from Wairau Bar, and especially to Richard Bradley for his work facilitating and supporting the research at Wairau Bar on behalf of Rangitane ki Wairau. We also thank the following people and institutions: Institute of Geological and Nuclear Sciences (GNS), Andy Brown, Jean Spinks and Frances Jacomb for assistance with laboratory work, Martin Jones for comments on the dating methodology.

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