A wiggle-match date for Polynesian settlement of New Zealand. (Method).

Hogg, Alan G. ; Higham, Thomas F.G. ; Lowe, David J. 等

Introduction

New Zealand was the last substantial landmass to be colonised by humans before the industrial age. Although it is now well established that the Polynesian settlers of New Zealand originated in central East Polynesia (e.g. Penny et al. 2002), the date of such settlement has proved controversial. An early transient contact c. 50-150 AD, based on Pacific rat-bone (Rattus exulans) dates obtained from natural sites, was proposed by Holdaway (1996, 1999) and Holdaway & Beavan (1999) on the premise that the rats, an introduced predator to New Zealand, accompanied the early Polynesians as a food source or stowaways (Roberts 1991; Matisoo-Smith et al. 1998). However, the reliability of the early rat-bone dates was disputed, especially as aberrant rat-bone dates were reported from several archaeological sites (Anderson 1996, 2000; Beavan & Sparks 1998; Smith & Anderson 1998; Hedges 2000; Higham & Petchey 2000). Brook (2000) suggested, from dating predation damage of the landsnail Placostylus ambagiosus, that the Pacific rat probably became established in northernmost North Island at around the same time as permanent Polynesian settlement. Most recently, Holdaway et al. (2002) provided support for the early rat-bone dates, and suggested that the Pacific rat was present in New Zealand well before permanent Polynesian settlement.

Various authors have shown that there are no human archaeological contexts in New Zealand which pre-date the 12th century AD (Anderson 1991; McFadgen 1994; McFadgen et al. 1994; Higham & Hogg 1997; Higham et al. 1999). Recent archaeological and radiocarbon evidence now suggests strongly that the earliest settlers in the New Zealand archipelago arrived around c. 1250-1300 AD (McFadgen et al. 1994; Higham & Hogg 1997; Higham et al. 1999). In addition, evidence from New Zealand's outlier islands (Norfolk, Kermadecs) supports the notion that southern Polynesia was all settled at virtually the same time in prehistory (Higham & Johnson 1996; Anderson et al. 2001). This evidence supports the socalled `late' settlement model first proposed by Anderson (1991).

Dating environmental events

Other dates for human settlement have been inferred from changes in environmental sequences. Short-lived, minor disturbances in the pollen record, including small increases in bracken (Pteridium esculentum) and other seral taxa, were attributed by Sutton (1987, 1994) to activities by a small but `archaeologically invisible' population of early Polynesian colonists prior to c. 1300 AD. His interpretation forms the basis for the `early' settlement model (Sutton 1987). However, critics have pointed out that such disturbances are indistinguishable from those resulting from natural background events, such as lightning-induced fires, impacts from volcanic eruptions, storms or droughts, and that these occurred throughout the Holocene with increasing frequency, and also in pre-Holocene pollen records (McGlone 1989; Wilmshurst et al. 1997; Ogden et al. 1998; Newnham et al. 1998a; McGlone & Wilmshurst 1999).

McGlone & Wilmshurst (1999) concluded that the first evidence for Polynesian environmental impact dates broadly to c. 1200-1400 AD based on palynological data from many sites throughout New Zealand. A similar finding was reported by Ogden et al. (1998). Opal phytolith data from tephra-palaeosol sequences in the Bay of Plenty in eastern North Island are also consistent with these results (Kondo et al. 1994; Sase & Hosono 1996), as are dunefield and sedimentological studies from northern and eastern North Island (McFadgen 1994; Wilmshurst 1997; Page & Trustrum 1997; Brook 1999; Horrocks et al. 2001a) and also isotope analyses on speleothems in northern South Island (Hellstrom et al. 1998). Taken together, the palaeoenvironmental research suggests that deforestation, beginning in the period c. 1200-1400 AD, occurred virtually simultaneously across much of New Zealand. Exacting greater precision has been hampered because of the limitations of radiocarbon calibration allied with the uncertainty associated with identifying the first human deforestation signals (McGlone & Wilmshurst 1999), and inadequate sampling resolution. Difficulties include the dating of recent events using radiocarbon where sediment contamination by in-washed carbon, in-built age, hard water or other factors may affect sample reliability (Elliot et al. 1997; Wilmshurst 1997; Wilmshurst et al. 1997; Newnham et al. 1998a, 1998b; McGlone & Wilmshurst 1999; Lowe et al. in press).

Using tephrochronology

To help resolve some of these problems, Newnham et al. (1998a) and Lowe et al. (2000, 2002) have promoted the role of tephrochronology--the use of tephra layers as marker beds to establish numerical or relative ages (Lowe & Hunt 2001). This method provides a way of circumventing the interpretative difficulties associated with radiocarbon dating at palaeoenvironmental (natural) and archaeological sites because tephra layers provide virtually instantaneous chronostratigraphic marker horizons, or isochrons, that can be correlated between sites independently of radiometric dating. That tephra deposits are found in both natural and archaeological sites means they have the capacity for linking such sites in an unambiguous manner that no other dating or correlative technique can provide.

A key event for human prehistory in New Zealand is the eruption of the Kaharoa Tephra, a geochemically distinctive, rhyolitic tephra layer originating from Mt Tarawera volcano in the Okataina Volcanic Centre, North Island (Figure 1) (Lowe et al. 1998). Widely dispersed over at least 30,000 [km.sup.2] of northern and eastern North Island, Kaharoa Tephra provides a unique `settlement horizon' (landnam) for prehistory in northern New Zealand. Although no cultural artefacts are recorded beneath it (Anderson 1991; Shepherd et al. 1997; Lowe et al. 2000; Horrocks et al. 2001b), the earliest human-induced environmental impacts, inferred largely from palynological data as described above, occur at around or just prior to its deposition (Newnham et al. 1998a; Lowe et al. 2000).

[FIGURE 1 OMITTED]

Dating the Kaharoa Tephra

Lowe et al. (1998) have determined a mean radiocarbon age for the deposition of the Kaharoa Tephra of 665 [+ or -] 15 radiocarbon years BP, but this corresponds to a wide range of calendar dates because of marked fluctuations of the calibration curves in the 14th century. The date of the initial Polynesian deforestation signals has therefore remained ambiguous because the Kaharoa eruption could have occurred at any time between c. 1290 AD and c. 1400 AD. This means initial settlement may have occurred towards the end of the 13th century or more than a century later.

One method of achieving a more precise date for the Kaharoa eruption event, and thus a date for the earliest settlement, is by `wiggle-matching' a known sequence of radiocarbon dates with the calibration curve. Samples taken from a known sequence, such as a tree ring series or superposed peat layers, are radiocarbon dated and the results fitted to the radiocarbon calibration curve using published statistical methods of best fit (Kojo et al. 1994; Christen & Litton 1995; van der Plicht et al. 1995; van Geel et al. 1996; Kilian et al. 1995, 2000; Speranza et al. 2000). The 1000-year-long Southern Hemisphere calibration curve enabled us to apply wiggle-matching to provide high- resolution calendar dates without the need to account for possible variations in the offset between Northern Hemisphere calibration curves and dated material from the Antipodes (McCormac et al. 1998a, 1998b; Hogg et al. 2002).

We obtained a carbonised log, 0.15 m in diameter, with the bark intact, from within near-source pyroclastic deposits (emplaced by hot, fast-moving, ground-hugging particulate gaseous flows) exposed at the Crater Rd section near Mt Tarawera (Figure 1). This section is a formally defined reference site (hypostratotype) for the Kaharoa Tephra Formation (see Froggatt & Lowe 1990 and Lowe et al. 1998, 2002 for details). The deposits (`unit Hpdc' in Nairn et al. 2001) encapsulating the log were emplaced early in the Kaharoa eruption episode. The duration of all the explosive eruption phases, which generated the distal tephra fall deposits (`Kaharoa Tephra'), is estimated at `days to weeks' (Nairn et al. 2001). The log was identified as PhyUocladus spp., generally known as the celery pine. This genus is usually well suited to dendrochronology (Norton & Palmer 1992; Newnham et al. 1999) and the rings proved suitable in our specimen. An inspection of the outermost ring revealed that it seemed to be completely formed, having both early wood and late wood. This suggests that the tree was killed during the period from late autumn to early spring (i.e. the period between growth cessation because of the onset of winter but before the start of spring growth). The exact month of the eruption within this period is impossible to determine, but our experience from contemporary tree-ring studies of the same species (e.g. Palmer et al. 1988) suggests that the period from May to September is likely.

Five contiguous ten-ring (i.e. decadal) blocks of carbonized wood were removed for high-precision radiocarbon dating. The first sample was the youngest, spanning ten annual rings obtained from the outside of the log, but excluding the bark. Subsequent samples were extracted contiguously towards the older, central part of the log. The methods and results of the radiocarbon analyses are given in Table 1, and Figure 2 shows the sequence of five dates, in their known order of age, matched to the wiggles of the high-precision Southern Hemisphere radiocarbon calibration curve.

[FIGURE 2 OMITTED]

The wiggle-matched date for the sample closest to the bark of the tree (Figure 2, no. 1) was 1308 [+ or -] 12 AD (20 error). However, a more accurate eruption date requires two other corrections. Firstly, five years must be added to each date because the 14C determination represents the average often rings. A second correction, adding one year, is a dating convention used in the Southern Hemisphere (growth in the austral summer begins in one year and ends in the next), and so the calendar year for a tree-ring is reckoned as the year that the new growth began (Schulman 1956). The calendar date we have determined for the eruption of Kaharoa Tephra therefore is 1314 [+ or -] 12 AD (2[sigma] error).

For comparison, we also used OxCal to date the death of the tree by wiggle matching using a Bayesian approach (Ramsey 1995, OxCal 3.5; see also Ramsey et al. 2001). We used the D_Sequence model with ten-year gaps, and a six-year gap to account for the final tree rings (Figure 3A). The wiggle match produced a result for the death of the tree of 1310-1320 AD (1[sigma]) and 1305-1325 AD (2[sigma]) (Figure 3B). This finding is identical to that derived using the first method and adds confidence in our date for the eruption.

[FIGURE 3 OMITTED]

Conclusion

Establishing a precise calendar date for the Kaharoa Tephra, a key settlement layer in northern New Zealand, is important for investigating patterns and processes of prehistoric human colonisation in both New Zealand and East Polynesia and of its associated anthropogenic environmental impacts. More precise dates for human settlement aid in understanding the process and rate of cultural change, and the demography of human population growth rates from founding colonisers (cf. Murray-Macintosh et al. 1998; Penny et al. 2002). We have derived a wiggle-match date of 1314 [+ or -] 12 AD (2[sigma] error) for the Kaharoa Tephra datum, and this acts as a settlement horizon for those areas where the tephra is known to occur in North Island. It remains possible of course that earlier sites await discovery outside the Kaharoa fallout zone.

Numerous archaeological sites in eastern and northern North Island have featured the Kaharoa Tephra (Lowe et al. 2000), and the absence of artefacts or cultural remains reported beneath it suggests that these sites must be younger than the wiggle-match date of 1314 [+ or -] 12 AD. Higham and Hogg (1997) showed that the earliest archaeological dates are indistinguishable in both main islands of New Zealand using radiocarbon, but because the Kaharoa Tephra is not currently traceable in the South Island, its use as a terminus ante quem for settlement is geographically circumscribed.

In the zone where it occurs, signals of environmental impact are essentially contemporary with or just pre-date the deposition of Kaharoa Tephra. Newnham et al. (1998a) examined eleven pollen profiles from peat or lacustrine deposits in northern and eastern North Island containing both Kaharoa Tephra and palynological indicators for the onset of significant deforestation (Figure 1). In the upper parts of pollen profiles, there are marked and sustained rises in bracken spores and charcoal, unprecedented in the Holocene record, and inferred to be the result of initial human firing (Newnham et al. 1998a; Lowe et al. 2000, 2002; Horrocks et al. 2001b). In a few profiles, the sustained rises in bracken and charcoal occur well after the Kaharoa Tephra datum but in the others the rises occur close to the time of its deposition. The earliest sustained rises begin in sediments only a few centimetres at most below the Kaharoa Tephra in four of the pollen profiles. Based on the chronostratigraphy of these profiles, all of which additionally contain the 232 [+ or -] 15 AD Taupo Tephra marker bed (Sparks et al. 1995; Lowe & de Lange 2000), we consider that the amount of time represented by this pre-Kaharoa (bracken-bearing) sediment probably represents a maximum of about fifty years (Lowe et al. in press).

This suggests very strongly that the earliest environmental impacts associated with initial Polynesian settlement in northern New Zealand (North Island) occurred in the second half of the 13th century AD, coincident with the earliest-known settlement dates from archaeological sites on both North Island and South Island.

Table 1. Radiocarbon measurements on carbonized Phyllocladus spp. log
from Kaharoa eruptives, Crater Rd. The carbonized wood samples of ten
tree-rings each were pretreated by the standard acid-base-acid
extraction procedure, with the NaOH step carried out in a nitrogen
atmosphere to minimize the possibility of incorporation of modern
carbon through absorption of atmospheric C[O.sup.2]. [sup.14]C was
measured by liquid scintillation counting of benzene using a Wallac
Quantulus 1220 spectrometer, especially optimized for high precision
([+ or -] 2-3 [per thousand]; [+ or -] ~ 16-24 [sup.14]C yr)
measurements (Higham & Hogg 1997; McCormac et al. 1998a, 1998b).
Benzene samples 8.5 ml in volume were counted for at least 7,500
minutes in Waikato 10 ml synthetic silica counting vials (Hogg 1992).

Laboratory [sup.14]C age Sample number
code (conventional (tree-rings
(Wk-) yr BP) number)

8204 728 [+ or -] 16 1 (1-10: sapwood)
8205 740 [+ or -] 16 2 (11-20)
8206 733 [+ or -] 16 3 (21-30)
8207 780 [+ or -] 16 4 (31-40)
8208 812 [+ or -] 16 5 (41-50)

Acknowledgements

We thank Ian Nairn for his help in sampling logs for dendrochronological evaluation and for information on the Kaharoa eruption, anonymous referees for their comments, and Martin Carver for his helpful editorial comments.

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Alan G. Hogg, Thomas F.G. Higham, David J. Lowe, Jonathan G. Palmer, Paula J. Reimer, & Rewi M. Newnham *

* Hogg, Waikato Radiocarbon Dating Laboratory, University of Waikato, Private Bag 3105, Hamilton, New Zealand. (Alan.Hogg@waikato.ac.nz)

Higham, Oxford Radiocarbon Accelerator Unit, Research Laboratory for Archaeology and the History of Art, University of Oxford, Oxford OX1 3QJ, England, U.K.

Lowe, Department of Earth Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand. Palmer, School of Archaeology and Palaeoecology, Queen's University of Belfast, Belfast BT7 INN, Northern Ireland, U.K.

Reimer, Center for Accelerator Mass Spectrometry L-397, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94550, U.S.A.

Newnham, Department of Geographical Sciences, University of Plymouth, Plymouth PL4 8AA, England, U.K.

Received 28 November 2001; Revised 18 January 2002.