Optical dating of quartz sediments and accelerator ...

© Journal of The Royal Society of New Zealand, Volume 32, Number 3, September 2002, pp 463-505

Optical dating of quartz sediments and accelerator mass spectrometry 14C dating of bone gelatin and moa eggshell: a comparison of age estimates for non-archaeological deposits in New Zealand Richard N. Holdaway1, Richard G. Roberts2, Nancy R. Beavan-Athfield3, Jon M. Olley4, and Trevor H. Worthy5 Abstract A consensus has not been reached on the validity of “old” (pre-Polynesian settlement) 14C ages for Pacific rat bones from New Zealand. As an independent test of their validity, we have applied optical dating techniques to fossiliferous sediments at three non-archaeological sites in the North and South Islands. In this paper, we report the optical ages obtained from quartz grains and compare them with a suite of accelerator mass spectrometry (AMS) 14C ages obtained from the bone gelatin of Pacific rats (Rattus exulans) and five species of bird (four herbivores and one omnivore). An AMS 14 C age is also reported for one sample of moa (Aves: Dinornithiformes) eggshell. All dated fossil remains were collected from known stratigraphic positions. Additional chronological control is provided by two known-age volcanic tephras at the Hukanui sites in the North Island. At the South Island site (Earthquakes #1), an infilled burrow provides independent age control, in that fossils inside the burrow should yield younger ages than the sediments and fossils outside the burrow. Bone preservation is uniformly good at all sites, as shown by surface detail, nitrogen content, and C:N ratios. In addition, amino acid profiles are consistent with those of collagen from modern Pacific rats and the laboratory rat collagen standard. Single-aliquot optical dating protocols were employed to avoid age overestimation due to incomplete bleaching of sediments before burial and to permit the identification of any post-depositional disturbance. At the Hukanui sites, 14C ages on bird bone and eggshell agree with the optical ages for the enclosing sediments, and both chronologies are consistent with the accepted ages for the overlying Taupo Ignimbrite (c. 1850 yr BP) and the underlying Waimihia Tephra (c. 3300 yr BP). Two 14C ages from Finsch’s duck (Chenonetta finschi) bones excavated from within the Taupo Ignimbrite agree with the accepted age of the ignimbrite. In addition, a gelatin sample from a Pacific rat bone reportedly excavated from beneath the Taupo Ignimbrite has a similar amino acid profile to that of modern collagen. This measure of good bone preservation provides confidence in the 14C age for this sample (1775 ± 93 yr BP), which accords with its position beneath the ignimbrite. At Earthquakes #1, five 14C determinations were made: three on Pacific rat, one on New Zealand pigeon

1Palaecol

Research, P.O. Box 16 569, Christchurch, New Zealand. Email: [email protected] of Geosciences, University of Wollongong, Wollongong, NSW 2522, Australia. 3Rafter Radiocarbon Laboratory, New Zealand Institute of Geological and Nuclear Sciences, P.O. Box 31 064, Lower Hutt, New Zealand. 4CSIRO Land and Water, P.O. Box 1666, Canberra, ACT 2601, Australia. 5Palaeofaunal Surveys, 2A Willow Park Drive, RD 11, Masterton, New Zealand. R01006 Received 11 April 2001; accepted 22 January 2002; published 27 September 2002 2School

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Journal of The Royal Society of New Zealand, Volume 32, 2002

(Hemiphaga novaeseelandiae), and one on South Island kokako (Callaeas cinerea). All five ages are consistent with the stratigraphic locations of the fossils, and with the optical ages for the enclosing sediments. We conclude that the early arrival of rats in New Zealand is supported by the array of chronological evidence presented here: the concordance of 14C and optical ages at each of the sites; the agreement between 14C ages on different materials and for a range of species; and the consistency of the 14C and optical ages with stratigraphic and other chronological markers. The Pacific rat was present in both the North and South Islands of New Zealand at least 800 years before permanent Polynesian settlement. Keywords radiocarbon dating; bone; comparison; optical dating; New Zealand

INTRODUCTION Radiocarbon (14C) dating has been the method of choice for determining numerical ages for events of significance to New Zealand prehistory. Such events include the time of human settlement (Anderson 1991; Higham et al. 1999), the initiation and course of human effects on the New Zealand vegetation and fauna (McGlone & Wilmshurst 1999; Holdaway & Jacomb 2000), and studies of faunal responses to climate change (Worthy 1997, 1998, 1999; Worthy & Holdaway 1993, 1994, 1995). Other dating methods, including uranium-series dating of speleothems (Hellstrom et al. 1998), obsidian hydration (Sheppard et al. 1996), tephrochronology, and luminescence techniques have also been used in archaeological and palaeoenvironmental research in New Zealand (see Newnham et al. 1999 for a recent review). Tephrochronology has been applied to the problems of first occupation and contemporaneity of events or sites (Newnham et al. 1998; Wilmshurst et al. 1999) but the ages of the critical tephras themselves rely principally on radiocarbon determinations or application of other radiometric techniques to material above, below, or within the tephra of interest. At least one apparently stratigraphically useful tephra, the Loisels Pumice (McFadgen 1985, 1994; Osborne et al. 1991) may have multiple sources of different ages (Shane et al. 1998). Optical and thermoluminescence dating has been applied to loess (Shulmeister et al. 1999; Lian et al. 2000) and dune sands (Shepherd & Price 1990; Duller 1994; Shulmeister & Kirk 1996), sometimes in conjunction with tephrostratigraphy (Pillans et al. 1993, 1996; Lian & Shane 2000). A major problem in applying radiocarbon dating to the very short archaeological chronology in New Zealand has been the apparent differences between results obtained from different materials. The considerable discussion on the relative merits of marine shell, charcoal, bone gelatin, and moa eggshell as dating material includes contributions by Trotter & McCulloch (1984), Anderson (1991, 1996), McFadgen et al. (1994), Higham (1994), Beavan-Athfield et al. (1999), Higham et al. (1999), Holdaway (1999), and Holdaway & Beavan (1999). A consensus has yet to be reached on the most reliable material for 14C dating. For example, Trotter & McCulloch (1984) strongly favoured moa bone collagen and, to a lesser extent, marine shell, and treated charcoal as being suspect because of inbuilt age from contaminants. Anderson (1991, 1996) favoured charcoal and marine shell at the expense of bone collagen, Higham et al. (1999) relied on moa eggshell, and Petchey (1999) further emphasised apparent problems with 14C dates on bone materials. Difficulties associated with calibration of radiocarbon dates on the short New Zealand human chronology have also been well canvassed (e.g., McFadgen 1982; McFadgen et al. 1994; Sparks et al. 1995). In view of the continuing debate on the best and most reliable material for 14C dating of New Zealand prehistory, it was no surprise that objections were raised (e.g., Anderson 1996; Petchey 1999) to accelerator mass spectrometry (AMS) 14C ages for bone gelatin from

Holdaway et al.—Comparison of age estimates by optical and radiocarbon dating

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Fig. 1 Arrangement of Hukanui sites (Hukanui #7 series and Hukanui Slab. Section A–A is shown in Fig. 2. TN, True North; MN, magnetic North; D4, D5, datum points for Hukanui Slab and Hukanui #7b sites; triangles, steeper slopes; broken lines, floor level outlines of shelter interiors. There are no connections between the interiors of the sites. Hatched areas are major slabs within sites, which restrict and control deposition areas.

Pacific rats (Rattus exulans) that suggested the rat arrived in New Zealand (with transient human visitors) at least 1000 years before Polynesian settlement (Holdaway 1996, 1999). One aspect of continuing research on the chronology of the effect of rats, humans, and other predators on the New Zealand biota has been to determine the reliability of bone gelatin as a dating material in archaeological and natural contexts (Beavan-Athfield et al. 1999; Holdaway 1999; Holdaway & Beavan 1999). For the present study, we further tested the reliability of AMS 14C dating of bone gelatin from Pacific rat bones by comparing optical (or optically stimulated luminescence (OSL)) ages on quartz grains from the bone-bearing sediments, with AMS 14C dates on the bones themselves from well-stratified sequences at several sites. In principle, optical dating of the time of sediment deposition should not only confirm or refute the radiocarbon age determinations, but it may also be able to provide evidence of bioturbation or other reworking of material within the sites. Bones of herbivorous birds and, in one instance, a moa eggshell sample were also dated. All samples were collected from stratigraphically controlled excavations of natural deposits. At two of the sites concerned, identified volcanic tephras from the Taupo Volcanic Centre were available as further stratigraphic controls. The relative proportions of amino acids characteristic of gelatin were measured in aliquots of gelatin from selected samples as a means of determining the quality of bone preservation.

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Fig. 2 Section at A–A in Fig. 1, from west, with Hukanui #7a projected onto same plane. T, Taupo Ignimbrite; W, Waimihia Tephra. A, present section; B, section as at time of emplacement of Waimihia Tephra (3280 ± 20 14C yr BP). Note lowered elevation of the floor of Hukanui #7a and #7b, and the stepped slope in front of the Hukanui Slab floor.

SITES Two of the three sites discussed here are at Puketitiri, inland Hawke’s Bay, south-eastern North Island; the third is in North Otago, South Island. The locations of the sites are shown in Holdaway & Beavan (1999, fig. 1, 2). Hukanui sites The disposition of the Hukanui #7 series and Hukanui Slab sites are shown in Fig. 1 and details of the individual sites are shown in Fig. 2–4. The processes of formation of the Hukanui sites are described in Holdaway & Beavan (1999). Briefly, they are formed under blocks of Pliocene limestone that have broken from a layer capping the ridge above and slid down the face on rubble consisting of shattered mudstone and limestone clasts. The structure of each shelter depends on the size and orientation of the blocks and the slope of the terrain on which they came to rest.


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Fig. 3 Detail of Hukanui Slab site, showing location of sections shown in Fig. 4, all viewed from the west. Abbreviations as in Fig. 1. Short broken lines within slab show where slab cut away in two stages, from western end. AMS 14C samples: solid circle, plan position of North Island piopio Turnagra tanagra humerus (HS/B1.30); hatched square, location of moa eggshell sample. Both samples from rust-red sediment (labelled “infill” in Fig. 4) just beneath Taupo Ignimbrite.

The slope is blanketed by two volcanic tephras originating in the Taupo Volcanic Zone about 90 km to the north-west. Almost all of the shelters contain a layer of Taupo Ignimbrite, which is Subunit Y7 of Unit 7, Taupo Tephra according to Wilson (1993) or Tpi using the nomenclature of Froggatt & Lowe (1990). It is the most recent of the large eruptions from the Taupo volcano, with a conventional 14C age of 1850 ± 10 yr BP (Froggatt & Lowe 1990; BP, before AD 1950). The older Waimihia Tephra (Unit S or Wm1) (3280 ± 20 14C yr BP, Froggatt & Lowe 1990) reached into the entrances of many shelters. The Waimihia Tephra is absent from the interior of the deeper shelters, as it was air fall and not emplaced by a horizontal flow like the ignimbrite. Details of the identification and ages of the tephras are given in Holdaway & Beavan (1999). Inland Hawke’s Bay is subject to intense, localised rainstorms associated with tropical cyclones passing down the east coast of the North Island. Individual catchments may be affected differently, and the return time for a significant storm in any one catchment can be decades or longer. Large storms can cause significant erosion and damage to areas of forest (Shaw 1983; Page et al. 1994). Soft sediments and developing soils on tephra layers are subject to slippage under heavy rain, and evidence for soil mass movement is widespread on the hillslope on which the Hukanui sites reside. The large blocks forming the sites tend to deflect large-scale movement from higher on the slope, but small-scale slumping or slippage of sediment between levels of the sites is possible. Hukanui Slab The deposits (Fig. 1–4) are situated beneath a large overhanging slab of limestone that came to rest at about 30° to horizontal (top outwards) on an old landslip on the southern face of Hukanui ridge (Holdaway & Beavan 1999, fig. 2) (NZMS260 V20/149111). The site is open to the south; the small flat area beneath it slopes away gradually and then more steeply to a

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Fig. 4 A–C, Sections of Hukanui Slab. Taupo, Taupo Ignimbrite; Waimihia, Waimihia Tephra; clasts, limestone blocks. A, section A–D in Fig. 3, showing location of 14C AMS samples (NZA6190, North Island piopio Turnagra tanagra bone; NZA6627, piece of moa eggshell, species unknown) relative to Taupo Ignimbrite as projected on plane of A–D, both collected from “infill” sediment. “Infill” is rust-red sediment rich in pumice lapilli. Luminescence samples (HS1, HS2) correspond to OSL positions shown in section on Fig. 3. Note that Waimihia Tephra is not present towards top of preWaimihia slope in front of clasts, nor within site, but that “infill” sediment extends as a thin layer within the site (left-hand of section). Dotted line within Taupo Ignimbrite indicates boundary between the lower, ground wave layer, rich in pumice blocks and lapilli, and the upper, “ash cloud” layer; their integrity indicates that the Taupo Ignimbrite has not been reworked; B, section at E–F in Fig. 3, showing inner edge of Waimihia Tephra, thickness of Taupo Ignimbrite, and “infill” of pumice-rich sediment; C, section X–X, location of NZA6190 sample 30 mm below Taupo Ignimbrite, in rust-red “infill” sediment; D, sections at entrance of Hukanui #7b; right-hand section from 1959 field notes by J. C. Yaldwyn (pers. comm. and 2002) at entrance; left-hand section, section from which luminescence samples (H#7b-1, H#7b-2) taken, October 1996, from a few centimetres outside entrance, beyond traces of the 1959 excavations. Location of NZA6636 and NZA6357 samples within site not known, except that collection noted as being from beneath Taupo Ignimbrite; E, section at 600 mm from eastern edge of Square 13 in Hukanui #7a (see Fig. 1), showing location of 14C samples NZA9050 (New Zealand pigeon Hemiphaga novaeseelandiae) and NZA9070 (kakapo Strigops habroptilus). NZA8559 and NZA10198 (Finsch’s duck, Chenonetta finschi) samples were collected from within Taupo Ignimbrite in Square 10.


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Fig. 5 Plan and section (at X–X in plan) of Earthquakes #1 site, North Otago (see Worthy 1998, fig. 3–6), adapted from Worthy (1998, fig. 6), together with calibrated AMS 14C ages on bone samples and optical ages of sediment deposition at two depths. Depths in mm below present surface. Dashed lines show approximate location of burrow fill. The horizontal dashed line at 300 mm indicates the plane of the photograph in fig. 5 of Worthy (1998) in the northern section of the excavation. OSL sample EQ125 was collected from undisturbed deposits at 250 mm depth north-west of the burrow, and samples E1/4 and E1/5 were collected at the same depth from another, undisturbed part of the site. AMS 14C samples: Pacific rat Rattus exulans (NZA5920, 5921, 5922); New Zealand pigeon Hemiphaga novaeseelandiae (NZA5923); South Island kokako Callaeas cinerea (NZA5927). All AMS ages are given as calibrated calendar ages BP. OSL samples: EQ1-25, EQ1-50. OSL ages are calendar ages BP.

vertical drop into a small stream. Unlike the enclosed sites of Hukanui Pool (Holdaway & Beavan 1999) and Hukanui #7a and 7b, there has been little accumulation of sediment from the roof of the shelter at Hukanui Slab. Most of the sediment is derived either from the debris of the slip that formed the shelter, or is allochthonous material that has poured into the site from above or as volcanic tephra. After the shelter was formed, little material was added to the sediment pile until the Waimihia Tephra was emplaced. Just within the drip line, there was a slight bank of limestone clasts and clay blocks (Fig. 3, 4) from the mudstone underlying the Te Waka limestone on the ridge. The surface outside the bank slopes away from the shelter. The Waimihia Tephra covered the terrain up to the front of the shelter, but was not deposited on or behind the bank under the overhang (Fig. 4). Towards the western end of the shelter, where the roof cover was less extensive, there was only a lower pile of limestone clasts resting on mudstone debris, and the Waimihia Tephra was deposited more deeply and extended further back in the shelter.

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At some time after the Waimihia Tephra was emplaced, a rusty red sediment, rich in pumice fines from the tephra, filled the space in front of the bank and spread thinly over the bank and the shallow depression towards the back of the shelter (Fig. 3, 4). This “infill” was particularly deep towards the western end of the shelter, and formed the surface at the time of the Taupo eruption. As the ignimbrite from that eruption was emplaced, it disturbed the ground surface in the shelter and fingers of sediment were entrained in the lower part of the ignimbrite layer as it settled. The Taupo Ignimbrite is grey and unweathered beneath the overhang of the shelter roof, but is weathered to an ochre shade with a surficial mottled redbrown and black layer where it has been exposed to processes of weathering and soil formation. Where unweathered, it can be clearly divided into a basal layer, rich in pumice lapilli and blocks, covered by a dense, fine grey (ash cloud) fraction (V. Neall pers. comm.). Much of the floor of the shelter above the Taupo Ignimbrite is covered by a slab fallen from the roof of the shelter (Fig. 3). The slab fell during or shortly after the Taupo eruption, because it lies directly on the surface of the tephra without evidence for further sedimentation between the time of tephra deposition and slab emplacement. The mottled red-brown/black surface layer was deeper beneath the slab, which effectively seals the sediments. Both the autochthonous, greyer sediment between the limestone clasts and the allochthonous red sediment layer are poorly fossiliferous. Fossils present include isolated bones of small birds, small pieces of moa eggshell, and the partial remains of one moa whose pelvis and some articulated lower neck vertebrae were located to the west of, and draped around, a limestone block to the west of the main slab. The block was emplaced before the tephras were deposited, and it is likely to have been part of the original floor of the shelter when the major blocks came to rest after the landslip. Hukanui #7b This is a small cave beneath a limestone slab 5 m north of, and c. 3 m above, Hukanui Slab (NZMS260 V20/149111). Although it is very close to Hukanui Slab, it is not connected to that site. Hukanui #7b was excavated in 1959 by W. H. Hartree, R. J. Scarlett, and J. C. Yaldwyn (Hartree, MS diary; records in the Museum of New Zealand Te Papa Tongarewa and Canterbury Museum). Sediments inside and outside the cave entrance were reported as being separated by a “sill rock” (Hartree MS diary). Hartree noted that “the ground sloped away from a big sill rock… Once over the sill rock we came across Post Taupo moa remains in near perfect condition” (Hartree MS diary, 28 May 1959). From this description and an examination on 31 May 1996 of the stratigraphy at the entrance of the site, the sill rock must have been located beneath the drip line but is no longer present. The 1959 excavation trench was visible in the walls of a test pit that was excavated on 22 October 1996 when the OSL samples were collected. The sill rock would have kept separate the sedimentation outside and inside the cave, in the same manner, and with the same effect, as the pile of clasts under the drip line of Hukanui Slab (Fig. 4A). Most of the material pouring over the lip above the

Fig. 6 Northern section at Earthquakes #1 site showing OSL sample tubes in situ. Only the ages for ➤ the top two samples (250 and 500 mm depths) are reported here. Microlaminations support consistency of optical ages with depth and our interpretation that bioturbation did not affect the deposit away from cliff face (apart from the burrow, which contained visibly coarser sediment and was richly fossiliferous; Worthy (1998, fig. 5)).


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entrance to Hukanui #7b would have accumulated outside the cave or continued on downslope over the lip of the cover rock of Hukanui Slab onto the surface outside that site, and thence down the steep slope below. The deposit outside the entrance of Hukanui #7b probably had much the same history of sedimentation as did that at the front of Hukanui Slab, except that individual sedimentation events, such as rain-induced slumping from the slope immediately above the entrances (Fig. 2B), could have occurred at different times. Deposition in Hukanui #7b differs, however, from that in Hukanui Slab as a result of the sediments being laid down inside a cave whose small entrance and low ceiling means that material eroded from the roof and walls forms most of the sediment pile. Before and after the Taupo eruption, the cave had been used as a nest site by small moa, probably Pachyornis mappini or Anomalopteryx didiformis: “Of interest was a moa nest above the Taupo pumice. Below we found two more moa nests one just above the other & the bones of a small…moa” (Hartree MS diary). These nest scoops are also shown in a section drawn at the time (J. C. Yaldwyn pers. comm.). Radiocarbon dates were obtained on gelatin from two bones reportedly removed from beneath the Taupo Ignimbrite by the excavators in 1959 (handwritten label in Museum of New Zealand Te Papa Tongarewa (MONZ); J. C. Yaldwyn pers. comm. and 2002). One bone was from a Pacific rat Rattus exulans, and the other a New Zealand pigeon Hemiphaga novaeseelandiae collected during the excavations and held in the MONZ collections. OSL samples were taken from two levels beneath the Taupo Ignimbrite (Fig. 4) in the southern face of the 1996 test pit (Fig. 1). Hukanui #7a This is a small cave with two entrances and levels joined at the north-western corner of the lower chamber. The lower entrance is about 15 m from Hukanui #7b, and there is presently no internal connection between the two caves (Fig. 1, 2). The post-Taupo sediments were excavated in May 1959 by Hartree, Scarlett, and Yaldwyn (Hartree MS diary) and R. N. Holdaway and T. H. Worthy excavated the Taupo Ignimbrite and pre-Taupo layers from December 1997 to December 1998. Bones of petrel and pigeon were on the surface of the intact sediment at the time of the initial excavations (Hartree MS diary). In 1997, a thick blanket of Taupo Ignimbrite still covered the entire floor of the lower chamber, except where the end of a large slab of limestone from the landslip protruded through the surface. The sediments beneath the ignimbrite were derived from erosion of the roof slab, and were richly fossiliferous in places, primarily with the remains of kills of the harrier (R. N. Holdaway & T. H. Worthy unpubl. data). Earthquakes #1 The Earthquakes #1 site is in North Otago, south-eastern South Island (NZMS260 I41/ 124901), where there are no Late Holocene tephras for stratigraphic control. The site is described and illustrated by Worthy (1998, fig. 3–6). It is a shallow deposit composed of very dry, mainly autochthonous, laminated sediments beneath a ledge on a low cliff of Oligocene limestone (Worthy 1998, fig. 3, 4). The deposit contains the remains of prey of laughing owls that nested on a ledge on the cliff above. Radiocarbon dates on bone gelatin samples from the site were reported by Worthy (1998), Holdaway (1999), and Holdaway & Beavan (1999). These dates were obtained from bones collected in the north and south excavations and beneath the test excavation (Worthy 1998, fig. 6) (Fig. 5). Samples for optical dating were collected in October 1996, from three levels (Fig. 5, 6) in the north-western corner of the northern excavation (Worthy 1998, fig. 6).


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METHODS Optical dating General principles Optical dating provides an estimate of the time since luminescent minerals, such as quartz and feldspar, were last exposed to sunlight (Huntley et al. 1985; Aitken 1998). Buried mineral grains will accumulate the effects of the ionising radiation flux to which they are exposed. The latter is due to the nuclear decay of the naturally occurring radioactive elements (“radionuclides”) 238U, 235U, and 232Th (and their daughter products) and 40K in the sample and surrounding material, and to cosmic rays. The effective rate of supply to the sample by radiation from these sources is termed the “dose rate”, while the total radiation dose received by the sample since it was last bleached by sunlight is termed the “palaeodose”. Optical ages (in calendar years) are calculated by dividing the palaeodose (in grays, Gy) by the dose rate (in Gy yr–1). Radionuclide concentrations can be measured using techniques such as neutron activation, X-ray fluorescence, inductively coupled plasma mass spectrometry, and alpha and gamma spectrometry. The corresponding dose rate is calculated using conversion factors (e.g., Olley et al. 1996; Adamiec & Aitken 1998) and the contribution from cosmic rays may be estimated from published equations (e.g., Prescott & Hutton 1994). The palaeodose can be measured using the thermoluminescence (TL) or OSL signal, but the latter is particularly advantageous for sediments transported by water or exposed only briefly to sunlight before burial (e.g., Roberts et al. 1998a, 1999; Olley et al. 1999) because the OSL signal is reset much more rapidly than the TL signal (Wintle 1997; Aitken 1998). In this study, we have exploited the light-sensitive OSL signal to minimise any problems from insufficient bleaching of the sediments before burial. Sample collection and preparation Sediment samples were collected using 18-mm-diameter stainless steel tubes inserted horizontally into the cleaned faces of the excavations (see Fig. 2, 3, 5, 6). The filled tubes were removed from the deposit and wrapped in black plastic for transport to the luminescence dating laboratory in Melbourne. At each sample location, four to six tubes were collected and three field gamma spectrometry measurements were made. The material removed during enlargement of the tube holes (to permit in situ spectrometry) was retained for laboratory determination of the field water content and for high-resolution gamma spectrometry analyses of the dried and powdered samples. Sediment samples were prepared for dating in subdued red (>590 nm) light to prevent inadvertent bleaching of the OSL signal. The outer few millimetres of material at both ends of the tubes may have been exposed to sunlight during sample collection, and were discarded. The remaining sediment was treated with hydrogen peroxide, hydrochloric acid, fluorosilicic acid, and fluoroboric acid to isolate the quartz and heavy minerals. The latter were removed by density separation using solutions of sodium polytungstate (2.7 g cm–3). Residual quartz grains of 90–125 mm diameter were isolated by dry sieving and finally etched in 40% hydrofluoric acid for 45 min. Grains were mounted on 10-mm-diameter stainless steel discs using a silicone oil spray as adhesive; each disc (aliquot) held ~ 1 mg (~ 800 grains) of quartz. The absence of significant feldspar contamination was checked using infrared stimulation (Aitken 1998).

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OSL measurement and palaeodose determination Luminescence measurements were made using an automated Risø TL/OSL reader and software version 4.65. Samples were optically stimulated by c. 25 mW cm–2 of green-plusblue (420–550 nm) light from a tungsten-halogen lamp fitted with a GG-420 filter and an interference filter. The OSL was detected by a Thorn-EMI 9235QA photomultiplier tube fitted with an HA-3 filter and two 3 mm U-340 filters (Bøtter-Jensen 1997). Laboratory irradiations were made using a calibrated 90Sr/90Y beta source (c. 0.024 Gy s–1) mounted on the reader. Palaeodose determinations were made using a regenerative-dose single-aliquot protocol and statistical models described in detail elsewhere (Roberts et al. 1998a; Galbraith et al. 1999). This protocol permits estimation of the palaeodose from a single aliquot of sample, which may consist of an individual grain (e.g., Murray & Roberts 1997; Roberts et al. 1998a,b, 1999, 2000; Olley et al. 1999) or several tens, hundreds, or thousands of grains. Quartz grains extracted from the samples discussed here were mostly too weakly luminescent to permit single-grain analyses. We show below that individual aliquots, each composed of c. 800 grains, produce palaeodose estimates of vastly differing precisions. A wide range of precisions is more characteristic of single-grain data than multiple-grain data. For example, multiple-grain aliquots of Australian quartz commonly give palaeodoses of uniform and high precision (e.g., Roberts et al. 1998a,b, 1999; Turney et al. 2001), because many of the grains on each aliquot emit luminescence. In this study, we interpret the wide range of precisions as evidence that very few grains in each aliquot are responsible for the OSL signal. We have therefore used experimental procedures and statistical models derived from those developed for single-grain investigations (Galbraith et al. 1999). Palaeodoses were obtained from the OSL signals arising from the natural (burial) dose SN, a subsequent test dose TN (3–4 Gy), a regenerative dose SR (3–7 Gy, chosen to approximately match the natural dose), and a second test dose TR (the same size as TN). The test dose signals are used to monitor for any changes in OSL sensitivity between the SN and SR cycles, and the palaeodose is calculated as (SN /SR) ¥ (TR /TN) ¥ regenerative dose. This formula is appropriate when the natural and regenerative doses lie in the initial linear region of the dose-response curve (0–10 Gy), as is typically the case for quartz (e.g., Roberts et al. 1999, fig. 14b; Murray & Wintle 2000, fig. 2b). We verified that linearity in the 0–10 Gy region is a good approximation for the quartz grains examined in this study, by using regenerative doses of 0, 2, 5, and 10 Gy to construct dose-response curves for a subset of aliquots of each sample. Optical stimulations were made for 100 s at 125°C and OSL emissions were integrated over the first 5 s of illumination, using the final 20 s to define the background count rate. Each aliquot was held (preheated) at a temperature of between 160 and 300°C for 10 s duration before measurement of the SN and SR signals, and each aliquot was heated to 160°C (but not held at this temperature) before measurement of the TR and TN signals. A “plateau test” (Wintle 1997; Aitken 1998) was made on each sample to check that preheat temperatures of 160–300°C yielded concordant palaeodoses, as reported for other samples using variants of this protocol (e.g., Murray & Roberts 1998; Roberts et al. 1998a,b, 1999; Murray & Mejdahl 1999; Murray & Wintle 2000; Turney et al. 2001). Each aliquot also received an additional two cycles of regenerative and test doses (using the same sizes of applied dose and the same heat treatments as before) to verify that the correct dose is calculated for aliquots that have received a known dose. For the second regenerative cycle, the dose is calculated from the formula (SR /SR2) ¥ (TR2 /TR) ¥ regenerative dose, where SR2 and TR2 are the OSL signals arising from the second regenerative dose and


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subsequent test dose, respectively. The corresponding OSL signals measured during the third regenerative cycle are SR3 and TR3. The data collected during the three regenerative cycles also provide a check on a critical assumption of the regenerative-dose protocol used here: that if thermal transfer effects (see Aitken 1998) are insignificant from traps deeper than 160°C then the test dose and regenerative dose OSL signals should vary by the same proportion, and a plot of the three pairs of data— i.e., (SR, TR), (SR2, TR2), and (SR3, TR3)—should be consistent with a line passing through, or close to, the origin (Galbraith et al. 1999; Roberts et al. 1999; Murray & Wintle 2000). Dose rate determination The effective dose rate to acid-etched quartz grains of 90–125 mm diameter consists of gamma, beta, internal alpha, and cosmic-ray components. Gamma dose rates were determined from the field gamma spectrometry measurements, after using the high-resolution gamma spectrometry analyses to investigate the equilibrium conditions of the 238U and 232Th decay chains. Beta dose rates were determined from the high-resolution gamma spectrometry analyses, using the dose rate conversion factors reported by Olley et al. (1996) and a dose-attenuation factor of 0.93 ± 0.03 (Mejdahl 1979). The gamma and beta dose rates were calculated for an assumed long-term mean water content (mass of water to mass of dry sample) of 32 ± 4% at Hukanui Slab (field water contents of 28–37%), 36 ± 4% at Hukanui #7b (field water contents of 32–40%), and 1.5 ± 0.5% at Earthquakes #1 (field water contents of 0.8–1.8%). The dose rates decrease, and the optical ages increase, by 0.7–0.8% for each 1% increase in water content. An internal alpha dose rate of 0.03 mGy yr–1 (typical for Australian quartz) was assumed for all samples; this constitutes