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Optical dating of paleosols bracketing the widespread Rotoehu tephra, North Island, New Zealand
Quaternary Science Reviews 19 (2000) 1649}1662
Optical dating of paleosols bracketing the widespread Rotoehu tephra, North Island, New Zealand Olav B. Lian *, Philip A. Shane School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand
Abstract Optical dating of the "ne-grained mineral fraction of paleosols developed on, or within, loess bracketing the widespread Rotoehu tephra at two sites in central North Island, New Zealand, was undertaken using infrared excitation of potassium feldspars. Optical ages suggest that Rotoehu tephra was deposited at ca 45 ka, and support several previous age determinations. Our optical ages are also consistent with individual K-Ar ages from bracketing lava #ows, though inconsistent with the currently preferred age for the tephra (64$4 ka) which is the weighted mean of them. Our data also suggests that the age of two other important time-stratigraphic markers, Tihoi and Tahuna tephras, estimated to have been deposited at 46 and 43 ka, respectively, may have instead been formed between 25 and 35 ka. In the light of this, the chronology of other North Island tephra beds older than 30 ka may have to be revised. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction New Zealand has a detailed record of Southern Hemisphere Late Quaternary climate change. The nature of climate change is recorded in loess, paleosols, alluvium, and lacustrine sediments (e.g., Pillans, 1991) that are well endowed with rhyolitic tephra beds that have been erupted from the Taupo Volcanic Zone (TVZ) in central North Island (Fig. 1). These tephra beds have been used extensively to constrain the age of lithostratigrahic units and to correlate sequences between basins (e.g., Froggatt and Lowe, 1990; Pillans et al., 1993). The tephrostratigraphic record for the last ca. 50 ka is well established, and the ages of most tephra beds younger than about 30 ka are constrained by radiocarbon ages on carbonised organic matter in proximal deposits, or on bounding organic-rich material such as peat. One of the largest eruptions during the Late Pleistocene originated from the Horohoro vent of the Okataina centre before &40 ka and produced the Rotoehu tephra and associated Rotoiti ignimbrite. Rotoehu tephra, "rst de"ned by Vucetich and Puller (1969), is an important, and widespread, time-
* Corresponding author. Present address: Department of Physics, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6. E-mail address: [email protected] (O.B. Lian).
stratigraphic marker, and is easily identi"ed throughout central North Island on the basis of glass chemistry and mineralogy; it contains the characteristic mineral cummingtonite which is rare in TVZ rhyolites. Cummingtonite has been found in abundance in only two other regional fall deposits, also from Okataina, both of which erupted in the Holocene. Rotoehu tephra is also important because it is the lowermost rhyolitic unit in the time interval during which other named tephra beds were deposited, stratigraphic order is established, and their source volcano is known (e.g., Froggatt and Lowe, 1990). The chronology of tephra beds, such as the Rotoehu, that erupted between ca. 30 and 200 ka remains problematic because not only are they of an age near, or beyond the age-limit of standard radiocarbon dating methods, but they are also too young to be dated using the routine "ssion-track methods on glass or zircon. Moreover, calc-alkaline tephra beds, such as those erupted from TVZ generally lack high-K phases that may be amenable to Ar}Ar dating. In this paper we report on optical dating of paleosols developed on or in loess that bracket Rotoehu tephra at two sites near Taupo. At one of the sites we also sampled loessic paleosol overlying Tihoi and Tahuna tephras, and underlying Kawakawa tephra. Loessic paleosols were selected for this study because loess is expected to have experienced extended exposure
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to direct sunlight prior to burial. Furthermore, it has been observed that in many cases mineral sediments extracted from paleosols have given reasonable luminescence ages, whereas the parent material has not. It is thought this is because of bioturbation, such as that which occurs during the soil-forming process, can be responsible for further sunlight exposure (e.g., Divigalpitiya, 1982; Huntley et al., 1983; Wintle and Catt, 1985; Forman et al., 1988; Berger and Mahaney, 1990). In this study, bioturbation would be expected to have ended soon after deposition of the overlying tephra beds, and thus our optical ages would probably more closely date the deposition of the tephra than the deposition of the loess from which the paleosol formed.
2. Previous attempts at dating Rotoehu tephra Because Rotoehu tephra was erupted near the older age limit of radiocarbon dating, it has proved di$cult to obtain an accurate and precise age (Froggatt and Lowe, 1990; Lowe and Hogg, 1995). Various dating techniques have been applied. Radiocarbon dating has yielded several ages younger than 40 ka, but there are also ages older than 40 ka. Whitehead and Ditchburn (1994) recalculated, and averaged, four previously reported radiocarbon ages and deduced from these data that the Rotoehu tephra formed at 35$3 ka. Lowe and Hogg (1995), however, point out that the mean radiocarbon age reported by Whitehead and Ditchburn should be disregarded because there is evidence that the samples dated were contaminated with younger carbon. They also add that Whitehead and Ditchburn overlooked several other radiocarbon ages that are older than 40 ka. Of these, two are `"nitea: one is from peat and yielded an age of 44,000$5300 yr BP (NZ-877), while the other, from wood found in paleosol beneath the tephra, gave an age of 41,700$3500 yr BP (NZ-1126) (Nairn and Kohn, 1973). Although this wood was rated as `optimala for radiocarbon dating by Froggatt and Lowe (1990, p. 107), its apparent age is near the limit of standard radiocarbon techniques and should therefore be considered with caution. Froggatt and Lowe report that the age for NZ-877 is almost certainly much greater than ca. 44 ka. Buhay et al. (1992) produced electron-spin resonance (ESR) ages from quartz extracted from Rotoiti ignimbrite. Their average age (45$8 ka) was based on those from Al centres (average, 49$8 ka) and Ti centres (average, 41$6 ka). Ota et al. (1989) used Th}U disequilibrium dating to produce an age of 71$6 ka for Rotoehu tephra. However, it has since been argued that their age is based on insu$cient data (e.g., Froggatt and Lowe, 1990; Lowe and Hogg, 1995) and that the true age is
`probably much youngera (Whitehead and Ditchburn, 1994, p. 382). Dating using the degree of racemization of aspartic acid extracted from peptides within tephra, loess and paleosols from a sedimentary sequence near Rotorua produced an age estimate of ca. 61 ka (no analytical uncertainty reported) for Rotoehu tephra (Kimber et al., 1994). However, it should be noted that their age for the Mamaku ignimbrite, which occurs at the base of their sequence, was ca. 140 ka, whereas it has since been shown that its age is 230$10 ka (Shane et al., 1996). At present, the most accepted age is probably the whole-rock K}Ar age of 64$4 ka from bracketing lava #ows from Mayor Island volcano (Fig. 1) (Wilson et al., 1992). However, we argue that it too should be considered tentative for the following reasons: (i) 64 ka is near the common lower limit of the K}Ar technique; (ii) K}Ar whole-rock ages are subject to potential xenolith and xenocryst contamination; and (iii) potential excess Ar, and K and Ar loss, are di$cult to assess. Furthermore, Wilson et al.'s age was derived by taking the weighted mean of ages obtained for two di!erent lava #ows * one overlies the tephra, while the other is beneath. They have therefore averaged the ages of two separate eruptive events, and their analytical uncertainty may be too small, as was pointed out by Lowe and Hogg (1995, p. 400). We think it more prudent to consider Wilson et al.'s ages separately: the whole-rock K}Ar age from the overlying lava (from their locality 1) is 67$11 ka, and those for the underlying lava are 63$5 (locality 2) and 63$9 (locality 3) (Wilson et al., 1992, p. 327). The ages from localities 1 and 2 are each the average of two age determinations on the same sample. Less-quantitative estimates for the age of Rotoehu tephra include 52$7 ka based on the tephra's stratigraphic position on raised marine terraces that have been correlated with previously dated sea-level positions (Berryman, 1992), and an age of &55 ka has been estimated from sedimentation rates in a deep-sea core collected o! the east coast of North Island (Pillans and Wright, 1992). These ages are supported by paleoecological data that indicate that the climatic interval that began just before deposition of Rotoehu tephra was characterised by cool interstadial conditions; this interval lasted until about the time of the emplacement of Omataroa tephra (ca. 28 ka), when the climate began to deteriorate (McGlone et al., 1984). This is consistent with Rotoehu tephra being deposited soon after the start of Oxygen Isotope Stage (OIS) 3, which commenced at about 60 ka (Martinson et al., 1987). Taking all of these studies into consideration, it is safe to conclude that Rotoehu tephra is older than &35 ka (based on the reported radiocarbon ages) but younger than &75 ka (based on the K}Ar age, at #2p, for the underlying lava #ow on Mayor Island).
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Fig. 1. Location of the study sites and features referred to in the text.
3. Lithostratigraphy and tephra identi5cation 3.1. Stratigraphy Sequences at Scott and Waihora roads were selected for this study (Fig. 1). Lithostratigraphic logs, sample locations, and textural characteristics are given in Fig. 2. The stratigraphy at Waihora Road was "rst established by Vucetich and Howorth (1976), and was designated the type section for Tihoi tephra. The measured section (Fig. 2) is about 3.5 m thick, rests on undi!erentiated ignimbrite, and consists of four loess units separated by four tephra beds. The loess units generally grade upward to paleosol. The lowermost tephra occurs &50 cm above the ignimbrite, and was identi"ed by Vucetich and Howorth (1976), on the basis of lithology, thickness, and mineralogy, as Rotoehu. Tihoi and Tahuna tephras were
identi"ed about 60 and 80 cm higher, respectively; Okaia tephra was found at the top of the sequence. The section extends another &2 m to the surface. The Scott Road section (Figs. 1 and 2) is exposed in a road cut. The stratigraphy in this locality was "rst documented by Pullar and Birrell (1973). The measured section is about 4.5 m thick and contains "ve tephra beds separated by "ve loess}paleosol units (Figs. 2 and 3). The sequence rests on welded Ongatiti ignimbrite that has been dated to ca. 1.2 Ma (Houghton et al., 1995). It is capped with Taupo ignimbrite that was deposited at ca. 1.8 ka (Froggatt and Lowe, 1990). The stratigraphy at Scott Road can be readily correlated with that at Waihora Road, about 23 km away (Fig. 1), but as an added precaution we have chosen to chemically identify some of the tephra beds; we have given these tephras laboratory designations 585, 587, and 588 (Fig. 2).
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Fig. 2. Lithostratigraphic logs of the Scott and Waihora road sections showing location of optical dating samples SRL1}4, and WRL1}2 and their apparent optical ages. At Scott Road, sample SRL1 (42% sand, 46% silt, 12% clay) was collected 10 cm below the base of the Rotoehu tephra bed, while sample SRL2 (15% sand, 58% silt, 26% clay) was collected in the overlying unit, 6 cm above the tephra. Sample SRL3 (49% sand, 43% silt, 68% clay) was taken 15 cm above the Tahuna tephra, and 11 cm below the Okaia tephra. Sample SRL4 (17% sand, 69% silt, 14% clay) was collected 24 cm below the Kawakawa tephra. Sample WRL1 (32% sand, 36% silt, 32% clay) was collected 9 cm below Rotoehu tephra, while sample WRL2 (12% sand, 41% silt, 48% clay) was collected 15 cm above the tephra. The calibrated age estimate for Kawakawa tephra is based on a radiocarbon age of 22.6 ka; see the text for discussion.
3.2. Tephra identixcation The lowermost tephra at Waihora Road has been previously identi"ed as Rotoehu by Vucetich and Howorth (1976). We have compared its glass chemistry and mineralogy with the lowermost tephra at Scott Road (our tephra 585). Both are calc-alkaline rhyolites, typical of those found in TVZ, and contain the diagnostic mineral cummingtonite. Both are characterised by low variance for most oxides (Table 1) when shared compositions are averaged (e.g., SiO $(0.21 wt.%,
FeO$(0.1 wt.%), suggesting that they are the product of a single eruptive event (e.g., Shane et al., 1996), and both have glass compositions that are the same as those reported by other workers for Rotoehu tephra (e.g., Pillans and Wright, 1992). Tephra beds 587 and 588 at Scott Road compositionally match the Tahuna and Okaia beds, respectively (Fig. 4, Table 1), and this is consistent with the stratigraphy found at Waihora Road by Vucetich and Howorth (1976). Based on this stratigraphy, the uppermost tephra unit at Scott Road is interpreted to be Kawakawa. The "ner material found at the
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Fig. 4. Individual glass shard analyses of tephra beds at Scott and Waihora roads sections compared to the "elds of reference sample compositions of known identity.
Table 1 Glass composition of tephra beds (wt%) at Waihora and Scott roads Re
Fig. 3. The loessic paleosol tephra sequence at Scott Road exposed up to the base of the Kawakawa tephra. The arrow points to the Rotoehu tephra bed. The knife (20 cm long) rests on Tahuna tephra, which in turn lies directly on Tihoi tephra. See Fig. 2 for sample locations.
base of this unit is probably fall deposit, while the overlying coarser material may be associated ignimbrite. As an additional criteria for identifying some of the tephra beds, we analysed Fe}Ti oxide spinel } rhombohedral pairs to estimate eruption temperatures and oxygen fugacities (fO ). These parameters are often char acteristic of particular eruptive events, even if the tephra beds display a similar glass composition (Shane, 1998). We analysed oxide pairs that are attached to ferromagnesian phenocrysts to ensure that they are co-magmatic. ¹}fO data were calculated following the procedure of Ghiorso and Sack (1991). Fe}Ti oxide pairs from the Rotoehu correlatives at Scott (tephra 585) and Waihora roads, and the Okaia correlative at Scott Road (tephra 588), produced temperatures and oxygen fugacities characteristic of these tephras (Fig. 5), thus con"rming their identi"cation. Rotoehu tephra is particularly distinctive, having a high fO relative to other tephras erupted from Okataina centre. ¹}fO data for sample 588 from Scott
SiO Al O TiO FeO MnO MgO CaO Na O K O Cl H O n
77.68 12.49 0.21 0.99 0.07 0.10 0.85 4.12 3.27 0.23 7.06 11
585 (0.18) (0.11) (0.03) (0.05) (0.03) (0.06) (0.05) (0.13) (0.16) (0.02) (1.63)
77.86 12.61 0.18 0.99 0.07 0.10 0.87 4.04 3.32 0.22 8.20 10
587 (0.17) (0.09) (0.03) (0.05) (0.05) (0.05) (0.03) (0.11) (0.07) (0.03) (1.72)
77.56 12.57 0.16 1.04 0.07 0.08 0.97 3.56 4.01 0.19 4.02 11
588 (0.18) (0.10) (0.05) (0.09) (0.04) (0.06) (0.05) (0.11) (0.05) (0.02) (1.79)
77.57 12.55 0.18 1.31 0.06 0.08 1.17 3.81 3.29 0.20 7.70 10
(0.21) (0.11) (0.07) (0.08) (0.03) (0.04) (0.06) (0.13) (0.11) (0.02) (1.89)
Note. Re"Rotoehu tephra at Waihora Road; 585, 587, and 588 are tephra beds from Scott Road. See Fig. 2 for sample locations. Compositions presented as mean and standard deviation (in parenthesis) from n shards. Recalculated to 100% on a volatile-free basis. All Fe as FeO. Water by di!erence. Analyses by a JEOL JXA-5A electron microprobe "tted with a Link Systems LZ-5 EDS detector. Absorbed current 0.6 nA at 15 kV. Beam defocused to 15 lm diameter.
Road con"rms its correlation to Okaia tephra, and show that the tephra has not been mis-identi"ed as another widespread Taupo tephra, such as Kawakawa or Tihoi (Fig. 5).
4. Optical dating 4.1. Introduction Luminescence dating (optical and thermoluminescence dating) works on the principle that minerals, such as
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Fig. 5. Temperature and oxygen fugacity estimates for tephra beds in this study compared to the typical ranges for tephra beds of similar age.
quartz and feldspar, contain impurities and structural defects, some of which can act as traps for free electrons. The rate at which such traps are "lled is proportional to the rate at which free electrons are produced, which is in turn proportional to the environmental dose rate due to the decay of radioactive elements within the mineral samples and in the surrounding sediment. For sediments within a few metres of the surface there can also be a signi"cant contribution from cosmic rays. Exposure to sunlight empties electrons from traps. During subsequent burial, electron traps become "lled. In practice, the radiation dose absorbed since the sediment was last exposed to sunlight is estimated, and, together with a measure of the ambient dose rate, the time elapsed since the sediment grains were last exposed to sunlight can be determined. Optical dating, introduced by Huntley et al. (1985), is superior to thermoluminescence (TL) dating because only electrons in light-sensitive traps are measured; for TL dating, electrons in light-sensitive and light-insensitive traps are measured together. Detailed accounts of the method can be found in Berger (1995), Wintle (1997), Aitken (1998), and Huntley and Lian (1999), while recent application-based reviews include those of Prescott and Robertson (1997), Roberts (1997), and Aitken (1998). 4.2. Sample collection and laboratory preparation All of our samples were composed of moist "ne sand, silt, and clay (Fig. 2), which allowed small blocks to be easily carved from the cleaned section face. Sample blocks were subsampled using the procedures outlined in Lian and Huntley (1999). The portions to be used for dating were prepared using standard methods, including immersion in hot (&903C) HNO for 40 min to assure removal of the organic material (Lian et al., 1995). The
4}11 lm diameter fraction was separated from the treated sediment fraction, and at least 70 aliquots were made using the procedures described in Lian et al. (1995). Because it was desired to compare the optical ages obtained from silt-sized potassium feldspars with those from sand-sized quartz, the prepared '11 lm diameter fractions were sieved to obtain grains in the 90}125 lm and 125}180 lm diameter size ranges. The quartz was isolated from the other minerals using a procedure similar to that outlined by Wintle (1997). For each sample, however, only 15}100 mg of quartz could be obtained in the desired size fractions, and this was not su$cient for routine dating. Single grain/aliquot dating (e.g., Galbraith et al., 1999; Roberts et al., 1999) of these quartz fractions will be undertaken in the near future. 4.3. Equivalent dose determination The &3.1 eV (400 nm) emission from potassium feldspars (e.g., Huntley et al., 1991; Krbetschek et al., 1997) was measured using a Ris+ TL-DA-15 Minisys system equipped with light-emitting diodes (TEMPT 484) that together delivered about 20 mW cm\ of &1.4 eV (infrared, 800}960 nm) photons to the sample (B+tter-Jensen, 1997). Luminescence was detected by an EMI 9235QA photomultiplier tube mounted behind a Schott BG-39 and a Kopp 5-58 optical "lter. The BG-39 "lter absorbs scattered 1.4 eV photons, while the 5-58 "lter selectively passes the 3.1 eV emission and blocks the 2.2 eV (570 nm) emission from plagioclase feldspars. To correct for the intrinsic variability in luminescence intensity between aliquots, each was initially given a 20 mJ cm\ exposure to 1.4 eV photons. Normalisation factors were calculated as the luminescence intensity of the individual aliquot divided by the average luminescence intensity of all aliquots in the set. Laboratory irradiations were from a Sr/Y source that delivered &6.7 Gy min\ of b radiation to the sample. The a-e$ciency (b-value) was estimated by exposing some of the aliquots to Am sources that delivered a particles to the sample at &0.15 lm\ min\. Laboratory irradiation populates electron traps that are thermally stable over geologic time, as well as those that are thermally unstable. To assure that only thermally stable traps are sampled during the "nal luminescence measurements, all of the aliquots in a set (except for several control aliquots) were heated (`preheateda) at 1403C for 7 d. Although a less severe preheat could probably have achieved the desired results, the 1403C/7 d protocol has been found to be consistently successful in several other studies (e.g., Huntley et al. 1993a; Ollerhead et al., 1994; Lian et al., 1995; Wolfe et al., 1995). For all of the samples, the additive-dose method, with thermal transfer correction (e.g., Huntley et al., 1993a;
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Huntley and Clague, 1996; Lian and Huntley, 1999) and the regeneration method (e.g., Huntley et al., 1993b; Huntley and Lian, 1999) were both used. Aliquots used to construct the thermal transfer curves, and the regeneration curves, were exposed to a quartz}halogen lamp (i.e., `bleacheda) behind a Schott RG-715 optical "lter that absorbs UV and most visible light (Huntley and Clague, 1996). This bleach was for 4 h and reduced the luminescence intensity of an unirradiated aliquot that had not been preheated to 3}5% of the intensity of a similar unbleached aliquot. After storage for 60 d at room temperature, the luminescence from all the aliquots was measured for 200 s. Dose-response curves were constructed using maximum likelihood "ts: additive-dose and regeneration data were consistently "tted with saturating exponentials, while the thermal-transfer correction data were "tted
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with straight lines. Examples are shown in Figs. 6 and 7. For the regeneration data, when a scaling parameter was included in the "t, in all but one case this parameter was within 1p of unity, which indicated that for most of the samples the laboratory bleach did not signi"cantly alter the luminescence sensitivity of the relevant minerals. For the additive-dose method, dose intercepts were calculated every 20 s, and in all but one case they were found to be constant with excitation time (Fig. 8). For sample SRL1 dose intercepts were found to gradually increase with excitation time, but this e!ect was not observed in the regeneration data. However, when the additive-dose data were "tted with a saturating exponential that included a linear term, or if a saturating exponential was used with the highest dose points omitted, this e!ect disappeared; the former "t was therefore used to determine the D .
Fig. 6. Luminescence decay as measured under infrared (1.4 eV) excitation for some of the samples. The insets show b-response curves for the "rst 20 s of excitation. For sample SRL1 the additive-dose data (solid points) are shown "tted with a saturating exponential#linear function; for all the other samples the "ts are saturating exponentials. In all cases, the thermal transfer data were "tted with straight lines. The D 's were taken as the dose intercept calculated using the luminescence integrated over the entire excitation time after a correction for the decay that resulted from the normalisation procedure. The luminescence intensity from a `naturala and `bleacheda aliquot that have not been preheated are indicated as Nnp and Bnp, respectively.
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Fig. 7. Luminescence decay as measured under infrared (1.4 eV) excitation for sample SRL4. The inset shows b-response curves for both the additive-dose (N#b) and regeneration (N#light#b) data for the "rst 20 s of excitation. The black points are the N#b data shifted along the dose axis for best "t, the shift being equal to the equivalent dose (D ) after correction for normalisation decay and incomplete laborat ory bleaching. The grey points are the N#b data before the shift. For each sample, the D used in the age calculation was that which was determined using the luminescence integrated over the entire excitation time. Nnp and Bnp are described in Fig. 6.
Dose intercepts were subsequently re-calculated using the luminescence recorded over the entire excitation time, and this was taken as the D after a correction for the decay resulting from the normalisation procedure (Table 2). For the regeneration procedure, the D 's were calculated in a similar manner with the scaling parameter "xed at unity, but were also corrected for incomplete laboratory bleaching (Table 2). 4.4. Anomalous fading tests For some feldspars, the luminescence that arises from traps "lled during laboratory irradiation, that are expected to be stable over geologic time, instead depends on the length of time between irradiation and measurement, the luminescence being smaller for longer delay times (e.g., Spooner, 1994; Aitken, 1998). This `anomalous fadinga can result in underestimation of the true optical age. To test if this e!ect was prominent in our samples, several aliquots were given a 24 h laboratory bleach, followed by a b dose equal to their D . All the aliquots were preheated together at 1403C for 7 d. The luminescence, ¸ , was measured for 20 s after 1 d, and " then again for 20 s after either 30 or 75 d of storage at room temperature. As a control, the luminescence, ¸ , of , several aliquots that had received no laboratory bleach or radiation, but were heated, was also measured. The heating and luminescence measurements of both sets were done together, and the latter were made using the
Fig. 8. Dose intercept versus excitation time for all the samples as determined by the additive-dose method with thermal transfer correction. Some of the points have been shifted slightly for clarity.
same apparatus and "lter combinations as for the other experiments. The fading ratio was taken as: (¸ / " ¸ )/(¸ /¸ ) where the subscripts 1 and 2 refer to the " , , "rst and second measurements, respectively. For each sample, this fading ratio was found to be consistent with unity (Table 2). 4.5. Dosimetry The environmental dose rate is proportional to the amount of a, b, and c radiation from the decay of
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Table 2 Equivalent doses (D ), b-values, dose rates, fading ratios, and optical ages D (Gy) Sample
Additive-dose
Regeneration
b-value (Gy lm)
DQ (Gy ka\)
DQ 2 (Gy ka\)
Fading ratio
Optical age (ka)
SRL1 SRL2 SRL3 SRL4 WRL1 WRL2
64$6 49$4 64$23 45$4 96$17 63$12
67$3 41$2 39$14 40$2 ss 67$5
0.93$0.03 0.76$0.05 1.19$0.19 1.02$0.05 1.06$0.08 0.73$0.09
0.10 0.11 0.14 0.15 0.09 0.09
1.50$0.05 1.52$0.07 1.87$0.14 1.93$0.07 2.28$0.13 1.97$0.12
0.999$0.016 1.001$0.052 1.022$0.033 1.003$0.053 0.986$0.031 1.014$0.031
44$3 30$2 21$8 22$2 42$8 34$3
b-value as de"ned by Huntley et al. (1988). DQ : dose rate due to cosmic rays (Prescott and Hutton, 1994). DQ : total dose rate (that due to cosmic rays plus that due to c, b, and a radiation). 2 Storage was for 30 d for samples SRL1, 2, and WRL1, and 75 d for the others. See the text for explanation. ss: sensitivity change detected in the regeneration data.
U, U, Th, their daughter products, and K in the sample and surrounding sediment matrix. Cosmicray radiation also contributes to the dose rate, and its e!ect is a function of the sample's depth beneath the surface. For this work, a subsample of the bulk sediment used for dating was dried and milled to a "ne powder, and was analysed for K by inductively coupled plasma atomic emission spectroscopy (ICP-AES), while U and Th were analysed by several methods that sample di!erent parts of the respective decay chains. Delayed neutron counting (DNC) and neutron activation analysis (NAA) give the concentrations of the parents, respectively (Table 3). Thick-source a counting (TSAC) gives U and Th equivalent concentrations (Table 4) by counting all the a particles in each chain (Huntley et al., 1986). Since a particles are attenuated by organic
matter, the samples were ashed (5003C for 24 h) prior to counting. In situ c ray spectroscopy (IGRS) gives the K content directly and allows equivalent concentrations of U and Th to be calculated from the measurement of c-rays from late daughters in each chain (Bi for U, and Tl for Th), from sediment up to about 50 cm away (Table 3). Together the various analyses can be used to determine the extent of radioactive disequilibrium in the U and Th decay chains, and to account for local inhomogeneity in the sediment matrix. 4.6. Results and evaluation of the dose rate For each sample, the U and Th concentrations as determined from DNC and NAA are consistent with those found from TSAC when the analytical
Table 3 K, U, and Th concentrations determined from laboratory analyses and in situ c-ray spectroscopy Sample
SRL1 SRL2 SRL3 SRL4 WRL1 WRL2
K (%)
0.79$0.04 1.00$0.05 0.80$0.04 0.96$0.05 0.97$0.05 0.98$0.05
Th (lg g\)
10.2$0.3 12.4$0.3 12.0$0.4 11.7$0.3 8.9$0.2 10.2$0.3
U (lg g\)
2.96$0.08 2.95$0.09 3.53$0.09 3.24$0.09 2.60$0.08 3.18$0.09
From in situ c-ray spectroscopy K (%)
Th (lg g\)
U (lg g\)
Apparent c dose rate (Gy ka\)
0.24$0.02 0.48$0.03 0.57$0.03 0.63$0.03 0.99$0.03 0.71$0.03
3.85$0.18 4.99$0.24 6.32$0.27 6.09$0.28 7.51$0.32 6.83$0.29
1.70$0.13 1.81$0.16 1.60$0.18 1.67$0.18 1.77$0.22 1.69$0.19
0.464$0.005 0.571$0.006 0.645$0.006 0.661$0.006 0.830$0.006 0.730$0.006
From ICP-AES. From NAA. From DNC. Equivalent concentrations calculated from the activities of Bi for U and Tl for Th, assuming secular equilibrium. Note: The values shown have not been corrected for the presence of water, and are thus smaller than those that would have been calculated if the sediments were dry. From counts in the energy range 0.8}2.6 MeV, corrected for relative proportions of K, U, and Th.
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Table 4 Thick-source a count (TSAC) rates, equivalent U and Th concentrations, organic and water contents Sample
SRL1 SRL2 SRL3 SRL4 WRL1 WRL2
TSAC
Water content
Total count rate (cm\ ks\)
Th count rate (cm\ ks\)
Th (lg g\)
U (lg g\) *
*
*
*
0.809$0.011 0.817$0.011 0.949$0.009 0.839$0.010 0.628$0.007 0.710$0.007
0.445$0.042 0.491$0.044 0.429$0.035 0.466$0.040 0.335$0.027 0.373$0.029
12.0$1.1 13.2$1.2 11.5$0.9 12.5$1.1 9.0$0.7 10.0$0.8
2.8$0.3 2.6$0.3 4.1$0.3 2.9$0.3 2.3$0.2 2.6$0.2
1.30 1.34 1.27 1.03 0.44 0.67
1.41 1.56 1.52 1.15 0.65 0.88
1.30$0.03 1.34$0.05 1.27$0.06 1.03$0.03 0.44$0.05 0.67$0.05
0.18 0.14 0.18 0.18 0.09 0.14
See Huntley et al. (1986). Note: samples used for TSAC were ashed (5003C for 24 h) before counting. Count rates have been corrected for sample re#ectivity using the model of Huntley and Wintle (1978). * and * , are the as-collected and saturated water contents, respectively; * is the water content used for the dose-rate calculation. * is the organic content. Organic and water contents are defnined as (mass organics)/(mass minerals) and (mass water)/(mass minerals), respectively. Organic content was found by ashing dry subsamples at 5003C for 24 h.
uncertainties are taken into account at 2p (most agree at 1p). This indicates the absence of signi"cant radioactive disequilibrium at the time of sample collection. This result was unexpected since disequilibrium has been found to be common in moist organic-rich environments, such as those studied by Lian et al. (1995) and Huntley and Clague (1996). For each of the samples in this study, the c dose rate as found from IGRS (Table 3) was consistently higher (by 4}22%) than that calculated from laboratory analysis (correction for sample moisture included). These di!erences have resulted from inhomogeneity and/or radioactive disequilibrium, but they are not signi"cant when the analytical uncertainties are taken into account. Because of the substantial organic content of these samples, dose rates due to a, b and c radiation (Table 2) were calculated using the modi"ed formulae introduced by Divigalpitiya (1982), which are those reported by Berger and Mahaney (1990) and Lian et al. (1995), but here we used the revised dose-rate conversion factors of Adamiec and Aitken (1998). In each case, the dose rates due to a and b radiation were taken as those calculated using the average value of Th found by NAA and TSAC, and the average value of U from DNC and TSAC, with uncertainties to cover any reasonable possibility. For the c dose rate, the average of that found from IGRS and that determined from laboratory analysis was used for the age calculation (Lian et al., 1995). The dose rate due to cosmic rays (Table 2) was estimated using present burial depths of the samples and the relationship of Prescott and Hutton (1994). Because all of our samples are from well-drained areas, the water content used for the doserate calculations (Table 2) was taken as the as-collected value, with an uncertainty that should account for wet and dry periods at $2p.
5. Optical ages For most of the samples, the D 's from the additive dose and regeneration methods are consistent. For these samples the D 's from both methods were averaged, and the optical ages were calculated by dividing the average D by the total dose rate (Table 2). For sample WRL1, a sensitivity change was detected in the regeneration data, so the D from the additive-dose method was used to calculate the optical age. For samples SRL3 and WRL2 the data were highly scattered, leading to a relatively large uncertainty in the D determined using both methods; for these samples only the D obtained from the regeneration method was considered in the age calculation. The optical ages date the time when the relevant mineral sediments were last exposed to sunlight. In this case, this event occurred sometime between "nal burial of the loess and the cessation of bioturbation, and the latter probably marks the time of deposition of the overlying tephra. The optical age for sample WRL1 is 42$8 ka, and that for sample WRL2 is 34$3 ka (Fig. 2). Sample SRL1 gave an optical age of 44$3 ka, while samples SRL2 and SRL3 yielded optical ages of 30$2 and 21$8 ka, respectively. The optical age for sample SRL4 is 22$2 ka. All of our optical ages are well within the currently accepted age range (ca. 0}150 ka) for the method (see e.g., Duller, 1994; Lian et al., 1995).
6. Discussion The most reliably dated unit in the sequences studied is the Kawakawa tephra. There are many radiocarbon ages
O.B. Lian, P.A. Shane / Quaternary Science Reviews 19 (2000) 1649}1662
associated with this tephra, and most of them have been assessed by Froggatt and Lowe (1990). The ages that they ranked as `optimala and `usefula (Froggatt and Lowe, 1990, p. 106) range between ca. 19.2 and 22.7 ka BP. The most reliable of these are four conventional ages, each from a small piece of charcoal, collected from four di!erent closely spaced locations within associated Oruanui ignimbrite. These range from 22,720> yr BP (Q-2668) \ to 22,470> yr BP (Q-2666) and give a mean age of \ 22,590$230 yr BP (Wilson et al., 1988; Froggatt and Lowe, 1990). More recently, ages of 20,700$150 yr BP (Wk2048) and 22,800$290 yr BP (Wk1826), from carbonate-rich mud sampled from a deep-sea core, have been obtained above and below the tephra, respectively (Pillans and Wright, 1992). Also, Pillans et al. (1993) reported an age of 22,700$230 yr BP (Wk1516) for charcoal found beneath Kawakawa tephra, and ages of 21,100$300 yr BP (NZ-7707) and 21,500$300 yr BP (NZ-6702) from charcoal directly above the tephra. These ages are in general agreement with those of Wilson et al. (1988), and thus the presently accepted radiocarbon age for Kawakawa tephra is 22.6 ka (e.g., Froggatt and Lowe, 1990; Pillans et al., 1993). The data of Bard et al. (1993) suggest that the calibrated (i.e., calendar) age for Kawakawa tephra is consistent with 24.5$1.0 ka. There have also been several luminescence ages on aeolian sediments bracketing Kawakawa tephra (see Shepherd and Price, 1990; Berger et al., 1994; Duller, 1994). These ages are based on both optical and TL methods and on various laboratory protocols. For these reasons the ages are di$cult to assess as a group. However, all but one of those reported for samples directly below (within 25 cm) of Kawakawa tephra are within 2p of 21$2 ka. Although this is broadly consistent with the calibrated radiocarbon age, there is indication that some luminescence ages are slightly too young. To attempt to explain this discrepancy, while accepting that the calibrated radiocarbon age is accurate over this time range, workers have suggested geological processes such as the downward translocation of younger grains (e.g., Berger et al., 1994, p. 324), or that long-term thermal fading has a!ected the luminescence signal (Duller, 1994). But the latter e!ect is probably insigni"cant on this time scale (Mejdahl, 1989; Duller, 1994). Nevertheless, the optical age of 22$2 (SRL4) for Kawakawa tephra at Scott Road is consistent with both the calibrated radiocarbon age and with the majority of reported luminescence ages, and this lends support to our laboratory procedures. The optical age for sample SRL1 suggests that Rotoehu tephra at Scott Road was deposited at, or slightly after, 44$3 ka. This is supported by the optical age for sample WRL1 (42$8 ka) from beneath the tephra at Waihora Road. Thus our age data for Rotoehu tephra are in agreement with the radiocarbon age (41.7$3.5 ka) reported by Nairn and Kohn (1973), the
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ESR age (45$8 ka) of Buhay et al. (1992), and the stratigraphy-based age estimate (52$7 ka) of Berryman (1992). Our age data are also consistent with Wilson et al.'s (1992) whole-rock K}Ar ages (63$5 and 63$9 ka) from lava underlying Rotoehu tephra, and our ages are consistent with Wilson et al.'s K}Ar age for the lava #ow overlying Rotoehu tephra (67$11 ka) if their $1p analytical uncertainty is taken into account at 2p. Our new ages are not consistent with Wilson et al.'s weightedmean K}Ar age for the tephra (64$4 ka), and they are also signi"cantly di!erent from Ota et al.'s (1989) Th}U age of 71$6 ka. Sample SRL3, from beneath Kawakawa and Okaia tephras, gave an optical age of 21$8 ka. The large analytical uncertainty in this age is mainly due to the relatively high degree of scatter in the dose}response data. This may have resulted from a slight darkening of the aliquots during the preheat treatment. The reason for such a colour change is presently unknown, but it has been observed before with similar malign results (e.g., Lian and Huntley, 1999). Sample SRL3 directly underlies Okaia tephra, and directly overlies Tahuna tephra. Okaia and Tahuna tephras have not been directly dated, but based on the information available in the literature (e.g., stratigraphic relationships, degree of paleosol development, and limiting radiocarbon ages), Froggatt and Lowe's (1990) best estimate for the age of Okaia and Tahuna tephras were ca. 23.5 and 43 ka, respectively. These age assignments are consistent with the optical age for sample SRL3. However, samples SRL2 and WRL2, both of which underlie Tahuna tephra, gave optical ages of 30$2 and 34$3 ka, respectively. This suggests that Tahuna tephra may be younger than ca. 35 ka, if our analytical uncertainties are taken into account at 2p. This conclusion also holds for Tihoi tephra, which previously has been estimated to be ca. 46 ka (e.g., Froggatt and Lowe, 1990). Hauparu tephra overlies Tahuna tephra at proximal sites around Okataina Centre, and the former has two reported radiocarbon ages associated with it: 39,000$5600 yr BP (NZ-3404) and 35,700$1300 yr BP (NZ-3405) (McGlone et al. 1984; Froggatt and Lowe, 1990), both of which are near the limit of conventional radiocarbon dating. Moreover, both these ages were determined from organic-rich mud that is prone to contamination with older material. More reliable ages come from Mangaone tephra, which stratigraphically overlies Hauparu tephra in the region. Mangaone tephra has several associated radiocarbon ages from soil, peat, and charcoal. Seven of these have been listed as `optimala by Froggatt and Lowe (1990, p. 107), and these range from 26,100$800 yr BP (NZ-1556) to 35,300$2200 yr BP (NZ-1812), and give an average radiocarbon age of about 30$2 ka. This, together with our optical ages, suggests that both Tahuna and Tihoi tephras were instead deposited between ca. 25 and 35 ka.
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7. Conclusions New optical age data suggest that the widespread Rotoehu tephra was deposited at 44$3 ka. There is good agreement between our optical age for Kawakawa tephra (22$2 ka) and its accepted age based on radiocarbon dating, and this supports our laboratory methods. The new optical age for Rotoehu tephra is in agreement with previous ages deduced from several other methods, but is signi"cantly younger than the currently preferred K}Ar whole-rock age of 64$4 ka which was derived from averaging di!erent lava #ow events. However, our optical ages are consistent with these data if the K}Ar ages for the individual events are considered separately. A younger post-60 ka age is also concordant with the occurrence of Rotoehu tephra in a sequence containing paleoecological information that is indicative of cold interstadial climate, consistent with OIS 3 (McGlone et al., 1984). Our optical ages also suggest that Tahuna and Tihoi tephras were deposited at some time between 25 and 35 ka. A mechanism that could potentially lead to our ages being too young is anomalous fading that was too subtle to be detected by our long-term storage experiments, which had analytical uncertainties between 1.6% (sample SRL1) and 5.3% (sample WRL1) (Table 2). However, our optical age for sample SRL4, collected beneath the welldated Kawakawa tephra, suggests that this potential e!ect was not a signi"cant source of error. It is also possible that our ages are underestimates because of the downward translocation of younger grains, as postulated by Berger et al. (1994) for some of their sites. But we did not observe evidence of signi"cant disturbance (e.g., solution pipes) in the loess/paleosol or tephra beds directly overlying our samples. More work is needed to re"ne the chronology for Rotoehu tephra and the other important marker beds found in the TVZ, and beyond. Single-crystal or stepheated Ar}Ar ages from the lava #ows bracketing Rotoehu tephra on Mayor Island would be useful to evaluate whether the lavas contain excess Ar and to test their homogeneity. Finally, the chronology of pre-30 ka tephra beds from Taupo and Okataina centres may have to be revised considering the optical ages associated with Tahuna and Tihoi tephras at our sites. This may have implications for eruption frequency, the chronologies of loess-paleosol sequences, marine and alluvial terrace chronologies, and palynological studies in New Zealand.
Acknowledgements We would like to thank D.J. Huntley for the use of his portable c-ray spectrometer and for discussion in the
"eld. Ningsheng Wang is thanked for careful preparation of the samples, and P.C. Froggatt kindly provided glass reference data for Tahuna and Okaia tephras. Optical dating was performed at the Luminescence Dating Laboratory, School of Earth Sciences, Victoria University of Wellington, while tephra analyses were undertaken in the Department of Geology, University of Auckland. D.J. Huntley and R.G. Roberts critically reviewed an earlier version of the manuscript and contributed to its improvement, while P.C. Froggatt and two anonymous journal reviewers provided valuable comments. This research was funded in part by a VUW internal research grant to OBL and a New Zealand Foundation for Research Science and Technology Fellowship to PAS. The Victoria University of Wellington Luminescence Dating Laboratory is supported in part by Public Good Science Fund (NZ) grant VIC808. References Adamiec, G., Aitken, M.J., 1998. Dose-rate conversion factors: update. Ancient TL 16, 37}50. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Bard, E., Arnold, M., Fairbanks, R.G., Hamelin, B., 1993. Th}U and C ages obtained by mass spectrometry on corals. Radiocarbon 35, 191}199. Berger, G.W., 1995. Progress in luminescence dating methods for Quaternary sediments. In: Rutter, N.W. Catto, N.R. (Eds.), Dating Methods for Quaternary Deposits, Vol. 2. Geological Association of Canada, GEOtext, pp. 81}104. Berger, G.W., Mahaney, W.C., 1990. Tests of thermoluminescence dating of buried soils from Mt. Kenya, Kenya. Sedimentary Geology 66, 45}56. Berger, G.W., Pillans, B.J., Palmer, A.S., 1994. Tests of thermoluminescence dating of loess from New Zealand and Alaska. Quaternary Science Reviews 13, 309}333. Berryman, K.R., 1992. A stratigraphic age of Rotoehu Ash and late Pleistocene climate interpretation based on marine terrace chronology, Mahia Peninsula, North Island, New Zealand. New Zealand Journal of Geology and Geophysics 35, 1}7. B+tter-Jensen, L., 1997. Luminescence techniques: instrumentation and methods. Radiation Measurements 27, 749}768. Buhay, W.M., Cli!ord, P.M., Schwarcz, H.P., 1992. ESR dating of the Rotoiti Breccia in the Taupo Volcanic Zone. New Zealand. Quaternary Science Reviews 11, 267}271. Divigalpitiya, W.M.R., 1982. Thermoluminescence dating of sediments. M.Sc. Thesis, Simon Fraser University, Burnaby, 93 pp. Duller, G.A.T., 1994. Luminescence dating using feldspars: a test case from southern North Island. New Zealand. Quaternary Geochronology (Quaternary Science Reviews) 13, 423}427. Forman, S.L., Jackson, M.E., McCalpin, J., Matt, P., 1988. The potential of using thermoluminescence to date buried soils developed on colluvial and #uvial sediments from Utah and Colorado. U.S.A. * preliminary results. Quaternary Science Reviews 7, 287}293. Froggatt, P.C., Lowe, D.J., 1990. A review of late Quaternary silicic and some other tephra formations from New Zealand: their stratigraphy, nomenclature, distribution, volume, and age. New Zealand Journal of Geology and Geophysics 33, 89}109. Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H., Olley, J.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia. Part I: experimental design and statistical models. Archaeometry 41, 339}364.
O.B. Lian, P.A. Shane / Quaternary Science Reviews 19 (2000) 1649}1662 Ghiorso, M.S., Sack, R.O., 1991. Fe}Ti oxide geothermometry: thermodynamic formulation and the estimation of intensive variables in silicic magmas. Contributions to Mineralogy and Petrology 108, 485}510. Houghton, B.F., Wilson, C.J.N., McWilliams, M.O., Lanphere, M.A., Weaver, S.D., Briggs, R.M., Pringle, M.S., 1995. Chronology and dynamics of a large silicic magmatic system: central Taupo Volcanic Zone. New Zealand. Geology 23, 13}16. Huntley, D.J., Clague, J.J., 1996. Optical dating of tsunami-laid sands. Quaternary Research 46, 127}140. Huntley, D.J., Lian, O.B., 1999. Using optical dating to determine when a sediment was last exposed to sunlight. In: Lemmen, D.S., Vance, R.E. (Eds.), Holocene Climate and Environmental Changes in the Palliser Triangle: A Geoscienti"c Context for Evaluating the Impacts of Climate change on the Southern Canadian Prairies. Geological Survey of Canada Bulletin 534, 211}222. Huntley, D.J., Wintle, A.G., 1978. Some aspects of alpha counting. PACT 2, 115}119. Huntley, D.J., Berger, G.W., Divigalpitya, W.M.R., Brown, T.A., 1983. Thermoluminescence dating of sediments. PACT 9, 607}618. Huntley, D.J., Berger, G.W., Bowman, S.G.E., 1988. Thermoluminescence responses to alpha and beta irradiations, and age determination when the high dose response is nonlinear. Radiation E!ects 105, 279}284. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W., 1985. Optical dating of sediments. Nature 313, 105}107. Huntley, D.J., Hutton, J.T., Prescott, J.R., 1993a. Optical dating using inclusions within quartz grains. Geology 21, 1087}1090. Huntley, D.J., Hutton, J.T., Prescott, J.R., 1993b. The stranded beachdune sequence of south-east South Australia: a test of thermoluminescence dating, 0}800 ka. Quaternary Science Reviews 12, 1}20. Huntley, D.J., Godfrey-Smith, D.I., Haskell, E.H., 1991. Light-induced emission spectra from some quartz and feldspars. Nuclear Tracks and Radiation Measurements 18, 127}131. Huntley, D.J., Nissen, M.K., Thompson, J., Calvert, S.E., 1986. An improved alpha scintillation counting method for determination of Th, U, Ra-226, Th-230 excess, and Pa-231 excess in marine sediments. Canadian Journal of Earth Sciences 23, 959}966. Kimber, R.W.L., Kennedy, N.M., Milnes, A.R., 1994. Amino acid racemization dating of a 140 000 year old tephra-loess-paleosol sequence on the Mamaku Plateau near Rotorua. New Zealand. Australian Journal of Earth Sciences 41, 19}26. Krbetschek, M.R., GoK tze, J., Dietrich, A., Trautmann, T., 1997. Spectral information from minerals relevant for luminescence dating. Radiation Measurements 27, 695}748. Lian, O.B., Huntley, D.J., 1999. Optical dating studies of postglacial aeolian deposits from the south-central interior of British Columbia. Canada. Quaternary Science Reviews 18, 1453}1466. Lian, O.B., Hu, J., Huntley, D.J., Hicock, S.R., 1995. Optical dating studies of Quaternary organic-rich sediments from southwestern British Columbia and northwestern Washington State. Canadian Journal of Earth Sciences 32, 1194}1207. Lowe, D.J., Hogg, A.G., 1995. Age of Rotoehu Ash*Comment. New Zealand Journal of Geology and Geophysics 38, 399}402. Martinson, D.G., Pisias, N.G., Hayes, J.D., Imbrie, J., Moore, T.C., Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000-year chronostratigraphy. Quaternary Research 27, 1}29. Mejdahl, V., 1989. How far back: life times estimated from studies of feldspars of in"nite ages. In: Aitken, M.J. (Ed.), Long and Short Range Limits in Luminescence Dating, Vol. 9. Research Laboratory for Archaeology and the History of Art, Oxford University, Occasional Publication pp. 49}54. McGlone, M.S., Howorth, R., Pullar, W.A., 1984. Late Pleistocene stratigraphy, vegetation and climate of the Bay of Plenty and
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Gisborne regions, New Zealand. New Zealand Journal of Geology and Geophysics 27, 327}350. Nairn, I.A., Kohn, B.P., 1973. Relationship of the Earthquake Flat Breccia to the Rotoiti Breccia, central North Island, New Zealand. New Zealand Journal of Geology and Geophysics 16, 269}279. Ollerhead, J., Huntley, D.J., Berger, G.W., 1994. Luminescence dating of sediments from Buctouche Spit. New Brunswick. Canadian Journal of Earth Sciences 31, 523}531. Ota, Y., Omura, A., Iwata, H., 1989. Th}U age of Rotoehu ash and its implications for marine terrace chronology of eastern Bay of Plenty, New Zealand. New Zealand Journal of Geology and Geophysics 32, 327}331. Pillans, B.J., 1991. New Zealand Quaternary stratigraphy: an overview. Quaternary Science Reviews 10, 405}418. Pillans, B., Wright, I., 1992. Late Quaternary tephrostratigraphy from the southern Harve Trough}Bay of Plenty, northeast New Zealand. New Zealand Journal of Geology and Geophysics 35, 129}143. Pillans, B., McGlone, M., Palmer, A., Mildenhall, D., Alloway, B., Berger, G., 1993. The Last Glacial Maximum in central and southern North Island, New Zealand: a paleoenvironmental reconstruction using the Kawakawa Tephra Formation as a chronostratigraphic marker. Palaeogeography, Palaeoclimatology, Palaeoecology 101, 283}304. Pullar, W.A., Birrell, K.S., 1973. Age and distribution of late Quaternary pyroclastic and associated cover deposits of the Rotorua and Taupo area, North Island, New Zealand. New Zealand Soil Survey Report 1, Part 1. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term time variations. Radiation Measurements 23, 497}500. Prescott, J.R., Robertson, G.B., 1997. Sediment dating by luminescence: a review. Radiation Measurements 27, 893}922. Roberts, R.G., 1997. Luminescence dating in archaeology: from origins to optical. Radiation Measurements 27, 819}892. Roberts, R.G., Galbraith, R.F., Olley, J.M., Yoshida, H. Laslett, G.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia. Part II: results and implications. Archaeometry. 41, 365}395. Shane, P., 1998. Correlation of rhyolitic pyroclastic eruptive units from the Taupo Volcanic Zone, New Zealand by Fe}Ti oxide compositional data. Bulletin of Volcanology 60, 224}228. Shane, P.A.R., Black, T.M., Alloway, B.V., Westgate, J.A., 1996. Early to middle Pleistocene tephrochronology of North Island, New Zealand: implications for volcanism, tectonism and paleoenvironments. Geological Society of America Bulletin 108, 915}925. Shepherd, M.J., Price, D.M., 1990. Thermoluminescence dating of late Quaternary dune sand, Manawatu/Horowhenua area, New Zealand: a comparison with C age determinations. New Zealand Journal of Geology and Geophysics. 33, 51}69. Spooner, N.A., 1994. The anomalous fading of infra-red-stimulated luminescence from feldspars. Radiation Measurement 23, 625}632. Vucetich, C.G., Howorth, R., 1976. Late Pleistocene tephrostratigraphy in the Taupo District, New Zealand. New Zealand Journal of Geology and Geophysics 19, 51}69. Vucetich, C.G., Puller, W.A., 1969. Stratigraphy and chronology of late Pleistocene volcanic ash beds in central North Island, New Zealand. New Zealand Journal of Geology and Geophysics 12, 784}837. Wolfe, S.A., Huntley, D.J., Ollerhead, J., 1995. Recent and late Holocene sand dune activity in southwestern Saskatchewan. In: Current Research 1995-B. Geological Survey of Canada, Ottawa, pp. 131}140. Whitehead, N.E., Ditchburn, R., 1994. Revision of some ages for the Rotoehu Ash. New Zealand Journal of Geology and Geophysics 37, 381}383.
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Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiation Measurements 27, 769}817. Wintle, A.G., Catt, J.A., 1985. Thermoluminescence dating of soil developed in Late Devensian loess at Pegwell Bay. Kent. Journal of Soil Science 36, 293}298.
Optical dating of paleosols bracketing the widespread Rotoehu tephra, North Island, New Zealand Olav B. Lian *, Philip A. Shane School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand
Abstract Optical dating of the "ne-grained mineral fraction of paleosols developed on, or within, loess bracketing the widespread Rotoehu tephra at two sites in central North Island, New Zealand, was undertaken using infrared excitation of potassium feldspars. Optical ages suggest that Rotoehu tephra was deposited at ca 45 ka, and support several previous age determinations. Our optical ages are also consistent with individual K-Ar ages from bracketing lava #ows, though inconsistent with the currently preferred age for the tephra (64$4 ka) which is the weighted mean of them. Our data also suggests that the age of two other important time-stratigraphic markers, Tihoi and Tahuna tephras, estimated to have been deposited at 46 and 43 ka, respectively, may have instead been formed between 25 and 35 ka. In the light of this, the chronology of other North Island tephra beds older than 30 ka may have to be revised. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction New Zealand has a detailed record of Southern Hemisphere Late Quaternary climate change. The nature of climate change is recorded in loess, paleosols, alluvium, and lacustrine sediments (e.g., Pillans, 1991) that are well endowed with rhyolitic tephra beds that have been erupted from the Taupo Volcanic Zone (TVZ) in central North Island (Fig. 1). These tephra beds have been used extensively to constrain the age of lithostratigrahic units and to correlate sequences between basins (e.g., Froggatt and Lowe, 1990; Pillans et al., 1993). The tephrostratigraphic record for the last ca. 50 ka is well established, and the ages of most tephra beds younger than about 30 ka are constrained by radiocarbon ages on carbonised organic matter in proximal deposits, or on bounding organic-rich material such as peat. One of the largest eruptions during the Late Pleistocene originated from the Horohoro vent of the Okataina centre before &40 ka and produced the Rotoehu tephra and associated Rotoiti ignimbrite. Rotoehu tephra, "rst de"ned by Vucetich and Puller (1969), is an important, and widespread, time-
* Corresponding author. Present address: Department of Physics, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6. E-mail address: [email protected] (O.B. Lian).
stratigraphic marker, and is easily identi"ed throughout central North Island on the basis of glass chemistry and mineralogy; it contains the characteristic mineral cummingtonite which is rare in TVZ rhyolites. Cummingtonite has been found in abundance in only two other regional fall deposits, also from Okataina, both of which erupted in the Holocene. Rotoehu tephra is also important because it is the lowermost rhyolitic unit in the time interval during which other named tephra beds were deposited, stratigraphic order is established, and their source volcano is known (e.g., Froggatt and Lowe, 1990). The chronology of tephra beds, such as the Rotoehu, that erupted between ca. 30 and 200 ka remains problematic because not only are they of an age near, or beyond the age-limit of standard radiocarbon dating methods, but they are also too young to be dated using the routine "ssion-track methods on glass or zircon. Moreover, calc-alkaline tephra beds, such as those erupted from TVZ generally lack high-K phases that may be amenable to Ar}Ar dating. In this paper we report on optical dating of paleosols developed on or in loess that bracket Rotoehu tephra at two sites near Taupo. At one of the sites we also sampled loessic paleosol overlying Tihoi and Tahuna tephras, and underlying Kawakawa tephra. Loessic paleosols were selected for this study because loess is expected to have experienced extended exposure
0277-3791/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 0 3 - 2
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to direct sunlight prior to burial. Furthermore, it has been observed that in many cases mineral sediments extracted from paleosols have given reasonable luminescence ages, whereas the parent material has not. It is thought this is because of bioturbation, such as that which occurs during the soil-forming process, can be responsible for further sunlight exposure (e.g., Divigalpitiya, 1982; Huntley et al., 1983; Wintle and Catt, 1985; Forman et al., 1988; Berger and Mahaney, 1990). In this study, bioturbation would be expected to have ended soon after deposition of the overlying tephra beds, and thus our optical ages would probably more closely date the deposition of the tephra than the deposition of the loess from which the paleosol formed.
2. Previous attempts at dating Rotoehu tephra Because Rotoehu tephra was erupted near the older age limit of radiocarbon dating, it has proved di$cult to obtain an accurate and precise age (Froggatt and Lowe, 1990; Lowe and Hogg, 1995). Various dating techniques have been applied. Radiocarbon dating has yielded several ages younger than 40 ka, but there are also ages older than 40 ka. Whitehead and Ditchburn (1994) recalculated, and averaged, four previously reported radiocarbon ages and deduced from these data that the Rotoehu tephra formed at 35$3 ka. Lowe and Hogg (1995), however, point out that the mean radiocarbon age reported by Whitehead and Ditchburn should be disregarded because there is evidence that the samples dated were contaminated with younger carbon. They also add that Whitehead and Ditchburn overlooked several other radiocarbon ages that are older than 40 ka. Of these, two are `"nitea: one is from peat and yielded an age of 44,000$5300 yr BP (NZ-877), while the other, from wood found in paleosol beneath the tephra, gave an age of 41,700$3500 yr BP (NZ-1126) (Nairn and Kohn, 1973). Although this wood was rated as `optimala for radiocarbon dating by Froggatt and Lowe (1990, p. 107), its apparent age is near the limit of standard radiocarbon techniques and should therefore be considered with caution. Froggatt and Lowe report that the age for NZ-877 is almost certainly much greater than ca. 44 ka. Buhay et al. (1992) produced electron-spin resonance (ESR) ages from quartz extracted from Rotoiti ignimbrite. Their average age (45$8 ka) was based on those from Al centres (average, 49$8 ka) and Ti centres (average, 41$6 ka). Ota et al. (1989) used Th}U disequilibrium dating to produce an age of 71$6 ka for Rotoehu tephra. However, it has since been argued that their age is based on insu$cient data (e.g., Froggatt and Lowe, 1990; Lowe and Hogg, 1995) and that the true age is
`probably much youngera (Whitehead and Ditchburn, 1994, p. 382). Dating using the degree of racemization of aspartic acid extracted from peptides within tephra, loess and paleosols from a sedimentary sequence near Rotorua produced an age estimate of ca. 61 ka (no analytical uncertainty reported) for Rotoehu tephra (Kimber et al., 1994). However, it should be noted that their age for the Mamaku ignimbrite, which occurs at the base of their sequence, was ca. 140 ka, whereas it has since been shown that its age is 230$10 ka (Shane et al., 1996). At present, the most accepted age is probably the whole-rock K}Ar age of 64$4 ka from bracketing lava #ows from Mayor Island volcano (Fig. 1) (Wilson et al., 1992). However, we argue that it too should be considered tentative for the following reasons: (i) 64 ka is near the common lower limit of the K}Ar technique; (ii) K}Ar whole-rock ages are subject to potential xenolith and xenocryst contamination; and (iii) potential excess Ar, and K and Ar loss, are di$cult to assess. Furthermore, Wilson et al.'s age was derived by taking the weighted mean of ages obtained for two di!erent lava #ows * one overlies the tephra, while the other is beneath. They have therefore averaged the ages of two separate eruptive events, and their analytical uncertainty may be too small, as was pointed out by Lowe and Hogg (1995, p. 400). We think it more prudent to consider Wilson et al.'s ages separately: the whole-rock K}Ar age from the overlying lava (from their locality 1) is 67$11 ka, and those for the underlying lava are 63$5 (locality 2) and 63$9 (locality 3) (Wilson et al., 1992, p. 327). The ages from localities 1 and 2 are each the average of two age determinations on the same sample. Less-quantitative estimates for the age of Rotoehu tephra include 52$7 ka based on the tephra's stratigraphic position on raised marine terraces that have been correlated with previously dated sea-level positions (Berryman, 1992), and an age of &55 ka has been estimated from sedimentation rates in a deep-sea core collected o! the east coast of North Island (Pillans and Wright, 1992). These ages are supported by paleoecological data that indicate that the climatic interval that began just before deposition of Rotoehu tephra was characterised by cool interstadial conditions; this interval lasted until about the time of the emplacement of Omataroa tephra (ca. 28 ka), when the climate began to deteriorate (McGlone et al., 1984). This is consistent with Rotoehu tephra being deposited soon after the start of Oxygen Isotope Stage (OIS) 3, which commenced at about 60 ka (Martinson et al., 1987). Taking all of these studies into consideration, it is safe to conclude that Rotoehu tephra is older than &35 ka (based on the reported radiocarbon ages) but younger than &75 ka (based on the K}Ar age, at #2p, for the underlying lava #ow on Mayor Island).
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Fig. 1. Location of the study sites and features referred to in the text.
3. Lithostratigraphy and tephra identi5cation 3.1. Stratigraphy Sequences at Scott and Waihora roads were selected for this study (Fig. 1). Lithostratigraphic logs, sample locations, and textural characteristics are given in Fig. 2. The stratigraphy at Waihora Road was "rst established by Vucetich and Howorth (1976), and was designated the type section for Tihoi tephra. The measured section (Fig. 2) is about 3.5 m thick, rests on undi!erentiated ignimbrite, and consists of four loess units separated by four tephra beds. The loess units generally grade upward to paleosol. The lowermost tephra occurs &50 cm above the ignimbrite, and was identi"ed by Vucetich and Howorth (1976), on the basis of lithology, thickness, and mineralogy, as Rotoehu. Tihoi and Tahuna tephras were
identi"ed about 60 and 80 cm higher, respectively; Okaia tephra was found at the top of the sequence. The section extends another &2 m to the surface. The Scott Road section (Figs. 1 and 2) is exposed in a road cut. The stratigraphy in this locality was "rst documented by Pullar and Birrell (1973). The measured section is about 4.5 m thick and contains "ve tephra beds separated by "ve loess}paleosol units (Figs. 2 and 3). The sequence rests on welded Ongatiti ignimbrite that has been dated to ca. 1.2 Ma (Houghton et al., 1995). It is capped with Taupo ignimbrite that was deposited at ca. 1.8 ka (Froggatt and Lowe, 1990). The stratigraphy at Scott Road can be readily correlated with that at Waihora Road, about 23 km away (Fig. 1), but as an added precaution we have chosen to chemically identify some of the tephra beds; we have given these tephras laboratory designations 585, 587, and 588 (Fig. 2).
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Fig. 2. Lithostratigraphic logs of the Scott and Waihora road sections showing location of optical dating samples SRL1}4, and WRL1}2 and their apparent optical ages. At Scott Road, sample SRL1 (42% sand, 46% silt, 12% clay) was collected 10 cm below the base of the Rotoehu tephra bed, while sample SRL2 (15% sand, 58% silt, 26% clay) was collected in the overlying unit, 6 cm above the tephra. Sample SRL3 (49% sand, 43% silt, 68% clay) was taken 15 cm above the Tahuna tephra, and 11 cm below the Okaia tephra. Sample SRL4 (17% sand, 69% silt, 14% clay) was collected 24 cm below the Kawakawa tephra. Sample WRL1 (32% sand, 36% silt, 32% clay) was collected 9 cm below Rotoehu tephra, while sample WRL2 (12% sand, 41% silt, 48% clay) was collected 15 cm above the tephra. The calibrated age estimate for Kawakawa tephra is based on a radiocarbon age of 22.6 ka; see the text for discussion.
3.2. Tephra identixcation The lowermost tephra at Waihora Road has been previously identi"ed as Rotoehu by Vucetich and Howorth (1976). We have compared its glass chemistry and mineralogy with the lowermost tephra at Scott Road (our tephra 585). Both are calc-alkaline rhyolites, typical of those found in TVZ, and contain the diagnostic mineral cummingtonite. Both are characterised by low variance for most oxides (Table 1) when shared compositions are averaged (e.g., SiO $(0.21 wt.%,
FeO$(0.1 wt.%), suggesting that they are the product of a single eruptive event (e.g., Shane et al., 1996), and both have glass compositions that are the same as those reported by other workers for Rotoehu tephra (e.g., Pillans and Wright, 1992). Tephra beds 587 and 588 at Scott Road compositionally match the Tahuna and Okaia beds, respectively (Fig. 4, Table 1), and this is consistent with the stratigraphy found at Waihora Road by Vucetich and Howorth (1976). Based on this stratigraphy, the uppermost tephra unit at Scott Road is interpreted to be Kawakawa. The "ner material found at the
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Fig. 4. Individual glass shard analyses of tephra beds at Scott and Waihora roads sections compared to the "elds of reference sample compositions of known identity.
Table 1 Glass composition of tephra beds (wt%) at Waihora and Scott roads Re
Fig. 3. The loessic paleosol tephra sequence at Scott Road exposed up to the base of the Kawakawa tephra. The arrow points to the Rotoehu tephra bed. The knife (20 cm long) rests on Tahuna tephra, which in turn lies directly on Tihoi tephra. See Fig. 2 for sample locations.
base of this unit is probably fall deposit, while the overlying coarser material may be associated ignimbrite. As an additional criteria for identifying some of the tephra beds, we analysed Fe}Ti oxide spinel } rhombohedral pairs to estimate eruption temperatures and oxygen fugacities (fO ). These parameters are often char acteristic of particular eruptive events, even if the tephra beds display a similar glass composition (Shane, 1998). We analysed oxide pairs that are attached to ferromagnesian phenocrysts to ensure that they are co-magmatic. ¹}fO data were calculated following the procedure of Ghiorso and Sack (1991). Fe}Ti oxide pairs from the Rotoehu correlatives at Scott (tephra 585) and Waihora roads, and the Okaia correlative at Scott Road (tephra 588), produced temperatures and oxygen fugacities characteristic of these tephras (Fig. 5), thus con"rming their identi"cation. Rotoehu tephra is particularly distinctive, having a high fO relative to other tephras erupted from Okataina centre. ¹}fO data for sample 588 from Scott
SiO Al O TiO FeO MnO MgO CaO Na O K O Cl H O n
77.68 12.49 0.21 0.99 0.07 0.10 0.85 4.12 3.27 0.23 7.06 11
585 (0.18) (0.11) (0.03) (0.05) (0.03) (0.06) (0.05) (0.13) (0.16) (0.02) (1.63)
77.86 12.61 0.18 0.99 0.07 0.10 0.87 4.04 3.32 0.22 8.20 10
587 (0.17) (0.09) (0.03) (0.05) (0.05) (0.05) (0.03) (0.11) (0.07) (0.03) (1.72)
77.56 12.57 0.16 1.04 0.07 0.08 0.97 3.56 4.01 0.19 4.02 11
588 (0.18) (0.10) (0.05) (0.09) (0.04) (0.06) (0.05) (0.11) (0.05) (0.02) (1.79)
77.57 12.55 0.18 1.31 0.06 0.08 1.17 3.81 3.29 0.20 7.70 10
(0.21) (0.11) (0.07) (0.08) (0.03) (0.04) (0.06) (0.13) (0.11) (0.02) (1.89)
Note. Re"Rotoehu tephra at Waihora Road; 585, 587, and 588 are tephra beds from Scott Road. See Fig. 2 for sample locations. Compositions presented as mean and standard deviation (in parenthesis) from n shards. Recalculated to 100% on a volatile-free basis. All Fe as FeO. Water by di!erence. Analyses by a JEOL JXA-5A electron microprobe "tted with a Link Systems LZ-5 EDS detector. Absorbed current 0.6 nA at 15 kV. Beam defocused to 15 lm diameter.
Road con"rms its correlation to Okaia tephra, and show that the tephra has not been mis-identi"ed as another widespread Taupo tephra, such as Kawakawa or Tihoi (Fig. 5).
4. Optical dating 4.1. Introduction Luminescence dating (optical and thermoluminescence dating) works on the principle that minerals, such as
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Fig. 5. Temperature and oxygen fugacity estimates for tephra beds in this study compared to the typical ranges for tephra beds of similar age.
quartz and feldspar, contain impurities and structural defects, some of which can act as traps for free electrons. The rate at which such traps are "lled is proportional to the rate at which free electrons are produced, which is in turn proportional to the environmental dose rate due to the decay of radioactive elements within the mineral samples and in the surrounding sediment. For sediments within a few metres of the surface there can also be a signi"cant contribution from cosmic rays. Exposure to sunlight empties electrons from traps. During subsequent burial, electron traps become "lled. In practice, the radiation dose absorbed since the sediment was last exposed to sunlight is estimated, and, together with a measure of the ambient dose rate, the time elapsed since the sediment grains were last exposed to sunlight can be determined. Optical dating, introduced by Huntley et al. (1985), is superior to thermoluminescence (TL) dating because only electrons in light-sensitive traps are measured; for TL dating, electrons in light-sensitive and light-insensitive traps are measured together. Detailed accounts of the method can be found in Berger (1995), Wintle (1997), Aitken (1998), and Huntley and Lian (1999), while recent application-based reviews include those of Prescott and Robertson (1997), Roberts (1997), and Aitken (1998). 4.2. Sample collection and laboratory preparation All of our samples were composed of moist "ne sand, silt, and clay (Fig. 2), which allowed small blocks to be easily carved from the cleaned section face. Sample blocks were subsampled using the procedures outlined in Lian and Huntley (1999). The portions to be used for dating were prepared using standard methods, including immersion in hot (&903C) HNO for 40 min to assure removal of the organic material (Lian et al., 1995). The
4}11 lm diameter fraction was separated from the treated sediment fraction, and at least 70 aliquots were made using the procedures described in Lian et al. (1995). Because it was desired to compare the optical ages obtained from silt-sized potassium feldspars with those from sand-sized quartz, the prepared '11 lm diameter fractions were sieved to obtain grains in the 90}125 lm and 125}180 lm diameter size ranges. The quartz was isolated from the other minerals using a procedure similar to that outlined by Wintle (1997). For each sample, however, only 15}100 mg of quartz could be obtained in the desired size fractions, and this was not su$cient for routine dating. Single grain/aliquot dating (e.g., Galbraith et al., 1999; Roberts et al., 1999) of these quartz fractions will be undertaken in the near future. 4.3. Equivalent dose determination The &3.1 eV (400 nm) emission from potassium feldspars (e.g., Huntley et al., 1991; Krbetschek et al., 1997) was measured using a Ris+ TL-DA-15 Minisys system equipped with light-emitting diodes (TEMPT 484) that together delivered about 20 mW cm\ of &1.4 eV (infrared, 800}960 nm) photons to the sample (B+tter-Jensen, 1997). Luminescence was detected by an EMI 9235QA photomultiplier tube mounted behind a Schott BG-39 and a Kopp 5-58 optical "lter. The BG-39 "lter absorbs scattered 1.4 eV photons, while the 5-58 "lter selectively passes the 3.1 eV emission and blocks the 2.2 eV (570 nm) emission from plagioclase feldspars. To correct for the intrinsic variability in luminescence intensity between aliquots, each was initially given a 20 mJ cm\ exposure to 1.4 eV photons. Normalisation factors were calculated as the luminescence intensity of the individual aliquot divided by the average luminescence intensity of all aliquots in the set. Laboratory irradiations were from a Sr/Y source that delivered &6.7 Gy min\ of b radiation to the sample. The a-e$ciency (b-value) was estimated by exposing some of the aliquots to Am sources that delivered a particles to the sample at &0.15 lm\ min\. Laboratory irradiation populates electron traps that are thermally stable over geologic time, as well as those that are thermally unstable. To assure that only thermally stable traps are sampled during the "nal luminescence measurements, all of the aliquots in a set (except for several control aliquots) were heated (`preheateda) at 1403C for 7 d. Although a less severe preheat could probably have achieved the desired results, the 1403C/7 d protocol has been found to be consistently successful in several other studies (e.g., Huntley et al. 1993a; Ollerhead et al., 1994; Lian et al., 1995; Wolfe et al., 1995). For all of the samples, the additive-dose method, with thermal transfer correction (e.g., Huntley et al., 1993a;
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Huntley and Clague, 1996; Lian and Huntley, 1999) and the regeneration method (e.g., Huntley et al., 1993b; Huntley and Lian, 1999) were both used. Aliquots used to construct the thermal transfer curves, and the regeneration curves, were exposed to a quartz}halogen lamp (i.e., `bleacheda) behind a Schott RG-715 optical "lter that absorbs UV and most visible light (Huntley and Clague, 1996). This bleach was for 4 h and reduced the luminescence intensity of an unirradiated aliquot that had not been preheated to 3}5% of the intensity of a similar unbleached aliquot. After storage for 60 d at room temperature, the luminescence from all the aliquots was measured for 200 s. Dose-response curves were constructed using maximum likelihood "ts: additive-dose and regeneration data were consistently "tted with saturating exponentials, while the thermal-transfer correction data were "tted
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with straight lines. Examples are shown in Figs. 6 and 7. For the regeneration data, when a scaling parameter was included in the "t, in all but one case this parameter was within 1p of unity, which indicated that for most of the samples the laboratory bleach did not signi"cantly alter the luminescence sensitivity of the relevant minerals. For the additive-dose method, dose intercepts were calculated every 20 s, and in all but one case they were found to be constant with excitation time (Fig. 8). For sample SRL1 dose intercepts were found to gradually increase with excitation time, but this e!ect was not observed in the regeneration data. However, when the additive-dose data were "tted with a saturating exponential that included a linear term, or if a saturating exponential was used with the highest dose points omitted, this e!ect disappeared; the former "t was therefore used to determine the D .
Fig. 6. Luminescence decay as measured under infrared (1.4 eV) excitation for some of the samples. The insets show b-response curves for the "rst 20 s of excitation. For sample SRL1 the additive-dose data (solid points) are shown "tted with a saturating exponential#linear function; for all the other samples the "ts are saturating exponentials. In all cases, the thermal transfer data were "tted with straight lines. The D 's were taken as the dose intercept calculated using the luminescence integrated over the entire excitation time after a correction for the decay that resulted from the normalisation procedure. The luminescence intensity from a `naturala and `bleacheda aliquot that have not been preheated are indicated as Nnp and Bnp, respectively.
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Fig. 7. Luminescence decay as measured under infrared (1.4 eV) excitation for sample SRL4. The inset shows b-response curves for both the additive-dose (N#b) and regeneration (N#light#b) data for the "rst 20 s of excitation. The black points are the N#b data shifted along the dose axis for best "t, the shift being equal to the equivalent dose (D ) after correction for normalisation decay and incomplete laborat ory bleaching. The grey points are the N#b data before the shift. For each sample, the D used in the age calculation was that which was determined using the luminescence integrated over the entire excitation time. Nnp and Bnp are described in Fig. 6.
Dose intercepts were subsequently re-calculated using the luminescence recorded over the entire excitation time, and this was taken as the D after a correction for the decay resulting from the normalisation procedure (Table 2). For the regeneration procedure, the D 's were calculated in a similar manner with the scaling parameter "xed at unity, but were also corrected for incomplete laboratory bleaching (Table 2). 4.4. Anomalous fading tests For some feldspars, the luminescence that arises from traps "lled during laboratory irradiation, that are expected to be stable over geologic time, instead depends on the length of time between irradiation and measurement, the luminescence being smaller for longer delay times (e.g., Spooner, 1994; Aitken, 1998). This `anomalous fadinga can result in underestimation of the true optical age. To test if this e!ect was prominent in our samples, several aliquots were given a 24 h laboratory bleach, followed by a b dose equal to their D . All the aliquots were preheated together at 1403C for 7 d. The luminescence, ¸ , was measured for 20 s after 1 d, and " then again for 20 s after either 30 or 75 d of storage at room temperature. As a control, the luminescence, ¸ , of , several aliquots that had received no laboratory bleach or radiation, but were heated, was also measured. The heating and luminescence measurements of both sets were done together, and the latter were made using the
Fig. 8. Dose intercept versus excitation time for all the samples as determined by the additive-dose method with thermal transfer correction. Some of the points have been shifted slightly for clarity.
same apparatus and "lter combinations as for the other experiments. The fading ratio was taken as: (¸ / " ¸ )/(¸ /¸ ) where the subscripts 1 and 2 refer to the " , , "rst and second measurements, respectively. For each sample, this fading ratio was found to be consistent with unity (Table 2). 4.5. Dosimetry The environmental dose rate is proportional to the amount of a, b, and c radiation from the decay of
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Table 2 Equivalent doses (D ), b-values, dose rates, fading ratios, and optical ages D (Gy) Sample
Additive-dose
Regeneration
b-value (Gy lm)
DQ (Gy ka\)
DQ 2 (Gy ka\)
Fading ratio
Optical age (ka)
SRL1 SRL2 SRL3 SRL4 WRL1 WRL2
64$6 49$4 64$23 45$4 96$17 63$12
67$3 41$2 39$14 40$2 ss 67$5
0.93$0.03 0.76$0.05 1.19$0.19 1.02$0.05 1.06$0.08 0.73$0.09
0.10 0.11 0.14 0.15 0.09 0.09
1.50$0.05 1.52$0.07 1.87$0.14 1.93$0.07 2.28$0.13 1.97$0.12
0.999$0.016 1.001$0.052 1.022$0.033 1.003$0.053 0.986$0.031 1.014$0.031
44$3 30$2 21$8 22$2 42$8 34$3
b-value as de"ned by Huntley et al. (1988). DQ : dose rate due to cosmic rays (Prescott and Hutton, 1994). DQ : total dose rate (that due to cosmic rays plus that due to c, b, and a radiation). 2 Storage was for 30 d for samples SRL1, 2, and WRL1, and 75 d for the others. See the text for explanation. ss: sensitivity change detected in the regeneration data.
U, U, Th, their daughter products, and K in the sample and surrounding sediment matrix. Cosmicray radiation also contributes to the dose rate, and its e!ect is a function of the sample's depth beneath the surface. For this work, a subsample of the bulk sediment used for dating was dried and milled to a "ne powder, and was analysed for K by inductively coupled plasma atomic emission spectroscopy (ICP-AES), while U and Th were analysed by several methods that sample di!erent parts of the respective decay chains. Delayed neutron counting (DNC) and neutron activation analysis (NAA) give the concentrations of the parents, respectively (Table 3). Thick-source a counting (TSAC) gives U and Th equivalent concentrations (Table 4) by counting all the a particles in each chain (Huntley et al., 1986). Since a particles are attenuated by organic
matter, the samples were ashed (5003C for 24 h) prior to counting. In situ c ray spectroscopy (IGRS) gives the K content directly and allows equivalent concentrations of U and Th to be calculated from the measurement of c-rays from late daughters in each chain (Bi for U, and Tl for Th), from sediment up to about 50 cm away (Table 3). Together the various analyses can be used to determine the extent of radioactive disequilibrium in the U and Th decay chains, and to account for local inhomogeneity in the sediment matrix. 4.6. Results and evaluation of the dose rate For each sample, the U and Th concentrations as determined from DNC and NAA are consistent with those found from TSAC when the analytical
Table 3 K, U, and Th concentrations determined from laboratory analyses and in situ c-ray spectroscopy Sample
SRL1 SRL2 SRL3 SRL4 WRL1 WRL2
K (%)
0.79$0.04 1.00$0.05 0.80$0.04 0.96$0.05 0.97$0.05 0.98$0.05
Th (lg g\)
10.2$0.3 12.4$0.3 12.0$0.4 11.7$0.3 8.9$0.2 10.2$0.3
U (lg g\)
2.96$0.08 2.95$0.09 3.53$0.09 3.24$0.09 2.60$0.08 3.18$0.09
From in situ c-ray spectroscopy K (%)
Th (lg g\)
U (lg g\)
Apparent c dose rate (Gy ka\)
0.24$0.02 0.48$0.03 0.57$0.03 0.63$0.03 0.99$0.03 0.71$0.03
3.85$0.18 4.99$0.24 6.32$0.27 6.09$0.28 7.51$0.32 6.83$0.29
1.70$0.13 1.81$0.16 1.60$0.18 1.67$0.18 1.77$0.22 1.69$0.19
0.464$0.005 0.571$0.006 0.645$0.006 0.661$0.006 0.830$0.006 0.730$0.006
From ICP-AES. From NAA. From DNC. Equivalent concentrations calculated from the activities of Bi for U and Tl for Th, assuming secular equilibrium. Note: The values shown have not been corrected for the presence of water, and are thus smaller than those that would have been calculated if the sediments were dry. From counts in the energy range 0.8}2.6 MeV, corrected for relative proportions of K, U, and Th.
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Table 4 Thick-source a count (TSAC) rates, equivalent U and Th concentrations, organic and water contents Sample
SRL1 SRL2 SRL3 SRL4 WRL1 WRL2
TSAC
Water content
Total count rate (cm\ ks\)
Th count rate (cm\ ks\)
Th (lg g\)
U (lg g\) *
*
*
*
0.809$0.011 0.817$0.011 0.949$0.009 0.839$0.010 0.628$0.007 0.710$0.007
0.445$0.042 0.491$0.044 0.429$0.035 0.466$0.040 0.335$0.027 0.373$0.029
12.0$1.1 13.2$1.2 11.5$0.9 12.5$1.1 9.0$0.7 10.0$0.8
2.8$0.3 2.6$0.3 4.1$0.3 2.9$0.3 2.3$0.2 2.6$0.2
1.30 1.34 1.27 1.03 0.44 0.67
1.41 1.56 1.52 1.15 0.65 0.88
1.30$0.03 1.34$0.05 1.27$0.06 1.03$0.03 0.44$0.05 0.67$0.05
0.18 0.14 0.18 0.18 0.09 0.14
See Huntley et al. (1986). Note: samples used for TSAC were ashed (5003C for 24 h) before counting. Count rates have been corrected for sample re#ectivity using the model of Huntley and Wintle (1978). * and * , are the as-collected and saturated water contents, respectively; * is the water content used for the dose-rate calculation. * is the organic content. Organic and water contents are defnined as (mass organics)/(mass minerals) and (mass water)/(mass minerals), respectively. Organic content was found by ashing dry subsamples at 5003C for 24 h.
uncertainties are taken into account at 2p (most agree at 1p). This indicates the absence of signi"cant radioactive disequilibrium at the time of sample collection. This result was unexpected since disequilibrium has been found to be common in moist organic-rich environments, such as those studied by Lian et al. (1995) and Huntley and Clague (1996). For each of the samples in this study, the c dose rate as found from IGRS (Table 3) was consistently higher (by 4}22%) than that calculated from laboratory analysis (correction for sample moisture included). These di!erences have resulted from inhomogeneity and/or radioactive disequilibrium, but they are not signi"cant when the analytical uncertainties are taken into account. Because of the substantial organic content of these samples, dose rates due to a, b and c radiation (Table 2) were calculated using the modi"ed formulae introduced by Divigalpitiya (1982), which are those reported by Berger and Mahaney (1990) and Lian et al. (1995), but here we used the revised dose-rate conversion factors of Adamiec and Aitken (1998). In each case, the dose rates due to a and b radiation were taken as those calculated using the average value of Th found by NAA and TSAC, and the average value of U from DNC and TSAC, with uncertainties to cover any reasonable possibility. For the c dose rate, the average of that found from IGRS and that determined from laboratory analysis was used for the age calculation (Lian et al., 1995). The dose rate due to cosmic rays (Table 2) was estimated using present burial depths of the samples and the relationship of Prescott and Hutton (1994). Because all of our samples are from well-drained areas, the water content used for the doserate calculations (Table 2) was taken as the as-collected value, with an uncertainty that should account for wet and dry periods at $2p.
5. Optical ages For most of the samples, the D 's from the additive dose and regeneration methods are consistent. For these samples the D 's from both methods were averaged, and the optical ages were calculated by dividing the average D by the total dose rate (Table 2). For sample WRL1, a sensitivity change was detected in the regeneration data, so the D from the additive-dose method was used to calculate the optical age. For samples SRL3 and WRL2 the data were highly scattered, leading to a relatively large uncertainty in the D determined using both methods; for these samples only the D obtained from the regeneration method was considered in the age calculation. The optical ages date the time when the relevant mineral sediments were last exposed to sunlight. In this case, this event occurred sometime between "nal burial of the loess and the cessation of bioturbation, and the latter probably marks the time of deposition of the overlying tephra. The optical age for sample WRL1 is 42$8 ka, and that for sample WRL2 is 34$3 ka (Fig. 2). Sample SRL1 gave an optical age of 44$3 ka, while samples SRL2 and SRL3 yielded optical ages of 30$2 and 21$8 ka, respectively. The optical age for sample SRL4 is 22$2 ka. All of our optical ages are well within the currently accepted age range (ca. 0}150 ka) for the method (see e.g., Duller, 1994; Lian et al., 1995).
6. Discussion The most reliably dated unit in the sequences studied is the Kawakawa tephra. There are many radiocarbon ages
O.B. Lian, P.A. Shane / Quaternary Science Reviews 19 (2000) 1649}1662
associated with this tephra, and most of them have been assessed by Froggatt and Lowe (1990). The ages that they ranked as `optimala and `usefula (Froggatt and Lowe, 1990, p. 106) range between ca. 19.2 and 22.7 ka BP. The most reliable of these are four conventional ages, each from a small piece of charcoal, collected from four di!erent closely spaced locations within associated Oruanui ignimbrite. These range from 22,720> yr BP (Q-2668) \ to 22,470> yr BP (Q-2666) and give a mean age of \ 22,590$230 yr BP (Wilson et al., 1988; Froggatt and Lowe, 1990). More recently, ages of 20,700$150 yr BP (Wk2048) and 22,800$290 yr BP (Wk1826), from carbonate-rich mud sampled from a deep-sea core, have been obtained above and below the tephra, respectively (Pillans and Wright, 1992). Also, Pillans et al. (1993) reported an age of 22,700$230 yr BP (Wk1516) for charcoal found beneath Kawakawa tephra, and ages of 21,100$300 yr BP (NZ-7707) and 21,500$300 yr BP (NZ-6702) from charcoal directly above the tephra. These ages are in general agreement with those of Wilson et al. (1988), and thus the presently accepted radiocarbon age for Kawakawa tephra is 22.6 ka (e.g., Froggatt and Lowe, 1990; Pillans et al., 1993). The data of Bard et al. (1993) suggest that the calibrated (i.e., calendar) age for Kawakawa tephra is consistent with 24.5$1.0 ka. There have also been several luminescence ages on aeolian sediments bracketing Kawakawa tephra (see Shepherd and Price, 1990; Berger et al., 1994; Duller, 1994). These ages are based on both optical and TL methods and on various laboratory protocols. For these reasons the ages are di$cult to assess as a group. However, all but one of those reported for samples directly below (within 25 cm) of Kawakawa tephra are within 2p of 21$2 ka. Although this is broadly consistent with the calibrated radiocarbon age, there is indication that some luminescence ages are slightly too young. To attempt to explain this discrepancy, while accepting that the calibrated radiocarbon age is accurate over this time range, workers have suggested geological processes such as the downward translocation of younger grains (e.g., Berger et al., 1994, p. 324), or that long-term thermal fading has a!ected the luminescence signal (Duller, 1994). But the latter e!ect is probably insigni"cant on this time scale (Mejdahl, 1989; Duller, 1994). Nevertheless, the optical age of 22$2 (SRL4) for Kawakawa tephra at Scott Road is consistent with both the calibrated radiocarbon age and with the majority of reported luminescence ages, and this lends support to our laboratory procedures. The optical age for sample SRL1 suggests that Rotoehu tephra at Scott Road was deposited at, or slightly after, 44$3 ka. This is supported by the optical age for sample WRL1 (42$8 ka) from beneath the tephra at Waihora Road. Thus our age data for Rotoehu tephra are in agreement with the radiocarbon age (41.7$3.5 ka) reported by Nairn and Kohn (1973), the
1659
ESR age (45$8 ka) of Buhay et al. (1992), and the stratigraphy-based age estimate (52$7 ka) of Berryman (1992). Our age data are also consistent with Wilson et al.'s (1992) whole-rock K}Ar ages (63$5 and 63$9 ka) from lava underlying Rotoehu tephra, and our ages are consistent with Wilson et al.'s K}Ar age for the lava #ow overlying Rotoehu tephra (67$11 ka) if their $1p analytical uncertainty is taken into account at 2p. Our new ages are not consistent with Wilson et al.'s weightedmean K}Ar age for the tephra (64$4 ka), and they are also signi"cantly di!erent from Ota et al.'s (1989) Th}U age of 71$6 ka. Sample SRL3, from beneath Kawakawa and Okaia tephras, gave an optical age of 21$8 ka. The large analytical uncertainty in this age is mainly due to the relatively high degree of scatter in the dose}response data. This may have resulted from a slight darkening of the aliquots during the preheat treatment. The reason for such a colour change is presently unknown, but it has been observed before with similar malign results (e.g., Lian and Huntley, 1999). Sample SRL3 directly underlies Okaia tephra, and directly overlies Tahuna tephra. Okaia and Tahuna tephras have not been directly dated, but based on the information available in the literature (e.g., stratigraphic relationships, degree of paleosol development, and limiting radiocarbon ages), Froggatt and Lowe's (1990) best estimate for the age of Okaia and Tahuna tephras were ca. 23.5 and 43 ka, respectively. These age assignments are consistent with the optical age for sample SRL3. However, samples SRL2 and WRL2, both of which underlie Tahuna tephra, gave optical ages of 30$2 and 34$3 ka, respectively. This suggests that Tahuna tephra may be younger than ca. 35 ka, if our analytical uncertainties are taken into account at 2p. This conclusion also holds for Tihoi tephra, which previously has been estimated to be ca. 46 ka (e.g., Froggatt and Lowe, 1990). Hauparu tephra overlies Tahuna tephra at proximal sites around Okataina Centre, and the former has two reported radiocarbon ages associated with it: 39,000$5600 yr BP (NZ-3404) and 35,700$1300 yr BP (NZ-3405) (McGlone et al. 1984; Froggatt and Lowe, 1990), both of which are near the limit of conventional radiocarbon dating. Moreover, both these ages were determined from organic-rich mud that is prone to contamination with older material. More reliable ages come from Mangaone tephra, which stratigraphically overlies Hauparu tephra in the region. Mangaone tephra has several associated radiocarbon ages from soil, peat, and charcoal. Seven of these have been listed as `optimala by Froggatt and Lowe (1990, p. 107), and these range from 26,100$800 yr BP (NZ-1556) to 35,300$2200 yr BP (NZ-1812), and give an average radiocarbon age of about 30$2 ka. This, together with our optical ages, suggests that both Tahuna and Tihoi tephras were instead deposited between ca. 25 and 35 ka.
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7. Conclusions New optical age data suggest that the widespread Rotoehu tephra was deposited at 44$3 ka. There is good agreement between our optical age for Kawakawa tephra (22$2 ka) and its accepted age based on radiocarbon dating, and this supports our laboratory methods. The new optical age for Rotoehu tephra is in agreement with previous ages deduced from several other methods, but is signi"cantly younger than the currently preferred K}Ar whole-rock age of 64$4 ka which was derived from averaging di!erent lava #ow events. However, our optical ages are consistent with these data if the K}Ar ages for the individual events are considered separately. A younger post-60 ka age is also concordant with the occurrence of Rotoehu tephra in a sequence containing paleoecological information that is indicative of cold interstadial climate, consistent with OIS 3 (McGlone et al., 1984). Our optical ages also suggest that Tahuna and Tihoi tephras were deposited at some time between 25 and 35 ka. A mechanism that could potentially lead to our ages being too young is anomalous fading that was too subtle to be detected by our long-term storage experiments, which had analytical uncertainties between 1.6% (sample SRL1) and 5.3% (sample WRL1) (Table 2). However, our optical age for sample SRL4, collected beneath the welldated Kawakawa tephra, suggests that this potential e!ect was not a signi"cant source of error. It is also possible that our ages are underestimates because of the downward translocation of younger grains, as postulated by Berger et al. (1994) for some of their sites. But we did not observe evidence of signi"cant disturbance (e.g., solution pipes) in the loess/paleosol or tephra beds directly overlying our samples. More work is needed to re"ne the chronology for Rotoehu tephra and the other important marker beds found in the TVZ, and beyond. Single-crystal or stepheated Ar}Ar ages from the lava #ows bracketing Rotoehu tephra on Mayor Island would be useful to evaluate whether the lavas contain excess Ar and to test their homogeneity. Finally, the chronology of pre-30 ka tephra beds from Taupo and Okataina centres may have to be revised considering the optical ages associated with Tahuna and Tihoi tephras at our sites. This may have implications for eruption frequency, the chronologies of loess-paleosol sequences, marine and alluvial terrace chronologies, and palynological studies in New Zealand.
Acknowledgements We would like to thank D.J. Huntley for the use of his portable c-ray spectrometer and for discussion in the
"eld. Ningsheng Wang is thanked for careful preparation of the samples, and P.C. Froggatt kindly provided glass reference data for Tahuna and Okaia tephras. Optical dating was performed at the Luminescence Dating Laboratory, School of Earth Sciences, Victoria University of Wellington, while tephra analyses were undertaken in the Department of Geology, University of Auckland. D.J. Huntley and R.G. Roberts critically reviewed an earlier version of the manuscript and contributed to its improvement, while P.C. Froggatt and two anonymous journal reviewers provided valuable comments. This research was funded in part by a VUW internal research grant to OBL and a New Zealand Foundation for Research Science and Technology Fellowship to PAS. The Victoria University of Wellington Luminescence Dating Laboratory is supported in part by Public Good Science Fund (NZ) grant VIC808. References Adamiec, G., Aitken, M.J., 1998. Dose-rate conversion factors: update. Ancient TL 16, 37}50. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Bard, E., Arnold, M., Fairbanks, R.G., Hamelin, B., 1993. Th}U and C ages obtained by mass spectrometry on corals. Radiocarbon 35, 191}199. Berger, G.W., 1995. Progress in luminescence dating methods for Quaternary sediments. In: Rutter, N.W. Catto, N.R. (Eds.), Dating Methods for Quaternary Deposits, Vol. 2. Geological Association of Canada, GEOtext, pp. 81}104. Berger, G.W., Mahaney, W.C., 1990. Tests of thermoluminescence dating of buried soils from Mt. Kenya, Kenya. Sedimentary Geology 66, 45}56. Berger, G.W., Pillans, B.J., Palmer, A.S., 1994. Tests of thermoluminescence dating of loess from New Zealand and Alaska. Quaternary Science Reviews 13, 309}333. Berryman, K.R., 1992. A stratigraphic age of Rotoehu Ash and late Pleistocene climate interpretation based on marine terrace chronology, Mahia Peninsula, North Island, New Zealand. New Zealand Journal of Geology and Geophysics 35, 1}7. B+tter-Jensen, L., 1997. Luminescence techniques: instrumentation and methods. Radiation Measurements 27, 749}768. Buhay, W.M., Cli!ord, P.M., Schwarcz, H.P., 1992. ESR dating of the Rotoiti Breccia in the Taupo Volcanic Zone. New Zealand. Quaternary Science Reviews 11, 267}271. Divigalpitiya, W.M.R., 1982. Thermoluminescence dating of sediments. M.Sc. Thesis, Simon Fraser University, Burnaby, 93 pp. Duller, G.A.T., 1994. Luminescence dating using feldspars: a test case from southern North Island. New Zealand. Quaternary Geochronology (Quaternary Science Reviews) 13, 423}427. Forman, S.L., Jackson, M.E., McCalpin, J., Matt, P., 1988. The potential of using thermoluminescence to date buried soils developed on colluvial and #uvial sediments from Utah and Colorado. U.S.A. * preliminary results. Quaternary Science Reviews 7, 287}293. Froggatt, P.C., Lowe, D.J., 1990. A review of late Quaternary silicic and some other tephra formations from New Zealand: their stratigraphy, nomenclature, distribution, volume, and age. New Zealand Journal of Geology and Geophysics 33, 89}109. Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H., Olley, J.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia. Part I: experimental design and statistical models. Archaeometry 41, 339}364.
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Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiation Measurements 27, 769}817. Wintle, A.G., Catt, J.A., 1985. Thermoluminescence dating of soil developed in Late Devensian loess at Pegwell Bay. Kent. Journal of Soil Science 36, 293}298.