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Distinction between the Youngest Toba Tuff and Oldest Toba Tuff from northern Sumatra based on the area density of spontaneous fission tracks in their glass shards
Quaternary Research 82 (2014) 388–393
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Distinction between the Youngest Toba Tuff and Oldest Toba Tuff from northern Sumatra based on the area density of spontaneous fission tracks in their glass shards John A. Westgate a,⁎, Nicholas J.G. Pearce b, Emma Gatti c, Hema Achyuthan d a
Department of Earth Sciences, University of Toronto, Toronto M5S 3B1, Ontario, Canada Department of Geography and Earth Science, Aberystwyth University, Wales SY23 3DB. UK c NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91106, USA d Department of Geology, Anna University, Chennai, India b
a r t i c l e
i n f o
Article history: Received 28 April 2014 Available online 5 August 2014 Keywords: Volcanic glass shards Spontaneous fission tracks Partial track fading Toba tuffs Acheulean artifacts Sumatra India
a b s t r a c t Determination of the area density of spontaneous fission tracks (ρs) in glass shards of Toba tephra is a reliable way to distinguish between the Youngest Toba Tuff (YTT) and the Oldest Toba Tuff (OTT). The ρs values for YTT, uncorrected for partial track fading, range from 70 to 181 tracks/cm2 with a weighted mean of 108 ± 5 tracks/cm2, based on 15 samples. Corrected ρs values for YTT are in the range of 77–140 tracks/cm2 with a weighted mean of 113 ± 8 tracks/cm2, within the range of uncorrected ρs values. No significant difference in ρs exists between YTT samples collected from marine and continental depositional settings. The uncorrected ρ s for OTT is 1567 ± 114 tracks/cm2 so that confusion with YTT is unlikely. The ρs values of the Toba tephra at Bori, Morgaon, and Gandhigram in northwestern India indicate a YTT identity, in agreement with geochemical data on their glass shards, the presence of multiple glass populations, and a glass fission-track age determination. Therefore, the view of others that OTT is present at these sites – and thereby indicates an early Pleistocene age for the associated Acheulean artifacts – is incorrect. © 2014 University of Washington. Published by Elsevier Inc. All rights reserved.
Introduction Violent volcanic eruptions have characterized the Toba caldera complex of northern Sumatra during the Quaternary (Chesner, 2012) resulting in extensive tephra deposits across Sumatra, India, Indian Ocean, Arabian Sea, East Africa, Malaysia, and the South China Sea (Fig. 1). Three major and widespread tephra sequences have been identified: the Youngest Toba Tuff (YTT, 75 ka), the Middle Toba Tuff (MTT, ~ 500 ka), and the Oldest Toba Tuff (OTT, ~800 ka) (Diehl et al., 1987; Chesner et al., 1991; Dehn et al., 1991; Hall and Farrell, 1995; Lee et al., 2004; Mark et al., 2014). The multidisciplinary benefits offered by these tephra deposits have long been appreciated (Acharyya and Basu, 1993) and vividly demonstrated more recently in a volume on applications of the YTT to problems in the Quaternary sciences (Petraglia et al., 2012). A prerequisite to the successful stratigraphic use of the Toba tuffs is an ability to recognize with confidence each of the three tuffs, YTT, MTT, and OTT. Acharyya and Basu (1993) argued that the numerous tephra occurrences across India are YTT based on the morphology of the glass ⁎ Corresponding author. E-mail address: [email protected] (J.A. Westgate).
http://dx.doi.org/10.1016/j.yqres.2014.07.001 0033-5894/© 2014 University of Washington. Published by Elsevier Inc. All rights reserved.
shards and chemical composition of bulk tephra and glass separates. They noted a strong similarity to YTT deposits in sediments of the Indian Ocean (Layer A, ODP 758, Dehn et al., 1991), Bay of Bengal (Ninkovitch et al., 1978), the youngest welded tuff at Siguragura (Fig. 1A), and tephra in Malaysia at the Kota Tampan site in the Lenggong Valley (near site 6 on Fig. 1A). The major and trace element composition of glass shards from many Toba tephra occurrences, as determined by using an electron microprobe (EMP) and solution ICPMS, respectively, led Westgate et al. (1998) to the same conclusion. This study showed that the major element composition of glass in YTT and MTT was very similar, but YTT had slightly higher average values for K2O and CaO, both units being readily distinguished on a CaO–Na2O– K2O ternary plot. These authors correlated the OTT to Layer E of the ODP 758 sedimentary core in the Indian Ocean (Fig. 1), observing a clear difference in glass composition with YTT, but later work showed that Layer D was more likely correlative, in which case separation of YTT from OTT on the basis of major elements in their glass shards was not possible (Chen et al., 2004; Lee et al., 2004). Smith et al. (2011) examined all the glass compositional data on the Toba tuffs and concluded that it was not possible to recognize each of the three Toba tuff units in this way, although the caveat was added that some of the compositional variation may be due to varied calibration strategies on the different
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Figure 1. Location of the Youngest Toba Tuff (YTT) and Oldest Toba Tuff OTT) samples. (A) Toba caldera and sampled sites; hatched area is Lake Toba. (B) Distribution of YTT and OTT tephra-fall deposits; black dots indicate locations of samples used in this study whereas circles show other Toba tephra sites. Additional data are given in Table 1. Arrow indicates north. Modified after Westgate et al. (2013). The YTT occurrence in sediments of Lake Malawi, East Africa, is not shown on this map (Lane et al., 2013).
EMP instruments. Instead, these authors discovered that YTT can be separated from OTT and MTT by its biotite composition, which has lower values of FeOt/MgO. In addition, OTT biotite crystals are relatively enriched in F, although there is overlap with YTT and MTT at lower concentrations. A comprehensive study on the composition of glass shards in the Toba tuffs was prompted by Smith et al.'s (2011) work (Westgate et al., 2013). The major and trace element compositions of glass shards in YTT, MTT, and OTT were determined, respectively, using an EMP and LA-ICP-MS system with the same shards being analyzed on both instruments to give a detailed compositional signature for each glass shard. All three Toba tuffs can be distinguished, based largely on the trace element composition of their glass shards. Four primary glass populations were found to be present in YTT, well defined by their Sr, Ba, and Y concentrations, whereas MTT and OTT have a single, distinctive glass population. Despite this large body of data on the characterization of the three Toba tuffs, differences of opinion still exist on the identity of the Toba tuff at some sites across India — mainly at sites with artifact-bearing
sediments (e.g. Mishra et al., 1995, 2009; Sangode et al., 2007; Gaillard et al., 2010). At least, this was true as of 2011, when Westaway et al. (2011) published their work. The essence of the controversy is the age of the artifacts as indicated by the associated Toba tephra bed. Is it YTT or OTT? These tephra beds differ in age by an order of magnitude, so recognition of the tephra is crucial to interpretation of the archeology. In an effort to resolve these conflicting views, we add a physical property of glass to the characterization/identification criteria, namely, the area density of spontaneous fission tracks. Fission tracks in glass are revealed by etching in hydrofluoric acid. They are conical pits formed by the preferential solution of glass along a latent fission track, which, in turn, is formed by the spontaneous fission of 238U or induced fission of 235U. Shapes, as seen under the optical microscope, vary from circular to elliptical, depending on the orientation of the cone axis with respect to the polished surface (Fig. 2). It should be an easy task to separate YTT from OTT based on the area density of spontaneous fission tracks in their glass shards because of the large difference in age and the broadly similar U contents, which are in
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Figure 2. Fission tracks in a glass shard of the Youngest Toba Tuff. Size of larger tracks is in the range of 6–8 μm. Lines on the glass shard have been produced by polishing to indicate an internal surface.
the range of 6–10 ppm (Westgate et al., 2013). More specifically, OTT glass has a U content of 9.17 ± 1.08 ppm (n = 24), and U in the YTT glass ranges from 9.97 ± 1.44 (Population I, n = 355) to 5.30 ± 0.66 ppm (Population IV, n = 100). After detailing the methods used in this investigation, we present the data on area density of spontaneous fission tracks (ρs), comment on the complicating factor of partial track fading (PTF) and its correction, and finally assess the usefulness of this new parameter in distinguishing between YTT and OTT.
points on glass is monitored. A point on glass is recorded when the center of the field of view, as indicated by cross-hairs in the eyepiece, is on glass as opposed to epoxy. An accurate value of ρs is achieved when this ratio value has stabilized with the associated error decreasing as more fission tracks and points on glass are counted. Correction for partial track fading (PTF) requires measurement of fission track “diameters,” which is done at 1000× magnification using an image analysis system. The diameter is measured for fission tracks with a circular shape, otherwise, the major axis of the ellipse is measured (Fig. 2). The correction procedure used in this report is taken from the experimental studies of Sandhu and Westgate (1995), who demonstrated a near 1:1 relationship between ρ/ρo and D/Do, where ρ and D are the measured area density and mean track diameter, respectively, and ρo and Do are the corresponding original values. If the sample is etched to give Do (=Di, the average diameter of induced fission tracks) in the range of 6–8 μm, the corrected spontaneous track area density (ρsc) is given by: ρsc ≈ ρs
Di Ds
In other words, this method of correction for PTF requires irradiation of an aliquot of the sample and its magnitude is given by the product of the measured area density of spontaneous tracks and the ratio value of the mean diameter of induced tracks over the mean diameter of spontaneous fission tracks (Ds). Area density of spontaneous fission tracks
Methods Determination of ρs required separation of the larger glass shards and mineral grains from the bulk tephra sample by sieving. Typically, the 0.25–0.125 mm (2–3φ units) size fraction was collected, but, in some cases, the 0.125–0.105 mm (3–3.25φ units) size fraction was required. The sample was then cleaned in an ultrasonic bath, washed in water, dried and resieved. Glass shards in each sample were separated from pumiceous pyroclasts and minerals with a Frantz Isodynamic Magnetic Separator, although heavy liquids were sometimes used for this purpose. The glass shards from each sample were then separately mounted in epoxy on glass slides and left to harden for at least three days. Polishing was done with a 0.3 μm alumina powder mixed with water, followed by a 6 μm diamond paste to give linear scratches on the surface of the glass shards to indicate an internal surface (i.e. 4π geometry) (Fig. 2). In other words, the fission tracks in the polished surface are the result of fission events from above and below that surface, the spontaneous tracks being the result of fission of 238U and the induced tracks by the fission of 235U during irradiation. Only fission tracks on an internal surface were counted (Wagner and Van den haute, 1992). Samples to be irradiated were packed in an aluminum can together with the NIST SRM 612 glass dosimeter and sent to the McMaster Nuclear Reactor at Hamilton, Ontario. All slides of a given sample were etched together in 24% HF in the same beaker at the same time, temperature and duration of etch being recorded. If the samples contained slides with glass shards that had been irradiated, that is, to give induced fission tracks, these were included in the same beaker so that both spontaneous and induced fission tracks were produced under exactly the same etching conditions. The etch time used was that which would give an average track diameter in the range of 6–8 μm. Fission tracks were counted using an optical microscope at 500 × magnification. Two methods can be used to determine their area density; an eyepiece graticule can be used if the glass shards are larger than 125 μm, but the point-counting technique is necessary for smaller shards (Naeser et al., 1982), and it was this latter method that was used for all the samples of Toba tephra analyzed in this study. During the point-counting operation the ratio of the number of tracks counted to
A very clear difference in the ρs of YTT and OTT is to be expected given their 40Ar/39Ar ages (Hall and Farrell, 1995; Mark et al., 2014). Table 1 shows this to be true. The ρs of OTT is more than an order of magnitude greater than that of the YTT samples, identified as such on the basis of their glass chemistry (Westgate et al., 2013). We were able to analyze only one sample of OTT, a distal occurrence in deepsea sediments of the Indian Ocean at site ODP 758 (Dehn et al., 1991). Several vitrophyre samples from the Toba caldera complex were examined, courtesy of C. Chesner, but they were devitrified, and OTT samples from other sites in the Indian Ocean, kindly sent to us by J. Pattan, were too small for determination of ρs (Pattan et al., 2010). One of the YTT samples comes from the same sedimentary core in the Indian Ocean that contained the OTT; all other YTT samples come from terrestrial sites at the caldera in northern Sumatra, Malaysia, and India. The YTT ρs values uncorrected for PTF range from 70 to 181 tracks/ cm2 and have a weighted mean and error of 108 ± 5 tracks/cm2, based on 15 samples (Table 1). No significant difference in ρs exists between samples collected from marine and terrestrial depositional settings. The uncorrected ρs of OTT is 1567 ± 114 tracks/cm2 so that confusion with YTT is unlikely when this parameter is known. Two localities in northwestern India are of special interest because of the association of Toba tuff with artifact-bearing sediments. They are the Bori site, first described by Kale et al. (1986) and shortly after by Korisettar et al. (1988), and the Morgaon site, described in detail by Mishra et al. (2009) (Fig. 1, Table 1). The tephra samples collected at both sites come from the same beds that were sampled by Westaway et al. (2011), and their uncorrected ρs values fall within the range of the YTT samples (Table 1). Luminescence dating studies at these two sites demonstrate that the Toba tephra has been reworked into its present stratigraphic position – events that occurred less than 24 ka at Bori – in accord with the earlier work of Horn et al. (1993) — and less than 37 ka at Morgaon (Biswas et al., 2013). Partial track fading Temperature is the dominant factor influencing the stability of latent fission tracks in natural glasses (Fleischer et al., 1965). With progressive
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Table 1 Area density of spontaneous fission tracks in glass shards of the Toba tuffs. Site number on Figure 1
Tephra bed
Sample number
Location name
Latitude, Longitude
Spontaneous track density, ρs,uncorrected t/cm2 ± 1σ
Spontaneous track density, ρs,corrected t/cm2 ± 1σ
Indian Ocean 1 1
OTT YTT
UT2059 UT1363
ODP site 758 ODP site 758
5°23.05′N, 90°21.67′E 5°23.05′N, 90°21.67′E
1567 ± 114 123 ± 34
nd nd
Toba Caldera 2 2 * 3 4 5 5
YTT YTT YTT YTT YTT YTT YTT
UT2319 UT2319 UT2315 UT2326 UT2328 UT2329 UT2327
Prapat Prapat Toba caldera Sipisupisu Siguragura Harranggaol Harranggaol
2°41.82′N 98°55.23′E 2°41.82′N 98°55.23′E – 2°55.01′N, 98°31.35′E 2°31.34′N, 99°16.24′E 2°53.88′N, 98°39.67′E 2°53.18′N, 98°39.69′E
127 135 101 109 125 181 158
Malaysia 6 7
YTT YTT
UT2317 UT2316
Gelok, Lenggong Valley Padang Terap
5°7.57′N, 100°59.36′E 6°19.87′N, 100°40.63′E
112 ± 11 93 ± 13
140 ± 13 (20%) nd
India 8 9 10 11 12
YTT YTT YTT YTT YTT
UT1070 UT1072 UT2387 UT2113 UT1069
Gandhigram Place, Purna Valley Bori, Kukdi Valley, Pune Morgaon, Karha Valley Jwalapuram, Jurreru Valley Pawlaghat Place, Narmada Valley
20°22.18′N, 77°0.87′E 19°06.77′N, 74°05.81′E 18°16.83′N, 74°19.26′E 15°19.33′N, 78°08.02′E 23°6.88′N, 79°23.17′E
70 107 91 79 120
77 ± 15 (9%) 135 ± 31 (21%) nd 103 ± 20 (17%) nd
± ± ± ± ± ± ±
± ± ± ± ±
35 18 18 26 21 23 21
14 25 19 15 22
nd nd nd nd nd nd nd
Notes: OTT is the Oldest Toba Tuff and YTT is the Youngest Toba Tuff; identity based on number and composition of glass populations (Westgate et al., 2013) with the exception of the Morgaon sample, which is based on the value of ρs. ODP is “Ocean Drilling Program.” Asterick in first column indicates precise location is unknown. Value in brackets of last column is percentage reduction of mean diameter of spontaneous tracks (see Fig. 3). Nd is “not determined” because an aliquot of sample was not irradiated (see text). The weighted mean and error of uncorrected values for YTT ρs is 108 ± 5 tracks/cm2 and that for the corrected values is 113 ± 8 tracks/cm2.
heating, latent tracks shorten and result in a reduced area track density, giving an age that is too young. Even at ambient surface temperatures, PTF takes place (Naeser et al., 1980). Therefore, all glass fission-track ages must be corrected for PTF. Examples of two tephra beds that have experienced PTF are shown in Figure 3; they come from sites 9 and 11, the Bori and Jwalapuram localitites, respectively (Fig. 1, Table 1). Their spontaneous tracks are distinctly smaller than the induced tracks. Several procedures have been developed to correct for PTF (Storzer and Wagner, 1969; Arias et al., 1981). In addition to the diameter correction method used in this study, there are a number of time — temperature combinations that can be used to correct for PTF
(Westgate et al., 2007), although conditions should be such as to minimize structural damage to the glass because of potential negative effects on its etching characteristics, a long time–low temperature combination being preferred. Four YTT ρs values, including that of the Toba tephra from the important Bori site, were corrected for PTF (Table 1). Each sample shows an increase in ρs over the uncorrected value, as expected, but the difference is small. Indeed, the range in corrected ρs values is within that for the uncorrected ρs values at the 1 σ level, specifically, 77 to 140 tracks/cm2 with a weighted mean and error of 113 ± 8 tracks/cm2. The concomitant reduction in Ds is in the range of 9%–21% (Table 1). In an effort to assess the reliability of the corrected ρs values listed in Table 1, we compare these data with that of a larger dataset, including tephra beds from eleven different countries, spanning tropical to arctic environments and ranging in age from late Pleistocene to middle Eocene (Table 2). The Di/Ds values range from 1.41 (early Pleistocene Opoli tephra, Iceland) to 1.10 (late Pleistocene Dominion Creek tephra, Yukon, Canada) with the mean and error of 50 samples being 1.22 ± 0.01, which gives an average percentage reduction of Ds of 18 ± 5% with a range of 9–29%. There is no correlation between age and magnitude of this correction (Table 2). The YTT samples fall within the spread of these data and support the view that the percentage reduction in Ds for YTT is likely not much greater than ~20%. More generally, under near-surface conditions and away from geothermal areas and sites proximal to younger igneous bodies, hydrated glass shards in tephra beds can survive with their latent fission tracks over time periods as long as 40 Ma — an ability that has probably not been fully appreciated previously. Discussion
Figure 3. Partial fading of fission tracks in glass shards of the Youngest Toba Tuff illustrated by size–frequency curves of samples from Bori (solid line) and Jwalapuram, India (dash-dot line). The mean diameter of spontaneous fission tracks (Ds) is smaller than that of the induced (Di). Specific data are as follows: Bori sample (UT1072), Ds = 5.59 μm, (n = 26), Di = 7.04 μm, (n = 255); Jwalapuram sample (UT2113), Ds = 5.28 μm, (n = 26), Di = 6.86 μm, (n = 253). Etching conditions were the same for both samples: etchant, 24% HF; duration of etching, 120 s; temperature, 24°C.
The possibility that extensive PTF has occurred in OTT samples to give ρs values comparable to those of YTT is very unlikely because of the very large difference in their uncorrected ρs (Table 1) and the observation based on a large dataset that PTF has reduced Ds by no more than 30% (Table 2). The uncorrected ρs values of YTT samples from India are very similar to that of the marine occurrence at the ODP 758 site, and
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Table 2 Partial fading of fission tracks in hydrated, silicic glass shards of some Cenozoic tephra beds. Sample number
Tephra name
Country where sample collected
Etching conditions HF:temperature:time (%:°C:s)
Mean diameter of spontaneous tracks, Ds (μm)
Mean diameter of induced tracks, Di (μm)
Reduction in Ds (%)
Di/ Ds
UT1094 UT1366 UT1366 UT1366 UT1366 UT1366 UT1366 UT1366 UT1418 UT1620 UT1545 UT19 UT1657 UT1622 UT1001 UT1624 UT1553 UT1636 UT1456 UT802 UT1786 UT1791 UT1163 UT86 UT1865 UT622 UT2034 UT2035 UT2115 UT2114 UT2181 UT2151 UT2183 UT2182 MR00-K05 UT2207 UT2210 UT2282 UT1395 UT2301 UT1396 UT1875 T7 T6 UT2273 UT2030 UT2317 UT2113 UT1070 UT1072
Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Maninjau Ignimbrite Mozume Roccastrada glass Mosquito Gulch Opoli unnamed Quartz Creek Paradise Hill Dago Hill Flat Creek Dominion Creek GI unnamed Gold Run Ototoka Lost Chicken Lost Chicken Little Timber unnamed unnamed Giraffe Giraffe Hoy-1 Kakuto Hoy-3 Hoy-2 unnamed Hollis Hollis 2 Ambrose Road BT-28 BT-27 BT-24 Chester Bluff unnamed unnamed North Birch Creek Fort Yukon Youngest Toba Tuff Youngest Toba Tuff Youngest Toba Tuff Youngest Toba Tuff
U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. Indonesia Japan Italy Canada Iceland Japan Canada Canada Canada Canada Canada U.S.A. U.S.A. Canada New Zealand U.S.A. U.S.A. Canada Chile Chile Canada Canada U.S.A. Japan U.S.A. U.S.A. Japan Canada Canada U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. Russia Russia U.S.A. U.S.A. Malaysia India India India
24:21:120 24:25:100 24:24:135 24:23:120 24:20:130 24:24:140 24:24:130 24:24:125 26:23:120 25:22:130 25:23:180 26:24:65 24:22:105 25:23:160 24: 22: 110 24:25:90 24:25:60 24:25:95 24:24:120 24:22:75 24:22:60 24:22:110 24:22:90 24:23:105 24:21:90 24:25:70 24: 24: 120 24: 24: 120 24: 24: 125 24: 24: 120 24: 24: 100 24: 24: 170 24: 22: 115 24: 22: 110 24: 23: 120 24: 23: 95 24: 23: 80 24: 22: 100 24: 22: 140 24: 23: 130 24: 25: 115 24: 23: 80 24: 22: 90 24: 22: 75 24: 22: 110 24: 25: 110 24: 22: 120 24: 24: 120 26: 26: 80 24: 24: 120
6.09 5.98 6.34 5.77 5.19 6.85 7.04 6.71 6.38 5.15 6.00 4.40 5.59 5.37 6.01 5.42 5.59 4.92 5.93 5.19 5.55 5.39 6.25 6.43 5.15 4.52 4.85 5.53 5.62 5.03 5.24 6.70 5.44 5.44 5.21 5.63 4.59 5.62 5.57 6.25 6.69 5.25 5.91 5.78 5.78 5.82 6.52 5.28 5.33 5.59
7.54 7.63 7.44 7.20 6.43 8.38 8.50 7.63 7.1 6.86 7.44 5.23 7.87 6.06 7.61 6.18 6.60 6.16 6.54 6.29 6.24 6.24 6.89 7.34 6.03 5.42 6.72 6.92 7.02 6.56 6.78 7.95 7.41 6.63 6.33 6.68 5.37 6.96 6.67 8.64 8.08 6.59 6.81 7.00 7.60 6.90 8.15 6.39 5.85 7.04
19 22 15 20 19 18 17 12 10 25 19 16 29 11 21 12 15 20 9 17 11 14 9 12 15 17 28 20 20 23 23 16 27 18 18 16 15 19 16 28 17 20 13 17 24 16 20 17 9 21
1.24 1.28 1.17 1.25 1.24 1.22 1.21 1.14 1.11 1.34 1.24 1.19 1.41 1.13 1.27 1.14 1.18 1.25 1.10 1.21 1.12 1.16 1.10 1.14 1.17 1.20 1.39 1.25 1.25 1.30 1.29 1.19 1.36 1.22 1.21 1.19 1.17 1.24 1.12 1.38 1.21 1.26 1.15 1.21 1.31 1.19 1.25 1.21 1.1 1.26
± ± ± ± ± ± ± ±
0.07 0.06 0.09 0.05 0.07 0.10 0.12 0.11
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.09 0.13 0.13 0.08 0.07 0.11 0.12 0.14 0.13 0.17 0.12 0.07 0.10 0.16 0.10 0.07 0.10 0.05 0.13 0.09 0.06 0.05 0.10 0.06 0.05 0.20 0.09 0.16 0.12 0.10 0.11 0.13 0.27 0.10 0.19 0.11 0.12 0.16 0.19
± 0.20
± ± ± ± ± ± ± ±
0.09 0.09 0.17 0.08 0.07 0.10 0.12 0.10
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.11 0.10 0.07 0.08 0.07 0.08 0.09 0.08 0.08 0.07 0.09 0.06 0.06 0.10 0.09 0.05 0.06 0.09 0.08 0.10 0.08 0.08 0.10 0.07 0.07 0.08 0.07 0.06 0.11 0.09 0.11 0.11 0.01 0.10 0.11 0.11 0.08 0.12 0.07
± 0.14
Age (Ma)
± ± ± ± ± ± ± ±
0.02 0.02 0.03 0.02 0.02 0.02 0.03 0.02
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.04 0.02 0.04 0.02 0.02 0.03 0.03 0.03 0.04 0.03 0.04 0.02 0.02 0.03 0.02 0.02 0.03 0.02 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.05 0.02 0.04 0.03 0.03 0.03 0.03 0.05 0.03 0.04 0.02 0.03 0.04 0.05
± 0.05
2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 0.05 0.76 2.44 1.38 2.16 1.07 2.82 1.4 3.02 nd 0.08 0.50 2.92 0.72 1.21 2.77 2.77 1.26 3.66 0.12 37.8 37.8 9.19 0.30 9.83 9.05 0.20 0.69 0.63 11.10 5.69 8.91 7.18 0.24 17.2 1.54 10 4.52 0.08 0.08 0.08 0.08
Notes: Glass shards with spontaneous and induced fission tracks were separately mounted in epoxy on glass slides, polished, and then etched under identical conditions in the same beaker at the same time. The percentage reduction in Ds is with respect to the original size of the fission tracks, which is given by Di (Sandhu and Westgate, 1995). The corrected spontaneous fission track density, as shown for YTT in Table 1, is determined by multiplying the measured spontaneous track density by the value of Di/Ds (Sandhu and Westgate, 1995). The standard error is given with the Ds and Di values. The weighted mean ± error for Di/Ds values on 8 samples of Huckleberry Ridge tephra is 1.22 ± 0.01; the corresponding value for the entire dataset is 1.218 ± 0.004. Further details on these samples can be obtained from the corresponding author. Nd is “not determined”.
again show that little fading of fission tracks has taken place in the continental tephra samples, despite the fact that they may have experienced daytime temperatures as high as 43°C (Westaway et al., 2011). Hence, the ρs values for the Toba tephra at Gandhigram, Bori, and Morgaon (Fig. 1, Table 1) indicate a YTT identity, which is compatible with (1) the glass fission-track age of the tephra at Gandhigram (Westgate et al., 1998), (2) the trace element composition of glass shards by solution ICP-MS at Gandhigram and Bori (Westgate et al., 1998), and (3) the multiple glass populations, as indicated by trace element concentrations, in the tephra bed at Gandhigram and Bori (Westgate et al., 2013). A contrary position is taken by Westaway et al. (2011), who believe that OTT is present at these three sites. Their argument is based on glass 40 Ar/39Ar dates, reversely magnetized fluvial sediments (Sangode et al.,
2007), and the presence of associated Acheulean (Lower Paleolithic) artifacts. The 40Ar/39Ar study has been criticized by Mark et al. (2014), who state that it “does not conform to the best practices adopted widely in the 40Ar/39Ar community.” In addition, several factors cast doubt on the reliability of the paleomagnetic study. Samples were taken from fissured clay with calcrete nodules, implying sediment disruption; weathering of titanomagnetites has resulted in overprinting of the original magnetic remanence by CRM (chemical remanence magnetization); sampled sections are less than 3 m in depth but many changes in magnetic polarity are recorded — 4 at Bori, 7 at Morgaon, and 8 at Gandhigram. The authors acknowledge the complexity of the paleomagnetic signals and the difficulty in their interpretation and make the point that more detailed work is required to constrain the magnetostratigraphy. Interestingly, Westaway et al. (2011) accept a
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YTT identity for the tephra bed at Kurnool, Jwalapuram, and Serdang in Malaysia (located just south of site 6 in Fig. 1) but the glass shards in the tephra bed at these sites have the same chemical characteristics as the tephra at Gandhigram, Bori, and Morgaon. In short, on the basis of the evidence at hand, we conclude that the tephra bed at Bori and Morgaon cannot be used to support a Lower Pleistocene age for the Acheulean artifacts found there, a view shared by Petraglia (2011). However, there is a need for further tephrochronological/ geochronological studies in northwestern India. In particular, attempts should be made to recover sanidine and biotite from the tephra beds for 40Ar/39Ar dating and geochemical characterization – a study comparable to that of Mark et al. (2014). Conclusions Determination of ρs in glass shards of Toba tephra beds is a reliable way to distinguish between YTT and OTT because of their large age difference, similar U contents, and the clearly defined nature of the fission tracks due in large part to the isotropic property of glass. Fears that PTF could negate the use of this method of discrimination are unfounded. Uncorrected ρs values of YTT in both marine and continental settings are very similar and their corrected values are within the range of uncorrected ρs determinations. Data at hand suggest that reduction of Ds is unlikely to be greater than 30% — true of numerous tephra beds in a variety of depositional settings and ranging in age from late Pleistocene to middle Eocene. The ρs values of the Toba tephra at Bori, Morgaon, and Gandhigram in northwestern India indicate a YTT identity, a result that is in agreement with published geochemical data on the glass shards, the presence of multiple glass populations, and a glass fission-track age determination. It follows, therefore, that OTT is not present at these sites and so cannot be used to demonstrate an early Pleistocene age for the associated artifacts. Acknowledgments Funds provided by the Natural Sciences and Engineering Research Council of Canada are gratefully acknowledged. We thank Craig Chesner (Eastern Illinois University, U.S.A.) and Jinnappa Pattan (CSIR, National Institute of Oceanography, Goa, India) for samples. Significant improvements to the manuscript were made following reviews by M. D. Petraglia (University of Oxford), J. Dodson (Associate Editor), and an anonymous reviewer. References Acharyya, S.K., Basu, P.K., 1993. Toba ash on the Indian subcontinent and its implications for correlation of Late Pleistocene alluvium. Quaternary Research 40, 10–19. Arias, C., Bigazzi, G., Bonadonna, F.P., 1981. Size corrections and plateau age in glass shards. Nuclear Tracks 5, 129–136. Biswas, R.H., Williams, M.A.J., Raj, R., Juyal, N., Singhvi, A.K., 2013. Methodological studies on luminescence dating of volcanic ashes. Quaternary Geochronology 17, 14–25. Chen, C.-H., Lee, M.-Y., Iizuka, Y., Dehn, J., Wei, K., Carey, S.N., 2004. First Toba supereruption revival: comment and reply. Geology 35, 54–55. Chesner, C.A., 2012. The Toba caldera complex. Quaternary International 258, 5–18. Chesner, C.A., Rose, W.I., Deino, A., Drake, R., Westgate, J.A., 1991. Eruptive history of Earth's largest Quaternary caldera (Toba, Indonesia) clarified. Geology 19, 200–203. Dehn, J., Farrell, J.W., Schmincke, H.-U., 1991. Neogene tephrochronology from Site 758 on northern Ninety East Ridge: Indonesian arc volcanism of the past 5 Ma. In: Weissel, J., Peirce, J., Taylor, E., Alt, J., et al. (Eds.), Proceedings Ocean Drilling Program, Scientific Results, 121, pp. 273–295. Diehl, J.F., Onstott, T.C., Chesner, C.A., Knight, M.D., 1987. No short reversals of Brunhes age recorded in the Toba tuffs, north Sumatra, Indonesia. Geophysical Research Letters 14, 753–756.
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Fleischer, R.L., Price, R.B., Walker, R.M., 1965. Effects of temperature, pressure and ionization on the formation and stability of fission tracks in minerals and glasses. Journal of Geophysical Research 70, 1497–1502. Gaillard, C., Mishra, S., Singh, M., Deo, S., Abbas, R., 2010. Lower and Early Middle Pleistocene Acheulian in the Indian subcontinent. Quaternary International 223–224, 234–241. Hall, C.H., Farrell, J.W., 1995. Laser 40Ar/39Ar ages of tephra from Indian Ocean deep-sea sediments: tie points for the astronomical and geomagnetic polarity time scales. Earth and Planetary Science Letters 133, 327–338. Horn, P., Müller-Sohnius, D., Storzer, D., Zöller, L., 1993. K-Ar, fission-track, and thermoluminescence ages of Quaternary volcanic tuffs and their bearing on Acheulian artifacts from Bori, Kukdi Valley, Pune district, India. Zeitschrift der Deutschen Geologischen Gesellschaft 144, 326–329. Kale, V.S., Ganjoo, R.K., Rajaguru, S.N., Ota, S.B., 1986. Discovery of an Acheulian site at Bori, District Pune. Bulletin of the Deccan College Postgraduate and Research Institute 45, 47–49. Korisettar, R., Mishra, S., Rajaguru, S.N., Gogte, V.D., Ganjoo, R.K., Venkatesan, T.R., Tandon, S.K., Somayajulu, B.L.K., Kale, V.S., 1988. Age of the Bori volcanic ash and Lower Palaeolithic culture of the Kukdi Valley, Maharashtra. Bulletin of the Deccan College Postgraduate and Research Institute 48, 135–138. Lane, C.S., Chorn, B.T., Johnson, T.C., 2013. Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka. Proceedings of the National Academy of Sciences 110, 8025–8029. Lee, M.-Y., Chen, C.-H., Wei, K.-Y., Iizuka, Y., Carey, S., 2004. First Toba supereruption revival. Geology 32, 61–64. Mark, D.F., Petraglia, M., Smith, V.C., Morgan, L.E., Barfod, D.N., Ellis, B.S., Pearce, N.J., Pal, J.N., Korisettar, R., 2014. A high-precision 40Ar/39Ar age for the Young Toba Tuff and dating of ultra-distal tephra: forcing of Quaternary climate and implications for hominin occupation of India. Quaternary Geochronology 21, 90–103. Mishra, S., Venkatesan, T.R., Rajaguru, S.N., Somayajulu, B.L.K., 1995. Earliest Acheulian industry from Peninsula India. Current Anthropology 36, 847–851. Mishra, S., Deo, S.G., Abbas, R., Naik, S., Shete, G., Agrawal, N., Rajaguru, S.N., 2009. Excavations at the early Acheulian site of Morgaon, Maharashtra (2000–2007). In: Paddayya, K., Joglekar, P.P., Basa, K.K., Sawant, R. (Eds.), Recent Research Trends in South Asian Archaeology. Deccan College Postgraduate and Research Institute, Pune, pp. 121–137. Naeser, C.W., Izett, G.A., Obradovich, J.D., 1980. Fission-track and K-Ar ages of natural glasses. United States Geological Survey Bulletin 1489, 31. Naeser, N.D., Westgate, J.A., Hughes, O.T., Péwé, T.L., 1982. Fission-track ages of late Cenozoic distal tephra beds in the Yukon Territory and Alaska. Canadian Journal of Earth Sciences 19, 2167–2178. Ninkovich, D., Shackleton, N.J., Abdel-Monem, A.A., Obradovich, J.D., Izett, G., 1978. K–Ar age of the late Pleistocene eruption of Toba, north Sumatra. Nature 276, 574–577. Pattan, J.N., Prasad, M.S., Babu, E.V.S.S.K., 2010. Correlation of the oldest Toba Tuff to sediments in the central Indian Ocean Basin. Journal of Earth System Science 119, 531–539. Petraglia, M.D., 2011. Assessment and critique of chronometric ages associated with “Early” Acheulean assemblages in the Indian subcontinent. In: Yi, S. (Ed.), Handaxes in the Imjin Basin. Seoul National University Press, Seoul, pp. 157–177. Petraglia, M.D., Korisettar, R., Pal, J.N., 2012. The Toba volcanic super-eruption of 74,000 year ago: climate change, environments, and evolving humans. Quaternary International 25, 1–4. Sandhu, A.S., Westgate, J.A., 1995. The correlation between reduction in fission-track diameter and areal track density in volcanic glass shards and its application in dating tephra beds. Earth and Planetary Science Letters 131, 289–299. Sangode, S.J., Mishra, S., Naik, S., Deo, S., 2007. Magnetostratigraphy of the Quaternary sediments associated with some Toba tephra and Acheulian artefact bearing localities in the western and central India. Gondwana Geological Magazine Special volume 10, 111–121. Smith, V.C., Pearce, N.J.G., Matthews, N.E., Westgate, J.A., Petraglia, M.D., Haslam, M., Lane, C.S., Korisettar, R., Pal, J.N., 2011. Geochemical fingerprinting of the widespread Toba tephra using biotite compositions. Quaternary International 246, 97–104. Storzer, D., Wagner, G.A., 1969. Correction of thermally lowered fission-track ages of tektites. Earth and Planetary Science Letters 5, 463–468. Wagner, G.A., Van den haute, P., 1992. Fission-Track Dating. Ferdinand Enke Verlag, Stuttgart (285 pp.). Westaway, R., Mishra, S., Deo, S., Bridgland, D.R., 2011. Methods for determination of the age of Pleistocene tephra, derived from eruption of Toba, in central India. Journal of Earth System Science 120, 503–530. Westgate, J.A., Shane, P.A.R., Pearce, N.J.G., Perkins, W.T., Korisettar, R., Chesner, C.A., Williams, M.A.J., 1998. All Toba tephra occurrences across Peninsular India belong to the 75,000 yr B.P. eruption. Quaternary Research 50, 107–112. Westgate, J.A., Naeser, N.D., Alloway, B.V., 2007. Fission-track dating. In: Elias, S. (Ed.), Encyclopedia of Quaternary Science. Elsevier, Amsterdam, pp. 651–672. Westgate, J.A., Pearce, N.J.G., Perkins, W.T., Preece, S.J., Chesner, C.A., Muhammad, R.F., 2013. Tephrochronology of the Toba tuffs: four primary glass populations define the 75-ka Youngest Toba Tuff, northern Sumatra, Indonesia. Journal of Quaternary Science 28, 772–776.
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Distinction between the Youngest Toba Tuff and Oldest Toba Tuff from northern Sumatra based on the area density of spontaneous fission tracks in their glass shards John A. Westgate a,⁎, Nicholas J.G. Pearce b, Emma Gatti c, Hema Achyuthan d a
Department of Earth Sciences, University of Toronto, Toronto M5S 3B1, Ontario, Canada Department of Geography and Earth Science, Aberystwyth University, Wales SY23 3DB. UK c NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91106, USA d Department of Geology, Anna University, Chennai, India b
a r t i c l e
i n f o
Article history: Received 28 April 2014 Available online 5 August 2014 Keywords: Volcanic glass shards Spontaneous fission tracks Partial track fading Toba tuffs Acheulean artifacts Sumatra India
a b s t r a c t Determination of the area density of spontaneous fission tracks (ρs) in glass shards of Toba tephra is a reliable way to distinguish between the Youngest Toba Tuff (YTT) and the Oldest Toba Tuff (OTT). The ρs values for YTT, uncorrected for partial track fading, range from 70 to 181 tracks/cm2 with a weighted mean of 108 ± 5 tracks/cm2, based on 15 samples. Corrected ρs values for YTT are in the range of 77–140 tracks/cm2 with a weighted mean of 113 ± 8 tracks/cm2, within the range of uncorrected ρs values. No significant difference in ρs exists between YTT samples collected from marine and continental depositional settings. The uncorrected ρ s for OTT is 1567 ± 114 tracks/cm2 so that confusion with YTT is unlikely. The ρs values of the Toba tephra at Bori, Morgaon, and Gandhigram in northwestern India indicate a YTT identity, in agreement with geochemical data on their glass shards, the presence of multiple glass populations, and a glass fission-track age determination. Therefore, the view of others that OTT is present at these sites – and thereby indicates an early Pleistocene age for the associated Acheulean artifacts – is incorrect. © 2014 University of Washington. Published by Elsevier Inc. All rights reserved.
Introduction Violent volcanic eruptions have characterized the Toba caldera complex of northern Sumatra during the Quaternary (Chesner, 2012) resulting in extensive tephra deposits across Sumatra, India, Indian Ocean, Arabian Sea, East Africa, Malaysia, and the South China Sea (Fig. 1). Three major and widespread tephra sequences have been identified: the Youngest Toba Tuff (YTT, 75 ka), the Middle Toba Tuff (MTT, ~ 500 ka), and the Oldest Toba Tuff (OTT, ~800 ka) (Diehl et al., 1987; Chesner et al., 1991; Dehn et al., 1991; Hall and Farrell, 1995; Lee et al., 2004; Mark et al., 2014). The multidisciplinary benefits offered by these tephra deposits have long been appreciated (Acharyya and Basu, 1993) and vividly demonstrated more recently in a volume on applications of the YTT to problems in the Quaternary sciences (Petraglia et al., 2012). A prerequisite to the successful stratigraphic use of the Toba tuffs is an ability to recognize with confidence each of the three tuffs, YTT, MTT, and OTT. Acharyya and Basu (1993) argued that the numerous tephra occurrences across India are YTT based on the morphology of the glass ⁎ Corresponding author. E-mail address: [email protected] (J.A. Westgate).
http://dx.doi.org/10.1016/j.yqres.2014.07.001 0033-5894/© 2014 University of Washington. Published by Elsevier Inc. All rights reserved.
shards and chemical composition of bulk tephra and glass separates. They noted a strong similarity to YTT deposits in sediments of the Indian Ocean (Layer A, ODP 758, Dehn et al., 1991), Bay of Bengal (Ninkovitch et al., 1978), the youngest welded tuff at Siguragura (Fig. 1A), and tephra in Malaysia at the Kota Tampan site in the Lenggong Valley (near site 6 on Fig. 1A). The major and trace element composition of glass shards from many Toba tephra occurrences, as determined by using an electron microprobe (EMP) and solution ICPMS, respectively, led Westgate et al. (1998) to the same conclusion. This study showed that the major element composition of glass in YTT and MTT was very similar, but YTT had slightly higher average values for K2O and CaO, both units being readily distinguished on a CaO–Na2O– K2O ternary plot. These authors correlated the OTT to Layer E of the ODP 758 sedimentary core in the Indian Ocean (Fig. 1), observing a clear difference in glass composition with YTT, but later work showed that Layer D was more likely correlative, in which case separation of YTT from OTT on the basis of major elements in their glass shards was not possible (Chen et al., 2004; Lee et al., 2004). Smith et al. (2011) examined all the glass compositional data on the Toba tuffs and concluded that it was not possible to recognize each of the three Toba tuff units in this way, although the caveat was added that some of the compositional variation may be due to varied calibration strategies on the different
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Figure 1. Location of the Youngest Toba Tuff (YTT) and Oldest Toba Tuff OTT) samples. (A) Toba caldera and sampled sites; hatched area is Lake Toba. (B) Distribution of YTT and OTT tephra-fall deposits; black dots indicate locations of samples used in this study whereas circles show other Toba tephra sites. Additional data are given in Table 1. Arrow indicates north. Modified after Westgate et al. (2013). The YTT occurrence in sediments of Lake Malawi, East Africa, is not shown on this map (Lane et al., 2013).
EMP instruments. Instead, these authors discovered that YTT can be separated from OTT and MTT by its biotite composition, which has lower values of FeOt/MgO. In addition, OTT biotite crystals are relatively enriched in F, although there is overlap with YTT and MTT at lower concentrations. A comprehensive study on the composition of glass shards in the Toba tuffs was prompted by Smith et al.'s (2011) work (Westgate et al., 2013). The major and trace element compositions of glass shards in YTT, MTT, and OTT were determined, respectively, using an EMP and LA-ICP-MS system with the same shards being analyzed on both instruments to give a detailed compositional signature for each glass shard. All three Toba tuffs can be distinguished, based largely on the trace element composition of their glass shards. Four primary glass populations were found to be present in YTT, well defined by their Sr, Ba, and Y concentrations, whereas MTT and OTT have a single, distinctive glass population. Despite this large body of data on the characterization of the three Toba tuffs, differences of opinion still exist on the identity of the Toba tuff at some sites across India — mainly at sites with artifact-bearing
sediments (e.g. Mishra et al., 1995, 2009; Sangode et al., 2007; Gaillard et al., 2010). At least, this was true as of 2011, when Westaway et al. (2011) published their work. The essence of the controversy is the age of the artifacts as indicated by the associated Toba tephra bed. Is it YTT or OTT? These tephra beds differ in age by an order of magnitude, so recognition of the tephra is crucial to interpretation of the archeology. In an effort to resolve these conflicting views, we add a physical property of glass to the characterization/identification criteria, namely, the area density of spontaneous fission tracks. Fission tracks in glass are revealed by etching in hydrofluoric acid. They are conical pits formed by the preferential solution of glass along a latent fission track, which, in turn, is formed by the spontaneous fission of 238U or induced fission of 235U. Shapes, as seen under the optical microscope, vary from circular to elliptical, depending on the orientation of the cone axis with respect to the polished surface (Fig. 2). It should be an easy task to separate YTT from OTT based on the area density of spontaneous fission tracks in their glass shards because of the large difference in age and the broadly similar U contents, which are in
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Figure 2. Fission tracks in a glass shard of the Youngest Toba Tuff. Size of larger tracks is in the range of 6–8 μm. Lines on the glass shard have been produced by polishing to indicate an internal surface.
the range of 6–10 ppm (Westgate et al., 2013). More specifically, OTT glass has a U content of 9.17 ± 1.08 ppm (n = 24), and U in the YTT glass ranges from 9.97 ± 1.44 (Population I, n = 355) to 5.30 ± 0.66 ppm (Population IV, n = 100). After detailing the methods used in this investigation, we present the data on area density of spontaneous fission tracks (ρs), comment on the complicating factor of partial track fading (PTF) and its correction, and finally assess the usefulness of this new parameter in distinguishing between YTT and OTT.
points on glass is monitored. A point on glass is recorded when the center of the field of view, as indicated by cross-hairs in the eyepiece, is on glass as opposed to epoxy. An accurate value of ρs is achieved when this ratio value has stabilized with the associated error decreasing as more fission tracks and points on glass are counted. Correction for partial track fading (PTF) requires measurement of fission track “diameters,” which is done at 1000× magnification using an image analysis system. The diameter is measured for fission tracks with a circular shape, otherwise, the major axis of the ellipse is measured (Fig. 2). The correction procedure used in this report is taken from the experimental studies of Sandhu and Westgate (1995), who demonstrated a near 1:1 relationship between ρ/ρo and D/Do, where ρ and D are the measured area density and mean track diameter, respectively, and ρo and Do are the corresponding original values. If the sample is etched to give Do (=Di, the average diameter of induced fission tracks) in the range of 6–8 μm, the corrected spontaneous track area density (ρsc) is given by: ρsc ≈ ρs
Di Ds
In other words, this method of correction for PTF requires irradiation of an aliquot of the sample and its magnitude is given by the product of the measured area density of spontaneous tracks and the ratio value of the mean diameter of induced tracks over the mean diameter of spontaneous fission tracks (Ds). Area density of spontaneous fission tracks
Methods Determination of ρs required separation of the larger glass shards and mineral grains from the bulk tephra sample by sieving. Typically, the 0.25–0.125 mm (2–3φ units) size fraction was collected, but, in some cases, the 0.125–0.105 mm (3–3.25φ units) size fraction was required. The sample was then cleaned in an ultrasonic bath, washed in water, dried and resieved. Glass shards in each sample were separated from pumiceous pyroclasts and minerals with a Frantz Isodynamic Magnetic Separator, although heavy liquids were sometimes used for this purpose. The glass shards from each sample were then separately mounted in epoxy on glass slides and left to harden for at least three days. Polishing was done with a 0.3 μm alumina powder mixed with water, followed by a 6 μm diamond paste to give linear scratches on the surface of the glass shards to indicate an internal surface (i.e. 4π geometry) (Fig. 2). In other words, the fission tracks in the polished surface are the result of fission events from above and below that surface, the spontaneous tracks being the result of fission of 238U and the induced tracks by the fission of 235U during irradiation. Only fission tracks on an internal surface were counted (Wagner and Van den haute, 1992). Samples to be irradiated were packed in an aluminum can together with the NIST SRM 612 glass dosimeter and sent to the McMaster Nuclear Reactor at Hamilton, Ontario. All slides of a given sample were etched together in 24% HF in the same beaker at the same time, temperature and duration of etch being recorded. If the samples contained slides with glass shards that had been irradiated, that is, to give induced fission tracks, these were included in the same beaker so that both spontaneous and induced fission tracks were produced under exactly the same etching conditions. The etch time used was that which would give an average track diameter in the range of 6–8 μm. Fission tracks were counted using an optical microscope at 500 × magnification. Two methods can be used to determine their area density; an eyepiece graticule can be used if the glass shards are larger than 125 μm, but the point-counting technique is necessary for smaller shards (Naeser et al., 1982), and it was this latter method that was used for all the samples of Toba tephra analyzed in this study. During the point-counting operation the ratio of the number of tracks counted to
A very clear difference in the ρs of YTT and OTT is to be expected given their 40Ar/39Ar ages (Hall and Farrell, 1995; Mark et al., 2014). Table 1 shows this to be true. The ρs of OTT is more than an order of magnitude greater than that of the YTT samples, identified as such on the basis of their glass chemistry (Westgate et al., 2013). We were able to analyze only one sample of OTT, a distal occurrence in deepsea sediments of the Indian Ocean at site ODP 758 (Dehn et al., 1991). Several vitrophyre samples from the Toba caldera complex were examined, courtesy of C. Chesner, but they were devitrified, and OTT samples from other sites in the Indian Ocean, kindly sent to us by J. Pattan, were too small for determination of ρs (Pattan et al., 2010). One of the YTT samples comes from the same sedimentary core in the Indian Ocean that contained the OTT; all other YTT samples come from terrestrial sites at the caldera in northern Sumatra, Malaysia, and India. The YTT ρs values uncorrected for PTF range from 70 to 181 tracks/ cm2 and have a weighted mean and error of 108 ± 5 tracks/cm2, based on 15 samples (Table 1). No significant difference in ρs exists between samples collected from marine and terrestrial depositional settings. The uncorrected ρs of OTT is 1567 ± 114 tracks/cm2 so that confusion with YTT is unlikely when this parameter is known. Two localities in northwestern India are of special interest because of the association of Toba tuff with artifact-bearing sediments. They are the Bori site, first described by Kale et al. (1986) and shortly after by Korisettar et al. (1988), and the Morgaon site, described in detail by Mishra et al. (2009) (Fig. 1, Table 1). The tephra samples collected at both sites come from the same beds that were sampled by Westaway et al. (2011), and their uncorrected ρs values fall within the range of the YTT samples (Table 1). Luminescence dating studies at these two sites demonstrate that the Toba tephra has been reworked into its present stratigraphic position – events that occurred less than 24 ka at Bori – in accord with the earlier work of Horn et al. (1993) — and less than 37 ka at Morgaon (Biswas et al., 2013). Partial track fading Temperature is the dominant factor influencing the stability of latent fission tracks in natural glasses (Fleischer et al., 1965). With progressive
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Table 1 Area density of spontaneous fission tracks in glass shards of the Toba tuffs. Site number on Figure 1
Tephra bed
Sample number
Location name
Latitude, Longitude
Spontaneous track density, ρs,uncorrected t/cm2 ± 1σ
Spontaneous track density, ρs,corrected t/cm2 ± 1σ
Indian Ocean 1 1
OTT YTT
UT2059 UT1363
ODP site 758 ODP site 758
5°23.05′N, 90°21.67′E 5°23.05′N, 90°21.67′E
1567 ± 114 123 ± 34
nd nd
Toba Caldera 2 2 * 3 4 5 5
YTT YTT YTT YTT YTT YTT YTT
UT2319 UT2319 UT2315 UT2326 UT2328 UT2329 UT2327
Prapat Prapat Toba caldera Sipisupisu Siguragura Harranggaol Harranggaol
2°41.82′N 98°55.23′E 2°41.82′N 98°55.23′E – 2°55.01′N, 98°31.35′E 2°31.34′N, 99°16.24′E 2°53.88′N, 98°39.67′E 2°53.18′N, 98°39.69′E
127 135 101 109 125 181 158
Malaysia 6 7
YTT YTT
UT2317 UT2316
Gelok, Lenggong Valley Padang Terap
5°7.57′N, 100°59.36′E 6°19.87′N, 100°40.63′E
112 ± 11 93 ± 13
140 ± 13 (20%) nd
India 8 9 10 11 12
YTT YTT YTT YTT YTT
UT1070 UT1072 UT2387 UT2113 UT1069
Gandhigram Place, Purna Valley Bori, Kukdi Valley, Pune Morgaon, Karha Valley Jwalapuram, Jurreru Valley Pawlaghat Place, Narmada Valley
20°22.18′N, 77°0.87′E 19°06.77′N, 74°05.81′E 18°16.83′N, 74°19.26′E 15°19.33′N, 78°08.02′E 23°6.88′N, 79°23.17′E
70 107 91 79 120
77 ± 15 (9%) 135 ± 31 (21%) nd 103 ± 20 (17%) nd
± ± ± ± ± ± ±
± ± ± ± ±
35 18 18 26 21 23 21
14 25 19 15 22
nd nd nd nd nd nd nd
Notes: OTT is the Oldest Toba Tuff and YTT is the Youngest Toba Tuff; identity based on number and composition of glass populations (Westgate et al., 2013) with the exception of the Morgaon sample, which is based on the value of ρs. ODP is “Ocean Drilling Program.” Asterick in first column indicates precise location is unknown. Value in brackets of last column is percentage reduction of mean diameter of spontaneous tracks (see Fig. 3). Nd is “not determined” because an aliquot of sample was not irradiated (see text). The weighted mean and error of uncorrected values for YTT ρs is 108 ± 5 tracks/cm2 and that for the corrected values is 113 ± 8 tracks/cm2.
heating, latent tracks shorten and result in a reduced area track density, giving an age that is too young. Even at ambient surface temperatures, PTF takes place (Naeser et al., 1980). Therefore, all glass fission-track ages must be corrected for PTF. Examples of two tephra beds that have experienced PTF are shown in Figure 3; they come from sites 9 and 11, the Bori and Jwalapuram localitites, respectively (Fig. 1, Table 1). Their spontaneous tracks are distinctly smaller than the induced tracks. Several procedures have been developed to correct for PTF (Storzer and Wagner, 1969; Arias et al., 1981). In addition to the diameter correction method used in this study, there are a number of time — temperature combinations that can be used to correct for PTF
(Westgate et al., 2007), although conditions should be such as to minimize structural damage to the glass because of potential negative effects on its etching characteristics, a long time–low temperature combination being preferred. Four YTT ρs values, including that of the Toba tephra from the important Bori site, were corrected for PTF (Table 1). Each sample shows an increase in ρs over the uncorrected value, as expected, but the difference is small. Indeed, the range in corrected ρs values is within that for the uncorrected ρs values at the 1 σ level, specifically, 77 to 140 tracks/cm2 with a weighted mean and error of 113 ± 8 tracks/cm2. The concomitant reduction in Ds is in the range of 9%–21% (Table 1). In an effort to assess the reliability of the corrected ρs values listed in Table 1, we compare these data with that of a larger dataset, including tephra beds from eleven different countries, spanning tropical to arctic environments and ranging in age from late Pleistocene to middle Eocene (Table 2). The Di/Ds values range from 1.41 (early Pleistocene Opoli tephra, Iceland) to 1.10 (late Pleistocene Dominion Creek tephra, Yukon, Canada) with the mean and error of 50 samples being 1.22 ± 0.01, which gives an average percentage reduction of Ds of 18 ± 5% with a range of 9–29%. There is no correlation between age and magnitude of this correction (Table 2). The YTT samples fall within the spread of these data and support the view that the percentage reduction in Ds for YTT is likely not much greater than ~20%. More generally, under near-surface conditions and away from geothermal areas and sites proximal to younger igneous bodies, hydrated glass shards in tephra beds can survive with their latent fission tracks over time periods as long as 40 Ma — an ability that has probably not been fully appreciated previously. Discussion
Figure 3. Partial fading of fission tracks in glass shards of the Youngest Toba Tuff illustrated by size–frequency curves of samples from Bori (solid line) and Jwalapuram, India (dash-dot line). The mean diameter of spontaneous fission tracks (Ds) is smaller than that of the induced (Di). Specific data are as follows: Bori sample (UT1072), Ds = 5.59 μm, (n = 26), Di = 7.04 μm, (n = 255); Jwalapuram sample (UT2113), Ds = 5.28 μm, (n = 26), Di = 6.86 μm, (n = 253). Etching conditions were the same for both samples: etchant, 24% HF; duration of etching, 120 s; temperature, 24°C.
The possibility that extensive PTF has occurred in OTT samples to give ρs values comparable to those of YTT is very unlikely because of the very large difference in their uncorrected ρs (Table 1) and the observation based on a large dataset that PTF has reduced Ds by no more than 30% (Table 2). The uncorrected ρs values of YTT samples from India are very similar to that of the marine occurrence at the ODP 758 site, and
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Table 2 Partial fading of fission tracks in hydrated, silicic glass shards of some Cenozoic tephra beds. Sample number
Tephra name
Country where sample collected
Etching conditions HF:temperature:time (%:°C:s)
Mean diameter of spontaneous tracks, Ds (μm)
Mean diameter of induced tracks, Di (μm)
Reduction in Ds (%)
Di/ Ds
UT1094 UT1366 UT1366 UT1366 UT1366 UT1366 UT1366 UT1366 UT1418 UT1620 UT1545 UT19 UT1657 UT1622 UT1001 UT1624 UT1553 UT1636 UT1456 UT802 UT1786 UT1791 UT1163 UT86 UT1865 UT622 UT2034 UT2035 UT2115 UT2114 UT2181 UT2151 UT2183 UT2182 MR00-K05 UT2207 UT2210 UT2282 UT1395 UT2301 UT1396 UT1875 T7 T6 UT2273 UT2030 UT2317 UT2113 UT1070 UT1072
Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Huckleberry Ridge Maninjau Ignimbrite Mozume Roccastrada glass Mosquito Gulch Opoli unnamed Quartz Creek Paradise Hill Dago Hill Flat Creek Dominion Creek GI unnamed Gold Run Ototoka Lost Chicken Lost Chicken Little Timber unnamed unnamed Giraffe Giraffe Hoy-1 Kakuto Hoy-3 Hoy-2 unnamed Hollis Hollis 2 Ambrose Road BT-28 BT-27 BT-24 Chester Bluff unnamed unnamed North Birch Creek Fort Yukon Youngest Toba Tuff Youngest Toba Tuff Youngest Toba Tuff Youngest Toba Tuff
U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. Indonesia Japan Italy Canada Iceland Japan Canada Canada Canada Canada Canada U.S.A. U.S.A. Canada New Zealand U.S.A. U.S.A. Canada Chile Chile Canada Canada U.S.A. Japan U.S.A. U.S.A. Japan Canada Canada U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. Russia Russia U.S.A. U.S.A. Malaysia India India India
24:21:120 24:25:100 24:24:135 24:23:120 24:20:130 24:24:140 24:24:130 24:24:125 26:23:120 25:22:130 25:23:180 26:24:65 24:22:105 25:23:160 24: 22: 110 24:25:90 24:25:60 24:25:95 24:24:120 24:22:75 24:22:60 24:22:110 24:22:90 24:23:105 24:21:90 24:25:70 24: 24: 120 24: 24: 120 24: 24: 125 24: 24: 120 24: 24: 100 24: 24: 170 24: 22: 115 24: 22: 110 24: 23: 120 24: 23: 95 24: 23: 80 24: 22: 100 24: 22: 140 24: 23: 130 24: 25: 115 24: 23: 80 24: 22: 90 24: 22: 75 24: 22: 110 24: 25: 110 24: 22: 120 24: 24: 120 26: 26: 80 24: 24: 120
6.09 5.98 6.34 5.77 5.19 6.85 7.04 6.71 6.38 5.15 6.00 4.40 5.59 5.37 6.01 5.42 5.59 4.92 5.93 5.19 5.55 5.39 6.25 6.43 5.15 4.52 4.85 5.53 5.62 5.03 5.24 6.70 5.44 5.44 5.21 5.63 4.59 5.62 5.57 6.25 6.69 5.25 5.91 5.78 5.78 5.82 6.52 5.28 5.33 5.59
7.54 7.63 7.44 7.20 6.43 8.38 8.50 7.63 7.1 6.86 7.44 5.23 7.87 6.06 7.61 6.18 6.60 6.16 6.54 6.29 6.24 6.24 6.89 7.34 6.03 5.42 6.72 6.92 7.02 6.56 6.78 7.95 7.41 6.63 6.33 6.68 5.37 6.96 6.67 8.64 8.08 6.59 6.81 7.00 7.60 6.90 8.15 6.39 5.85 7.04
19 22 15 20 19 18 17 12 10 25 19 16 29 11 21 12 15 20 9 17 11 14 9 12 15 17 28 20 20 23 23 16 27 18 18 16 15 19 16 28 17 20 13 17 24 16 20 17 9 21
1.24 1.28 1.17 1.25 1.24 1.22 1.21 1.14 1.11 1.34 1.24 1.19 1.41 1.13 1.27 1.14 1.18 1.25 1.10 1.21 1.12 1.16 1.10 1.14 1.17 1.20 1.39 1.25 1.25 1.30 1.29 1.19 1.36 1.22 1.21 1.19 1.17 1.24 1.12 1.38 1.21 1.26 1.15 1.21 1.31 1.19 1.25 1.21 1.1 1.26
± ± ± ± ± ± ± ±
0.07 0.06 0.09 0.05 0.07 0.10 0.12 0.11
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.09 0.13 0.13 0.08 0.07 0.11 0.12 0.14 0.13 0.17 0.12 0.07 0.10 0.16 0.10 0.07 0.10 0.05 0.13 0.09 0.06 0.05 0.10 0.06 0.05 0.20 0.09 0.16 0.12 0.10 0.11 0.13 0.27 0.10 0.19 0.11 0.12 0.16 0.19
± 0.20
± ± ± ± ± ± ± ±
0.09 0.09 0.17 0.08 0.07 0.10 0.12 0.10
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.11 0.10 0.07 0.08 0.07 0.08 0.09 0.08 0.08 0.07 0.09 0.06 0.06 0.10 0.09 0.05 0.06 0.09 0.08 0.10 0.08 0.08 0.10 0.07 0.07 0.08 0.07 0.06 0.11 0.09 0.11 0.11 0.01 0.10 0.11 0.11 0.08 0.12 0.07
± 0.14
Age (Ma)
± ± ± ± ± ± ± ±
0.02 0.02 0.03 0.02 0.02 0.02 0.03 0.02
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.04 0.02 0.04 0.02 0.02 0.03 0.03 0.03 0.04 0.03 0.04 0.02 0.02 0.03 0.02 0.02 0.03 0.02 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.05 0.02 0.04 0.03 0.03 0.03 0.03 0.05 0.03 0.04 0.02 0.03 0.04 0.05
± 0.05
2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 0.05 0.76 2.44 1.38 2.16 1.07 2.82 1.4 3.02 nd 0.08 0.50 2.92 0.72 1.21 2.77 2.77 1.26 3.66 0.12 37.8 37.8 9.19 0.30 9.83 9.05 0.20 0.69 0.63 11.10 5.69 8.91 7.18 0.24 17.2 1.54 10 4.52 0.08 0.08 0.08 0.08
Notes: Glass shards with spontaneous and induced fission tracks were separately mounted in epoxy on glass slides, polished, and then etched under identical conditions in the same beaker at the same time. The percentage reduction in Ds is with respect to the original size of the fission tracks, which is given by Di (Sandhu and Westgate, 1995). The corrected spontaneous fission track density, as shown for YTT in Table 1, is determined by multiplying the measured spontaneous track density by the value of Di/Ds (Sandhu and Westgate, 1995). The standard error is given with the Ds and Di values. The weighted mean ± error for Di/Ds values on 8 samples of Huckleberry Ridge tephra is 1.22 ± 0.01; the corresponding value for the entire dataset is 1.218 ± 0.004. Further details on these samples can be obtained from the corresponding author. Nd is “not determined”.
again show that little fading of fission tracks has taken place in the continental tephra samples, despite the fact that they may have experienced daytime temperatures as high as 43°C (Westaway et al., 2011). Hence, the ρs values for the Toba tephra at Gandhigram, Bori, and Morgaon (Fig. 1, Table 1) indicate a YTT identity, which is compatible with (1) the glass fission-track age of the tephra at Gandhigram (Westgate et al., 1998), (2) the trace element composition of glass shards by solution ICP-MS at Gandhigram and Bori (Westgate et al., 1998), and (3) the multiple glass populations, as indicated by trace element concentrations, in the tephra bed at Gandhigram and Bori (Westgate et al., 2013). A contrary position is taken by Westaway et al. (2011), who believe that OTT is present at these three sites. Their argument is based on glass 40 Ar/39Ar dates, reversely magnetized fluvial sediments (Sangode et al.,
2007), and the presence of associated Acheulean (Lower Paleolithic) artifacts. The 40Ar/39Ar study has been criticized by Mark et al. (2014), who state that it “does not conform to the best practices adopted widely in the 40Ar/39Ar community.” In addition, several factors cast doubt on the reliability of the paleomagnetic study. Samples were taken from fissured clay with calcrete nodules, implying sediment disruption; weathering of titanomagnetites has resulted in overprinting of the original magnetic remanence by CRM (chemical remanence magnetization); sampled sections are less than 3 m in depth but many changes in magnetic polarity are recorded — 4 at Bori, 7 at Morgaon, and 8 at Gandhigram. The authors acknowledge the complexity of the paleomagnetic signals and the difficulty in their interpretation and make the point that more detailed work is required to constrain the magnetostratigraphy. Interestingly, Westaway et al. (2011) accept a
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YTT identity for the tephra bed at Kurnool, Jwalapuram, and Serdang in Malaysia (located just south of site 6 in Fig. 1) but the glass shards in the tephra bed at these sites have the same chemical characteristics as the tephra at Gandhigram, Bori, and Morgaon. In short, on the basis of the evidence at hand, we conclude that the tephra bed at Bori and Morgaon cannot be used to support a Lower Pleistocene age for the Acheulean artifacts found there, a view shared by Petraglia (2011). However, there is a need for further tephrochronological/ geochronological studies in northwestern India. In particular, attempts should be made to recover sanidine and biotite from the tephra beds for 40Ar/39Ar dating and geochemical characterization – a study comparable to that of Mark et al. (2014). Conclusions Determination of ρs in glass shards of Toba tephra beds is a reliable way to distinguish between YTT and OTT because of their large age difference, similar U contents, and the clearly defined nature of the fission tracks due in large part to the isotropic property of glass. Fears that PTF could negate the use of this method of discrimination are unfounded. Uncorrected ρs values of YTT in both marine and continental settings are very similar and their corrected values are within the range of uncorrected ρs determinations. Data at hand suggest that reduction of Ds is unlikely to be greater than 30% — true of numerous tephra beds in a variety of depositional settings and ranging in age from late Pleistocene to middle Eocene. The ρs values of the Toba tephra at Bori, Morgaon, and Gandhigram in northwestern India indicate a YTT identity, a result that is in agreement with published geochemical data on the glass shards, the presence of multiple glass populations, and a glass fission-track age determination. It follows, therefore, that OTT is not present at these sites and so cannot be used to demonstrate an early Pleistocene age for the associated artifacts. Acknowledgments Funds provided by the Natural Sciences and Engineering Research Council of Canada are gratefully acknowledged. We thank Craig Chesner (Eastern Illinois University, U.S.A.) and Jinnappa Pattan (CSIR, National Institute of Oceanography, Goa, India) for samples. Significant improvements to the manuscript were made following reviews by M. D. Petraglia (University of Oxford), J. Dodson (Associate Editor), and an anonymous reviewer. References Acharyya, S.K., Basu, P.K., 1993. Toba ash on the Indian subcontinent and its implications for correlation of Late Pleistocene alluvium. Quaternary Research 40, 10–19. Arias, C., Bigazzi, G., Bonadonna, F.P., 1981. Size corrections and plateau age in glass shards. Nuclear Tracks 5, 129–136. Biswas, R.H., Williams, M.A.J., Raj, R., Juyal, N., Singhvi, A.K., 2013. Methodological studies on luminescence dating of volcanic ashes. Quaternary Geochronology 17, 14–25. Chen, C.-H., Lee, M.-Y., Iizuka, Y., Dehn, J., Wei, K., Carey, S.N., 2004. First Toba supereruption revival: comment and reply. Geology 35, 54–55. Chesner, C.A., 2012. The Toba caldera complex. Quaternary International 258, 5–18. Chesner, C.A., Rose, W.I., Deino, A., Drake, R., Westgate, J.A., 1991. Eruptive history of Earth's largest Quaternary caldera (Toba, Indonesia) clarified. Geology 19, 200–203. Dehn, J., Farrell, J.W., Schmincke, H.-U., 1991. Neogene tephrochronology from Site 758 on northern Ninety East Ridge: Indonesian arc volcanism of the past 5 Ma. In: Weissel, J., Peirce, J., Taylor, E., Alt, J., et al. (Eds.), Proceedings Ocean Drilling Program, Scientific Results, 121, pp. 273–295. Diehl, J.F., Onstott, T.C., Chesner, C.A., Knight, M.D., 1987. No short reversals of Brunhes age recorded in the Toba tuffs, north Sumatra, Indonesia. Geophysical Research Letters 14, 753–756.
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