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All Toba Tephra Occurrences across Peninsular India Belong to the 75,000 yr B.P. Eruption
QUATERNARY RESEARCH ARTICLE NO.
50, 107–112 (1998)
QR981974
All Toba Tephra Occurrences across Peninsular India Belong to the 75,000 yr B.P. Eruption John A. Westgate Physical Sciences Division, University of Toronto, Scarborough, Ontario M1C 1A4, Canada
Philip A. R. Shane Department of Geology, University of Auckland, Auckland, New Zealand
Nicholas J. G. Pearce and William T. Perkins Institute of Earth Studies, University of Wales, Aberystwyth, SY23 2DB, United Kingdom
Ravi Korisettar Department of History and Archaeology, Karnatak University, Dharwad 580 003, India
Craig A. Chesner Department of Geology, Eastern Illinois University, Charleston, Illinois 61920
Martin A. J. Williams Mawson Graduate Centre for Environmental Studies, University of Adelaide, Adelaide, SA 5005, Australia
and Subhrangsu K. Acharyya Geological Survey of India, Calcutta-700016, India Received December 4, 1997
INTRODUCTION A controversy currently exists regarding the number of Toba eruptive events represented in the tephra occurrences across peninsular India. Some claim the presence of a single bed, the 75,000-yr-old Toba tephra; others argue that dating and archaeological evidence suggest the presence of earlier Toba tephra. Resolution of this issue was sought through detailed geochemical analyses of a comprehensive suite of samples, allowing comparison of the Indian samples to those from the Toba caldera in northern Sumatra, Malaysia, and, importantly, the sedimentary core at ODP Site 758 in the Indian Ocean—a core that contains several of the earlier Toba tephra beds. In addition, two samples of Toba tephra from western India were dated by the fission-track method. The results unequivocally demonstrate that all the presently known Toba tephra occurrences in peninsular India belong to the 75,000 yr B.P. Toba eruption. Hence, this tephra bed can be used as an effective tool in the correlation and dating of late Quaternary sedimentary sequences across India and it can no longer be used in support of a middle Pleistocene age for associated Acheulian artifacts. q 1998 University of Washington. Key Words: Toba tephra; volcanic glass; major and trace elements; fission-track age; Acheulian artifacts; India.
Rhyolitic tephra predating late Pleistocene (26,000– 12,000 yr B.P.) alluvium was recognized in Quaternary sediments of the Son Valley, north-central India in 1980 (Williams and Royce, 1982; Williams and Clarke, 1984, 1995) and later identified as having been derived from Toba, northern Sumatra (Rose and Chesner, 1987). Later, a similar tephra bed was discovered along the Kukdi River at Bori, western India—a sample that has received considerable attention (Korisetter et al., 1987). Additional Toba tephra occurrences have been discovered in recent years, giving a distribution that spans peninsular India (Acharyya and Basu, 1993) (Fig. 1). The number of Toba eruptive events represented in these tephra occurrences is presently debated. Some claim the presence of a single bed, the 75,000-yr-old Toba tephra (Acharyya and Basu, 1993, 1994; Shane et al., 1995); others argue that dating and archaeological evidence point to the presence of earlier Toba tephra (Mishra et al., 1995; Mishra and Rajaguru, 1994, 1996). Given that several
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large-magnitude eruptions have occurred at Toba during the last million years (Chesner et al., 1991), a necessary prerequisite for the effective stratigraphic use of the Toba tephra occurrences across India is an understanding of their relationship to the eruptions at Toba. Three major rhyolitic tuffs of Quaternary age have been identified at the Toba caldera (Chesner et al., 1991). The Youngest Toba tuff (YTT) has a mean 40Ar/39Ar age of 73,000 { 4000 yr, the Middle Toba tuff (MTT) is 501,000 { 5000 yr and the Oldest Toba tuff (OTT) is 840,000 { 30,000 yr. Each of these eruptions shed an extensive blanket of co-ignimbritic ash over the Indian Ocean, as demonstrated by tephra layers in the sedimentary core recovered at ODP Site 758 (Fig. 1). Here, the four uppermost tephra layers (A, C, D, and E) are believed to be related to the Toba eruptions (Dehn et al., 1991). Specifically, layer A is correlated to YTT, layer C to MTT, and layer E to OTT. Compositional and chronological controls strongly support this interpretation. Glass compositions of YTT and layer A overlap and the same is true for OTT and layer E (Dehn et al., 1991; Chesner, 1988). Plagioclase in layer C is very similar in composition to that in the MTT vitrophyre and the distinctive biotite in layer C also occurs in MTT (Dehn et al., 1991; Chesner, 1988). Using the astronomically derived geomagnetic polarity time scale (ADGPTS) (Shackleton et al., 1990) and assuming a uniform sedimentation rate between the dated levels in the core (Farrell and Janecek, 1991), the age of layer A is about 75,000 yr, layer C is about 540,000 yr, and E is 840,000 yr. A laser 40Ar/39Ar age of 800,000 { 20,000 yr for layer D, which lies just below the Bruhnes–
TABLE 1 Location of Samples Site number in Fig. 1
Sample number
Locality name
1 2 3 3 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
UT1298 UT778 UT1363 UT1364 UT1365 UT1362 UT1358 UT1071 UT1134 UT1135 UT1136 UT1137 UT1138 UT1359 UT1300 UT1069 UT1299 UT1070 UT1361 UT1068 UT1072 UT1360
Toba caldera, Sumatra; YTT Serdang, Selangor, Malaysia; YTT ODP 758, layer A; YTT ODP 758, layer C; MTT ODP 758, layer E; OTT Goguparhu, Vansadhara Pitha mohul, Mohanadi Son Valley Son Valley Son Valley Son Valley Son Valley Son Valley Ramnagar, Son Valley Karnool area Pawlaghat Place, Narmada River Guruwara, Narmada Gandhigram Place, Purna River Purna Basin Bori, Kukdi River, Pune Bori Place, Kukdi River Kukdi River, Pune
Matuyama boundary but above layer E, is in excellent agreement with the ADGPTS age estimate for layer E (Hall and Farrell, 1995). Tephra was probably transported to the middle part of the Indian Ocean during each of these Toba eruptions, given the tephrostratigraphic record at ODP 758. Here, we attempt to answer the question: in which cases did tephra reach the Indian subcontinent in sufficient quantity to be preserved as a discernible bed in the sedimentary record? COMPOSITION OF GLASS SHARDS
FIG. 1. Map showing localities of Toba tephra in northern Sumatra, Malaysia, the Indian Ocean, and the Indian subcontinent. Analyzed samples come from the numbered sites, defined in Table 1.
An attempt to answer this question was made by analyzing the composition of glass shards in the Indian Toba tephra samples and comparing the results to those obtained for samples from Toba caldera, Malaysia, and layers A, C, and E at ODP Site 758 (Table 1, Fig. 1). Several studies have shown that the chemical composition of glass is very useful in identifying tephra beds or distinguishing between them (Westgate and Gorton, 1981). The major-element composition of the glass shards was determined using a Cameca SX50 wavelength dispersive electron microprobe and all the samples were run in a single batch under the same instrumental calibration conditions, thereby optimizing the possibility of distinguishing differences between the samples. The concentration of trace elements in the glass was determined by the fully quantitative solution ICP-MS method using a VG Elemental PlasmaQuad II/ with a modified high sensitivity interface. Calibration was achieved using multi-element syn-
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UT1363
Layer A
Layer E UT1365
Layer C UT1364
ODP 758
UT1069
UT1070
UT1299
UT1359
UT1358
UT1300
UT1361
India UT1362
UT1071
UT1072
UT1134
UT1135
UT1068
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Note. Analyses done on a Cameca SX-50 wavelength dispersive electron microprobe operating at 15 kV accelerating voltage, 20-mm beam diameter, 6nA beam current, and counting times of 20–55 s with Na and Al being analyzed first, as recommended by Morgan and London (1996). Standardization achieved by use of mineral and glass standards. Analyses recast to 100% on a water-free basis. All samples are rhyolites (IUGS classification, Le Bas et al., 1986). n, number of analyses; FeOt , total iron oxide as FeO; H2O, water by difference; standard deviation is given in brackets.
SiO2 77.70 (0.18) 77.71 (0.29) 77.68 (0.27) 77.57 (0.20) 76.05 (0.41) 77.78 (0.24) 77.63 (0.29) 77.81 (0.17) 77.76 (0.23) 77.76 (0.23) 77.68 (0.24) 77.69 (0.27) 77.67 (0.22) 77.64 (0.12) 77.57 (0.13) 77.58 (0.29) 77.71 (0.26) 77.62 (0.22) TiO2 0.06 (0.05) 0.06 (0.06) 0.08 (0.05) 0.06 (0.05) 0.15 (0.08) 0.05 (0.05) 0.05 (0.04) 0.05 (0.04) 0.06 (0.04) 0.03 (0.04) 0.05 (0.04) 0.09 (0.05) 0.07 (0.05) 0.05 (0.04) 0.05 (0.04) 0.07 (0.06) 0.08 (0.08) 0.07 (0.06) Al2O3 12.21 (0.12) 12.16 (0.12) 12.27 (0.13) 12.24 (0.15) 12.94 (0.22) 12.26 (0.14) 12.21 (0.18) 12.02 (0.12) 12.06 (0.12) 12.10 (0.14) 12.12 (0.16) 12.09 (0.15) 12.14 (0.18) 12.14 (0.06) 12.16 (0.11) 12.12 (0.11) 12.04 (0.14) 12.20 (0.14) FeOt 0.83 (0.06) 0.89 (0.07) 0.84 (0.06) 1.02 (0.07) 1.23 (0.07) 0.86 (0.05) 0.87 (0.07) 0.86 (0.07) 0.88 (0.07) 0.87 (0.04) 0.88 (0.07) 0.88 (0.04) 0.89 (0.05) 0.87 (0.06) 0.92 (0.03) 0.90 (0.06) 0.85 (0.05) 0.90 (0.07) MnO 0.09 (0.04) 0.07 (0.04) 0.08 (0.04) 0.04 (0.03) 0.09 (0.04) 0.05 (0.03) 0.06 (0.04) 0.03 (0.02) 0.06 (0.03) 0.06 (0.03) 0.05 (0.03) 0.05 (0.04) 0.06 (0.05) 0.05 (0.04) 0.07 (0.04) 0.06 (0.04) 0.07 (0.04) 0.07 (0.04) MgO 0.04 (0.02) 0.05 (0.02) 0.05 (0.02) 0.04 (0.02) 0.14 (0.02) 0.04 (0.02) 0.06 (0.02) 0.05 (0.01) 0.05 (0.02) 0.05 (0.02) 0.06 (0.02) 0.06 (0.02) 0.05 (0.01) 0.04 (0.01) 0.05 (0.01) 0.06 (0.02) 0.05 (0.02) 0.05 (0.02) CaO 0.71 (0.04) 0.74 (0.07) 0.79 (0.07) 0.63 (0.09) 0.89 (0.07) 0.80 (0.05) 0.80 (0.07) 0.75 (0.08) 0.76 (0.08) 0.80 (0.08) 0.83 (0.07) 0.82 (0.05) 0.75 (0.08) 0.77 (0.07) 0.83 (0.06) 0.78 (0.09) 0.74 (0.09) 0.79 (0.09) Na2O 3.18 (0.08) 3.24 (0.10) 3.16 (0.11) 3.55 (0.20) 4.07 (0.11) 3.08 (0.11) 3.13 (0.16) 3.24 (0.11) 3.21 (0.11) 3.08 (0.21) 3.22 (0.11) 3.14 (0.16) 3.24 (0.15) 3.22 (0.16) 3.17 (0.12) 3.10 (0.12) 3.14 (0.17) 3.17 (0.12) K 2O 5.03 (0.10) 4.93 (0.21) 4.88 (0.13) 4.73 (0.22) 4.17 (0.13) 4.94 (0.17) 5.05 (0.18) 5.03 (0.13) 5.02 (0.14) 5.12 (0.09) 4.96 (0.18) 5.04 (0.08) 4.98 (0.13) 5.09 (0.19) 5.06 (0.07) 5.18 (0.16) 5.17 (0.15) 5.00 (0.10) Cl 0.15 (0.03) 0.14 (0.04) 0.16 (0.03) 0.12 (0.03) 0.27 (0.04) 0.13 (0.02) 0.13 (0.04) 0.14 (0.02) 0.15 (0.03) 0.14 (0.04) 0.14 (0.03) 0.13 (0.03) 0.14 (0.03) 0.12 (0.02) 0.12 (0.04) 0.15 (0.03) 0.14 (0.02) 0.14 (0.03) n 26 20 16 18 20 17 25 10 11 11 10 11 19 11 10 9 10 20 H 2O 0.9 (0.7) 2.2 (0.6) 1.2 (0.6) 2.3 (0.5) 2.4 (1.0) 2.9 (1.4) 2.7 (0.6) 1.5 (0.5) 1.1 (0.4) 1.2 (0.9) 1.4 (0.7) 1.2 (0.4) 1.3 (0.5) 2.5 (0.9) 2.7 (0.6) 2.0 (0.4) 3.9 (3.0) 2.5 (1.2)
Malaysia YTT
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Toba caldera YTT
TABLE 2 Average Major-Element Composition of Glass Shards from Toba Tephra Beds in Sumatra, Malaysia, India, and the Indian Ocean
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FIG. 2. Partial plot of CaO–Na2O–K2O ternary diagram showing glass compositional similarities and differences between tephra samples listed in Table 1.
thetic standards (Pearce et al., 1997). Again, most samples were run in a single batch but a few were analyzed later; the consistency of results being monitored by inclusion of samples from the earlier batch. Glass in OTT (layer E) has lower SiO2 and K2O and higher FeOt , CaO, and Na2O than the other samples. Glass in MTT (layer C) and YTT (samples from Toba caldera, Malaysia, and layer A) are very similar in their major-element composition, although YTT has slightly higher average values for K2O and CaO (Table 2). Each unit is readily distinguished on a CaO–Na2O–K2O ternary plot with only marginal overlap between YTT and MTT. The Indian samples coincide exactly with the YTT field (Fig. 2). The trace-element content in the glass also serves to differentiate these three stratigraphic units (Table 3). Although the rare-earth element (REE) profiles are broadly similar (Fig. 3), YTT can be recognized by its lower light REE (La, Ce, Pr, Nd, Sm) content, higher heavy REE (Ho, Er, Tm, Yb, Lu) content, and larger Eu anomaly. The Indian samples closely track the YTT profile (Fig. 3). Furthermore, YTT glass has the highest Rb and Th values, mimicked by the Indian samples. Thus, the chemical data for the glass demonstrates that all the Indian tephra samples analyzed in this study relate to the 75,000-yr-old YTT. AGE DATA
Support for the presence of older Toba tephra in India has come from dating studies of the tephra at Bori (locality 18, Fig. 1, Table 1) (Mishra et al., 1995; Korisettar et al., 1989; Horn et al., 1993). Highly discrepant ages have been obtained. Two K–Ar age determinations for the bulk tephra have a mean age of 1.38 myr (Korisettar et al., 1989). The same method applied to a glass separate gave an age of 0.538 { 0.047 myr and a glass-fission-track measurement gave an age of 0.64 { 0.29 myr (Horn et al., 1993). 40Ar/ 39 Ar age estimates based on the bulk tephra range from 1.81 to 1.04 myr; corresponding values for the magnetic fraction
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TABLE 3 Average Trace-Element Composition of Glass Shards from Toba Tephra Beds in Sumatra, Malaysia, India, and the Indian Ocean ODP 758
Toba caldera YTT
Malaysia YTT
Layer A
Layer C
Layer E
UT1298
UT778
UT1363
UT1364
UT1365
UT1069
UT1070
UT1299
UT1359
UT1358
UT1300
UT1361
UT1362
UT1071
UT1072
UT1135
UT1068
265 28 39 73 12.2 9.51 117 20.35 40 4.92 18.2 4.21 0.30 4.92 0.92 6.25 1.35 3.90 0.77 4.88 0.88 3.42 1.15 30.3 6.04
240 36 32 75 16.4 7.52 236 21.35 42 4.73 17.0 3.63 0.36 4.72 0.82 5.46 1.16 3.59 0.64 3.90 0.67 3.12 1.59 25.4 4.82
247 42 30 81 15.3 8.51 370 27.25 53 5.82 20.3 4.18 0.60 5.72 0.86 4.41 1.03 3.07 0.56 3.81 0.63 3.23 1.90 29.7 5.39
186 43 27 79 14.4 5.47 533 27.14 55 6.38 22.80 4.83 0.79 6.34 0.91 4.74 0.97 2.82 0.49 3.14 0.49 3.16 1.03 22.70 3.66
155 118 22 121 8.1 7.77 441 33.14 63 7.22 24.70 4.47 1.01 6.84 0.81 3.95 0.77 2.18 0.35 2.48 0.44 3.92 0.50 20.9 5.01
213 48 30 78 16.0 8.0 376 27.28 52 5.81 20.10 4.13 0.43 4.02 0.71 4.65 1.02 3.02 0.51 3.52 0.61 3.17 1.39 29.5 5.40
214 47 29 80 13.90 7.48 415 26.85 51 5.70 19.13 4.09 0.42 4.05 0.72 4.59 1.01 3.04 0.52 3.44 0.57 3.15 1.34 27.4 4.83
223 46 31 79 13.0 7.44 374 26.16 49 5.45 19.3 4.29 0.44 4.15 0.73 4.91 1.09 3.45 0.51 3.64 0.63 3.39 1.15 27.5 4.97
220 35 31 69 12.0 8.26 271 24.10 47 5.30 17.0 4.23 0.35 3.93 0.67 5.03 1.13 3.20 0.47 4.08 0.46 2.92 0.94 28.0 5.07
212 39 29 74 13.9 8.60 361 25.85 51 5.77 19.5 4.16 0.37 3.96 0.71 4.29 1.01 2.96 0.53 3.57 0.63 3.23 1.62 30.9 5.53
213 45 30 78 14.3 7.68 412 27.79 53 5.86 20.0 3.88 0.38 3.89 0.71 4.67 1.02 3.01 0.48 3.78 0.60 3.28 1.48 28.4 5.04
208 45 29 80 11.1 7.60 430 27.53 52 5.87 19.3 3.90 0.41 3.83 0.72 4.40 0.99 3.07 0.47 3.78 0.60 3.14 0.83 27.9 4.79
221 37 31 78 11.7 7.74 288 23.03 45 5.14 19.0 3.69 0.34 4.25 0.72 5.20 1.02 3.19 0.57 3.93 0.61 2.88 0.77 26.9 4.92
220 45 31 78 14.9 7.63 380 26.55 51 5.74 20.1 4.33 0.42 4.33 0.74 4.73 1.04 3.15 0.52 3.74 0.59 3.10 1.46 28.2 5.01
214 51 29 74 12.3 7.12 327 24.24 46 5.25 18.3 3.95 0.39 4.06 0.72 4.64 0.96 3.06 0.51 3.52 0.59 2.98 0.69 26.2 4.84
213 54 29 78 12.7 7.56 395 27.08 51 5.72 19.6 4.17 0.50 4.73 0.73 4.57 0.94 3.07 0.49 3.55 0.56 2.99 1.08 27.2 4.50
213 48 29 77 14.9 7.60 439 28.40 53 5.90 19.6 4.12 0.41 3.84 0.67 4.56 0.98 2.83 0.49 3.34 0.57 3.32 1.28 28.1 4.99
Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U
India
Note. Samples were analyzed at the University of Wales, Aberystwyth. Fully quantitative solution ICP-MS analyses were performed using a VG Elemental ICP-MS PlasmaQuad II/ with a modified high sensitivity interface and calibration was achieved using multi-element synthetic standards. Details of the analytical procedures and standards are given in Pearce et al. (1997). Concentration reported is the average of five analyses of the same solution. Relative standard deviations for rare-earth elements are less than 1% and most of the other trace elements have RSDs less than 5%.
are 16.7 to 15.5 myr and for the nonmagnetic fraction 0.68 to 0.54 myr, the preferred age being 0.67 { 0.03 myr (Mishra et al., 1995). The thermoluminescence (TL) age of the glass is 23,400 { 2400 yr (Horn et al., 1993). All age estimates based on bulk tephra are too old due to
FIG. 3. Chondrite-normalized (Nakamura, 1974) rare-earth-element composition of glass in Toba tephra samples across India, YTT (UT1363, UT778, UT1298), MTT (UT1364), and OTT (UT1365).
the presence of detrital contaminants, introduced when the tephra was locally reworked by streams into a bed more than 1 m thick (Korisettar, 1994). The young TL age most likely dates the last reworking event at the Bori site. The age based on a glass concentrate is also suspect because of the difficulty in obtaining pure separates in silt-grade tephra. The authors note a 2% contaminant fraction and admit that their result has to be considered a maximum age because the feldspars might be inherited xenocrysts (Horn et al., 1993). The glassfission-track age must be considered a very qualitative result because the glass surface area scanned was not measured; no distinction was made between the observed glass surface and the epoxy in which the glass was embedded. Curiously, the close correspondence in the average size of the induced and spontaneous fission tracks shows that little, if any, track fading has occurred in the hydrated glass shards, a likely condition for a very young rhyolitic tephra in India but not for one of middle Pleistocene age. An inherent weakness of the 40Ar/39Ar step-heating dating approach for bulk samples is ignorance of the phases or combination of phases that are contributing to the Ar release. Crystals of the same mineral that have a similar origin and chemical composition—but differ in age—are likely to have a similar closure temperature (Bogaard, in press). Another difficulty is that the pre-
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TABLE 4 Glass–Fission-Track Age of Toba Tephra in India and Malaysia
Sample number
Site number
UT1070
16
UT1069
14
UT778
2
Spontaneous track density (t/cm2) 69.8 { 13.7 (26) 120.1 { 22.3 (29) 78.7 { 11.1 (50)
Corrected spontaneous track density (t/cm2) 76.8 { 15.06 154.9 { 28.8 —
Induced track density (104 t/cm2)
Track density on muscovite detector over dosimeter glass (104 t/cm2)
17.57 { 0.17 (11384) 24.64 { 0.29 (7259) 26.79 { 0.19 (19976)
60.49 { 0.49 (15486) 60.49 { 0.49 (15486) 72.45 { 0.61 (14081)
Etching conditions HF:temp:time (%):7C:s
Ds /Di or Di/D*s
Uncorrected age (103 yr)
Corrected age (103 yr)
26:26:80
1.10 { 0.04*
76 { 15
84 { 16
26:22:155
1.29 { 0.06*
94 { 17
121 { 22
26:21:145
1.07 { 0.04
—
68 { 10
Note. The population–subtraction method was used. Samples UT1070 and UT1069 were corrected for partial track fading by the track-size method (Sandhu and Westgate, 1995). The published isothermal plateau age of UT778 is included here for completeness (Chesner et al., 1991; Westgate, 1989). Ages calculated using the zeta approach (Hurford and Green, 1983) and lD Å 1.551 1 10010 yr01. Zeta value is 318 { 3 based on six irradiations at the McMaster Nuclear Reactor, Hamilton, Ontario, using the NIST SRM 612 glass dosimeter and the Moldavite tektite glass Lhenice locality with an 40Ar/ 39Ar plateau age of 15.21 { 0.15 myr (Staudacher et al., 1982). Ds , mean spontaneous track diameter and Di , mean induced track diameter. Mean track diameters are in the range of 6–8 mm. Number of tracks counted is given in brackets. Samples UT1070 and UT778 dated by JW and UT1069 by PS. Geochemical data (see text) indicates that these three samples relate to the same eruption whose weighted mean fission-track age is 79,000 { 8000 yr.
ferred 40Ar/39Ar plateau age of 0.67 { 0.03 myr of the ‘‘nonmagnetic’’ fraction of the Toba tephra at Bori is significantly different from the age of known large-magnitude eruptions at Toba (Chesner et al., 1991). Our fission-track dating of the Indian Toba tephra takes advantage of recent methodological developments that give accurate and precise ages for fine-grained tephra using their hydrated glass shards (Westgate, 1989; Sandhu and Westgate, 1995). Track counts are accumulated by examination of individual shards so that any contaminant material encountered can be readily ignored. A homogeneous glass shard population is indicated by the low dispersion of the major-element data (Fig. 2) and U content (4.99 { 0.27 ppm). Toba tephra from two sites in western India were dated: UT1069 at Pawlaghat Place, Narmada River, and UT1070 at Gandhigram Place, Purna River (Fig. 1, Table 1). Both samples contained enough glass shards coarser than 125 mm for a statistically meaningful age determination. The results are given in Table 4. UT1070 is within 1s and UT1069 is within 2s of the age of YTT. The major- and trace-element composition of glass shards in the Toba tephra at Bori (UT1068) are indistinguishable from those of UT1069 and UT1070. Thus, it follows that the Toba tephra at Bori is equivalent to YTT.
no longer be used in support of a middle Pleistocene age for associated Acheulian artifacts (Mishra et al., 1995) that have been reworked into their present fluvial context. ACKNOWLEDGMENTS We thank C. M. Hall and J. W. Farrell for tephra samples from ODP Site 758, C. Cermigniani for support on the microprobe aspect of this study, and L. Hill for conducting the sample dissolution for the ICP-MS analyses. James Hester and Bhaskar Deotare helped CAC and RK, respectively. Reviews by Stephen Self and William Rose are gratefully acknowledged. This work was funded by an NSERC operating grant to JAW.
REFERENCES Acharyya, S. K., and Basu, P. K. (1993). Toba ash on the Indian subcontinent and its implications for correlation of late Pleistocene alluvium. Quaternary Research 40, 10–19. Acharyya, S. K., and Basu, P. K. (1994). Reply to comments by S. Mishra and S. N. Rajaguru and by G. L. Badam and S. N. Rajaguru on ‘‘Toba ash on the Indian subcontinent and its implication for the correlation of late Pleistocene alluvium.’’ Quaternary Research 41, 400–402. Bogaard, P. van den. (in press). 40Ar/39Ar ages of Plio-Pleistocene fallout tephra layers and volcaniclastic deposits in the sedimentary aprons of Gran Canaria and Tenerife (ODP Sites 953, 954 and 956). Proceedings Ocean Drilling Program, Scientific Results. Chesner, C. A. (1988). ‘‘The Toba Tuff and Caldera Complex, Sumatra, Indonesia: Insights into Magma Bodies and Eruptions.’’ Ph.D. thesis, Michigan Technological University, Houghton.
CONCLUSION
New chemical and chronological data show that all the Toba tephra occurrences across peninsular India belong to the 75,000-yr-old YTT bed; the purported older occurrence at Bori is discounted. As a result, this tephra bed can be used as an effective tool in the correlation and dating of late Quaternary sediments in India. Furthermore, this tephra can
Chesner, C. A., Rose, W. I., Deino, A., Drake, R., and Westgate, J. A. (1991). Eruptive history of Earth’s largest Quaternary caldera (Toba, Indonesia) clarified. Geology 19, 200–203.
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Dehn, J., Farrell, J. W., and Schmincke, H.-U. (1991). Neogene tephrochronology from Site 758 on northern Ninetyeast Ridge: Indonesian arc volcanism of the past 5 Ma. Proceedings Ocean Drilling Program, Scientific Results 121, 273–295.
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Farrell, J. W., and Janecek, T. R. (1991). Late Neogene paleoceanography and paleoclimatology of the northeast Indian Ocean (ODP Site 758). Proceedings Ocean Drilling Program, Scientific Results 121, 297–355.
Morgan, G. B., and London, D. (1996). Optimizing the electron microprobe analysis of hydrous alkali aluminosilicate glasses. American Mineralogist 81, 1176–1185.
Nakamura, N. (1974). Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochimica Cosmochimica Acta 38, 757–775. Pearce, N. J. G., Perkins, W. T., Westgate, J. A., Gorton, M. P., Jackson, S. E., Neal, C. R., and Chenery, S. P. (1997). A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 partially certified glass standard reference materials. Geostandards Newsletter 21, 115–144. Rose, W. I., and Chesner, C. A. (1987). Dispersal of ash in the great Toba eruption, 75 ka. Geology 15, 913–917. Sandhu, A. S., and 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. Shackleton, N. J., Berger, A., and Peltier, W. R. (1990). An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677. Transactions of the Royal Society of Edinburgh 81, 251–261. Shane, P. A. R., Westgate, J. A., Williams, M. A. J., and Korisettar, R. (1995). New geochemical evidence for the Youngest Toba tuff in India. Quaternary Research 44, 200–204. Staudacher, T. H., Jessberger, E. K., Dominik, B., Kirsten, T., and Schaeffer, O. A. (1982). 40Ar/39Ar ages of rocks and glasses from the No¨rdlinger Ries Crater and the temperature history of impact breccias. Journal Geophysics 51, 1–11. Westgate, J. A., and Gorton, M. P. (1981). Correlation techniques in tephra studies. In ‘‘Tephra Studies’’ (S. Self and R. S. J. Sparks, Eds.), pp. 73– 94. Reidel, Dordrecht. Westgate, J. A. (1989). Isothermal plateau fission-track ages of hydrated glass shards from silicic tephra beds. Earth and Planetary Science Letters 95, 226–234. Williams, M. A. J., and Clarke, M. F. (1984). Late Quaternary environments in north central India. Nature 308, 633–635. Williams, M. A. J., and Clarke, M. F. (1995). Quaternary geology and prehistoric environments in the Son and Belan Valleys, north-central India. Geological Society of India Memoir 32, 282–308. Williams, M. A. J., and Royce, K. (1982). Quaternary geology of the middle Son Valley, north central India: implications for prehistoric archaeology. Palaeogeography, Palaeoclimatology, Palaeoecology 38, 139–162.
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Hall, C. M., and 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., Mu¨ller-Sohnius, D., Storzer, D., and Zo¨ller, 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. Hurford, A. J., and Green, P. F. (1983). The zeta calibration of fission-track dating. Isotope Geoscience 1, 285–317. Korisettar, R. (1994). Quaternary alluvial stratigraphy and sedimentation in the upland Deccan region, western India. Man and Environment XIX, 29–41. Korisettar, R., Venkatesan, T. R., Mishra, S., Rajaguru, S. N., Somayajulu, B. L. K., Tandon, S. K., Gogte, V. D., Ganjoo, R. K., and Kale, V. S. (1989). Discovery of a tephra bed in the Quaternary alluvial sediments of Pune district (Maharashtra), peninsular India. Current Science 58, 564–567. Le Bas, M. J., LeMaitre, R. W., Streckeisen, A., and Zanellin, B. (1986). A chemical classification of volcanic rocks based on the total alkali-silica diagram. Journal of Petrology 27, 745–750. Mishra, S., and Rajaguru, S. N. (1996). Comment on ‘‘New geochemical evidence for the Youngest Toba tuff in India’’. Quaternary Research 46, 340–341. Mishra, S., and Rajaguru, S. N. (1994). Comment on ‘‘Toba ash in the Indian subcontinent and its implication for the correlation of late Pleistocene alluvium.’’ Quaternary Research 41, 396–397. Mishra, S., Venkatesan, T. R., Rajaguru, S. N., and Somayajulu, B. L. K. (1995). Earliest Acheulian industry from peninsular India. Current Anthropology 36, 847–851.
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All Toba Tephra Occurrences across Peninsular India Belong to the 75,000 yr B.P. Eruption John A. Westgate Physical Sciences Division, University of Toronto, Scarborough, Ontario M1C 1A4, Canada
Philip A. R. Shane Department of Geology, University of Auckland, Auckland, New Zealand
Nicholas J. G. Pearce and William T. Perkins Institute of Earth Studies, University of Wales, Aberystwyth, SY23 2DB, United Kingdom
Ravi Korisettar Department of History and Archaeology, Karnatak University, Dharwad 580 003, India
Craig A. Chesner Department of Geology, Eastern Illinois University, Charleston, Illinois 61920
Martin A. J. Williams Mawson Graduate Centre for Environmental Studies, University of Adelaide, Adelaide, SA 5005, Australia
and Subhrangsu K. Acharyya Geological Survey of India, Calcutta-700016, India Received December 4, 1997
INTRODUCTION A controversy currently exists regarding the number of Toba eruptive events represented in the tephra occurrences across peninsular India. Some claim the presence of a single bed, the 75,000-yr-old Toba tephra; others argue that dating and archaeological evidence suggest the presence of earlier Toba tephra. Resolution of this issue was sought through detailed geochemical analyses of a comprehensive suite of samples, allowing comparison of the Indian samples to those from the Toba caldera in northern Sumatra, Malaysia, and, importantly, the sedimentary core at ODP Site 758 in the Indian Ocean—a core that contains several of the earlier Toba tephra beds. In addition, two samples of Toba tephra from western India were dated by the fission-track method. The results unequivocally demonstrate that all the presently known Toba tephra occurrences in peninsular India belong to the 75,000 yr B.P. Toba eruption. Hence, this tephra bed can be used as an effective tool in the correlation and dating of late Quaternary sedimentary sequences across India and it can no longer be used in support of a middle Pleistocene age for associated Acheulian artifacts. q 1998 University of Washington. Key Words: Toba tephra; volcanic glass; major and trace elements; fission-track age; Acheulian artifacts; India.
Rhyolitic tephra predating late Pleistocene (26,000– 12,000 yr B.P.) alluvium was recognized in Quaternary sediments of the Son Valley, north-central India in 1980 (Williams and Royce, 1982; Williams and Clarke, 1984, 1995) and later identified as having been derived from Toba, northern Sumatra (Rose and Chesner, 1987). Later, a similar tephra bed was discovered along the Kukdi River at Bori, western India—a sample that has received considerable attention (Korisetter et al., 1987). Additional Toba tephra occurrences have been discovered in recent years, giving a distribution that spans peninsular India (Acharyya and Basu, 1993) (Fig. 1). The number of Toba eruptive events represented in these tephra occurrences is presently debated. Some claim the presence of a single bed, the 75,000-yr-old Toba tephra (Acharyya and Basu, 1993, 1994; Shane et al., 1995); others argue that dating and archaeological evidence point to the presence of earlier Toba tephra (Mishra et al., 1995; Mishra and Rajaguru, 1994, 1996). Given that several
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large-magnitude eruptions have occurred at Toba during the last million years (Chesner et al., 1991), a necessary prerequisite for the effective stratigraphic use of the Toba tephra occurrences across India is an understanding of their relationship to the eruptions at Toba. Three major rhyolitic tuffs of Quaternary age have been identified at the Toba caldera (Chesner et al., 1991). The Youngest Toba tuff (YTT) has a mean 40Ar/39Ar age of 73,000 { 4000 yr, the Middle Toba tuff (MTT) is 501,000 { 5000 yr and the Oldest Toba tuff (OTT) is 840,000 { 30,000 yr. Each of these eruptions shed an extensive blanket of co-ignimbritic ash over the Indian Ocean, as demonstrated by tephra layers in the sedimentary core recovered at ODP Site 758 (Fig. 1). Here, the four uppermost tephra layers (A, C, D, and E) are believed to be related to the Toba eruptions (Dehn et al., 1991). Specifically, layer A is correlated to YTT, layer C to MTT, and layer E to OTT. Compositional and chronological controls strongly support this interpretation. Glass compositions of YTT and layer A overlap and the same is true for OTT and layer E (Dehn et al., 1991; Chesner, 1988). Plagioclase in layer C is very similar in composition to that in the MTT vitrophyre and the distinctive biotite in layer C also occurs in MTT (Dehn et al., 1991; Chesner, 1988). Using the astronomically derived geomagnetic polarity time scale (ADGPTS) (Shackleton et al., 1990) and assuming a uniform sedimentation rate between the dated levels in the core (Farrell and Janecek, 1991), the age of layer A is about 75,000 yr, layer C is about 540,000 yr, and E is 840,000 yr. A laser 40Ar/39Ar age of 800,000 { 20,000 yr for layer D, which lies just below the Bruhnes–
TABLE 1 Location of Samples Site number in Fig. 1
Sample number
Locality name
1 2 3 3 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
UT1298 UT778 UT1363 UT1364 UT1365 UT1362 UT1358 UT1071 UT1134 UT1135 UT1136 UT1137 UT1138 UT1359 UT1300 UT1069 UT1299 UT1070 UT1361 UT1068 UT1072 UT1360
Toba caldera, Sumatra; YTT Serdang, Selangor, Malaysia; YTT ODP 758, layer A; YTT ODP 758, layer C; MTT ODP 758, layer E; OTT Goguparhu, Vansadhara Pitha mohul, Mohanadi Son Valley Son Valley Son Valley Son Valley Son Valley Son Valley Ramnagar, Son Valley Karnool area Pawlaghat Place, Narmada River Guruwara, Narmada Gandhigram Place, Purna River Purna Basin Bori, Kukdi River, Pune Bori Place, Kukdi River Kukdi River, Pune
Matuyama boundary but above layer E, is in excellent agreement with the ADGPTS age estimate for layer E (Hall and Farrell, 1995). Tephra was probably transported to the middle part of the Indian Ocean during each of these Toba eruptions, given the tephrostratigraphic record at ODP 758. Here, we attempt to answer the question: in which cases did tephra reach the Indian subcontinent in sufficient quantity to be preserved as a discernible bed in the sedimentary record? COMPOSITION OF GLASS SHARDS
FIG. 1. Map showing localities of Toba tephra in northern Sumatra, Malaysia, the Indian Ocean, and the Indian subcontinent. Analyzed samples come from the numbered sites, defined in Table 1.
An attempt to answer this question was made by analyzing the composition of glass shards in the Indian Toba tephra samples and comparing the results to those obtained for samples from Toba caldera, Malaysia, and layers A, C, and E at ODP Site 758 (Table 1, Fig. 1). Several studies have shown that the chemical composition of glass is very useful in identifying tephra beds or distinguishing between them (Westgate and Gorton, 1981). The major-element composition of the glass shards was determined using a Cameca SX50 wavelength dispersive electron microprobe and all the samples were run in a single batch under the same instrumental calibration conditions, thereby optimizing the possibility of distinguishing differences between the samples. The concentration of trace elements in the glass was determined by the fully quantitative solution ICP-MS method using a VG Elemental PlasmaQuad II/ with a modified high sensitivity interface. Calibration was achieved using multi-element syn-
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UT1363
Layer A
Layer E UT1365
Layer C UT1364
ODP 758
UT1069
UT1070
UT1299
UT1359
UT1358
UT1300
UT1361
India UT1362
UT1071
UT1072
UT1134
UT1135
UT1068
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Note. Analyses done on a Cameca SX-50 wavelength dispersive electron microprobe operating at 15 kV accelerating voltage, 20-mm beam diameter, 6nA beam current, and counting times of 20–55 s with Na and Al being analyzed first, as recommended by Morgan and London (1996). Standardization achieved by use of mineral and glass standards. Analyses recast to 100% on a water-free basis. All samples are rhyolites (IUGS classification, Le Bas et al., 1986). n, number of analyses; FeOt , total iron oxide as FeO; H2O, water by difference; standard deviation is given in brackets.
SiO2 77.70 (0.18) 77.71 (0.29) 77.68 (0.27) 77.57 (0.20) 76.05 (0.41) 77.78 (0.24) 77.63 (0.29) 77.81 (0.17) 77.76 (0.23) 77.76 (0.23) 77.68 (0.24) 77.69 (0.27) 77.67 (0.22) 77.64 (0.12) 77.57 (0.13) 77.58 (0.29) 77.71 (0.26) 77.62 (0.22) TiO2 0.06 (0.05) 0.06 (0.06) 0.08 (0.05) 0.06 (0.05) 0.15 (0.08) 0.05 (0.05) 0.05 (0.04) 0.05 (0.04) 0.06 (0.04) 0.03 (0.04) 0.05 (0.04) 0.09 (0.05) 0.07 (0.05) 0.05 (0.04) 0.05 (0.04) 0.07 (0.06) 0.08 (0.08) 0.07 (0.06) Al2O3 12.21 (0.12) 12.16 (0.12) 12.27 (0.13) 12.24 (0.15) 12.94 (0.22) 12.26 (0.14) 12.21 (0.18) 12.02 (0.12) 12.06 (0.12) 12.10 (0.14) 12.12 (0.16) 12.09 (0.15) 12.14 (0.18) 12.14 (0.06) 12.16 (0.11) 12.12 (0.11) 12.04 (0.14) 12.20 (0.14) FeOt 0.83 (0.06) 0.89 (0.07) 0.84 (0.06) 1.02 (0.07) 1.23 (0.07) 0.86 (0.05) 0.87 (0.07) 0.86 (0.07) 0.88 (0.07) 0.87 (0.04) 0.88 (0.07) 0.88 (0.04) 0.89 (0.05) 0.87 (0.06) 0.92 (0.03) 0.90 (0.06) 0.85 (0.05) 0.90 (0.07) MnO 0.09 (0.04) 0.07 (0.04) 0.08 (0.04) 0.04 (0.03) 0.09 (0.04) 0.05 (0.03) 0.06 (0.04) 0.03 (0.02) 0.06 (0.03) 0.06 (0.03) 0.05 (0.03) 0.05 (0.04) 0.06 (0.05) 0.05 (0.04) 0.07 (0.04) 0.06 (0.04) 0.07 (0.04) 0.07 (0.04) MgO 0.04 (0.02) 0.05 (0.02) 0.05 (0.02) 0.04 (0.02) 0.14 (0.02) 0.04 (0.02) 0.06 (0.02) 0.05 (0.01) 0.05 (0.02) 0.05 (0.02) 0.06 (0.02) 0.06 (0.02) 0.05 (0.01) 0.04 (0.01) 0.05 (0.01) 0.06 (0.02) 0.05 (0.02) 0.05 (0.02) CaO 0.71 (0.04) 0.74 (0.07) 0.79 (0.07) 0.63 (0.09) 0.89 (0.07) 0.80 (0.05) 0.80 (0.07) 0.75 (0.08) 0.76 (0.08) 0.80 (0.08) 0.83 (0.07) 0.82 (0.05) 0.75 (0.08) 0.77 (0.07) 0.83 (0.06) 0.78 (0.09) 0.74 (0.09) 0.79 (0.09) Na2O 3.18 (0.08) 3.24 (0.10) 3.16 (0.11) 3.55 (0.20) 4.07 (0.11) 3.08 (0.11) 3.13 (0.16) 3.24 (0.11) 3.21 (0.11) 3.08 (0.21) 3.22 (0.11) 3.14 (0.16) 3.24 (0.15) 3.22 (0.16) 3.17 (0.12) 3.10 (0.12) 3.14 (0.17) 3.17 (0.12) K 2O 5.03 (0.10) 4.93 (0.21) 4.88 (0.13) 4.73 (0.22) 4.17 (0.13) 4.94 (0.17) 5.05 (0.18) 5.03 (0.13) 5.02 (0.14) 5.12 (0.09) 4.96 (0.18) 5.04 (0.08) 4.98 (0.13) 5.09 (0.19) 5.06 (0.07) 5.18 (0.16) 5.17 (0.15) 5.00 (0.10) Cl 0.15 (0.03) 0.14 (0.04) 0.16 (0.03) 0.12 (0.03) 0.27 (0.04) 0.13 (0.02) 0.13 (0.04) 0.14 (0.02) 0.15 (0.03) 0.14 (0.04) 0.14 (0.03) 0.13 (0.03) 0.14 (0.03) 0.12 (0.02) 0.12 (0.04) 0.15 (0.03) 0.14 (0.02) 0.14 (0.03) n 26 20 16 18 20 17 25 10 11 11 10 11 19 11 10 9 10 20 H 2O 0.9 (0.7) 2.2 (0.6) 1.2 (0.6) 2.3 (0.5) 2.4 (1.0) 2.9 (1.4) 2.7 (0.6) 1.5 (0.5) 1.1 (0.4) 1.2 (0.9) 1.4 (0.7) 1.2 (0.4) 1.3 (0.5) 2.5 (0.9) 2.7 (0.6) 2.0 (0.4) 3.9 (3.0) 2.5 (1.2)
Malaysia YTT
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Toba caldera YTT
TABLE 2 Average Major-Element Composition of Glass Shards from Toba Tephra Beds in Sumatra, Malaysia, India, and the Indian Ocean
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FIG. 2. Partial plot of CaO–Na2O–K2O ternary diagram showing glass compositional similarities and differences between tephra samples listed in Table 1.
thetic standards (Pearce et al., 1997). Again, most samples were run in a single batch but a few were analyzed later; the consistency of results being monitored by inclusion of samples from the earlier batch. Glass in OTT (layer E) has lower SiO2 and K2O and higher FeOt , CaO, and Na2O than the other samples. Glass in MTT (layer C) and YTT (samples from Toba caldera, Malaysia, and layer A) are very similar in their major-element composition, although YTT has slightly higher average values for K2O and CaO (Table 2). Each unit is readily distinguished on a CaO–Na2O–K2O ternary plot with only marginal overlap between YTT and MTT. The Indian samples coincide exactly with the YTT field (Fig. 2). The trace-element content in the glass also serves to differentiate these three stratigraphic units (Table 3). Although the rare-earth element (REE) profiles are broadly similar (Fig. 3), YTT can be recognized by its lower light REE (La, Ce, Pr, Nd, Sm) content, higher heavy REE (Ho, Er, Tm, Yb, Lu) content, and larger Eu anomaly. The Indian samples closely track the YTT profile (Fig. 3). Furthermore, YTT glass has the highest Rb and Th values, mimicked by the Indian samples. Thus, the chemical data for the glass demonstrates that all the Indian tephra samples analyzed in this study relate to the 75,000-yr-old YTT. AGE DATA
Support for the presence of older Toba tephra in India has come from dating studies of the tephra at Bori (locality 18, Fig. 1, Table 1) (Mishra et al., 1995; Korisettar et al., 1989; Horn et al., 1993). Highly discrepant ages have been obtained. Two K–Ar age determinations for the bulk tephra have a mean age of 1.38 myr (Korisettar et al., 1989). The same method applied to a glass separate gave an age of 0.538 { 0.047 myr and a glass-fission-track measurement gave an age of 0.64 { 0.29 myr (Horn et al., 1993). 40Ar/ 39 Ar age estimates based on the bulk tephra range from 1.81 to 1.04 myr; corresponding values for the magnetic fraction
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TABLE 3 Average Trace-Element Composition of Glass Shards from Toba Tephra Beds in Sumatra, Malaysia, India, and the Indian Ocean ODP 758
Toba caldera YTT
Malaysia YTT
Layer A
Layer C
Layer E
UT1298
UT778
UT1363
UT1364
UT1365
UT1069
UT1070
UT1299
UT1359
UT1358
UT1300
UT1361
UT1362
UT1071
UT1072
UT1135
UT1068
265 28 39 73 12.2 9.51 117 20.35 40 4.92 18.2 4.21 0.30 4.92 0.92 6.25 1.35 3.90 0.77 4.88 0.88 3.42 1.15 30.3 6.04
240 36 32 75 16.4 7.52 236 21.35 42 4.73 17.0 3.63 0.36 4.72 0.82 5.46 1.16 3.59 0.64 3.90 0.67 3.12 1.59 25.4 4.82
247 42 30 81 15.3 8.51 370 27.25 53 5.82 20.3 4.18 0.60 5.72 0.86 4.41 1.03 3.07 0.56 3.81 0.63 3.23 1.90 29.7 5.39
186 43 27 79 14.4 5.47 533 27.14 55 6.38 22.80 4.83 0.79 6.34 0.91 4.74 0.97 2.82 0.49 3.14 0.49 3.16 1.03 22.70 3.66
155 118 22 121 8.1 7.77 441 33.14 63 7.22 24.70 4.47 1.01 6.84 0.81 3.95 0.77 2.18 0.35 2.48 0.44 3.92 0.50 20.9 5.01
213 48 30 78 16.0 8.0 376 27.28 52 5.81 20.10 4.13 0.43 4.02 0.71 4.65 1.02 3.02 0.51 3.52 0.61 3.17 1.39 29.5 5.40
214 47 29 80 13.90 7.48 415 26.85 51 5.70 19.13 4.09 0.42 4.05 0.72 4.59 1.01 3.04 0.52 3.44 0.57 3.15 1.34 27.4 4.83
223 46 31 79 13.0 7.44 374 26.16 49 5.45 19.3 4.29 0.44 4.15 0.73 4.91 1.09 3.45 0.51 3.64 0.63 3.39 1.15 27.5 4.97
220 35 31 69 12.0 8.26 271 24.10 47 5.30 17.0 4.23 0.35 3.93 0.67 5.03 1.13 3.20 0.47 4.08 0.46 2.92 0.94 28.0 5.07
212 39 29 74 13.9 8.60 361 25.85 51 5.77 19.5 4.16 0.37 3.96 0.71 4.29 1.01 2.96 0.53 3.57 0.63 3.23 1.62 30.9 5.53
213 45 30 78 14.3 7.68 412 27.79 53 5.86 20.0 3.88 0.38 3.89 0.71 4.67 1.02 3.01 0.48 3.78 0.60 3.28 1.48 28.4 5.04
208 45 29 80 11.1 7.60 430 27.53 52 5.87 19.3 3.90 0.41 3.83 0.72 4.40 0.99 3.07 0.47 3.78 0.60 3.14 0.83 27.9 4.79
221 37 31 78 11.7 7.74 288 23.03 45 5.14 19.0 3.69 0.34 4.25 0.72 5.20 1.02 3.19 0.57 3.93 0.61 2.88 0.77 26.9 4.92
220 45 31 78 14.9 7.63 380 26.55 51 5.74 20.1 4.33 0.42 4.33 0.74 4.73 1.04 3.15 0.52 3.74 0.59 3.10 1.46 28.2 5.01
214 51 29 74 12.3 7.12 327 24.24 46 5.25 18.3 3.95 0.39 4.06 0.72 4.64 0.96 3.06 0.51 3.52 0.59 2.98 0.69 26.2 4.84
213 54 29 78 12.7 7.56 395 27.08 51 5.72 19.6 4.17 0.50 4.73 0.73 4.57 0.94 3.07 0.49 3.55 0.56 2.99 1.08 27.2 4.50
213 48 29 77 14.9 7.60 439 28.40 53 5.90 19.6 4.12 0.41 3.84 0.67 4.56 0.98 2.83 0.49 3.34 0.57 3.32 1.28 28.1 4.99
Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U
India
Note. Samples were analyzed at the University of Wales, Aberystwyth. Fully quantitative solution ICP-MS analyses were performed using a VG Elemental ICP-MS PlasmaQuad II/ with a modified high sensitivity interface and calibration was achieved using multi-element synthetic standards. Details of the analytical procedures and standards are given in Pearce et al. (1997). Concentration reported is the average of five analyses of the same solution. Relative standard deviations for rare-earth elements are less than 1% and most of the other trace elements have RSDs less than 5%.
are 16.7 to 15.5 myr and for the nonmagnetic fraction 0.68 to 0.54 myr, the preferred age being 0.67 { 0.03 myr (Mishra et al., 1995). The thermoluminescence (TL) age of the glass is 23,400 { 2400 yr (Horn et al., 1993). All age estimates based on bulk tephra are too old due to
FIG. 3. Chondrite-normalized (Nakamura, 1974) rare-earth-element composition of glass in Toba tephra samples across India, YTT (UT1363, UT778, UT1298), MTT (UT1364), and OTT (UT1365).
the presence of detrital contaminants, introduced when the tephra was locally reworked by streams into a bed more than 1 m thick (Korisettar, 1994). The young TL age most likely dates the last reworking event at the Bori site. The age based on a glass concentrate is also suspect because of the difficulty in obtaining pure separates in silt-grade tephra. The authors note a 2% contaminant fraction and admit that their result has to be considered a maximum age because the feldspars might be inherited xenocrysts (Horn et al., 1993). The glassfission-track age must be considered a very qualitative result because the glass surface area scanned was not measured; no distinction was made between the observed glass surface and the epoxy in which the glass was embedded. Curiously, the close correspondence in the average size of the induced and spontaneous fission tracks shows that little, if any, track fading has occurred in the hydrated glass shards, a likely condition for a very young rhyolitic tephra in India but not for one of middle Pleistocene age. An inherent weakness of the 40Ar/39Ar step-heating dating approach for bulk samples is ignorance of the phases or combination of phases that are contributing to the Ar release. Crystals of the same mineral that have a similar origin and chemical composition—but differ in age—are likely to have a similar closure temperature (Bogaard, in press). Another difficulty is that the pre-
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TABLE 4 Glass–Fission-Track Age of Toba Tephra in India and Malaysia
Sample number
Site number
UT1070
16
UT1069
14
UT778
2
Spontaneous track density (t/cm2) 69.8 { 13.7 (26) 120.1 { 22.3 (29) 78.7 { 11.1 (50)
Corrected spontaneous track density (t/cm2) 76.8 { 15.06 154.9 { 28.8 —
Induced track density (104 t/cm2)
Track density on muscovite detector over dosimeter glass (104 t/cm2)
17.57 { 0.17 (11384) 24.64 { 0.29 (7259) 26.79 { 0.19 (19976)
60.49 { 0.49 (15486) 60.49 { 0.49 (15486) 72.45 { 0.61 (14081)
Etching conditions HF:temp:time (%):7C:s
Ds /Di or Di/D*s
Uncorrected age (103 yr)
Corrected age (103 yr)
26:26:80
1.10 { 0.04*
76 { 15
84 { 16
26:22:155
1.29 { 0.06*
94 { 17
121 { 22
26:21:145
1.07 { 0.04
—
68 { 10
Note. The population–subtraction method was used. Samples UT1070 and UT1069 were corrected for partial track fading by the track-size method (Sandhu and Westgate, 1995). The published isothermal plateau age of UT778 is included here for completeness (Chesner et al., 1991; Westgate, 1989). Ages calculated using the zeta approach (Hurford and Green, 1983) and lD Å 1.551 1 10010 yr01. Zeta value is 318 { 3 based on six irradiations at the McMaster Nuclear Reactor, Hamilton, Ontario, using the NIST SRM 612 glass dosimeter and the Moldavite tektite glass Lhenice locality with an 40Ar/ 39Ar plateau age of 15.21 { 0.15 myr (Staudacher et al., 1982). Ds , mean spontaneous track diameter and Di , mean induced track diameter. Mean track diameters are in the range of 6–8 mm. Number of tracks counted is given in brackets. Samples UT1070 and UT778 dated by JW and UT1069 by PS. Geochemical data (see text) indicates that these three samples relate to the same eruption whose weighted mean fission-track age is 79,000 { 8000 yr.
ferred 40Ar/39Ar plateau age of 0.67 { 0.03 myr of the ‘‘nonmagnetic’’ fraction of the Toba tephra at Bori is significantly different from the age of known large-magnitude eruptions at Toba (Chesner et al., 1991). Our fission-track dating of the Indian Toba tephra takes advantage of recent methodological developments that give accurate and precise ages for fine-grained tephra using their hydrated glass shards (Westgate, 1989; Sandhu and Westgate, 1995). Track counts are accumulated by examination of individual shards so that any contaminant material encountered can be readily ignored. A homogeneous glass shard population is indicated by the low dispersion of the major-element data (Fig. 2) and U content (4.99 { 0.27 ppm). Toba tephra from two sites in western India were dated: UT1069 at Pawlaghat Place, Narmada River, and UT1070 at Gandhigram Place, Purna River (Fig. 1, Table 1). Both samples contained enough glass shards coarser than 125 mm for a statistically meaningful age determination. The results are given in Table 4. UT1070 is within 1s and UT1069 is within 2s of the age of YTT. The major- and trace-element composition of glass shards in the Toba tephra at Bori (UT1068) are indistinguishable from those of UT1069 and UT1070. Thus, it follows that the Toba tephra at Bori is equivalent to YTT.
no longer be used in support of a middle Pleistocene age for associated Acheulian artifacts (Mishra et al., 1995) that have been reworked into their present fluvial context. ACKNOWLEDGMENTS We thank C. M. Hall and J. W. Farrell for tephra samples from ODP Site 758, C. Cermigniani for support on the microprobe aspect of this study, and L. Hill for conducting the sample dissolution for the ICP-MS analyses. James Hester and Bhaskar Deotare helped CAC and RK, respectively. Reviews by Stephen Self and William Rose are gratefully acknowledged. This work was funded by an NSERC operating grant to JAW.
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CONCLUSION
New chemical and chronological data show that all the Toba tephra occurrences across peninsular India belong to the 75,000-yr-old YTT bed; the purported older occurrence at Bori is discounted. As a result, this tephra bed can be used as an effective tool in the correlation and dating of late Quaternary sediments in India. Furthermore, this tephra can
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