QUATERNARY RESEARCH 50, 107–112 (1998)ARTICLE NO. QR981974

All Toba Tephra Occurrences across Peninsular IndiaBelong 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

INTRODUCTIONA controversy currently exists regarding the number of Toba erup-

tive events represented in the tephra occurrences across peninsularRhyolitic tephra predating late Pleistocene (26,000–

India. Some claim the presence of a single bed, the 75,000-yr-old12,000 yr B.P.) alluvium was recognized in Quaternary sedi-Toba tephra; others argue that dating and archaeological evidencements of the Son Valley, north-central India in 1980 (Wil-suggest the presence of earlier Toba tephra. Resolution of this issueliams and Royce, 1982; Williams and Clarke, 1984, 1995)was sought through detailed geochemical analyses of a comprehen-and later identified as having been derived from Toba, north-sive suite of samples, allowing comparison of the Indian samples toern Sumatra (Rose and Chesner, 1987). Later, a similarthose from the Toba caldera in northern Sumatra, Malaysia, and,

importantly, the sedimentary core at ODP Site 758 in the Indian tephra bed was discovered along the Kukdi River at Bori,Ocean—a core that contains several of the earlier Toba tephra beds. western India—a sample that has received considerable at-In addition, two samples of Toba tephra from western India were tention (Korisetter et al., 1987). Additional Toba tephra oc-dated by the fission-track method. The results unequivocally demon- currences have been discovered in recent years, giving astrate that all the presently known Toba tephra occurrences in penin- distribution that spans peninsular India (Acharyya and Basu,sular India belong to the 75,000 yr B.P. Toba eruption. Hence, this 1993) (Fig. 1). The number of Toba eruptive events repre-tephra bed can be used as an effective tool in the correlation and

sented in these tephra occurrences is presently debated.dating of late Quaternary sedimentary sequences across India andSome claim the presence of a single bed, the 75,000-yr-oldit can no longer be used in support of a middle Pleistocene age forToba tephra (Acharyya and Basu, 1993, 1994; Shane et al.,associated Acheulian artifacts. q 1998 University of Washington.

1995); others argue that dating and archaeological evidenceKey Words: Toba tephra; volcanic glass; major and trace ele-point to the presence of earlier Toba tephra (Mishra et al.,ments; fission-track age; Acheulian artifacts; India.1995; Mishra and Rajaguru, 1994, 1996). Given that several

107 0033-5894/98 $25.00Copyright q 1998 by the University of Washington.

All rights of reproduction in any form reserved.

AID QR 1974 / a613$$$$21 07-09-98 17:09:20 qral AP: QR

108 WESTGATE ET AL.

TABLE 1large-magnitude eruptions have occurred at Toba during theLocation of Sampleslast million years (Chesner et al., 1991), a necessary prereq-

uisite for the effective stratigraphic use of the Toba tephraSite number Sampleoccurrences across India is an understanding of their rela-

in Fig. 1 number Locality nametionship to the eruptions at Toba.

Three major rhyolitic tuffs of Quaternary age have been 1 UT1298 Toba caldera, Sumatra; YTT2 UT778 Serdang, Selangor, Malaysia; YTTidentified at the Toba caldera (Chesner et al., 1991). The3 UT1363 ODP 758, layer A; YTTYoungest Toba tuff (YTT) has a mean 40Ar/39Ar age of3 UT1364 ODP 758, layer C; MTT73,000 { 4000 yr, the Middle Toba tuff (MTT) is 501,0003 UT1365 ODP 758, layer E; OTT

{ 5000 yr and the Oldest Toba tuff (OTT) is 840,000 { 4 UT1362 Goguparhu, Vansadhara30,000 yr. Each of these eruptions shed an extensive blanket 5 UT1358 Pitha mohul, Mohanadi

6 UT1071 Son Valleyof co-ignimbritic ash over the Indian Ocean, as demonstrated7 UT1134 Son Valleyby tephra layers in the sedimentary core recovered at ODP8 UT1135 Son ValleySite 758 (Fig. 1). Here, the four uppermost tephra layers (A,9 UT1136 Son Valley

C, D, and E) are believed to be related to the Toba eruptions 10 UT1137 Son Valley(Dehn et al., 1991). Specifically, layer A is correlated to 11 UT1138 Son Valley

12 UT1359 Ramnagar, Son ValleyYTT, layer C to MTT, and layer E to OTT. Compositional13 UT1300 Karnool areaand chronological controls strongly support this interpreta-14 UT1069 Pawlaghat Place, Narmada Rivertion. Glass compositions of YTT and layer A overlap and15 UT1299 Guruwara, Narmada

the same is true for OTT and layer E (Dehn et al., 1991; 16 UT1070 Gandhigram Place, Purna RiverChesner, 1988). Plagioclase in layer C is very similar in 17 UT1361 Purna Basin

18 UT1068 Bori, Kukdi River, Punecomposition to that in the MTT vitrophyre and the distinctive19 UT1072 Bori Place, Kukdi Riverbiotite in layer C also occurs in MTT (Dehn et al., 1991;20 UT1360 Kukdi River, PuneChesner, 1988). Using the astronomically derived geomag-

netic polarity time scale (ADGPTS) (Shackleton et al., 1990)and assuming a uniform sedimentation rate between thedated levels in the core (Farrell and Janecek, 1991), the age

Matuyama boundary but above layer E, is in excellent agree-of layer A is about 75,000 yr, layer C is about 540,000 yr,ment with the ADGPTS age estimate for layer E (Hall andand E is 840,000 yr. A laser 40Ar/39Ar age of 800,000 {Farrell, 1995). Tephra was probably transported to the mid-20,000 yr for layer D, which lies just below the Bruhnes–dle part of the Indian Ocean during each of these Tobaeruptions, given the tephrostratigraphic record at ODP 758.Here, we attempt to answer the question: in which cases didtephra reach the Indian subcontinent in sufficient quantity tobe preserved as a discernible bed in the sedimentary record?

COMPOSITION OF GLASS SHARDS

An attempt to answer this question was made by analyzingthe composition of glass shards in the Indian Toba tephrasamples and comparing the results to those obtained forsamples from Toba caldera, Malaysia, and layers A, C, andE at ODP Site 758 (Table 1, Fig. 1). Several studies haveshown that the chemical composition of glass is very usefulin identifying tephra beds or distinguishing between them(Westgate and Gorton, 1981). The major-element composi-tion of the glass shards was determined using a Cameca SX-50 wavelength dispersive electron microprobe and all thesamples were run in a single batch under the same instrumen-tal calibration conditions, thereby optimizing the possibilityof distinguishing differences between the samples. The con-centration of trace elements in the glass was determined bythe fully quantitative solution ICP-MS method using a VGFIG. 1. Map showing localities of Toba tephra in northern Sumatra,Elemental PlasmaQuad II/ with a modified high sensitivityMalaysia, the Indian Ocean, and the Indian subcontinent. Analyzed samples

come from the numbered sites, defined in Table 1. interface. Calibration was achieved using multi-element syn-

AID QR 1974 / a613$$$$21 07-09-98 17:09:20 qral AP: QR

109TOBA TEPHRA IN PENINSULAR INDIA

FIG. 2. Partial plot of CaO–Na2O–K2O ternary diagram showing glasscompositional similarities and differences between tephra samples listed inTable 1.

thetic standards (Pearce et al., 1997). Again, most sampleswere run in a single batch but a few were analyzed later;the consistency of results being monitored by inclusion ofsamples from the earlier batch.

Glass in OTT (layer E) has lower SiO2 and K2O and higherFeOt , 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 compo-sition, although YTT has slightly higher average values forK2O and CaO (Table 2). Each unit is readily distinguished ona CaO–Na2O–K2O ternary plot with only marginal overlapbetween YTT and MTT. The Indian samples coincide ex-actly with the YTT field (Fig. 2). The trace-element contentin the glass also serves to differentiate these three strati-graphic units (Table 3). Although the rare-earth element(REE) profiles are broadly similar (Fig. 3), YTT can berecognized 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 trackthe YTT profile (Fig. 3). Furthermore, YTT glass has thehighest Rb and Th values, mimicked by the Indian samples.Thus, the chemical data for the glass demonstrates that allthe Indian tephra samples analyzed in this study relate tothe 75,000-yr-old YTT.

AGE DATA

Support for the presence of older Toba tephra in India hascome 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 ob-tained. Two K–Ar age determinations for the bulk tephrahave a mean age of 1.38 myr (Korisettar et al., 1989). Thesame method applied to a glass separate gave an age of0.538 { 0.047 myr and a glass-fission-track measurementgave an age of 0.64 { 0.29 myr (Horn et al., 1993). 40Ar/

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39Ar age estimates based on the bulk tephra range from 1.81to 1.04 myr; corresponding values for the magnetic fraction

AID QR 1974 / a613$$$$22 07-09-98 17:09:20 qral AP: QR

110 WESTGATE ET AL.

TABLE 3Average Trace-Element Composition of Glass Shards from Toba Tephra Beds in Sumatra, Malaysia, India, and the Indian Ocean

ODP 758Tobacaldera MalaysiaYTT YTT Layer A Layer C Layer E India

UT1298 UT778 UT1363 UT1364 UT1365 UT1069 UT1070 UT1299 UT1359 UT1358 UT1300 UT1361 UT1362 UT1071 UT1072 UT1135 UT1068

Rb 265 240 247 186 155 213 214 223 220 212 213 208 221 220 214 213 213Sr 28 36 42 43 118 48 47 46 35 39 45 45 37 45 51 54 48Y 39 32 30 27 22 30 29 31 31 29 30 29 31 31 29 29 29Zr 73 75 81 79 121 78 80 79 69 74 78 80 78 78 74 78 77Nb 12.2 16.4 15.3 14.4 8.1 16.0 13.90 13.0 12.0 13.9 14.3 11.1 11.7 14.9 12.3 12.7 14.9Cs 9.51 7.52 8.51 5.47 7.77 8.0 7.48 7.44 8.26 8.60 7.68 7.60 7.74 7.63 7.12 7.56 7.60Ba 117 236 370 533 441 376 415 374 271 361 412 430 288 380 327 395 439La 20.35 21.35 27.25 27.14 33.14 27.28 26.85 26.16 24.10 25.85 27.79 27.53 23.03 26.55 24.24 27.08 28.40Ce 40 42 53 55 63 52 51 49 47 51 53 52 45 51 46 51 53Pr 4.92 4.73 5.82 6.38 7.22 5.81 5.70 5.45 5.30 5.77 5.86 5.87 5.14 5.74 5.25 5.72 5.90Nd 18.2 17.0 20.3 22.80 24.70 20.10 19.13 19.3 17.0 19.5 20.0 19.3 19.0 20.1 18.3 19.6 19.6Sm 4.21 3.63 4.18 4.83 4.47 4.13 4.09 4.29 4.23 4.16 3.88 3.90 3.69 4.33 3.95 4.17 4.12Eu 0.30 0.36 0.60 0.79 1.01 0.43 0.42 0.44 0.35 0.37 0.38 0.41 0.34 0.42 0.39 0.50 0.41Gd 4.92 4.72 5.72 6.34 6.84 4.02 4.05 4.15 3.93 3.96 3.89 3.83 4.25 4.33 4.06 4.73 3.84Tb 0.92 0.82 0.86 0.91 0.81 0.71 0.72 0.73 0.67 0.71 0.71 0.72 0.72 0.74 0.72 0.73 0.67Dy 6.25 5.46 4.41 4.74 3.95 4.65 4.59 4.91 5.03 4.29 4.67 4.40 5.20 4.73 4.64 4.57 4.56Ho 1.35 1.16 1.03 0.97 0.77 1.02 1.01 1.09 1.13 1.01 1.02 0.99 1.02 1.04 0.96 0.94 0.98Er 3.90 3.59 3.07 2.82 2.18 3.02 3.04 3.45 3.20 2.96 3.01 3.07 3.19 3.15 3.06 3.07 2.83Tm 0.77 0.64 0.56 0.49 0.35 0.51 0.52 0.51 0.47 0.53 0.48 0.47 0.57 0.52 0.51 0.49 0.49Yb 4.88 3.90 3.81 3.14 2.48 3.52 3.44 3.64 4.08 3.57 3.78 3.78 3.93 3.74 3.52 3.55 3.34Lu 0.88 0.67 0.63 0.49 0.44 0.61 0.57 0.63 0.46 0.63 0.60 0.60 0.61 0.59 0.59 0.56 0.57Hf 3.42 3.12 3.23 3.16 3.92 3.17 3.15 3.39 2.92 3.23 3.28 3.14 2.88 3.10 2.98 2.99 3.32Ta 1.15 1.59 1.90 1.03 0.50 1.39 1.34 1.15 0.94 1.62 1.48 0.83 0.77 1.46 0.69 1.08 1.28Th 30.3 25.4 29.7 22.70 20.9 29.5 27.4 27.5 28.0 30.9 28.4 27.9 26.9 28.2 26.2 27.2 28.1U 6.04 4.82 5.39 3.66 5.01 5.40 4.83 4.97 5.07 5.53 5.04 4.79 4.92 5.01 4.84 4.50 4.99

Note. Samples were analyzed at the University of Wales, Aberystwyth. Fully quantitative solution ICP-MS analyses were performed using a VGElemental 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 samesolution. 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 the presence of detrital contaminants, introduced when theto 0.54 myr, the preferred age being 0.67{ 0.03 myr (Mishra tephra was locally reworked by streams into a bed more thanet al., 1995). The thermoluminescence (TL) age of the glass 1 m thick (Korisettar, 1994). The young TL age most likelyis 23,400 { 2400 yr (Horn et al., 1993). dates the last reworking event at the Bori site. The age based

All age estimates based on bulk tephra are too old due to on a glass concentrate is also suspect because of the difficultyin obtaining pure separates in silt-grade tephra. The authorsnote a 2% contaminant fraction and admit that their resulthas to be considered a maximum age because the feldsparsmight be inherited xenocrysts (Horn et al., 1993). The glass-fission-track age must be considered a very qualitative resultbecause the glass surface area scanned was not measured;no distinction was made between the observed glass surfaceand the epoxy in which the glass was embedded. Curiously,the close correspondence in the average size of the inducedand spontaneous fission tracks shows that little, if any, trackfading has occurred in the hydrated glass shards, a likelycondition for a very young rhyolitic tephra in India but notfor one of middle Pleistocene age. An inherent weakness ofthe 40Ar/39Ar step-heating dating approach for bulk samplesis ignorance of the phases or combination of phases that arecontributing to the Ar release. Crystals of the same mineralthat have a similar origin and chemical composition—but

FIG. 3. Chondrite-normalized (Nakamura, 1974) rare-earth-elementdiffer in age—are likely to have a similar closure tempera-composition of glass in Toba tephra samples across India, YTT (UT1363,

UT778, UT1298), MTT (UT1364), and OTT (UT1365). ture (Bogaard, in press). Another difficulty is that the pre-

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111TOBA TEPHRA IN PENINSULAR INDIA

TABLE 4Glass–Fission-Track Age of Toba Tephra in India and Malaysia

Track density onCorrected muscovite detector Etching

Ds /DiSpontaneous spontaneous Induced over dosimeter conditions Uncorrected CorrectedorSample Site track density track density track density glass HF:temp:time age age

number number (t/cm2) (t/cm2) (104 t/cm2) (104 t/cm2) (%):7C:s Di/D*s (103 yr) (103 yr)

UT1070 16 69.8 { 13.7 76.8 { 15.06 17.57 { 0.17 60.49 { 0.49 26:26:80 1.10 { 0.04* 76 { 15 84 { 16(26) (11384) (15486)

UT1069 14 120.1 { 22.3 154.9 { 28.8 24.64 { 0.29 60.49 { 0.49 26:22:155 1.29 { 0.06* 94 { 17 121 { 22(29) (7259) (15486)

UT778 2 78.7 { 11.1 — 26.79 { 0.19 72.45 { 0.61 26:21:145 1.07 { 0.04 — 68 { 10(50) (19976) (14081)

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 theMcMaster Nuclear Reactor, Hamilton, Ontario, using the NIST SRM 612 glass dosimeter and the Moldavite tektite glass Lhenice locality with an 40Ar/39Arplateau age of 15.21 { 0.15 myr (Staudacher et al., 1982). Ds , mean spontaneous track diameter and Di , mean induced track diameter. Mean trackdiameters 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 ‘‘non- no longer be used in support of a middle Pleistocene agefor associated Acheulian artifacts (Mishra et al., 1995) thatmagnetic’’ fraction of the Toba tephra at Bori is significantly

different from the age of known large-magnitude eruptions have been reworked into their present fluvial context.at Toba (Chesner et al., 1991).

Our fission-track dating of the Indian Toba tephra takes ACKNOWLEDGMENTSadvantage of recent methodological developments that giveaccurate and precise ages for fine-grained tephra using their 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,hydrated glass shards (Westgate, 1989; Sandhu and West-and L. Hill for conducting the sample dissolution for the ICP-MS analyses.gate, 1995). Track counts are accumulated by examinationJames Hester and Bhaskar Deotare helped CAC and RK, respectively.

of individual shards so that any contaminant material en- Reviews by Stephen Self and William Rose are gratefully acknowledged.countered can be readily ignored. A homogeneous glass This work was funded by an NSERC operating grant to JAW.shard population is indicated by the low dispersion of themajor-element data (Fig. 2) and U content (4.99 { 0.27 REFERENCESppm). Toba tephra from two sites in western India weredated: UT1069 at Pawlaghat Place, Narmada River, and Acharyya, S. K., and Basu, P. K. (1993). Toba ash on the Indian subconti-UT1070 at Gandhigram Place, Purna River (Fig. 1, Table nent and its implications for correlation of late Pleistocene alluvium.

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