Methods for determination of the age of Pleistocene tephra, derived from eruption of Toba, in central India

Abstract

Tephra, emplaced as a result of Pleistocene eruption of the Indonesian 'supervolcano' Toba, occurs at many localities in India. However, the ages of these deposits have hitherto been contentious; some workers have argued that these deposits mark the most recent eruption (eruption A, ca 75 ka), although at some sites they are stratigraphically associated with Acheulian (Lower Palaeolithic) artefacts. Careful examination of the geochemical composition of the tephras, which are composed predominantly of shards of rhyolitic glass, indicates that discrimination between the products of eruption A and eruption D (ca 790 ka) of Toba is difficult. Nonetheless, this comparison favours eruption D as the source of the tephra deposits at some sites in India, supporting the long-held view that the Lower Palaeolithic of India spans the late Early Pleistocene. In principle, these tephra deposits should be dateable using the K-Ar system; however, previous experience indicates contamination by a small proportion of ancient material, resulting in apparent ages that exceed the true ages of the tephras. We have established the optimum size-fraction in which the material from Toba is concentrated, 53-61 μm, and have considered possible origins for the observed contamination. We also demonstrate that Ar-Ar analysis of four out of five of our samples has yielded material with an apparent age similar to that expected for eruption D. These numerical ages, of 809 ± 51, 714 ± 62, 797 ± 45 and 827 ± 39 ka for the tephras at Morgaon, Bori, Gandhigram and Simbhora, provide a weighted mean age for this eruption of 799 ± 24 ka (plus-or-minus two standard deviations). However, these numerical ages are each derived from no more than 10-20% of the argon release in each sample, which is not ideal. Nonetheless, our results demonstrate that it is feasible, in principle, to date this difficult material using the Ar-Ar technique; future follow-up studies will therefore be able to refine our preparation and analysis procedures to better optimize the dating.

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

“…7) yield 40 Ar/ 39 Ar ages that are indistinguishable from the previously reported radio-isotopic 40 Ar/ 39 Ar age for YTT, 73 AE 4 ka , relative to the use of the same standard ages and decay constants (see discussion in Section 6.2). Our data contradict the data of Westaway et al (2011) who suggest an approximate age of 800 ka (comparable to an OTT age) for tephra throughout the Middle Son Valley. The data supports the work of Smith et al (2011a) who correlated the distal biotite shards from both sites to the proximal Toba YTT deposits.…”

“…The current 40 Ar/ 39 Ar study of the ash is the first robust chronometric study of the ash deposits in the Son and Jurreru River valleys that confirm the previous data and inferences about the age of the deposits, thus allaying criticisms that the ash could represent the OTT event (e.g., Westaway et al, 2011). The YTT ash deposits associate with Middle Palaeolithic assemblages in the Son Valley (Jones and Pal, 2009;Petraglia et al, 2012) and the Jurreru Valley Petraglia et al, 2012).…”

“…The OSL ages are apparently erroneous and claims that grains have been 'mixed in slump deposits' find no stratigraphic support. Westaway et al (2011) have suggested the Middle Son Tephra is OTT, which, if true, would necessitate significant revision of geological, environmental, and archaeological data in the region.…”

“…However, owing to poor stratigraphic correlation, the large distances between sites, and the controversial nature of the archaeological assemblages, a more extensive 40 Ar/ 39 Ar dating programme that adopts the same robust approach outlined here is required to test the hypothesis that more than one distal Toba ash fall event is preserved in India. Certainly the work of Westaway et al (2011) at other sites (e.g., Bori tephra) requires thorough examination and testing with renewed data collection. Work at the NERC Argon Isotope Facility (SUERC) on the provenance of these deposits is on going.…”

Discovery of Youngest Toba Tuff localities in the Sagileru Valley, south India, in association with Palaeolithic industries

“…Geochemical analyses of India tephra deposits have confirmed the presence of YTT in the Jurreru, Son, Purna, Mahanadi, Kukdi, Vansadhara, Barakar and Narmada Valleys (Acharyya and Basu, 1993;Shane et al, 1995;Westgate et al, 1998;Petraglia et al, 2007Petraglia et al, , 2012Pearce et al, 2014). Ash deposits identified in a number of other valleys are assumed to also comprise YTT horizons, such as in the Indravati, Brahmani, and Nagavali valleys (Acharyya and Basu, 1993), although some investigators consider that ash in some sites in India correlate with the 0.84 Ma Older Toba Tuff (OTT) (Westaway et al, 2011). Yet, assertions about the presence of OTT deposits in India have been called into question on the basis of ArgoneArgon dating (Mark et al, 2014), differences in geochemistry, including distinctions of biotite crystals Smith et al, 2011), and area density of spontaneous fission tracks in glass shards .…”