Individual glass shard trace element analyses confirm that all known Toba tephra reported from India is from the c. 75‐ka Youngest Toba eruption

Abstract

Uncertainty over the identity and age of Toba tephras across peninsular India persists, with radiometric age dates contradicting earlier compositional data, which have been used to identify this important stratigraphic marker as the Youngest Toba Tuff (YTT). To address this issue, new single glass shard analyses have been performed for samples from Morgaon and Bori (north‐western India), which have recently been dated at c. 800 ka. These, and indeed all Toba tephra samples thus far analysed from India, show the presence of four populations of glass shards (defined by their Ba/Y ratio), which uniquely identifies them as products of the c. 75‐ka Youngest Toba eruption. Confirmation that the YTT fingerprint is characteristic comes from new analyses of Oldest Toba Tuff (OTT) glass shards from five sites in the Indian Ocean. These are compositionally identical to Layer D from the ODP site 758 sediment core (c. 800 ka), and belong to a single, low‐Ba population, clearly different from YTT. These analyses show that there is essentially no reworked OTT material in the YTT eruption, and indicate unequivocally that all known Toba tephra occurrences in India belong to the c. 75‐ka Youngest Toba eruption.

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).…”

Tephra glass chemistry provides storage and discharge details of five magma reservoirs which fed the 75 ka Youngest Toba Tuff eruption, northern Sumatra

“…When Ba and Sr concentrations are plotted against many other incompatible elements (e.g. Rb, Cs, Nb, Ta, Zr, Hf, U, Th, REE), MTT and YTT consist of several glass shard populations (Westgate et al , ; Pearce et al , ; Westgate and Pearce, ). These are distinct within individual eruptions, but show some overlap between eruptions (e.g.…”

“…Assigning distal Toba tephra to a particular eruption using glass shard major element compositional data has proved difficult because OTT, MTT and YTT have almost identical compositions (Westgate et al , ; Smith et al , ; Gatti et al , ). Nonetheless, some have suggested that major element discrimination is possible; for example, by using FeO (Westaway et al , ), but this has subsequently been discounted (Gatti et al , ; Pearce et al , ; Westgate and Pearce, ). Biotite major element composition provides a means to distinguish these eruptions (Smith et al , ), and biotite is often found in distal deposits, where small flakes travel easily within the eruption cloud.…”

“…It is not possible to distinguish unequivocally between YTT, OTT or MTT using just major element glass shard chemistry, but they can be separated using glass trace element compositions, biotite compositions or spontaneous fission track density (Smith et al , ; Westgate et al , ; Pearce et al , ; Westgate and Pearce, ).…”

“…Glass trace element compositions, however, clearly separate YTT, MTT and OTT (Fig. ), and while there are significant overlaps, the range and distribution of glass compositions within an eruption clearly identifies each unit: a single shard may not have a uniquely identifiable composition, but several to tens of shards will define fields which are characteristic (Westgate et al , ; Pearce et al , ; Westgate and Pearce, ). In terms of Ba vs Y, OTT has a single low‐Ba population, rarely exceeding 300 ppm, and while both MTT and YTT also contain glass shards with low Ba contents, their compositions extend to >1500 ppm Ba.…”