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

Abstract

Determination of the area density of spontaneous fission tracks (ρ _s) 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 ρ _svalues for YTT, uncorrected for partial track fading, range from 70 to 181 tracks/cm ²with a weighted mean of 108 ± 5 tracks/cm ², based on 15 samples. Corrected ρ _svalues for YTT are in the range of 77–140 tracks/cm ²with a weighted mean of 113 ± 8 tracks/cm ², within the range of uncorrected ρ _svalues. No significant difference in ρ _sexists between YTT samples collected from marine and continental depositional settings. The uncorrected ρ _sfor OTT is 1567 ± 114 tracks/cm ²so that confusion with YTT is unlikely.

The ρ _svalues 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.