Methods for the Identification of the Three Major Eruptions from the Toba Complex, Indonesia

Abstract:

Tephra provide a valuable tool for palaeoenvironmental reconstructions when correctly identified. Here we present three approaches to discriminate between the Oldest, Middle and Younger Toba Tuff (OTT, MTT and YTT, respectively), these being three major Quaternary eruptions of the Toba volcano in Sumatra, Indonesia. Toba ash forms an important stratigraphic marker across peninsular India, and it has been widely used for archaeological and palaeoenvironmental studies. Major element ratios in YTT show post-depositional diagenetic effects from ash-environment interaction. Na₂O/K₂O varies up to 50% between marine and terrestrial sites, because of seawater induced alkali migration and site-specific water pH. However, major elements cannot discriminate between OTT and YTT, but these can be identified using trace elements data. Compiling several hundreds analyses of selected rare elements, four glass populations are recognized, which define the YTT and represent different stages of magma evolution. In contrast, OTT has one, homogeneous, low Ba glass population. Thus, the presence of multiple glass populations now allows the YTT to be clearly distinguished from OTT and MTT. This discrimination was used to resolve the issue surrounding the age of Toba ash from Morgaon, a tephra deposit in western India recently dated at c. 809 ka (Ar-Ar). New LA-ICP-MS analyses revealed that the Morgaon tephra contains the same four glass populations that define YTT, indicating this cannot be OTT. In addition, a study of the area density of spontaneous fission tracks (ρs) in glass shards of the Morgaon tephra sample (91±19 tracks/cm²) are within the range of the YTT (70-181 tracks/cm²) and well below the OTT (1567 tracks/cm²), again confirming its YTT origins. Indeed, all other Toba tephra occurrences from India thus far analysed for trace elements and spontaneous FT density show a YTT origin.