Geochemical fingerprinting of the widespread Toba tephra using biotite compositions

Toba caldera, Sumatra, is one of the largest and most explosive volcanoes on Earth, erupting some of the most voluminous volcanic deposits. Chronologically these eruptions are ∼790 ka Older Toba Tuff, ∼500 ka Middle Toba Tuff, and ∼74 ka Younger Toba Tuff. Ash from these eruptions is dispersed over the entire region from India, through Malaysia to Indonesia where it forms isochronous markers that have been documented in a variety of sedimentary sequences including palaeoclimate archives and archaeological sites. The chemistry of the volcanic glass shards usually enables distal volcanic ash units to be correlated to a known eruption. However, the magmas involved in the three largest Toba eruptions are compositionally similar and consequently the glass chemistry that is used to differentiate eruption units cannot be used to identify distal deposits. Here we show that the composition of biotite crystals, which occur with glass in the distal deposits, can be used to fingerprint the deposits of the Younger Toba Tuff. Biotite in Younger Toba Tuff has a lower FeO/MgO (2.1-2.6) than in the products of older eruptions (2.8-3.7). Correlations using these distinct biotite compositions indicate that the ash found in Malaysia and in archaeological sites in India was from the ∼74 ka Younger Toba Tuff eruption.