Characterisation of Late Pleistocene Tephra in Deep Sea Sediments of Arabian Sea

Many sediment cores raised from the deep Arabian Sea, west of Lakshadweep Ridge, record the occurrence of 10-38 cm thick tephra layers, 35 to 195 cm below sea floor. These layers arc difficult, mainly due to admixing with biogenic debris, The non-carbonate coarse fraction of these tephra beds consists mostly of transparent fresh glass shards of K-rich, high silica rhyolitic composition, with some pumice fragments and crystals. The glass shard morphology is dominated by bubble wall type that often shows triple junctions. Individual analyses of glass shards from these volcanic ash layers is comparable to those of the proximal (Malaysia) and distal (Bay of Bengal and Indian subcontinent) tephra beds associated with the 75 ka Toba caldera cruption in north Sumatra. Characterisation on the basis of shard morphology and chemical composition demonstrates that all the tephra layers identified in the sediment cores from this study are correlative to the Youngest Toba Tuff. This new occurrence of youngest Toba ash layers in Arabiiin Sea, together with its identification further north in Murray Ridge and recent discovery of YTT glasses in the South China Sea and Central Indian Ocean Basin expand the previously known dispersal of Toba ash to much longer distances, both towards east and west from the source, and suggest that the volume of erupted magma was larger than the previously interpreted.