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et al., 2012 ). This eruption is considered one of the largest volcanic eruptions of the
past 2 million years. It produced at least 2,500-3,000 km 3 of dense rock equivalent
(DRE) of pyroclastic ejecta, of which at least 800-1,000 km 3 was ash (Rose and
Chesner, 1987 ; Chesner et al., 1991 ;Buhring and Sarnthein, 2000 ), and the eruption
covered peninsular India (located roughly 3,000 km from Toba) in a layer of volcanic
ash initially 10-15 cm thick (Williams et al., 2009a ), termed the Youngest Toba
Tephra, or YTT (Acharyya and Basu, 1993 ; Shane et al., 1995 ; Shane et al., 1996 ;
Westgate et al., 1998 ). By way of comparison, the eruption of Krakatoa in 1883
produced no more than 20 km 3 of ejecta, and the 1815 eruption of Tambora produced
30-33 km 3 (Foden, 1986 ;Selfetal., 2004 ). The YTT has been recovered from
marine cores in the Bay of Bengal (Ninkovich et al., 1978a ; Ninkovich et al., 1978b ;
Ninkovich, 1979 ), the Indian Ocean to at least 14
south of the equator, the Arabian
Sea and the South China Sea (Pattan et al., 1999 ;Buhring and Sarnthein, 2000 ;
Song et al., 2000 ; Liu et al., 2006 ). Buhring and Sarnthein ( 2000 ) noted that because
the YTT continues to be found further and further from source, the initial DRE ash
volume estimate is likely to be an underestimate, a conclusion endorsed by Williams
( 2012a ) and supported by the very recent discovery of YTT crypto-tephra in a core
in Lake Malawi, some 7,300 km from Toba volcano (Chorn, 2012 ;Laneetal., 2013 ).
(Crypto-tephra are volcanic ash layers invisible to the eye but evident in geochemical
analysis.)
Westgate et al. ( 1998 ) analysed the major-element composition of the YTT glass
shards, as well as their trace element and rare earth element content. They found that
the YTT could be clearly distinguished from both the Oldest Toba tuff (OTT) and the
Middle Toba tuff (MTT), dated respectively to 840,000
°
±
30,000 years and 501,000
±
5,000 years ago. The great value of the 74 ka Toba ash outcrops in India is that
they provide an isochronous marker bed (i.e., one that is of the same age and was laid
down at the same time) across the subcontinent, allowing inferred environmental and
climatic changes from before and after the eruption to be compared (Williams et al.,
2009a ; Williams et al., 2010a ). For example, the vegetation growing in semi-arid
north-central India consisted of forest before the eruption and of grassland or open
woodland after it, and the pollen grains preserved in a marine core in the Bay of
Bengal also show a reduction in forest pollen in the sediment above the YTT layer in
the core (Williams et al., 2009a ). However, because the YTT has been to some degree
reworked, some workers have questioned its value as an isochronous marker bed (Gatti
et al., 2011 ). In response to this, one can argue that for relatively pure ash (80-90 per
cent of the host sediment) to have accumulated in depressions in the landscape as a
result of run-off and mass movement, it seems likely that such processes would have
occurred quite soon after the deposition of the primary air fall ash, probably no more
than a few years later. Distinguishing between primary and secondary ash layers is
not always easy. In fact, the recently acquired age of 73.88
±
0.32 ka for the YTT
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