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Sumatra 74,000 years ago (Williams et al.,
2009b
; Williams et al.,
2010a
; Williams,
2012b
; Williams,
2012c
). The precise age is 73.88
0.32 ka (Storey et al.,
2012
).
This eruption was one to two orders of magnitude larger than the historic and highly
destructive eruptions of Tambora (1815), Krakatau (1883), Agung (1963) and Pinatubo
(1991) (Chesner et al.,
1991
). Apart from killing many people, these historic eruptions
were followed by a 0.5
±
C or so drop in mean global temperature, which persisted for
one or more years (Rampino and Self,
1992
; Kelly et al.,
1996
;Parkeretal.,
1996
).
Some recent eruptions have had additional effects, of which sustained cooling of the
ocean surface is perhaps the most significant (Gleckler et al.,
2006
). The Pinatubo
eruption in the Philippines was followed by a weakening of the regional hydrologic
cycle and prolonged drought (Trenberth and Dai,
2007
). A further apparent effect of
the Pinatubo eruption was a change in the mode of the Arctic Oscillation (Stenchikov
et al.,
2002
). (The Arctic Oscillation is controlled by surface atmospheric pressure
and is reflected in periodic southward extensions of cold Arctic air into mid-latitudes,
which have major impacts on North American weather).
In light of these impacts from quite minor volcanic eruptions, we might expect
some significant impacts on regional and global temperature and precipitation arising
from the 74 ka Toba eruption. Ninkovich et al. (
1978a
;
1978b
;
1979
) were the first
to report ash from the 74 ka Toba eruption in marine sediment cores collected from
the Bay of Bengal. Williams and Royce (
1982
) also found volcanic ash in the Son
Valley of north-central India. This was the first time any Quaternary ash had ever
been reported from India, and it soon led to discoveries of late Quaternary volcanic
ash across the continent (Acharyya and Basu,
1993
). Williams and Clarke (
1995
)
compared the
87
Sr/
86
Sr values preserved in the Son Valley ash with those obtained by
Whitford (
1975
) from welded tuffs around the Toba caldera and found that they were
identical and related to the most recent (74 ka) eruption.
Geochemical analysis of the volcanic ash by Shane et al. (
1995
;
1996
) andWestgate
et al. (
1998
) confirmed that the ash deposits across peninsular India all belonged to this
74 ka Toba eruption, which mantled India in a layer of ash 10-15 cm thick. In order to
determine the possible impact of this event on the vegetation and climate of India and
the wider region, the
°
13
C values in pedogenic carbonate nodules in paleosols above
and beneath the 74 ka ash layer were analysed across a 400 km transect in north-
central India (Ambrose et al.,
2007c
; Williams et al.,
2009b
). Before the eruption, this
part of India supported woodland and forest; after it, the forest was replaced by open
woodland and grassland. Analysis of the pollen preserved in sediments above and
beneath the 74 ka volcanic ash within a marine core from the Bay of Bengal revealed
a similar pattern of vegetation change (Williams et al.,
2009b
). Other workers have
concluded that the Toba eruption had little or no impact in India and the wider region
(Gathorne-Hardy and Harcourt-Smith,
2003
; Petraglia et al.,
2007
), a view that has
not gone unchallenged (Williams et al.,
2010a
). However, as Williams (
2012c
) was at
pains to point out, much of the existing work on Toba lacks the chronologic precision
