Quaternary Science Reviews 21 (2002) 1593–1609
Limited global change due to the largest known
Quaternary eruption, Toba E74 kyr BP?
Clive Oppenheimer*
Department of Geography, Cambridge University, Downing Place, Cambridge CB2 3EN, UK
Received 21 September 2001; accepted 2 December 2001
Abstract
The E74 kyr BP ‘‘super-eruption’’ of Toba volcano in Sumatra is the largest known Quaternary eruption. On the basis of
preserved deposits, the eruption magnitude has been estimated at E7 1015 kg (E2800 km3 of dense magma). The largest sulphate
anomaly in the Greenland Ice Sheet Project 2 core has been identified as fallout from Toba’s stratospheric aerosol veil. Correlation
of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the Toba eruption might have played a role in triggering
a millennium of cool climate prior to Dansgaard-Oeschger event 19, although a comparable stadial preceded event 20. A possible
6 yr duration ‘‘volcanic winter’’ immediately following the eruption has also been proposed as the cause of a putative bottleneck in
human population supporting, in a general way, the ‘‘Garden of Eden’’ model for the origin of modern humans. However, along
with counter arguments regarding the timing of any demographic crash, there remain major gaps in our understanding of the
E74 kyr BP Toba eruption that hinder attempts to model its global atmospheric and climatic, and hence human consequences.
The tephra record reveals basic aspects of the eruption style but calculations of the duration, and hence intensity and plume height
of the event, are poorly constrained. Furthermore, estimates of the sulphur yield of the erupting magma, central to predictions
of its atmospheric and climatic impacts, vary by two orders of magnitude (3.5–330 1010 kg). Previous estimates of globally
averaged surface cooling of 3–51C after the eruption are probably too high; a figure closer to 11C appears more realistic. The
volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the Toba
super-eruption. r 2002 Published by Elsevier Science Ltd.
1. Introduction
The Late Pleistocene eruption of the Younger Toba
Tuff (YTT) may have been the greatest single volcanic
cataclysm in the Quaternary. It expelled an estimated
7 1015 kg (or 2800 km3 dense rock equivalent, DRE, at
2500 kg m3) of rhyolitic magma and made a sizeable
contribution to the 100 30 km caldera complex occupied today by Lake Toba in northern Sumatra (Fig. 1;
Rose and Chesner, 1987, 1990; Chesner and Rose,
1991). The size of the eruption (3500 times greater than
the largest historic eruption, the 1815 outburst of
Tambora volcano, Indonesia) and its approximate
coincidence with global environmental and climatic
change at the end of Oxygen Isotope Stage (OIS) 5a
led to speculation that it accelerated the transition into
the last Ice Age (Rampino and Self, 1992, 1993a). It has
also been suggested that global cooling, triggered by the
*Tel.: +44-1223-719-712; fax: +44-1223-333-392.
E-mail address: [email protected] (C. Oppenheimer).
0277-3791/02/$ - see front matter r 2002 Published by Elsevier Science Ltd.
PII: S 0 2 7 7 - 3 7 9 1 ( 0 1 ) 0 0 1 5 4 - 8
YTT eruption, precipitated an environmental catastrophe that resulted in near extinction of contemporaneous human populations (Gibbons, 1993; Rampino
and Self, 1993b; Ambrose, 1998; Rampino and Ambrose, 2000; and cited in Harpending and Rogers, 2000;
Hewitt, 2000). In view of the significance of these
palaeodemographic implications, and a range of recently available tephrostratigraphic, ice core and petrologic evidence that bears on the possible consequences of
the eruption, it is timely to re-evaluate critically the basis
for the claims made for Toba. The aim of this paper is
also to stimulate broad, interdisciplinary research across
the Quaternary science community that will advance our
understanding of the global effects of very large
eruptions.
2. Age of the YTT
The YTT was not the first large eruption from Toba.
It was preceded by the Haranggoal dacite (1.2 Myr BP),
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C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
Fig. 1. The 100 30 km2 Toba caldera is situated in north-central Sumatra around 200 km north of the equator. It is comprised of four overlapping
calderas aligned with the Sumatran volcanic chain. Repeated volcanic cataclysms culminated in the stupendous expulsion of the Younger Toba Tuff
around 74 kyr ago. The lake area is E1100 km2. Samosir Island formed as a result of subsequent uplift above the evacuated magma reservoir. Such
‘‘resurgent domes’’ are typically seen as the concluding phase of a large eruption. Landsat Enhanced Thematic Mapper Plus (ETM+) ‘‘browse
images’’ for path/row 128/58 (6 September 1999) and 129/58 (21 January 2001) from http://landsat7.usgs.gov/. Copyright USGS.
the Oldest Toba Tuff (840 kyr BP) and Middle Toba
Tuff (501 kyr BP) (Chesner and Rose, 1991; Chesner
et al., 1991). Dates obtained for the YTT have
tended to be remarkably consistent given the very
different techniques that have been used to obtain them
(Table 1). The best current estimate for its age is
7472 kyr. There was some controversy surrounding a
more recent explosive eruption of Toba, at around
30 kyr BP. This had been suggested by fission-track
measurements of a zircon crystal isolated from a 90 cm
thick ash deposit at Selangor in Malaysia, 350 km from
Toba, and apparently corroborated by 14C dates on
wood fragments extracted from sediments directly
beneath this ash. However, the lack of potential
horizons on Sumatra, or in deep-sea cores, that correlate
with this age, and the chemical similarity of the YTT
and Selangor tephra, led Chesner et al. (1991) to doubt
the interpretation. Their own subsequent fission-track
analysis on a sample of the Malaysian ash yielded an age
of 6877 kyr, within analytical error of other dates on
known YTT material.
3. Deposits of the YTT eruption
The eruption is thought to have ensued as the roof of
a large magma chamber, located up to 10 km deep in the
crust, began to founder (Chesner, 1998; although
Beddoe-Stephens et al., 1983, estimated a shallower
depth for the magma body of around 3–4 km; recent
seismic tomographic investigations have suggested the
presence of two principal melt regions between the lake
and a depth of 10 km, Masturyono et al., 2001). This
opened up ring fractures, which fed massive outpourings
of pyroclastic flows. If this interpretation is correct, it
implies that the caldera formed in a piecemeal fashion
(e.g., Moore and Kokelaar, 1998) during the eruption,
and not catastrophically, afterwards. Supporting evidence is provided by the generally symmetrical distribution of YTT around the present day lake, and the
absence of plinian tephra fall deposits from an eruption
above a central vent. This latter feature of the YTT is
unlike most studied examples of large caldera-forming
eruptions, which begin with a plinian phase. There is
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
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Table 1
Published age determinations for the YTT
Location
Material
Age (kyr BP)
Technique
Reference
Malaysia
Greenland
South China Sea
Arabian Sea
Samosir island
Prapat
Indian Ocean
Sigurapura
Prapat
Ash (glass shards)
Sulphate
Foraminifera
Foraminifera
Ignimbrite (sanidine)
Pumice (sanidine)
Foraminifera
Ignimbrite (biotite)
Quartz crystals
6877
7175a
71
72.4–74.675
7374
7473
75
75712
81717
Fission-track
Ice core stratigraphy
18
O/16O stratigraphy
18
O/16O stratigraphy
Ar–Ar
K–Ar
18
O/16O stratigraphy
K–Ar
ESR
Chesner et al. (1991)
Zielinski et al. (1996)
Huang et al. (2001)
Schulz et al. (1998)
Chesner et al. (1991)
Ninkovitch et al. (1978b)
Ninkovitch (1979)
Ninkovitch et al. (1978b)
Wild et al. (1999)
a
70 kyr using the official GISP2 timescale, 68 kyr using the refined Meese et al. (1997) timescale (see Fig. 4 caption).
some room for doubt with this conclusion, however.
Although no pumice fallout has been found preserved
beneath the base of the massive outflow sheets of
ignimbrite, which cover >20,000 km2 of northern
Sumatra, it is conceivable that the great erosive power
of the pyroclastic flows could have scoured away any
deposits from an initial plinian phase.
There is no shortage of tephra fall deposit from the
Toba eruption, however (Fig. 2). The pyroclastic flows
generated immense co-ignimbrite (‘‘phoenix’’) clouds of
fine ash and entrained (and heated) air, as the denser
pyroclasts settled out. These must have risen from
multiple sources across a wide area around the subsiding
caldera. They are crucial for understanding the potential
global atmospheric and climatic impacts of the eruption,
since they would have been capable of carrying large
quantities of sulphur and other volatiles into the
stratosphere.
The YTT ignimbrites form a well-developed outward
sloping (1–41) plateau (Knight et al., 1986). Much of the
Fig. 2. Distribution of recovered YTT phoenix cloud ash-fall deposits. Recent studies have fingerprinted deep-sea tephra layers south of India and in
the South China Sea. Data from Ninkovitch et al. (1978a, b), Acharyya and Basu (1993), Pattan et al. (1999), Buhring
.
et al. (2000), Gasparotto et al.
(2000), and Song et al. (2000).
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C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
YTT is un-welded and contains pumice blocks up to
80 cm in size. Up to 400 m thickness of YTT is exposed
in the caldera walls enclosing Lake Toba. A posteruption resurgent dome occupies the central part of the
caldera—the western part of the dome is the island of
Samosir (Fig. 1). Knight et al. (1986) believed Samosir
island to be composed of Middle Toba Tuff, challenging
the YTT magnitude estimate made by Rose and Chesner
(1987). Chesner et al. (1991) dated tuff from the island at
7374 kyr BP, confirming it as part of the YTT and
supporting the original volume calculation, which was
partly based on the thickness of tuffs on Samosir.
Taking the average thickness of intracaldera deposits to
be 400 m indicated a mass of 2.5 1015 kg (1000 km3,
dense-rock-equivalent, DRE) of tephra within the
caldera alone.
The co-ignimbrite clouds drifted above both the
Indian Ocean and South China Sea depositing 10 cm
or more thickness of ash on at least 1% of the surface
area of the planet. Toba ash has been identified in many
deep-sea sediment cores (Fig. 3) and on the Indian subcontinent, up to 3100 km from the vent, providing a
valuable time marker for palaeontological and palaeoanthropological studies. The minimum mass of this
tephra fall deposit was estimated as 2 1015 kg (a DRE
volume of 800 km3; Rose and Chesner, 1987). To
estimate the volume of the YTT itself, Rose and
Chesner (1987) assumed a mean thickness of 50 m
outside the caldera covering an area of >20,000 km2
(>2.5 1015 kg or 1000 km3 DRE) and added this to the
intracaldera fill. The total minimum mass of the entire
deposit comes to 7 1015 kg, at least 30% of which is in
the co-ignimbrite cloud fallout.
Once more people began looking for it, the YTT
turned up further afield (Fig. 2). Two independent
studies, based on deep-sea cores retrieved from the
South China Sea, recently extended the known distribution of the YTT, and may suggest that the Rose and
Chesner (1987) calculation of the eruption magnitude is
an under-estimate. The conventional interpretation of
the many Quaternary tephra layers found in this region
is that they derive from volcanoes in the Philippines,
such as Pinatubo. Song et al. (2000) recovered volcanic
glass bubble walls and mica crystals about 16 m beneath
the seabed off south-eastern Vietnam, 1500 km northeast of Toba. The tephra layer was about 2 cm thick and
fell within the oxygen isotope date range of 64.1–
79.3 kyr BP. Chemical fingerprinting of the >63 mmsized glass shards matched them closely with previously
identified distal Toba tephra.
Buhring
.
et al. (2000) discovered a 3 cm thick layer of
rhyolitic ash in sediment cores even further away, some
1800 km northeast of Toba, retrieved from the seabed
between Borneo and Indochina. The ash was dated to
E74 kyr BP and is composed almost entirely of glass
shards. Analyses of individual fragments indicated SiO2
Fig. 3. SEM micrographs of glass shards from the YTT layer in
Bengal Fan sediment cores (Gasparotto et al., 2000): (a) assorted
vesicular and bubble wall fragments (100 mm scale bar shown), (b)
individual bubble wall at higher magnification (scale bar divisions are
100 mm). Photographs courtesy of Giorgio Gasparotto, University of
Bologna.
contents of 75–78 wt% and similar major element
compositions to published analyses of YTT. These two
occurrences are the first YTT deposits found substantially eastwards of the caldera. They suggest that the
magnitude of the eruption might be significantly greater
than previously estimated. The dispersal of the ash
cloud across the South China Sea may point to eruption
during the northern hemisphere summer monsoon when
south-westerly winds prevail at low- to middle-tropospheric levels, transporting ash from lower parts of the
co-ignimbrite clouds.
Deep-sea coring in the Central Indian Basin has
exhumed further suspected Toba tephra, up to 1500 km
south of previous occurrences (Pattan et al., 1999), and
further investigations have revealed YTT tephra on the
western continental margin of India (Pattan et al., 2001)
and in the Arabian Sea (Schulz et al., 1998). These new
finds push dispersal of >63 mm glass shards well into the
southern hemisphere to latitudes of 141S, and westwards
as far as longitude 641E. Analyses of putative Toba
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
tephra from India, Sumatra, Malaysia, and, importantly, an Ocean Drilling Program sediment core located
halfway between Sumatra and Sri Lanka that contains
Oldest and Middle Toba tephra as well as YTT, indicate
that all the Toba ash so far found on peninsular India
belongs to the YTT (Shane et al., 1995, 1996; Westgate
et al., 1998) and not to some of the earlier eruptions
(Mishra and Rajaguru, 1996). (If correct, this clears up a
palaeoanthropological controversy concerning ages of
stone tools dated by their stratigraphic association with
respect to Toba deposits.) Toba ash on the eastern coast
of India is found in several metre-thick layers within
alluvial sections and with grain sizes of a few millimetres
(Acharyya and Basu, 1993). These considerable thicknesses, however, reflect post-deposition reworking.
4. Eruption parameters: intensity, duration
The granulometry of feldspar crystals in Indian Ocean
YTT tephra horizons was investigated by Ledbetter and
Sparks (1979) to estimate the eruption duration. By
computing settling velocities for the crystals in water,
and from an estimate of the contemporary water depth,
they concluded that the eruption lasted between 9 and
14 days. Combined with the eruption magnitude
estimate, this suggests a mean eruption intensity of
around 7 109 kg s1 of magma, 1–3 orders of magnitude greater than the magma discharge rates calculated
by Carey and Sigurdsson (1989) for a number of historic
and prehistoric plinian eruptions.
Woods and Wohletz (1991) developed a model for the
dynamics of co-ignimbrite clouds and concluded that
their rise height is less sensitive to intensity than for
plinian plumes. Consequently, an eruption like Tambora 1815, which was characterised by fountain
collapse, propelled its co-ignimbrite clouds to only
about the same height (around 23 km) as the lower
! in 1982. Taking
intensity plinian column of El Chichon
the intensity of the Toba eruption as 7.1 109 kg s1,
they estimated the plume height for the YTT cloud as
3275 km. Rampino and Self (1993a) considered this to
be an under-estimate, suggesting that for peak intensities
approaching 1010 kg s1, column heights may have
reached 40 km.
In any case, these calculations of plume height cannot
be reliable when it is considered that neither the
magnitude nor duration of the eruption are well
constrained. In particular, the estimate of eruption
duration may need to be reconsidered. Since the model
based on the vertical size grading in deep-sea tephra
layers, elaborated by Ledbetter and Sparks (1979), there
has been increasing recognition of the importance of ash
aggregation during airborne transport. There have also
been observations and models of the surprisingly fast
sedimentation of ash clumps through the water column
1597
(Weisner et al., 1995; Carey, 1997). These effects
probably have significant consequences for grain size
profiles in tephra layers settling on the seabed.
Unfortunately, all these uncertainties necessarily propagate into any modelling of the chemical and radiative
impacts of Toba’s stratospheric aerosol veil.
Gasparotto et al. (2000) studied thirteen piston cores
containing YTT tephra, retrieved from the Bengal deep
sea fan off the coast of Bangladesh and India (Fig. 3).
Detailed granulometry of sub samples within the layer
revealed two grain size populations. This had been
observed previously by Ninkovitch et al. (1978a) who
considered the possibility that these reflected different
transport dynamics of a plinian plume and phoenix
cloud. However, as there is no evidence for plinian style
activity, an alternative explanation is that aggregation of
particles in a co-ignimbrite cloud caused them to fall out
of the plume at the same distance as larger, individual
particles. As they dropped through the water, these
aggregates could have broken up so that the final
deposit includes two grain size populations. The
different wind dispersal patterns suggested by the
widespread tephra distribution (Fig. 2) add further
complexity to understanding and modelling observed
grain size spectra.
5. Impacts on the atmosphere and climate
Assessing the impacts of Toba on the Earth system is
highly challenging given the uncertainties in the key
parameters for the eruption (intensity, height, magnitude), and amounts of gaseous sulphur species released.
Initial interest focused on the apparent coincidence of
the eruption with the onset of the Last Glaciation.
Rampino and Self (1992) dwelled on the coincidence of
the YTT eruption with the transition to glacial climate
at the OIS 5a-4 boundary at 67.5 kyr BP. They argued
that Toba caused a ‘‘volcanic winter’’—a global mean
surface temperature drop of 3–51C—which accelerated
the glacial transition from warm to cold temperatures of
the last glacial cycle. They also hinted that falling sealevel accompanying the incipient glaciation may have
even played a role in triggering the eruption by reducing
the load on Toba’s magma reservoir promoting overpressure within it. However, given the relatively shallow
depth of the Toba magma chamber, Toba’s distance
from the sea (>100 km), and the low strength of the
crust, a more likely mechanism whereby climate change
could have triggered the eruption may be via the impact
of sea-level change on seismicity along the Great
Sumatra Fault that runs the length of the island,
accommodating oblique convergence between the Eurasian and Australasian plates. Major earthquakes have
been implicated as eruption triggers (e.g., Linde and
Sacks, 1998), and the location of Toba itself may be
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C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
related to an apparent restraining bend in the Great
Sumatra Fault which could have promoted the development of large, long-lived magma reservoirs there in
the first place (Wark and McCaffrey, 2001).
Rampino and Self (1992) argued that although global
cooling was already underway at the time of the YTT, it
had been sluggish up to that point. North Atlantic ocean
surface temperatures did not cool significantly, retarding
ice sheet growth. But the aerosol veil from Toba, they
believed, was sufficient to overcome this slow start and
push climate into fully glacial conditions. They suggested that the regional climatic response of Canada to
the volcanic forcing played a particularly important
role, reporting climate model results that indicated
around a 121C reduction in summer temperatures in
Canada. This would have favoured the growth of the
Laurentide ice sheet, providing a strong positive feedback to the combined Milankovitch and volcanic
forcing at an especially sensitive time.
Since publication of this work, improved evidence for
the timing of the eruption in relation to regional-tohemispheric scale climate changes has emerged from the
Greenland ice core record and high-resolution marine
cores. In addition, understanding of the atmospheric
and climatic impacts of volcanic eruptions has advanced
considerably, largely through investigations of the
effects of the Mt. Pinatubo eruption (e.g., Minnis et al.,
1993; McCormick et al., 1995). In particular, these have
highlighted the importance of eruptive volatile budgets.
5.1. Volatile yields
There is no doubt from the size of the Toba caldera
and extent of ignimbrite and co-ignimbrite fall deposits
that the YTT was the result of a monumentally large
eruption. Critical for evaluation of its potential climatic
impacts, however, are estimation of the gaseous sulphur
yield (Table 2) and resulting sulphate aerosol loading of
the stratosphere (Robock, 2000). Rose and Chesner
(1990) estimated gas contents of the YTT magma to
have been about 0.05 wt% H2S and 3 wt% H2O. It is
significant that the primary sulphur species released
would have been H2S because of the reducing conditions
of the magma. Once in the atmosphere, though,
oxidation to SO2 would presumably have been rapid.
Scaling the H2S content by the estimated total eruption
magnitude (7 1015 kg), they determined an eruptive
emission of 3.5 1012 kg of H2S (adding that this could
be a minimum figure if the magma contained a free
vapour phase containing sulphur prior to the eruption).
This is equivalent to >1013 kg of sulphate/water aerosol.
However, it should not be assumed that all of this
sulphur would have reached the stratosphere, since the
co-ignimbrite clouds only account for an estimated
800 km3 of the deposit. Scaling the sulphur content by
this value suggests a sulphate aerosol loading of
Table 2
Estimates of sulphur yields for the YTT eruption
Method
Sulphur yield
(kg of S)
Reference
Mineral chemistry of YTT
3.3 1012
Ice core sulphate deposition
Experimental petrology
0.57–1.1 1012
3.5 1010
Rose and Chesner
(1990)
Zielinski et al. (1996)
Scaillet et al. (1998)
o4 1012 kg. Scavenging of SO2 or H2S by ice or ash
particles or aggregates or by liquid droplets in the
plume, and subsequent deposition, would have reduced
the stratospheric loading further still (Rose and Chesner, 1990; Oppenheimer et al., 1998; Textor, 1999).
A relationship between a volcano’s sulphur yield (MS )
and the ensuing maximum surface temperature anomaly, DT; for the northern hemisphere was proposed by
Sigurdsson (1990) based on historical climate observations
DT¼ 5:9 105 MS0:31 :
ð1Þ
Eq. (1) yields a DT of 3.2 to 4.51C for this range of
sulphur yield. Rampino and Self (1992, 1993a, b)
essentially used this equation, as well as linear scaling
from the observed temperature drop following the 1815
Tambora eruption, to arrive at an estimated 3–51C
hemispheric surface temperature fall (they assumed an
aerosol loading of around 1012 kg). Such relationships
should be used cautiously, however. Not only are they
based on sometimes poorly constrained estimates of
temperature anomalies and sulphur yields, they may
also fail to represent adequately several factors that
influence the climate changing potential of an eruption
(e.g., plume and upper atmospheric dynamics). Physical
and chemical processes in stratospheric volcanic clouds
may act in a ‘‘self-limiting’’ way. Pinto et al. (1989)
applied simplified aerosol photochemical and microphysical models to show that, for a 1011 kg injection of
SO2, condensation and coagulation produce larger
aerosol particles, which are less effective at scattering
incoming sunlight, and also sediment more rapidly.
Combined, these effects lessen the expected magnitude
and duration of climate forcing expected from supereruptions, certainly if based on linear extrapolations of
observations of cooling from eruptions like Pinatubo.
An independent estimate of the sulphur yield from the
YTT eruption was obtained from the Greenland ice core
record (Fig. 4). Zielinski et al. (1996) observed sulphate
spikes at 69.4, 71, 72, and 73.6 kyr BP in the Greenland
Ice Sheet Project 2 (GISP2) core. At this depth, the error
in ages is E75 kyr. The 71 kyr BP anomaly (70 kyr BP
using the latest GISP2 chronology, Fig. 4) was by far the
strongest, with concentrations of volcanic sulphate 3–5
times the other nearby peaks. In fact, it is the largest
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
1599
Fig. 4. Oxygen isotope and total sulphate measurements for the GISP2 core between 60 and 80 kyr. The sulphate measurements (lower curve, in ppb
by mass) are un-corrected for non-volcanic contributions but the putative Toba spike is very clear. The upper curves indicate oxygen isotopic
measurements (more negative values indicate lower temperatures), which were made at much coarser temporal resolution (2 m sections of core). The
thick and thin lines indicate ages for the top and bottom of the measured layers, respectively. Dansgaard-Oeschger events 19 and 20 are labelled.
Note that the cold spell between them is not that different in magnitude and duration from the one immediately prior to event 20. Ages correspond to
the official GISP2 ‘‘Meese/Sowers’’ timescale which dates the YTT eruption to 70 kyr BP. The refined timescale elaborated by Meese et al. (1997)
which is arguably more accurate, places the eruption date a little later at 68 kyr BP. These differ slightly from the original sulphate record published
by Zielinski et al. (1996), which dated the YTT eruption at 71 kyr BP. Data from Stuiver et al. (1995, 1997) and Mayewski et al. (1997) and provided
by the National Snow and Ice Data Center, University of Colorado at Boulder, and the World Data Center-A for Paleoclimatology, National
Geophysical Data Center, Boulder, Colorado. For a review of ice core and other records of volcanism, see Zielinski (2000).
sulphate anomaly reported in the entire 110 kyr record,
and this was considered befitting for the largest known
Late Quaternary eruption. The spike was also close to
the beginning of the last glacial period, consistent with
the timing identified by other records. It was accordingly
designated as the YTT layer. They estimated the
sulphate/water aerosol loading from the volcanic
sulphate concentrations (given by the difference of total
and background sulphate concentrations, {½SO2
4 total and ½SO2
;
respectively)
in
each
of
five
con4 background
secutive 1.5 yr samples of the core containing the
identified YTT layer
Maerosol ¼ 3:2 109 f½SO2
4 total ½SO2
4 background g L rice k;
ð2Þ
where L is the length of the ice layer, rice its density, and
k is a thinning factor (approximately 16 for this depth of
ice) to account for compaction of the annual layers. The
constant multiplier converts the flux of volcanic sulphate
(in kg km2 per 1.5 yr) for each sample into an aerosol
loading, using a calibration based on nuclear bomb
fallout, and taking into account the mass difference
between H2SO4 and sulphate/water aerosol. Summing
the five samples yielded an estimate of 2.3–4.7 1012 kg
of aerosol, in reasonable agreement with the earlier
calculations based on mineral chemistry of the YTT.
However, more recent work based on experimental
petrology presents an intriguing challenge to these
estimates. In general, silicic magmas degas less sulphur
than mafic magmas. What made the silicic magmas
! (1982) and Pinatubo (1991)
erupted by El Chichon
atypical was their highly oxidising conditions. Recalling
that H2S rather than SO2 was stable in the YTT magma
implies a very different environment for evolution of
magmatic volatiles. The reducing conditions and the
relatively low temperature of the YTT magma could
indicate that Toba’s sulphur yield was actually remarkably low. Based on an extrapolation of experimental
data, Scaillet et al. (1998) estimated that only around
3.5 1010 kg of sulphur would have been liberated by
the YTT magma on eruption, 2–3 orders of magnitude
lower than the previous estimates. A 3.5 1010 kg
sulphur yield is only three times that measured for
Pinatubo’s eruption, which was probably o0.2% of the
size of the YTT event in terms of tephra production.
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
This leaves a problem of reconciling the massive ice
core sulphate anomaly with such a modest sulphur
emission. Firstly, there is potentially a circular argument
in the identification of the YTT ice core horizon. One of
the main reasons for its designation was that the largest
sulphate anomaly in the GISP2 core (five times bigger in
terms of concentration than the Tambora 1815 horizon)
was considered ‘‘appropriate for the great size of the
[YTT] eruption and the estimated large amount of
atmospheric loading associated with it’’. If the YTT was
only Tambora-sized in terms of sulphur output then it
would not have left so impressive a mark in the
Greenland core. Scaillet et al. (1998) point out that the
dating accuracy at this depth of the ice core is only
about 75 kyr and suggested therefore that the 71 kyr BP
ice core signal might be the cumulative imprint of
several eruptions with sulphur yields comparable to
Krakatau or Pinatubo. Ten Krakatau-sized aerosol
events occurring at the rate of one per century, they
claimed, could leave a 460 ppb signal similar to that
identified as Toba’s. But this explanation does not work
because the ‘‘YTT’’ sulphate anomaly spans only 6 yr,
not 1 kyr, and so is consistent with fallout from a single
event.
Another possibility is that the 71 kyr BP anomaly
records a very large Icelandic or other high northern
latitude, sulphur-rich eruption. The problem with this
argument is that such an event should have deposited
identifiable glass shards in the horizon—none has been
found. This is even hard to explain if the layer is
genuinely that of the YTT. Even shards from much
smaller eruptions, further south than Toba, have been
identified in the Greenland ice core, for example, those
belonging to the 1600 eruption of Huyanaputina
volcano, Peru, at 161S (de Silva and Zielinski, 1998).
This could reflect several factors including the different
dynamics of co-ignimbrite plumes, lower altitudes of
injection into the stratosphere, or retarded poleward
circulation of the cloud, allowing ash to fall out before
the volcanic cloud reached high latitudes, but is
surprising nevertheless.
The low yield calculated by Scaillet et al. (1998),
though, is sensitive to the assumed depth of the magma
reservoir (they used the 10 km estimate of Chesner,
1998). Such a large chamber as must have been involved
in the YTT eruption may have had considerable vertical
extent (as the present magmatic system appears to have
according to Masturyono et al., 2001) with the
possibility that sulphur was considerably more abundant at greater depth in the more mafic roots of the
system. Another potential source of sulphur in eruption
columns is from hydrothermal fluids and minerals
incorporated into the erupting fluid as the conduit and
vent widens (Oppenheimer, 1996). Any contribution
from this source would be neglected by the approach
taken by Scaillet et al. (1998), which considers only
pre-eruptive sulphur co-existing as a fluid phase in the
magma, or dissolved in the melt.
As it stands, we can only conclude that we need better
petrologic understanding of the YTT eruption, improved estimates of the magnitude of the co-ignimbrite
fall deposits, the ‘‘smoking gun’’ of YTT glass in the ice
core, and especially more reliable figures for the sulphur
yield, before being able to verify the correspondence
between the YTT eruption and atmospheric and climatic
change. This uncertainty should be kept in mind while
reading the following sections on the YTT eruption’s
atmospheric and climatic impacts.
5.2. Modelling the impacts on the atmosphere of a
super-eruption
The only atmospheric model to date for the YTT
eruption was presented by Bekki et al. (1996). They used
a global-scale, two-dimensional model that had been
developed initially for studies of aircraft emissions, and
simulated the eruption by a 6 1012 kg SO2 injection
into the tropical stratosphere at altitudes between 25
and 35 km. The model indicated considerably enhanced
residence times of SO2 in the stratosphere compared
with Pinatubo (Fig. 5). Only after 5–6 yr in the model
did SO2 concentrations return to background levels in
the stratosphere. The reason for this is that the volcanic
SO2 mops up all the available OH radicals in oxidising
to sulphate aerosol, long before all the SO2 has been
consumed. The remaining gaseous SO2 effectively has to
wait until more OH is produced photochemically.
12
10
11
10
Mass of sulfur (kg)
1600
Toba
10
10
9
10
Pinatubo
8
10
10
Aerosol background
7
SO 2 background
0
1
2
3
4
5
6
7
8
9
10
Time after eruption (yr)
Fig. 5. Total mass of SO2 and sulphate aerosol in the stratosphere
(heavy solid and dotted lines, respectively) modelled for a 6 1012 kg
stratospheric injection of SO2. Observed SO2 and aerosol mass for the
1991 Pinatubo eruption are shown for comparison. The much larger
amount of SO2 in the Toba simulation soaks up all available oxidants
in the stratosphere leading to a much longer lifetime of SO2 and, in
turn, prolonging the manufacture of sulphate aerosol. Data from Read
et al. (1993) and Bekki et al. (1996).
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
However, production of OH is also retarded because the
aerosol already formed backscatters solar ultraviolet
radiation. In the case of a much smaller input of
sulphur, such as Pinatubo’s, the oxidation rate of SO2 to
SO2
4 is proportional to the product of the concentrations of SO2 and OH. For the Toba model, in the first 2
years after the eruption when SO2 concentrations are
very high, the OH concentration is itself inversely
proportional to the SO2 concentration. As a consequence, the oxidation rate is slow and roughly constant,
and the SO2 cloud long-lived.
As sulphate forms in the gas phase, it condenses to
form sulphate aerosol. The aerosol is believed to form
by spontaneous nucleation of new particles condensing
from the gas phase rather than by condensation on to
existing aerosol. This ensures that the aerosol maintains
small (sub-micron) sizes resulting in a longer residence
time in the stratosphere, and stronger backscattering of
solar short-wave radiation. The combination of slow
oxidation of a long-lived SO2 gas cloud, and gas-phase
nucleation of aerosol results in a longer-lived layer of
finer sized, radiatively effective aerosol. In fact, the
model predicts that the peak aerosol loading due to the
YTT eruption did not occur until 3 yr after the eruption,
and that the volcanically-enhanced stratospheric aerosol
persisted for almost a decade. The radiative effects of
this longer-lasting aerosol may have been sufficient to
overcome the thermal inertia of the oceans, triggering a
longer-term climatic cooling.
5.3. Ice cores
The GISP2 record also pointed to a longer-lived
aerosol layer. The high-resolution sampling of the
putative YTT layer led Zielinski et al. (1996) to conclude
that sulphate deposition lasted about 6 yr (Fig. 6).
Possible effects of shearing of the ice layer or diffusion
of the acid were discounted because there were neither
symmetry nor repeat signals in this part of the core.
Initially, they attributed this extended fallout to the
possibility of multiple eruptions from Toba but the
observation is consistent with the model prediction from
Bekki et al. (1996) of sustained replenishment of
sulphate aerosol due to the slow oxidation of a single
massive SO2 cloud.
Taking the intermediate values for sulphate loading
from Zielinski et al. (1996), indicates annual sulphate/
water aerosol loadings of 1.8–6.5 1011 kg. Inserting the
equivalent sulphur amounts into Eq. (1) suggests mean
surface temperature anomalies of between 0.91C and
1.31C, much less than the values computed from an
instantaneous release of >1012 kg of aerosol. These
values coincide with results of oxygen isotopic analyses
of the sediment immediately above the YTT layer in a
South China Sea core, which suggest that sea-surface
temperatures dropped by E11C (Song et al., 2000),
1601
although these could reflect conditions 10s or 100s yr
after the event. This finding has been reinforced more
recently by Huang et al. (2001) who used the unsaturation index of long-chain ketones (produced by various
single-celled algae) to estimate sea-surface temperatures
in the South China Sea following the YTT eruption. The
high sedimentation rates in this region (E20 cm kyr1)
permit 100 yr resolution. Their analyses indicate a 11C
sea-surface temperature anomaly that lasted for about
1 kyr immediately following the eruption. This seasurface temperature change appears consistent with
conclusions reached by Rousseau and Kukla (2000).
They suggested that a possible explanation for a rapid
monsoon retreat, documented by a rapid switch from
soil complexes to loess deposition in the Chinese Loess
Plateau, is the sudden rearrangement of the oceanic
conveyor belt triggered by the YTT eruption. Huang
et al. (2001) also note the coincidence of increased East
Asia winter monsoon intensity, increased ice-rafted
debris in North Pacific sediments (Kotilaninen and
Shackleton, 1995), and decreased d18O in the Greenland
ice core (Zielinski et al., 1996), implicating the following
chain of events: polar cooling, expansion of northern
hemisphere ice sheets, increasing intensity of the East
Asian monsoon, cooling of China and the tropical
Pacific, and reduction of atmospheric water concentration. They concluded that such processes could have
acted to prolong the short term cooling initiated by the
Toba event. de Garidel-Thoron et al. (2001) have also
noted a teleconnection between the East Asian winter
monsoon and Greenland climate.
The high temporal resolution, multi-parameter measurements of the GISP2 core enable a detailed inspection of the relative timings of the sulphate impulses and
climatic changes, that is much more difficult with even
the highest-resolution oxygen isotope records from the
deep-sea sediment cores containing YTT glass. The first
observation is that while the Toba spike coincides with a
1 kyr cool period between interstadials 19 and 20, it is
separated by the 2 kyr long, and dramatic warming
event (Lang et al., 1999) of interstadial 19, from the
prolonged (9 kyr) major glacial which began around
67.5 kyr BP. This undermines the early argument
that the YTT eruption played a role in initiating the
Last Glaciation, though leaves open the possibility that
it is implicated in the E1 kyr cold period prior to
interstadial 19.
It remains difficult to ascertain, for certain, the YTT
eruption’s climatic consequences. To begin with, it is
clear from the glaciochemical record that climate was
already unstable prior to the eruption and the main
glacial period. For example, interstadial 20 is preceded
by an 800 yr period of cold climate, apparent from low
d18O, high Ca2+, and very low ECM signals (Fig. 1 in
Zielinski et al., 1996). Also apparent from the GISP2
records, Ca2+ levels began increasing, and ECM drops,
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
1602
Years
15
12
9
6
3
0
500
2000
SO42-
400
300
1000
200
Ca2+
[Ca2+] (ppb)
[SO42-] (ppb)
1500
500
100
0
0
2590.95
2591.05
2591.15
2591.25
Depth (m)
Fig. 6. High temporal resolution sulphate (corrected for sea-salt contributions) and calcium ion concentrations spanning the Toba horizon in the
GISP2 ice core. The sampling was carried out for 1.5 cm thick slices of core which are equivalent to E1.5 yr each. From Zielinski et al. (1996).
just before the YTT spike. This suggests that the 71 kyr
BP stadial might have happened even without Toba. On
the other hand, the timing of the eruption during a
phase of high climate sensitivity may have been
sufficient to amplify the initial cooling impulse. Ca2+
fluctuations at the very start of the 71 kyr BP stadial
provide some evidence for this hypothesis. No other
stadials reveal a pulse of Ca2+ deposition prior to the
main peak of the cooling event as the 71 kyr BP episode
does. Zielinski et al. (1996) concluded that ‘‘eruptions of
[Toba’s] magnitude can significantly modify atmospheric conditions to the point of playing a role in
forcing climatic cooling on century timescalesyAn
eruption of Toba’s magnitude occurring today would
have devastating repercussions (i.e., a true volcanic
winter)’’.
5.4. Other atmospheric impacts
Evidence for other atmospheric perturbations arising
from the Toba eruption has come to light from further
ice core analysis. Yang et al. (1996) recorded timeresolved concentrations of chloride, nitrate, and the
ratio of Cl to Na+, in the GISP2 YTT layer, and
compared them with the sulphate spike (Fig. 7). The
pulse of aerosol fallout was found to coincide with a low
concentration of nitrate (about 5 ppb, the lowest in the
entire GISP2 record, and compared with a mean of
about 83 ppb prior to the eruption), and low concentrations of Cl (25 ppb, compared with a pre-eruption
mean of 66 ppb). These depletions of chloride and
nitrate, and an accompanying low ratio of Cl to Na+
(0.75, compared with a pre-eruptive value of 1.9)
indicate strong impacts on tropospheric chemistry. The
primary source of Cl to the ice caps is from deposition
of sea salt (NaCl). Reaction between sea salt aerosol and
H2SO4 in the atmosphere is thought to yield most of the
atmosphere’s complement of HCl
2NaCl þ H2 SO4 ¼ 2HClðgÞ þ Na2 SO4 :
ð3Þ
This reaction is sometimes responsible for lowering the
Cl/Na+ ratio in the sea salt transported to the ice caps
relative to the mean value for seawater of about 1.8. The
additional H2SO4 produced by Toba may have abstracted even more Cl from sea salt, further reducing the
Cl and Cl/Na+ ratio in the ice core. Chloride
concentrations reach a peak following the sulphate
spike (Fig. 7), possibly as a result of build-up of
atmospheric chlorine due to the limited solar ultraviolet
radiation passing through the aerosol veil. Usually,
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
1603
Years
15
12
9
6
3
0
2000
120
SO42Cl-
100
80
1000
60
[Cl-] (ppb)
[SO42-] (ppb)
1500
40
500
20
0
0
3
2000
SO42Cl-/Na+
2
1000
1
Cl-/Na+
[SO42-] (ppb)
1500
500
0
0
2000
200
1500
150
1000
100
500
50
0
2590.95
[NO3-] (ppb)
[SO42-] (ppb)
SO42NO3-
0
2591.05
2591.15
2591.25
Depth (m)
Fig. 7. High temporal resolution sulphate (as in Fig. 6), chloride, and nitrate concentrations, and the chloride/sodium ionic ratio for the Toba
horizon in the GISP2 ice core. The timescale is the same as for Fig. 6. From Yang et al. (1996).
1604
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
chlorine formed by reaction between sea salt and ozone
undergoes rapid photochemical conversion to HCl. As
the volcanic aerosol veil dissipated and solar short-wave
radiation again penetrated into the troposphere, the
reservoir of accumulated chlorine would have been
rapidly photochemically converted to HCl, resulting in a
pulse of Cl deposition seen in the ice core record.
The low nitrate levels can be explained by Toba’s
cooling effect on the lower atmosphere. Reduced
heating would lessen thermal convection, decreasing
the frequency of electrical storms that are a key source
of NOx. The combination of less NOx with reduced
production of OH radicals (which oxidise NO and NO2
to NO
3 ) would result in low atmospheric nitrate. This
result is at odds with earlier modelling efforts, which
suggested that significant denitrification of the polar
lower stratosphere would occur after the Toba eruption.
The lost nitrogen should end up being scavenged by the
polar snow leaving a peak of NO
3 in the ice core,
contrary to observations.
There are some significant caveats to this work
however. It is possible that the apparent perturbations
to ice core chemistry do not reflect contemporary
atmospheric chemistry but rather post-deposition reactions occurring in the highly acidic YTT ice horizon,
and, to an extent, the conclusions hinge on the
assumption that, given the size of the YTT eruption,
there must have been an atmospheric chemistry perturbation.
! and Pinatubo eruptions resulted in
The El Chichon
global reductions of stratospheric ozone of up to 10%
(McCormick et al., 1995), and it is important to consider
whether the more prolonged and concentrated YTT
aerosol veil might have had major impacts on upper
atmospheric ozone (Tie and Brasseur, 1995). However,
the chlorine activated by the stratospheric aerosol veils
of these recent eruptions would have been primarily
anthropogenic (i.e., not a factor in the Middle Palaeolithic). There is likely to have been an impact on
stratospheric ozone chemistry, however, due to the
levels of solar radiation intercepted by SO2. This would
have reduced the photochemical production of ozone,
and Bekki et al. (1996) have modelled localised 30–60%
reductions in stratospheric ozone within 3 yr of the
eruption. What this might have meant in terms of
biologically significant levels of ultraviolet-B penetrating
to the surface is difficult to assess, however, since the
gaseous SO2 and the aerosol veil in the stratosphere
would have counteracted the effect of ozone loss by
absorbing and scattering ultraviolet radiation.
6. Impacts on the terrestrial environment
Rose and Chesner (1990) likened the aftermath of the
eruption to an ‘‘enormous fire’’, covering up to
30,000 km2, arising from the widespread ignition of
vegetation. They judged that the outflow sheets of the
YTT would have had temperatures up to 5501C when
they came to rest, compared with an initial temperature
of 710–7701C. From observations following Pinatubo’s
1991 eruption (e.g., Torres et al., 1996), we can imagine
that these deposits remained at high temperatures for
years given their great thicknesses (up to several 100 m),
and that their lack of welding would have resulted in
frequent remobilisation and generation of secondary
pyroclastic flows. There must have also been many
months of explosions due to the interaction between the
hot deposits and rain or surface water.
Rampino and Ambrose (2000) suggested that soot
from the burnt vegetation, as well as very fine ash and
sulphate particles, would have contributed to the aerosol
veil spreading across the globe in the weeks after the
eruption. They likened the environmental consequences
to those predicted from studies of nuclear winter, and
asteroid impacts on the Earth. The low light levels
reaching the Earth’s surface due to the turbid aerosol
veil would have reduced photosynthesis, which, coupled
with low surface temperatures, would have killed off
much vegetation. They suggested that tropical forests
would have been most vulnerable to chilling, with
potentially severe and lasting damage resulting from
temperature drops of up to 101C. Temperate forests may
have fared little better with both coniferous and
deciduous trees suffering, especially during cold springs
and summers when new shoots are less resistant to
chilling.
In the oceans, Rampino and Ambrose (2000) argued
that the tephra sedimenting through the water column
would have scavenged out nutrients. This effect
combined with reduced sunlight reaching sea-level, and
changes in atmospheric circulation, could have limited
surface productivity in the oceans. They considered that
‘‘given the magnitude of the effects of volcanic winter on
global climate and primary productivity it would be
remarkable if this cataclysmic event did not affect
tropical humans and other species’’. However, recent
analyses of leachates of fresh ash fall from the modest
2000 eruption of Hekla in Iceland suggest that ash
fallout can actually fertilise the oceans by supplying
macronutrients and ‘‘bioactive’’ trace metals (Frogner
et al., 2001).
7. Human impacts
Investigation of the palaeodemography of Homo
through the Pleistocene has been tackled from many
perspectives, including palaeoanthropology, archaeology, numerical modelling, and, most recently, a range of
genetic studies. Around the time of the eruption,
Neanderthals inhabited Eurasia and the Levant, modern
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
humans occupied Africa, the Arabian peninsula and
parts of central Asia, ‘‘Homo heidelbergensis’’ was in
China, and Homo erectus in southeast Asia. Some
authors contend that modern humans are the ancestors
of a small founder population (several thousand
individuals) surviving one or more population ‘‘bottlenecks’’ at some point in the Pleistocene (for a review, see
Harpending and Rogers, 2000). The timing and duration of this event are controversial, not least because of
large confidence limits in the application of the
‘‘molecular clocks’’ behind many of the estimates—
and there are arguments that refute the evidence for any
significant bottlenecks occurring after the beginning of
the Homo lineage (e.g., Hawks et al., 2000; Hawks and
Wolpoff, 2001; Wolpoff et al., 2001).
Elaborating on arguments for the origins of modern
humans (e.g., Harpending et al., 1993; Rogers and
Jorde, 1995), Ambrose (1998) and Rampino and
Ambrose (2000) have argued that 6 years of volcanic
winter caused by the YTT eruption, and a consequent
millennium of cold climate and its global environmental
consequences, precipitated a severe bottleneck, implying
an abrupt and recent differentiation of human populations. Ambrose (1998) suggested that Neanderthals
might have faired better in these climatic conditions
due to their more cold-tolerant anatomy (Stringer and
Gamble, 1993; note that Neanderthals survived up to
around 30 kyr BP, e.g., Bolus and Conard, 2001).
Release from the bottleneck would have been facilitated
by invention of improved Later Stone Age technology,
and increasing access to food supplies around 50 kyr BP
(Harpending et al., 1993; Ambrose, 1998), though
McBrearty and Brooks (2000) have argued for a far
earlier and more gradual development of tool technology, long distance trade, and other putative aspects of
behavioural modernity. The equatorial region of Africa
would have offered the most extensive refugia for
survival of humans during this period, and therefore
have sustained the highest populations through the
bottleneck.
There are a number of difficulties with this hypothesis. Population bottlenecks at various times are turning
up in the lineages of all kinds of creatures including
elephant seals, pin-worms, koalas, fruit flies and
anchovies. What would lend weight to the ‘‘volcanic
winter-Garden of Eden’’ scenario (Ambrose, 1998)
would be contemporaneous bottlenecks in the ancestry
of great apes. However, the genetic variation in humans
reveals a very different pattern to the African great apes,
apparently undermining suggestions of parallel demographic histories (Gagneux et al., 1999). The possible
exception to this is the eastern chimpanzee clade.
Harpending and Rogers (2000) point out that a species
could survive an environmental catastrophe like the
YTT eruption in two or more refugia, resulting in far
more genetic variance than a species that survived in
1605
only one, ‘‘thus, a few species that showed a different
pattern would not refute the hypothesis of a Pleistocene
bottleneck, but a few showing the same pattern would
greatly strengthen it’’.
Perhaps a more substantive problem is that on the
basis of the well-constrained date for the YTT, 74 kyr
BP, and the broader age range proposed for the
bottleneck, one cannot even conclude with any confidence that the eruption actually preceded the bottleneck. In fact, the widely accepted median estimate for
the bottleneck age based on mitochondrial DNA studies
is about 200 kyr (Harpending and Rogers, 2000; Hawks
et al., 2000; Hawks and Wolpoff, 2001).
There is no firm evidence, therefore, linking the YTT
eruption to a human demographic crash. Nor is the
climate around 74 kyr BP uniquely cold by Quaternary
standards. The GISP2 oxygen isotope record reveals
comparably low temperatures to 70 kyr BP at 36 and
33 kyr BP. The direct impacts of the eruption (inundation by ash, etc.) would surely have been devastating,
but at a regional rather than global scale. If Toba is to
be held responsible for a more global impact on human
demography, however, one would have to argue that the
eruption occurred at a particularly sensitive time for
other reasons, or the suddenness of the impacts played a
role. Interestingly, if Swisher et al. (1996) have correctly
identified and dated the Ngandong fossils from Java, the
eruption does not appear even to have extinguished the
last vestiges of Homo erectus populations, which they
claim survived up to 30–50 kyr BP, although both the
interpretation of these fossils as Homo erectus, and the
young dates have been challenged (e.g., Grun
. and
Thorne, 1997; Wolpoff et al., 2001, respectively). It is
also relevant to consider that there are approximately 10
super-eruptions (involving eruption of 300–3000 km3
DRE magma) per 106 yr (Pyle, 2000; Simkin and
Siebert, 2000), i.e., there have been more than 20 such
events during the Homo lineage. Especially in view
of its potentially low sulphur yield (Scaillet et al., 1998),
the eruption of the YTT may not have been the
most climatically significant volcanic event of the
Quaternary.
Without a clear picture even of the relative timing of
events, we cannot at present establish a causal chain.
Just because the YTT event is the largest known Late
Quaternary eruption does not by itself implicate it in a
global human population crash. There are often many
contending explanations for demographic demise. With
the repeated, profound and prolonged global climatic
change and environmental stress associated with interglacial–glacial transitions, we may not have to look too
hard for an underlying cause for any putative near
extinction of Homo and for proposed expansions and
contractions of human populations throughout the
Quaternary (e.g., Lahr and Foley, 1998, who associate
the bottleneck with OIS 6; Hewitt, 2000).
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C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
8. Discussion and conclusions
Probing the published investigations of the YTT
eruption reveals that a number of conclusions have been
based on unreliable assumptions and inferences. In
particular, model outputs are sensitive to poorly
constrained inputs. For example, the model elaborated
by Bekki et al. (1996) assumed a stratospheric injection
of 6 1012 kg of SO2 but the total aerosol loading
computed by Zielinski et al. (1996) is 0.7–3.5 1012 kg,
corresponding to 0.46–2.3 1012 kg of SO2, up to an
order of magnitude less. Scaillet et al. (1998) suggest an
even lower sulphur yield. A smaller SO2 pulse can be
expected to produce a shorter-lived SO2 cloud and hence
less radiatively active aerosol. The eruption intensity
estimate was based partly on size grading of tephra in
deep-sea cores (Ledbetter and Sparks, 1979). But the
observed grading could reflect variable column dynamics and wind dispersal patterns, clumping of the ash
as it settled through the water column, or possibly even
bioturbation of sediment after deposition. If the derived
eruption duration is unreliable, then the intensity is
unreliable. If the intensity is uncertain then the column
heights estimated for the YTT are hard to evaluate. In
other words, there remain major unresolved issues that
limit current attempts to model the atmospheric and
climatic effects of the eruption. Advances in modelling
will require new approaches to documenting the
eruption style, and the temporally and spatially (vertically) resolved sulphur yield.
Assuming that the YTT fallout has been correctly
identified in the GISP2 sulphate stratigraphy, we can
discount earlier suggestions that the eruption triggered
the last Ice Age, but still have the possibility that it was
responsible for the onset of a millennium of cool climate
prior to Dansgaard-Oeschger event 19. On the other
hand, a comparable stadial preceded event 20.
What kinds of impact might the eruption have had on
contemporary human populations? Can any credence be
given to suggestions that the eruption precipitated a
crash in human population numbers? While the arguments proposed by Rampino and Ambrose (2000) and
Ambrose (1998) are plausible, they are not yet compelling because, in addition to the problems in estimating
the global and regional climatic impacts of the eruption,
it is hard to compare a fairly robust date for the YTT
eruption itself, with controversial and uncertain dates
for the putative demographic bottleneck. Furthermore,
the timing of the YTT is at the limit of radiocarbon
dating, perhaps holding out limited hope of advances
coming from archaeology. Resolving the sequences of
events and the relationships between cause and effect is
difficult enough for the Late Bronze Age ‘‘Minoan’’
eruption of Santorini, a mere 3.5 kyr ago.
So what can realistically be achieved in the next years?
There is an increasingly clear indication of the rich
tephra record of the YTT eruption in the oceans, and to
a lesser extent, on land. The deep-sea sediment
stratigraphy often appears disturbed and the ash
horizons are thin but given refined models for ash
deposition in deep water, new studies of YTT occurrences, their thickness, componentry and granulometry
could provide a strong basis for re-evaluating the total
mass of tephra, atmospheric transport, plume rise
heights, and eruption intensity vs. time. It could shed
light on the fractions of sulphur entrained into the
stratosphere and troposphere, the time of year of the
eruption, and suggest analogies with modern equatorial
explosive eruptions (e.g., the 1994 eruption of Rabaul,
Papua New Guinea).
The ice cores provide a further repository of
information on the eruption. Conclusive identification
of YTT-like glass shards would corroborate designation
of the YTT sulphate spike. More high temporal
resolution analyses might also yield additional information on atmospheric dynamics and climate response.
Perhaps some new information might come from the
Antarctic ice core records.
The experimental work of Scaillet et al. (1998) may
point the way to new petrological methods that will
ultimately yield more reliable estimates of the sulphur
(and halogen) yield of the eruption. New melt inclusion
analyses of volatiles would also help to corroborate or
refute the conclusions reached by Scaillet et al. (1998), at
least concerning the availability of volatiles from
syneruptive degassing. Hard data on all these aspects
of the YTT eruption, along with advances in understanding the contemporary climate and atmospheric
dynamics, and their response to the eruption, based on a
wide range of high-temporal resolution palaeoclimate
records, will be essential to parameterise any models for
the eruption’s potential global consequences. Development of such models will open up many avenues of
research into volcano-atmosphere-climate interactions.
For example, currently we have only a limited understanding of the implications of atmospheric injection of
sulphur gases by phoenix clouds rather than a central
eruption column, or of the primary release of H2S rather
than SO2. Other fruitful research directions include
modelling the atmospheric environments that could
permit large amounts of volcanic chlorine or bromine to
reach the stratosphere, and quantifying the oceanatmosphere-cryosphere interactions and system thresholds whereby short-term (a few years) volcanic cooling
can trigger millennial scale cold periods (e.g., Pollack
et al., 1976).
In addition to the approaches described above, we
should also be open to broad, integrated multi- and
inter-disciplinary investigations of the YTT eruption
involving both novel and conventional approaches (high
resolution palaeoclimate records, sedimentology, palynology, archaeology, geochemistry, climate modelling,
C. Oppenheimer / Quaternary Science Reviews 21 (2002) 1593–1609
terrestrial ecology, etc.). While there is clearly
much work still to do, the rewards of an improved
understanding of the YTT eruption and its aftermath
will be highly significant, not least for the insights
to be gained into the potential effects of the next ‘‘supereruption’’.
Acknowledgements
I am grateful to Rob Foley, Marta Lahr, David Pyle,
Bruno Scaillet and Milford Wolpoff for discussions and
comments; to Bill Rose and John Westgate for beneficial
reviews of the manuscript; to Giorgio Gasparotto for
the SEM images; and Owen Tucker for assistance with
the illustrations; and Jim Rose for overall editorial
input.
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