Could the Toba eruption have affected migrating humans?

Around 73 thousand years ago a supervolcano in Sumatra erupted on a scale unprecedented in the last 2 million years. It left a 100 by 30 km elliptical caldera now occupied by Lake Toba, and explosively ejected 2800 of magma, about 800  km3 falling as ash as far afield as the Greenland ice cap. Although ice-core records show little if any sign of associated climate change in polar regions, the vast amount of ash and sulfate aerosols blasted into the stratosphere must have had some ‘global winter’ effect. Large areas of South Asia were blanketed by thick beds of ash. Human migration from Africa into Eurasia was probably underway at the time, indeed stone tools are found directly beneath and above the Toba ash in southern India and Malaysia. Some palaeoanthropologists have seen the stresses imposed by the Toba eruption as possible means of reducing the entire human population to a mere few thousand: a genetic ‘bottleneck’ that could have led to rapid evolution among surviving generations that may have shaped changes in human behaviour and culture.

Landsat image of Lake Toba, the largest volcan...
Landsat image of Lake Toba, the largest volcanic crater lake in the world. (credit: Wikipedia)

There is a widening range of views on the climate changes that may have followed Toba. It has even been suggested that global mean surface temperature fell by as much as 10°C (Robock, A. et al. 2009. Did the Toba volcanic eruption of ∼74 ka B.P. produce widespread glaciation? Journal of Geophysical Research: Atmospheres, v. 114, DOI: 10.1029/2008JD011652), although not so far as to produce a worldwide glacial surge but sufficient to devastate vegetation. This bleak look back to a critical point in human affairs resulted from modeling of the effects of a global reflective cloud of ash and sulfate. A later modeling study factored in particle and aerosol sizes (Timmreck, C. et al. 2010. Aerosol size confines climate response to volcanic super-eruptions. Geophysical Research Letters, v. 37, doi:10.1029/2010GL045464) to give a less dramatic, but still severe maximum global cooling due to Toba of ~3.5°C.

The focus has now shifted from modelling to a more direct look at the environmental effects of the Toba super-eruption, preserved in sediments beneath Lake Malawi in southern Africa (Lane, C.S. et al. 2013. Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka. Proceedings of the National Academy of Science, v. 110, doi/10.1073/pnas.1301474110). The sediments contain a thin ash layer that is very different from those produced by East African Rift volcanism but chemically and texturally similar to the Toba ash from the Indian Ocean and India. The sediments, diatom fossils and chemical biomarkers immediately above the ash show little sign of a significant temperature fall. At most it records a 1.5°C fall, and the authors conclude little chance of a human genetic bottleneck among Africans living at the time.

There is clearly a conflict between results of modeling and real-world climatic data, which is interesting in its own right. But the Malawi findings do not rule out ‘bottlenecks’ resulting from severe stress in South Asia where the ash itself would have severely affected game and vegetation for long enough to face migrating human bands with the prospect of starvation. Obviously, some survived to move on and to leave their tools behind on top of the Toba Ash.

Toba ash and calibrating the Pleistocene record

Landsat image of Lake Toba, the largest volcan...
Landsat image of the Lake Toba caldera, Sumatra (credit: Wikipedia)

The largest volcanic catastrophe during the evolution of humans formed the huge caldera at Lake Toba near the Equator in Sumatra about 70 thousand years ago. Explosive action erupted 2800 cubic kilometres of magma, of which 800 km3 was deposited as thick ash across most of South Asia and the northern Indian Ocean. Sulfates derived from the gas emissions by Toba form clear ‘spikes’ in ice cores from both Greenland and Antarctica. Its effects were global through the mixing of sulfate aerosols in the stratosphere of both hemispheres, encouraged by its position close to the Equator. By reflecting incoming solar energy the aerosols resulted in a century-long 10°C fall in temperature over the Greenland ice cap. Such global cooling almost certainly affected anatomically modern humans, but it is possible that in South Asia Toba had an even more devastating effect.

The Toba ash at the Jwalapuram excavations in South India(Photo credit: Sanjay P. K. via Flickr)

At several sites in the Indian state of Tamil Nadu and in Malaysia Toba ash has buried artifacts that arguably may have been made by the earliest modern emigrants from Africa. Immediately above the ash are yet more tools that suggest humans did survive the eruption. Palaeoanthropologists have argued that the stress of Toba’s environmental effects on all hominins living at the time may have resulted in population crashes from which only the fittest individuals emerged. Major evolutionary changes have been ascribed to ‘bottlenecks’ of that kind to result in changes in human behaviour detectable from the archaeological record, such as the creation of completely new kinds of tools, art and language.  However, recent finds in Africa suggest that many such shifts are much older than Toba.

Perhaps Toba’s greatest contribution to palaeoanthropology is that it is an easily recognised event in the geological record, but compared with its sulfate spike in the Greenland ice core at ~71 ka the existing radiometric dates have uncertainties of several thousand years. Using the latest 40Ar/39Ar dating methods on fresh crystals of sanidine (volcanic K-feldspar) from new excavations in Malaysia these uncertainties have been reduced significantly (Storey, M. et al. 2012. Astronomically calibrated 40Ar/39Ar age for the Toba supereruption and global synchronization of late Quaternary records. Proceedings of the National Academy of Sciences, v. 109, p. 19684-18688 ). The sulfate peak and the ash can now be attributed to an age of 73.88 ± 0.32 ka; better than a golden spike in Late Pleistocene stratigraphy. The ice-cores have a check on chronology just beyond the limit of counting annual layering, as do ocean sediment cores for a time older than 14C can ever achieve. Toba now links too with events recorded by the precise U-Th series dating of cave deposits