Friday, 7 February 2014

Have Volcanic Super-Eruptions Impacted on the Course of Human History?By Prof Melanie Leng

The ancient Toba caldera (a basin left by
the volcano after eruption) from space is
now occupied by a huge lake (@NASA)
Today Melanie Leng, an isotope geochemist and palaeoclimatologist at the BGS, tells us about the Toba super volcano in Sumatra, Indonesia, which has erupted on many occasions over geological time. 

One very destructive eruption of Toba occurred around 74,000 years ago, an event that has been estimated to have ejected thousands of cubic kilometres of volcanic ash and rocks into the atmosphere which spread far and wide both North and West of Sumatra on prevailing winds. Evidence for the volcanic ash is seen today on the Indian subcontinent and in the Arabian and South China Seas. The ejection of so much ash and volcanic gases into the atmosphere is thought to have reflected the suns heat and there is evidence that global cooling occurred or a ‘volcanic winter’ for example in geochemical data from the Greenland ice cores. In other parts of the world the Toba eruption appears to have little impact, life went on undisturbed, in the African Great Lakes for example. One of the big unanswered questions about Toba is whether the eruption impacted on human populations. There are theories suggesting human population decreases (a genetic bottleneck) and a significant arid event could have altered human migration patterns for example.

Volcanic ash called tephra is made up of fragments
of glassy material that are ejected during volcanic
eruptions and can be used to identify volcanic
To assess the global influence of the eruption we need to look for evidence for environmental change at the time of the eruption around the globe*. From the southern Mediterranean (Balkans) we investigated lake sediments deposited 74,000 years ago for this evidence. Detailed geological detective work on the sediments using high magnification microscopes have so far not detected any evidence for volcanic ash from Toba. However we have found other clues from within the lake sediments that show a response to several years/decades of cold (volcanic winter) conditions. Shell fragments transported to the central part of the lake indicate lake level drop (cold dry conditions) and the geochemistry of the sediments suggest that the lake became more evaporated (less rainfall); while pollen preserved in the sediments shows trees died back as the SE Mediterranean experienced dark, cold summers and long cold winters.

In the context of human migration it is important to understand the climate of the southern Mediterranean (a conduit for humans leaving Africa) at this important period in history. Did the Toba eruption completely change the course of human history? Well the geology of the lake sediments definitely suggest harsher winters, so maybe this was a good reason to slow human migration? Isn't geology amazing? 

Prof Melanie Leng (@MelJLeng) is an isotope geochemist and palaeoclimatologist at the BGS. 

This research is in collaboration with scientists in Germany and Italy.
*Wagner, B., Leng, M.J., Wilke, T., Bohm, A., Panagiotopoulos, K., Vogel, H., Lacey, J.H., Zanchetta, G, Sulpizio, R. 2014. Distinct lake level stand in Lake Prespa (SE Europe) at the time of the 74 (75) ka Toba eruption. Climate of the Past, 10, 261-267.

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