| Youngest Toba eruption | |
|---|---|
 Artist's impression of the eruption from about 42 kilometres (26 mi) above Northern Sumatra | |
| Volcano | Lake Toba Caldera |
| Date | 75,000 ± 900 years BP |
| Type | Ultra-Plinian |
| Location | Sumatra, Indonesia 2.6845°N 98.8756°ECoordinates: 2.6845°N 98.8756°E |
| VEI | 8 |
| Impact | Second-most recent supervolcanic eruption; impact disputed |
 Lake Toba is the resulting crater lake | |
The Youngest Toba eruption was a supervolcanic eruption that occurred around 75,000 years ago at the site of present-day Lake Toba in Sumatra, Indonesia. It is one of the Earth's largest known explosive eruptions. The Toba catastrophe theory holds that this event caused a global volcanic winter of six to ten years and possibly a 1,000-year-long cooling episode.
In 1993, science journalist Ann Gibbons posited that a population bottleneck occurred in human evolution about 70,000 years ago, and she suggested that this was caused by the eruption. Geologist Michael R. Rampino of New York University and volcanologist Stephen Self of the University of Hawaii at Manoa support her suggestion. In 1998, the bottleneck theory was further developed by anthropologist Stanley H. Ambrose of the University of Illinois at Urbana–Champaign. Both the link and global winter theories are controversial. The Youngest Toba eruption is the most closely studied supervolcanic eruption.
Supervolcanic eruption
The Youngest Toba eruption occurred at the present location of Lake Toba in Indonesia, about 75,000 ± 900 years BP according to potassium argon dating. This eruption was the last and largest of four eruptions of the Lake Toba Caldera during the Quaternary period, and is also recognized from its diagnostic horizon of ashfall, the Youngest Toba tuff. It had an estimated volcanic explosivity index of 8 (the highest rating of any known eruption on Earth); it made a sizable contribution to the 100 × 35 km caldera complex. Dense-rock equivalent (DRE) estimates of eruptive volume for the eruption vary between 2,000 km3 and 3,000 km3; the most common DRE estimate is 2,800 km3 (about 7×1015 kg) of erupted magma, of which 800 km3 was deposited as ash fall.
The erupted mass was, at the very least, 12 times greater than that of the largest volcanic eruption in recent history, the 1815 eruption of Mount Tambora in Indonesia, which caused the 1816 "Year Without a Summer" in the Northern Hemisphere. Toba's erupted mass deposited an ash layer of about 15 centimetres (5.9 in) thick over the whole of South Asia. A blanket of volcanic ash was also deposited over the Indian Ocean, the Arabian Sea, and the South China Sea. Deep-sea cores retrieved from the South China Sea have extended the known reach of the eruption, suggesting that the 2,800 km3 calculation of the erupted mass is a minimum value or even an underestimate. Based on new methods (crystal concentration and exponential), the Lake Toba Caldera possibly erupted as much as 3,200 km3 of ignimbrite and co-ignimbrite.
Volcanic winter and global cooling computer models
Geologist Michael R. Rampino and volcanologist Stephen Self argue that the eruption caused a "brief, dramatic cooling or 'volcanic winter'", which resulted in a drop of the global mean surface temperature by 3–5 °C (5.4–9.0 °F). Evidence from Greenland ice cores indicates a 1,000-year period of low δ18O and increased dust deposition immediately following the eruption. The eruption may have caused this 1,000-year period of cooler temperatures (stadial), two centuries of which could be accounted for by the persistence of the Toba stratospheric loading. Rampino and Self believe that global cooling was already underway at the time of the eruption, but that the process was slow; the Youngest Toba tuff "may have provided the extra 'kick' that caused the climate system to switch from warm to cold states". Although Clive Oppenheimer rejects the hypothesis that the eruption triggered the last glaciation, he agrees that it may have been responsible for a millennium of cool climate prior to the 19th Dansgaard–Oeschger event.
According to Alan Robock, who has also published nuclear winter papers, the Toba eruption did not precipitate the last glacial period. However, assuming an emission of six billion tons of sulphur dioxide, his computer simulations concluded that a maximum global cooling of approximately 15 °C (27 °F) occurred for three years after the eruption, and that this cooling would last for decades, devastating life. Because the saturated adiabatic lapse rate is 4.9 °C/1,000 m (1.5 °C/1,000 ft, 2.7 °F/1,000 ft) for temperatures above freezing, the tree line and the snow line were around 3,000 m (9,800 ft) lower at this time. The climate recovered over a few decades, and Robock found no evidence that the 1,000-year cold period seen in Greenland ice core records had resulted from the Toba eruption. In contrast, Oppenheimer believes that estimates of a drop in surface temperature by 3–5 °C are probably too high, and he suggests that temperatures dropped only by 1 °C. Robock has criticized Oppenheimer's analysis, arguing that it is based on simplistic T-forcing relationships.
Despite these different estimates, scientists agree that a supervolcanic eruption of the scale at the Lake Toba Caldera must have led to very extensive ash-fall layers and injection of noxious gases into the atmosphere, with worldwide effects on weather and climate. In addition, the Greenland ice core data display an abrupt climate change around this time, but there is no consensus that the eruption directly generated the 1,000-year cold period seen in Greenland or triggered the last glaciation.
Physical data contradicting the winter hypothesis
In 2013, archaeologists, led by Christine Lane, reported finding a microscopic layer of glassy volcanic ash in sediments of Lake Malawi, and definitively linked the ash to the 75,000-year-old eruption at the Lake Toba Caldera, but found no change in fossil type close to the ash layer, something that would be expected following a severe volcanic winter. They concluded that the eruption did not significantly alter the climate of East Africa, attracting criticism from Richard Roberts. Lane explained, "We examined smear slides at a 2-mm interval, corresponding to subdecadal resolution, and X-ray fluorescence scans run at 200-µm intervals correspond to subannual resolution. We observed no obvious change in sediment composition or Fe/Ti ratio, suggesting that no thermally driven overturn of the water column occurred following the Toba supereruption." In 2015, a new study on the climate of East Africa supported Lane's conclusion that there was "no significant cooling associated with Mount Toba".
Genetic bottleneck theory
Genetic bottleneck in humans
The Youngest Toba eruption has been linked to a genetic bottleneck in human evolution about 70,000 years ago, which may have resulted in a severe reduction in the size of the total human population due to the effects of the eruption on the global climate. According to the genetic bottleneck theory, between 50,000 and 100,000 years ago, human populations sharply decreased to 3,000–10,000 surviving individuals. It is supported by some genetic evidence suggesting that today's humans are descended from a very small population of between 1,000 and 10,000 breeding pairs that existed about 70,000 years ago.
Proponents of the genetic bottleneck theory (including Robock) suggest that the Youngest Toba eruption resulted in a global ecological disaster, including destruction of vegetation along with severe drought in the tropical rainforest belt and in monsoonal regions. A 10-year volcanic winter triggered by the eruption could have largely destroyed the food sources of humans and caused a severe reduction in population sizes. These environmental changes may have generated population bottlenecks in many species, including hominids; this in turn may have accelerated differentiation from within the smaller human population. Therefore, the genetic differences among modern humans may reflect changes within the last 70,000 years, rather than gradual differentiation over hundreds of thousands of years.
Other research has cast doubt on a link between the Lake Toba Caldera and a genetic bottleneck. For example, ancient stone tools in southern India were found above and below a thick layer of ash from the Youngest Toba eruption and were very similar across these layers, suggesting that the dust clouds from the eruption did not wipe out this local population. Additional archaeological evidence from southern and northern India also suggests a lack of evidence for effects of the eruption on local populations, leading the authors of the study to conclude, "many forms of life survived the supereruption, contrary to other research which has suggested significant animal extinctions and genetic bottlenecks". However, evidence from pollen analysis has suggested prolonged deforestation in South Asia, and some researchers have suggested that the Toba eruption may have forced humans to adopt new adaptive strategies, which may have permitted them to replace Neanderthals and "other archaic human species".
Additional caveats include difficulties in estimating the global and regional climatic impacts of the eruption and lack of conclusive evidence for the eruption preceding the bottleneck. Furthermore, genetic analysis of Alu sequences across the entire human genome has shown that the effective human population size was less than 26,000 at 1.2 million years ago; possible explanations for the low population size of human ancestors may include repeated population bottlenecks or periodic replacement events from competing Homo subspecies.
Genetic bottlenecks in other mammals
Some evidence points to genetic bottlenecks in other animals in the wake of the Youngest Toba eruption. The populations of the Eastern African chimpanzee, Bornean orangutan, central Indian macaque, cheetah and tiger, all recovered from very small populations around 70,000–55,000 years ago.
The separation of the nuclear gene pools of eastern and western lowland gorillas has been estimated to have occurred about 77,700 years ago.
Migration after Toba
The exact geographic distribution of anatomically modern human populations at the time of the eruption is not known, and surviving populations may have lived in Africa and subsequently migrated to other parts of the world. Analyses of mitochondrial DNA have estimated that the major migration from Africa occurred 60,000–70,000 years ago, consistent with dating of the Youngest Toba eruption to around 75,000 years ago.
Criticism
A 2018 study by Chad Yost and colleagues of cores from Lake Malawi dating to the period of the Youngest Toba eruption showed no evidence of a volcanic winter, and they argue that there was no effect on African humans. In the view of John Hawks, the study confirms evidence from a variety of studies that the eruption did not have a major climatic effect or any effect on human numbers.
