Youngest Toba eruption

The Toba eruption (sometimes called the Toba supereruption or the Youngest Toba eruption) was a supervolcanic eruption that occurred about 74,000 years ago during the Late Pleistocene at the site of present-day Lake Toba in Sumatra, Indonesia. It was the last in a series of at least four caldera-forming eruptions at this location, with the earlier known caldera having formed around 1.2 million years ago. This last eruption had an estimated VEI of 8, making it the largest-known explosive volcanic eruption in the Quaternary, and one of the largest known explosive eruptions in the Earth's history.

Chronology of the Toba eruption
The exact year of the eruption is unknown, but the pattern of ash deposits suggests that it occurred during the northern summer because only the summer monsoon could have deposited Toba ashfall in the South China Sea. The eruption lasted perhaps 9 to 14 days. The most recent two high-precision argon–argon datings dated the eruption to 73,880 ± 320 and 73,700 ± 300 years ago. Five distinct magma bodies were activated within a few centuries before the eruption. The eruption commenced with small and limited air-fall and was directly followed by the main phase of ignimbrite flows. The ignimbrite phase is characterized by low eruption fountain, but co-ignimbrite column developed on top of pyroclastic flows reached a height of 32 km. Petrological constraints on sulfur emission yielded a wide range from $10,000,000,000,000$ to $1,000,000,000,000,000 g$, depending on the existence of separate sulfur gas in the Toba magma chamber. The lower end of estimate is due to the low solubility of sulfur in the magma. Ice core records estimate the sulfur emission on the order of $100,000,000,000,000 g$.

Effects of the eruption
Bill Rose and Craig Chesner of Michigan Technological University have estimated that the total amount of material released in the eruption was at least 2800 km3 —about 2000 km3 of ignimbrite that flowed over the ground, and approximately 800 km3 that fell as ash mostly to the west. However, as more outcrops become available, the most recent estimate of eruptive volume is 3800 km3 dense-rock equivalent (DRE), of which 1800 km3 was deposited as ash fall and 2000 km3 as ignimbrite, making this eruption the largest during the Quaternary period. Previous volume estimates have ranged from 2000 km3 to 6000 km3. Inside the caldera, the maximum thickness of pyroclastic flows is over 600 m. The outflow sheet originally covered an area of 20000-30000 km2 with thickness nearly 100 m, likely reaching into the Indian Ocean and the Straits of Malacca. The air-fall of this eruption blanketed the Indian subcontinent in a layer of 5 cm ash, the Arabian Sea in 1 mm, the South China Sea in 3.5 cm, and Central Indian Ocean Basin in 10 cm. Its horizon of ashfall covered an area of more than 38000000 km2 in 1 cm or more thickness. In Sub-Saharan Africa, microscopic glass shards from this eruption are also discovered on the south coast of South Africa, in the lowlands of northwest Ethiopia, in Lake Malawi, and in Lake Chala. In South China, Toba tephras is found in Huguangyan Maar Lake.

The subsequent collapse formed a caldera that filled with water, creating Lake Toba. The island in the center of the lake is formed by a resurgent dome.

Climate at time of eruption
Greenland stadial 20 (GS20) is a millennium-long cold event in the north Atlantic ocean that started around the time of Toba eruption. The timing of the initiation of GS20 is dated to 74.0–74.2 kyr, and the entire event lasted about 1,500 years. It is the stadial part of Dansgaard–Oeschger event 20 (DO20), commonly explained by an abrupt reduction in the strength of the Atlantic meridional overturning circulation (AMOC). Weaker AMOC caused warming in Southern Ocean and Antarctica, and this asynchrony is known as bipolar seesaw. The start of GS20 cooling event corresponds to the start of Antarctic Isotope Maxima 19 (AIM19) warming event. GS20 was associated with iceberg discharges into the North Atlantic, thus it was also named Heinrich stadial 7a. Heinrich events tend to be longer, colder and with weaker AMOC in the Atlantic ocean than other DO stadials. From 74 to 58 kyr, Earth transitioned from interglacial MIS 5 to glacial MIS 4, experiencing cooling and glacial expansion. This transition is a part of Pleistocene interglacial-glacial cycle driven by variations in the earth's orbit. Ocean temperature cooled by 0.9 C-change. Sea level fell 60 m. Northern Hemisphere ice sheets embarked on significant expansion and surpassed the extent of Last Glacial Maximum in eastern Europe, Northeast Asia and the North American Cordillera. Southern Hemisphere glaciation grew to its maximum extent during MIS 4. Australasian region, Africa and Europe were characterized by increasingly cold and arid environment.

Possible climate records of eruption
While Toba eruption occurred in the backdrop of rapid climate transitions of GS20 and MIS 4 triggered by changes in ocean currents and insolation, whether the eruption played any role in accelerating these events is much more debated. South China Sea marine records of climate, sampled at every centennial interval, shows 1 C-change cooling above Toba ash layer for a thousand year but the authors concede that it may just be GS20. Arabian Sea marine records confirm that Toba ash occurred after the onset of GS20 but also that GS20 is not colder than GS21 in the records, from which authors conclude that the eruption did not intensify GS20 cooling. Dense sampling of environmental records, at every 6–9 year interval, in Lake Malawi, show no cooling-induced change in lake ecology and in grassy woodlands after the deposition of Toba ash, but cooling-forced aridity killed high elevation afromontane forests. The Lake Malawi studies concluded that the environmental effects of the eruption were mild and limited to less than a decade in East Africa, but these studies are questioned due to sediment mixing which would have diminished the cooling signal. Environmental records from a Middle Stone Age site in Ethiopia, however, shows that a severe drought occurred concurrently with Toba ash layer which altered early human foraging behaviours.

No Toba ash has been identified in ice core records, but four sulfate events within the ice strata have been proposed to possibly represent the deposition of aerosols from Toba eruption. One sulfate event at 73.75–74.16 kyr, which has all the characteristics of the Toba eruption, is among the largest sulfate loadings that have ever been identified. In the ice core records, GS20 cooling was already underway by the time of sulfate deposition, nonetheless a 110-year period of accelerated cooling followed the sulfate event, and the authors interpret this acceleration as AMOC weakened by the Toba eruption.

Climate modeling
The modeled climate effects of the Toba eruption hinges on the mass of sulfurous gases and aerosol microphysical processes. Modeling on an emission of $850,000,000,000,000 g$ of sulfur, which is 100 times the 1991 Pinatubo sulphur, volcanic winter has a maximum global mean cooling of 3.5 C-change and returns gradually within the range of natural variability 5 years after the eruption. An initiation of 1,000-year cold period or ice age is not supported by the model. Two other emission scenarios, $100,000,000,000,000 g$ and $1,000,000,000,000,000 g$, are investigated using state-of-art simulations provided by the Community Earth System Model. Maximum global mean cooling is 2.3 C-change for the lower emission and 4.1 C-change for the higher emission. Strong decrease in precipitation occurs in high emission. Negative temperature anomalies return to less than 1 C-change within 3 and 6 years for each emission scenario after the eruption. But so far no model can simulate aerosol microphysical processes with sufficient accuracy, empirical constraints from historical eruptions suggest that aerosol size may substantially reduce magnitude of cooling to less than 1.5 C-change no matter how much sulfur emitted.

Toba catastrophe theory
The Toba catastrophe theory holds that the eruption caused a severe global volcanic winter of six to ten years and contributed to a 1,000-year-long cooling episode, resulting in a genetic bottleneck in humans. However, some physical evidence disputes the association with the millennium-long cold event and genetic bottleneck, and some consider the theory disproven.

History
In 1972, an analysis of human hemoglobins found very few variants, and to account for the low frequency of variation human population must have been as low as a few thousand until very recently. More genetic studies confirmed an effective population on the order of 10,000 for much of human history. Subsequent research on the differences in human mitochondrial DNA sequences dated a rapid growth from a small effective population size of 1,000 to 10,000, sometime between 35 and 65 kyr.

In 1993, science journalist Ann Gibbons posited that population growth was suppressed by the cold climate of the last Pleistocene Ice Age, possibly exacerbated by the Toba super-eruption which at the time was dated to between 73 and 75 kyr near the beginning of glacial period MIS 4. The subsequent explosive human expansion was believed to be the result of the end of the ice age. Geologist Michael R. Rampino of New York University and volcanologist Stephen Self of the University of Hawaiʻi at Mānoa supported her theory. In 1998, anthropologist Stanley H. Ambrose of the University of Illinois Urbana-Champaign hypothesized that the Toba eruption caused a human population crash to only a few thousand surviving individuals, and the subsequent recovery was suppressed by the global glacial condition of MIS 4 until the climate eventually transitioned to the warmer condition of MIS 3 about 60,000 years ago, during which rapid human population expansion occurred.

Possible effects on Homo
At least two other Homo lineages, H. neanderthals, and Denisovans, survived the Toba eruption and subsequent MIS 4 ice age, as their latest presence are dated to ca. 40 kyr, and ca. 55 kyr. Other lineages including H. floresiensis, H. luzonensis, and Penghu 1 may have also survived through the eruption. More recently, reconstructions of human demographic history using whole-genome sequencing  and discoveries of archaeological cultures with Toba ash layer  add further light to how humans had fared during the eruption and the following GS20 and MIS 4 ice age.

Human demographic history
Recent analysis applies Markov model to the complete set of genetic material to infer human population history. In non-African populations, studies recover a long-term steep decline in numbers starting 200 kyr and reaching the lowest point around 40–60 kyr. During this bottleneck non-African populations experienced 5- to 15-fold reduction, with only 1,000–3,000 remaining individuals at 50 kyr, consistent with the earliest mtDNA studies. This severe non-African contraction is consistent with founder effect caused by Out-of-Africa dispersal. As a small group with a size of a few thousand people migrated from the African continent into the Near East, the drastic reduction in numbers imprinted on non-African genomic diversity. Genetic analysis identified 56 selective sweeps related to cold adaptations in non-African populations, of which 31 sweeps occurred during 72–97 kyr. This event of closely timed selections is named Arabian Standstill and may have been caused by the severe cold arid conditions from the onset of MIS 4 and exacerbated by Toba super-eruption.

African populations experienced a slightly earlier, milder bottleneck and recovered earlier. Luhya and Maasai people attained their lowest numbers around 70–80 kyr, while Yoruba people reached a nadir around 50 kyr, though the long-term declining trend already started before 200 kyr. The estimated remaining effective population sizes are around 10,000 individuals, larger than the estimated non-African size during their bottleneck. Unlike the non-African populations, there is no consensus as to the cause of African bottleneck. Proposed causes include climatic deterioration (from MIS 5, Toba eruption, GS20 and/or MIS 4), reduction in substructure across African populations, and founder effects from the dispersal within Africa.

Earlier 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 crashes or periodic replacement events from competing Homo subspecies. Whole-genome analysis similarly recovers very low African population sizes around 1 million years ago. This 1 million year old bottleneck is thought to have been caused by severe ice age MIS 22 which marked the mid-Pleistocene climate transition with widespread aridity across Africa.

Archaeological studies
Other research has cast doubt on an association between the Toba Caldera Complex and a genetic bottleneck. For example, ancient stone tools at the Jurreru Valley in southern India were found above and below a thick layer of ash from the Toba eruption and were very similar across these layers, suggesting that the dust clouds from the eruption did not wipe out this local population. However, another site in India, the Middle Son Valley, exhibits evidence of a major population decline and it has been suggested that the abundant springs of the Jurreru Valley may have offered its inhabitants unique protection. At the Jurreru Valley in southern India, Middle Paleolithic stone tools below the Toba ash layer are dated by OSL to 77±4 kyr, while the age of stone tools above the ash layer is constrained to be no older than 55 kyr. This age gap is suspected to be due to the removal of post-eruption sediments or decimation of the local population until re-occupation at 55 kyr. Additional archaeological evidence from southern and northern India also suggests a lack of evidence for effects of the eruption on local populations, causing 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, some researchers have questioned the techniques utilized to date artifacts to the period subsequent to the Toba supervolcano. The Toba Catastrophe also coincides with the disappearance of the Skhul and Qafzeh hominins. 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".

Genetic bottlenecks in other mammals
Some evidence indicates population crashes of other animals after the Toba eruption. The populations of the Eastern African chimpanzee, Bornean orangutan, central Indian macaque, cheetah and tiger, all expanded from very small populations around 70,000–55,000 years ago.