Volcanic supereruption 74,000 years ago in Indonesia
The Toba eruption (sometimes called the Toba supereruption or the Youngest Toba eruption) was a supervolcaniceruption that occurred about 74,000 years ago during the Late Pleistocene[1] at the site of present-day Lake TobainSumatra, 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.[2] 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.
Youngest Toba eruption
Artist's impression of early stages of eruption from about 42 km (26 mi) above northern Sumatra
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.[4] The eruption lasted perhaps 9 to 14 days.[5] The most recent two high-precision argon–argon datings dated the eruption to 73,880 ± 320[6] and 73,700 ± 300 years ago.[7] Five distinct magma bodies were activated within a few centuries before the eruption.[8][9] The eruption commenced with small and limited air-fall and was directly followed by the main phase of ignimbrite flows.[10] The ignimbrite phase is characterized by low eruption fountain,[11] but co-ignimbrite column developed on top of pyroclastic flows reached a height of 32 km (20 mi).[12]Petrological constraints on sulfur emission yielded a wide range from 1×1013to1×1015g, depending on the existence of separate sulfur gas in the Toba magma chamber.[13][14] The lower end of estimate is due to the low solubility of sulfur in the magma.[13]Ice core records estimate the sulfur emission on the order of 1×1014g.[15]
Bill Rose and Craig Chesner of Michigan Technological University have estimated that the total amount of material released in the eruption was at least 2,800 km3 (670 cu mi)[16]—about 2,000 km3 (480 cu mi) of ignimbrite that flowed over the ground, and approximately 800 km3 (190 cu mi) that fell as ash mostly to the west. However, as more outcrops become available, the most recent estimate of eruptive volume is 3,800 km3 (910 cu mi) dense-rock equivalent (DRE), of which 1,800 km3 (430 cu mi) was deposited as ash fall and 2,000 km3 (480 cu mi) as ignimbrite, making this eruption the largest during the Quaternary period.[17] Previous volume estimates have ranged from 2,000 km3 (480 cu mi)[5] to 6,000 km3 (1,400 cu mi).[18] Inside the caldera, the maximum thickness of pyroclastic flows is over 600 m (2,000 ft).[19] The outflow sheet originally covered an area of 20,000–30,000 km2 (7,700–11,600 sq mi) with thickness nearly 100 m (330 ft), likely reaching into the Indian Ocean and the Straits of Malacca.[10] The air-fall of this eruption blanketed the Indian subcontinent in a layer of 5 cm (2.0 in) ash,[20] the Arabian Sea in 1 mm (0.039 in),[21] the South China Sea in 3.5 cm (1.4 in),[4] and Central Indian Ocean Basin in 10 cm (3.9 in).[22] Its horizon of ashfall covered an area of more than 38,000,000 km2 (15,000,000 sq mi) in 1 cm (0.39 in) or more thickness.[17]InSub-Saharan Africa, microscopic glass shards from this eruption are also discovered on the south coast of South Africa,[23] in the lowlands of northwest Ethiopia,[24]inLake Malawi,[25] and in Lake Chala.[26]InSouth China, Toba tephras is found in Huguangyan Maar Lake.[27]
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.
Greenland stadial 20 (GS20) is a millennium-long cold event in the north Atlantic ocean that started around the time of Toba eruption.[28] The timing of the initiation of GS20 is dated to 74.0–74.2 kyr, and the entire event lasted about 1,500 years.[28][29] 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.[30][31] The start of GS20 cooling event corresponds to the start of Antarctic Isotope Maxima 19 (AIM19) warming event.[32] GS20 was associated with iceberg discharges into the North Atlantic, thus it was also named Heinrich stadial 7a.[33] Heinrich events tend to be longer, colder and with weaker AMOC in the Atlantic ocean than other DO stadials.[30]
From 74 to 58 kyr, Earth transitioned from interglacial MIS 5 to glacial MIS 4, experiencing cooling and glacial expansion.[34][35] This transition is a part of Pleistocene interglacial-glacial cycle driven by variations in the earth's orbit.[36] Ocean temperature cooled by 0.9 °C (1.6 °F).[37] Sea level fell 60 m (200 ft).[38] Northern Hemisphere ice sheets embarked on significant expansion and surpassed the extent of Last Glacial Maximumineastern Europe, Northeast Asia and the North American Cordillera.[39] Southern Hemisphere glaciation grew to its maximum extent during MIS 4.[40]Australasian region, Africa and Europe were characterized by increasingly cold and arid environment.[41][42][43]
While Toba eruption occurred in the backdrop of rapid climate transitions of GS20 and MIS 4 triggered by changes in ocean currents and insolation,[44][28] 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 (1.8 °F) cooling above Toba ash layer for a thousand year but the authors concede that it may just be GS20.[45] 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.[46] 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,[25][47] but cooling-forced aridity killed high elevation afromontane forests.[48] The Lake Malawi studies concluded that the environmental effects of the eruption were mild and limited to less than a decade in East Africa,[47] but these studies are questioned due to sediment mixing which would have diminished the cooling signal.[49] 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.[24]
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.[50][32][51] 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.[51] 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.[15]
The modeled climate effects of the Toba eruption hinges on the mass of sulfurous gases and aerosol microphysical processes. Modeling on an emission of 8.5×1014g of sulfur, which is 100 times the 1991 Pinatubo sulphur, volcanic winter has a maximum global mean cooling of 3.5 °C (6.3 °F) 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.[52][53] Two other emission scenarios, 1×1014g and 1×1015g, are investigated using state-of-art simulations provided by the Community Earth System Model. Maximum global mean cooling is 2.3 °C (4.1 °F) for the lower emission and 4.1 °C (7.4 °F) for the higher emission. Strong decrease in precipitation occurs in high emission. Negative temperature anomalies return to less than 1 °C (1.8 °F) within 3 and 6 years for each emission scenario after the eruption.[54] 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 (2.7 °F) no matter how much sulfur emitted.[55]
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 bottleneckinhumans.[56][57] However, some physical evidence disputes the association with the millennium-long cold event and genetic bottleneck, and some consider the theory disproven.[58][48][59][60][61]
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.[62] More genetic studies confirmed an effective population on the order of 10,000 for much of human history.[63][64] 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 35,000 and 65,000 years ago.[65][66][67]
The large magnitude of the Toba eruption has been known since 1939, and various techniques dated the timing of the event to 73,000 to 75,000 years ago.[5] A study published in 1993 suggested that the eruption accelerated climate and environmental transition from the last interglacial period MIS 5 to the last glacial period MIS 4.[68]
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 eruption. The subsequent explosive human expansion was believed to be the result of the end of the ice age.[69] Geologist Michael R. RampinoofNew York University and volcanologist Stephen Self of the University of Hawaiʻi at Mānoa supported her theory.[70] 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.[56]
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,[71] and ca. 55 kyr.[72] Other lineages including H. floresiensis,[73]H. luzonensis,[74] and Penghu 1[75] may had also survived through the eruption. More recently, reconstructions of human demographic history using whole-genome sequencing[76][77][78] and discoveries of archaeological cultures with Toba ash layer[79][23][24] add further light to how humans had fared during the eruption and the following GS20 and MIS 4 ice age.
The Toba eruption has been associated with a genetic bottleneck in human evolution about 70,000 years ago;[80][81] it is hypothesized that the eruption resulted in a severe reduction in the size of the total human population due to the effects of the eruption on the global climate.[82] According to the genetic bottleneck theory, between 50,000 and 100,000 years ago, human populations decreased to 3,000–10,000 surviving individuals.[83][84] It is supported by some genetic evidence suggesting that modern humans are descended from a very small population of between 1,000 and 10,000 breeding pairs that existed about 70,000 years ago.[85][86]
Proponents of the genetic bottleneck theory (including Robock) suggest that the 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.[87] These environmental changes may have generated population bottlenecks in many species, including hominids;[88] this in turn may have accelerated differentiation from within the smaller human population. Therefore, the genetic differences among modern humans may represent changes within the last 70,000 years, rather than gradual differentiation over hundreds of thousands of years.[89]
Additional caveats include difficulties in estimating the global and regional climatic effects of the eruption and lack of conclusive evidence for the eruption preceding the crash.[90] 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 crashes or periodic replacement events from competing Homo subspecies. (If these results are accurate, then, even before the emergence of Homo sapiens in Africa, Homo erectus population was unusually small when the species was spreading around the world.)[91]
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,[92] consistent with dating of the Toba eruption to about 75,000 years ago.[citation needed]
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.[93][94][95] 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.[96] 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".[97] However, some researchers have questioned the techniques utilized to date artifacts to the period subsequent to the Toba supervolcano.[98] The Toba Catastrophe also coincides with the disappearance of the Skhul and Qafzeh hominins.[99] 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".[100][101]
Some evidence indicates population crashes of other animals after the Toba eruption. The populations of the Eastern African chimpanzee,[102]Bornean orangutan,[103] central Indian macaque,[104]cheetah and tiger,[105] all expanded from very small populations around 70,000–55,000 years ago.
^Ambrose, S. H. (2019), "Chapter 6 chronological calibration of Late Pleistocene Modern Human dispersals, climate change and Archaeology with Geochemical Isochrons", in Sahle, Yonatan; Reyes-Centeno, Hugo; Bentz, Christian (eds.), Modern Human Origins and Dispersal, Kerns Verlag, pp. 171–213
^Jones, Sacha. (2012). Local- and Regional-scale Impacts of the ~74 ka Toba Supervolcanic Eruption on Hominin Population and Habitats in India. Quaternary International 258: 100-118.
^National Geographic- Did early humans in India survive a supervolcano?
^ Shea, John. (2008). Transitions or Turnovers? Climatically-forced Extinctions of Homo sapiens and Neanderthals in the East Mediterranean Levant. Quaternary Science Reviews 27: 2253-2270.
Ninkovich, D.; N.J. Shackleton; A.A. Abdel-Monem; J.D. Obradovich; G. Izett (7 December 1978). "K−Ar age of the late Pleistocene eruption of Toba, north Sumatra". Nature. 276 (5688): 574–577. Bibcode:1978Natur.276..574N. doi:10.1038/276574a0. S2CID4364788.
Steiper, M.E. (2006). "Population history, biogeography, and taxonomy of orangutans (Genus: Pongo) based on a population genetic meta-analysis of multiple loci". Journal of Human Evolution. 50 (5): 509–522. doi:10.1016/j.jhevol.2005.12.005. PMID16472840.
Williams, Martin A.J.; Stanley H. Ambrose; Sander van der Kaars; Carsten Ruehlemann; Umesh Chattopadhyaya; Jagannath Pal; Parth R. Chauhan (30 December 2009). "Environmental impact of the 73 ka Toba super-eruption in South Asia". Palaeogeography, Palaeoclimatology, Palaeoecology. 284 (3–4). Elsevier: 295–314. Bibcode:2009PPP...284..295W. doi:10.1016/j.palaeo.2009.10.009.
Journey of Mankind by The Bradshaw Foundation – includes discussion on Toba eruption, DNA and human migrations
Geography Predicts Human Genetic Diversity ScienceDaily (Mar. 17, 2005) – By analyzing the relationship between the geographic location of current human populations in relation to East Africa and the genetic variability within these populations, researchers have found new evidence for an African origin of modern humans.
