The Toba eruption was a super-volcano eruption c 74 ka (late-Pleist.) at the present-day Lake Toba in Sumatra, one of the largest known explosive eruptions in the Earth's history. Did this event cause a severe global volcanic winter of 6-10 yrs, and contribute to a 1000-year-long cooling episode, leading to a genetic bottleneck in humans?

A nr of genetic studies have revealed: 50 ka, the human ancestor population greatly expanded from only a few 1000 individuals. Journalist Ann Gibbons has posited: the low population size was caused by the Toba eruption. Geologist Michael Rampino (NY Univ) & volcanologist Stephen Self (Univ.Hawaiʻi Mānoa) have supported her suggestion, in 1998, further developed by anthropologist Stanley Ambrose (Univ.Illinois Urbana-Champaign), but some physical evidence disputes the links with the millennium-long cold event & genetic bottleneck: some consider the theory disproven.

The Toba eruption 73.88 ka ± 0.32 (high-precision K-Ar dating) was the last & largest of 4 Quaternary eruptions of the Toba Caldera Complex. It is also recognized from its diagnostic horizon of ash-fall, the Toba tuff. It had an estimated Volcanic explosivity index VEI of 8 (the highest rating on the scale), it made a sizable contribution to the 100 km × 35 km caldera complex.

Based on known distribution of ash fall & pyro-clastic flows, eruptive volume was estimated to be at least 2800 km3 dense-rock equivalent DRE: 800 km3 was deposited as ash-fall. Computational ash dispersal models suggested possibly 5300 km3 DRE. Even larger 6000 km3 DRE has been suggested, based on lost & eroded ash from pyroclastic flows. The Toba eruption was the largest explosive volcanic eruption known in the Quaternary.

The eruption was of exceptional intensity, completed within 9-14 days. Toba's erupted mass deposited an ash layer of c 15 cm thick over India. A blanket of volcanic ash was also deposited over the Indian Ocean, the Arabian Sea & the S.China Sea. Glass shards from this eruption have also been discovered in E.Africa.

Climatic effects
By analyzing climate proxies & simulating climate forcing, researchers can gain insights into the immediate climatic effects of the Toba eruption. But there are limitations to both approaches. In sedimentary records where the Toba tuff does not serve as a marker horizon, it cannot pinpoint the exact section that records the environmental conditions immediately following the eruption. In sedimentary records that do have the Toba tuff as a marker horizon, the sedimentation rate may be too low to capture the short-term climatic effects of the eruption. OTOH, results of climate models entirely depend on the volatile budget of erupted magma, hence varies accordingly to the assumed volatile budget.

Climate proxy
Toba tephra-layer in marine sediments coincides δ18O marine isotope stage MIS-5a to -4 boundary, marking a climatic transition from warm to cold, caused by change in ocean circulation & drop in atmospheric CO2-concentration (Dansgaard-Oeschger event). Geologist Michael Rampino volcanologist Stephen Self hypothesized: Toba eruption accelerated this shift. Testing this hypothesis required higher resolution sedimentary records.
2 marine sediment cores Toba marker horizon retrieved from the N-Indian Ocean & the S.China Sea showed no pronounced cooling, or a 0.8–1.0°C cooling in the centuries following eruption. The core resolution was insufficient to ascertain that the cooling was caused by Toba eruption: the 2 events could be decades or centuries apart in the core. But a severe cooling of only a few years is not expected to appear in these sediment records of centennial resolution. Yet the marine sedimentary records support: Toba had only a minor impact on the time scales longer than a century.
In Greenland ice-cores, a large sulfate spike that appeared between Dansgaard–Oeschger event 19 & 20 was possibly related to Toba eruption. The δ18O-values of the ice-cores indicate a 1000-yr cooling immediately following the sulfate signal. But high-resolution δ18O excluded the possibility of a >1-century-long cooling impact of the eruption, and ruled out that Toba triggered the cooling as it was already underway.
Insufficient resolution in marine sediments bearing the Toba tuff has hindered the assessment of any short-term effects that may have lasted for <1 century.
In 2013, a microscopic layer of Toba ash was reported in sediments of Lake Malawi. Together with the high sedimentation rate of the lake & Toba marker horizon, several teams have reconstructed the local environment after Toba eruption at sub-decadal resolution of ~6–9 yrs. The sediments in core display no clear evidence of cooling, and no unusual deviations in concentrations of climate-sensitive ecological indicators: the duration of the Toba-cooling must have been shorter than the sampling resolution of ~6–9 yrs, or too small in magnitude in E.Africa.

Climate model
The mass of sulfurous gases emitted during Toba eruption is a crucial parameter when modeling its climatic effects.
Assuming an emission of 1.7 billion tonnes of SO2 (100x the 1991 Pinatubo sulphur), the modeled volcanic winter has max.global mean cooling of −3.5°C, and gradually returns within the range of natural variability 5 yrs after the eruption. An initiation of 1000-yr cold period or ice-age is not supported by the model.
In a 2021 study, 2 other emission scenarios, 0.2 billion tonnes & 2 billion tonnes of SO2 (10 & 100x of Pinatubo resp.) are investigated, using state-of-art simulations provided by the Community Earth System Model. Max.global mean cooling is −2.3°C for a 0.2 billion tonnes SO2-release, and −4.1°C for a 2 billion tonnes SO2 release. Negative Tp anomalies return to <−1°C within 3 & 6 yrs for each emission scenario after the eruption.
Petrological studies of Toba magma constrained: the mass of sulfuric acid aerosols from Toba eruption represents c 2–5x the sulfuric acid aerosols generated during 1991 Pinatubo eruption. The studies suggest: previous modelings of global Tp perturbations following Toba eruption were excessive. Ice-core records of atm.sulfur injection during the period during which the Toba eruption occurred contain 3 large injections that are 10–30x the Pinatubo sulfur.

Genetic bottleneck hypothesis in humans
The Toba eruption has been linked to a genetic bottleneck in human evolution x 70 ka: did the eruption result in a severe reduction in the size of the total human population due to the global climate effects of the eruption? did human populations between 50-100 ka sharply decrease to 3000-10,000 surviving individuals? Some genetic evidence suggests: today's humans are descended from a very small population of between 1000 & 10,000 breeding pairs c 70 ka.
Proponents of the genetic bottleneck theory (incl. Robock) suggest: the Toba eruption resulted in a global ecol.disaster: destruction of vegetation + severe drought in the tropical rain-forest belt & in monsoonal regions. A 10-yr volcanic winter triggered by the eruption could have largely destroyed human food-sources, and caused a severe reduction in population sizes. These environmental changes may have generated population bottlenecks in many spp incl. hominids, this in turn may have accelerated differentiation from within the smaller human population: the genetic differences among modern humans may reflect changes within the last 70 ka, rather than gradual differentiation over 100s of 1000s of yrs.
Other research has cast doubt on a link between the Toba Caldera Complex & a genetic bottleneck, e.g. ancient stone tools at the Jurreru Valley (S-India) were found above & below a thick layer of ash from the Toba eruption, and were very similar across these layers: did the dust clouds from the eruption not wipe out this local population? But another site in India, the Middle Son Valley, exhibits evidence of a major population decline: did the abundant springs of the Jurreru Valley offer its inhabitants unique protection? Additional archaeol.evidence from S- & N-India also suggests a lack of evidence: the authors conclude: "many forms of life survived the super-eruption, contrary to other research which has suggested significant animal extinctions & genetic bottlenecks". But 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 & Qafzeh hominins. Evidence from pollen-analysis has suggested prolonged de-forestation in S.Asia: did the Toba eruption force humans to adopt new adaptive strategies, which may have permitted them to replace Neanderthals & "other archaic human spp"?
Additional caveats include difficulties in estimating the global & regional climatic impacts of the eruption, and lack of conclusive evidence for the eruption preceding the bottleneck. Genetic analysis of Alu-sequences across the entire human genome has shown: the effective human population size was <26,000 at 1.2 Ma, possible explanations for the low population size of human ancestors may include repeated population bottlenecks, or periodic replacement events from competing Homo subspp.

Genetic bottlenecks in other mammals
Some evidence points to genetic bottlenecks in other animals in the wake of the Toba eruption. The populations of the E-African chimp, Bornean orang, C-Indian macaque, cheetah & tiger, all recovered from very small populations c 70–55 ka.

Migration after Toba
The exact geographic distribution of AMH 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 mtDNA have estimated that the major migration from Africa occurred 60–70 ka, cons.x the Toba eruption c 75 ka.

littor...@gmail.com wrote:
> The Toba eruption was a super-volcano eruption c 74 ka (late-Pleist.) at the present-day Lake
> Toba in Sumatra, one of the largest known explosive eruptions in the Earth's history.
> Did this event cause a severe global volcanic winter of 6-10 yrs, and contribute to a
>1000-year-long cooling episode, leading to a genetic bottleneck in humans?

The thing about Toba is that it's NOT a theory at all.

We know it erupted. We know it was MASSIVE. We know what the consequences
would have been. We know countless animals, including Homo would have died.

A bottleneck is impossible to escape. It really always came down to how tight
that bottleneck was, how few made it past it...

There's this brain-lock in paleo anthropology that always insists on an either-or
mode of thinking..Toba was either a massive dying off of Homo or Homo went
about their business without a care.

To me it's similar to Aquatic Ape. They all agree with Aquatic Ape, they all
agree that humanity was on the coast, following the water's edge, exploiting
the sea -- COASTAL DISPERSAL -- and they all deny it. Which is stupid.

We know there was a Toba bottleneck. But, how big was it? And we know
that Aquatic Ape was real. But, when did it start and how significant was it?

IMPORTANT: After a climate upheaval like Toba, the best place to be is
along the coast. The sea is a massive battery, storing and releasing the
sun's energy. The result is that it moderates temperature. Along the water
is cooler when it's hot, and warmer than it's cold. Because when it's hot,
water absorbs energy. When it's cold the water releases energy.

This is why deserts can be so hot during the day & cold at night. They're
very dry, so there's no moisture to absorb energy during the day and
none being released at night...

So when Toba exploded, the population best suited for survival was
the Aquatic Ape parent group, not the child populations inland.

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