Discussion
Comment on “Environmental impact of the 73 ka Toba super-eruption in South Asia” by M.A.J. Williams, S.H. Ambrose, S. van der Kaars, C. Ruehlemann, U. Chattopadhyaya, J. Pal and P.R. Chauhan [Palaeogeography, Palaeoclimatology, Palaeoecology 284 (2009) 295–314]

https://doi.org/10.1016/j.palaeo.2010.03.057Get rights and content

Abstract

Williams et al. [Williams, M., Ambrose, S.H., van der Kaars, S., Ruehlemann, C., Chattopadhyaya, U., Pal, J.N., Chauhan, P., 2009, Environmental impact of the 73 ka Toba super-eruption in South Asia. Palaeogeography, Palaeoclimatology, Palaeoecology, 284, 295–314] recently reported Late Pleistocene palaeoenvironment indicators from terrestrial sites in central India and a marine core from the Bay of Bengal. They correlate isotopic data from soil carbonates and pollen from the marine core using occurrences of the Youngest Toba Tuff (YTT), a widespread tephra erupted from the Sumatran Toba caldera ∼ 74,000 years ago. They conclude that the Toba eruption caused both climatic cooling and prolonged deforestation in South Asia. However, consideration of the bases for these claims reveals that the YTT eruption is used as a chronological marker of cooling at the end of Dansgaard–Oeschger interstadial 20, and no evidence is provided that the eruption caused or prolonged this process. Furthermore, while the presented data add to the South Asian palaeoenvironmental literature, the applicability of the results to questions of impact on human and other populations is overstated.

Introduction

We welcome the recent publication of South Asian palaeoenvironmental data from before and after the ∼ 74 ka Youngest Toba Tuff (YTT) super-eruption by Williams et al. (2009). As noted by these authors, reliable climatic and environmental information dated to around the time of this eruption is scarce in South Asia, and accumulation of relevant data is necessary to disentangle the effects of cyclical climatic oscillations from unique events such as the YTT.

However, the results and conclusions of the Williams et al. (2009) study suffer from a significant disjuncture in causal reasoning, including unsubstantiated links between one particular cold period identified in the Greenland isotopic record and the Toba eruption. The consequence is a lack of demonstrated influence of the YTT event over the reported palaeoenvironmental changes. Here we briefly discuss three main issues that arise from the conclusions of the study: first, the acceptance by Williams et al. that the Toba eruption was the cause of the cold period between Dansgaard–Oeschger (D–O) interstadials 20 and 19; second, the use of the results obtained during the study to make claims for Toba's influence on South Asia-wide vegetation changes; and third, discussion of Toba's possible effects on Late Pleistocene human and mammalian populations.

Section snippets

Toba and D–O 20

The last glacial period saw a number of periods of rapid large-scale warming followed by variably-paced cooling, known as Dansgaard–Oeschger events following their initial identification in the oxygen isotope record of Greenland ice cores (GRIP and GISP2) (Dansgaard et al., 1993, Grootes et al., 1993). The dramatic increase in temperature that marks the commencement of D–O 20 is dated between 77 and 72 ka in a number of oxygen isotopic records from widespread locations including Greenland, China

Toba's impact on environments

As Williams et al. fail to differentiate the cooling effects of the Toba eruption from the cyclic signal of D–O 20, it follows that their claim that Toba was the cause of attendant South Asian environmental effects associated with cooling during this period is unsupported. The central assertion made by Williams et al. (2009) is that the YTT event caused a change in terrestrial vegetation at three sites (Khunteli, Rehi and Hirapur) in north-central India, from C3-dominated forests to mainly C4

Toba's impact on populations

Despite the lack of change directly attributable to Toba, Williams et al. extend their conclusions to discuss the possible effects of the eruption on human and other mammalian populations, including those outside of South Asia. The notion that ancestral human populations suffered a Late Pleistocene decrease in effective population size (a population ‘bottleneck’) is an established one, but it remains contentious as to the size and timing of the decrease (Gathorne-Hardy and Harcourt-Smith, 2003

Conclusion

Williams et al. have produced a useful addition to the study of the effects of thick YTT accumulations on local flora in north-central India, as well as the possible effects of Dansgaard–Oeschger interstadial 20, and the subsequent glacial OIS 4, on South Asian environments. However, in failing to unequivocally associate any of the environmental, genetic, or social changes discussed by them with the Late Pleistocene Toba super-eruption, they run the risk of misdirecting the debate in which they

Acknowledgements

We thank Craig Chesner and an anonymous reviewer for their helpful comments on an earlier draft of this paper.

References (53)

  • S. McBrearty et al.

    The revolution that wasn't: a new interpretation of the origin of modern human behavior

    Journal of Human Evolution

    (2000)
  • C. Oppenheimer

    Limited global change due to the largest known Quaternary eruption, Toba 74 kyr BP?

    Quaternary Science Reviews

    (2002)
  • H. Schulz et al.

    The Toba volcanic event and interstadial/stadial climates at the Marine Isotopic Stage 5 to 4 transition in the northern Indian Ocean

    Quaternary Research

    (2002)
  • M. Steiper

    Population history, biogeography, and taxonomy of orangutans (Genus: Pongo) based on a population genetic meta-analysis of multiple loci

    Journal of Human Evolution

    (2006)
  • K.E. Westaway et al.

    Homo floresiensis and the late Pleistocene environments of eastern Indonesia: defining the nature of the relationship

    Quaternary Science Reviews

    (2009)
  • M. Williams et al.

    Environmental impact of the 73 ka Toba super-eruption in South Asia

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (2009)
  • M. Zhao et al.

    A millennial-scale U K′/37 sea-surface temperature record from the South China Sea (8°N) over the last 150 kyr: monsoon and sea-level influence

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (2006)
  • R.B. Alley

    Wally was right: predictive ability of the North Atlantic “conveyor belt” hypothesis for abrupt climate change

    Annual Review of Earth and Planetary Sciences

    (2007)
  • S.H. Ambrose et al.

    Environmental impact of the 73 ka Toba eruption reflected by paleosol carbonate carbon isotope ratios in central India

    Quaternary International

    (2007)
  • J. Antos et al.

    Recovery of forest understories buried by tephra from Mount St. Helens

    Vegetatio

    (1985)
  • D. Brandon-Jones

    Pre-glacial Bornean primate impoverishment and Wallace's line

  • J. Caswell et al.

    Analysis of chimpanzee history based on genome sequence alignments

    PLoS Genetics

    (2008)
  • A. Cohen et al.

    Ecological consequences of early Late Pleistocene megadroughts in tropical Africa

    Proceedings of the National Academy of Sciences

    (2007)
  • W. Dansgaard et al.

    Evidence for general instability of past climate from a 250-kyr ice record

    Nature

    (1993)
  • D. Erwin et al.

    Testing for causal relationships between large pyroclastic volcanic eruptions and mass extinctions

    Geophysical Research Letters

    (1992)
  • R.N. Fuller et al.

    The role of refugia and dispersal in primary succession on Mount St. Helens, Washington

    Journal of Vegetation Science

    (2003)
  • Cited by (39)

    • The prelude to the Holocene: tropical Asia during the Pleistocene

      2021, Holocene Climate Change and Environment
    • The Toba tephra as a late Quaternary stratigraphic marker: Investigations in the Sagileru river basin, Andhra Pradesh, India

      2019, Quaternary International
      Citation Excerpt :

      Proponents of extreme change in scenarios have concluded that ecosystems and Homo sapien populations were devastated as a result of the ∼74 ka volcanic activity (Ambrose, 1998, Gathorne-Hardy and Harcourt-Smith, 2003). The duration and degree to which this eruption impacted on global climates, terrestrial palaeoenvironments and the impact on the Homo sapien populations has also been a topic of intense debate (Ambrose, 1998, 2003; Petraglia et al., 2007; Haslam and Petraglia, 2010; Jones, 2012, Williams 2012a, b). However, others have also cautioned against assuming the triggering of extreme global climate impacts given the limited constraints on the sulphur yield of the eruption and on the date of any palaeodemographic ‘bottleneck’ in anatomically modern human populations (Oppenheimer, 2002).

    • Subdecadal phytolith and charcoal records from Lake Malawi, East Africa imply minimal effects on human evolution from the ∼74 ka Toba supereruption

      2018, Journal of Human Evolution
      Citation Excerpt :

      The magnitude of the Indonesian Mount Toba supereruption at ∼74 ka and its temporal proximity to a Late Pleistocene human population bottleneck and to various models of anatomically modern human (AMH) dispersal out of Africa have made this eruption the subject of much debate (Oppenheimer, 2002; Ambrose, 2003; Gathorne-Hardy and Harcourt-Smith, 2003; Haslam and Petraglia, 2010; Balter, 2010; Williams et al., 2010; Williams, 2012; Mark et al., 2013; Roberts et al., 2013; Lane et al., 2013b; Haslam, 2014).

    View all citing articles on Scopus
    View full text