Palaeogeography, Palaeoclimatology, Palaeoecology
DiscussionComment 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]
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.
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