The article by Lee et al. (2004) suggests that at least one “supereruption” from the Toba caldera, Sumatra, was associated with a climatic warming, rather than a cooling trend. The background to this is that Rampino and Self (1992, 1993) originally suggested that the supereruption that produced the younger Toba tuff ca. 74 ka could have helped to usher in the last glacial period (isotope stage 4). This long-term climatic feedback effect would be in addition to the severe short-term climatic effects or “volcanic winter” predicted for that eruption. Lee et al. (2004) have studied deep-sea ashes in the South China Sea that they contend were produced by the smaller older Toba tuff eruption at a revised age of 788 ka, which they then propose was not involved in a long-term cooling episode. We here raise three points that challenge the results of their study.
The identification of the tephra in the South China Sea core by Lee et al. is based on correlation with layer D in Ocean Drilling Program (ODP) Site 758 in the Indian Ocean. The correlation rests on stratigraphic position and major element glass chemistry, and appears sound. However, the major premise of the paper depends on the assumption that layer D (ca. 800 ka) in the ODP Site 758 core is older Toba tuff. They provide no mineralogical or major and trace element geochemical data on glass shards, nor are there data in other literature, to support this correlation. Previous workers who have studied the Toba ashes in deep-sea cores think that layer E (not D) in ODP Site 758 represents the older Toba tuff eruption (840 ka), but this is based only upon the apparent age of the ashes (Dehn et al., 1991; Westgate et al., 1998; Pattan et al., 1999).
In Lee et al.'s (2004) study, ash shards in South China Sea cores are correlated with other presumed Toba ashes in deep-sea cores. They do not provide the obvious correlation to prove their case—there is no geochemical data linking the deep-sea ashes to older Toba tuff on land in Sumatra. Thus, their proposed 788 ka age for older Toba tuff, which is more than 50 k.y. younger than dates on older Toba tuff on land (840 ka), is also not substantiated. The age of older Toba tuff (840 ka) fits better with layer E (also 840 ka) in ODP Site 758 (found just below layer D).
Comments about the dispersal of layer D/South China Sea tephra being as wide as that of the ca. 74 ka younger Toba tuff are unfounded. Ash in the Central Indian Ocean basin reported by Pattan et al. (1999) is contained within sediment younger than 200 ka based on microfossils and Th isotopes. Thus, they cannot match older eruptions such as layer D (800 ka). Similarly, ash on land in India has been dated using isothermal plateau fission-track methods at younger than 100 ka (Westgate et al., 1998). Again, correlation to younger Toba tuff is indicated (not older Toba tuff or layers E or D). The dispersal of the layer D/South China Sea tephra is limited to areas between the sites of ODP 758 and MD972142 at the most, and is therefore not comparable to the much wider dispersal of the younger Toba tuff ash. The volume of the eruption that generated the layer D/South China Sea ash may also be considerably smaller than younger Toba tuff, because, to date, there is no demonstration of correlation to on-land pyroclastic flow deposits.
In summary, the proposed younger age for the older Toba tuff is not substantiated, and thus the correlation of the older Toba tuff eruption with any effects on Pleistocene climate remains unknown.
Another point is that the much larger volume younger Toba tuff is now possibly recognized as a large acidity peak in Greenland ice cores (Zielinski et al., 1996), which occurs within a brief cold period prior to the major cooling from isotope stage 5 to 4. Oppenheimer (2002) presents a review of why direct correlations of the younger Toba tuff eruption with climatic and demographic phenomena must be viewed with caution until more precise age dating of the various events and better understanding of the volcanic eruption are available. These reservations also apply to interpretations of the long-term environmental impact of the considerably more ancient older Toba tuff event.