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

Abstract

The ≈74 kyr BP “super-eruption” of Toba volcano in Sumatra is the largest known Quaternary eruption. On the basis of preserved deposits, the eruption magnitude has been estimated at ≈7×10 15 kg (≈2800 km 3 of dense magma). The largest sulphate anomaly in the Greenland Ice Sheet Project 2 core has been identified as fallout from Toba's stratospheric aerosol veil. Correlation of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the Toba eruption might have played a role in triggering a millennium of cool climate prior to Dansgaard-Oeschger event 19, although a comparable stadial preceded event 20. A possible 6 yr duration “volcanic winter” immediately following the eruption has also been proposed as the cause of a putative bottleneck in human population supporting, in a general way, the “Garden of Eden” model for the origin of modern humans. However, along with counter arguments regarding the timing of any demographic crash, there remain major gaps in our understanding of the ≈74 kyr BP Toba eruption that hinder attempts to model its global atmospheric and climatic, and hence human consequences. The tephra record reveals basic aspects of the eruption style but calculations of the duration, and hence intensity and plume height of the event, are poorly constrained. Furthermore, estimates of the sulphur yield of the erupting magma, central to predictions of its atmospheric and climatic impacts, vary by two orders of magnitude (3.5–330×10 10 kg). Previous estimates of globally averaged surface cooling of 3–5°C after the eruption are probably too high; a figure closer to 1°C appears more realistic. The volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the Toba super-eruption.