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

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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 ≈7×1015 kg (≈2800 km3 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 (3.5–330×1010 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.

Introduction

The Late Pleistocene eruption of the Younger Toba Tuff (YTT) may have been the greatest single volcanic cataclysm in the Quaternary. It expelled an estimated 7×1015 kg (or 2800 km3 dense rock equivalent, DRE, at 2500 kg m−3) of rhyolitic magma and made a sizeable contribution to the 100×30 km caldera complex occupied today by Lake Toba in northern Sumatra (Fig. 1; Rose and Chesner (1987), Rose and Chesner (1990); Chesner and Rose, 1991). The size of the eruption (3500 times greater than the largest historic eruption, the 1815 outburst of Tambora volcano, Indonesia) and its approximate coincidence with global environmental and climatic change at the end of Oxygen Isotope Stage (OIS) 5a led to speculation that it accelerated the transition into the last Ice Age (Rampino and Self (1992), Rampino and Self (1993a)). It has also been suggested that global cooling, triggered by the YTT eruption, precipitated an environmental catastrophe that resulted in near extinction of contemporaneous human populations (Gibbons, 1993; Rampino and Self, 1993b; Ambrose, 1998; Rampino and Ambrose, 2000; and cited in Harpending and Rogers, 2000; Hewitt, 2000). In view of the significance of these palaeodemographic implications, and a range of recently available tephrostratigraphic, ice core and petrologic evidence that bears on the possible consequences of the eruption, it is timely to re-evaluate critically the basis for the claims made for Toba. The aim of this paper is also to stimulate broad, interdisciplinary research across the Quaternary science community that will advance our understanding of the global effects of very large eruptions.

Section snippets

Age of the YTT

The YTT was not the first large eruption from Toba. It was preceded by the Haranggoal dacite (1.2 Myr BP), the Oldest Toba Tuff (840 kyr BP) and Middle Toba Tuff (501 kyr BP) (Chesner and Rose, 1991; Chesner et al., 1991). Dates obtained for the YTT have tended to be remarkably consistent given the very different techniques that have been used to obtain them (Table 1). The best current estimate for its age is 74±2 kyr. There was some controversy surrounding a more recent explosive eruption of Toba,

Deposits of the YTT eruption

The eruption is thought to have ensued as the roof of a large magma chamber, located up to 10 km deep in the crust, began to founder (Chesner, 1998; although Beddoe-Stephens et al., 1983, estimated a shallower depth for the magma body of around 3–4 km; recent seismic tomographic investigations have suggested the presence of two principal melt regions between the lake and a depth of 10 km, Masturyono et al., 2001). This opened up ring fractures, which fed massive outpourings of pyroclastic flows.

Eruption parameters: intensity, duration

The granulometry of feldspar crystals in Indian Ocean YTT tephra horizons was investigated by Ledbetter and Sparks (1979) to estimate the eruption duration. By computing settling velocities for the crystals in water, and from an estimate of the contemporary water depth, they concluded that the eruption lasted between 9 and 14 days. Combined with the eruption magnitude estimate, this suggests a mean eruption intensity of around 7×109 kg s−1 of magma, 1–3 orders of magnitude greater than the magma

Impacts on the atmosphere and climate

Assessing the impacts of Toba on the Earth system is highly challenging given the uncertainties in the key parameters for the eruption (intensity, height, magnitude), and amounts of gaseous sulphur species released. Initial interest focused on the apparent coincidence of the eruption with the onset of the Last Glaciation. Rampino and Self (1992) dwelled on the coincidence of the YTT eruption with the transition to glacial climate at the OIS 5a-4 boundary at 67.5 kyr BP. They argued that Toba

Impacts on the terrestrial environment

Rose and Chesner (1990) likened the aftermath of the eruption to an “enormous fire”, covering up to 30,000 km2, arising from the widespread ignition of vegetation. They judged that the outflow sheets of the YTT would have had temperatures up to 550°C when they came to rest, compared with an initial temperature of 710–770°C. From observations following Pinatubo's 1991 eruption (e.g., Torres et al., 1996), we can imagine that these deposits remained at high temperatures for years given their great

Human impacts

Investigation of the palaeodemography of Homo through the Pleistocene has been tackled from many perspectives, including palaeoanthropology, archaeology, numerical modelling, and, most recently, a range of genetic studies. Around the time of the eruption, Neanderthals inhabited Eurasia and the Levant, modern humans occupied Africa, the Arabian peninsula and parts of central Asia, “Homo heidelbergensis” was in China, and Homo erectus in southeast Asia. Some authors contend that modern humans are

Discussion and conclusions

Probing the published investigations of the YTT eruption reveals that a number of conclusions have been based on unreliable assumptions and inferences. In particular, model outputs are sensitive to poorly constrained inputs. For example, the model elaborated by Bekki et al. (1996) assumed a stratospheric injection of 6×1012 kg of SO2 but the total aerosol loading computed by Zielinski et al. (1996) is 0.7–3.5×1012 kg, corresponding to 0.46–2.3×1012 kg of SO2, up to an order of magnitude less.

Acknowledgements

I am grateful to Rob Foley, Marta Lahr, David Pyle, Bruno Scaillet and Milford Wolpoff for discussions and comments; to Bill Rose and John Westgate for beneficial reviews of the manuscript; to Giorgio Gasparotto for the SEM images; and Owen Tucker for assistance with the illustrations; and Jim Rose for overall editorial input.

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