Limited effect of the Quaternary's largest super-eruption (Toba) on land mammals from Southeast Asia

Abstract

The effect of the Toba super-eruption at ∼74 kyr on the mammals of Southeast Asia is examined. Although few Late Pleistocene sites from Southeast Asia have been described, an analysis of those which pre- and post-date Toba reveals relatively few species became extinct following the eruption. It is suggested that species survived in refugia immediately following the eruption, and that they repopulated vast areas following a probable short period (i.e. decades to century) of environmental devastation. This study suggests that mammals are more robust at coping with catastrophic events than previously acknowledged, and questions the perceived human monopoly in overcoming ecological adversity.

Introduction

Lake Toba, Sumatra, a 100×30 km caldera complex, is the site of the Quaternary's largest explosive eruption (Fig. 1). This super-eruption expelled an estimated 7×10¹⁵ kg of rhyolitic magma (Rose and Chesner, 1987, Rose and Chesner, 1990; Chesner and Rose, 1991) and injected ∼10¹⁵ g of fine ash into the stratosphere (Zielinski et al., 1996; Bühring et al., 2000) over an estimated period of 9–14 days (Ledbetter and Sparks, 1979). Rhyolitic ash deposits have been found as far afield as the seabed between Borneo and Indochina (Bühring et al., 2000), and suspected Toba tephra have been found on the Indian sub-continent, more than 3000 km away from Sumatra (Oppenheimer, 2002). Dating of the various deposits associated with Toba has consistently returned ages of approximately 74±2 kyr (Oppenheimer, 2002).

Climatic impacts of the Toba eruption are related to the emission of stratospheric sulphur (Ambrose, 2003). Estimates of these emissions range from 2.3 to 4.7×10¹² kg (Zielinski et al., 1996) to 3.5×10¹⁰ kg (Scaillet et al., 1998), a difference of two orders of magnitude. Atmospheric modeling using the upper range of sulphuric emission suggests a 5–6 year period of high stratigraphic sulphate aerosol abundance (Bekki et al., 1996). Predictions of temperature changes resulting from the Toba eruption range from no more than those observed for previous ice age oscillations (Oppenheimer, 2002; Gathorne-Hardy and Harcourt-Smith, 2003) to those associated with a nuclear winter (Rampino and Ambrose, 2000; Rampino and Self, 1992, Rampino and Self, 1993). In the latter scenario, regional temperatures are expected to drop ∼10–15 °C below normal, while global decreases of 3–5 °C could be expected (Rampino and Ambrose, 2000). Simulations of super-eruptions suggest that climatic conditions resulting from volcanic aerosol are strongly dependent on the season of eruption (Timmreck and Graf, 2006).

Climatic changes associated with Toba could have resulted in a volcanic winter of such magnitude as to devastate vegetation and cause a human population bottleneck (Rampino and Self, 1992, Rampino and Self, 1993; Ambrose, 1998; Rampino and Ambrose, 2000). In modeling the effects of the eruption on vegetation, Rampino and Self, 1992, Rampino and Self, 1993 utilized studies of the effects of a nuclear winter on the environment (e.g. Harwell and Hutchinson, 1985; Harwell, 1984). Such studies suggest near complete decimation of above-ground tropical plants, and little better survival of deciduous and temperate forests (Harwell et al., 1985). These studies, however, have little to say on the survivability of mammals caught in nuclear winter (e.g. Hutchinson et al., 1985; Harwell et al., 1985) beyond lamenting their probable demise. Studies of the effects of volcanic super-eruptions suggest that they alone would be insufficient in producing mass extinction events (Erwin and Vogel, 1992; Prothero, 2004). However, even proponents of the volcanic winter hypothesis have not claimed mass extinctions would follow on from Toba (Ambrose, 2003). Given the magnitude and devastation associated with Toba, however, one would expect that regional extinctions would follow, especially if the ecological devastation posited for the volcanic winter were accurate. Species especially vulnerable to temperature changes and those dependent on tropical vegetation should, in particular, be adversely affected by the Toba super-eruption.