About 73,000 years ago in what is now Sumatra, Indonesia, Mount Toba exploded in the largest eruption since the appearance of Homo sapiens. This enormous eruption (a supereruption of a supervolcano in the new parlance) spewed 2800 km3 of pyroclastic material from a 100 by 30 km caldera. That is a hell of a lot – almost three times more than Yellowstone 600,000 years ago and even more than the giant (super) Yellowstone Huckleberry Ridge eruption of 2 million years ago. Toba was probably the biggest single explosive eruption in tens of millions of years.
Lake Toba, Sumatra. The white dashed line marks the extent of the 73 ka caldera, which is 100 km long and 30 km wide. Landsat image from NASA.
Martin Williams of the University of Adelaide is the lead author of an upcoming paper on the eruption’s environmental impact. Stanley Ambrose of the University of Illinois at Urbana-Champaign, a co-author on the study, suggested in a 1998 paper that the eruption correlates to a catastrophic drop in human population. There is strong genetic evidence that such a bottleneck occurred and reduced the human population to perhaps 1,000-10,000 breeding pairs during the Middle Stone Age; the time constraints are broad but they allow the Toba eruption as a cause. From my reading of the literature, this idea is fairly well accepted, though there is by no means a consensus.
The eruption covered much of the Indian subcontinent with 10-15 cm of ash and released huge amounts of aerosols like hydrogen sulfide and sulfur dioxide into the atmosphere. There’s good evidence from geochemical markers in ice cores that the eruption had the immediate short-term effect of causing a 6-year volcanic winter – a more extreme version of the years following the 1815 Tambora, 1883 Krakatau, and 1991 Pinatubo eruptions. This 6-year period marks the beginning of the coldest 1,800 years of the Pleistocene, although the relationship between the eruption and the longer term cooling is controversial.
The main contribution of the Williams et al. (2009) study is that it directly demonstrates environmental changes from the eruption across the Indian subcontinent. By examining pollen in a marine core from the Bay of Bengal and the isotopic signature of soil carbonate minerals from across central India, they show a sudden and lasting cooling and drying coincident with the ashfall. The pollen counts from the core showed a reduction in trees and wet-adapted ferns. The carbonate isotopic signatures indicate almost pure C3 habitats (like forests) below the ash layers and show a large component of C4 habitats (grasslands and mixed forested grasslands) within and above the ash layers.
The authors present a strong temporal correlation between their findings and data from ice cores in Greenland, deep lake sediment cores in Africa, and fossil soils in China, all of which indicate global cooling, falling lake levels, and the disruption of monsoons. There doesn’t seem to be much doubt that the Toba eruption played a large role in this. The longer-term cooling, however, is still problematic. In the short term, aerosols reflect and scatter solar energy and widespread ash cover increases the albedo of the surface, also reflecting sunlight. In the longer term, you need to invoke increased albedo from snowfall in high latitudes, changing of ocean currents, or other feedback mechanisms that are plausible but not as well constrained.
Williams, M., Ambrose, S., van der Kaars, S., Ruehlemann, C., Chattopadhyaya, U., Pal, J., & Chauhan, P. (2009). Environmental impact of the 73ka Toba super-eruption in South Asia Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2009.10.009
Ambrose, S. (1998). Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans Journal of Human Evolution, 34 (6), 623-651 DOI: 10.1006/jhev.1998.0219
0 comments:
Post a Comment