Mt. Toba being slowly revisited.

All,

Anyone who has taken more than just a passing interest in Stanley Ambrose’s Mt. Toba hypothesis (see below, second abstract) should have a look at Oppenheimer’s recent statement regarding the actual impact this massive volcanic eruption could have had on the global climate and environment, and by extension, how it might have influenced human evolution, around 74,000 years ago.

Jacques Cinq-Mars

Oppenheimer, Clive. 2002. Limited global change due to the largest known Quaternary eruption, Toba 74kyr BP? Quaternary Science Reviews 21(14-15): 1593-1609.

Abstract:

The 74kyr 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?1015kg (2800km3 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 6yr 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 74kyr 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?1010kg). 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.

Ambrose, Stanley H. 1998. Late Pleistocene human population bottlenecks, volcanic winter, and differenciation of modern humans. Journal of Human Evolution 35:115-118.

Abstract:

The "Weak Garden of Eden" model for the origin and dispersal of modern humans (Harpending et al., 1993) posits that modern humans spread into separate regions from a restricted source, around 100 ka (thousand years ago), then passed through population bottlenecks. Around 50 ka, dramatic growth occurred within dispersed populations that were genetically isolated from each other. Population growth began earliest in Africa and later in Eurasia and is hypothesized to have been caused by the invention and spread of a more efficient Later Stone Age/Upper Paleolithic technology, which developed in equatorial Africa. Climatic and geological evidence suggest an alternative hypothesis for Late Pleistocene population bottlenecks and releases. The last glacial period was preceded by one thousand years of the coldest temperatures of the Later Pleistocene (c. 71-70 ka), apparently caused by the eruption of Toba, Sumatra. Toba was the largest known explosive eruption of the Quaternary. Toba's volcanic winter could have decimated most modern human populations, especially outside of isolated tropical refugia. Release from the bottleneck could have occurred either at the end of this hypercold phase, or 10,000 years later, at the transition from cold oxygen isotope stage 4 to warmer stage 3. The largest populations surviving through the bottleneck should have been found in the largest tropical refugia, and thus in equatorial Africa. High genetic diversity in modern Africans may thus reflect a less severe bottleneck rather than earlier population growth. Volcanic winter may have reduced populations to levels low enough for founder effects, genetic drift and local adaptations to produce rapid population differentiation. If Toba caused the bottlenecks, then modern human races may have differentiated abruptly, only 70 thousand years ago.