T309 Understanding African Environmental History through Continental Scientific Drilling: Past Successes and Future Opportunities

Wednesday, 25 October 2017
8:00 AM - 12:00 PM
Room 307/308

SUB-DECADAL PHYTOLITH AND CHARCOAL RECORDS FROM LAKE MALAWI, EAST AFRICA, INDICATE THAT THE TOBA SUPERERUPTION AT 75 KA WAS NOT SEVERE ENOUGH TO CAUSE A HUMAN GENETIC BOTTLENECK

Paper number: T309-3
Program: Technical Sessions
Time: 8:35 AM – 8:55 AM

Authors

Abstract

The temporal proximity of the ~75 ka Toba supereruption to a putative 100­–50 ka human population bottleneck is the basis for the Volcanic Winter/Weak Garden of Eden hypothesis, which states that the eruption caused a 6-year-long global volcanic winter and reduced human populations to fewer than 10,000. To test this hypothesis, we sampled two cores collected from Lake Malawi with cryptotephra previously fingerprinted to the Toba supereruption. Phytolith and charcoal samples were continuously collected at ~3–4 mm (~8–9 yr) intervals above and below the Toba cryptotephra position, with no stratigraphic breaks. For samples synchronous or proximal to the Toba interval, we found no change in low elevation tree cover, or in cool climate C3 and warm season C4 xerophytic and mesophytic grass abundance that is outside of normal variability. A spike in locally derived charcoal and xerophytic C4 grasses immediately after the Toba eruption indicates reduced precipitation and die-off of at least some afromontane vegetation, but does not signal volcanic winter conditions. A review of Toba tuff melt inclusion studies suggest a Tambora-like 50 to 100 Mt SO2 atmospheric injection; however, most Toba climate models use SO2 values that are 1 to 2 orders of magnitude higher, thereby significantly overestimating the amount of cooling. A review of recent human genomic studies finds no support for a genetic bottleneck at or near ~75 ka, as consistently low populations had been declining since ~150 ka. The putative 100­–50 ka human genetic bottleneck is now converging on ~50 ka and is being attributed to an out-of-Africa founder effect and not a population reduction bottleneck. We find no support for the Toba catastrophe hypothesis and conclude that the Toba supereruption did not 1) produce a 6-year-long volcanic winter in eastern Africa, 2) cause a genetic bottleneck among African human populations, or 3) bring humanity to the brink of extinction.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-298043

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