Subdecadal phytolith and charcoal records from Lake Malawi, East Africa imply minimal effects on human evolution from the ~74 ka Toba supereruption
Chad L. Yost, Lily J. Jackson, Jeffery R. Stone, Andrew S. Cohen, 2018. Journal of Human Evolution, v. 116, p. 75-94.
Our study uses phytoliths and microcharcoal from lake sediments to determine the effects of the Indonesian Mount Toba supereruption at ~75 ka on the climate and vegetation of East Africa. We worked with sediment cores from the north basin and central basin, which are separated by ~100 km. Phytolith and charcoal samples were continuously collected at ~3–4 mm (~8–9 yr) intervals above and below the Toba cryptotephra position in each core, with no stratigraphic breaks.
A total of 100 samples (50 from each core) were analyzed, representing ~100 years before and ~200 years after the eruption. Phytolith extractions utilized wet oxidation, clay removal, and density separation steps. Microspheres were added for concentration calculations. It should be noted that each dried sample weighed about 0.27 grams, but yielded more than enough extract for sufficient phytolith counts. In retrospect, 2 mm intervals could have been analyzed.
A total of 34 phytolith morphotypes were identified. Phytoliths darkened from burning were carefully identified from those darkened naturally due to dissolution or incomplete silicification. Fungal spores, chrysophyte cysts, cryptotephra, and microcharcoal particles were also counted. In addition to reporting relative abundances for all morphotypes, grass functional type, Iph, D/P°, and Ic indices were also reported. Although the clay separation steps removed some of the diatom load, the most challenging part of the phytolith analysis was the high concentration of diatoms that made some samples very time consuming to count.
For samples synchronous or proximal to the Toba interval, we found no change in low elevation tree cover, or in C3, C4 xerophytic, or C4 mesophytic grass abundances that were outside of normal variability. For core 2A from the north basin, 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 higher elevation afromontane vegetation, but does not signal volcanic winter conditions hypothesized by some. These results have implications concerning a putative human genetic bottleneck attributed to the Toba supereruption.