YOUNGEST TOBA TUFF (~74 KA) CRYPTOTEPHRA IDENTIFIED IN AN ARCHAEOLOGICAL SITE AT PINNACLE POINT, SOUTH AFRICA

Cryptotephra was identified in a sedimentary sequence at Pinnacle Point Site PP5-6 (PP5-6), South Africa, and occur as small glass shards less than 100 µm in size. This site contains a continuous stratigraphic record of early modern human occupation extending from ~90-50 ka based on single grain OSL dating. Shards are intimately mixed with sediment and are sparse (3 shards/gram maximum). The first shards occur in nearly the same stratigraphic position in three separate sampling profiles and define an isochron with two measured OSL ages of 72.6 ± 5.6 ka and 72.8 ± 5.8 ka with a Bayesian modeled age interval of 75.4–70.9 ka. The cryptotephra in PP5-6 display a vertical shard profile with a few peaks and a tail above with fluctuating, but diminishing-upwards, shard content. Major element analyses performed on several shards using an electron microprobe indicate that they are rhyolite with SiO₂ varying from 72.15 to 77.56 wt. %. To identify the source of the shards, we searched the VOGRIPA, Smithsonian and PetDB databases for moderate to large rhyolitic eruptions occurring between 86-66 ka and made comparisons to all available compositional data. Volcanic ash can travel significant distances and even moderate-sized eruptions can produce far-traveling tephra, so we searched records of known volcanism beyond Africa to Antarctica, South America, and Indonesia. To account for possible reworking of older cryptotephra into younger deposits, we widened our time window to include eruptions ~700 ka to the present, including tephra found within the Siple A ice core from west Antarctica, the Rungwe volcanic province in Tanzania, and the Turkana basin in Kenya and Ethiopia. None of these eruptions produced tephra that matched the chemistry of shards at PP5-6 and/or were too young to contribute tephra to PP5-6. Major element chemistry of the cryptotephra is most similar to distal tephra of the ~74 ka eruption of Toba volcano in Indonesia (the youngest Toba tuff or YTT) providing, for the first time, evidence that YTT tephra may have reached southern Africa. The discovery of YTT cryptotephra at PP5-6 provides a powerful new method to create a precise chronological tie-point between archaeological sites and to help test hypotheses concerning the impact of the Toba eruption on ancient human populations.