The Search for Toba in Africa


The Indian Ocean from Pinnacle Point 5-6, South Africa

Approximately 74 thousand years ago (ka), the Toba caldera erupted in Sumatra. Since the magnitude of this eruption was first established, its effects on climate, environment and humans have been debated. We discovered microscopic glass shards characteristic of the Youngest Toba Tuff—ashfall from the Toba eruption—in two archaeological sites on the south coast of South Africa, a region in which there is evidence for early human behavioral complexity. An independently derived dating model supports a date of approximately 74 ka for the sediments containing the Youngest Toba Tuff glass shards. By defining the input of shards at both sites, which are located nine kilometers apart, we are able to establish a close temporal correlation between them. Our high-resolution excavation and sampling technique enable exact comparisons between the input of Youngest Toba Tuff glass shards and the evidence for human occupation. Humans in this region thrived through the Toba event and the ensuing full glacial conditions, perhaps as a combined result of the uniquely rich resource base of the region and fully evolved modern human adaptation.

from Smith et al. 2018 in Nature


Cryptotephra in archaeological sites in Europe


The Arma Veirana site, northwestern Italy. Photo by Jayde Hirniak.

Chemical characterization of cryptotephra is critical for temporally linking archaeological sites. We describe cryptotephra investigations of two Middle-Upper Paleolithic sites from north‐west Italy, Arma Veirana and Riparo Bombrini. Cryptotephra are present as small (<100 µm) rhyolitic glass shards at both sites, with geochemical signatures rare for volcanoes in the Mediterranean region. Two chemically distinct shard populations are present at Arma Veirana (P1 and P2). P1 is high silica rhyolite (>75 wt.%) with low FeO (<1 wt.%) and a K2O/ Na2O > 1 and P2 is also high silica rhyolite (>75 wt.%) but with higher FeO (2.33–2.65 wt.%). Shards at Riparo Bombrini (P3) are of the same composition as P1 shards at Arma Veirana, providing a distinct link between deposits at both sites. Geochemical characteristics suggest three possible sources for P1 and P3: eruptions from Lipari Island (56–37.7 ka) in Italy, the Acigöl volcanic field (200–20 ka) in Turkey and the Miocene Kirka‐Phrigian caldera (18 Ma) in Turkey. Eruptions from Lipari Island are the most likely source for P1,3 cryptotephra. This study highlights how cryptotephra can benefit archaeology, by providing a direct link between Arma Veirana and Riparo Bombrini as well as other deposits throughout the Mediterranean.

from Hirniak et al. 2020 in the Journal of Quaternary Science

Cryptotephra in the Las Vegas Formation


Collecting the Las Vegas Formation at Tule Springs Fossil Beds National Monument.

Cryptotephra are small glass shards (<80 microns) of volcanic origin that occur invisibly in sediments and can be used to create precise isochrons in geological deposits and archaeological sites.  We discovered two peaks of cryptotephra from a 100-cm interval of Member D2 of the Las Vegas Formation in Tule Springs Fossil Beds National Monument just north of Las Vegas, Nevada. Shards are sparse (<10 shards/gram) and small (60-100 microns) and display blocky and cuspate shapes. Samples were processed in the Cryptotephra Laboratory for Archaeological and Geological Research at UNLV.

Major element chemistry by electron microprobe indicates that the shards are high-silica rhyolite (>75 wt. % SiO2) with FeO < 1 wt. %, providing a unique major element signature that correlates to Wilson Creek tephra that erupted from the Mono Craters in eastern California.  The Wilson Creek section contains 19 tephra layers that are indistinguishable using major elements but have distinctive trace element signatures. The upper peak of D2 shards provided high quality trace element data by LA-ICP-MS at Michigan State University and display light rare-earth element (REE) enrichment, a deep Eu anomaly, and a relatively flat heavy REE signature when normalized to chondrite.  This signature (especially high La, Ce and La/Sm) indicates that the shards correlate with Wilson Creek tephra unit15. The discovery of this Wilson Creek 15 in Tule Springs Fossil Beds National Monument greatly extends the fallout range of this tephra unit and suggests that it might be found at other sites in southern Nevada. The application of cryptotephra analysis to the Las Vegas Formation will allow for the precise dating of fossiliferous horizons in this unit and among other Late Pleistocene localities.

from Smith et al. 2019 GSA Abstract Phoenix National Meeting