FINGERPRINTING A FOOTPRINT-BEARING VOLCANIC ASH IN NORTHERN TANZANIA

Hominin footprint sites in Africa are rare but record critical information about early human locomotion and behavior. In northern Tanzania, a recent hominin trackway has been uncovered southwest of Lake Natron. The multiple footprint impressions are preserved in a tuff that was produced by the eruption of one of four neighboring volcanoes: Oldoinyo Lengai, Kerimassi, Gelai, or Ela Nairobi. Since the mineralogy of volcanic ash can be specific to the source volcano, a comparison of the complex mineral assemblages produced by each of the surrounding volcanoes to the footprinted tuff will “fingerprint” the source of the target ash. Thin-section and chemical analysis (i.e., X-ray diffraction and laser-ablation optical emission spectroscopy) of extrusive volcanic samples will be used to compare the composition of these neighboring volcanoes to the trackway-bearing ash. Preliminary observation of hand samples suggests that the footprinted tuff is the product of Oldoinyo Lengai, an active carbonatite volcano. This volcano is located to the south of the trackway site and produces nephelinite and carbonatite ash. According to Ar⁴⁰/Ar³⁹ analysis, the ash is approximately 120ka making it the oldest known modern human trackway and placing Homo sapiens in eastern Africa at a much earlier date then previously thought.