A pre-Columbian obsidian trade from secondary fluvial sources supported by new geochemical data from the Alto Coca Reserve and Sumaco sites (Napo Province, Ecuador)

New obsidian samples (fragments of pebbles and flakes), mixed with ceramic sherds, were found in 10–25 cm cm deep excavations in the Alto Coca Reserve (Napo Province, Ecuador) upon a flat platform on a ridgetop when digging postholes and drainage ditches. Charcoal associated with both obsidian and pottery fragments, attesting to a possible settlement and human pathways (i.e. coluncos), were dated between 1040 and 1210 AD by the AMS radiocarbon (14C) method. Representative samples of obsidian found in the Alto Coca Reserve were analyzed by INAA (major elements) and ICP-OES-MS (trace elements), together with four additional obsidian samples collected from the Sumaco Volcano area, located at the same longitude but ca. 45 km to the south. The Sumaco site (dated by radiocarbon between 1398 and 1451 AD) was preliminary investigated by Santi et al. (in J Archaeol Sci 37:1753–1760. https:// doi: 10. 1016/j. jas. 2010. 01. 034, 2010). Major and trace elements analyses of all the samples closely match with obsidian pebbles collected by Knight et al. (in J Archaeol Sci 38:1069–1079. https:// doi. org/ 10. 1016/j. jas. 2010. 12. 002, 2011) along the Cosanga River and its tributaries, in the eastern piedmont of the Cordillera Real. In particular, they correspond to the secondary fluvial sources of obsidian named “Cosanga A” and “Cosanga B” (as defined by the literature; e.g. Knight et al. in J Archaeol Sci 38:1069–1079. https:// doi. org/ 10. 1016/j. jas. 2010. 12. 002, 2011) most likely deriving from erosional processes of primary sources involving the obsidian-bearing deposits of the Las Caucheras area (5–10 km to the east and north-east of the Cosanga town) where several rhyolite eruptive centers (mostly domes) are related to the youngest activity of the Aliso Volcano. The extensive outcrops of the Andean highlands primary sources of obsidian from the well-known pre-Columbian mines at Sierra de Guamaní, located at the crest of the Eastern Cordillera, are not correlated at all with the investigated samples. The geochemical signatures of the obsidian from the Alto Coca Reserve and Sumaco area therefore support a pre-Columbian exploitation of the secondary fluvial sources of obsidian, along the Cosanga-Quijos Valleys, where obsidian pebbles were collected and brought eastward, towards the Amazonian area.