Increasing data (INAA) on Ecuadorian obsidian artifacts: preliminary provenance and a clue for pre-Columbian eastward trade.

In this work we carried out INAA major (Na, K, Ca and Fe %) and trace (ppm) elements (plus Mn by FAAS analysis) of 15 obsidian samples (waste flakes) coming from an unknown archaeological site (14C-AMS age of 1425 AD) located on the south-eastern flank of the back-arc Sumaco volcano, in the Sub-Andean Ecuador (to the east of the Cordillera Real) and from two already known pre-Columbian archaeological localities: La Florida (Quito) and Milan (Cayambe). Literature compositional data of the Ecuadorian obsidian outcrops (Bigazzi et al., 1992, Asaro et al., 1994, Bellot-Gurlet et al., 1999, 2008) provide some constraints on the provenance of the analyzed waste flakes, even though different methods of analyses make comparisons a difficult task. Concerning the obsidian artifacts of La Florida and Milan, they come from the well known Sierra de Guamanì obsidian sources (Cordillera Real). By contrast, the obsidian fragments of the Sumaco settlement show Mn-rich, U- Th-poor compositions and relatively high Nb/Zr ratios, these characters being compatible with obsidian erratic pebbles recently discovered by Bellot-Gurlet et al. (2008) in some river banks of the Amazonian foothills draining the easternmost flanks of the Antisana volcano in the Cordillera Real as well. In this way, the obsidian artifacts found at the Sumaco site reinforce the opinion of Bellot-Gurlet et al. (2008) that Ecuadorian source inventory is not yet exhaustive. These preliminary inferences of provenance for the Sumaco obsidian findings would however need to be furtherly tested with the same analytical methods on both artifacts and sources. The Antisana volcano, located 30 km to WNW of the Amazonian foothills, seems to be the best candidate to find out additional primary outcrops of obsidian sources. Obsidian ancient trade was well established in Ecuador among the Andean people of the Cordillera Real and between them and people of the Pacific coast, whereas an eastward trade toward the rainforest people was never documented. In this framework, the archaeometric study of the obsidian samples of the 1425 AD site of Sumaco, located well eastward of the Cordillera Real, is of paramount importance in tracing the ancient routes of the obsidian trade toward the Amazonian region. It can not be also excluded that sub-Andean and Amazonian people direcly took advantage from obsidian secondary sources (i.e. within the river banks of the Amazonian foothills) rather than procurements from primary outcrops in the Cordillera Real.