The timing of postglacial coastal adaptations in Eastern Iberia: A Bayesian chronological model for the El Collado shell midden (Oliva, Valencia, Spain)

Abstract

El Collado is an open air site containing evidence of 14 burials and a shell midden archaeological deposit with different phases of human occupation dated to the Early Holocene. Previous studies have produced 14 radiocarbon dates using bone collagen samples from human burials. However, no attempt has been made to date the stratigraphic sequence to address the formation time of the shell midden and establish a chrono-stratigraphic framework for the study of bio-archaeological and cultural assemblages. We critically evaluate the available radiocarbon record of the site and present 6 new AMS radiocarbon dates on ungulate bones with anthropic fractures and bivalves from the three major stratigraphic horizons. Then, we integrate our results with previously published AMS radiocarbon determinations on human samples into a chronological Bayesian model, constraining the radiocarbon distributions with prior information about the samples stratigraphic provenance. Finally, we correlate the results of the chronological model with the regional archaeological sequence and the available data on coastal evolution at regional and local scales. The vertical distribution of the radiocarbon dates makes evident some stratigraphic disturbance caused by the repeated presence of pit graves, agricultural activities and low stratigraphic control of the excavation process. However, once the problematic determinations are identified as outliers, the remaining radiocarbon dates grouped according the major stratigraphic divisions, and the calibrated distributions modelled-constrained, the resulting Bayesian phase model reveal high agreement index (Ac = 103). The shell midden formation took place from the bottom of the sequence, spanning 1022–1965 calibrated years (CI 95.4%). The occupations documented at Phase 1 (Level IV) are dated to the Early Holocene (9828–9551 cal BP). The Phase 2 (Level II), encompass radiocarbon evidence of both burial and occupational activities dated to 9437 to 8477 cal BP, spanning 779–985 calibrated years (CI 95.4%). Finally, the Phase 3 (Level I) records Late Mesolithic occupations deposited between 8509 and 8391 cal BP for the start boundary and 8499–8060 cal BP for the end boundary (CI:95.4%). There are no individual radiocarbon dates during or postdating the chronological span of the 8.2 ka climatic event (8300–8140 cal BP).