Mesolithic human occupation and seasonality: sclerochronology, δ 18 O isotope geochemistry, and diagenesis verification by Raman and LA-ICP-MS analysis of Argyrosomus regius (meagre) sagittae otoliths from layer 1 of Cabeço da Amoreira Mesolithic shell midden (Muge, Portugal)

Abstract

We present preliminary sclerochronological analysis on 15 Argyrosomus regius (meagre) otoliths collected from two different human occupation levels from the Cabeço da Amoreira shell midden (Muge valley, Portugal). The otoliths were sectioned and observed under a reflected light stereomicroscope to examine seasonal growth rings. Carbonates collected from individual growth rings subsampled with a micromill sampling device were analyzed for their stable oxygen isotope content with the objective of determining the predominant season of capture and therefore season of site use, environmental conditions, and sites of resource procurement (local versus regional). The otoliths’ stable isotope records show clear seasonality and a season of capture estimate that seems consistent with a “good season” (warmer season, i.e., from spring to late summer/beginning of autumn in this area) site use in the last occupation layer (the great majority of otoliths are from layer 1), except for four samples giving heaver oxygen isotope (δ18O) values that suggest colder conditions and only one with markedly positive δ18O values. Potential effect of diagenesis on the otolith records was also assessed through coupled laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) and by micro-X-Ray diffraction (μ-XRD). Results revealed the presence of only aragonite and no traces of calcite, providing no evidence of diagenesis that could significantly alter isotope results and lead to erroneous interpretations. The implications of these results are discussed and compared with data from other archeological sites, as well as data from micromorphology regarding continuity or interruption of site use and other faunal remains.