Oxygen isotopic analysis of archaeological shells to detect seasonal use of wetlands on the southern Pacific Coast of Mexico

Oxygen isotopic ratios in modern and archaeological marsh clam shells (Polymesoda radiata) from the Acapetahua Estuary in southwestern Mexico record large scale salinity fluctuations caused by alternating wet and dry seasons. Thus, prehistoric patterns of rainfall can be reconstructed and the season of molluscan death can be estimated. Changes in the oxygen isotopic patterns preserved in marsh clam shells from late Archaic period (c. 3000-1800 BC) archaeological deposits indicate that the season of shellfish harvesting changed dynamically through time in this region. Based on this study, and other lines of archaeological evidence, we argue that hunter-gatherers during the early stages of the late Archaic period visited locations in the littoral zone throughout the year with a focus during dry season months. Through the late Archaic period a general trend occurred toward wet season use of these locations. This culminated at the end of the late Archaic period with the exclusive use of the littoral zone during wet season months. These data indicate a fundamental shift in the way these estuarine locations were being used. We argue that people living in this region altered their overall subsistence strategy during the late Archaic period due to scheduling conflicts that occurred with the adoption of maize agriculture.