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 Mexic o record large scale salinity fluctuations caused by alternating wet a nd dry seasons. Thus, prehistoric patterns of rainfall can be reconstr ucted and the season of molluscan death can be estimated. Changes in t he oxygen isotopic patterns preserved in marsh clam shells from late A rchaic 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 e vidence, 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 Arch aic period a general trend occurred toward wet season use of these loc ations. This culminated at the end of the late Archaic period with the exclusive use of the littoral zone during wet season months. These da ta indicate a fundamental shift in the way these estuarine locations w ere being used. We argue that people living in this region altered the ir overall subsistence strategy during the late Archaic period due to scheduling conflicts that occurred with adoption of maize agriculture.