Arrkamrnerry zyxwvutsr 37. zyxwvutsrqpon I (1995), 157-170. Printed in Great Britain zyxwvu MIDDLE HOLOCENE PERIODICITIES IN RAINFALL INFERRED FROM OXYGEN AND CARBON ISOTOPIC FLUCTUATIONS IN PREHISTORIC TROPICAL ESTUARINE MOLLUSC SHELLS* D. KENNETT Department of Anthropology, University zyxwvu of California Santa Barbara, Santa Barbara, CA 93106, U.S.A. and B. VOORHIES Department of Anthropology. University of Colorado at Boulder, Boulder, CO 80309. U.S.A. zyx Stable oxygen and carbon isotopic ratios in modern and archaeological estuarine mollusc shells, Polymesoda radiata, change in accordance with seasonal salinityfluctuations in the Acapetahua estuary located on the Pacijic coast of southern Mexico. This region receives zy N 3000mm ofprecipitation annually, most during a wet season between April and October. The changingflux of fresh water and organic detritus into the estuary causes large changes in the oxygen and carbon isotopic composition of the estuarine waters and in the carbonate precipitated by P. radiata. Oxygen isotopic ratios in the shells of molluscs collected by late Archaic period populations (5000-4000 BP) in this region indicate that patterns of rainfall were similar to today. Modern shells, however, exhibit much more negative carbon isotopic values than observed in prehistoric shells. This change may be associated with the input of modern fertilizers into the estuary. KEYWORDS: MESOAMERICA, ARCHAIC PERIOD, OXYGEN ISOTOPIC ANALYSIS. CARBON ISOTOPIC ANALYSIS, MOLLUSC SHELLS, POLYMESODA RADIATA INTRODUCTION This paper reports palaeoclimatic information based on stable oxygen and carbon isotopic analysis of estuarine mollusc shells found in late Archaic period (5000-4000 BP) archae- ological deposits located on the Pacific coast of south-western Mexico (Fig. 1). These shell deposits represent some of the earliest evidence for coastal occupation in Mesoamerica, accumulating during a time when human populations were making the economic transition from hunting and gathering to agriculture. The primary goal of this study was to determine the environmental context of this important economic transformation. The oldest-known sites in this area consist of six prominent shell mounds situated within the coastal wetlands. One of these, Cerro de las Conchas, has been dated to c. 6000 BP. This mound is presently located at the inland margin of a large (30-60 sq km; Helbig 1976,219), freshwater swamp (El Hueyate or Cantileiia); however, associated mollusca are either estuarine or marine, indicating large palaeoenvironmental changes since its formation. This site is being investigated by others (Clark 1994) and is not discussed here. Our study focuses on material from the site of Tlacuachero, one of five shell mound sites that constitute sizeable islands in the Acapetahua estuary immediately to the north-west *Received zyxwvutsrqp 10 January 1994, accepted 1 June 1994 157