Environmental changes in central Mesoamerica in the Archaic and Formative periods

This study aims to improve the understanding of climatic fluctuations in the southern Basin of Mexico (BM) during the Archaic (11,500–4,000 cal bp) and Formative (4,000–2,000 cal bp) archaeological periods, when early sedentary settlements at the Zohapilco and Atlapulco sites developed from ∼8,000–7,500 cal bp. This study is based on the analysis of diatoms, pollen, microalgae, charcoal, magnetic susceptibility, geochemistry (organic carbon and titanium) and micromorphology in sediments from a section in a 5 m deep trench at Tulyehualco in the southern BM, close to the Zohapilco and Atlapulco archaeological sites. The chronology is based on five radiocarbon dates and on the correlation with two dated tephra layers. Abundant diatoms and microalgae attest for a period of high lake levels during the deglaciation (17,000–11,000 cal bp), followed by lowering lake levels during the Greenlandian (11,000–8,200 cal bp) when summer insolation peaked, and more oxidative subaerial conditions likely caused poor pollen preservation. Micromorphological analysis showed soil development processes and poor diatom and pollen preservation that confirmed subaerial (dry land) conditions during the 8,200 cal bp cold event. Zohapilco and Atlapulco were first occupied during this period of environmental difficulty. Better diatom and pollen preservation suggest a return to wetter conditions at ~ 7,500–6,500 cal bp, but unusually high charcoal concentrations could reflect human impact in the landscape since the mid Archaic. The diatom record showed that the late Formative (2,000 cal bp, 50 bc) abandonment of sites in the BM was associated with a new reduction in lake levels and a sedimentation hiatus.