Late Pleistocene and Holocene environmental history of the Iguala Valley, Central Balsas Watershed of Mexico

Abstract

The origin of agriculture was a signal development in human affairs and as such has occupied the attention of scholars from the natural and social sciences for well over a century. Historical studies of climate and vegetation are closely associated with crop plant evolution because they can reveal the ecological contexts of plant domestication together with the antiquity and effects of agricultural practices on the environment. In this article, we present paleoecological evidence from three lakes and a swamp located in the Central Balsas watershed of tropical southwestern Mexico that date from 14,000 B.P. to the modern era. [Dates expressed in B.P. years are radiocarbon ages. Calibrated (calendar) ages, expressed as cal B.P., are provided for dates in the text.] Previous molecular studies suggest that maize (Zea mays L.) and other important crops such as squashes (Cucurbita spp.) were domesticated in the region. Our combined pollen, phytolith, charcoal, and sedimentary studies indicate that during the late glacial period (14,000-10,000 B.P.), lake beds were dry, the climate was cooler and drier, and open vegetational communities were more widespread than after the Pleistocene ended. Zea was a continuous part of the vegetation since at least the terminal Pleistocene. During the Holocene, lakes became important foci of human activity, and cultural interference with a species-diverse tropical forest is indicated. Maize and squash were grown at lake edges starting between 10,000 and 5,000 B.P., most likely sometime during the first half of that period. Significant episodes of climatic drying evidenced between 1,800 B.P. and 900 B.P. appear to be coeval with those documented in the Classic Maya region and elsewhere, showing widespread instability in the late Holocene climate.

Fig. 1.

Map of Mexico showing the location of the Iguala region and sites mentioned in the text.

Fig. 2.

Pollen diagram showing the percentage of taxa identified in Core 2 at Ixtacyola. In this and other diagrams, (Po) and (S) before radiocarbon determinations indicate that the material dated was pollen and sediment, respectively. A few grains of Quercus were found between 5.3 and 1.5 m. Fern percentages generally range from 2% to 11%, including in basal levels, with a maximum of 35% at 5.3 m. The Pozo Sur edge hole also contains a good pollen record above 4.5 m. Arboreal taxa not recorded in the center hole and present in the Pozo Sur edge sediments are Moraceae, Sapotaceae, Anacardiaceae, Prosopis, Ilex, and Liquidambar.

Fig. 3.

Phytolith diagram from Ixtapa showing the percentage of taxa present. In this and other diagrams, (Ph) before radiocarbon determinations indicates that the material dated was phytoliths.

Fig. 4.

Pollen diagram from Tuxpan showing the percentage of taxa present. The “Other Arboreal” class includes Alchornea and Bunchosia. The pollen sum does not include aquatic taxa, which are almost entirely from Typha. Fern percentages generally range from 1% to 8%. Sediments between 5.7 and 4.4 m from Core 5 were not analyzed because, due to Core 5 having a more extended sediment record, they represent deposits of the same age as at the base of Core 2. Hence, a continuous sequence is displayed.