Tepexpan revisited: A multiple proxy of local environmental changes in relation to human occupation from a paleolake shore section in Central Mexico

Abstract

Building up a scenario of Late Pleistocene–Holocene environmental change and human–landscape interactions in Central Mexico – one of the key areas for the natural and cultural history of America – requires development of local paleoenvironmental reconstructions. We studied the Tepexpan section at the edge of Texcoco a paleolake, near the mouth of Teotihuacan Valley as a multiproxy record of the environmental dynamics at the shore in the Basin of Mexico throughout the period of human occupation. The section is located in an ecotone affected by intensive and variable geomorphic processes and includes lacustrine, fluvial and volcanic sediments as well as paleosols. Our chronological scale is based on 6 AMS 14C dates from pollen concentrates and paleosol organic matter. The lower segment of the section dominated by the lacustrine sediments yielded pollen spectra; in the upper segment the record is based on the pedogenetic characteristics of paleosols. Different proxies agree in demonstrating the general trend (although some reversals are apparent) of decreasing effective moisture since MIS3: it is reflected first in the increase of herbaceous pollen after 27 ka BP, the decrease of lake level, the cessation of lacustrine sedimentation and beginning of marsh soil development at 10 ka BP and finally, the shift from hydromorphic to dryland semiarid pedogenesis in the Late Holocene. We assume that this trend was climatically controlled, whereas the deposition of sedimentary layers enriched with tephra are related to the pulses of volcanic activity; the pedosedimentary features associated with the upper soil are human-induced. Comparing the proposed scheme of environmental change with the archaeological record we propose that the initial settlers, Late Paleolithic hunters, could have utilized the wet swampy meadows which expanded on the Basin bottom as the lake level lowered and provided the niche for large herbivores during the Pleistocene–Holocene transition. Development of agriculture and the network of permanent settlements are believed to be linked to the formation of well drained soils. In the methodological perspective the study shows the potential of ecotones as highly dynamic and sensitive systems that may archive valuable paleoecological records.