摘要
A soil chronosequence study is presented in a beach ridge system at the mouth of the Usumacinta and Grijalva Rivers, Mexico, formed since a strong decrease in the rate of sea level rise some 6000 years ago. Four sites were selected for the chronosequence study: Simón Sarlat (SS), Rancho Magdaleno 1 and 2 (RM1, RM2), Cocoteros (CC), and Playa Cocoteros (PC), located on beach ridges that range in age between 5000 years old to recently formed. The soils show poor to moderate degree of development, and the change in the pedogenic properties has been clearly evaluated. The most pronounced changes are the following, from the older (SS, RM2) to the younger soils (PC, CC): color (becoming yellowish brown in the better developed soils); an increase in the amount of organic matter, clay, and Fed (iron extracted with dithionite solution); a better structure with more stable aggregates; a decrease in the Feo proportion (Fe extracted with oxalate solutions); and an increase in the content of fine magnetic particles (formed by pedogenesis). This area was densely populated and thus affected by human activities since the pre-Classic period (800–300 BC), as many archaeological sites have been registered. We evaluated soil agronomic quality, in order to understand the ancient land use. The results show that the soils in this area, no matter their age, have low agronomic quality, which must have limited their use by ancient population. However, the strategic position near main trade routes, near rich aquatic resources, and a low susceptibility to flooding were favorable for human settlement on the beach ridges.