Geoscience Reference
In-Depth Information
immense irrigation systems and a complex social organization in the Sonoran Desert
A prolonged bimodal drought from about 1130 to 1180 was associated chrono-
logically with a series of human behavioral and organizational changes throughout
the Southwest: Anasazi groups withdrew from the peripheries of their maximum
range and from upland areas as previously scattered groups aggregated into larger
settlements in better watered lowland localities, the Chacoan regional system ended
with the depopulation of its Chaco Canyon core to be succeeded by more local-
ized polities, the Hohokam Sedentary Period pattern gave way to that of the Classic
Period, and many others. The late thirteenth century saw widespread environmental
degradation, including massive arroyo cutting, falling alluvial groundwater levels,
emigration from the Four Corners area began before the environmental crisis of
the late 1200s, and by the close of the thirteenth century much of the Anasazi cul-
tural area on the Colorado Plateau was abandoned. Although highly unfavorable
environmental conditions can certainly be documented for that time, agent-based
modeling of environmental and social interactions among Anasazi households in
that the carrying capacity of the environment was not entirely depleted by the end of
the thirteenth century. This outcome suggests that the Anasazi abandonment of the
Four Corners area must have involved social or cultural considerations in addition
to the environmental crisis of the time.
One of the most challenging problems in American archaeology concerns the
decline of Classic Period city-states in Mesoamerica during the late first millennium
AD, including the abandonment of Teotihuacan in central Mexico (ca. AD 750) and
the large urban centers in the Mayan lowlands (ca. AD 770-950). The Terminal
Classic Period (AD 750-950) has been recognized in the archaeological record by a
decline in the production of fine manufactured goods, the end of large construction
projects, the collapse of large-scale trade networks, the abandonment of large urban
centers, and the general depopulation of the region. The cause of Classic Period
decline is unclear, but drought, human degradation of the environment, disease,
warfare, and collapse of the social order needed to sustain the complex exchange
The North American tree-ring network for the first millennium is extremely
sparse and limited largely to the American West. No tree-ring chronologies more
than 1000 years long have yet been developed for Mesoamerica near the cen-
ter of the cultural changes during the Terminal Classic Period. Many of the
longest Western chronologies have been developed for high-elevation conifers
such as bristlecone pine and limber pine, some of which exhibit ambiguous
precipitation-sensitive tree-ring chronology at El Malpais, New Mexico, arguably
one of the most important tree-ring chronologies ever produced. The El Malpais
chronology is based on long-lived Douglas-fir and ponderosa pine trees and
