Geoscience Reference
In-Depth Information
impacts on the southwest
Archaeologists and climate researchers are increasingly ot en teaming up to
understand the environmental conditions during key periods of past climate
or environmental change. h e environmental conditions can provide con-
text and breadth for their understanding of the lives and fates of the ancient
inhabitants of the West.
One such collaboration has been in the Four Corners region of the
Southwest, where archaeologists have long observed the so-called “Anasazi
collapse.” Tree-ring studies from this region show unusually narrow rings
during the periods AD 1020-1050, AD 1130-1180, and AD 1276-1299.
Each of these series of narrow rings rel ects periods of slow growth, indi-
cating drought, which corresponds to dramatic population declines in
the Southwest as interpreted from archaeological evidence. For example,
researchers studying the Medieval drought in New Mexico have pro-
duced tree-ring records that focus on summer rainfall for the region
spanning the past 1,200 years. h eir results reveal that the droughts from
AD 1130 to 1180 and AD 1276 to 1299 were tied to failures of the summer
monsoon rains.
A similar result was found in a pollen study from southwestern Colorado
that focused on the prevalence of piñon pines, a common species in the area
that depends on summer monsoon rains for seedling germination. h e pol-
len record reveals a sharp reduction in seedling success in the region lasting
over 400 years, from AD 1250 to 1650, indicating more failure of summer
monsoon rains and less seedling establishment.
Larry Benson and his colleagues at the U.S. Geological Survey
have recently examined the possible impact of this failure of the summer
monsoons on the Ancestral Pueblo by comparing many proxy records of
climate from throughout the region—including the tree-ring records in New
Mexico and the pollen study from Colorado—to the archaeological records
of Ancestral Pueblo migrations. Benson found that major human migrations
occurred during prolonged periods of drought, particularly when the sum-
mer monsoons failed. To better understand how the droughts would have
af ected these populations, the team applied a model of maize production
based on Zuni and Hopi agriculture as the closest approximation of the early
Ancestral Pueblo agricultural practices. In this region, the crops were planted
in the spring and depended on summer rains for water. h e models show
that, during multiple years of summer drought, annual harvests of maize
