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650-Year Drought TriggeredAncient City's Abandonment
by Laura Geggel, Staff Writer | January 30, 2015 01:54pm ET
Credit: Tripti Bhattacharya
View full s ize image
A lake near Cantona where
researchers collected sediment
samples to learn about the area's his tory of
drought between A.D. 500 and A.D. 1150.
A once-thriving Mesoamerican
metropolis dried up about 1,000
years ago when below-average
rainfall triggered centuries-long
droughts that largely prompted
people to abandon the city for
greener opportunities, a new study
finds.
Scientists havelong debated whether
it was droughtor cultural forces that
led to the abandonment of Cantona,
a once-fortified city located just east
of modern-day Mexico City. Few
details were known about its past
climate, which prompted researchers to take a closer look at the
weather
conditions that affected the pre-Columbian city in
Mesoamerica.
In its heyday, about 90,000 people lived in Cantona, which is
located in a dry
volcanic basin. The area provided vast amounts of valuable
obsidian, volcanic
glass used for trade and making sharp tools for hunting and
farming. But
people deserted the city between A.D. 900 and A.D. 1050,
research shows.
[History's 10 Most Overlooked Mysteries]
To investigate why, geographers assessed the climate before and
after
Cantona's collapse. They took sediment cores and samples from
Aljojuca, a
lake about 20 miles (32 kilometers) from the city.
As a closed lake basin, Aljojuca enabled the scientists to track
the past
climate in the region. The researchers examined the relationship
between
different oxygen isotopes, or variants of the element, in the
water, and
determined how much precipitation and evaporation was taking
place at the
lake. The ratio of the isotopes was high, indicating the area
had drier
summers, the scientists said. Analyses of other compounds in the
sediment
samples yielded similar results.
Overall, Cantona still had wet summers and dry winters, but its
regularmonsoon
seasonwas disturbed by frequent long-term droughts, which likely
harmed the
area's crops and water supply, the researchers said. Moreover,
the droughts
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lasted hundreds of years.
A 650-year period of frequent droughts plagued the area from
about A.D. 500
to about A.D. 1150, they found. This dry period wasn't isolated,
but part of a
period of droughts in modern-day Mexico's highlands that lasted
from about
200 B.C. until A.D. 1300, just before theAztec empiretook
power.
"The decline of Cantonaoccurred during this dry interval, and we
conclude that
climate change probably played a role, at least towards the end
of the city's
existence," lead researcher Tripti Bhattacharya, a graduate
student of
geography at the University of California, Berkeley,said in a
statement.
Yet, Cantona's population actually grew during the early part of
the dry period.
It's possible that political turmoil and droughts elsewhere
drove more people to
the fortified city, Bhattacharya said.
"In a sense the area became important because of the increased
frequency of
drought," Roger Byrne, an associate professor of geography at
the University
of California, Berkeley, said in a statement. "But when the
droughts continued
on such a scale, the subsistence base for the whole area changed
and people
just had to leave. The city was abandoned."
The study was published online Jan. 26 in thejournal the
Proceedings of the
National Academy of Sciences.
http://www.pnas.org/content/early/2015/01/21/1405653112.abstracthttp://newscenter.berkeley.edu/2015/01/27/long-dry-spell-doomed-mexican-city-1000-years-ago/http://www.livescience.com/26765-aztecs-changed-dna-people.html
