Ancient rodent middens record past tropical rain in "absolute" desert

Jan. 6, 2000


(Contact: Julio Betancourt, 520-670-6821, ext. 109, or 520- 219-6531,
jlbetanc@usgs.gov. Jay Quade , 520-794-0454, jquade@geo.arizona.edu.)


Where does one go to find information about tropical rainfall patterns since
the last major glaciation? Try the driest desert of South America.

That's the approach taken by University of Arizona geoscientists Julio
Betancourt and Jay Quade. With assistance from several UA students, they are combing the
Atacama Desert in northern Chile for clues on regional climate changes over the
past 40,000 years.
Their findings may force the scientific community to rethink the accepted views
on the timing of wet and dry periods in the region -- and the importance of the
tropics during times of climate change. Their research is funded by the
InterAmerican Institute, the National Geographic Society and the National Science
Foundation.

Why look in the desert for signs of past rainfall?

"The Atacama is an obvious place to do it because it's so conspicuous. It's not
like the Amazon where you have to hack your way through and the outcrops are
covered with vegetation," explained Quade. Outcrops are surface geological
exposures.

At the same time, summer rainfall in the northern Atacama Desert flows from the
same atmospheric circulation patterns that define climate in the Amazon, as
Betancourt pointed out.

Still, the region bears small resemblance to the tropical rain forests -- or
even the relatively lush Sonoran Desert here. The researchers use the phrase
"absolute desert" to describe the barrenness they find once they move down from the
mountains into the Atacama Desert.

"Below about 3,000 meters in altitude, plants become so widely spaced that they
disappear from the landscape, depending on where you're standing," Betancourt
observed. "But if you increased precipitation in this area, you would get the
expansion of plants into this absolute desert, along with the animals who follow
the vegetation."

These animals include rodents, many of whom have a peculiar habit of urinating
in their nests. The end result is a hardened deposit, known as a midden, that
can persist for 40,000 years or more.

Scientists can analyze the materials in these ancient rodent middens -- leaves,
twigs, seeds, bones and insects -- to reconstruct vegetation zones for the
area, which in turn reveal climatic trends at the time of the middens' creation.
Researchers then use radiocarbon techniques to date the midden material and thereby
put the climate on a timeline.

Betancourt and other UA researchers have led the North American midden research
effort, collecting and analyzing more than 2,000 packrat middens from western
North America over the years. So he expected to find interesting information on
climate by expanding this data base into South American deserts, where at least
four different rodent species use similar nest-building techniques.

He didn't expect his findings to challenge the current thinking on the timing
of climate change in the region, however.

Other researchers had concluded earlier that regional lakes were drying up
about 11,600 calendar years ago. Yet radiocarbon dates showed many of the 400
Atacama middens collected by Betancourt and team were constructed between 10,500 and
11,800 calendar years ago and are full of summer-flowering grasses, indicating a
stronger monsoon at the end of the period geologists call the Younger Dryas.

The Younger Dryas -- named after a flower, not a moisture condition,
incidentally -- is generally recognized as the time when ice sheets briefly readvanced
over North America and Europe.

Betancourt called in Quade, a colleague also working at the UA Desert Lab on
Tumamoc Hill, whose expertise includes interpreting vegetation signals from soil
carbonates and reconstructing water table heights from spring deposits.

Upon joining the Atacama effort, Quade and some of his students headed for
ancient spring deposits left high and dry along mountain fronts in the Atacama
Desert.

"Think of them as bathtub rings," Quade said of the spring deposits. "When the
water table was high, it left a ring on the canyon wall. It's the same stuff you
get on your pipes or the bottom of your water cooler."

The dates returned for the spring deposits support Betancourt's interpretation
of climate.

"We're seeing higher water tables between 14,000 and 9,100 years ago, and 2,000
to 7,000 years ago, right when everybody else says it's dry," Quade said.

The UA researchers maintain that their own radiocarbon dates will stand the
test of time better than the information gleaned earlier from lake deposits.

"In lakes, there's the 'hard water effect,' which means aquatic organisms are
actually taking up old water," Betancourt noted. They incorporate carbon from the
older water into their tissue and shells, which are later dated by geologists.
"So the dates are not very trustworthy."

Although the debate may sound academic at first glance, it could influence
modern interpretations of climate change.

"The climate changes at the end of the Younger Dryas might have been happening,
basically, in a few decades," Betancourt explained. "This question has bearing
on our future. Is it possible within the current regime to get such rapid
changes in climate?"

Also, the findings support an emerging line of thinking that the tropics play
an important role in large-scale climate change. For the past several decades,
many scientists have viewed the tropics as climatically stagnant and the polar
regions as the driving force behind global climatic shifts.

Quade agrees that their work could have bearing on modern thinking, as
international leaders depend on scientists to use knowledge of past climate along with
Global Circulation Models to predict how climate could change in the future.

As he asked rhetorically: "How can you build a model if you don't have the
dates right?"

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