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                                                                                SCIENCE, Page 61

Nature's Time Capsules


Nests built long ago by the humble pack rat provide tantalizing
peeks into the earth's past

By LEON JAROFF


    "Part way up we came to a high cliff and in its face were
niches . . . and in some of them we found balls of a glistening
substance looking like pieces of variegated candy . . . it was
evidently food of some sort, and we found it sweet but sickish,
and those who were hungry, making a good meal of it, were a
little troubled with nausea afterwards."

    -- from the diary of a lost prospector
       in the Gold Rush of 1849


    Nausea? Little wonder. The glistening balls mistaken for
a snack that day in Nevada were later identified as pack-rat
middens -- globs of crystallized pack-rat urine containing
sticks, plant fragments, bones and animal dung. Still, while the
middens failed to make the grade as cuisine, they have begun to
excel in another role -- as a kind of natural time capsule.

    From the well-preserved contents of middens, scientists
using radiocarbon dating can peer thousands of years into the
past to discern when climates changed, why civilizations
withered and how plants and animals migrated.

    Pack-rat middens are found in arid regions of North and
Central America and take shape when the acquisitive rodent, like
its human namesake, collects and carries home virtually all the
trash it can find. It piles the debris in its den, where it
becomes saturated with urine. As the urine evaporates in the dry
climate, it crystallizes, gradually enveloping the collection
and forming a large, hard clump. Protected from the elements,
the pack rat's trophies, like insects entombed in amber, are
preserved for millenniums.

    "A pack-rat midden is a snapshot of the flora and fauna
existing within about 50 m [164 ft.] of the midden at the time
it was accumulating," explains Peter Wigand, a paleoecologist
at the University of Nevada's Desert Research Institute.
Scientists can pin down the approximate time the snapshot was
taken by radiocarbon dating of a preserved twig or fecal pellet;
the technique can date specimens that are more than 40,000
years old. And by studying middens of different vintages in the
same area, researchers can in effect create a movie from a
sequence of snapshots, showing changes in local ecosystems.

    The analysis of middens is emerging as a distinct
scientific specialty. Its handful of practitioners have already
published a 472-page tome on the subject (Packrat Middens;
University of Arizona) and have considered naming the specialty
paleo nidology, which roughly translated means "study of old
nests."

    By whatever name, the investigation of middens is paying
off with a host of new insights about the past. Using midden
evidence of tree growth and distribution in the Mojave Desert,
botanist W. Geoffrey Spaulding of the University of Washington
determined that average desert temperatures during the height
of the last Ice Age, about 18,000 years ago, were 6 degreesC (11
degrees F) colder than they are today.

    In a midden study covering 11,000 years of vegetation
change in New Mexico's Chaco Canyon, Julio Betancourt of the
U.S. Geological Survey and Thomas Van Devender of the
Arizona-Sonora Desert Museum found evidence that could explain
why a once thriving Anasazi Indian community was abandoned 800
years ago. Simply stated, the Indians eventually used all the
surrounding pine trees for their dwellings and firewood,
depleting the woodland and eroding the farmland vital to the
tribe's survival.

    Middens made by hyraxes -- rodents found in Africa and the
Middle East -- have provided similar evidence that human
clearing of surrounding forests and shrubbery led to the sudden
collapse in A.D. 900 of the ancient metropolis of Petra, in what
is now Jordan.

    Middens can reveal changes in the heavens as well as on
earth. That was demonstrated by hydrologist Fred Phillips of the
New Mexico Institute of Mining and Technology, who checked an
ancient pack-rat midden for evidence of cosmic-ray bombardment
of the earth. He knew that highly energetic cosmic-ray particles
create the radioisotope chlorine 36 when they strike argon atoms
in the atmosphere, and that the isotope finds its way into
plants and the urine of mammals, including the pack rat.

    With the aid of radiochemist Pankaj Sharma of the
University of Rochester, he compared the amount of the isotope
in the midden urine with contemporary values, and concluded that
cosmic-ray bombardment was 41% more intense 21,000 years ago
than it is now. This suggests that the earth's magnetic field,
which acts as a partial barrier to cosmic rays, was then
considerably weaker. One implication: terrestrial life had been
-- and could someday again be -- exposed to higher doses of
dangerous radiation from space.

    Researchers are gleaning other secrets from plant leaves
preserved in the middens. At the end of the last Ice Age, for
example, plant structures called stomata, which are used to
process carbon dioxide, were far denser than they are today.
This suggests that the ancient atmosphere contained much less
carbon dioxide. Middens have even more to reveal. The
well-preserved plant and animal DNA in midden specimens promises
to be a bonanza for genetic researchers.

    History does not record if the band of nauseated
Forty-Niners eventually reached California or how they fared in
their quest. Yet on that day long ago in Nevada, they had
already struck gold.