Nature's Time Capsules

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 paleonidology, 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.