Nature's Time Capsules

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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.