해석 제발..부탁드립니다...꼭 필요합니다... 저에게는 너무 어려워서요...ㅜ.ㅜ Nature's Time Capsules 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 50m[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.