![]() Produced when stratospheric atoms are bombarded by cosmic rays, chlorine 36 washes into the lower atmosphere in the form of rain, which packrats ingest when it falls on desert plants. After dissolving urine from 10,000-year-old nests near Yucca Mountain, Nev., Mitch Plummer, a hydrogeologist at the Idaho National Laboratory, found that the samples had surprisingly high levels of chlorine 36 isotopes. Middens contain markers of other types of environmental change as well. “If we hadn’t done this work, we’d still be in a Stone Age of ecology when it comes to this region.” “Before scientists started studying these middens, people assumed the Southwestern desert had been in place for millions of years,” Dr. ![]() Still, researchers have collected enough data to correct quite a few misconceptions. ![]() So when we see changes in plant life from one of these middens to the next, it doesn’t always indicate a climate shift.” “You go to a cliff face and collect data from one midden, then look at a second and third one a few miles away, each with subtle differences in ecology. “The limitation of the midden method is that in most cases, it’s hard to get samples from the same site spanning a long period of time,” Dr. Van Devender/Arizona Sonora Desert Museum, via USGS Middens like the one at top contain all kinds of debris, including needles, above. As warming occurs, the plants move back uphill some even disappear from the middens altogether. The agave can ordinarily be found near cliff edges, but “during the Younger Dryas cold snap, it was not growing at higher elevations than Lake Mead,” Dr. During cooler periods, for instance, most vegetation migrates downhill. If a climate shift occurs, changes in these norms are recorded in the contents of middens at different altitudes. Some plants, like junipers, usually grow relatively high in the mountains, while water-retaining species like agave and prickly pear are found at lower elevations. When researchers find a new nest in a cave or an outcropping, they pick it apart like prospectors - an extremely smelly process - and analyze the plants they find for carbon-isotope ratios that identify them as members of a particular species. “It crystallizes and becomes solid.” Like hardened amber, the solidified waste is an ideal matrix to keep plant fragments intact for thousands of years. “Packrats don’t drink water, so their urine is very viscous,” Dr. To turn grasses and leaf bits into sturdy walls, the rats use the only natural glue available: their own urine. Cole said, “and the debris helps seal up the crevice so larger animals can’t get in.” “The rats leave piles of debris in front of the entrance to preserve moisture,” Dr. The packrats’ middens, hulking structures up to 10 feet wide, serve as trash heaps, climate-controlled homes and defensive ramparts. Cole recently proved that a miniature ice age known as the Younger Dryas, long thought to have been confined to the North Atlantic, was also felt in the American Southwest. By analyzing preserved ancient plants and scat from a variety of middens dating back 12,000 years, Dr. Packrats, which look like brown squirrels with Dumbo ears, are skilled home builders, and their massive nests, known as middens, can last 10,000 to 20,000 years (though they are not usually inhabited the entire time).įor that reason, the middens serve as time capsules of desert ecology. Cole, an ecologist for the United States Geological Survey, the nests were precious cargo. Spaulding, a geologist with the engineering company CH2M Hill, and Dr. “Thank goodness for those Vietnam-vet pilots.” “The chopper gave this sickly shudder as we made our way back across the chasm of the canyon,” Dr. ![]() Cole lifted off from a high plateau in the Grand Canyon, their helicopter laden with so many packrat nests that it could barely climb. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |