National Geographic : 2001 Sep
decay in 5,730 years. After another 5,730 years, only a quarter of it would be left. (Physicists call these 5,730-year periods the half-life of carbon 14.) Plants and animals that are alive and absorbing carbon dioxide from the air have constant levels of both carbon 12 and carbon 14. But as soon as they die, the supply of carbon 14, which decays back to nitrogen 14 at a known rate, begins to dwindle. By compar ing the carbon 14 level to the total amount of carbon in the material, scientists can calculate how long ago the plant or animal died. Fossils older than 40,000 years have so little carbon 14 left in them that scientists have had to search for other ways to determine their age. A geologist named Gifford Miller from the University of Colorado showed me around a site at Lake Victoria in southern Australia where he used two new dating techniques to get around the limits of carbon 14. Lake Victoria is bounded by an enormous crescent of high dunes, piled up over tens of thousands of years. Under swarms of pink breasted galah cockatoos, Miller and I hiked the rippled sands. Signs of Australia's history, unburied by scouring winds, were everywhere. We saw rusted shell casings left from Royal Australian Air Force training runs in the 1940s. From deeper layers of the dunes-and farther back in time-came piles of mussel shells that had been collected from the lake by Aborigines. Spearpoints lay nearby along with the bones of kangaroo and emu the Aborigines hunted. Descending into a gully, we walked down toward the water and further back through time. "Here is the extinction layer, I think," Miller said, stamping a layer of clay. Below it paleontol ogists have found the skeletons of hippo-size marsupials, kangaroos ten feet tall, marsupial lions-a collection of giants. There's a debate in Australia over how those giants became extinct. Did humans wipe them out, or was it a climate change? The first step in solving the mystery is to decipher the age of the fossils, but there's not enough carbon 14 left in them to measure their age accurately. So Miller has become a connoisseur of new clocks. "There it is. Genyornis," he said, picking up a fragment the size of his fingernail. Genyornis is the name of one of the vanished monsters: a 400-pound flightless bird. Miller held a piece of an eggshell from one of them-the color of putty, with small dimples on its surface. He has amassed a collection of thousands of similar shell fragments from many sites in southeastern Australia. It turns out they're IspcTcnqe Urnu 6ola Ridu to so iu Pts iu toaro Uranium~~~ ~ ~ ~ ~ ~ seis ieqiibim o 0,00yer Cabn1 0t 000 er Raito Ex osr Tehi us'Fiso rc Shr7au nse c n ltclysin lt u iecne0t 0,0 er Electron~~~~~ spnrsoac 163t 1mlio er Ote Technique *edohrnlg 0 to 1200 years i Y., * ?15 S. !SOS:J . .3 !