Dino-death: flash broil or slow steam.

Dino-death: Flash broil or slow steam

The dinosaurs didn't stand a chance of surviving the postulated asteroid impact 66 million years ago, according to two new theories. One researcher reports evidence that the impact generated a moist greenhouse effect, transforming savannas into ran forests and drastically upsetting the environment. Other scientists suggest the impact turned the atmosphere into a broiler that sparked wildfires worldwide.

Whether or not these scenarios did occur, they demonstrate how far researchers have traveled from the simple impact hypothesis, proposed in 1979 after scientists detected evidence that a meteorite or comet hit the Earth long ago. The original theory held that an impact created a global dust cloud that blocked out sunlight, cooled the planet and arrested photosynthesis, killing a significant number of species.

While he has found evidence supporting the "impact winter" idea, Jack A. Wolfe, a paleobotanist with the U.S. Geological Survey in Denver, has also discovered signs that a moist greenhouse followed the cold times. Wolfe compared the shapes of modern leaves from different environments to leaf fossils found in sediments before and after the time of the proposed impact. The abrupt transition from small, rounded leaves to large, pointed ones suggests precipation quadrupled and mean annual temperatures soared by 10 degrees C in the interior of North America, he reports in the Jan. 11 NATURE. If the cold didn't kill off vulnerable species, the warm, moist conditions would have finished the job, Wolfe says.

H. Jay Melosh of the University of Arizona in Tucson and his colleagues used theoretical calculations to come up with a different scenario. Melosh proposes that material from the impact body and the impact site should have flown up past Earth's atmosphere and then fallen back down in tiny pieces that moved much faster than scientists had estimated. Speeding through the atmosphere, the billions of glowing bits would heat Earth's surface enough to ignite wildfires, the group suggests in the Jan. 18 NATURE. They would also cause extremely acidic rain, which would kill off many ocean organisms, Melosh says. Some researchers have reported evidence of massive fires from that time, but Wolfe says he sees no such signs in sediments he has examined.