Science: Lab notes

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Climate clues found in dinosaurs' teeth

The average adult human struggles to maintain tooth enamel, the hard crystal coating that protects our teeth from gritty foods and invading bacteria. Dinosaur teeth, on the other hand, often retain their enamel for millions of years. "In fact," says geophysicist Henry Fricke of the Carnegie Institution's Geophysical Laboratory, "it is often the only stuff that is decently preserved in the ancient rock beds that palaeontologists comb for dinosaur fossils."

Now scientists are probing the ancient tooth enamel for information about the temperature and weather on Earth during the Mesozoic era, over 65 million years ago. By combining geology, biology and chemistry, Fricke and his colleagues at the Geophysical Laboratory and Macalester College have developed new ways of analysing the oxygen content of the enamel for clues.

Recently, they presented the first results of their study of climate conditions on land during the Late Cretaceous era (about 75 million years ago). They found that temp- eratures in middle latitudes may not have been greatly different from those of today. However, data from Madagascar indicate that sub-tropical latitudes may have been warmer and more arid than today.

The researchers measured small differences in the chemical composition of oxygen found in dinosaur enamel. The levels of two oxygen isotopes reflect their proportions in the water the dinosaurs drank. Oxygen isotope ratios in rainwater are strongly influenced by climatic factors - mainly temperature. Fortunately, the authors noted, the oxygen isotope ratio of tooth enamel does not change over the millions of years that a dinosaur fossil is buried, so it is possible to use this ratio to infer the surface temperature when the dinosaur was actually alive.

Primordial crude

Tiny drops of oil have been discovered preserved in 3- billion-year-old rocks from Australia, South Africa and Canada. This suggests that oil-forming microbes were plentiful not long after the origin of life itself. Indeed, these oil-forming bacteria may be among the most primitive and earliest of the Earth's inhabitants.

Adriana Dutkiewicz of CSIRO Petroleum, Birger Rasmussen from the University of Western Australia and Roger Buick of the University of Sydney studied thin sections of sandstone up to 3 billion years old. The sandstone contains bitumen nodules, which formed when oil flowed through the rocks and became immobilised when it came into contact with radioactive materials. Most of these nodules measure less than one hundredth of a millimetre across.

The oldest oil had previously been dated to 1.5 billion years ago. By extending this the findings have the potential both to add new insights into the evolution of life, and to expand the areas of petroleum exploration. "Early oil generation was evidently widespread, since we found these in rocks from three continents," says Dutkiewicz. "This study also reveals that aquatic life at the dawn of evolution was more abundant than was previously thought. It's quite amazing when you consider that the oldest rocks are 3.8 billion years old and the oldest fossils 3.5 billion years old."

Past earthquakes revealed via cosmic rays

A new method of dating earthquakes, based on the chemical effects of cosmic rays, may help us to predict future earthquakes on certain faults. According to Marek Zreda, a geochemist at the University of Arizona, and Jay Noller, a seismologist at Vanderbilt University, it is possible to date earthquakes by measuring trace amounts of the isotope chlorine-36 on surfaces of exposed bedrock.

The presence of chlorine-36 is the result of chemical changes in surface elements that react with neutrons and muons produced by cosmic rays in the Earth's atmosphere. These cosmic rays, produced in deep space and deflected into Earth's atmosphere by its magnetic field, bombard our planet at a rate that is predictable for any given latitude and altitude. The longer a rock surface has been exposed to the air, the more chlorine-36 it accumulates.

After measuring the amounts of the chemical at different heights on a rock surface exposed by a fault, the researchers calculated that six earthquakes had struck the area in the past 24,000 years. They were even able to date them, to between 400 and 23,800 years ago. And knowing the pattern of past earthquakes at any given fault, they say, may help in predicting when the next one will hit.

! All items are adapted from `Scientific American' magazine. Visit Scientific American's website at www.sciam.com

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