Science comes in two styles. The first happens in the head—Einstein is the icon for this, thinking physics during his dull day job at the Swiss Patent Office and creating relativity theory from thin air. In geology the exemplar is Darwin, pondering his notes and slowly perfecting a breakthrough version of evolutionary theory.
The other style of science happens in the eye, so to speak, and Galileo is its icon. He heard about a new instrument, the telescope, built one, and turned it toward the skies. The papers he published after that nailed down earth-shaking new insights about the planets and the universe. In geology, an example might be Victor Vacquier, who invented a state-of-the-art magnetometer in the 1950s and took it to sea off Washington state. The magnetic map he made led directly to the theory of seafloor spreading, then to plate tectonics. The insights were not possible before the instrument created the data.
Meet the Mass Spectrometer
Two more people of this type are Zachary Sharp and Thure Cerling, who published a new example of instrument-based science in the March 1998 issue of Geology. (Here's the abstract.) The instrument in this case is the latest version of the mass spectrometer, and its target is a single fossil horse tooth.
The mass spectrometer is basically an instrument that sorts atoms by weight. A century of improvements (chronicled at this history page from the Scripps Institute) has given the mass spectrometer fantastic capabilities. Measurements that once needed large samples and intricate chemical manipulations can now be done with a brief shot of laser light on a pinprick-sized sample. Now excellent data is available not just for every element in a sample, but for every isotope of each element.
We are only beginning to explore what's possible with such information. The instrument helps us determine ever more precise ages for rocks and fossils, for instance, through isotopic dating. But Sharp and Cerling used the mass spectrometer to study ancient weather.
The data in their study is the abundances of telltale isotopes of carbon and oxygen. A small fraction of carbon-13, about 0.2 percent, is mixed with ordinary carbon-12. Different kinds of plants, when they fix carbon dioxide from the air through photosynthesis, favor the heavy carbon to different degrees. Likewise, about 3 percent of oxygen-18 is mixed with ordinary oxygen-16. Water with the heavy oxygen evaporates slightly more slowly than ordinary water. This makes rainwater slightly lighter than groundwater or seawater.
The Tape-Recorder Teeth
What an animal eats and drinks ends up in its bones and teeth. The teeth of horses and other grazing animals grow for several years creating an environmental record along their length, a "tape recorder" of the changing seasons. Thus, fossil teeth can be useful records of the environment in which the animal lived and died.
Sharp and Cerling collected two fossil horse teeth, one from an Ice Age horse in North Dakota and the other from a much older horse species, 6.8 million years old, in northern Texas. Then they zapped the teeth from one end to the other in the mass spectrometer, taking measurements every millimeter of the oxygen and carbon isotope abundances.
Their carbon measurements showed a wavelike pattern, corresponding to the change in grasses in the horse's diet over the course of a year. The oxygen data showed the same pattern, corresponding to the changes in rainwater with the seasons. Moreover, the Ice Age horse's tooth showed less variation in the oxygen data than the older tooth, reflecting the fact that the horse drank from a large lake whose oxygen isotope balance stayed steadier throughout the year than the dry Texas plains.
This proves at least three new pieces of knowledge. First is that with the right technique, we can study not just the average climates of the distant past but their seasonal patterns, too. Second is that by always choosing data from the same time of year, we can remove the seasonal variation to focus exclusively on long-term climate trends. Third is that now we can tell if a tooth is giving us good data by looking for that annual cycle—if it's not there, then the fossil has suffered from alteration (diagenesis). And we've also learned a little more about those horses. (For more about ancient horses, visit the Fossil Horse Cybermuseum.)
Of course, those three discoveries open up three new avenues of inquiry, three new sets of questions. That's science for you, whether it's Einstein's or Galileo's variety.
PS: Mass spectrometers are everywhere these days, not just in the geology lab. The FBI uses them, and the food-and-drug people too. Here's a report of using one to detect adulterated honey.
