Ancient Heights: Leaf fossils track elevation changes

Using altitude-dependent differences in fossil leaves, geologists have developed a tool that they say can track land elevations over geologic epochs. The scientists plan to use the new technique to better chronicle the rise and fall of mountain ranges.

BREATHING HOLES Pores that take in carbon dioxide appear in this 330-fold magnified view of a present-day California black oak leaf, stained orange for clarity. McElwain/Field Museum

Trees’ leaves breathe in carbon dioxide through tiny pores, called stomata, that can be seen with a microscope in both modern and fossilized samples. At higher altitudes, the leaves grow with more tightly packed pores to compensate for the thinner air. Jennifer C. McElwain of the Field Museum in Chicago has taken advantage of this anatomical adaptation to turn fossil leaves into a measure of ancient altitudes. She details the strategy in the December Geology.

McElwain used leaves from living California black oak, or Quercus kelloggii. Unlike most trees, it can grow from near sea level all the way to 2,500 meters. Using a microscope, McElwain counted the pores per square millimeter for leaves collected at different altitudes. She then developed an equation relating the density of pores to the altitude at which the plant lived.

The black oak has been around at least 24 million years, so its fossilized leaves may provide information on elevation changes since the end of the Oligocene period.

The new technique for measuring ancient altitudes has several advantages. One current method examines a range of species appearing together in the fossil record to estimate the altitude at which they lived. But variables such as temperature and rainfall can confound the analysis, a problem not found with the pore-density method. Another method now in use examines bubbles in cooled lava, but it applies only in volcanic regions, and its altitude determinations can be specified only within a range of about 450 m.

The pore-density method, in contrast, can distinguish differences of 300 m or less. “It’s precision gets better as you go up in altitude,” says Dana Royer of Pennsylvania State University in State College, who studies variations in the concentration of carbon dioxide over geologic time. The increased precision with altitude of McElwain’s new technique is “the opposite of just about any other method,” Royer says.

Changing concentrations of carbon dioxide can also affect leaf-pore density. But scientists can determine ancient carbon dioxide concentrations independently using, for example, isotope measurements on plankton fossils.

McElwain plans to make her technique more versatile by using fossil leaves from other species. One of her goals is to get a more precise date for the emergence of the Himalayas, now estimated at 40 to 50 million years ago. Geologists suggest that these mountains may have had a major impact on the Earth’s climate.

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