Spores record changes in ozone concentration.

The chemistry of plant spores on a frigid Antarctic island chronicles decreasing concentrations of ozone over the region in recent decades. Scientists may take advantage of that trend to learn about ancient extinctions.

At far-southern latitudes, the concentration of stratospheric ozone--an atmospheric constituent that provides protection against the sun's ultraviolet radiation--has dropped about 14 percent in the past 4 decades, says Barry Lomax of the University of Sheffield in England. Over that same period, the concentration of an ultraviolet-absorbing pigment in the spores of a particular club moss has more than tripled. Lomax and his collaborators studied spores from South Georgia Island, a tiny landmass near Antarctica.

The protective pigment, called sporopollenin, and several of its long-lived breakdown products absorb a specific wave-length of ultraviolet radiation, says Lomax. That makes these substances easy for researchers to measure.

Analyses of the compounds in ancient spores of various ages could identify long-term fluctuations in atmospheric ozone. Such changes could have been caused by major volcanic eruptions, the impact of large extraterrestrial objects, or upheavals in atmospheric chemistry--all of which scientists suspect may be associated with some of Earth's previous mass extinctions.--S.P.