Lightning Strikes Give Clues About Ancient Climate

New research indicates that lumps of glass that are created when lightning hits sandy ground can preserve information about ancient climate.

Lightning flashes occur about 65 times per second worldwide. The equivalent of a quarter ton of TNT's worth of energy is released by one bolt. Lightning flashes heat the air to about 30,000 degrees Celsius. That's about five times hotter than the sun's surface. When a bolt hits a sandy surface, the sand and other materials can be melted and fused together into masses of glass. These masses are called fulgurites, according to Rafael Navarro-Gonzalez, a geochemist from Mexico City. Their name comes from "fulgur," the Latin word for lightning.

Thunderstorms are common in many parts of the world, but they are very rare in the deserts of southwestern Egypt. Of the storms' rarity, Navarro-Gonzalez says, "Satellite data gathered between 1998 and 2005 detected little, if any, lightning in that area." However, the sands are littered with fulgurites. This proves that lightning frequently struck in the past.

Navarro-Gonzalez and his colleagues studied samples of a fulgurite that as found in 1999. They determined that it had formed 15,000 years ago. They measured the luminescent glow that the glass' minerals gave off when it was heated. Defects in the material are created over time by cosmic rays and radioactive elements in the soil. The heated fulgurite glows brighter the more defects it has.

In analyzing gasses trapped in the fulgurite, the researchers revealed that the landscape has changed greatly since ancient times. It is little more than sand now, but it was hospitable shrubs and grasses 15,000 years ago.

The tests revealed a small amount of argon. Today, argon is the most abundant inert gas in our atmosphere. In today's atmosphere, argon is about 25 times more prevalent than carbon dioxide. In the fulgurite gases, on the other hand, there was about 100 times more carbon dioxide than argon. This abundance of carbon dioxide was created when lightning struck organic material in once-fertile soil.

The carbon in the glass is similar to the kind generated by grasses and shrubs in hot, arid climates. Today, plants that would generate this kind of carbon lie 600 miles south of the research site, in Niger.

This evidence suggests that at the end of the last ice age, about 15,000 years ago, the climate in southwestern Egypt was similar to the one Niger has today.

Fulgurites are mainly glass, aren't very susceptible to erosion, and are chemically stable. This makes them long-lasting indicators of climate, according to Barbara Sponholz, a physical geographer from Germany.

Source:

"Stroke of Good News: A Wealth of Data From Petrified Lightning" ( http://www.sciencenews.org/articles/20070217/fob5.asp)