Lightning Glass Yields Climate Clues

Jan. 31, 2007 — A glassy stick made by lightning striking Libyan sands has now been used to date the lightning and detect the ancient climate.

It turns out that the root-like glass "fulgurite" structure made of melted and welded sand from the Libyan Desert contains miniscule bubbles of trapped gases from the time it was made. These gases, carbon dioxide (CO2), carbon monoxide (CO) and nitric oxide (NO) harbor hints of the type of plants that were zapped along with the sandy ground some 15,000 years ago.

Careful analysis of those gases and the weights of the carbon atoms in them reveal the plants were probably the sort which used what’s called C4 photosynthesis — best for plants living in hot, arid climates. This, in turn, implies that the semi-arid Sahel region of today reached much further north during the Pleistocene (1.8 million to 10,000 years ago).

In other words, the Libyan Desert was once a lot less arid than it is today.

That so much information can come from fulgurites — which have long been little more than fragile curiosities — is a surprise even to the researchers conducting the work.

"Up to now no one was interested in doing actual research on them," said Rafael Navarro-González, a researcher at Mexico City’s Universidad Nacional Autónoma de México. "No one had even attempted to date fulgurites."

Navarro-González is the lead author of a paper on fulgurites in the February issue of the journal Geology. His team, composed of scientists from France, India, Niger and the United States, used a process called thermoluminescence to date how long the fulgurite has been around.

"That’s amazing," observed lightning researcher Martin Uman of the University of Florida at Gainesville.

Uman agrees that few people have looked very closely at ancient fulgurites. For his part, he and his colleagues actually make new fulgurites of a sort by triggering lightning with cable-tethered rockets that send the lightning into drums containing whatever material they want to experimentally zap. That makes him something of an expert on fulgurites, at least the modern kind.

"Fulgurites are hollow, shiny on the inside and rough on the outside," said Uman. "They are very fragile." So the discovery that they can stick around for thousands of years is impressive, he said.

As for the change in Libyan climate, from semi-arid to hyper-arid, that’s not exactly a surprise, of course, said Navarro-González. After all, the area where the fulgurite was found is known for having virtually no lightning storms at all under the current climate regime.

The extending of the Sahel more than 600 miles north has also been suggested by other researchers looking at other lines of evidence, like pollen, Navarro-González said. So the fulgurites of Libya are confirmation of that other work.

Elsewhere, however, fulgurites could reveal surprises. For this reason Navarro-González hopes his work will spur others to take a closer look at fulgurites that have been sitting in drawers and display cases for decades.