Fill ‘er up . . . with a few tons of wheat

If you think it takes a lot to fill up your vehicle’s gas tank, you’re right. A new analysis suggests that each gallon of gasoline is derived from an amount of ancient plant matter equivalent to the vegetation in a midsize wheat field.

Most of today’s oil started out millions of years ago as tiny aquatic plants called phytoplankton, says Jeffrey S. Dukes, an ecologist at the Carnegie Institution of Washington in Stanford, Calif. After those organisms died, the forces of heat, pressure, and time transformed their carbon-based remains into a power-packed fossil fuel.

There are about 4.14 kilograms of carbon in a gallon of gasoline, says Dukes. To calculate how much aquatic plant matter it would take to supply that carbon, he analyzed nature’s oil-making process. Many plants decompose right after they die, so their carbon is immediately recycled into ecosystems, primarily into carbon dioxide. The carbon from other plants may eventually end up in oil trapped in untappable pockets.

Overall, only about one carbon atom out of every 10,750 from ancient phytoplankton ended up in recoverable oil, says Dukes. Therefore, he calculates, it takes about 44,500 kg of carbon from ancient plant matter to make a gallon of gasoline.

Because plants are only about one-half carbon, the carbon in each gallon of gasoline corresponds to a whopping 89 metric tons of vegetation, says Dukes. That’s approximately the same amount of plant matter–roots, stalks, and all–that grows in a 40-acre wheat field just so people can drive 30 kilometers, he adds. Dukes reports the results of his analysis in the November Climatic Change.


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