Cobra Effect · Earth
Dating the Earth, and finding lead everywhere
How measuring the age of the Earth uncovered lead pollution everywhere.
7 cards, read aloud in 3:20, with a test and sources.
In the late 1940s, Clair Patterson set out to help measure the age of the Earth.
He was a young chemist working for Harrison Brown at the University of Chicago. The method used lead. Uranium slowly turns into lead, at a steady rate. Measure the different kinds of lead in very old material, and you can work out how long ago it formed. But Patterson’s lead measurements kept coming out too high.
The extra lead was contamination, and it was everywhere.
Lead was in the air, in dust, in glassware, and in the chemicals he used. Tiny traces from his surroundings were swamping the lead he was trying to measure. Patterson spent years learning to keep lead out of his samples. He went on to build one of the first ultra-clean laboratories, scrubbing and sealing out lead wherever he could.
He measured lead from a meteorite found at a crater in Arizona.
Meteorites are leftovers from the birth of the solar system, and formed at about the same time as the Earth. Patterson used fragments of the Canyon Diablo meteorite, from Meteor Crater in Arizona. He took his purified samples to a sensitive instrument at the Argonne National Laboratory, near Chicago. By 1953 he had the measurements he needed.
In 1956 Patterson put the age of the Earth at about four and a half billion years.
His figure was four point five five billion years. Earlier estimates had varied widely, and many were far too young. His result has held up remarkably well ever since. By then he was working at Caltech, in California.
Patterson then asked where all the lead around us had come from.
He found that surface water in the ocean off southern California held far more lead than deep water. Lead was arriving from the air, and the likely source was car exhaust. Since the 1920s, a lead compound invented by Thomas Midgley had been added to petrol to help engines run smoothly. In 1965 he warned that people were carrying far more lead than was natural.
Snow and ice from Greenland showed how much lead had risen.
Patterson measured lead in layers of snow and ice laid down over thousands of years. Recent snow held many times more lead than ice from thousands of years before. He faced strong opposition from the lead industry. The United States began phasing out leaded petrol in the 1970s, and banned it for cars on the road in 1996.
So when your measurement keeps coming out wrong, ask what else is getting in.
Patterson’s contamination problem turned out to be a discovery in its own right. Solving it gave the age of the Earth, and revealed lead pollution across the planet. As leaded petrol was phased out, the amount of lead in people’s blood fell sharply. The last leaded petrol for cars anywhere in the world was used up in 2021.
Sources
- Clair Patterson, the Caltech scientist who led the charge against lead, Caltech Magazine. The clean lab, the age of the Earth, and the long fight over leaded petrol.
- Clair Cameron Patterson: a biographical memoir, National Academy of Sciences. A detailed account of his life and work by those who knew it.
- EPA takes final step in phaseout of leaded gasoline, US Environmental Protection Agency. The 1996 announcement, with the history of the phase-out.
Nearby ideas
- The rocks that revealed deep time. How rocks on a Scottish shore showed that the Earth is immensely old.
- The ozone hole and the treaty that worked. How a warning about spray-can chemicals led to a treaty every country signed.
- The clay layer that pointed to an asteroid. How a thin layer of clay led to an asteroid, and a crater in Mexico.
- The curve that tracks carbon dioxide. How patient measurements on a Hawaiian volcano revealed carbon dioxide rising.
- Ancient air trapped in ice. How bubbles and layers in polar ice record hundreds of thousands of years of climate.
- The continents that drift. How matching coasts, fossils and rocks suggested that continents move.
- The magnetic stripes on the sea floor. How stripes of magnetism in the sea floor showed that the oceans are spreading.