That CC>2 works its way down into the biosphere, where it's taken up by plants. Since humans, animals, and plants comprise the same food chain, as long as they are alive they have a constant amount of radiocarbon in them. The actual amount is gradual y decreasing due to radioactive decay, but is being replenished through food intake, or through photosynthesis in the case of plants. This equilibrium exists as long as an organism is alive. When it dies, decay becomes the only active process. Radiocarbon dating is a method that determines the point in time at which this disequilibrium started."

LaManche raised both palms in a gesture of skepticism. "Five thousand-plus years. How can such a slow process be of value with recent remains?"

"A fair question. It's true that Carbon 14 dating has been used primarily by archaeologists, and has been shown to be quite reliable. But the technique is based on a number of assumptions, one of which is that the atmospheric level of radiocarbon has remained constant over time. Data inconsistent with that assumption can actual y be used to give the process wider applicability."

"How so?"

"That's where it gets interesting. Studies have documented signif

« 49 »

S

icant anomalies in radiocarbon data for certain time periods. Two perturbations have taken place over the past eighty years, both of which were caused by human activity."

LaManche leaned back, interlaced his fingers, and laid his hands on his chest. A hint for brevity? I did some mental abridgment.

"The period from about 1910 to 1950 is characterized by a decrease of atmospheric radiocarbon, probably due to the release into the atmosphere of the products of combustion of fossil fuels such as oil, coal, and natural gas."

"Why?"

"Because of their great age, fossil fuels contain no detectable radiocarbon. They are said to be dead. Since the combustion of these fuels releases carbon dioxide devoid of radiocarbon, the relative amount of Carbon 14 in the atmosphere drops."

"Out."

"But beginning about 1950, the atmospheric testing of thermonuclear weapons reversed this downward trend."

"The radiocarbon in living things increased."

"Dramatical y. From 1950 to 1963, the values rose to about 85 percent above contemporary reference levels. In 1963, an international agreement halted atmospheric nuclear testing by most nations, and biospheric radiocarbon levels began settling into a new equilibrium."

"Such fol y." LaManche wagged his head sadly.

"These permutations are known as the fossil fuel and atomic bomb effects."

LaManche stole a glance at his watch.

"The bottom line is that artificial or 'bomb' Carbon 14 can be used to determine if someone died before or after the period of atmospheric nuclear testing."

"How is this test done?"

"There are actual y two methods. With the standard radiometric technique, materials are analyzed by synthesizing the sample carbon to benzene, then measuring the Carbon 14 content in a scintil ation spectrometer."

"And the other method?"