Monday Mourning

“During my lifetime?”

“Do not grow defensive. I am not fighting you.”

I drew a deep breath.

“There is one test which might resolve the question.”

“Tell me.”

“You’ve heard of Carbon 14 dating?”

“I know it is used to assign age to organic materials, including human bone. I do not know how it works.”

“Radiocarbon, or Carbon 14, is an unstable isotope. Like all radioactive substances, it decays by releasing subatomic particles at a uniform rate.”

LaManche’s eyes stayed heavy on mine.

“In about 5,730 years half of a population of radiocarbon atoms will have reverted to nitrogen.”

“That is the half-life.”

I nodded. “After 11,460 years, a fourth of the original amount of radiocarbon remains. After another 5,730 years, only an eighth remains, and so on.”

LaManche did not interrupt.

“The amount of radiocarbon in the atmosphere is really tiny. There’s only about one radiocarbon atom for every trillion stable carbon atoms. Radiocarbon is constantly being created in the upper atmosphere by cosmic bombardment of nitrogen. Some of the nitrogen converts to radiocarbon, which immediately oxidizes to CO₂. That CO₂ 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 gradually 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 actually be used to give the process wider applicability.”

“How so?”

“That’s where it gets interesting. Studies have documented significant 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.”

“Oui.”

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

“The radiocarbon in living things increased.”

“Dramatically. 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 folly.” 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 actually 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 scintillation spectrometer.”

“And the other method?”

“With the other method results are derived from reduction of the sample carbon to graphite. The graphite is then tested for Carbon 14 content in an accelerator mass spectrometer.”

For several seconds LaManche said nothing. Then, “How much bone is required?”