probably heard of Carbon-14, also written C14.
C14 is a radioactive form of carbon produced in the
outer atmosphere when cosmic rays transform atoms of regular nitrogen. Being
radioactive and thus unstable, C14 "decays" at
a certain predictable rate. Its decay rate is such that half of any given quantity of it
will convert back to nitrogen in about 5,730 years. Then half of the remainder will be
gone in the next 5,568 years, etc. The fancy way of saying this is that C14's half life is 5,568 years. There's a good site on the
Web for finding more detailed
information on C14 dating.
C14's remarkable behavior is very useful to us because of two further fabulous facts: First, despite its strange manner of having a half life, it behaves just like regular carbon as it cycles through the ecosystem, sometimes as a constituent of the gas carbon dioxide, or CO2, sometimes as a carbohydrate inside a plant's body, or whatever.
The second fabulous fact about C14 is that since it both appears and disappears at given rates, and surely has been doing so ever since Earth had an atmosphere with nitrogen in it, it can be assumed that the ratio between regular carbon and C14 in the atmosphere has long remained the same.
Therefore, imagine this: A typical green plant spends its day "photosynthesizing" -- using energy from sunlight to convert water and carbon dioxide into its "food," which later becomes part of the plant's own body. The vast majority of the carbon comprising the carbon dioxide used during photosynthesis is regular carbon, but a certain small percentage is C14. This means that, ultimately, the carbon in the plant's body itself will be composed of a certain predictable quantity of C14.
Now imagine that the plant dies, decays into a kind of compost, and a snail eats the compost. The result will be that the snail's body will ultimately be composed of essentially the same ratio of C14 to regular carbon as the plant.
Then the snail dies and is buried beneath... loess, let's say. Now, after a certain time we dig up the fossil snail's shell, analyze the shell's C14 content, and find that it is exactly half of the C14 found in a living snail's body. Therefore...
Since C14's half life is 5,568 years, we can estimate that our fossil snail lived approximately that many years ago. If the C14 content had been exactly one-quarter of that found in a living snail, we'd estimate an age of about 11,100 years.
Unfortunately, after about 30,000 years the concentration of C14 in fossil specimens becomes so small that it becomes chancy to estimate times from the small concentrations. However, for something that lived 20,000-years ago, C14 dating works great!
It's good to have this confidence in C14 dating up to the 20,000-year mark because during the mid 1960s our much-mentioned team of investigators from Millsaps College in Jackson, Mississippi, headed by J.O Snowden, Jr. and Richard R. Priddy, pulled up to a considerable-size road cut on the U.S. Highway 61 Bypass at Vicksburg, Mississippi -- a road cut through unmistakably classic loess -- scraped away part of the road cut's weathered face, and found some white, thumbnail-size fossil snails, just like the ones that once caused B. Shimek's heart to flutter in our "Snails were telling us" section.
In this road-cut loess at Vicksburg, fossil snail shells lay embedded at various levels, some at the top, some at the bottom, and some in between. The investigators gingerly pried shells from various levels, and lovingly stored them out of harm's way. Eventually the shells were checked for their C14 content both at the U.S. Department of Agriculture Sedimentation Laboratory in Oxford, Mississippi, and by a company called Isotopes, Inc., of Westwood, New Jersey.
When the reports on estimated dates-of-snail-death came back, the numbers harmonized with previous estimates with the loess's age, they confirmed aspects of a magnificent story, they were simply glorious to see.
The snail-shell samples had been taken at five different levels. The topmost sample taken were judged as 17,850 years old, give or take 380 years. The middle sample was placed at 19,250 years old, give or take 350 years. The lowest sample registered at 25,300 years old, give or take 1,000 years.
In short, the snails tell us that our loess was deposited during a long period between about 25,000 and 18,000 years ago, which is toward the end of the last Ice Age.
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