MadSci Network: Earth Sciences

Re: Is radiocarbon dating the best current method available? Why/Whynot?

Area: Earth Sciences
Posted By: Thomas M. Greiner, faculty, Anatomy / Physical Anthropology, New York Chiropractic College
Date: Thu Oct 23 14:10:00 1997
Area of science: Earth Sciences
ID: 876927559.Es
Is radiocarbon dating the best current method available?

The only answer to this question is: “it depends.”

Radiocarbon dating is based on the fact that the Carbon atom is 
incorporated into every living thing (hence organic chemistry). It also 
makes an assumption — that the ratio between radioactive carbon (C14) and 
“normal” carbon (C12) has remained constant in the environment. That means 
that as long as we are alive, the ratio of C14 to C12 in our bodies will be 
the same as in the world around us. When we die, we no longer bring new 
carbon atoms into our bodies. That means that C14 will decay (it turns into 
Nitrogen 14) and alter the ratio between the two types of carbon. The lower 
the ratio (less C14 because of decay) the longer amount of time has gone by 
since death. Since C14 has a known half-life (rate of decay) we can 
calculate the time since death of an organic specimen by observing its 
carbon isotope ratio.

That’s great — however, there are several problems. 

1. Radiocarbon dating only works on things that were organic (things that 
were alive) and that still have organic (carbon based) material in them. 
This dating method does not work on fossils, since the fossilization 
process completely replaces the organic material with inorganic rock.

2. Radiocarbon dating only works on relatively recent specimens. The old 
rule of thumb was that radiocarbon could only date things back to about 
100,000 years ago. Recent technological advances have increased the time 
depth (I’m not sure how far back it goes now), but it still wouldn’t work 
on very old things (things that are several million years old).

3. There are many opportunities for carbon contamination. If plant roots 
grow into your specimen (even if it happened so long ago you can’t tell 
that it happened) they bring in “young” carbon. The result of radiocarbon 
dating will be a younger date than is actually true. If the specimen is 
contaminated with oil (or something like that) “old” carbon will be 
introduced. The date will be much older than is actually true.

4. The central assumption that the ratio between C12 and C14 is constant in 
nature, and always has been, may not be valid. Years of burning fossil 
fuels have increased the amount of C12 in our the environment.  Sunspots, 
or some other cosmic event, may increase the amount of C14.
(There is a story that someone "dated" an artifact that was found near 
a nuclear explosion test site. The atomic blast shifted the isotope ratio, 
so that radiocarbon dating suggested that the artifact came from 
the future! This story may not be true, but it does illustrate the problem 
of shifting carbon isotope ratios in the environment).

So the real answer to your question is that radiocarbon dating is a useful 
method of dating archaeological (and sometimes paleontological) specimens. 
It has benefits and drawbacks, as do all the other dating techniques. 
Scientists try to use as many different types of dating methods that they 
can so that the limitations will cancel each other and provide a better 
estimate of the actual age of a specimen.

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