How accurate is radiocarbon dating?
For recent materials it usualy as accurate as a 200 year range but with
older materials this range gets wider. Materials at the limit of the range
that can be measured may only be dated to a 500 or 1000 year range. You
will see some dates which quote only a +_ 10 or 15 years error. This reflects
very good measurement of the isotope ratios. However there is a limit to
accuracy which arises from uncertainty over the exact levels of the isotope
ratios in the past which puts a higher limit on the really useful accuracy.
This is with the best materials and methods. With samples which are smaller than desirable or some of the less accurate methods the accuracy falls off.
Sometimes with trees that have been preserved intact a series of dates spread over 50 or more tree rings can be matched to wiggles in the calibration curve and very accurate dates can result.
How can you tell if the date you give an object is correct?
The only universal method is to do a second measurement on a second sample. Other ways are to have a series of dates which are in a stratigraphic order and check that the ages come out in the same order.
The problem with a lot of samples is that the method tells you when the carbon was fixed from the atmosphere but that event may have been a long time before the sample came to be preserved and if that latter event is what you are trying to date there may be a time gap to be considered of indeterminate length. So for instance charcoal in an archaeological site could be from a shrub which grew over the 10 years prior to it begin burnt, or it could be from a piece of driftwood - wood from a large tree that grew 500 years previously.
With some archaeological materials there are links to historical ages - for instance in Europe some eastern Mediterranean sites have Egyptian material which can be dated from its historic record. This provides a cross check. In places where a lot of dating has been done there is usually a fair expectation of what the age of a new site will be from the cultural material in the site. This provides a cross-check.
Then three are some other dating methods which can ion occasions provide a check. These include thermoluminesence dating, obsidian hydration dating and fission track dating.
Are there any methods available that you feel are more exact?
The best ones in archaeology are dendrochronology where patterns of droughts in tree rings in wood can be matched to a master sequence of rings and give almost exact ages. There needs to be suitable species and climates to do this. To date it is best in the American southwest and in western Europe. Of course you need samples of wood from large trees preserving many rings.
The obsidian hydration methods and thermoluminesence can be pretty good
In parts of the world there are lake deposits laid down in annual layers (varves) Where materials can be related stratigraphically to these they are pretty good too.
How do the changes in the Earth's climate effect the amount of carbon
They don't. The amount of C14 in the atmosphere at any time depends on the amount produced in the upper atmosphere. This in turn depends on the strength of the sun's magnetic field. That varies a bit over a long cycle and with some much shorter cycles as well. Climate doesn't come into that. The amount of C12 does vary with climate but then so does the amount of the other stable isotope C13. Natural things take up the carbon directly or indirectly from the atmosphere when they are alive. The amount the take up does not matter. It is the starting ratio of C12 to C14 that matters. So even if the organism takes up more C14 say because the climate is warmer it will also take up more C14 so the ratio will not be changed.
Actually there are some effects where the rate of take up varies with
the molecular number of the carbon but we can correct for this by also
checking the C12 to C13 ratio. If that is distorted a bit from the natural
level then a correction can be made to adjust for the distortion to the
C12 to C14 ratio and hence the potential error is corrected.
How long does it take to date something using this method?
The sample preparation can take a couple of days - it depends on the
method being used. The actual testing takes a few hours with some methods,
a few minutes with others.
The slow ones are called gas counting and liquid scintillation. The fast one is accelerator dating.
Are there any other materials being experimented with using a similar
There are several radioisotope methods used by geologists. Potassium
Argon (K Ar) is the best known. It is much less accurate that C14 but can
extend back to the beginning of the earth's history.
How difficult is it to perform the dating procedure?
Its quite difficult. There need to be tests run on standards to check
the calibration of the equipment and needs to be inter-laboratory tests
on blind samples to check the results on typical samples. The chemical
pre-preparation is tricky for some samples.
Statistical checks need to be run on the figures produced.
For accelerator dating you need an accelerator which costs millions of dollars and needs to be housed in a large building.
Its not a matter of "plug and play" Well qualified scientists expert
in chemistry and physics are needed.
About how many people usually work together while dating an object?
Are there any objects that cannot be dated using this technique?
The limit on age is about 30,000 years - so it is only relevant to a
small part of the geological past but a larger part of our human past.
Materials that can be dated must have incorporated atmospheric carbon by
a known route. There can be problems - animals that live on intertidal
algae are one - you don't know whether the carbon they used is from the
atmosphere or the sea. Deep sea animals are another problem - as are marine
animals which might have lived in a zone of the sea where there is up welling
of deep ocean water which make them have carbon as if they lived in the
Freshwater animals and plants from areas where there is limestone are suspect - they may have a lot of "old" carbon in their makeup.
Materials which have been contaminated - by old carbon from petroleum based solvents or preservatives are no good as are any contaminated by modern carbon - or contaminated by modern manufactured C14 - used for "labeling" in some modern experiments and also produced in much greater than normal amounts in the atmosphere during the atmospheric testing of thermonuclear weapons.
Lastly anything which does not have a very close contextual association with the event to be dated cannot be used other than to find an age limit.