How accurate is carbon dating?

Question

(Please no religious answers)

Answer 1

(m)

Can We Prove That Carbon Dates Are Accurate?
There are two ways to do this. We can date things for which historians know a "right answer". And, we can date things that have been dated by some other method.

Historians don't have "right answers" for really old things. However, carbon dating has done well on young material like the Dead Sea Scrolls, the Minoan ruins, and acacia wood from the tomb of the pharoah Zoser.

Some corals can be carbon dated, and also dated by another radioactive material, Thorium-230. Pollen found in the Greenland icecap has been carbon dated, and also dated by counting ice layers. The three methods confirm each other.

Trees grow a thick ring in a good year, and grow a thin ring in a bad year. It is sometimes possible to match up tree-ring patterns between different trees. When enough suitable trees are found, living or dead, the matching is completely accurate. Then, we have wood for which we know the right answer.

So, carbon dating has been calibrated against the rings of California bristlecone pines, and Irish bog oaks, and the like. When this was first done, it turned out that carbon dating had been giving too-young dates for early civilizations. Apparently, the production of C14 by the Sun has changed by several percent across the last 10,000 years. We know (from other measurements) that the Sun hasn't fluctuated by more than 10 percent in the last million years. However, even this small an adjustment was a bit of a shock. For example, Stonehenge suddenly became older than the Pyramids, instead of younger.

Since then, several other calibrations have been done, which confirm and extend the tree-ring one. Some were done by finding lakes with atmospherically derived carbon in their annual layers of silt (called varves). In those particular lakes, the varves can be counted, and the varves can also be carbon dated. See below for details about the 45,000 annual varves in Lake Suigetsu.



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Are There Inaccurate Carbon Dates?
Yes. There are three kinds.

The first kind are datings of things that should't be carbon dated. For example, polar bears that eat seals aren't getting their carbon from an atmospheric source.

The second kind are datings on contaminated samples, or on samples which are a mixture. Old samples contain much less C14, so the measured date of older samples is strongly affected by even small amounts of contamination.

The third kind are dates which were measured before the 1970's. In the 70's:

much better measurement equipment was introduced.
the tree-ring calibration eliminated the assumption about the Sun being constant.
procedures for avoiding and recognizing contamination were established.

Answer 2

Not very. Carbon 14 dating is based on the assumption that all geological and physical processes remained constant on the earth, a concerpt referred to as 'Uniformatarianism'. Nothing could be further from the truth. The earth has had intermittent periods of violent volcanic activity, tectonic activity, numerous ice ages, catastrophic floods that inumdated entire land masses, magnetic pole weakening and flipping just to mention a few. Even variences in solar activity can affect carbon 14 deposition.

Answer 3

By knowing the half life of an isotope you can determine how old a material is by looking at the ratio of the parent isotope and it's daughter isotope. Carbon-14's daughter isotope is Nitrogen. The half life of C-14 is 5730 years, so we can effectively date materials up to 100,000 years before there is no longer enough tracable carbon to measure.

The problem in the case of carbon dating is in how carbon is absorbed into organic material, and how it is created. The earth is shielded from solar radiation by it's magnetosphere. C-14 forms when radiation penetrates the magnetosphere and knocks neutrons out of nuclei in the upper atmosphere. These neutrons then bump into ordinary Nitrogen atoms this creates Carbon-14. Radioactive C-14 combines with Oxygen just like regular Carbon-12 to form Carbon Dioxide which is then taken into plants and animals like you and me when we breathe. When an organism dies, it no longer takes in C-14 and so the Carbon clock starts ticking.

The problem is, that different plants will absorb C-14 differently then they absorb regular C-12. In addition, fluxuations in the magnetosphere will cause more or less C-14 to be produced, affecting the overall Carbon Nitrogen and Oxygen ratios.

It addition, other isotopes used to date, such as Potassium (K) can also affect parent-daughter ratios when they are created. The isotope of K-40's daughter isotope is Argon-40. It has been observed that Argon may also be present when K-40 is formed. This has actually been observed in Volcanic eruptions in New Zealand in the mid-1900's. The presence of Argon affects the parent:daughter ratio making materials appear much older or younger than they really are.

Another problem is the presence of C-14 in fossils older than 100,000 years old. As mentioned earlier, Carbon-14 is only effective in dating up to 100,000 years old. Normally to date these fossils we evaluate the strata in which we find them... if we find a fossil in layers that we date by other methods to be several million years old, then we can presume the fossil lived about the time that layer was made. However some of these fossils have been shown to have very high amounts of Carbon-14 in them. This would indicate that either the Carbon was somehow absorbed afterwards, or that it is not as old as the strata it was found.

As to the implications of these findings, time will tell as we come to a better understanding of our dating methods, and the universe in which we exist.

Answer 4

Well, according to my religion, carbon does not exist. Just kidding.

The type of carbon dating I always hear about is carbon-14 dating. Carbon-14 is preferred, I believe, because it is a stable molecule and decays at a very predictable rate (half the carbon-14 in an object will decay in about 5500years or so). So, does carbon-14 dating give you the age of an object down to the minute? No, but it does give you a very good 'ballpark' figure.

The way it works is that 'they' measure how much carbon-14 has decayed since the object died and use the decay rate to determine how long it took for that much carbon-14 to decay. What I've always wondered, though, is how 'they' know how much carbon-14 was there to begin with....

Hope this helps.

Answer 5

The best-known absolute dating technique is carbon-14 dating, which archaeologists prefer to use. However, the half-life of carbon-14 is only 5730 years, so the method cannot be used for materials older than about 70,000 years.

Answer 6

I dont think you will find a straight forward answer to this question, because it would depend on things like contamination from other sources, like microbes or mending, meddling etc...

The ratios of c14/c12 will skew the results, and this will of course vary drastically on the particular circumstances of the experiment you are interested in.

However, given this if all things were perfect, then c-14 decay is predictable, and usually accurate to within +/- 2% at 10,000 years

Answer 7

That's an excellent question. It's most valid for time periods about equal to C14 half-life. It becomes very difficult to measure such low levels at much greater times.

You can search to get the info.

Wikipedia says that it's good for up to 60,000 years. Here's a refernce:

http://en.wikipedia.org/wiki/Carbon_dating

Answer 8

It depends if you bring flowers when you first arrive.