# Understanding carbon dating **URL:** \ **Category:** Factual Questions\ **Created:** [June 8, 2016, 5:40pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844 "2016-06-08T17:40:34Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![obbn](https://avatars.discourse-cdn.com/v4/letter/o/d6d6ee/32.png) [@obbn](https://boards.straightdope.com/u/obbn)\ **Post date:** [June 8, 2016, 5:40pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/1 "2016-06-08T17:40:34Z")
Hello Everyone, I just read an Article on Cracked that researches have carbon dated Stonehenge and have discussed that it couldn’t have been built by the Druids as it pre dates them. What I don’t get is how they can tell when a structure like Stonehenge was built using carbon dating. If they are dating the stones, surely those were around many centuries before being plucked out of the ground by the builders. How does dating the stones tell you anything about when it was built? ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [June 8, 2016, 5:47pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/2 "2016-06-08T17:47:08Z")
Yeah, you can’t date the stones themselves. However, you can date organic material in the dirt that the stone sits on, which should get you pretty close to when the stone was put into place. > [@](#): > > The dig unearthed about 100 pieces of organic material from the original bluestone sockets, now buried under the monument. Of these, 14 were selected to be sent for modern carbon dating, at Oxford University. From here: [BBC NEWS | Science/Nature | Dig pinpoints Stonehenge origins](http://news.bbc.co.uk/2/hi/science/nature/7625145.stm) ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [June 8, 2016, 5:47pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/3 "2016-06-08T17:47:20Z")
Carbon dating can only be used on objects that were once part of a living thing. C-14 is naturally occurring in the atmosphere, due to Solar radiation. This is incorporated into living tissue due when the creature (or plant) eats or breathes. Once it dies, the C-14 decays, and the ratio of C-14 to C-12 is what dates the time of death. So, rock’s can’t be carbon dated, but it’s possible that something like lichens on them could be. ---
**Author:** ![silenus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/silenus/32/166_2.png) [@silenus](https://boards.straightdope.com/u/silenus)\ **Post date:** [June 8, 2016, 5:48pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/4 "2016-06-08T17:48:27Z")
They don’t date the stones. They date organic residue _associated_ with the stones. Usually fire pits, wooden structures, food remains, etc. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [June 8, 2016, 5:49pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/5 "2016-06-08T17:49:53Z")
They’re not dating the stones, they’re dating organic remains found around the stones. Stone itself cannot, of course, be carbon dated since it contains no carbon. It’s been known for a long time that Stonehenge predated the druids by thousands of years. The earliest known reference to the druids was around 200 BC. The first version of Stonehenge was built about 3000 years earlier. The association of Stonehenge with the druids was a guess by John Aubrey in the mid 1600s. The theory was disproved soon after, but somehow it has stuck in the public mind. –Mark ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [June 8, 2016, 6:02pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/6 "2016-06-08T18:02:27Z")
> [@markn\_1](#): > > The association of Stonehenge with the druids was a guess by John Aubrey in the mid 1600s. The theory was disproved soon after, but somehow it has stuck in the public mind. “Which is solid evidence that druids existed for thousands of years longer than anyone originally thought!”-A response I’ve gotten more than once when correcting wooish people about the date Stonehenge was created. :smack: ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [June 8, 2016, 6:10pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/7 "2016-06-08T18:10:39Z")
> [@markn\_1](#): > > They’re not dating the stones, they’re dating organic remains found around the stones. Stone itself cannot, of course, be carbon dated since it contains no carbon. Plenty of rocks contain carbon–Limestone for instance. > **[Carbonate rock](https://en.wikipedia.org/wiki/Carbonate_rock)** > > Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolostone), which is composed of mineral dolomite (CaMg(CO3)2). They are usually classified based on texture and grain size. Importantly, carbonate rocks can exist as metamorphic and igneous rocks, too. When recrystallized carbonate rocks are metamorphosed, mar Ca... But carbonate rocks can’t be dated by carbon because the half-life of carbon-14 is so short. Carbonate rocks that formed millions of years ago won’t have usefully detectable levels of carbon-14. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [June 8, 2016, 7:01pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/8 "2016-06-08T19:01:20Z")
Since others have covered the main points, I’ll note that carbon dating depends on there being a more or less constant amount of carbon-14 in the atmosphere. I.e. that the natural decay of carbon 14 is balanced by its creation by solar radiation. But in the 1950s somebody set off a bunch of nuclear bombs that basically doubled the amount of C-14 in the atmosphere and it has yet to return to baseline. This means that in the future, things that died from about the 1950s to the 2050s will seem “younger” than they actually are because they actually started with more C-14. It also makes it possible to identify when various[living](http://www.smithsonianmag.com/smart-news/nuclear-bombs-made-it-possible-to-carbon-date-human-tissue-20074710/?no-ist) tissues were formed. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [June 8, 2016, 7:11pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/9 "2016-06-08T19:11:49Z")
Also keep in mind that this is dependent upon atmospheric carbon, which explains why you get odd readings from, say, freshwater clamshells which get their carbon from dissolved carbonates in the water. ---
**Author:** ![sbunny8](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@sbunny8](https://boards.straightdope.com/u/sbunny8)\ **Post date:** [June 8, 2016, 7:49pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/10 "2016-06-08T19:49:46Z")
> [@leahcim](#): > > I’ll note that carbon dating depends on there being a more or less constant amount of carbon-14 in the atmosphere. Not quite. Carbon dating merely depends on knowing what the carbon-14 to carbon-12 ratios have been in the past. If the ratio has fluctuated, but we know how it has fluctuated, we can take that into account when doing the calculations. [Radiocarbon calibration - Wikipedia](https://en.wikipedia.org/wiki/Calibration_of_radiocarbon_dates) ---
**Author:** ![glowacks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/glowacks/32/5548_2.png) [@glowacks](https://boards.straightdope.com/u/glowacks)\ **Post date:** [June 8, 2016, 11:00pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/11 "2016-06-08T23:00:29Z")
Yes, carbon-dating has to be calibrated somewhat, much like the distances to far-away galaxies. We take an object we have knowledge of the age of from another source and take a sample of carbon from that object and determine how much C-14 is left. For objects near to it in age, the assumption of constant proportions of atmospheric carbon isotopes will be closer to correct than it will be over longer periods. We can then string these observations together to form a calibration curve as linked to above. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [June 10, 2016, 5:00pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/12 "2016-06-10T17:00:44Z")
> [@engineer\_comp\_geek](#): > > Yeah, you can’t date the stones themselves. However, you can date organic material in the dirt that the stone sits on, which should get you pretty close to when the stone was put into place. But couldn’t that organic material have been embedded in that dirt for a lo-o-o-o-o-ng time before the stone was put there? ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [June 10, 2016, 5:13pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/13 "2016-06-10T17:13:42Z")
> [@CookingWithGas](#): > > But couldn’t that organic material have been embedded in that dirt for a lo-o-o-o-o-ng time before the stone was put there? Yes it could. Other problems could be roots digging in to your sample. There are all sorts of ways your sample could get contaminated and you have to be extremely careful or your sample is worthless. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [June 10, 2016, 5:33pm UTC](https://boards.straightdope.com/t/understanding-carbon-dating/756844/14 "2016-06-10T17:33:14Z")
> [@CookingWithGas](#): > > But couldn’t that organic material have been embedded in that dirt for a lo-o-o-o-o-ng time before the stone was put there? They are not just digging up dirt underneath the stones and sampling it at random. They are looking for organic material that skilled archaeologists can reliably assign to the time of construction. The original research all seems to be hidden behind paywalls, but this article describes the samples as > [@](#): > > …carbonised plant remains and bone in their trench… [https://www.newscientist.com/blogs/shortsharpscience/2008/09/was-stonehenge-an-ancient-cent.html](https://www.newscientist.com/blogs/shortsharpscience/2008/09/was-stonehenge-an-ancient-cent.html)