What makes carbon 14

His results perfectly matched the known dates of the items he had scanned. It was a brilliant undertaking for which Libby was awarded the Nobel prize for chemistry in , though he was lucky in one sense.

Libby assumed that the rate of carbon production in the atmosphere had been constant for the past few tens of thousands of years. In fact, it has varied fairly widely, thanks to changes in sunspot activity, atmospheric nuclear bomb tests and rising emissions of carbon dioxide from fossil fuels.

These have to be taken carefully into account when estimating ages, scientists now realise, though the underlying basis of radiocarbon dating remains sound. More recently, radiocarbon dating has changed from simply measuring the radioactivity emitted by carbon nuclei to directly counting numbers of atoms of the isotope in a sample.

This is done using a technique called accelerator mass spectrometry AMS , which has allowed scientists to date bones, artefacts and other carbon-based items from the tiniest sample. One example involving the use of carbon resulted in the overturning of the idea that past western European cultures had depended on practices and ideas that began in the Middle East and slowly disseminated westwards with the spread of farming.

Radiocarbon dating revealed a very different picture and showed that the neolithic cultures of Britain, France and central Europe must have evolved independently. Later, the technique was used by laboratories in Britain, Switzerland and the United States to date the flax used to weave the Turin shroud. This cloth, marked with the negative image of a bearded man, was believed by some to be the burial shroud in which Jesus was wrapped after crucifixion.

Using only a few fragments of cloth, scientists dated it to AD. Over the years, uses of carbon have spread well beyond dating ancient artefacts. Drugs can be labelled with carbon and followed as they pass through the body in order to test their safety and efficacy.

Other researchers have used the isotope to trace the way in which plants convert carbon dioxide into sugar, revealing the intricate processes underpinning photosynthesis. In addition, carbon has been exploited to study plankton and other forms of sea life, revealing how the waters of the oceans circulate in a great interconnected web of currents that sweep round the planet.

For good measure, carbon is now playing a major role in uncovering how climates have changed on Earth over tens of thousands of years, work of immense importance as scientists struggle to understand how rising carbon emissions are now triggering dangerous global heating.

Certainly, it is hard to exaggerate the impact it has had on science. Yet its discoverers, Kamen and Ruben, both fared badly in the wake of their breakthrough. Intermediate levels of 14 C can represent either mixtures of modern and dead carbon or carbon that was fixed from the atmosphere less than 50, years ago. Signals of this kind are often used by chemists studying natural environments.

A hydrocarbon found in beach sediments, for example, might derive from an oil spill or from waxes produced by plants. If isotopic analyses show that the hydrocarbon contains 14 C at atmospheric levels, it's from a plant. If it contains no 14 C, it's from an oil spill. If it contains some intermediate level, it's from a mixture of both sources. What is Carbon Dating? How does Radiocarbon work? Scientific American Editor Michael Moyer explains the process of radiocarbon dating.

The Radiocarbon Collaborative. See Renfrew for more details. So, creationists who complain about double rings in their attempts to disprove C dating are actually grasping at straws. If the Flood of Noah occurred around BC, as some creationists claim, then all the bristlecone pines would have to be less than five thousand years old. This would mean that eighty-two hundred years worth of tree rings had to form in five thousand years, which would mean that one-third of all the bristlecone pine rings would have to be extra rings.

Creationists are forced into accepting such outlandish conclusions as these in order to jam the facts of nature into the time frame upon which their "scientific" creation model is based. Question: Creationist Thomas G. Barnes has claimed that the earth's magnetic field is decaying exponentially with a half-life of fourteen hundred years. Not only does he consider this proof that the earth can be no older than ten thousand years but he also points out that a greater magnetic strength in the past would reduce C dates.

Now if the magnetic field several thousand years ago was indeed many times stronger than it is today, there would have been less cosmic radiation entering the atmosphere back then and less C would have been produced. Therefore, any C dates taken from objects of that time period would be too high.

How do you answer him? Answer: Like Cook, Barnes looks at only part of the evidence. What he ignores is the great body of archaeological and geological data showing that the strength of the magnetic field has been fluctuating up and down for thousands of years and that it has reversed polarity many times in the geological past.

So, when Barnes extrapolates ten thousand years into the past, he concludes that the magnetic field was nineteen times stronger in BC than it is today, when, actually, it was only half as intense then as now.

This means that radiocarbon ages of objects from that time period will be too young, just as we saw from the bristlecone pine evidence. Question: But how does one know that the magnetic field has fluctuated and reversed polarity?

Aren't these just excuses scientists give in order to neutralize Barnes's claims? Answer: The evidence for fluctuations and reversals of the magnetic field is quite solid. Bucha, a Czech geophysicist, has used archaeological artifacts made of baked clay to determine the strength of the earth's magnetic field when they were manufactured.

He found that the earth's magnetic field was 1. See Bailey, Renfrew, and Encyclopedia Britannica for details. In other words, it rose in intensity from 0. Even before the bristlecone pine calibration of C dating was worked out by Ferguson, Bucha predicted that this change in the magnetic field would make radiocarbon dates too young.

This idea [that the fluctuating magnetic field affects influx of cosmic rays, which in turn affects C formation rates] has been taken up by the Czech geophysicist, V. Bucha, who has been able to determine, using samples of baked clay from archeological sites, what the intensity of the earth's magnetic field was at the time in question. Even before the tree-ring calibration data were available to them, he and the archeologist, Evzen Neustupny, were able to suggest how much this would affect the radiocarbon dates.

Renfrew, p. There is a good correlation between the strength of the earth's magnetic field as determined by Bucha and the deviation of the atmospheric radiocarbon concentration from its normal value as indicated by the tree-ring radiocarbon work. As for the question of polarity reversals, plate tectonics can teach us much. It is a fact that new oceanic crust continually forms at the mid-oceanic ridges and spreads away from those ridges in opposite directions.

When lava at the ridges hardens, it keeps a trace of the magnetism of the earth's magnetic field. Therefore, every time the magnetic field reverses itself, bands of paleomagnetism of reversed polarity show up on the ocean floor alternated with bands of normal polarity.

These bands are thousands of kilometers long, they vary in width, they lie parallel, and the bands on either side of any given ridge form mirror images of each other. Thus it can be demonstrated that the magnetic field of the earth has reversed itself dozens of times throughout earth history.

Barnes, writing in , ought to have known better than to quote the gropings and guesses of authors of the early sixties in an effort to debunk magnetic reversals. Before plate tectonics and continental drift became established in the mid-sixties, the known evidence for magnetic reversals was rather scanty, and geophysicists often tried to invent ingenious mechanisms with which to account for this evidence rather than believe in magnetic reversals.

However, by , sea floor spreading and magnetic reversals had been documented to the satisfaction of almost the entire scientific community. Yet, instead of seriously attempting to rebut them with up-to-date evidence, Barnes merely quoted the old guesses of authors who wrote before the facts were known. But, in spite of Barnes, paleomagnetism on the sea floor conclusively proves that the magnetic field of the earth oscillates in waves and even reverses itself on occasion.

It has not been decaying exponentially as Barnes maintains. Answer: Yes. When we know the age of a sample through archaeology or historical sources, the C method as corrected by bristlecone pines agrees with the age within the known margin of error. For instance, Egyptian artifacts can be dated both historically and by radiocarbon, and the results agree.

At first, archaeologists used to complain that the C method must be wrong, because it conflicted with well-established archaeological dates; but, as Renfrew has detailed, the archaeological dates were often based on false assumptions. The method does not count beta particles but the number of carbon atoms present in the sample and the proportion of the isotopes. Not all materials can be radiocarbon dated.

Most, if not all, organic compounds can be dated. Samples that have been radiocarbon dated since the inception of the method include charcoal , wood , twigs, seeds , bones , shells , leather , peat , lake mud, soil , hair, pottery , pollen , wall paintings, corals, blood residues, fabrics , paper or parchment, resins, and water , among others. Physical and chemical pretreatments are done on these materials to remove possible contaminants before they are analyzed for their radiocarbon content.

The radiocarbon age of a certain sample of unknown age can be determined by measuring its carbon 14 content and comparing the result to the carbon 14 activity in modern and background samples.

The principal modern standard used by radiocarbon dating labs was the Oxalic Acid I obtained from the National Institute of Standards and Technology in Maryland. This oxalic acid came from sugar beets in When the stocks of Oxalic Acid I were almost fully consumed, another standard was made from a crop of French beet molasses.

Over the years, other secondary radiocarbon standards have been made. Radiocarbon activity of materials in the background is also determined to remove its contribution from results obtained during a sample analysis.