Safety issues of carbon dating

Water in the primary coolant circuit of pressurised water reactors contains excess hydrogen that combines with oxygen from radiolysis. In this reducing environment, compounds such as methane CH 4 and ethane C 2 H 6 form. Most of the carbon released in a pressurised water reactor is in the form of alkanes. Various estimations indicate that the annual production rate for a light water reactor pressurised or boiling water reactor is between 0. The rest is released during reprocessing, or remains in the fuel cladding and is later disposed of as solid waste Garnier-Laplace et al.

Spent nuclear fuel 14 C is released during the dissolution step in reprocessing plants. Depending on the operating mode, these releases are continuous or discontinuous. Commissioning of the UP3 and UP plants at La Hague resulted in increased annual gaseous 14 C releases starting in the early s.

In , the gaseous releases of carbon at the site corresponded to 1. Carbon in fuel cladding is not released during dissolution and remains trapped. It is disposed of later as solid waste. At the Sellafield plant in the UK in , the 14 C gaseous releases reached 3. In research, carbon is widely used in carbonate form for isotopic labelling of molecules. The activities used are greater than 1 GBq. For example, carbon is used to study metabolic dysfunction related to diabetes and anaemia.

It can also be used as a marker to track the metabolism of new pharmaceutical molecules. More generally, carbon can be used to uncover new metabolic pathways, and to identify their normal functioning and any departures from it, e. It is assumed that all 14 C used for labelling molecules will be released into the atmosphere as CO 2. This estimation is based on the results of a US study.

In the terrestrial environment, the consensus relatively well supported by observations is that the specific activity, expressed in becquerels of 14 C per kilogram of total carbon, is constant in the environmental components and at equilibrium with the specific activity of atmospheric CO 2 Roussel-Debet et al. These activities have slowly decreased since then by less than 0. In aquatic environments, the specific activity of 14 C varies with its dilution in carbon substances, particularly carbonates from old sedimentary rocks lacking carbon Unlike the terrestrial environment, 14 C in freshwater ecosystems is not in equilibrium with atmospheric CO 2: Based on the specific activity and the total proportion of carbon in the various environmental matrices air, plants, animals and thus food products , the activity concentration for the 14 C in these matrices can be estimated Figure 2.

The more carbon the product contains sugars, oils, grains, etc. Carbon activity concentration range for food products. Carbon thus has the highest environmental activities amongst the radionuclides released from nuclear facilities. In milk and meat, this contamination is also significant although much less so, probably due to a feeding component outside the area influenced by the atmospheric releases.

The carbon addition around nuclear power plants atmospheric releases of 0. This low level is the result not only of low releases, but also of a clear predominance of releases in the form of methane CH 4 , which plants cannot assimilate. In rivers, the carbon released by nuclear power plants is diluted in the dissolved stable carbon from carbonates, which are found in sediment.

This significantly decreases the specific activity of carbon in physical components. For reasons that remain to be elucidated, fish do not seem to benefit from these dilution phenomena. Carbon in an environmental sample may be quantified by activity measurement or by atom counting. These two destructive techniques require converting the sample to CO 2 Maro et al.

The carbon contained in the test portion is transformed to carbon dioxide from which a sample is prepared for measurement by liquid scintillation AFNOR, Two sample preparation methods are mainly used: The sample is placed in a cellulose cone, which is inserted in a platinum filament.


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The entire unit is placed in a combustion chamber. Voltage applied to the ends of the filament in the presence of O 2 causes combustion of the sample.

Carbon - C

This mixture is eluted from the column by the scintillation liquid and then collected for measurement. The oxydiser allows to prepare several samples per day for counting. The test portions are generally less than 0. They must be rich enough in carbon to undergo a complete oxidation. Combustion yield must be determined on a reference sample labelled for 14 C. This reference sample must be as close as possible in nature and composition to the samples to be analysed.

The 14 C naturally contained in the combustion cone cellulose contributes to the increase in background and thus in higher measurement uncertainty. Background must thus be determined as precisely as possible. The expression of the sample's activity in Bq of 14 C per kg of carbon also requires measuring its elementary carbon content, generally by gas chromatography.

This uncertainty can, however, be reduced by increasing the test portions or by combining the measurements of several test portions from the same sample. The sample is burned in the presence of under pressure oxygen in a combustion bomb. The CO 2 produced is then reduced by a heated reaction with lithium to obtain lithium carbide Li 2 C 2 , the hydrolysis of which produces acetylene C 2 H 2 , which is trimerised by catalysis in benzene C 6 H 6.

The counting vial is prepared by weighing out synthesised benzene and scintillants. Spectroscopy-quality benzene is added if needed. The activity of the 14 C present in the vial is then measured using liquid scintillation. The test portions consist of 7 to 10 g of finely ground, dry sample. The chemical processing time for one sample is 3 days, 2 more days being necessary for counting.

This method is suitable for solid dry samples containing high carbon and for water matrices in the form of carbonate e. For water matrices, CO 2 is extracted from the sample by acid attack e. The rest of the protocol does not change. The analysis methods involving oxydiser or benzene synthesis are not well suited for carbon-poor matrices, such as soil and sediment. The carbon present in the sample is extracted in the form of ions. The carbon ions are accelerated, sorted by mass in a magnetic field which alters their trajectory.

They are then counted. After decarbonation and combustion of the sample, the CO 2 obtained is reduced by H 2 in the presence of powdered iron. Freon is used in cooling systems. Other metallic carbides have important uses as heat-resistants and metal cutters. Elemental carbon is of very low toxicity. Health hazard data presented here is based on exposures to carbon black, not elemental carbon. Chronic inhalation exposure to carbon black may result in temporary or permanent damage to lungs and heart. Pneumoconiosis has been found in workers engaged in the production of carbon black.

Skin conditions such as inflammation of the hair follicles, and oral mucosal lesions have also been reported from skin exposure. Carbon 14 is one of the radionuclides involved in atmospheric testing of nuclear weapons, which began in , with a US test, and ended in with a Chinese test. It is among the long-lived radionuclides that have produced and will continue to produce increased cancers risk for decades and centuries to come.

It also can cross the placenta, become organically bound in developing cells and hence endanger fetuses.

Carbon (C) - Chemical properties, Health and Environmental effects

Digestion consist of breaking these compounds down into molecules than can be adsorbed to the wall of the stomach or intestine. There they are trasported by the blood to sites where they are utilized or oxidised to release the energy they contain. Back to periodic table of elements. For more information on carbon's place in the environment, move to the carbon cycle.

Home Periodic table Elements Carbon. Carbon Carbon is unique in its chemical properties because it forms a number of components superior than the total addition of all the other elements in combination with each other. Carbon in the environment Carbon and its components are widely distributed in nature.

Health effects of carbon Elemental carbon is of very low toxicity. Some simple carbon compound can be very toxic, such as carbon monoxide CO or cyanide CN-. The latter may be explained away due to various mechanisms. Let us consider processes that could cause uranium and thorium to be incorporated into minerals with a high melting point.

I read that zircons absorb uranium, but not much lead. Thus they are used for U-Pb dating.

But many minerals take in a lot of uranium. It is also known that uranium is highly reactive.

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To me this suggests that it is eager to give up its 2 outer electrons. This would tend to produce compounds with a high dipole moment, with a positive charge on uranium and a negative charge on the other elements. This would in turn tend to produce a high melting point, since the atoms would attract one another electrostatically.

I'm guessing a little bit here. There are a number of uranium compounds with different melting points, and in general it seems that the ones with the highest melting points are more stable. I would suppose that in magma, due to reactions, most of the uranium would end up in the most stable compounds with the highest melting points. These would also tend to have high dipole moments. Now, this would also help the uranium to be incorporated into other minerals. The electric charge distribution would create an attraction between the uranium compound and a crystallizing mineral, enabling uranium to be incorporated.

But this would be less so for lead, which reacts less strongly, and probably is not incorporated so easily into minerals. So in the minerals crystallizing at the top of the magma, uranium would be taken in more than lead. These minerals would then fall to the bottom of the magma chamber and thus uranium at the top would be depleted.

It doesn't matter if these minerals are relatively lighter than others.

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The point is that they are heavier than the magma. Two kinds of magma and implications for radiometric dating It turns out that magma has two sources, ocean plates and material from the continents crustal rock. This fact has profound implications for radiometric dating. Mantle material is very low in uranium and thorium, having only 0.

The source of magma for volcanic activity is subducted oceanic plates. Subduction means that these plates are pushed under the continents by motions of the earth's crust. While oceanic plates are basaltic mafic originating from the mid-oceanic ridges due to partial melting of mantle rock, the material that is magma is a combination of oceanic plate material and continental sediments. Subducted oceanic plates begin to melt when they reach depths of about kilometers See Tarbuck, The Earth, p.

In other words, mantle is not the direct source of magma. Further, Faure explains that uraninite UO sub2 is a component of igneous rocks Faure, p. Uraninite is also known as pitchblende. According to plate tectonic theory, continental crust overrides oceanic crust when these plates collide because the continental crust is less dense than the ocean floor.

As the ocean floor sinks, it encounters increasing pressures and temperatures within the crust. Ultimately, the pressures and temperatures are so high that the rocks in the subducted oceanic crust melt. Once the rocks melt, a plume of molten material begins to rise in the crust. As the plume rises it melts and incorporates other crustal rocks. This rising body of magma is an open system with respect to the surrounding crustal rocks. It is possible that these physical processes have an impact on the determined radiometric age of the rock as it cools and crystallizes.

Time is not a direct measurement. The actual data are the ratios of parent and daughter isotopes present in the sample. Time is one of the values that can be determined from the slope of the line representing the distribution of the isotopes. Isotope distributions are determined by the chemical and physical factors governing a given magma chamber.

Rhyolites in Yellowstone N. Most genetic models for uranium deposits in sandstones in the U. Most of the uranium deposits in Wyoming are formed from uraniferous groundwaters derived from Precambrian granitic terranes. Uranium in the major uranium deposits in the San Juan basin of New Mexico is believed to have been derived from silicic volcanic ash from Jurassic island arcs at the edge of the continent.

From the above sources, we see that another factor influencing radiometric dates is the proportion of the magma that comes from subducted oceanic plates and the proportion that comes from crustal rock. Initially, we would expect most of it to come from subducted oceanic plates, which are uranium and thorium poor and maybe lead rich. Later, more of the crustal rock would be incorporated by melting into the magma, and thus the magma would be richer in uranium and thorium and poorer in lead.

So this factor would also make the age appear to become younger with time. There are two kinds of magma, and the crustal material which is enriched in uranium also tends to be lighter. For our topic on radiometric dating and fractional crystallization, there is nothing that would prevent uranium and thorium ores from crystallizing within the upper, lighter portion of the magma chamber and descending to the lower boundaries of the sialic portion.

The upper portion of the sialic magma would be cooler since its in contact with continental rock, and the high melting point of UO sub 2 uranium dioxide, the common form in granite: The same kind of fractional crystallization would be true of non-granitic melts. I think we can build a strong case for fictitious ages in magmatic rocks as a result of fractional cystallization and geochemical processes.

As we have seen, we cannot ignore geochemical effects while we consider geophysical effects. Sialic granitic and mafic basaltic magma are separated from each other, with uranium and thorium chemically predestined to reside mainly in sialic magma and less in mafic rock. Here is yet another mechanism that can cause trouble for radiometric dating: As lava rises through the crust, it will heat up surrounding rock. Lead has a low melting point, so it will melt early and enter the magma. This will cause an apparent large age.

Uranium has a much higher melting point. It will enter later, probably due to melting of materials in which it is embedded. This will tend to lower the ages. Mechanisms that can create isochrons giving meaningless ages: Geologists attempt to estimate the initial concentration of daughter product by a clever device called an isochron.

Let me make some general comments about isochrons. The idea of isochrons is that one has a parent element, P, a daughter element, D, and another isotope, N, of the daughter that is not generated by decay. One would assume that initially, the concentration of N and D in different locations are proportional, since their chemical properties are very similar.

Note that this assumption implies a thorough mixing and melting of the magma, which would also mix in the parent substances as well. Then we require some process to preferentially concentrate the parent substances in certain places. Radioactive decay would generate a concentration of D proportional to P. By taking enough measurements of the concentrations of P, D, and N, we can solve for c1 and c2, and from c1 we can determine the radiometric age of the sample. Otherwise, the system is degenerate. Thus we need to have an uneven distribution of D relative to N at the start.

If these ratios are observed to obey such a linear relationship in a series of rocks, then an age can be computed from them. The bigger c1 is, the older the rock is. That is, the more daughter product relative to parent product, the greater the age. Thus we have the same general situation as with simiple parent-to-daughter computations, more daughter product implies an older age.

This is a very clever idea. However, there are some problems with it. First, in order to have a meaningful isochron, it is necessary to have an unusual chain of events. Initially, one has to have a uniform ratio of lead isotopes in the magma. Usually the concentration of uranium and thorium varies in different places in rock.

This will, over the assumed millions of years, produce uneven concentrations of lead isotopes. To even this out, one has to have a thorough mixing of the magma. Even this is problematical, unless the magma is very hot, and no external material enters. Now, after the magma is thoroughly mixed, the uranium and thorium will also be thoroughly mixed. What has to happen next to get an isochron is that the uranium or thorium has to concentrate relative to the lead isotopes, more in some places than others.

So this implies some kind of chemical fractionation. Then the system has to remain closed for a long time. This chemical fractionation will most likely arise by some minerals incorporating more or less uranium or thorium relative to lead. Anyway, to me it seems unlikely that this chain of events would occur. Another problem with isochrons is that they can occur by mixing and other processes that result in isochrons yielding meaningless ages. Sometimes, according to Faure, what seems to be an isochron is actually a mixing line, a leftover from differentiation in the magma.

Fractionation followed by mixing can create isochrons giving too old ages, without any fractionation of daughter isotopes taking place. To get an isochron with a false age, all you need is 1 too much daughter element, due to some kind of fractionation and 2 mixing of this with something else that fractionated differently. Since fractionation and mixing are so common, we should expect to find isochrons often. How they correlate with the expected ages of their geologic period is an interesting question.

There are at least some outstanding anomalies. Faure states that chemical fractionation produces "fictitious isochrons whose slopes have no time significance. As an example, he uses Pliocene to Recent lava flows and from lava flows in historical times to illustrate the problem. He says, these flows should have slopes approaching zero less than 1 million years , but they instead appear to be much older million years. Steve Austin has found lava rocks on the Uinkeret Plateau at Grand Canyon with fictitious isochrons dating at 1.

Then a mixing of A and B will have the same fixed concentration of N everywhere, but the amount of D will be proportional to the amount of P. This produces an isochron yielding the same age as sample A. This is a reasonable scenario, since N is a non-radiogenic isotope not produced by decay such as lead , and it can be assumed to have similar concentrations in many magmas.

Magma from the ocean floor has little U and little U and probably little lead byproducts lead and lead Magma from melted continental material probably has more of both U and U and lead and lead Thus we can get an isochron by mixing, that has the age of the younger-looking continental crust. The age will not even depend on how much crust is incorporated, as long as it is non-zero. However, if the crust is enriched in lead or impoverished in uranium before the mixing, then the age of the isochron will be increased.

If the reverse happens before mixing, the age of the isochron will be decreased. Any process that enriches or impoverishes part of the magma in lead or uranium before such a mixing will have a similar effect. So all of the scenarios given before can also yield spurious isochrons. I hope that this discussion will dispel the idea that there is something magical about isochrons that prevents spurious dates from being obtained by enrichment or depletion of parent or daughter elements as one would expect by common sense reasoning.

So all the mechanisms mentioned earlier are capable of producing isochrons with ages that are too old, or that decrease rapidly with time. The conclusion is the same, radiometric dating is in trouble. I now describe this mixing in more detail.

Suppose P p is the concentration of parent at a point p in a rock. The point p specifies x,y, and z co-ordinates. Let D p be the concentration of daughter at the point p.

Let N p be the concentration of some non-radiogenic not generated by radioactive decay isotope of D at point p. Suppose this rock is obtained by mixing of two other rocks, A and B. Suppose that A has a for the sake of argument, uniform concentration of P1 of parent, D1 of daughter, and N1 of non-radiogenic isotope of the daughter. Thus P1, D1, and N1 are numbers between 0 and 1 whose sum adds to less than 1.


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Suppose B has concentrations P2, D2, and N2. Let r p be the fraction of A at any given point p in the mixture. So the usual methods for augmenting and depleting parent and daughter substances still work to influence the age of this isochron. More daughter product means an older age, and less daughter product relative to parent means a younger age. In fact, more is true. Any isochron whatever with a positive age and a constant concentration of N can be constructed by such a mixing.

It is only necessary to choose r p and P1, N1, and N2 so as to make P p and D p agree with the observed values, and there is enough freedom to do this. Anyway, to sum up, there are many processes that can produce a rock or magma A having a spurious parent-to-daughter ratio. Then from mixing, one can produce an isochron having a spurious age. This shows that computed radiometric ages, even isochrons, do not have any necessary relation to true geologic ages. Mixing can produce isochrons giving false ages.

But anyway, let's suppose we only consider isochrons for which mixing cannot be detected. How do their ages agree with the assumed ages of their geologic periods? As far as I know, it's anyone's guess, but I'd appreciate more information on this. I believe that the same considerations apply to concordia and discordia, but am not as familiar with them.

It's interesting that isochrons depend on chemical fractionation for their validity. They assume that initially the magma was well mixed to assure an even concentration of lead isotopes, but that uranium or thorium were unevenly distributed initially. So this assumes at the start that chemical fractionation is operating. But these same chemical fractionation processes call radiometric dating into question. The relative concentrations of lead isotopes are measured in the vicinity of a rock.

The amount of radiogenic lead is measured by seeing how the lead in the rock differs in isotope composition from the lead around the rock. This is actually a good argument. But, is this test always done? How often is it done? And what does one mean by the vicinity of the rock? How big is a vicinity? One could say that some of the radiogenic lead has diffused into neighboring rocks, too. Some of the neighboring rocks may have uranium and thorium as well although this can be factored in in an isochron-type manner.

Furthermore, I believe that mixing can also invalidate this test, since it is essentially an isochron.

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