(The isotopes of a chemical element are the various configurations of its atoms. There are three carbon isotopes in nature: 12C, 13C and 14C. These are three varieties of the same chemical element, carbon, whose nuclei contain the same number of protons (six), but a different number or neutrons (six, seven and eight, respectively). Thus, besides having the same chemical properties, the isotopes have different atomic mass: twelve, thirteen and fourteen, respectively). Almost 99% of atmospheric CO2, contains the less heavy carbon, 12C. A small part is somewhat heavier , 1.1% of CO2, since it contains 13C. Finally, in a very small proportion, there is also in the atmosphere a type of CO2 that contains 14C, which is radioactive and unstable and whose applications have typically being in paleochronology.)

Carbon–14

14C ,containing 6 protons and 8 neutrons, has the peculiarity that it is an unstable isotope and little by little mutates into nitrogen 14N, containing 7 protons and 7 neutrons, and disappears according to the reaction:

C = N + ß + neutrino

However, in the atmosphere this loss is compensated by new atoms of 14C that are formed continuouslly as a product of the collision between cosmic neutrons and other atmospheric nitrogen atoms:

neutron + N = C + H

These neutrons are part of the galactic or cosmic radiation that reaches the earth’s atmosphere after traveling across the solar system. The collision of cosmic rays with 14N atoms, and therefore, the production of 14C, is maximum at an altitude of 15 km. Rapidly, the 14C atoms formed in this manner are oxydated to CO2 , diffused and mixed throughout the atmosphere with the rest of CO2. The processes of disintegration and formation of 14C are compensated in a quasi (short temporal scale) equilibrium so that the concentration of 14C in the atmosphere is more or less constant.

Carbon-14 dating

Calculation of the lost 14C in dead organisms is used to date the fossils. Indeed, live plants absorb carbon from atmospheric CO2 during photosynthesis and expel it during respiration. In this form, the tissues of live plants – and those of live animals (humans included) that are fed by these plants – are continuously interchanging 14C with the atmosphere. This makes the ratio 14C/12C of carbon contained in organic tissue of living organisms similar to the ratio in the atmosphere. So, when the animal or plant dies, the interchange with the atmosphere stops and so does the replacement of carbon in its tissues. From this moment on the percent of 14C of organic dead matter starts to deplete, since it decays into 14N and is not replaced.

The mass of 14C of any fossil drops at an exponential rate, and the process is well studied. It is known that at 5,730 years of death of a living organism the quantity of 14C in its fossil remains has been reduced to half and at 57,300 years it is only 0.01% of the amount when alive.

Knowing the difference between the proportion of 14C that should contain a fossil if it were still alive (similar to that in the atmosphere at the moment of its death) and what it actually contains, it is possible to date its death.

The amount and percent of 14C is calculated by measuring the emission of b particles in the sample. The method is viable for not very old fossils (less than 60,000 years) since for older dates the emission of b particles are of too low intensity and difficult to measure, so significant error may exist.

In practice, the dating of fossils is complicated because the atmospheric concentration of 14C has varied substantially during time. This makes it necessary to know not only the amount of 14C remaining in the fossil find, but also the atmospheric concentration existing when the specimen was alive (figure).

The variations of 14C in the last 11,800 years are known with sufficient precision thanks to dendrochronology, that is, tree ring analysis and age recognition by counting of the rings. Beyond that time, the data are poor and imprecise and cannot be based on the study of fossil trees. Nevertheless, the period has been extended more recently to 50,000 years through the analysis of 14C contained in the laminar sediments of lake and ocean beds, as for example the bed of Cariaco Basin in Venezuela (figure), and up to 45,000 years on the basis of stalagmite in a submerged cave in the Bahamas.

During the last ten thousand years there has been a decline in 14C carbon concentration in the atmosphere due to a variation in the geomagnetic field that has strengthened the protective shield against the cosmic rays. With some fluctuation in the last 10,000 years, this reduction has been approximately 15% with respect to the 1950 levels.

But aside from this decline, in shorter time scales of centuries or less, the variations in the concentration of atmospheric 14C are due to other causes: 1) changes in solar activity and 2) variations in oceanic ventilation. Because of their climatological interest we discuss these two causes.

Carbon-14 and solar activity

The solar wind, linked to the intensity of solar radiation emission, intercepts part of the galactic cosmic radiation – responsible for the formation of 14C – before it reaches the earth. So, when in a fossil (or in the wood of a tree ring), whose age is known by other methods, an anomaly is found with respect to the percent of 14C that it should contain, it indicates that when this fossil was alive (or the tree ring grew) there could be an anomaly in the production of atmospheric 14C. Therefore, also in the intensity of the cosmic radiation reaching the earth at that time. The arrival of more or less cosmic rays depends inversely on the intensity of the solar wind that intercepts it. Therefore eventually, the anomalies detected in 14C can be attributed to the excursions in solar emissivity.

The eras where there was higher production of 14C correspond to times of less solar activity (and more incident cosmic radiation). Additionally, if an increase in Beryllium 10 is produced, an isotope of Beryllium also of cosmogenic origin, the hypothesis of reduced solar activity is reinforced. That is the case of the minima of Wolf, Sporer and Maunder occurring during the course of the last millennium (figure).

And the reverse, the eras of less production of 14C should be related to times of high solar activity. According to some paleoclimatologists, a long intense drought that occurred between 750 and 1025 of our age, and that coincides with a low production of 14C (and high solar activity) seen in the lake bed sediments of Yucatan originated the decline of the Mayan civilization (figure). It seems that those centuries were also

Carbon-14 and changes in oceanic ventilation

There can be important variations in atmospheric concentration of 14C if sea ventilation changes drastically. There is continuous interchange of CO2 between the atmosphere and the oceans. So, once the CO2 is absorbed by water and penetrates the ocean, it can remain trapped for centuries in it and therefore its carbon becomes poorer in 14C. In this manner the CO2 returned to the atmosphere in deep water upwelling processes contain carbon poor in 14C, which also results in the reduction of the concentration of 14C in atmospheric CO2.

When the exchange cycle of carbon between the atmosphere and the ocean is modified, so suffers the concentration of 14C, in the atmosphere as well as in the surface of the ocean. For example, at the beginning of the Younger Dryas, there was a strong increase in the concentration of atmospheric 14C, since the atmosphere stopped receiving 14C-depleted CO2 from the sea, as it had been receiving previously, during the warm Bølling – Allerød. With the advent of the new oceanic situation the Atlantic, ventilation was reduced, since the thermohaline circulation – as in the cold glacial times – had once more been weakened. This increase has also been documented on the surface of the sea thanks, for example, to the measurements of 14C performed in plankton fossil foraminifera conserved in the sedimentary layers of the marine bottom at Cariaco Basin in Venezuela.

The differences observed today between the age of dissolved carbon in tropical waters and the age of carbon in waters at high latitudes, could also give clues about variations in the oceanic circulation (figure). The apparent age (difference with respect to the atmosphere) of the Tropical and North Atlantic superficial water reservoirs, by analysis of their 14C/12C ratio, is of some 400 years, while at high latitudes of the North and South Pacific is of some 1200 years. This difference is caused by the type and intensity of oceanic thermohaline circulation existing today. So, through the study of 14C contained in calcareous fossil foraminifera collected at different regions, we can learn about changes that occurred in the ages of the different oceanic reservoirs (for example during the last deglaciation). In this manner we can reach conclusions about variations in the thermohaline circulation of that era.