Kurioza naukowe / Scientific curiosities ISSN 1176-7545; rok VIII; No 1816
Jedno zdumienie dziennie...
kołowy 5.500 lat temu w Bronocicach.
stosowana metoda to tak zwane datowanie radiowęglowe.
Opracowana została przez amerykańskiego badacza
Willarda Libbyego (1908-1980) w połowie ubiegłego
wieku. Libby otrzymał za to nagrodę Nobla z chemii w
Wheeled traffic 5,000 years ago in Bronowice (2)
The method used today is the so-called ‘radioactive dating’. It was developed by the American scientist William Libby (1908-1980) in the middle of the last century. Libby was for this awarded the Nobel Prize in chemistry in 1960.
It is worthwhile to again outline the principles of the method. Carbon, found in the atmosphere as CO2, is in the form of 3 isotopes, i.e. atoms very similar physically and indistinguishably chemically: 12C, 13C and 14C. Isotope 14Cis radioactive, it disintegrates with statistical regularity emitting weak beta particles. All radioactive isotopes have a strictly defined half-life period (T1/2). Radioactive carbon 14C is created in the upper atmosphere upon the bombardment of atoms of nitrogen 14N with cosmic rays, and is uniformly distributed by atmospheric currents throughout the atmosphere. It is relatively rare, it is thought that for every 14C atom there are 1012 atoms of 12C , but, despite this, it is easily identified due to its radioactivity. 14C is created at the rate of 1-2 atoms per second per 1 cm2 of earth’s surface, and because it is constantly subject to the process of disintegration, its concentration in the atmosphere remains constant. And so all living organisms contain proportionally as much radioactive carbon as is contained in the atmosphere, but only as long as life processes continue. After death plants cease to exchange gases, there is no influx of fresh carbon, and the accumulated carbon 14C disintegrates in accordance with its half-life period, which is 5,739 years. It must be noted that all carbon in animal organisms is derived from plants. The discovery in the mined material of reduced quantities of 14C allows us to calculate the moment when life functions ceased in the subject material. Of course, with time 14C will disappear almost completely, which sets out the limits of the application of the method. The method is applicable to the assessment of biological material no older than 60,000 years. This is a convenient period for anthropology. The matter would be straight-forward, if cosmic radiation were uniform over the entire past of the earth. This, unfortunately, had not been the case. The degree of irradiation depends on solar activity (sun spots), and so in estimating age one has to apply corections to this variable. There may also occur changes in the content of 14C as a result of volcanic activity or the movement of very old carbon accumulated in the seas (change in water temperature).
Here the so-called dendrochronology comes with help. It is known that most trees form annual growth rings, the thickness of which depends on the climatic conditions at the moment when these rings have been created. Counting the rings makes possible to determine the absolute age of the timber, and taking into consideration the interlocking profiles of growth rings of different ages one can create ‘dendrochronological chains’. Such chains are known which represent thousands of years. So in order to correct for the unevenness in the content of 14C in the atmosphere one takes samples of wood with rings of an accurately determined age, finds its radioactivity, and thus determine the content of 14C in the atmosphere in the subject period.
There has recently been a disturbance in the content of 14C in the atmosphere due to nuclear explosions and the mass burning of oil and coal, processes which change the 14C content in the atmosphere. This does not apply to anthropological specimens, in which an exchange of carbon with the atmosphere does not occur. Nevertheless this can give rise to amusing errors. Scientists had once obtained milk samples in order to determine the content of 14C in contemporary biological matter. A very careful analysis proved the milk to be over 1200 years old, but the cows were alive, and to the eye quite young! It turned out that they browsed near a freeway and the air was filled with non-radioactive carbon from the petrol fumes…
In self-respecting institutions occupied with
radioactive carbon dating the above factors are, as much
as possible, taken into account. Nevertheless there is
yet another factor which is overlooked and does not
allow itself to be taken into account, though it does
play a role in the above matter. This is the so-called
‘isotope effect’. Although carbon 14C
differs from 12C only very slightly, hardly two units of
atomic weight, this is enough to result in an
appreciable difference in the dynamics of involving the
isotopes in the chemical reactions. Carbon 12C,
being the lighter, is more inclined to take part in
chemical and physicochemical reactions. Because life
processes are an enormous chain of enzyme reactions, and
each of these is composed of many steps, what happens is
a fractionalising of the chemical compounds in the
metabolic process. The more steps in the chain of
reactions, the less 14C will be
found in the product. And thus the relatively closest to
atmospheric conditions is the carbon contained in
plants, the content then falls in herbivorous animals,
and falls further in carnivorous animals. The
differences are very minute, but sufficient to determine
the position in the food-chain (13C
is also suitable in this research). But this makes for a certain difficulty in
age determination of organic
The age of the bone material found in the pit in which were discovered the rest of the vase was determined in the renowned specialist workshop in Groningen in Holland, where they certainly would have taken into account all the then factors contributing to the accuracy of the analysis (1993). According to the analysis the Bronowice vase is over 5,500 years old, expressing this in the manner accepted in radiocarbon dating: 3637-3373, median 3520 BCE, year 1993.