by Neville Agnew
Senior Principal Project Specialist - Getty Conservation Institute

Our society is time-obsessed. Perhaps it has long been so. Some cultures answer a question about the age of the world, stars, and life with the simple response ‘forever’ which from the standpoint of the duration of the existence of life on our planet is a good enough one. It is, however, not satisfying to the modern mind which wants a time – scale to impose order on events of the physical world. The puzzle and debates about the age of the earth gained momentum in the late 18th century when scientific methods began to be used by eminent persons. No longer was Bishop Ussher’s date for the creation acceptable in the European world – a clock was needed. Archaeologists want to be able to date events ‘securely’ and this can usually be done by linking sequences in excavations to known dates, much like dendrochronology, the science of dating by means of trees using the annual growth rings in the wood. But if no written record exists, as is usual with archaeological sites and rock art, and when the cultures that made the art no longer survive there is an impasse. Rock art is not excavated, it is simply there on the rock face, without stratigraphic context, which appears to be a reason for its neglect as a worthy discipline by archaeologists.

Ingenuity was applied to attempt estimates of the age of the world. Geology provides a relative clock – sedimentary rocks offer a partial answer: younger ones lie above older, ‘all the way down,’ except when they have been overturned, even inverted; but this was a clue of sorts, not an answer. From the thickness of sedimentary deposits, sometimes a mile or more thick, it was clear, at least to geologists, that the earth must be vastly older than received wisdom stated it to be. It was clear too that sedimentary rocks are secondary ones, derived from older rocks by natural processes of weathering, erosion, and deposition. And under the sediments generally lie metamorphic and igneous rocks and they must be older yet. There was the problem also of fossils and the mystery of marine fossils, some species are still alive today like certain mollusks, found high in the Andes, as noted by Darwin.

To say that theories contended at the time is an understatement. In the 19th century Lord Kelvin (Fig. 1) lent his formidable intellect to the problem and had come up with the answer: initially, 20 million years, based on an estimated rate of cooling of the earth (before radioactivity was discovered and its heating effect on the core), and he stuck vigorously to his belief in the face of mounting evidence to the contrary while disparaging the geological evidence. In 1906, when radioactive experiments were reported that seemed to disprove his ideas he responded angrily in The Times, which drew a retort: ‘Lord Kelvin’s letter will of course receive respectful attention … but it is also known that his brilliantly original mind has not always submitted patiently to the task of assimilating the work of others by the process of reading’ (The Dating Game – One Man’s Search for the Age of the Earth by Cherry Lewis. 2000. Cambridge University Press, pp. 11-12). Other methods tried to use the salinity of the ocean based on the idea that originally there was no salt in the seas and that it accumulated over time as rivers leached soluble salts from the rocks.

The last decades of the 19th century surely were among the greatest ones in the history of science as they allowed the breakthroughs in physics that followed. Mendeleev had drawn up the periodic table of the elements which predicted correctly the existence of hitherto unknown elements – the true test of scientific validity; X-rays were discovered in 1895 and a year later radioactivity by Becquerel, and there was the clock, though it was not yet recognized as such. So rapid have been the advances that today it is astonishing that within 50 years of its discovery radioactivity would be used not only for dating of the past but also in the first true weapon of mass destruction. The key to radioactivity as a clock is the immutability of radioactive decay. No matter the state of chemical combination of the radioactive element, or its temperature and pressure, the clock ticks at the same rate characterized by its half-life, the time taken for half the atoms present to decay by radioactive emission yielding, usually, another element close to the parent’s position in the periodic table. Thus, uranium decays via a series of radioactive daughter elements to the element lead; carbon-14 to nitrogen-14; a radioactive isotope of potassium and its decay product argon comprise a pair suitable for dating events of the order of a few million years, while carbon-14 (half-life 5730 years) is well suited to dating archaeological materials up to an age of about 50,000 years, beyond which not enough remains to allow meaningful detection. Each radioactive element or isotope of that element has its own half-life; some extraordinarily brief, from fractions of a second, others to hundreds of million years. What it is within the atomic nucleus that dictates half-life is something of a mystery though this is not a concern for dating; suffice it to say half-lives of all the radioactive elements are accurately known. Geologists utilize the long half-life elements for dating rocks, and the age of the earth, while archaeologists have found carbon-14 essential for their purposes.

The discoverer of carbon-14, Martin Kamen, died aged 89 in 2002. Carbon-14 is an isotope of the predominant form of non-radioactive carbon (that is carbon-12) formed in the upper atmosphere by cosmic ray bombardment of nitrogen. As such, carbon-14 pervades all living organisms through photosynthesis and the food chain to achieve a steady state condition – so yes, we are all radioactive to some small degree. When organisms die they no longer assimilate carbon-14 and the clock starts to tick. Kamen’s biography Radiant Science, Dark Politics – A Memoir of the Nuclear Age recounts how he lost his academic position at the University of California during the communist witch hunts in the aftermath of World War II, when he was publicly labeled a suspected spy. Charges against him proved groundless and he later received the Enrico Fermi Award for his contributions to physics. The utility of carbon-14 for dating was quick to follow: by 1949 Willard Libby began the development of radiocarbon dating (Fig. 2). That history too is fascinating because the method was validated against organic materials of known date such as ancient Egyptian artefacts and annual tree rings.

From the perspective of archaeological dating carbon-14 initially had one serious drawback – sample size. Gram size quantities were required for the radioactivity to be accurately measured. When precious artefacts are to be dated few archaeologists or museum curators are willing to have amounts of this order taken, whether an Egyptian mummy, the Shroud of Turin, or a rock art painting with an organic binding medium. Today, AMS (accelerator mass spectrometry) carbon-14 dating is used. This requires a miniscule amount of sample since the mass spectrometer precisely counts the number of radioactive carbon-14 atoms present, not the gross radioactivity (Fig. 3).

One would imagine with the exquisite sensitivity and accuracy of the AMS method the problems of dating organic carbon, that is, carbon from formerly living sources, as distinct from ‘dead’ coal or graphite, would be completely settled. This is not the case, however. The problem now is one of contamination by living sources such as pollen spores, fungi, and bacteria all of which skew the results making the source material appear younger than it really is, while ‘dead’ carbon, if present, has the opposite effect. This problem has in turn been addressed by a variety of ingenious sample preparation procedures to purge the contaminants. These include plasma oxidation and a sophisticated statistical method.

Beyond the carbon-14 method, many other physical techniques have been tried, debated, contested and sometimes discarded or are still in the process of being evaluated. For rock art, these include optically stimulated luminescence (OSL), a proven technique for buried artefacts (Fig. 4); uranium-thorium dating, another radioactive method; and others less reliably accurate, but which may suffice to bracket the age of the art (Fig. 5). Stylistic dating has been extensively used too. This relates rock art of known date to similarly styled art elsewhere. It has been roundly denounced by some, but is not discredited – on the contrary, it is a useful qualitative technique: this looks like that rock art (of known date) and so is likely to be of similar date. When different scientific techniques are applied to rock art and yield similar ages one can have confidence in the validity of the dates.

Scientific dating continues to be an active area of research and application for rock art and in many other fields as well. One of the great names in the debate about the age of the earth was the geologist Arthur Holmes who early on adopted the radioactive clock provided by the uranium-lead series. He vindicated the geologists who had been thoroughly intimidated by Kelvin who had used thermodynamics and refused to consider the geological and biological evolutionary evidence. Though Kelvin was a great scientist and is rightly recognized for his work in thermodynamics by having the absolute temperature scale named after him, the Kelvin scale in which zero degrees Kelvin is some minus 273 degrees Celsius (our everyday temperature scale itself named after the Swedish astronomer of that name), dismissed geologists and thought they were ‘confused’. Geologists themselves, of course, are not always blameless having stalled for decades on the question of continental drift. Such is the nature of humanity and the ultimately self-correcting beauty of science – in Kamen’s phrase, ‘radiant science’.

So what is the age of the earth? It may come as no surprise that it is still not quite settled, but it is agreed to be about four and a half billion years – a significant expansion of the gap between Ussher’s 4004 BCE date and Kelvin’s 20 million. But then, of course, they used very different methods.

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