Carbon Dating & Archaeology

The RadioActive Clock

by Ashley M. Richter
August 17, 2009
Time is relative. Different cultures around the world record time in different fashions. According to the Gregorian calendar, it is the year 2009 AD. But according to the Hebrew calendar it is 5769. The Kaliyuga Hindu Calendar maintains it is 5110, the Islamic calendar 1430 and the Persian, 2630. Chances are, right now, you have a Gregorian calendar stuck to your wall. This calendar, with the months January through December, is a business standard used in many places round the world to define the year: one which hearkens back to Christian and Roman Imperial precedents. But other timekeeping methods exist and are still used in the modern world, circumventing the easy processing of dates and history between cultures.

Throughout history, time has been defined in a variety of ways: by everything from the current ruler, or empire, or not defined at all. For periods without a historic record, attempts have been made to categorize tool kits, pottery styles, and architectural forms into regional timelines. Some ill-fated attempts to define time even attempted to count backwards through the genealogies of the Bible, establishing a series of dates which remain a cause of confusion. These various chronologies and their inherent inconsistencies, known as ‘relative dates,’ are a constant series of hurdles in the quest of historians and archaeologists to record mankind’s existence on earth.

However, in the 1940s, the organization of time was transformed by the revelation of radiometric dating and the subsequent creation of a scientific chronology of humankind, known as ‘absolute dating’. Dates could be assigned based upon scientific evidence rather than on the inconsistent mathematics, historical comparisons and simulated typologies of artifacts that had previously regulated time. The most well known and oft used form of radiometric dating is radiocarbon dating. But how does radiocarbon dating actually work? It has helped define the ages of man in ways never thought possible and led the way for a vanguard of scientific techniques that have further defined time for humanity and beyond. But what does it actually do and how much can it tell us?

Radiocarbon dating is a side benefit of a naturally occurring scientific process. Living organisms absorb a proportional amount of radioactive carbon fourteen isotopes to what is constantly present in the earth’s atmosphere. When that organism dies, the carbon fourteen decays at a known exponential rate: making it possible to calculate the approximate time when the organism died based on how much carbon fourteen remains in a sample of the dead material. It can date a variety of materials, ranging from, but not necessarily limited to: bone, shell, charcoal, soft tissue, horn, teeth, ivory, hair, blood, wool, silk, leather, paper, parchment, insects, coral, metal if there is charcoal present in it, and sometimes dirt. Carbon dating assumes a variety of things about the natural world in order to work. Among other scholarly scientific suppositions, it assumes that the amount of carbon fourteen in the atmosphere has remained constant bar minor recent fluctuations due to the industrialization of the past few centuries and our impact on the environment. And also, rather importantly, the laws of radioactive decay hypothesize that once a living organism is dead, it no longer interacts with anything in its environment which would affect the speed of its radioactive decay.

With all the technical terms and mathematical physics equations taken out, carbon dating sounds pretty easy right? Wrong: it involves a complex process of collecting a useful sample, dating it properly, and calibrating the scientific dates to ones recognizable in the outside world. At the beginning of the process, it is important to remember that only certain materials can be tested using carbon dating, i.e. the remains of living organisms. Sites like Stonehenge, Chichén Itzá, and Rapa Nui, where the focus is on large stone monuments, cannot be dated unless corroborating evidence can be found to assign a possible date. Such was the case at these three sites, where wooden and pollen elements could be dated, providing a speculative chronology for the sites as a whole, but even these are subject to error and constant scrutiny by the academic community. The dates given for Rapa Nui attract particular debate as they have been used to establish the time of the migration to the island. With carbon dating the type of sample and the placement of it within the site are very important. Some samples might be degraded or out of context within the site: meaning a spurious date might be assigned. And samples must be collected carefully, as often they have been in stable environments prior to their unearthing by industrious archaeologists and may be easily degraded in the open air or attacked by moisture and sunlight once gathered. Old school radiocarbon dates used to be collected using Geiger counters to establish the amount of radiation they were emitting. But nowadays, once a sample is successfully collected there are several forms of mass spectrometers and other equipment which can more accurately measure the carbon fourteen level of a sample. However, a number of things can easily go wrong during this stage of the process and the labs that calculate radiocarbon dates are subject to constant scrutiny to ensure that they are up to par; but even so, samples sent to different labs often produce slightly various results.

It is when a sample is measured that the real complications begin: as the process to assign a meaningful date to the scientific chronology is rather erudite. The date assigned to a sample will initially be given as a raw b.p. or “before present.” This dates the sample to ‘X’ amount of years before present; the ‘present’ being set at the year 1950 on the Gregorian calendar. This date can then be calibrated with dendrochronology, sediment cores, and/or other dating methods to ensure maximum accuracy and account for discrepancies in the amount of carbon fourteen in the atmosphere over the past few centuries. The newly calibrated result is then given as a more absolute B.P. which can be correlated to whichever calendar system necessary (though is most often calibrated to the Gregorian B.C/A.D or B.C.E./C.E. dates (raw dates that are calibrated to the same calendar system are written without capital letters as b.c./ a.d. or b.c.e./c.e)). With every sample there is a margin of error. These are established by a variety of elements, including but not limited to: the quality of the sample, the quality of the lab, and the age of the sample. Younger samples have a larger margin of error than older samples. Often this margin of error is negligible in establishing a general chronology. But when a very precise timeline is needed, it can be very frustrating to have a margin of thirty years thwarting your efforts.

Regardless of these issues, carbon dating is still one of the most effective tools in the archaeologist’s kit. It has provided illumination where none was once thought possible. The historians of one hundred years ago could only dream of such a wonderful, albeit frightening atomic clock ticking away, helping to mark the passing of the years and the ages of man. No scientific technique is perfect, despite sometimes obsequious media coverage of their capabilities. But radiocarbon dating tries its best; and can often serve as a base for additional scientific techniques which can clarify results further. It is a vital part in the investigation and preservation of our past and a lovely bit of analysis to compliment digital records of monuments. It places the plants, animals, and people of yore into an understandable and verifiable context. And not just at individual sites, it places events and movements like the spread of agriculture or the rise of domesticated animals into a datable context. A context understood all over the world because it is broadcast on its own scientific timeline. A timeline which does not conflict with the separate ways of keeping time that humans have devised the world over.

Bowman, S. 1990. Interpreting the Past: Radiocarbon Dating. Berkeley: University of California Press.
Mook, W.G. & Waterbolk, H.T., 1985. Handbook for Archaeologists No. 3 Radiocarbon Dating. Strasbourg: European Science Foundation.
RadioCarbon: An International Journal of Cosmogenic Isotope Research
RadioCarbon Web Info
Renfrew, C. & Bahn, P. 2000. Archaeology: Theories, Methods, and Practices (3rd edition). London: Thames & Hudson.

Donate $5 to CyArk

Help CyArk preserve more sites by making a small donation.

The <a href="">Moai of Ahu Tongariki</a> on Rapa Nui would not be datable by radioactive metrics without corroborating organic materials which are used to provide associative dates for the stone statures.
The craters of Rapa Nui like <a href="">Rano Kau</a> pictured provided valuable carbon dating samples to create a chronology of the craftsmanship of Rapa Nui's famous statues.
Likewise, the <a href="">Red House</a> at Chichén Itzá would not have been datable were it not for corroborating organic materials in their framework which allowed dates to be assigned to the structures.
Related Projects:
Chichén Itzá
Rapa Nui