Friday, March 27, 2015

How Old is Old? – Part II

Continuing with the understanding of radiocarbon dating (Carbon-14), and the time clock Willard F. Libby (left) invented to read the ages of the past used constantly by archaeologists and anthropologists in determining the age of past civilizations.
    Once again, to understand how the radiocarbon dating time clock works, and its affect on our undertanding of the age of various ruins found throughout the Americas, cosmic rays enter the earth's atmosphere in large numbers every day and when one collides with an atom in the atmosphere, it can create a secondary cosmic ray in the form of an energetic neutron. When these energetic neutrons collide with a nitrogen-14 (seven protons, seven neutrons) atom it turns into a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons). Since Nitrogen gas makes up about 78 percent of the Earth's air, by volume, a considerable amount of Carbon-14 is produced. The carbon-14 atoms combine with oxygen to form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis. Animals and people take in carbon-14 by eating the plants and/or the animals.
Red Arrow: When cosmic rays bombard the earth’s atmosphere, they produce neutrons. Green Arrow: These excited neutrons then collide with nitrogen atoms in the atmosphere, changing them into radioactive carbon-14 atoms; Blue Arrows: Plants absorb this carbon-14 during photosynthesis. When animals eat the plants, the carbon-14 enters their bodies. The carbon-14 in their bodies breaks down to nitrogen-14 and escapes at the same rate as new carbon-14 is added. So the level of carbon-14 remains stable; Yellow Arrow: When an animal dies the carbon-14 continues to break down to nitrogen-14 and escapes, while no new carbon-14 is added. By comparing the surviving amount of carbon-14 to the original amount, scientists can calculate how long ago the animal died
    It is also interesting to note that between 1945 and 1947, Libby decided to pursue the radiocarbon dating project in secret, though he broadcast the subject of his ultimate goal during a Christmas party in 1946, which astounded one of his colleagues, James Arnold, who had been working under Libby’s direction isolating the first millicurie of reactor-produced Carbon-14.
    It should also be noted that at this time Carbon-14 had not yet been isolated in nature, yet Libby’s now announced plan was to date archaeological history with an isotope of carbon. However, when his pre-mature announcement was given, it upset the archaeological world who wanted to know why the physicist was getting involved in archaeology and trying to tell them how to date their finds. By his own admission, Libby stated: “I have no competence in the field of archaeology.” This led to a two-year series of diverse encounters between the physicists and the humanists (the latter being the American archaeologist’s  intellectual orientation). That is, it described the problems in cross-discipline communication, i.e., communicating to archaeologists the concepts of chemistry and physics involved with Carbon-14.
    So, for the next two years, from 1947 to 1949, Libby, Anderson and Arnold worked more closely with archaeologists to help them better understand the dating process Libby wanted to create and how that would benefit archaeology. This involved the creation of the “Supper Conferences,” a dinner held every two weeks by the Viking Fund, a supplier of funds for risk-involvement research. These dinners drew anthropologists on the Eastern Seaboard to attend cocktails and dinner for informal discussions—an innovative way for conveying novel ideas across disciplines in an informal mode of conversation.
    It was explained that once equilibrium in the atmosphere reached equilibrium (about 40,000 to 60,000 years after creation (however, after 50,000 years, there is so little Carbon-14 left in any specimen that it is very hard, almost impossible, to calculate its age beyond that point, so a figure of 50,000 years is usually given as the end date), then no more carbon would enter the system after death. That, then, would allow the scientist to determine how old it was at death by measuring the half-life cycle (the standard way of expressing the decay rate was called “the half-life”), so in knowing how much Carbon-14 element was left in the artifact, it could be determined how many “half-life” cycles had passed and date the artifact within a reasonable time frame—say, to 500 B.C., plus or minus 150 years (meaning the artifact died between 650 B.C. and 350 B.C.
Looking at the blue line only, we can see how many of the 100 Carbon-14 atoms remain during 10 half-life cycles ending at 57,300 years. As an example, 75% of Carbon-14 is gone after two half-life cycles, or 11,460 years
    It was also explained that this dating was possible since after plants and animals perish, they no longer replace molecules damaged by radiocarbon decay. Instead, the radiocarbon atoms in their bodies slowly decay away, so the ratio of carbon-14 atoms to regular carbon atoms will steadily decrease over time. This is based, of course, on the Earth having achieved equilibrium, i.e., being at least 50,000 years old.
    But what if it was not that old? What if during its life, a plant or animal was not yet exchanging carbon with its surroundings and its carbon did not have the same proportion of Carbobn-14 as the bioshphere and the carbon exchange reservoir? What if there was still Carbon-14 entering the atmosphere and building up? How then could the clock be set to equilibrium?
The Carbon Exchange Reservoir. It is based on the belief that the Earth is older than 50,000 years and that equilibrium of Carbon-14 had already built up in the atmosphere 
    But if Carbon-14 had not reached Equilibrium in the atmosphere, that would mean that the Earth was less than 50,000 years old!
    To test his theory and the “clock” he had developed, Libby took several Egyptian artifacts that were already datable by other techniques and tested them. A committee of advisers consisting of Donald Collier, Richard Foster Flint, Frederick Johnson, and Froelich Rainey was appointed to select the samples for use and to help collect them. These distinguished gentlemen worked hard for several years, assisting and collecting the samples and advising Libby and his team. The research in the development of the dating technique consisted of two stages—the historical and the prehistorical epochs. As Libby said in his 1960 Nobel Lecture: “The first shock Dr. Arnold and I had was when our advisers informed us that history extended back only to 5,000 years. We had thought initially that we would be able to get samples all along the curve back to 30,000 years, put the points in, and then our work would be finished.”
    Libby knew that in reading statements in books that such and such a society or archeological site was 20,000 years old, he was surprised to learn that these ancient dates were not really known dates, but numbers the archeologists came up with. “In fact,” Libby added , “it is at about the time of the First Dynasty in Egypt [3200 B.C.] that the first historical date of any real certainty has been established. So we had, in the initial stages, the opportunity to check against knowns, principally Egyptian artifacts, and in the second stage we had to go into the great wilderness of prehistory to see whether there were elements of internal consistency which would lead one to believe that the method was sound or not.”
    What he found was even more of a shock!
(See the next post, “How Old is Old? – Part III,” to see how and why Libby’s clock was set to read the wrong time for radiocarbon dating and what impact that has on our understanding the past and the age of the Earth)

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