Thursday, April 9, 2015

How Far Back Can We Measure Dates? Part VI

Continuing from the previous five posts regarding radiocarbon dating techniques and how they have skewed our understanding of the past and its age, and more specifically continuing with the last post on the gaps found in tree-ring overlap dating and its effect on using dendrochronology (the method of dating wood by analyzing the growth ring pattern) to extend radiocarbon dating back to B.C. times.
    First of all, in temperate climates wood cells that are produced in the beginning of the growing season are larger and have thinner walls than the cells produced in the latter part of the growing season. The density difference between early and late growth produces visible features known as tree rings.
The darker circle is the thinner walls showing the early growth, with the lighter color between the later growth of a year
    To being with, and as has been pointed out earlier, using radioactive carbon to determine age is a complex process. The method is based on the slow disintegration of Carbon-14, and the less Carbon-14 present in a sample, the older it will date. To determine a date, one must have data concerning:
1. The present content of Carbon-14 in the specimen (determined as the ratio of isotope 14 to isotope 12 (C-14/C-12) or as the number of Carbon-14 atom transformations per second per gram of carbon;
2. The rate at which Carbon-14 spontaneously converts to Nitrogen-14;
3. The Carbon-14 content (C-14/C-12 ratio) at the beginning of the time period related to the age.
    At the best laboratories the Carbon- 14/Carbon-12 ratio can be determined to about one-thousandth of the value that characterizes contemporary plants and animals. The most recent determination of the spontaneous Carbon-14 conversion rate indicates that, within an uncertainty of about ± 30 years, in 5715 years half of an initial amount of Carbon-14 will have converted into Nitrogen-14. At this rate of conversion approximately 57,000 years would be required for the Carbon-14/Carbon-12 ratio to diminish one-thousand-fold. The initial Carbon-14/Carbon-12 ratio is not accessible to experimental determination, and must be assumed.
    Accordingly, any Carbon-14 age is based on an assumption.
The radioactive decay of Carbon-14 after 35,000 years leaves an amount that is almost impossible to measure and after 50,000 years non-existent for measuring
    The simplest calibration base for the initial Carbon-14 is the assumption that throughout all past time accessible to Carbon-14 dating, the Carbon-14/Carbon-12 ratio in the active carbon exchange system has been the same as it is at present. With this calibration base a specimen for which the Carbon-14/Carbon-12 ratio is 0.001 times that of corresponding contemporary material has a 57,000 year radiocarbon "age." Radiocarbon ages obtained in this simple, direct way may be classified as "radiocarbon isotope ages."
    However, there is good evidence that the proportion of Carbon-14 has varied over time, and a more reliable calibration base is the Carbon-14/Carbon-12 ratio found in artifacts that have a precise and accurate historical (calendric) age. A base established in this manner requires guessing by interpolation for Carbon-14/Carbon-12 ratios that fall between values that have been calibrated by historical dates. Also it is insecure for extrapolation beyond the oldest firmly established historical calibration points.
When comparing an Irish tree ring dating collection, which is not consistently known, gaps are filled in by switching to other tree ring chronologies, i.e., (red arrow) shows a gap between English collections and South English Roman collection between 200 and 400 A.D.; (blue arrow) shows a gap of about 100 years between the Belfast QUB data collection and the Late B.C. period collection 
    In the comparison of the above and below graphs—illustrating The NorthEnglishRoman and SouthEnglishRoman collections—a critical step (Southwark), a 500-year span with a 250 year overlap (with Teeshan), was used to bridge the gap; however, the QUB-material contains only one single stem marked Southward,” and the dating is not sure.
However, the 100 year gap (blue arrow in Table “A”) of the Belfast AD collection is linked to the Late BC collection through a series of six different collections (Garry, Dorsey, Corlea, Carlisle, Southward, and Teeshan), each of which has to have correct and exact overlapping tree rings; consequently, we are not overlapping 100 years, but an actual span of exact chronologies of some 600 years (13 B.C. to 581 A.D.)
    However, because of the 300-year span from 103 A.D. to 372 B.C., matching the Late B.C. chronology (Table “A”), it was considered a “good match” toward Late B.C. by its designer, Mike G.L. Baillie (“A Slice Through Time – Dendrochronology and precision dating,” 1995)
    Others, however, consider this chronology, with this dating, that BelfastLong overlaps LateBC by 316 years, is a very unsatisfactory correlation (corr. 0.17, TT 3.0). This means that some deeper analysis is required. Especially when we consider a one-year gap that unexpectedly occurred. That is, the Long chronology and LateBC only overlap via SwanCarr and that there is a one-year gap between the two meeting Garry Bog/Ballymacombs More collections, the collection in the Long chronology ending at -948 and the collection in LateBC starting at -946. We find -946 as the oldest ring in our Ballymacombs4 (of LateBC), but -836 instead of -948 as the youngest ring of our Ballymacombs3 (of BelfastLong).
    Certainly, it is not difficult to see the inherent problems associated with trying to overlap tree ring dating and why those who create these chronologies are very secretive about what they do and how they do it. The problem lies in the fact that Dendrochronology has a serious organizational problem that impedes its development as a scientific discipline and tends to compromise its results. This is the problem of proprietary data. When a person or organization has made a reference curve, then in many cases they will not publish it, but keep it as an in-house trade secret and offer their paid services as dendrochronologists, dendrochronology becomes a black box into which customers stick samples, and out of which dates come, but only the owner of the black box can evaluate the process going on inside. Or, and far more importantly, know whether or not the response is actually accurate. This is of course a deeply unscientific state of things. And regardless of the scientific issue, then it seems that since dendro reference curves are produced with public funding, they should be published on-line as a public resource.
    However, that is another matter.
    Yet, for older dates the most satisfactory calibration base is still considered to be the Carbon-14/Carbon-12 ratio of wood stated above that has been dated by dendrochronology (tree-ring dating).
(See the next post, “How Far Back Can We Measure Dates? Part VII,” to see how this patching and floating of tree-ring dates has uncovered a huge gap in the dating sequence of tree-ring in the Middle Ages)

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