How Far Back Can We Measure Dates? Part VII

Continuing from the previous six posts regarding radiocarbon dating techniques and how they have skewed our understanding of the past and its ages, and more specifically continuing with the last post about the tree ring dating of dendrochronology to extend radiocarbon dating back to B.C. times. 
    First of all, for many years it was assumed that the content of Carbon-14 in the atmosphere was constant. We now know that the Earth and solar magnetic fields are changing over time. This means that the flux of cosmic rays impinging on the atmosphere varies, and therefore so does the rate of Carbon-14 it produces. That makes it necessary to calibrate the Carbon-14 dates according to other techniques. One such technique is the dendrochronology, or tree-ring dating (dendros, Greek for tree, chronos, for time).

Left: In a tree cross section, the first year of growth is the inside or central ring; Right: 1) First year of growth (center ring), the latest ring is the one nearest the outer bark; 2) Rainy season (not the thicker or wider ring); 3) Dry season (narrower ring); and 4) Dark Area is a scar from a forest fire
    These rings result from the change in growth speed through the seasons of the year, with each ring usually, but not always, marking the passage of one year in the life of the tree. This technique works best in temperate (moderate) climates where four seasons can be noted, as opposed to the tropics, typically between 40º and about 60 or 70º. It is also important to note that through tree ring dating, one can only date back a few hundred years as very old trees are rare.
    Variation in the width of these rings results from year-by-year variation in the conditions favorable to growth of a particular portion of a tree. By assuming that a similar variation in the pattern of ring thickness between samples represents growth during the same period of time, the ring-width patterns of many wood specimens can be combined into a single master dendrochronological sequence that 1) has an average growth-ring width variation pattern for periods of overlapping growth, and 2) extends the time range beyond the time span of any one component.

Comparing wood cross sections from different trees, looking for ring overlaps. While it sounds easy enough, the process is very complex and highly guarded and extremely difficult to verify
    Extension of the time range is accomplished by matching an upper portion of the ring-width sequence in one specimen with the lower portion of another specimen. The Bristlecone Pine master dendrochronological sequence that has been foundational for Carbon-14 calibration has been based on 81 living-wood and 118 dead-wood specimens from California.

These specimens (bristlecone pine, lodgepole pine, Jeffrey pine and Ponderosa, were found in the White Mountains, which is a fault block mountain range facing the Sierra Nevada Range across the upper Owens Valley, running from Mono Lake in the north along the Owens River to  Crowley Lake, Bishop, and south to Owens Lake. Bristlecone pine, of course, is the oldest known living tree in the world, at 5063 years old.
    This basic pattern for dendrochronological calibration of Carbon-14 age was set by C. W. Ferguson in 1969 (“A 7,104-year annual tree ring chronology for bristlecone pine,” Pinus aristata, from the White Mountains, California. 1969 Tree-Ring Bulletin 29(3-4), pp3-29).
    A calibration that falls within a time span that has been established by wood specimens that have been dated by unquestioned historical records (usually by cross-referencing Carbon-14 ages) can be relied on to give a high precision estimate of real time. But because of the uncertainty in matching a wood specimen against a master sequence only on the basis of growth-ring patterns, there is uncertainty regarding the validity of a master tree-ring sequence in a range that has been extrapolated beyond an unquestioned historical reference point.
    The process of switching from Carbon-14 dating sequences to dendrochronology for verification purposes is fraught with difficulties and errors, yet scientists correct  the raw data from radiocarbon dating determinations so as to give what they consider to be a more accurate real-time age. This is necessary because of the uncertainty about the original concentration of carbon-14, which must be assumed to calculate a radiocarbon age. In order to determine what real-time age should be associated with a radiocarbon age, the radiocarbon data are often compared to historical and tree-ring data that are considered to be more reliable indicators of time. Tree-ring data are especially important in the correction process for dates older than 1000 BC. Extensive lists of correlation between radiocarbon data and tree-ring data have been published.

Left: Unnamed Bristlecone pine dated to 5064 years old; Right: Methuselah Bristlecone pine dated to 4845 years old in 2013
    However there is a problem. It appears that the tree-ring chronology that has been established to adjust the raw carbon-14 determinations is a fragile structure. Our oldest living trees are only about 5000 years old. The oldest (left) dates to 5064 years old, remains unnamed, and is located in the White Mountains of California, near to what had been the oldest, Methuselah (right) that was been dated to 4845 years old. Another Bristlecone pine, named Prometheus was cut down in 1964, judged to have been between 4900 and 5000 years old (the count, taken several times, is still disputed).
    For objects or specimens beyond these days (about 3000 B.C.) are corrects often based on attempts to match the thickness variations of tree rings in old wood samples. If a similar pattern of variation in tree-ring thickness is found in two pieces of wood, the two are assumed to have grown at the same time. By comparing many pieces of wood and combining matches, tree-ring chronologies of over 11,000 years extent have been proposed for use in correcting carbon-14 dates. The reliability of the system is dependent on the correctness of the tree-ring matches, — and here there is considerable uncertainty. Statistical tests show that it is easy to get significant matches of tree-ring patterns at various juxtapositions between samples of wood. More sophisticated statistical tests are being developed to correct for this problem. However, these tests were not used when the original dendrochronological correction scheme for carbon-14 dates was established. It appears that this original scheme is subject to reevaluation.

Lowell Observatory on Mars Hill in Flagstaff, Arizona
    To understand this system, we need to know a little about it. First of all, Andrew Ellicott Douglass, an American astronomer, discovered the process of tree-ring dating in 1894 at the age of 27 while working at the Lowell Observatory as Percival Lowell’s chief assistant. This discipline became known as dendrochronology, the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Lowell and Douglass teamed up regarding this process, but they clashed several times over Douglass’ opinion that Lowell used data selectively and thus unscientifically and inaccurately to prove his theories. Lowell eventually lost patience with Douglass and sacked him for his opinion in 1901. Dendrochronology was later assimilated into the Earth Sciences where it now specializes in Lacunar Amnesia (Ignoring the gaps).
    Dendrochronology has three main areas of application:
1. Paleoecology, which involves the study of fossil organisms and their associated remains that are then used to interpret their life cycle, living interactions, natural environment, communities, and manner of death and burial (paleoenvironment, most prominently climate);
2. Archaeology and the history of art and architecture, where it is used to date old panel paintings on wood, buildings, etc.;
3. Radiocarbon dating, where it is used to calibrate radiocarbon ages, particularly in conjunction with Carbon-14 dating.
(See the next post, “How Far Back Can We Measure Dates? Part VIII,” to see how this patching and floating of tree-ring dates has uncovered a huge gap in the dating sequence of tree-ring sequences in the Middle Ages)