Tree ring dating is possible because

Calibration is not only done before an analysis but also on analytical results as in the case of radiocarbon dating —an analytical method that identifies the age of a material that once formed part of the biosphere by determining its carbon content and tracing its age by its radioactive decay. Carbon is a naturally occurring isotope of the element carbon. Results of carbon dating are reported in radiocarbon years, and calibration is needed to convert radiocarbon years into calendar years. It should be noted that a BP notation is also used in other dating techniques but is defined differently, as in the case of thermoluminescence dating wherein BP is defined as AD It is also worth noting that the half-life used in carbon dating calculations is years, the value worked out by chemist Willard Libby, and not the more accurate value of years, which is known as the Cambridge half-life. Although it is less accurate, the Libby half-life was retained to avoid inconsistencies or errors when comparing carbon test results that were produced before and after the Cambridge half-life was derived.
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Dendrochronology - Tree Ring Records of Climate Change

Dendrochronology is the formal term for tree-ring dating, the science that uses the growth rings of trees as a detailed record of climatic change in a region, as well as a way to approximate the date of construction for wooden objects of many types. As archaeological dating techniques go, dendrochronology is extremely precise: if the growth rings in a wooden object are preserved and can be tied into an existing chronology, researchers can determine the precise calendar year—and often season—the tree was cut down to make it. Radiocarbon dates which have been calibrated by comparison to dendrochronological records are designated by abbreviations such as cal BP, or calibrated years before the present. Tree-ring dating works because a tree grows larger—not just height but gains girth—in measurable rings each year in its lifetime.
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How Old Tree Rings And Ancient Wood Are Helping Rewrite History

Cornell archaeologists are rewriting history with the help of tree rings from year-old trees, wood found on ancient buildings and through analysis of the isotopes especially radiocarbon dating and chemistry they can find in that wood. By collecting thousands of years worth of overlapping tree rings, with each ring representing a tree's annual growth, the researchers have created long-term records in the eastern Mediterranean that allow them to precisely date such seminal milestones in history as when Hammurabi, "the law-giver," reigned, when the massive Santorini volcanic eruption occurred, and the timelines of the Bronze and Iron ages, as well as many more recent events. Dendrochronology is the science of comparing growth patterns in tree trunks to date past events or climate changes. Cornell's dendrochronology laboratory now holds more than 40, tree-ring samples, including many from the eastern Mediterranean. Trees of the same species from the same geographical area have fairly similar ring patterns, Manning said, because they are exposed to similar climatic conditions.
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This chronometric technique is the most precise dating tool available to archaeologists who work in areas where trees are particularly responsive to annual variations in precipitation, such as the American Southwest. Developed by astronomer A. Douglass in the s, dendrochronology—or tree-ring dating—involves matching the pattern of tree rings in archaeological wood samples to the pattern of tree rings in a sequence of overlapping samples extending back thousands of years.
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