Tree annual ring dating principle, sampling method and analysis method

Dating is one of the important aspects of archaeological analysis. There are many methods of dating in the field of archaeology, and tree-ring dating is the most accurate method of dating, which can be accurate to the year and even to a certain season. Dendrochronology, also known as Tree–ring Dating, is a study of the tree-ring chronology. The scientific tree-ring chronology is the American astronomer Dr. Douglass in the 20th century. The initial research was established. He used the tree-ring dating method to determine the tree-rings of the remaining trees in the Indian ruins, and clarified the age of the site, so this method was established in the prehistoric chronology of the United States. Since the establishment of the scientific tree-ring chronology, the tree-ring chronology has made great progress. In establishing long-term chronology of annual rings, many countries have established chronologies of different lengths, including two long-term chronologies of annual rings, and one is a sample of wood unearthed from archaeological sites in the southwestern United States. The prehistoric chronology framework of a region has established the chronology of the Pinus aristata annual rings for tens of thousands of years, and the other is the establishment of an uninterrupted 1030 eucalyptus tree (Quercus) that can be extended throughout the Holocene. Annual ring chronology. The use of long-sequence annual ring chronology not only dates the remains of the Neolithic Age, but also stipulates the wood samples of ancient and ancient art, and corrects the 14C era, speculating on the age of past events, river diversion, speculation Past social economic and cultural conditions, settlement history and architectural history. In short, in the field of archaeology, tree-ring chronology has two main functions, one is to use the tree ring analysis to determine the age of past human cultural remains, and the other is to carry out past climate (including temperature, precipitation) and environment. Reconstruction and research. Therefore, in order to establish a long-sequence annual ring chronology as soon as possible, it is necessary to systematically introduce the principles, analysis methods and sampling methods of the tree-ring chronology, so that archaeologists can understand and master them in order to obtain the ideal wood. sample.

The principle of a tree-ring chronology <br> The formation of tree trunks has growth activities every year. In the spring, the layers of cells are split quickly, and the walls are thin and thin. The material is loose and light, called springwood. From summer to In autumn, the activity of forming layers gradually decreases, the cells divide and grow slowly, and the small wall thickness is dense and the color is called autumn material. The annual ring of the tree is the concentric circle of wood and densely spaced on the trunk cross section. The width of each annual ring includes the spring and autumn materials of the year. Most temperate tree species form an annual ring, so the number of annual rings indicates the age of the tree. The width of the annual rings is closely related to the climatic conditions of the corresponding growing year. In the dry years, the trees grow slowly and the annual rings are narrow. The annual ring is wide. Different individuals of the same species in the same climatic zone have the same pattern of width and width of the annual rings in the same period. If the annual ring map of the inner layer of a trunk is consistent with the annual pattern of the outer layer of another trunk, it means that the two have a common growth period, and the growth years can be connected to each other. If we use the existing standing trees or the tree samples of the known cutting age as the time base, and the earlier samples overlap with some of the annual ring maps, they can be connected, so that they can always be connected, and even can be connected to ancient times. So that you can build a long sequence of tree rings.
Once a long sequence of tree ring chronologies is established, the wood of the unknown age can be analyzed and dated. If a wood sample is taken from an archaeological remains, a tree wheel analysis of the wood sample is first performed to establish a tree wheel map of the wood sample, if the wood sample is the same as the wood species of the established synthetic annual chronology In the climatic zone, a unique coincidence position can be found by comparing the long-term principle with the long-sequence tree chronology to determine the absolute age of the wood sample.
In the warm and humid European region, there is basically no missing wheel in the tree wheel, and the above cross dating method is often used. In areas with dry and semi-arid climates, there are more rounds in the tree-rings, often using the American skeleton dating method, and making appropriate adjustments based on China's actual conditions.

The analysis method of the second tree-ring chronology (1) Cross-scheduled work procedure Before the establishment of the sample, all samples should be evaluated to further understand the direction, clarity, scarring, and disease of each sample annual ring. Etc., select the part of the normal growth, which is not only conducive to the determination and identification of the false annual ring, lost annual ring, accurate dating, not easy to make mistakes when measuring, and in the annual ring analysis, if there is doubt, it is easy to check back.
The cross-year work procedure is as follows:
The 1 year round mark will polish the sample from the medulla to the bark, draw a small dot with a mechanical pencil every 10 years, draw two small dots in the vertical direction every 50 years, and draw 3 in the vertical direction every 100 years. smaller.
2 The skeleton drawing is generally based on the cross-caling method of the Tree Ring Research Laboratory of the University of Arizona, which is the skeleton schematic method for dating the tree rings. The method takes the narrow wheel in the tree wheel width sequence as the "bone" of the sequence, and recognizes it on the coordinate paper in the form of the length of the vertical line. If the annual ring is relatively narrower than the annual rings adjacent to the two sides, the longer the vertical line marked on the corresponding year position of the coordinate paper, and the annual width of the average width is not marked, indicated by blank, extremely wide The annual rings are marked with the letter W. The narrow wheel profile identified on the coordinate paper in this way is considered to be the "skeleton" of the actual wheel width variation. Draw a skeleton diagram for each sample.
3 Comparison First compare the two tree cores on the same tree, whether the narrow wheels coincide. If the front part overlaps and the latter part does not overlap, then after moving one or several annual rings, the skeletons overlap. May be missing, return to the microscope to reconfirm. Once determined, compare it to another sample in the same way. Until the number of annual rings of all samples is accurate.
The determination of the 4th century is known for the core of the living tree. The age of the outermost ring is known. Because the previous steps are accurate, the growth year of each annual ring can be determined.
If the annual ring skeleton of the ancient wood sample overlaps with the annual ring skeleton of the modern sample, then the growth period of each annual ring can be determined.
The value here is that the final year of the sample is based on the anatomical features of the tree wheel. For example, if the sample core was collected in spring and summer of 2005, the tree has begun to grow. Under the microscope, the tube of the light-colored coniferous species or the broad-leaved tree species is seen between the last complete round and the bark. The last annual measurement was the year before the deforestation or the year before the sampling year, which is 2004. If the sample core was collected from the fall of 2005 to the winter, the trees have stopped growing. Under the microscope, there is only dark-colored late-maturing cells between the last complete round and the bark, indicating that the last annual ring measured is the age of tree felling or It is the age of sampling, that is, 2005.
For archaeological samples, it is important to know whether the outermost ring is the age of felling. Because you know the age of the felling, you can determine the age of the site.
Regrettably, the excavated wood in archaeological sites often rots on the surface. I don’t know how many annual rings have been lost. In this case, we can only say with certainty that after the trees die or are cut down in the outermost years. .
So how do you determine the age of the site?
The first case: If it is determined that the sample of the absolute age has bark, it can be based on the age of the outermost ring of the wood, at least the upper limit of the remains can be locked, that is, the age of the site is not earlier than the outermost wood. In the age of the annual ring, if the wood is used for the current cut, then the age of the remains can be determined.
Case 2: If most of the samples in a site end in the same year, it means that the outer xylem of these samples did not rot, because it is impossible for most samples to rot the same number of rings, which can also set the upper limit of this remains. If the wood is currently in use, then the age of the remains can be determined.
The third case: If a wormhole is found in the xylem outside the sample, it means that the outermost ring is close to the bark. Because the worms are generally new in the xylem and phloem. These worms erode the bark of a tree that has just been felled, a weak tree, or a tree that has died for other reasons. The annual ring depth of insect erosion is usually only a few annual rings, so their presence means that the outermost ring is close to the felling year. The age of the outermost ring of this sample is close to the age of the site.
The fourth situation: the sample has a part of the sapwood, according to the relationship between the sapwood and the heartwood to determine how many annual rings are lost, and estimate the age of the annual ring near the bark. The age of the outermost ring of this sample is estimated to be close to the age of the site.
The fifth situation: the sample only has the heart material. It is difficult to determine how many annual rings are lost in the xylem. The age of the outermost ring of this sample is quite different from the age of the site.
(2) After the measurement sample of the sample is determined, it is measured with a LINTAB tree wheel width measuring instrument manufactured by Frank Rinn, which has a measurement accuracy of 0.01 mm. Or measure the wheel width with a precision of 0.01 mm using the University Model 4 tree wheel width gauge manufactured in the United States.
(iii) Establishment of the tree ring chronology Before establishing the tree ring chronology, first use the COFECHA computer program dedicated to checking the sample or tree plate dating and wheel width measurements or the Gleichl?ufigkeit statistic check The accuracy of the tree wheel width value, using ARSTAN software to establish a tree wheel width index sequence.

Three Sampling Methods <br> The first step in tree wheel analysis is to obtain a suitable wood sample. Samples come from a wide range of sources, from the ages, to modern and ancient ones; in nature they may be artistic, archaeological, and fossilized. From the shape it may be a disc or a wedge or a strip or a tree core. In addition, it may be a dry sample or a wet sample.

(1) Sampling of live tree samples for live tree sampling, it is best to follow the following principles: selection is less affected by human influence; subject to a climatic factor, such as temperature or rainfall; growing in arid, semi-arid areas, poor growing conditions Forest margin wood and isolated wood; tree samples of alpine areas and high altitude forest ceilings.
In order to ensure minimal damage to the trees, the sample is sampled using a thinner (4.3 mm diameter) growth cone for sampling. The sample core is placed in a paper straw or plastic straw and marked with an oil-based pen on the straw. The advantage of the paper straw is that the moisture in the sample core can be sufficiently volatilized to avoid moldy mold core and protect the sample core. If the sample is placed in a plastic pipette, a number of small holes must be cut in the pipe wall to facilitate the evaporation of water from the core.
(2) Collection of ancient wood samples Any wood with an annual ring number of more than 100 rounds or even charcoal and fossil wood can be used as a sample for tree wheel analysis. For the ancient wood unearthed in the archaeological site, the tree tray is collected with a chain saw. For those who need to save the structure and are not easy to saw the tree plate, the wood core can be collected. The tree tray is the best tree round analysis sample. In general, take a 2~3cm thick tree tray. Special attention is paid to the fact that if the logs still have well-preserved bark or wood with intact sapwood, be sure to take care when sampling. Also, do not take the tree tray in the area where the trunk has side branches, because the annual rings are extremely irregular. . If the transportation is not convenient, the wedge-shaped sample can also be taken, that is, the annual ring comparison rule is selected on the transverse section of the trunk, and the annual ring is not too narrow, taking 1/4 or 1/8 of the original disk, because if the annual ring is too narrow, it is easy. Produce a discontinuous growth wheel or a broken wheel. Before taking the tree tray, you can use the hand-held magnifying glass to observe the quality of the sample. When observing the sample, the specimen is observed to face the light. If the surface is coated with water, use the refraction of light to make it clearer. At the same time, pay attention to the direction. The wood specimen must be near the side of the bark and close to the marrow. While approaching the side, the ray is perpendicular to the chest, not to look over, and not to observe the ray parallel to the chest. If the archaeological site has charcoal, the charcoal sample can be wrapped in cotton or immersed in a polyethylene glycol (carbon wax) solution. If the archaeological excavated wood is wet and soft, wrap it in plastic sheet or freeze it.
The larger the number of samples, the better, because in the cross dating, data deviation due to the false growth wheel and the discontinuous growth wheel can be excluded. Moreover, large sample sizes are also important for the final statistical analysis. However, taking a large number of samples inevitably requires a lot of manpower, material resources, and talent. Therefore, if a large number of logs are unearthed, it is best to sample on wood with obvious bark and sapwood, and the number of samples is 30. If less wood is unearthed, it is best to take one sample per wood or even two samples on one wood. Samples should be numbered and recorded while samples are being taken. The purpose of our sampling is not only to understand the age of the sample, but also to understand as much as possible about the environment at the time and its impact on people's lives. Therefore, the numbering and recording of the samples is crucial. The number of the sample is preferably the same as the number of the archaeological excavation. The sample record should record the sampling location, sample code, tree species, location of the sample in the site (including horizontal and vertical locations), sampler, sampling date, reason for sample discovery (excavation, construction, road), etc. You should also draw a plan of the sampling site, depict the spatial relationship between the sample and other objects, and infer the possible age of the sample based on the coexisting relics, preferably with a photo taken. In summary, provide as much information as possible about the sample in order to analyze the results of the tree-ring study. The sample registration form should be consistent with the record on the sample.
The above is a detailed description of the principle, analysis and sampling methods of the tree ring year. I hope to work with multidisciplinary experts to build a longer annual chronology. We should always remember that only by completing the tree-ring chronology that can be extended to prehistoric times, we can use its precise determination of the potential of the year in many aspects, dating the archaeological sites, and accurately knowing what people are doing in what time. To begin to understand the relationship between man and the natural environment, and to use the finely determined wood for isotope, elemental and other analysis to correct the 14C era.