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The Technique of Crossdating. Crossdating Building a composite chronology with samples is possible if the crossdating between all samples is significantly.

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Presentation on theme: "The Technique of Crossdating. Crossdating Building a composite chronology with samples is possible if the crossdating between all samples is significantly."— Presentation transcript:

1 The Technique of Crossdating

2 Crossdating Building a composite chronology with samples is possible if the crossdating between all samples is significantly strong. Building a composite chronology with samples is possible if the crossdating between all samples is significantly strong.

3 Preparing samples for Crossdating and Measuring With the aid of a stereo microscope… Mark every tenth ring with a fine point pencil using a single dot (.). Every 50 years, use two dots (..). Every 100 years, use three (…). Every 1000 years gets ? The location of false rings, micro rings, and locally absent rings that may be easy to miss while measuring should also be noted on the sample. These problematic rings should be annotated using locator marks on the core mount itself and written instructions to the measurement technician. For example: 1585a for an absent 1585 ring; 1748m for a micro 1748 ring. On the core itself, a micro ring is marked using dots on either side of the ring. A locally absent ring is marked using offset dots. Preparing samples for Crossdating and Measuring With the aid of a stereo microscope… Mark every tenth ring with a fine point pencil using a single dot (.). Every 50 years, use two dots (..). Every 100 years, use three (…). Every 1000 years gets ? The location of false rings, micro rings, and locally absent rings that may be easy to miss while measuring should also be noted on the sample. These problematic rings should be annotated using locator marks on the core mount itself and written instructions to the measurement technician. For example: 1585a for an absent 1585 ring; 1748m for a micro 1748 ring. On the core itself, a micro ring is marked using dots on either side of the ring. A locally absent ring is marked using offset dots. Crossdating

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5 Direct-visual inspection When inspecting your cores, select the best 10 or more representative samples, free of exaggerated periods of suppressed growth or severe injury. These are called type cores. Visually compare the common period of growth under a microscope looking for similar patterns of small and large rings, intra-annual features, latewood variation, or any other consistently found annual markers. Once the first two are dated against each other, bring in another and continue until all the initial samples have been dated. By now, significant pointer years or features should be all accounted for and the remaining samples from the site can be dated. When inspecting your cores, select the best 10 or more representative samples, free of exaggerated periods of suppressed growth or severe injury. These are called type cores. Visually compare the common period of growth under a microscope looking for similar patterns of small and large rings, intra-annual features, latewood variation, or any other consistently found annual markers. Once the first two are dated against each other, bring in another and continue until all the initial samples have been dated. By now, significant pointer years or features should be all accounted for and the remaining samples from the site can be dated. Crossdating

6 This is a graphical technique that begins with the visual examination of a tree-ring sequence under the microscope. The dendrochronologist then selects the narrower rings out of a sequence (or window) of rings. Different dendrochronologists have different windows. A popular number of rings to inspect is a decade or 10 rings. Simply choose the narrower rings from that sequence. The dendrochronologist then assigns a weight between 1 and 10 for the narrow rings. The narrower the ring, the higher the weight. This is a graphical technique that begins with the visual examination of a tree-ring sequence under the microscope. The dendrochronologist then selects the narrower rings out of a sequence (or window) of rings. Different dendrochronologists have different windows. A popular number of rings to inspect is a decade or 10 rings. Simply choose the narrower rings from that sequence. The dendrochronologist then assigns a weight between 1 and 10 for the narrow rings. The narrower the ring, the higher the weight. Skeleton plotting Crossdating

7 Use this scheme: slightly narrow rings get weights of 1 to 3 narrow rings get weights of 4 to 6 very narrow rings get values of 7 to 9 a weight of 10 is reserved for extremely narrow rings a missing ring (the ultimate narrow ring) is also given a weight of 10. On graph paper, plot only the narrow rings by creating bold lines equal in length to the weight you assigned to them. A missing ring will get a line weight of 10 but the line should be dashed. In addition, the skeleton plot may be enhanced with the addition of any relevant intra-annual information (e.g., latewood width, latewood color, trauma cells, barrier zones, etc.). Use this scheme: slightly narrow rings get weights of 1 to 3 narrow rings get weights of 4 to 6 very narrow rings get values of 7 to 9 a weight of 10 is reserved for extremely narrow rings a missing ring (the ultimate narrow ring) is also given a weight of 10. On graph paper, plot only the narrow rings by creating bold lines equal in length to the weight you assigned to them. A missing ring will get a line weight of 10 but the line should be dashed. In addition, the skeleton plot may be enhanced with the addition of any relevant intra-annual information (e.g., latewood width, latewood color, trauma cells, barrier zones, etc.). Skeleton plotting Crossdating

8 The narrowest rings are connected (blue lines) with the longest weights in the skeleton plot (8). The widest ring is connected (green line) with a "b" mark. Note the red line which points to a ring of average width, but it seems narrow compared to the preceding ring. Perhaps it merits a small mark because of the large year-to-year difference The narrowest rings are connected (blue lines) with the longest weights in the skeleton plot (8). The widest ring is connected (green line) with a "b" mark. Note the red line which points to a ring of average width, but it seems narrow compared to the preceding ring. Perhaps it merits a small mark because of the large year-to-year difference Crossdating [ The weights are measured vertically here.

9 One advantage of using skeleton plots for crossdating is that the rate of growth of individual trees is essentially standardized. In the example above, the three trees have different growth rates. The same 20 years are shown for each. But, note that their skeleton plots all look the same. In other words, differences in growth rates of trees do not matter. One advantage of using skeleton plots for crossdating is that the rate of growth of individual trees is essentially standardized. In the example above, the three trees have different growth rates. The same 20 years are shown for each. But, note that their skeleton plots all look the same. In other words, differences in growth rates of trees do not matter. Crossdating

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11 Once a series is plotted, it can be compared to a skeleton plot created for a master or reference chronology for the area. The only difference in these master plots is that their lines go down below the horizontal zero line. Notice below that the same weight scheme is used. For example, the extremely narrow ring for 1919 got a weight of 10. Once a series is plotted, it can be compared to a skeleton plot created for a master or reference chronology for the area. The only difference in these master plots is that their lines go down below the horizontal zero line. Notice below that the same weight scheme is used. For example, the extremely narrow ring for 1919 got a weight of 10. Crossdating

12 - Here, we have three skeleton plots for three cores. Lets create a master by compositing the weights from these three cores…

13 - Now, how does this master composite plot compare to the middle series? Pretty good match!

14 - Now, how does this master composite plot compare to the top series? Another pretty good match!

15 Crossdating can also be used to extend a chronology back in time by overlapping older and older samples.

16 Cabin rings Reference chronology A very important application of crossdating is the absolute dating of wood taken from archaeological sites or remnant pieces of wood that can extend a chronology back in time.

17 Reference chronology Cabin rings Cutting date of 1820 A very important application of crossdating is the absolute dating of wood taken from archaeological sites or remnant pieces of wood that can extend a chronology back in time.

18 One of the best educational resources for learning how to create and use skeleton plots is Paul Sheppards Java-based crossdating web site. In the plot below, the top graph does not match the bottom graph. Key in on triple narrow pairs!

19 Ideally, you will move your test skeleton plot one ring at a time (either to the left or to the right) until you find a match, but key in on those obvious ring patterns (like the three pairs of narrow back-to-back rings)! And theres your match! Your 61 rings date from 1853 to 1913!

20 Check your crossdating skills! Whats wrong in the plot being tested below? You can see that these three narrow rings appear offset by a year. Why? Easy. The ring for the year 1689 is locally absent on your core!


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