Crossdating: From Principle to Technique

Crossdating

Crossdating 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 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.

Crossdating Skeleton plotting 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.

Crossdating Skeleton plotting 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.).

Crossdating 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 weights are measured vertically here.

Crossdating 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

Crossdating 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.”

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

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

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

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

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. Cabin rings Reference chronology 1700 1750 1800 1850 1900 1950

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. Cabin rings Cutting date of 1820 Reference chronology 1700 1750 1800 1850 1900 1950

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

And there’s your match! Your 61 rings date from 1853 to 1913! 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 there’s your match! Your 61 rings date from 1853 to 1913!

Check your crossdating skills Check your crossdating skills! What’s 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!