Crossdating: From Principle to Technique

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Crossdating From Principle to Technique
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Crossdating
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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.

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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.

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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.

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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.).

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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.
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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.

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Crossdating
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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.

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

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• Now, how does this master composite plot compare
  to the middle series?

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

Another pretty good match!
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• Crossdating can also be used to extend a
  chronology back in time by overlapping older and
  older samples.

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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.
Reference chronology
1700 1750 1800
1850 1900 1950
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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.
Reference chronology
1700 1750 1800
1850 1900 1950
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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.
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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!
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Check your crossdating skills! Whats wrong in
the plot being tested below?