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Tree Rings

July 30, 2012

For the past thirty years, my family has been protecting the environment in one small way. Every year, we harvest our own Christmas tree from a local Christmas tree farm. How is this good for the environment? First, it eliminates the manufacture and transportation of the artificial tree we would buy in its place. The manufacture includes toxic and non-biodegradable materials, and the long distance transport (usually from China) involves a fossil fuel burn.

A natural tree sequesters carbon from the atmosphere while it grows and provides a habitat for wildlife. The tree we cut each year is replaced by another that's planted on the farm, so the process is sustainable. Sure, our tree is discarded afterwards, but its decomposition in a landfill takes a lot longer than the time it took to grow, so there really is a net carbon sequestration.

When our children were younger, we would save a disk-shaped slice from the tree stump as a record of the tree. We would count the growth rings,a method of finding the age of a tree, and we would look for wide rings and narrow rings indicative of better and worse growing seasons. We were practicing dendrochronology, the scientific method of dating using tree rings.

Examples of tree growth rings

Left image, a slice from the 1990 Gualtieri Christmas tree. The rings are hard to see after 22 years, but Christmas trees are generally eight years old when cut. Right image, growth rings of a tree in Pacific Spirit Regional Park, Vancouver, by Lawrence Murray, via Wikimedia Commons)

Since humans use a lot of wood, there's a record of climate embedded in many dwellings. There's even an inverse method, called dendroarchaeology, by which you can date a structure by looking at the growth rings in its wood. You just do a cross-correlation of the ring patterns in the wood with an historical record of growth rings, and out pops your date.

Tree growth rings have more information than what's present in their width. The 14C isotope of carbon holds clues to their age and environment. An increase in 14C concentration can be the result of a radiation event, such as a solar flare, or cosmic rays. One interesting example of such a 14C excess is reported in a recent article in Nature by scientists at the Solar-Terrestrial Environment Laboratory and the Center for Chronological Research, Nagoya University, Japan.[1-4]

The Japanese scientists measured the 14C concentration in the growth rings of cedar trees from 750 AD to 820 AD with an ultimate resolution of a single year. They found that 14C increased by a phenomenal 1.2% for the years 774-775, much larger than any affect caused by solar radiation. These results are consistent with coarser resolution measurements for North American and European trees.[1] Also in concurrence are 10Be measurements for Antarctic ice cores at the same time.[4]

14C is produced when cosmic rays transform 14N to 14C.[2] This reaction is fuel to a lesser extent by natural solar radiation, and this enables 14C dating, since the isotope is replenished continually. The annual variation in solar production of 14C is just 0.05%, or a twentieth of that seen for the 774-775 time period.[3-4]

The one problem with this observation, however, is that there are no dramatic signs, such as auroras, recorded in human history that mark this event. One scenario, a nearby supernova explosion, appears to be ruled out. Two supernovas, one in 1006, and the 1054 supernova that caused the Crab Nebula, were observed on Earth, and even these were too small to cause a spike in 14C.[2-3] Both of these supernovas were visible, even in daytime skies, for a period of weeks.[3]

Composite image of the Crab Nebula, NGC 1952

Composite image of the Crab Nebula, NGC 1952. This image combines visible, infrared and X-ray images by various NASA telescopes.

Image credits: NASA/ESA/J.Hester & A.Loll (optical); NASA/JPL-Caltech/R.Gehrz (Infrared); NASA/CXC/J.Hester (X-ray)

(Via Wikimedia Commons))

One possibility is that the source erupted in southern skies, and it was not observed; however, its remnant would have been detected by now.[2-3] Roger of Wendover, a thirteenth century English chronicler, wrote about events, such as signs in the sky after sunset (auroras), that might indicate a solar flare.[3-4] Daniel Baker, a space physicist at the Laboratory for Atmospheric and Space Physics of the University of Colorado (Boulder, Colorado) thinks that a solar flare is possibility;[2] while Igor Moskalenko, an astrophysicist at Stanford University, is quoted in the Daily Mail as saying, "I cannot imagine a single flare which would be so bright... It may be a series of weaker flares over the period of one to three years."[3]

In the midst of all this uncertainty is an hypothesis by Jonathon Allen, a biochemistry major at the University of California at Santa Cruz, who decided to Google historical references for those dates.[5-6] The historical record is sparse, but he did find a mention of an event for 744 in the Anglo-Saxon Chronicle, a history of England written in the ninth century. An online version of this Chronicle was available from the Avalon Project hosted by Yale University.[6]
"Her oðywde read Cristes mæl on heofenum æfter sunnan setlgange..."[7]

"This year also appeared in the heavens a red crucifix, after sunset..."[8]
The red color may indicate that the object is hidden behind interstellar dust clouds[5-6] Its appearance at sunset would put the object's location too near the Sun to be observed easily.[6]

Portion of the first page of the Anglo Saxon Peterborough Chronicle

Portion of the first page of the Anglo Saxon Peterborough Chronicle, c. 1150. This is one of a number of annals that are collectively known as the Anglo-Saxon Chronicle. (Via Wikimedia Commons))


  1. Fusa Miyake, Kentaro Nagaya, Kimiaki Masuda & Toshio Nakamura, "A signature of cosmic-ray increase in ad 774–775 from tree rings in Japan," Nature, vol. 486, no. 7402 (June 14, 2012), pp. 240-242.
  2. Richard A. Lovett, "Mysterious radiation burst recorded in tree rings, Nature News, June 3, 2012.
  3. Eddie Wrenn, "Mystery over the giant cosmic explosion of 774AD, which has left absolutely no trace - except deep within the bark of two cedar trees," Daily Mail (UK), June 4, 2012.
  4. Jessica Griggs, "Vast cosmic event leaves record in ancient trees," New Scientist, June 3, 2012.
  5. Eddie Wrenn, "Does a single report of a 'red cross in the skies' over Britain in AD774 prove a supernova exploded in the heavens?," Daily Mail (UK), June 29, 2012.
  6. Richard A. Lovett, Supernova Could Have Caused Mysterious 'Red Crucifix' in the Sky in A.D. 774," Scientific American, June 27, 2012.
  7. Tony Jebson, The Anglo-Saxon Chronicle We Site, December 5, 2006.
  8. The Anglo-Saxon Chronicle : Eighth Century, Lillian Goldman Law Library, Yale University

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