DENDROCHRONOLOGY:
annual layers of wood

History: A.E. Douglas, astronomer, founder of tree-ring science and University of Arizona Tree-ring Laboratory. Interested in sunspots and climate
  • 1901 rediscovered CROSSDATING, Living trees, small rings 21 rings inside bark, but in stump small, small rings 11 in from bark (tree cut 10 yr ago). (1737 Fr. Duhamel and Buffon frost-ring for 1709)
  • 1914 Carnegie Inst. Wash. funded dating beams from archeological site, Aztec NM. Developed 100 yr "Floating Chronology." National Geographic Soc. grant to develope master chronology
  • 1922 Sequoia Chronology shows 22-yr sunspot cycle.
  • 1929 Charred Beam HH-39 tied in Floating Chronology.
  • 1937 Laboratory of Tree Ring Research established at UA
Wood Morphology
  • Bark, Phloem, Cambium, Xylem
  • Late Wood: dense, dark; deposited in late summer/fall
  • Early Wood: large cells and thin walls, spring


Principles
  1. species must have clearly defined annual rings
  2. species must have Circuit Uniformity
  3. rings must vary from year to year
Techniques:
  1. Tree Coring, stumps, slabs
  2. Ring Counting: often false rings or missing rings in one tree
      precision (repeated counting)
      accuracy (true date)
  3. Crossdating: process of matching rings of trees in an area based on patterns of ring widths produced by regional climate. More accurate age than ring counting Methods of Crossdating:
    1. sample with increment borer, core glued to board
    2. count rings under dissecting scope
    3. skeleton plotting: graphical technique for matching
      • plot small rings as wide lines on paper
      • paste graph paper strips together and pattern match
    4. prepare local chronology by matching trees
    5. standardization: de-trending individual growth curves
    6. calibaration and verification: producing a climatic history
    7. combine local curves to produce regional chronology
Calibaration and Verification


Experimental Design
    sensitivity: variability in ring width (better crossdating)
    complacency: lack of variability
COMPLACENT SENSITIVE


World's Oldest Living Things

Tree Ring Growth Model

    Rt = f(Gt, Ct, D1t, D2t, Et)
      Gt = Age-related growth trend
      Ct = Climate-related growth
        Dendroclimatology: precipitation, temperature, atm. pressure, runoff. e.g., Large forest fires in AZ and NM after dry springs associated with El Niño events. Swetnam and Betancourt (1990)
      D1t = Disturbance-ralated growth due to local factors within the stand
        Dendroecology: pollution insects (e.g., tussock moth, spruce budworm), air pollution
      D2t = Disturbance-ralated growth due to regional factors outside the stand
        Dendrochronology Dating fires, floods, earthquakes, volcanism, erosion & deposition rates.
          Dating of archeological sites: date when tree died that was used for construction beams = "vegas." Chaco Canyon main construction period AD 1030 - 1100, spruce and fir transported > 75 km (Betancourt et al., 1986)
      Et = "Error" Variations due to factors other than Gt, Ct, D1t, D2t,
Application (Ct)


Radiocarbon Dating of Tree Rings:

Radiocarbon dating assumes a constant ratio of 14C/12C in the atmosphere. Dating of tree-rings shows that this ratio has fluctuated through time. U-Th dates on Barbados Corals corroborate the treering chronology and extend it to 30K, max deviation, 3.5 K too young, about 20 K (Bard et al., 1990)
Radiocarbon Dating of 20-yr, 10-yr and 1-yr ring intervals

North America
    Bristlecone Pine
    Giant Sequoia
    Douglas Fir
Europe: (Ireland, Scotland, England, Germany)
    Oaks and pines in Bogs
"Delta C fourteen"
Deviations expressed as difference between measured 14C and amount that would result in 1:1 radiocarbon-calendar age:
    Δ 14C ‰



0-100 ya. 14C age too old (subtract yrs from 14C date)
100-700 ya. 14C age too young (add years to 14C date)
700-2500 ya. 14C age too old (subtract years from 14C date)
2500-8000 ya. 14C age too young (add years to 14C date)

Variations in the ratio of 14C/12C
    Rc = f(A, I, S, V)
  • Atomic Bomb tests doubled 14C concentration in atm.
  • Industrial Revolution has increased 12C concentration in atm. by 1/3
  • Solar variability
  • Variations in Earth's Geomagnetic Field


READINGS

HOMEWORK

Links:
Henri's Ultimate Tree Ring Pages
Tree Ring Glossary