History: A.E. Douglas, astronomer, founder of tree-ring science and University of
Arizona Tree-ring Laboratory. Interested in sunspots and climate
annual layers of wood
- 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
- Bark, Phloem, Cambium, Xylem
- Late Wood: dense, dark; deposited in late summer/fall
- Early Wood: large cells and thin walls, spring
- species must have clearly defined annual rings
- species must have Circuit Uniformity
- rings must vary from year to year
- Tree Coring, stumps, slabs
- Ring Counting: often false rings or
missing rings in one tree
precision (repeated counting)
accuracy (true date)
- 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:
- sample with increment borer, core glued to board
- count rings under dissecting scope
- skeleton plotting: graphical technique for matching
- plot small rings as wide lines on paper
- paste graph paper strips together and pattern match
- prepare local chronology by matching trees
- standardization: de-trending individual growth curves
- calibaration and verification: producing a climatic history
- combine local curves to produce regional chronology
|Calibaration and Verification|
sensitivity: variability in ring width (better crossdating)
complacency: lack of variability
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
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.
Et = "Error" Variations due to factors other than Gt, Ct, D1t, D2t,
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)
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
Europe: (Ireland, Scotland, England, Germany)
"Delta C fourteen"
Deviations expressed as difference between measured 14C
and amount that would result in 1:1 radiocarbon-calendar age: