1/27 notesGroup 1: Meet at Tree-Ring Lab 9:30

(west stadium 104)Group 2: Meet at RadioCarbon Lab

9:30(Physics & Atmo Main Lobby)

• Start tree-ring crossdating activity today– Due next Thurs Feb 3

• Reading for dating lecture– E Haury: HH-39: Recollections of a

Dramatic Moment in Southwestern Archaeology

– Focus on “bridging the gap”

• "Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure. We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess.”Rasmus Nyerup, 1802

• Emil Haury: Recollections … (Reading)– “… descriptions of ruins, study of pots

and pans, and efforts to recreate ancient history were sterile without a valid sense of time.”

Dating Terms

• Precision

• Accuracy• Labels--

– 500BC– 2500b.p.– but AD 2008

Know what is being Dated !

• Dated Event– Tree-ring growth– Decay of element/death of organism– Movement of magnetic pole

• Target Event– Building construction– Site occupation– Last use of fireplace (hearth)

Dating Techiques:

• Relative– (before, after, earlier, later)– Stratigraphy (think layer cake; Grand

Canyon)– Seriation– Changes in style (think changes

in car bodies; video games, etc)

Both used extensively in Southwest—but must be “calibrated” against some known date

Dating Techniques (Pertinent to SW)• Absolute:

– Radiometric•Archaeomagnetic—burned things

500B.C.•14C for recent organic material:

35,000 bpboth– calibrated with

Dendrochronology—• Dendrochronology

• Trees, about the last 2000 years

•Crossdating: outside activity #1•Due Tuesday

Archaeomagnetic DatingHearths,Burned clay walls, etc. (not movable objects)

N Pole (geographic)

N Pole (Magnetic)

YearFrom: Michaels, J. 1973

Southwest

• Wood, charcoal• Cloth (Shroud of Turin, etc.)• Bone and antler• Peat, organic-bearing sediments• Marine and fresh-water shell• Carbonate deposits

– Caliche (CaCO3)

UA has a GREAT Radiocarbon lab

14C (Radiocarbon dating)Dates Organic Stuff

(not rocks)

Radiocarbon Dating

• 14C is formed in the upper atmosphere through the effect of cosmic ray neutrons upon 14N. The reaction is:

14N + n 14C + p(Where n is a neutron and p is a proton)

• The radiocarbon method is based on the rate of decay of the radioactive or unstable carbon isotope 14

• 12C: stable, 98.9% of all Carbon• 13C: stable, 1.1% of all Carbon• 14C: radioactive, ~1 x 10-10 % of all

Carbon

Carbon

Think ratios--- ½, ¼, etc.

• Variations atmospheric 14C production– Natural– over time due to solar– Post bomb

• Postdepositional contamination– percolating groundwater– incorporation of older or younger

carbon– contamination in field or

laboratory• Determination of half-life

– Now 5730 ± 40 years

14C Dating Errors

Andrew EllicottDouglass

• 1867 - 1962• Astronomy• 1894: Flagstaff

to build Lowell Observatory

• Sunspots a lifelong interest – impact on Earth climate?

• 1906: Tucson as UA Professor of astronomy, physics, geography, dean, acting president

• Founder of LTRR - 1938

• 1929: Natl. Geog. Magazine:– Secrets of the SW

Solved by Talkative Tree Rings

• 1904: observed a ring pattern in Flagstaff sawmill logs– 1904, 1902, 1899, 1896, 1894,

and 1891 were narrow • Compared, confirmed other

Flagstaff trees– oral history• Flagstaff chronology• 1911: Prescott trees confirmed the

pattern• Climate caused? Solar variation?

Douglass

Long Chronologies

• SW: 2000-3000 years• Bristlecone pine of West:

– Oldest tree known was 4900 years old– Chronology now at 8700 years

• Europe:– Oak chronologies at ~10,000 years– Composed of short segments

Another GREAT discovery at the UA

The 2nd Radiocarbon Revolution (1960s)

Outside Activity: Skeleton Plotting• Access through our course web pagehttp://www.ltrr.arizona.edu/skeletonplot/introcrossdate.htm• Read explanatory pages• Try skeleton plotting for yourself• When you succeed:

– alt-print screen– Edit-Paste (or Paste Special) into a word document– Fill in page with text:

• Insightful observations on crossdating• MUST HAVE: Your plot, Master plot, Answer box

(begin/end year), comments

• Due next Tuesday 2/2

Some Tips

• Your homework should look like this– paragraph here , blah, blah

Answer box

core

Your plotMaster Plot

WARNING: Easy to cheat, Easier to See if you cheated!!!

Steps: Plotting

Mark ONLY small rings on graph paperFrom bottom up

Steps: Looking at the Master

Align Your graph paper with the Master so Every Ring Matches