Ways to build chronology (decreasing resolution)
LAYERS
- trees
- stalagmites
- ice cores
- Lake sediment (n.b. Datum)
RADIOMETRIC DATING
- C
- U-Th
- K-At
COSMOGENIC NUCLIDES
Living part of tree =
Cambium
How is radiocarbon produced?
Cosmic ray-derives neutrons bombard N atoms
What affects radiocarbon?
Solar wind = deflects cosmic rays
Which increases with sunspots
So active sun = less radiocarbon
The cycle of radiocarbon
Plants <> atmosphere
Animals <> food
Die = 14C beta decays to 14N
Problems with radiocarbon
HALF LIFE
- 5,700 years
- Max dateable 50,000 years
CALIBRATION CURVE
- b/c varies with solar activity (can’t solve equation)
- slow changes in geomagnetic field = gradual increase in 14C production rate
RESERVOIR EFFECTS - corals/stalagmites - add dead C with 14C = appears older - dead anthropogenic C (dilutes 14C) “Corrected for reservoir of x years, ... calendar years”
BOMB SPIKE
Partial Test Band Treaty 10/10/1963
Before present =
Before 1950
Secular equilibrium =
Activity ratios of any isotopes in decay chain = 1
Ancient samples
Youngest
More than 500ka
Furthest from secular equilibrium (1)
U-Th principles
Th is insoluble
U6+ is soluble
Stalagmites/corals/shells contain no Th = daughter deficiency dating
Radioactive daughter = use activity
Positives of U-Th
Date carbonates up to 500,000 years B.P.
In calendar years without calibration curve
Problems with U-Th
Detrital Th (232Th) May contaminate 230/232 ratio
Elements for Cosmogenic include surface exposure dating
10Be
26Al
26Cl
10Be
1.5 am
O/mg/si/fe spallation
Quartz/olivine/magnetite
26Al
0.7ma
Si/Al/Fe spallation
Quartz/olivine
36Cl
0.3ma
Ca/K spallation
35Cl neutron capture
Whole rock
Problems with Cosmogenic
Different surfaces bombarded different rates
Erratics
Shielded by ice
