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Absolute Dating
Shmulik Marco and Neta Wechsler
Relative dating
Relative dating
Biochronology (relative dating, limited to fossil record)
Every parent atom of a particular type has the same probability of decaying to a specified daughter atom.Decay only depends on structure of atomic nucleus - NOT on external conditions.If N = number of parent atoms at time "t"andN0 is number of parent atoms present at start
where is the DECAY CONSTANT which is constant for a particular parent-daughter system.
The basic principle
1
, 2ln
21 t
Rb-> beta decay->Sr
Sr has several isotopes: 84, 85, 86, 87, 88
Only 87Sr has a radiogenic source – 87Rb
87Rb87Sr
How much 87Sr was at the beginning and how much came from 87Rb?
][ 870
870
8787tt RbRbSrSr
life half age, t; 870
87 t
t eRbRb
tt
t
t
tt
Rbt
SrSr
yyt
et
eRbSrSr
870
8787
99
870
8787
)10 t, 1070( 1
]1[
086
87
086
087
086
87
Sr
Rbt
Sr
Sr
Sr
Sr tt
For this method, t1/2 = 47 Byr
Rb-Sr MethodRb-Sr MethodAn example from some of the oldest rocks on Earth. A suite of different samples from a group of granites and gneisses were analyzed. These had different concentrations of parent rubidium 87 and so generated different amounts of the daughter (strontium 87). The resulting data plot on a straight line, called an isochron. This means that all samples have seen the same amount of time. If different samples had formed at different times (a random collection of rocks on earth) then the data would not fall on a straight line (no single isochron).
086
87
086
087
086
87
Sr
Rbt
Sr
Sr
Sr
Sr tt 0
86
87
086
087
086
87
Sr
Rbt
Sr
Sr
Sr
Sr tt
Dating meteorites with the 87Rb 87Sr method – the age of the Solar system
K-> beta decay->Ar40K – 89% -> 40Ca, 11% -> 40Ar
For this method, t1/2 = 1.93Byr
cek
ycey
.
10
. , 10 11 1058.01096.4
K-> beta decay->ArParent against daughter concentrations. In this hypothetical example there are no daughter atoms present in the original rock but a certain amount of parent atoms. Consequently a line passing through this point to the origin has zero slope. After time a number of parent atoms have decayed and the number of daughters increase. Eventually there will be no parents and only daughters - the composition will plot on the daughter axis. Note therefore the gradient of line connecting any composition withthe origin increases
with time. Or - the gradient of the line is a function of time and also a function of the decay constant.
Carbon datingCarbon dating
Carbon has 3 isotopes:Carbon has 3 isotopes:
1212CC – stable – stable1313CC – stable – stable
1212C:C:1313C = 98.89 : 1.11C = 98.89 : 1.11
1414CC – radioactive – radioactive
Abundance: Abundance: %10 8
RadiocarbonRadiocarbon
Forms:Forms: in the upper atmospherein the upper atmosphere
Decays:Decays:
tt ½ ½ = 5730 yr= 5730 yr..
pCnN 1414
NC 1414
tt eCC
01414
21
2ln
t
Half life Half life timetime
Half life Half life timetime
MeasuredMeasured
ConstantConstant
CalculatedCalculated
??????
DatingDating
Dating methods:Dating methods:
** Conventional wayConventional way
** Accelerator mass spectrometry (AMS) Accelerator mass spectrometry (AMS)
teAA 0
Bowman S., 1990
)ln(0A
At
AssumptionsAssumptions
** Constant production rateConstant production rate
** Constant and rapid mixingConstant and rapid mixing
** Constant exchange rateConstant exchange rate
** Constant reservoirs’ sizeConstant reservoirs’ size
** Same concentration in all parts of the biosphereSame concentration in all parts of the biosphere
** Decay determines the radiocarbon concentrationDecay determines the radiocarbon concentration
Problems concerning the Problems concerning the radiocarbon dating:radiocarbon dating:
-- Atmospheric Atmospheric 1414C variationsC variations
-- Alteration effectsAlteration effects
-- ContaminationContamination
-- Source or reservoir effectsSource or reservoir effects
Bowman S., 1990
Atmospheric 14C Atmospheric 14C variationsvariations
** Natural variationsNatural variations
- - Secular changesSecular changes
-- Sun spotsSun spots
** Men caused Men caused variations:variations:
-- Fossil fuel effectFossil fuel effect
-- Bomb effectBomb effect
Calibration curveCalibration curve
Hans Suess calibration curve – based on dendrochronologyHans Suess calibration curve – based on dendrochronology
Bowman S., 1990
Source or reservoir effectsSource or reservoir effects
Marine effectMarine effect
Hard water effectHard water effect
Volcanic effectVolcanic effect
Lake Kinneret reservoir time by Stiller M. et al., 2001
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