Chronology Development Flashcards
Different ways to build a chronology?
• Counting annual layers (tree rings, stalagmites, ice cores, lake sediment)
• Radiometric dating (radiocarbon, U-Th, K-Ar)
• Cosmogenic Nuclides
(decreasing resolution)
What is a floating chronology?
dated substance without a known start date (datum)
What is Radiometric dating (radiocarbon) useful for?
- Useful for dating organic material
- Useful for dating ‘once off’ samples (wood particle in alluvial fan)
- Useful for dating some lake sediments
- Useful for dating shells
- Useful in archaeology
V good for organic samples
Carbon isotopes
Carbon has two stable isotopes 12C and 13C
14C is radioactive
14C is produced by the bombardment of nitrogen atomic in the atmosphere by cosmic ray-derived neutrons
This 14C is oxidised into CO2 and is taken in by plants
• Plants are in equilibrium with atmospheric radiocarbon
• Animals are in equilibrium with their food
• Once living organisms die, 14C beta decays to 14N
• Beta decay: neutron changes to a proton by losing electron
14C production variability issues?
• Cosmic rays are made of subatomic particles, including atomic nuclei and neutrons
• Strike the Earth’s atmosphere constantly
• Cosmic ray flux is stable
• Influence modulated by solar wind – sun = variable star therefore not stable output
• Radiocarbon production therefore affected by solar wind and solar activity
Solar wind:
• Increases when there are more sunspots
• Mostly consists of a proton plasma
• Deflects cosmic rays
• More active sun = less cosmic rays = more C14 made
Radiometric dating overview and method? (Chemistry)
Abundance of radionuclide; C, depends on:
• initial concentration; C0
• decay constant; l, (rate of parent nuclide change)
• time; t
C = C0 e-lt
Radioactive parent has finite lifetime:
solve
C = C0 e-lt
for t.
If we know the present concentration (C), the initial concentration (Co)(constant?), and the decay constant (l):
C = C0 e-lt
We can easily solve for t
Radiocarbon currently measured on accelerator mass spectrometer (AMS)
Half-life is the time it takes for half the material to decay
Half-life of radiocarbon is ca. 5700 years.
Radiocarbon activity negligible after 6/7 half-lives.
So oldest datable material using 14C dating is 50,000 years
Problem with Radiometric dating: Solar activity
Production rate changes according to solar activity – it is not constant
Can’t solve for T – we need to come up with a calibration curve
• First the radiocarbon concentration is measured by AMS
• Then that is matched to the appropriate concentration on the calibration curve, which provides the true date
- Count rings ; know age
- Then radio carbon date to calibration curve; can then solve unknown
- Problem is that curve leads too points with multiple age values
Problem with Radiometric dating: MF
14C production rate also controlled by slow changes in Earth’s geomagnetic field
• Magnetic field affects cosmic rays
o Weakening filed means more radioactive carbon is being formed
Reservoir effects on carbon dating
- Some radiocarbon systems (corals, stalagmites) not only consist of organic carbon, but also of carbon contributions from limestone bedrock (stalagmites) or bicarbonate ions in seawater (corals)
- This adds ‘dead’ carbon (old carbon with no 14C) into the system, making the date appear older than it really is.
- Only way around this problem is use radiocarbon and another dating method to estimate the reservoir of ‘dead’ carbon
- So, a final radiocarbon date is is called ‘corrected’ (for reservoir effects) and is in ‘calendar years’ (production rate is taken into account)
Delta 14C = [(Asample /Astandard)-1] x 1000
industrial revolution coal burning leads to diluting atmosphere with co2 which is not aged
Bomb Issue?
- Atomic bomb testing released huge amounts of 14C
- Because current bomb spike highs of 14C leads to high, incorrect values
- All dates are reported as years before 1950 (even if it says present)
pMC is percent modern carbon
D14C of sample is divided against standard and converted to percentage
pMC = 100% is 1950
U-Series Dating equation and explanation?
Activity = concentration x decay constant (l)
• decay constant (l) = 0.693/half-life
Secular equilibrium is when the activity ratios of any isotopes in a decay chain are equal to 1: this only reached in very ancient systems
- Th is insoluble
- U6+ is soluble (UO2++)
- So, stalagmites, corals, and shells start out with uranium but no thorium
- Just depositing uranium
- Daughter- deficiency dating method
Decay equation could be used
C = C0 e-lt
Where Co is the initial concentration of parent isotope, C is the measured concentration
Activity Ratios explained?
instead of measuring using concentrations, activity ratios are used
• The idea is that the youngest samples will be the furthest from secular equilibrium (ancient samples, activity ratios = 1)
• Age equation more complicated than simple decay equation
Close to 0 = young
Close to 1 = old
1 = older than 500ka
Pros and cons of activity ratios
• 500,000 year maximum
• Produces dates in calendar years without need of a calibration curve
• Only major issue is that detrital Th may contaminate samples
o Th232 means Th230 will be present which make the sample look older
• If 230Th/232Th activity ratio is less than ~20 date may not be useable
• Detrital Th correction may be applied (based on ‘best guess’ of detrital Th contribution)
Cosmogenic nuclide surface exposure dating:
- Cosmogenic nuclides most widely utilized for geologic applications are the radionuclides: 10Be, 26Al, and 36Cl.
- 10Be is a radionuclide with a half-life of 1.5 Myr, is primarily produced by spallation from O, Mg, Si, and Fe, and is most commonly measured in quartz, olivine and magnetite.
- 26Al is a radionuclide with a half-life of 0.7 Ma, is primarily produced by spallation from Si, Al, and Fe, and is most commonly measured in quartz and olivine.
- 36Cl is a radionuclide with a half-life of 0.3Ma, is mostly formed by spallation from Ca and K and by neutron capture from 35Cl, and is commonly measured in whole rock samples
Problems with cosmogenic nuclide dating:
- Different surfaces bombarded by cosmic rays at different rates
- The surface might have shifted (glacial erratic)
