Discussion regarding loess

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Discussion regarding loess

• Chinese loess
• Principal (continuous) sources are the Tibetan
  plateau and Chinese deserts
• Thick deposits that continue over 3 Myrs
• Soil units are developed during times when loess
  flux decreased (but not stopped)
• Chronology
• Luminescence, 14C, ash
• 10Be and magnetic susceptibility
• Atmospheric 10Be and a measurement of the
  magnetization
• Climate
• 10Be and mag. Susceptibility correlate with 18O
  record

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Flavours of Dating Methods

• Isotopic
• Radiometric
• radioactive parent produces daughter isotope
• Cosmogenic
• Isotopes produced directly from cosmic rays
• Nucleogenic
• Fission track, luminescence
• Biometric
• Layers, rings, life cycles
• Marrying techniques
• Stratigraphy, tracers, correlables

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?106 year
105 year
103-104 year
10-102 year
Atmospheric 10Be (subd. zones)
U-Pb, U-Th/He paleomag reversal stratigraphy,
fission track biostratigraphy cosmo 38Ar
Rb-Sr, Sr-curve
Ar/Ar, cosmo , 21Ne, 14C soil development
U-series (speleothems, soil CO) Anthro tracers
TL, Electron Spin Resonance (ESR) Amino Acid
Racemization (AAR) rock varnish dating
Dendrochronology, 14C 210Pb, cosmogenic 10Be,
26Al, 36Cl, 3He, anthro. Tracers elemental Pb
and W, 137Cs 3H meteoric 7Be, 10Be marrying
techniques OSL U-series, repeat photos remote
sensing GPS lichens magnetic secular
variation correlational techs. stable isotopes,
tephrochronology ice cores varves Paleovegetatio
n archaeology direct erosional measurements on
decade scale
Geomorphology
32Si 4He
32Si, 39Ar, 38Ar, 36Ar, 81Kr
Hydrogeology
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Quaternary Dating At Dalhousie

• 40Ar/39Ar K-Ar (Reynolds)
• Fission track (Zentilli)
• Luminescence (OSL, TL) (Godfrey-Smith)
• Exposure dating (Gosse)
• (U-Th)/He (Reynolds, Gosse, Zentilli)
• Radiocarbon
• 210Pb

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Principles of Radiometric Dating

• Radioactive Decay
• Spontaneous emission of particle from nucleus.
• Transforms atom from one isotope into another.

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Radiometric Dating
First T½
Second T½
5730 yr
17190 yr
45840 yr (Eighth T½) 0.4 remaining
11460 yr
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Radiometric Dating

• Different isotopes have different half-lives
• 14C 5.7 kyr
• 235U 704 Myr
• 40K 1251 Myr
• 238U 4468 Myr
• 87Rb 48.8 Gyr

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A typical radiometric dating method
Parent
Daughter
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GCR

• Galactic Cosmic Radiation
• 83 p, 13 a, 3 e-, heavies, GKWEs
• 0.1 to 1020 eV per nucleon
• (energy gained by electron thru potential
  difference of 1V)
• 0.3 eV nitrogen molecule in air
• 50 eV baseball in World Series
• 157 eV radiogenic b- from 14C decay
• 1.33 MeV radiogenic b- from 40K decay

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Cascade of secondary radiation
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Cosmic rays
N2 O2
N
78 21
p
14
C
O CO
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2
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Radiocarbon Dating
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Radiocarbon

• 14C12C nearly constant in atm and organisms that
  are in equilibrium with atm
• After death, 14C decays, so ratio decreases
• Plants, animals, soils organic material
• Compare ratio of atm to fossil carbon to get age
• Decades to 90 ka

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Radiocarbon Dating

• But
• Atm ratio is not constant over time
• magnetic field effect CALIBRATION
• Not all life is in equilibrium with atm
• 1400 year old whale! RESERVOIR EFFECT

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Present state of 14C calibration gt 10 ka
Hughen et al 2004
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Terrestrial cosmogenic nuclides

• TCN
• Exposure dating of rock and sediment surfaces
  (lavas, fault scarps, boulders, terraces, fans,
  beaches)
• 30 years to millions of years

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Peggys Cove, Nova Scotia
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In situ production of TCN

• Spallation, thermal neutrons, muonic interactions
• C P T Ci
• C concentration of nuclide (atoms g-1)
• P sum of all production mechanism rates (atoms
  g-1 yr-1)
• T exposure time (yr-1) (ignore decay)
• ( at the surface )
• i ( initial concentration inherited from exposure
  prior to T)

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Compare both methods
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(No Transcript)
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11.0
11.5
YD
LOI
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10Be concentration from boulders
Exposure age 12.4 ka
X 100,000 atoms/g
10Be mean 6.26 atom/g 10Be CV 0.040 atom/g,
1s1.3
Sample ID