Isotopes

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Lecture 9 Isotopes
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Look For the Following Key Ideas

• Elements consist of isotopes with the same number
  of protons, but differing numbers of neutrons
• Stable isotopes provide a variety of
  information on the Earth system including
  paleo-conditions, ecosystem dynamics, global
  productivity, weathering rates
• Radioisotopes provide a variety of information
  on the Earth system including ocean circulation,
  sedimentation, process rates CLOCKS

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Overview

• Isotopes
• overview
• Stable Isotopes
• C, H, N, O, S, Sr
• Radioisotopes
• C, H, U-series

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Isotope Systematics
Isotope Identical number of protons, different
number of neutrons. Identical chemistry,
different rates. 2 Flavors stable (280) and
radioactive (1700 ? 67)
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Isotope Systematics
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Isotope Systematics
The wonders of mass spectrometry allow very
precise measurements. Sensible units are needed.
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Isotope Systematics
C, H, O, N, S (Standards) C Pee Dee
belemnite carbonate (PDB) H/O Standard Mean
Ocean Water (V-SMOW) S Diablo Canyon
Troilite (a meteor) N Atmospheric N2
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Isotope Systematics
d13C () ( - 1) 1000
13C/12Csample
13C/12Cstandard
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Group Problem 1 (10 min)
Pretend I provide each group with three samples
of distilled water (equal volumes) with the
following d2H values -400, 0, and 600.
Calculate the D/H ratios for each of these
samples. Assume the D/H of SMOW is
1.5010-4. Now pretend you mix the three samples
together and calculate the resulting D/H. From
the D/H, calculate the resulting d2H.
d2H () ( - 1) 1000
2H/1Hsample
2H/1HSMOW
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Some Uses of Stable Isotopes

• Isotope Ratios Vary in Nature
• ? Kinetic Processes (KIE)
• ? Thermodynamic Processes (EIE)

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Some Uses of Stable Isotopes

• ? Kinetic Processes (KIE)
• Lighter Isotope Reacts Faster
• Quantum Mechanical

12CO2 H2O ? 12CH2O O2 (kC12)
13CO2 H2O ? 13CH2O O2 (kC13)
kC12 gt kC13
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Some Uses of Stable Isotopes

• ? Equilibrium Processes (EIE)
• Equilibrium Achieved
• Thermodynamic effect

HDO H2 ?? H2O HD (exchange)
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Some Uses of Stable Isotopes

• 13C/12C Biological Pump, Trophic..
• 2H/1H Temp/Ice Volume
• 15N/14N Trophic Interactions
• 18O/16O Temperature/Ice Volume
• 34S/32S Atmospheric O2
• 87Sr/86Sr Global Weathering

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Some Uses of Stable Isotopes

• Rayleigh Process After changing form/phase, each
  increment is isolated from contact with the
  initial form/phase.

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Some Uses of Stable Isotopes
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Some Uses of Stable Isotopes

• Carbon Source
• Photosynthesis
• C3 plants (annual) d13C -21 to -35
• C4 plants (perrenials) d13C -11 to -15
• Aquatic plants d13C -11 to -39

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Biological Pump
122CO2 138H2O 16HNO3 H3PO4 ?
(CH2O)80(CH2)42(NH3)16(H3PO4) 175O2
B
(1-f)B
CO2 HNO3 H3PO4
fB
(CH2O)80(CH2)42(NH3)16(H3PO4)
Burial and Preservation of OM.
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Biological Pump
122CO2 138H2O 16HNO3 H3PO4 ?
(CH2O)80(CH2)42(NH3)16(H3PO4) 175O2
B
(1-f)B
CO2 HNO3 H3PO4
fB
(CH2O)80(CH2)42(NH3)16(H3PO4)
Burial and Preservation of OM.
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Isotopes

• Imbalance of protons and neutrons ? nuclear
  instability

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Radioisotopes

• Imbalance of protons and neutrons ? nuclear
  instability
• B- emission (electron N ? P)
• B emission (positron P ? N)
• Electron Capture (P ? N)
• Alpha Emission (He nucleus)
• Gamma ray (just energy)

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Group Problem 2 (8 min)

• 238U is a common radioisotope of Uranium. 238U
  decays through a complex process of 6b- decays
  and 8a decays. What are the final daughter
  nuclei?
• On a plot of atomic number vs. Neutron number,
  plot out all possible decay pathways. All
  pathways produce 234Th, 234U, 218Po, and 210Pb
  as intermediates. 210Po is never formed.

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Some Uses of Radioisotopes

• 40K/40Ar Dating ancient rocks
• 238U/234Th Particle scavenging
• 226Ra/222Rn Air-Sea gas exchange
• 226Ra/220Pb Sedimentation Rates
• 14C Mixing, age dating
• 3H Modern mixing

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Mathematically

• dN/dt -lN (activity rate first order)
• ln(N/No) -lt
• N/No e-lt
• N Noe-lt
• No atoms at t 0
• l decay constant

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Group Problem 3 (8 min)

• Derive the mathematical expression for half life.
• Determine the mathematical relationship between
  the half life (T1/2) and the mean lifetime (t
  1/l). What fraction of an isotope has decayed
  after 1 mean lifetime?

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Secular Equilibrium

• Parent and Daughter(s)
• Parent is long lived
• No chemical separation for several half lives
• lN(p) lN(d)
• ? Deviations are useful
• lN activity

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14C Special Focus

• Production 14N n ? 14C p
• Decay 14C ? 14N b- neutrino
• l 1.2110-4 y-1

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14C Special Focus

• Natural Prodcution rate of 14C

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14C Special Focus

• The Bomb Spike

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14C Special Focus