Stable Isotope Ecology

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Stable Isotope Ecology

• Lecture 3
• Oxygen and Hydrogen Isotopes

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Fractionations during evaporation and
condensation of water.
Equilibrium effects (Evaporation and
Condensation) aliquid-vaporRliquid/Rvapor Empiri
cal Equations for hydrogen and oxygen 103lnahydr
ogen24.844(106/Tk2)-76.248(103/Tk)52.612 103lna
oxygen 1.137(106/Tk2)-0.4156(103/Tk)-2.0667 K
inetic effects (Evaporation) Diffusion of water
molecules across the boundary layer DknO14.2(1-h
) DknH12.5(1-h)
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In a body of water the vapor generated by
evaporation is therefore a combination of
equilibrium effects (between the liquid and vapor
within the boundary layer) and the diffusion of
water molecules across the boundary layer. Given
by the following equation Exercise. Given a
body of water having the dD and d18O value of 0,
what is the isotopic composition of the vapor
generated by evaporation when the temperature is
25oC and the relative humidity is 85.
dvapordliquid Deq(T)-Dkn(RH)
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The condensation of liquid water from vapor
involves no diffusive fractionation and its given
by the following equation
dliquiddvapor Deq(T)
Exercise. Given a body of vapor having the dD and
d18O value of 0, what is the isotopic
composition of the liquid generated by
condensation when the temperature is 25oC?
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If body of water moisture undergoing condensation
is infinite, there will be no significant
isotopic effect on the remaining vapor body
during loss of water by condensation. However,
most likely it is a finite pool and as water
molecules having the heavier isotopes of oxygen
and hydrogen condense out, the remaining vapor
mass will become progressively depleted. This is
mathematically formalized in the Rayleigh
distillation equations.
dvapor(f) dvapor(0) Deq.ln f
The same is true for a finite body of water
undergoing evaporation i.e. its isotopic
composition will be affected by the preferential
loss of the lighter isotopes. The body of water
will become progressively enriched.
dliquid(f) dliquid(0) - Deq.ln f
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EXERCISE

• Lets assume that the ocean is an infinite pool of
  water relative to the loss by evaporation, it is
  at 25oC and the relative humidity is 85. Lets
  generate a moisture mass and progressively
  extract rainwater from it as it moves either
  inland or to higher latitude and determine the
  isotopic composition of the rainfall along its
  path.
• Generating the moisture mass dvapordliquid
  Deq(T)-Dkn(RH)
• Condensation at f1, 0.90, 0.80, 0.70..0.00001
  and using the Rayleigh distillation equation for
  both hydrogen and oxygen.
• dvapor(f) dvapor(0) Deq.ln f
• What is the dD and d18O composition of rainfall
  from this progressively depleted moisture mass?
  Plot it in graph with vertical axis as dD and
  horizontal as d18O values. What is the slope and
  what is the intercept of this line?

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EXERCISE
Lets assume a small lake created by rainfall
during the wet season with the dD and d18O values
of -30 and -5 respectively. During the dry
season (temp 30oC and RH 0.50) this lake
begins to evaporate and loose water (i.e. its a
finite pool). What is the isotopic composition
of the lake water as the remaining fraction is 1,
0.90, 0.80 .0.0001? Plot the meteoric water line
and then plot the values of the lake water as it
undergoes evaporation. What is the slope of the
line?
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Wilcox et al. Journal of Hydrology 293 (2004) 1-19
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Wilcox et al. Journal of Hydrology 293 (2004) 1-19
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Water Utilization by Coastal Hardwood Hammocks in
the Florida Keys. Ish-Shalom et al. 1992
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At steady state the isotopic composition of the
vapor is the same as that of the stem water.
dev dst
The isotopic composition (either dD or d18O
value) of water in the evaporative surface of a
leaf at steady state is given by the following
equation
desdstDeqDknh(da-Dkn-dst)
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Moreira et al. 1997
Harwood et al. 1999
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• Oxygen isotope ratio of leaf water, however, is
  not homogeneous. There is
• 1) A stream of isotopically enriched water in the
  stomatal cavity diffusing back to the vein having
  the isotopic abundance of
• dewdstDeqDknh(da-Dkn-dst).
• 2) A stream of isotopically depleted water moving
  by convective flow from the xylem towards the
  leaf evaporative surface having the isotopic
  composition of the stem water (dst)
• These opposing streams of water are characterized
  by the Peclet equation
• pLE/CD
• Where L is the effective path length, E is the
  transpiration rate, C is the molar density of
  water and D is the diffusivity.

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The isotopic ratio of leaf water is actually a
mixture of enriched and non-enriched water
according to the equation below
Where alpha is
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pLE/CD
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Yakir Sternberg 2000
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Farquhar et al. 1993
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Dole Effect
The Dole effect is defined as the difference
between the d18O value of atmospheric oxygen and
seawater. The d18O of the atmospheric oxygen is
23.5 and a function of terrestrial and marine
photosynthesis and respiration as well as
stratospheric chemical reactions. Dole
effect(fraction of land contribution the O2
cycle x land Dole effect)(fraction of ocean
contribution to O2 cycle x marine Dole
effect)-stratospheric diminuition. The
Terrestrial Dole effect is defined as
FTGPPxdLFTR(dATM-DTR) The Marine Dole effect
is defined as FMGPPXdMLFSORx(dATMDSE-DSOR)FDOR
x(dATMDSE-DDOR) Stratospheric contribution is
due reaction of O2 and CO2 in the stratosphere.
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Coherence between insolation and Dole effect. Ice
Core Records. Higher insolation ? Higher d18O
of leaf water?Higher d18O of terrestrial Dole
effect. Bender et al. 1994.
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Incorporation of oxygen and hydrogen isotopes in
biomass (cellulose).
Not much is known about the mechanisms of
hydrogen isotope labeling of cellulose, but much
more is known about oxygen isotope labeling in
cellulose. We observe, however, that the
hydrogen isotope ratios of cellulose are much
higher for CAM plants compared to C3 and C4
plants.
Sternberg DeNiro 1983
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Hydrogen isotope ratios of cellulose can
potentially be used to identify plants having an
intermediary CAM-C3 photosynthetic mode.
Sternberg et al. 1984
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Whereas hydrogen from water is the only possible
hydrogen source for cellulose, oxygen has three
potential sources CO2, H2O and O2. Analysis of
cellulose from aquatic plants reveals however
that the d18O values of cellulose is 27 higher
than that of water.
DeNiro Epstein 1981
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First hypothesis (pre-photosynthesis
equilibration) proposed that cellulose acquires
one oxygen from water and two from carbon dioxide
equilibrated with water which has a d18O value
41 greater than water. d18Ocell1/3xd18Owater
2/3xd18OCO2 1/3xd18Owater
2/3x(d18Owater41)
d18Owater27
If this was the case there should be a
temperature effect on the oxygen isotope
fractionation of cellulose.
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DeNiro Epstein 1981
No temperature effect was observed in the
fractionation. What is the expected change in
fractionation from 15 to 25oC according to the
previous model?
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An alternate hypothesis (post-photosynthesis
equilibration) is that there is carbonyl oxygen
equilibration during carbohydrate synthesis.
Sternberg DeNiro 1983
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Evidence for post-photoysnthetic labeling of
cellulose by the 18O of water.
Complete Exchange, slope1
d18O of Cellulose Synthesized from Glucose
No Exchange, slope0
d18O of Water
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