Stable Isotopes Raleigh distillation10107

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Stable Isotopes Raleigh distillation 10/1/07 a
nd water isotopes

• Lecture outline
• mass balance
• Raleigh distillation
• 3) the hydrological cycle
• 4) dD and d18O variability

spectrometer light intake
Chalk cliffs formed in Cretaceous
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Mass balance of stable isotopes
Principle stable isotopes are CONSERVED, unlike
radioactive isotopes Therefore, if one reservoir
is enriched, the other reservoir must be depleted
R (reservoir size) is expressed in moles d
represents the delta value for a given reservoir,
expressed in per mil
Example What was the glacial-interglacial sea
level change? Given G-I d18O change 1.3
(SMOW) present-day ocean 0 (SMOW) glacial
ice caps averaged -35
D sea level 140m
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Continual fractionation in a closed system
Raleigh distillation
ex rainfall from cloud
IF PRODUCT REMOVED (cannot re-equilibrate w/
parent liquid)
original vapor
first drop
enriched phase (a ? 1)
equilibrium
vapor becomes lighter
rain becomes lighter
next vapor
next drop
enriched phase
equilibrium
TIME
TIME
next vapor
next drop
enriched phase
equilibrium
final vapor
final drop
enriched phase
NO FRACTIONATION FOR LAST DROP. . . Why?
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Raleigh distillation model

• We can track the progression
• of the vapor-rainfall if we know
• the initial isotopic ratio of the vapor
• the fraction of vapor remaining
• where
• RV is the isotopic ratio of the vapor
• RV0 is the initial isotopic ratio of the vapor
• ? is the fraction of vapor remaining
• is the fractionation factor
• Who wants to derive the formula for
• the Rrain as a function of ??

After Dansgaard, 1964
If the a of vapor to liquid is 1.0092, what is
the a of liquid to vapor?
NOTE fractionation increasing because T(cloud)
decreasing
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Raleigh distillation in the real world
If the tropics are the source of all cloud
moisture, then the d18O of rainfall _________
from equator to pole.
What also happens as you move from equator to
pole? This effect would ________ the d18O of
rainfall at the poles.
What other natural systems might be
characterized by Raleigh fractionation?
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The Hydrosphere
How do 18O, 16O (d18O) and 2H, 1H (dD) move
through this system?
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Water Isotopic Variations
NOTE water isotopes are always reported with
respect to SMOW
Ocean d18O 0 2 dD 0 16 Lake
Michigan d18O -7 dD -54 Lake Chad d18O
-20 dD -110 Dead Sea d18O 4.4 dD
0
What processes explain these variations?
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Water Isotopic Fractionation review from last
lecture
Reminder Oxygen and hydrogen isotopes are
strongly fractionated as they move through the
hydrological cycle, because of the large
fractionation associated with evaporation/condens
ation. This fractionation is temperature-dependent
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GNIP global network of isotopes in precipitation
Rainwater samples are routinely collected for
d18O and dD analysis all over the world. The data
are stored and managed by GNIP, and used to study
the processes that fractionate water isotopes.
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Water Isotopic Fractionation some data
d18O of rain near SMOW in tropics,
highly depleted in high-latitudes
d18O of rain decreases far from vapor source
(Raleigh) and is heavier during winter
(temperature)
Rozanski, 1993
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Temperature effect on the d18O of precipitation
holds for both spatial T variability and
temporal variability
Rozanski, 1993
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But what if we add all the GNIP global d18Oprecip
data?
A bit more complicated, but generally
strong relationship.
However, what is happening at higher
temperatures?
Rozanski, 1993
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The so-called amount effect more rain,
heavier d18O
NOTE only in tropics (lt30? N and S), where
deep convection takes place
Empirical relationship meaning.? It would be
difficult to explain a vapor source at 1, when
the tropical oceans are 0. Thought to be
linked to increased evaporation of raindrop in
dry, under-saturated environment (i.e. vapor is
-9 ish, but the raindrop is enriched as it falls
from the sky) Mechanism still unknown
need atmospheric modelers help.
Dansgaard, 1964 Rozanski, 1993
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Surface Water Salinity-d18O relationship - general
Global precipitation
So d18O of surface waters, like salinity, is also
correlated to evaporation precipitation.
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Surface Water Salinity-d18O relationship - tropics
Fairbanks et al., 1997
Slope of d18O-salinity relationship is 0.273 in
the deep tropics (lt5? N and S), vs. 0.45
elsewhere. Why?
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The Global Meteoric Water Line what happens
to d18O happens to dD, but with a different a
annual mean dD vs. d18O of precipitation
But month-to-month variations at a given site
fall off this line deuterium excess
Craig, 1961 Rozanski, 1993
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Why dont all waters fall on the GMWL? Or. why
do different source waters have different
deuterium excess values?
Fact water vapor above the ocean is -13 in
d18O, not the -9.2 expected from equilibrium
fractionation. Why?

• Planetary boundary layer
• the layer where exchange occurs
• between the surface and the free
• atmosphere
• evaporation not purely
• equilibrium process
• what other type of fractionation
• is involved?

Given the potential for complicated boundary
layer physics, its a wonder that the GMWL exists
at all!
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Deuterium excess
Humid regions will show smaller departures from
GMWL than arid regions. Generally interpreted
as a proxy for the source of the moisture.
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Modeling water isotopes in the hydrosphere
Full atmospheric General Circulation Model (GCM)
with water isotope fractionation included.
Noone, D., 2002
Goal quantify physical processes associated
with water isotope variability Applications
atmospheric mixing, vapor source regions, impact
of climate variability on hydrological cycle,
interpretation of paleoceanographic records