Stable Isotopes – Raleigh distillation
and water isotopes
10/4/12
Lecture outline:
1) mass balance
2)
Raleigh distillation
3)
the hydrological cycle
4)
D and 18O variability
spectrometer light
intake
Chalk cliffs formed in Cretaceous
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
Rt d R t   0 Ri di
i
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 18O change = +1.3‰ (SMOW)
present-day ocean = 0‰ (SMOW)
glacial ice caps averaged -35‰
Vo *0‰  Vi * 35‰  +  Vgo *1.3‰ 
Vi  Vo  Vgo
 sea level = 140m
Continual fractionation in a closed system: Raleigh distillation
ex: rainfall from cloud
original
vapor
enriched phase ( ? 1)
equilibrium
first
drop
enriched phase
equilibrium
next
drop
next vapor
enriched phase
equilibrium
next
drop
final vapor
enriched phase
final
drop
rain
becomes
lighter
TIME
next vapor
TIME
vapor
becomes
lighter
IF PRODUCT REMOVED
(cannot re-equilibrate w/ parent liquid)
NO FRACTIONATION
FOR LAST DROP. . . Why?
Raleigh distillation model
We can track the progression
of the vapor-rainfall if we know:
1. the initial isotopic ratio of the vapor
2. the fraction of vapor remaining
liquid
RV
 f  1
RV0
18O (‰
where
RV is the isotopic ratio of the vapor
RV0 is the initial isotopic ratio of the vapor
f is the fraction of vapor remaining
vapor
α is the fractionation factor
We can also derive the formula for
the Rrain as a function of α:
RR
   f  1 
RV0
If the  of vapor to liquid is 1.0092,
what is the  of liquid to vapor?
After Dansgaard, 1964
NOTE: fractionation increasing
because T(cloud) decreasing
Raleigh distillation in the real world
If the tropics are the source of all cloud moisture, then
the 18O of rainfall _________ from equator to pole.
What also happens as you
move from equator to pole?
This effect would ________
the 18O of rainfall at the poles.
What other natural systems might be
characterized by Raleigh fractionation?
The Hydrosphere
How do 18O, 16O (18O) and 2H, 1H (D) move through this system?
Water Isotopic Variations
Ocean
18O
Lake Michigan
18O = -7‰
D = -54‰
Lake Chad
18O = -20‰
D = -110‰
Dead Sea
18O = +4.4‰
D = 0‰
= 0 ± 2‰
D = 0 ± 16‰
What processes
explain these
variations?
NOTE: water isotopes are always reported
with respect to SMOW
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/condensation. This fractionation is temperature-dependent.
GNIP – global network of isotopes in precipitation
Rainwater samples are routinely collected for 18O and D analysis all over the world.
The data are stored and managed by GNIP, and used to study the processes that
fractionate water isotopes.
Water Isotopic Fractionation – some data
Rozanski, 1993
18O of rain
near SMOW
in tropics, highly
depleted in
high-latitudes
18O of rain
decreases
far from vapor source (Raleigh)
and is heavier during winter (temperature)
Temperature effect on the 18O of precipitation
holds for both
spatial T variability
and temporal variability
Rozanski, 1993
But what if we add all the GNIP global 18Oprecip data?
A bit more complicated,
but generally strong
relationship.
However, what is
happening at
higher temperatures?
Rozanski, 1993
The so-called “amount” effect: more rain, heavier d18O
NOTE: only in tropics (<30° 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)
Dansgaard, 1964
Rozanski, 1993
Mechanism still unknown – need
atmospheric modeler’s help.
Surface Water Salinity-18O relationship - general
d 18O  0.45* S  15.5
Global precipitation
So 18O of surface waters, like salinity,
is also correlated to evaporation – precipitation.
Surface Water Salinity-18O relationship - tropics
d 18O  0.273* S  9.4
Fairbanks et al., 1997
Slope of 18O-salinity relationship is 0.273 in the deep tropics (<5° N and S),
vs. 0.45 elsewhere. Why?
The “Global Meteoric Water Line” – what happens to 18O
happens to D, but with a different 
annual mean dD vs. d18O of precipitation
But month-to-month variations
at a given site fall off this line –
“deuterium excess”
d  d D  8* d 18O
d D  8* d 18O  10
Craig, 1961
Rozanski, 1993
Why don’t 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 18O, not the -9.2‰ expected from
equilibrium fractionation. Why?
Planetary boundary layer
-evaporation not purely
equilibrium process
-what other type of fractionation
is involved?
1-3km
the layer where exchange occurs
between the surface and the free
atmosphere
Water
Given the potential for complicated boundary layer physics, it’s a wonder that the
GMWL exists at all!
Deuterium excess
Humid regions will show smaller
departures from GMWL than arid
regions.
Generally interpreted as a proxy
for the “source” of the moisture.
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