Water Isotopes in the Hydrosphere I
10/10/05
Lecture outline:
1) the hydrological cycle
2)
dD and d18O variability
3)
fractionation processes
4)
d18O, dD of precipitation
5)
modeling d18Oprecip
spectrometer light
intake
The Hydrosphere
How do 18O, 16O (d18O) and 2H, 1H (dD) move through this system?
Water Isotopic Variations
Ocean
d18O
Lake Michigan
d18O = -7‰
dD = -54‰
Lake Chad
d18O = -20‰
dD = -110‰
Dead Sea
d18O = +4.4‰
dD = 0‰
= 0 ± 2‰
dD = 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 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.
Water Isotopic Fractionation – some data
Rozanski, 1993
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)
Temperature effect on the d18O of precipitation
holds for both
spatial T variability
and temporal variability
Rozanski, 1993
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
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-d18O relationship - general
d 18O  0.45* S  15.5
Global precipitation
So d18O of surface waters, like salinity,
is also correlated to evaporation – precipitation.
Surface Water Salinity-d18O relationship - tropics
d 18O  0.273* S  9.4
Fairbanks et al., 1997
Slope of d18O-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 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”
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 d18O, 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