Contents of Session 4
Aquatic Physics II
–
• Stable Isotopes of Water in the Hydrological Cycle
– Global Meteoric Water Line (GMWL)
– Effects on stable isotopes in precipitation:
4. Stable Isotopes in the Global Water
Cycle
• Temperature effect
• Altitude and continental effect
• Amount effect
Werner Aeschbach-Hertig
Institute of Environmental Physics
University of Heidelberg
Literature: Mook (2001) Vol. 1, ch. 7; Vol. 2, ch. 3,4
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Institut für Umweltphysik
Universität Heidelberg
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
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Institut für Umweltphysik
Stable Isotopes of Water
in the Hydrological Cycle
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
The Global Meteoric Water Line (GMWL)
The Global Meteoric Water Line (GMWL)
Empirical Finding (Craig, 1961):
Isotopic composition of precipitation from all over the world,
plotted in δ2H versus δ18O graphs, are strongly correlated
according to the equation:
δ 2 H = 8 ⋅ δ 18 O + 10
(in ‰)
The slope of 8 is similar to the ratio of the equilibrium fractionations
The intercept of 10 ‰ is called deuterium excess (d-excess)
from Mook, 2001
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Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
Institut für Umweltphysik
GMWL
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
GMWL
from Clark & Fritz, 1997 5
Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
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Universität Heidelberg
Universität Heidelberg
from Mook, 2001
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Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
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Evaporation line
Origin of the GWML (1)
• First step: Evaporation from the ocean
• Mixed equilibrium and kinetic fractionation (Craig-Gordon)
Evaporation shifts isotopic composition along a line with
slope SE < 8 (depending on humidity).
Equilibrium at surface
δv = δl − εl / v
Total fractionation
ε tot = ε l / v + ε diff
δ of evaporation flux
δE =
δ l − hδ a − ε tot
1− h
Slope of evaporation line
h: Relative humidity at water temperature
Institut für Umweltphysik
⎡ h (δ a − δ l ) + ε tot ⎤⎦ 2 H
SE = ⎣
⎡⎣ h (δ a − δ l ) + ε tot ⎤⎦ 18 O
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Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
from Mook, 2001
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Institut für Umweltphysik
Origin of the GWML (2)
Universität Heidelberg
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Origin of the GWML (3)
• Second step: Rayleigh-type condensation in clouds
• Approximately equilibrium fractionation: slope ~ 8
from Clark & Fritz, 1997 10
from Clark & Fritz, 1997 9
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
Institut für Umweltphysik
Universität Heidelberg
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Progressive Rainout of Atmospheric Vapour
Explanation of the GMWL
• Slope of ~ 8 due to ~ equilibrium conditions during
condensation of precipitation in clouds
• Deuterium excess of ~ 10 ‰ due to mean relative humidity
of ~ 85 % during evaporation from the ocean
Vapour pressure [mbar]
Institut für Umweltphysik
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d-excess depends on
humidity during evaporation
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⇒ indicator of conditions in
moisture source region
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Temperature [°C]
0
0
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
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15
20
25
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from Mook, 2001
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Institut für Umweltphysik
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Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
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Universität Heidelberg
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Temperature Effect
Temperature Effect
Relationships between
temperature T (in °C)
and δ18O (in ‰):
Dansgaard (1964):
δ18O = 0.695T - 13.6
Yurtsever (1975):
δ18O = 0.521T - 15.0
Slope ~ 0.6 ‰ °C-1
from Dansgaard, 1964, Tellus 16: 436-468
Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
from Rozanski et al., 1993, In: Climatic Change in Continental Isotopic Records
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Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Comparison of seasonal and temperature effect
Seasonal Effect
from Clark & Fritz, 1997 15
Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
Universität Heidelberg
from Mook, 2001 16
Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
Continental Effect
Continental Effect
Rainout of oceanic water vapour: Increasing depletion of precipitation
Contours of δ2H in precipitation over Europe
from Siegenthaler, 1997, In: Lectures in Isotope Hydrology
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Universität Heidelberg
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Continental Effect
Altitude Effect
from Clark & Fritz, 1997 19
Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
from Mook, 2001
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Institut für Umweltphysik
Slope of Altitude Effect
Simple estimate:
Lapse rate:
Temperature effect:
Altitude effect:
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
Amount Effect
Light/early rainfall tends
to be enriched compared
to heavy rainfall.
dT/dz ~ -0.5 °C/100m
dδ18O/dT ~ 0.6 ‰/°C
dδ18O/dz ~ -0.3 ‰/100m
In part due to evaporation
of falling rain drops.
Also due to dynamics in
convective storm cells.
Important effect in
tropical regions!
from Clark & Fritz, 1997 21
Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
from Mook, 2001
Institut für Umweltphysik
Operated by IAEA and WMO; http://isohis.iaea.org/
from Clark & Fritz, 1997 23
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
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GNIP: Global Network of Isotopes in Precipitation
Amount Effect
Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
from Rozanski et al., 1993, In: Climatic Change in Continental Isotopic Records
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Universität Heidelberg
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Global distribution of stable isotopes
http://isohis.iaea.org/userupdate/Waterloo/index.html 25
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Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
http://isohis.iaea.org/userupdate/Waterloo/index.html 26
Institut für Umweltphysik
Physics of Aquatic Systems II, 4. Stable Isotopes – Water Cycle
Universität Heidelberg
Summary
• Water isotopes in the hydrological cycle
– δ2H and δ18O correlated along Global Meteoric Water Line
– Position of precipitation on GMWL related to temperature
– GMWL explained by non-equilibrium evaporation (d-excess)
and equilibrium condensation during rainout (slope)
• Factors influencing stable isotopes in precipitation
– Temperature effect due to progressive rainout with cooling
– Continental and altitude effects due to progressive rainout
– Amount effect can be dominant in tropics
– No strict, exact physical relationships, but very useful
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Universität Heidelberg
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