12. Assessing Ecosystem-Level Water Relations Through Stable Isotope Ratio Analyses
183
Meteoric Water Line
The isotopic composition of ocean water is approximately oD = 0 and 0 18 0 = O. Since the bonding
is stronger when heavier isotopes are involved, the
evaporating water vapor is depleted in the heavier
isotopes relative to its water source. Under equilibrium conditions and at 25°C, the water vapor is depleted by approximately -74%0 and - 9.2%0 for
oD and 0 18 0, respectively, relative to the water
source (Gat and Gonfiantini 1981). When this
moisture condenses from clouds during precipitation events, the precipitation is slightly enriched in
the heavier isotopes relative to the vapor remaining
in the clouds. As a consequence, subsequent precipitation events from this cloud produce precipitation that is progressively more depleted in the
heavier isotopes. The slope of the relationship of
oD and 0 1S 0 in these precipitation events should
follow vapor pressure differences (i.e., ~OD/~OISO
""" 74/9.2 """ 8).
The observed relationship between oD and 0 1S 0
of precipitation collected from stations throughout
the world generally follows a slope of8 (Fig. 12.1).
This regression is known as the meteoric water line
(MWL). While local variations do exist in this relationship, the MWL serves as a foundation for predicting the isotopic composition of precipitation in
0
-100
0
-200
~
6
(.()
-300
00 = 8 • 0 18 0 + 10
-400
-500 -50
-40
-30
-20 -10
0
different locations and allowing measurement of
one isotope without always requiring the second
isotopic measurement. Because the isotopic composition of subsequent precipitation events along a
storm trajectory will follow Rayleigh distillationkinetics, the oD and 0 1S 0 of precipitation will become progressively more negative as the storm
moves inland (Mazor 1991). The isotopic depletion
of water from inland regions serves as a valuable
signal that can is recorded in both plants and animals. The oD and 0 1S 0 of precipitation are related
to temperature (Gat and Gonfiantini 1981). Summer precipitation events have higher oD and 0 1S 0
values than winter precipitation events, which provides a basic mechanism for creating isotopic variation throughout the soil moisture profile (Mazor
1991; Gat 1996).
Evaporative Enrichment
While the isotopic composition of incoming precipitation follows the MWL, evaporating bodies
(e.g., lakes, leaves, soils) within the ecosystem tend
to exhibit a ~OD/~01S0 slope that is substantially
less than 8 (see Fig. 12.1). Both the upper soil layers and leaves exhibit a strong evaporative enrichment relative to the input source water under nat0
-40
-80
-160
-20
-10
0
10
20
0 18 0, %0
FIGURE 12.1. Left, A plot of the relationships between hydrogen isotope and oxygen isotope ratios for precipitation
globally. Right, A plot of the relationships between hydrogen isotope and oxygen isotope ratios for an evaporating
soil and leaf water values of different leaves in the same environment. (Data from Allison et al. [1983], Craig [1961],
and Flanagan et al. [1991a].)
183
Meteoric Water Line
The isotopic composition of ocean water is approximately oD = 0 and 0 18 0 = O. Since the bonding
is stronger when heavier isotopes are involved, the
evaporating water vapor is depleted in the heavier
isotopes relative to its water source. Under equilibrium conditions and at 25°C, the water vapor is depleted by approximately -74%0 and - 9.2%0 for
oD and 0 18 0, respectively, relative to the water
source (Gat and Gonfiantini 1981). When this
moisture condenses from clouds during precipitation events, the precipitation is slightly enriched in
the heavier isotopes relative to the vapor remaining
in the clouds. As a consequence, subsequent precipitation events from this cloud produce precipitation that is progressively more depleted in the
heavier isotopes. The slope of the relationship of
oD and 0 1S 0 in these precipitation events should
follow vapor pressure differences (i.e., ~OD/~OISO
""" 74/9.2 """ 8).
The observed relationship between oD and 0 1S 0
of precipitation collected from stations throughout
the world generally follows a slope of8 (Fig. 12.1).
This regression is known as the meteoric water line
(MWL). While local variations do exist in this relationship, the MWL serves as a foundation for predicting the isotopic composition of precipitation in
0
-100
0
-200
~
6
(.()
-300
00 = 8 • 0 18 0 + 10
-400
-500 -50
-40
-30
-20 -10
0
different locations and allowing measurement of
one isotope without always requiring the second
isotopic measurement. Because the isotopic composition of subsequent precipitation events along a
storm trajectory will follow Rayleigh distillationkinetics, the oD and 0 1S 0 of precipitation will become progressively more negative as the storm
moves inland (Mazor 1991). The isotopic depletion
of water from inland regions serves as a valuable
signal that can is recorded in both plants and animals. The oD and 0 1S 0 of precipitation are related
to temperature (Gat and Gonfiantini 1981). Summer precipitation events have higher oD and 0 1S 0
values than winter precipitation events, which provides a basic mechanism for creating isotopic variation throughout the soil moisture profile (Mazor
1991; Gat 1996).
Evaporative Enrichment
While the isotopic composition of incoming precipitation follows the MWL, evaporating bodies
(e.g., lakes, leaves, soils) within the ecosystem tend
to exhibit a ~OD/~01S0 slope that is substantially
less than 8 (see Fig. 12.1). Both the upper soil layers and leaves exhibit a strong evaporative enrichment relative to the input source water under nat0
-40
-80
-160
-20
-10
0
10
20
0 18 0, %0
FIGURE 12.1. Left, A plot of the relationships between hydrogen isotope and oxygen isotope ratios for precipitation
globally. Right, A plot of the relationships between hydrogen isotope and oxygen isotope ratios for an evaporating
soil and leaf water values of different leaves in the same environment. (Data from Allison et al. [1983], Craig [1961],
and Flanagan et al. [1991a].)