Geoscience Reference
In-Depth Information
18
O
of oxygen, and is
δ
≈+
5.5
. If atmospheric oxygen was in equilibrium with seawa-
18
O would be very similar to SMOW. Actually, its value is more like
ter, its
δ
≈+
23.5
.
This is the Dole effect, which reflects the control of photosynthesis and respiration.
Fractionation of
16
Ofrom
18
O between liquid water and vapor is about one order of
magnitude smaller t
han fo
r hydrogen
iso
topes, which is what is roughly expected from
the comparison of
√
18
1 and
√
2
1. At the temperatures of interest for environ-
mental studies, vapor/liquid
18
O/
16
O fractionation is only mildly temperature dependent:
−
/
16
−
/
1
−
25
◦
C. Because temperature is the only
variable and is common to both systems, it is expected that
25
◦
C,
at 0
◦
C, and
15
at
−
−
12
−
9
at
+
18
O should be strongly
δ
D and
δ
18
O
correlated in rain waters (and snow). The linear relation
10 discovered by
Craig in 1961 and called the Global Meteoric Water Line (GMWL) can be explained by
a process of fractional condensation of water vapor upon migration of moist equatorial air
toward the poles. The SMOW does not plot on this line simply because water vapor is iso-
topically fractionated with respect to surface seawater. Let us write the Rayleigh equation
for the number of moles
n
of each isotope left in the atmosphere with
D
i
being the partition
coefficient of isotope
i
between water and vapor as:
δ
D
=
8
δ
+
D
18
O
liq
1
dln
f
dln
n
18
O
vap
=
vap
−
(3.33)
/
D
16
O
liq
1
dln
f
dln
n
16
O
vap
=
vap
−
(3.34)
/
in which
f
is the mass fraction of vapor left; and subtract the second equation from the
first. We obtain:
dln
18
O
16
O
vap
=
D
18
O
liq
vap
dln
f
D
16
O
liq
vap
−
(3.35)
/
/
vap
1
dln
f
18
O
16
O
D
16
O
liq
/
=
α
−
(3.36)
/
liq
/
vap
where we have simply introduced the original definition
(3.2)
of
. A similar expression
holds for D/H. We now observe that, because
16
O and H are the dominant oxygen and
hydrogen species in both the liquid and the vapor,
D
16
O
liq
α
vap
and
D
liq
/
vap
are both very close
/
to unity and introduce the approximation d ln D
1000.
After this replacement has been made, we divide the same expression for D/H by the last
equation, and we get the slope of the GMWL as:
/
H
=
dln
(
1
+
δ
D
/
1000
)
≈
d
δ
D
/
d
D
/
H
α
vap
−
1
δ
D
liq
/
vap
≈
(3.37)
d
δ
18
O
18
O
16
O
/
α
−
1
liq
/
vap
From the equation describing the isotopic evolution of the vapor, we now proceed to derive
the
18
O relationship in precipitations. Equation
(3.24)
shows that rain and snow sam-
ples are just shifted from the vapor line towards more positive values by the factors 100
ln
δ
D-
δ
18
O
18
O
D
/
H
/
α
α
vap
for
y
but the slope of the precipitations is identical
to that of the vapor. This fully justifies the correlation between
for
x
and 1000 ln
liq
/
vap
liq
/
18
O observed by
δ
D and
δ
Craig.
