Geoscience Reference
In-Depth Information
70
1.0
ϕ
H
3
BO
3
ϕ
B(OH)
4
0.9
60
0.8
50
0.7
δ
11
B
H
3
BO
3
0.6
Surface
ocean
40
0.5
δ
11
B
B(OH)
4
30
0.4
Modern sedimen-
tary carbonates
0.3
20
0.2
10
0.1
0
0.0
6
7
8
9
10
11
12
pH
Figure 7.8
Isotopic composition of dissolved boron and boron in sedimentary carbonates as a function of the
pH of seawater. The
11
B values are defined with reference to a standard boric acid (SRM 951).
δ
)
4
. There is an isotopic
fractionation of
11
B of 22 per mil between the two species. The
11
B refer to each species H
3
BO
3
and B
The fractions
ϕ
and
δ
(
OH
)
4
incorporated in
the sedimentary carbonate is a measure of the pH of the ocean from which limestone forms.
11
BofB
δ
(
OH
7.8 Precipitation, rivers, weathering, and erosion
heating of the oceans between the tropics produces water vapor. When the hot, moist air
rises, the water vapor condenses into fine droplets forming clouds, but only falls to the
ground as rain when the droplets coalesce to attain sufficient size. The masses of hot air
migrate toward the poles and as they cool precipitation progressively drains the moisture
from the atmosphere. This can be seen from the measurement of isotopic concentrations
of hydrogen and oxygen in precipitation, i.e. rain (meteoric) water and snow (
Fig. 7.9
).
Because heavy isotopes concentrate preferentially in liquid water and ice, progressive con-
densation of atmospheric vapor produces a very clear poleward depletion of precipitation in
the heavier isotopes of hydrogen and oxygen. A correlation ensues between
δ
D (a measure
of the
2
H/
1
H ratio) and
18
O of rain or snow, corresponding to this progressive depletion
of deuterium (D or
2
H) and
18
O in accordance with the linear approximation of fractional
condensation (see
Chapter 3
):
δ
D
(
T
)
d
δ
D
α
−
1
18
O
=
(7.46)
d
δ
18
O
(
T
)
α
−
1
where coefficients
measure the given isotope fractionation between liquid and vapor
at temperature
T
of the clouds. Strictly, this expression should be corrected to allow for
α
