Geoscience Reference
In-Depth Information
α
H
2
CO
3
.
alkalinity of the rivers and the ocean. Use
Appendix F
to calculate the residence time
of alkalinity in the ocean.
7. A measurement on a surface ocean sample gives
Alk
2.30 meq kg
−
1
and
=
CO
2
=
2.15 mmol kg
−
1
. A similar measurement on a deep-water sample gives
Alk
=
2.45
2.40 mmol kg
−
1
. Calculate
CO
2
3
,
HCO
3
, and pH in
meq kg
−
1
and
CO
2
=
9.2.
8. Write a weathering reaction of enstatite (Mg
2
SiO
6
) by water
each seawater sample. Explain the differences. Use p
K
2
=
to serpentine
Mg
3
Si
2
O
5
(OH)
4
.
9. Aqueous silica is present in fresh water as H
2
SiO
3
and HSiO
3
. Silicic acid H
2
SiO
3
precipitates as amorphous silica following the reaction SiO
2
(solid)
+
H
2
O
=
H
2
SiO
3
HSiO
3
H
+
(log
K
=−
2.7) and dissociates following the reaction H
2
SiO
3
=
+
(log
=−
9.6). Calculate the abundance of each soluble species in equilibrium with
amorphous silica as a function of pH.
10. Complexation of Zn
2
+
K
in seawater. We define a complexation constant
β
n
of a metal
ion M in solution as:
[
ML
n
]
[
M
][
L
]
n
β
n
=
For Zn
2
+
, the decimal logarithm of complexation constants are (1) for OH
−
:5.0
(
n
4); and (2) for CO
2
3
:3.3(
n
1).
What are the relative proportions of the main species of Zn in seawater for a water
sample in which
Alk
=
1), 11.1 (
n
=
2), 13.6 (
n
=
3), 14.8 (
n
=
=
2.15 mmol kg
−
1
(assume
that the second dissociation constant of carbonic acid is 9.2). Hint: write the sum
of all
2.35 meq kg
−
1
=
and
CO
2
=
the Zn species and factor [Zn
2
+
], calculate [Zn
2
+
]/
Zn,
then the other
species.
11. What are the sources of alkalinity in river water? Which one can be considered as
recycled and which is juvenile?
12. Show how
Eq. 7.46
giving the slope of the meteoritic
18
O correlation in
δ
D-
δ
meteoritic waters can be derived from
Eq. 2.31
.
13. From the definition of the isotopic fractionation factor
18
O between liquid water
α
and vapor, show that
D
16
O
18
O, where
D
stands for partition
coefficient. For fractionation of oxygen isotopes during water vapor condensation, use
1, and
D
18
O
≈
≈
α
16
O
res
≈
18
O
16
O
0
18
O
1
, where
f
is the fraction
of original
16
O (and therefore of original water vapor) left in the atmosphere. What is
the corresponding relationship in
f
α
−
/
/
δ
units? Establish similar relationships for the D/H
ratio. Remember that liquid water is enriched in the heavier isotope.
14. The west coast of the Americas is fringed by elevated coastal mountain ranges. Explain
how, in a regime of west winds, the isotopic composition of oxygen and hydrogen in
precipitations changes with elevation.
15. A core of salty seawater depleted in nutrients and relatively oxygenated is observed in
the South Atlantic at a depth of 2200 m about 200 km east off the Brasilian continental
slope. What is the origin of the core water?
