Geoscience Reference
In-Depth Information
2.5
2.4
2.3
2.2
2.1
2.0
2.0
2.1
Dissolved inorganic carbon (mmol l
-1
)
2.2
2.3
2.4
2.5
D
A
T
: D
C
T
Process
CO
2
outgassing
Silicate weathering CaSiO
3
+2CO
2
+H
2
O => Ca
2+
+ 2HCO
-
+ SiO
2
Calcium carbonate precipitation Ca
2+
+CO
2
-
=> CaCO
3
0 : 1
2 : 0
-2 : -1
vector sum
0 : 0
Figure 4.2
Three fundamental processes that control the marine carbonate system and inl uence the saturation state of calcite (Ω
c
) of seawater on
geological timescales. See text for details.
Chapter 2); here we consider those processes that
affect total alkalinity (
A
T
), total dissolved inorganic
carbon (
C
T
), or both simultaneously. Figure 4.2
depicts one such solution for the marine carbonate
system. Given a range of plausible
A
T
and
C
T
values,
functions of equal saturation state (here calculated
for calcite, but a similar reasoning applies to arago-
nite and magnesian calcite) can be drawn. Three
primary processes control the l uid earth carbonate
system over long timescales:
serves to raise Ω in seawater and both increases
the rate of carbonate precipitation and promotes
precipitation and preservation of carbonate
minerals in areas of the oceans that were previ-
ously undersaturated.
(3) Finally, carbonate mineral precipitation pro-
vides the mathematical complement to CO
2
outgassing and silicate weathering by consum-
ing
A
T
and
C
T
with a slope of -2.
It is not by chance that these fundamental processes
have a Δ
A
T
:Δ
C
T
vector sum that equals zero: they do
not operate independently of one another. CO
2
out-
gassing and silicate weathering are connected via
the silicate weathering feedback (e.g. Walker
et al
.
1981), and because the global oceans have a i nite
and stable water volume they produce carbonate
minerals to alleviate inputs of dissolved inorganic
carbon and total alkalinity. This forms the basis for
a set of negative, or stabilizing, feedbacks on Ω in
seawater (a form of 'carbonate compensation'). The
(1) CO
2
produced from solid earth sources (volca-
noes and metamorphism) and the weathering
of sedimentary rocks increases
C
T
, but does not
affect
A
T
. This process works to lower Ω in sea-
water and will slow the rate of carbonate pre-
cipitation or even begin to promote dissolution
of carbonate sediments.
(2) Chemical weathering of silicate minerals con-
sumes protons (derived from CO
2
via carbonic
acid) and increases
A
T
, but not
C
T
. This process
