Geoscience Reference
In-Depth Information
Sand
Mud
1
1
0
0
-1
-1
-2
-2
-3
-3
-4
-4
-5
-5
-6
-6
5.5
6
6.5
7
7.5
8
5.5
6
6.5
7
7.5
8
pH
NBS
pH 5.6
pH 6.5
pH 7.3
pH 8 (Control)
Figure 9.2
Impact of extreme seawater acidii cation (pH
NBS
of 5.6, 6.5, 7.3, and 8) on depth proi les in sand and mud (modii ed from Widdicombe
et al. 2009 ).
microbial processes on pH (excluding CO
2
produc-
tion), results in pH reduction from typical seawater
values of 8.2 on the free scale (Soetaert
et al.
2007 ).
Soetaert
et al.
(2007) showed that the magnitude of
the acid-base effect of various biogeochemical proc-
esses on pH (excluding CO
2
production) was of the
order of 0.001 pH units per mole of substrate
reduced or oxidized. The effect of CO
2
on pH is of
the same order of magnitude evaluated using the
CO2SYS software (Lewis and Wallace 1998). This
observation suggests that the reduced pH com-
monly found in marine sediments is not due to CO
2
production alone and that acid-base reactions play
a signii cant role in determining sediment pH.
However, stoichiometric considerations dictate that
during aerobic respiration of organic matter the
molar production of CO
2
(which reduces pH)
exceeds the molar production of NH
3
and PO
4
3-
(which increase pH). Specii cally, the production of
CO
2
from respiration of organic matter with a molar
C:N:P ratio of approximately 106:16:1 (Redi eld
et al.
1963), exceeds production of NH
3
and PO
4
3-
by
one and two orders of magnitude, respectively, on a
molar basis. The aerobic respiration of organic mat-
ter therefore results in a net decrease of pH (Soetaert
et al.
2007). Furthermore, adsorption of Mn, NH
3
,
and PO
4
3-
onto minerals may remove substantial
amounts of these solutes from porewaters (Berner
-
+
-
CH O
+
NO
+
H
+
e
®+
CO
N
1
2
2
3
2
2(g )
+
2H O (denitrification)
2
-
2
+
CH O
+
MnO
+
2e
®+
CO
Mn
2
2
2
H O (Mn reduction)
+
2
-
-
2
+
CH O
+
Fe(OO)
+
e
®
CO
+
Fe
2
2
+
H O (Fe reduction)
2
2
-
+
-
CH O SO 3H CO HS
H O (sulphate reduction)
+
+
®+
2
4
2
+
2
4H
+
CO
®+
CH
2H O (methanogenesis)
2
2
4
2
It is important to note that all of these processes,
with the exception of methanogenesis, produce CO
2
and therefore lead to a reduction in pH.
In addition to CO
2
production, many microbial
processes consume or produce ions which are also
involved in acid-base equilibria and these ions will
have an additional effect on sediment pH (Soetaert
et al.
2007). For example, microbial nitrii cation
(
NH
+®
2O
HNO
+
) consumes ammonia
H O
3
2
3
2
N
+ +
(NH
3
, a weak base:
+®
) and pro-
duces nitric acid (HNO
3
) thereby reducing the pH
(increasing [H
+
]). With the exception of Mn and Fe
reduction, the acid-base (non-CO
2
) effect of these
NH
H
3
4
