Environmental Engineering Reference
In-Depth Information
capable to produce reliable data for individual sulphur intermediates in natural
waters at low concentrations and on sampling and preparation methods without
oxidation artefacts.
8. ALKALINITY AND C/S RATIOS
Alkalinity usually increases in the anoxic zone and in the pore water of
marine sediments [19, 21, 27, 38, 52, 82, 101]. Total alkalinity A
T
is a sum of
several components such as hydrocarbonate (or carbonate) A
C
and borate A
B
alkalinities, both typical for anoxic and oxic waters. Silicate A
Si
, hydrosulfide
(sulfide) A
S
, phosphate A
P
, and ammonium A
N
alkalinities add to the total
alkalinity A
T
in the anoxic water:
A
T
=
A
C
+
A
S
+
A
B
+
A
Si
+
A
P
+
A
N
or
[
HCO
3
]
2[
CO
2
3
]
[
HS
−
]
2[
S
2
−
]
[
B
(
OH
)
4
]
[
Si
(
OH
)
3
]
A
T
=
+
+
+
+
+
+
[
HPO
2
4
]
2[
PO
3
4
]
[
OH
−
]
[
H
+
]
+
+
[
NH
3
]
+
−
(1)
Total alkalinity is calculated using in situ pH and dissociation constants for
particular species in the equation (1) [21, 56]. Here we have used new alkalinity
data obtained by Makkaveev (Shirshov Institute of Oceanology RAS, Moscow)
in the north-eastern Black Sea during several cruises of
R/V Akvanavt
in 1999-
2001. Close relationships between A
T
and hydrogen sulphide S
T
=
(H
2
S+
HS
−
+S
2
−
) have been observed during several years in the Black Sea anoxic
water (Fig. 10):
Σ
In 1999: A
T
= 3304 + 3.11 S
T
(n = 134)
In 2000: A
T
= 3312 + 3.05 S
T
(n = 145)
In 2001: A
T
= 3302 + 3.04 S
T
(n = 180)
Average: A
T
= 3306 + 3.06 S
T
(n = 459).
The average A
T
of 4457 µM and total hydrogen sulphide S
T
of 376 µMin
the bottom layer of 1700-2200 m were used to calculate the increase of total
alkalinity (
A
T
) and its components in the layer from 100 to 2000 m (Table 4).
The contribution of different components in the total alkalinity changes with
depth. The sulfide component is responsible for up to 312 µM (or 27%) of
the total alkalinity increase
∆
A
T
in the anoxic zone. Silicate, phosphate and
ammonium components increase with depth too, but their relative contribu-
tion is insignificant compared to A
S
. The role of borate alkalinity decreases
insignificantly with depth.
The stoichiometry of the organic matter decomposition suggested by
Richards [75] is closely connected with the alkalinity concept:
∆
