Environmental Engineering Reference
In-Depth Information
N
2
Excess (
µ
g-at l
-1
)
(N
2
:Ar)
n
1.00
1.01
1.02
1.03
-
5 0 5 0 5 025
0
0
500
1000
1500
2000
2500
3000
Figure 5. N:Ar
ratio profiles (left) from the four WOCE stations (open symbols) as well as those in the ODZ (Solid symbols) and
Composite N
2
excess profile (right). Symbols are as follows 28
o
S, open tria ngles; 10
o
S open circle s; 1
o
S open inverte d triangles;
8
o
N open square s; Jan.98, solid square s; Dec.98, solid triang les; Apr.96, solid inverte d triangles; and Sept. 99, solid
diamonds. The cross hatched area shows the approx
imate boundaries of the ODZ (Redrawn from [28]).
This is close to the upper end of the range (0.4-3.5 N yr
−
1
) for sedimentary
denitrification for the Arabian Sea reported by Naik [65]. Sulfate reduction is
probably the dominant carbon oxidation pathway in the sediment bordering
the ODZ. Moreover, there are several strains of sulfide oxidizing bacteria, e.g.
Thioploca, Beggiatoa and Thiomargarita
, that derive energy from the reduction
of NO
3
−
to NH
4
+
[49] and mats
Thioploca
, are known to occur in sediments of
the Arabian Sea ODZ [84]. The NH
4
+
produced by these processes will diffuse
into the overlying water where anammox could convert it to N
2
[20]. It may be
noted that the chemoautotrophic reduction of NO
3
−
to NH
4
+
does not involve
regeneration of PO
4
3
−
. It should be noted however, that because sedimentary
processes also regenerate PO
4
3
, they could only contribute to the excess N
2
to the extent that nitrogen is preferentially recycled relative to phosphorus.
ODZ sediments have been found to support phosphogenesis with the apatite
precipitation rate varying between 0.08 and 1.04µmole P cm
−
2
yr
−
1
[83]. Thus,
one might expect higher N/P export ratio from the ODZ sediments than ex-
