Environmental Engineering Reference
In-Depth Information
the ODZ's of the eastern tropical North Pacific [3, 8], it appears that open-
ocean-water-column denitrification may display a relatively constant isotope
fractionation.
Interestingly, the NO
3
−
isotope composition at the base of the mixed layer
is characterized by a relative depletion of
15
N(δ
15
N
∼
6‰). In non-ODZ
areas the trend toward the surface is exactly the opposite, i.e., the δ
15
Nof
NO
3
−
increases toward the surface due to the preferential uptake of
14
NO
3
−
by phytoplankton [2, 86]. Thus, there appears to be a requirement for a source
of isotopically light fixed nitrogen in the Arabian Sea mixed layer. This light
fixed nitrogen must either be laterally advected to the central Arabian Sea or
generated locally through nitrogen fixation, which produces fixed nitrogen with an
isotopic signature near 0‰ [94]. The principal water mass that forms the upper
portion of the thermocline, characterized by light δ
15
N-NO
3
−
values, is the
Arabian Sea high salinity surface water. This water is formed in the northern
Arabian Sea during winter [53]. The nitrate content of this water is expected to
be determined by local processes (regeneration from organic matter and vertical
supply from the deeper layers) rather than the long range transport from the
south. Hence nitrogen fixation appears to be the most likely process responsible
for the observed decrease in δ
15
NofNO
3
in near surface waters.
Brandes et al. [8] estimated the amount of nitrogen fixation required to match
the observed NO
3
−
isotope distribution. They assumed the water upwelled to
the mixed layer originated at 200 m, which had an isotopic composition of
11‰. If all the upwelled NO
3
−
were utilized by primary producers in the
mixed layer above 80 m, then a simple mixing calculation suggested that 40%
of the fixed nitrogen must have come from nitrogen fixation. This gave them a
rate of 6 TgN y
1
. Because newer primary production data [6] is about twice as
great as estimates [79] used by Brandes et al., this might be a low estimate for
N-fixation. Conversely, the upwelling depth chosen by Brandes et al., is deeper
than that suggested by the analysis of USJGOFS data [63], which would lower
Brandes et al.'s estimate. Nevertheless, the nitrogen fixation rate in the Arabian
Sea appears to be lower by several times than the denitrification rate indicating
that on balance the Arabian Sea is a substantial sink of fixed nitrogen.
Dual isotopic analysis of N
2
O also shows a pattern of enrichment of both
15
N and
18
O (relative to the more abundant
16
O) - sometimes exceeding 30‰
(versus air) and 60‰ (versus Standard Mean Ocean Water), respectively - in
strongly denitrifying waters (Fig. 9; [72, 73, 98]). In general, the maxima in
δ
18
O are associated with the minimum of N
2
O concentration. At
sites located at the periphery of the perennial denitrification zone where the
intermediate N
2
O concentration minimum was weakly developed, enrichments
in the two heavier isotopes were not as great as at those at strongly denitrify-
ing locations [72]. It is obvious, therefore, that the isotopic anomalies in the
denitrifying waters result from the preferential loss of light N
2
OtoN
2
.
15
N and δ
