Environmental Engineering Reference
In-Depth Information
Figure 8.
15
NO
3
−
from profiles taken in the heart of the
ODZ (Data from Brandes 1996). The cross hatched area shows the approximate boundaries of
the ODZ.
Composite profiles of δ
15
N
2
and δ
a maximum of about 15‰, more or less coincident with the SNM. Above the
SNM the δ
15
NofNO
3
−
decreases to about 6‰ at the base of the mixed layer.
The large enrichment of
15
NinNO
3
−
within the heart of ODZ is undoubtedly
due to fractionation during denitrification, especially as it is corroborated by
the mirror-image decrease in δ
15
NofN
2
(Fig 8). Laboratory culture studies
suggest a fractionation factor, ε, for denitrification of 17-20‰ [25, 61]. Given
the data shown in Fig. 8 and nitrate deficits calculated from hydrographic
data, Brandes et al. [8] fitted the Rayleigh fractionation equation, to arrive at
a fractionation factor of 22‰ (Rayleigh fraction:
15
N-NO
3
−
=10
3
(α - 1)ln
f
NO
3
, where α is the ratio of reaction rates of
15
N and
14
N and
f
NO
3
is the
fraction of the initial NO
3
−
removed by denitrification). The Rayleigh fraction
equation represents a closed system; however, Brandes et al. [8] also used an
open-system advection-reaction model to calculate a fractionation factor of
25‰. Altabet et al. [3] reported a similar, but slightly higher, value for ε.As
theses values are very similar to fractionation factors calculated similarly for
∆
δ
