Environmental Engineering Reference
In-Depth Information
where
14
k and
15
k are the rate constants for reaction of molecules containing the
light and heavy isotopes, respectively. Using this convention, “normal” isotopic
fractionation that discriminates against the heavy isotope has a fractionation
factor greater than unity. The inverse convention is also used by some authors
[e.g., 35], requiring careful attention when comparing fractionation factors
reported in different publications.
While the fractionation factor describes the relative reaction rates of the dif-
ferent isotopic species, the isotopic discrimination factor, , provides a simpler
expression of the magnitude of fractionation:
1000 (2)
To a good approximation, the discrimination factor is equal to the instantaneous
difference in δ
=
(α
−
1)
∗
15
N between the substrate and product of a reaction
15
N
product
(3)
as long as residual substrate remains and is undergoing reaction. As a reaction
progresses in a closed system, both the residual substrate and the product
formed will become progressively enriched in
15
N, following a typical Rayleigh
fractionation trajectory (Fig. 1). Conservation of mass and isotopes requires that
theδ
15
N
substrate
=
δ
−
δ
15
N of the combined substrate and product pool remain constant throughout
the course of reaction. Note that complete conversion of substrate to product
will leave no measurable isotopic imprint since the accumulated product will
have exactly the same δ
15
N as the initial pool of substrate even if the reaction
itself discriminates strongly between isotopes (Fig. 1). The effect of isotopic
fractionation is thus observable only under conditions of partial consumption
of the available substrate pool.
1.2 Isotopic Fractionation in the Water Column
Deep water NO
3
−
is the dominant pool of combined nitrogen in the ocean,
and its isotopic composition integrates a variety of processes and inputs. In
OMZs, suboxic conditions promote denitrification as a respiratory pathway
supporting heterotrophic microbial growth. Both field and laboratory exper-
iments have shown that denitrification discriminates strongly (20 - 30 ‰)
between the stable isotopes of nitrogen [6, 16, 57]. As a result, the partial
denitrification characteristic of many pelagic systems (e.g., the Eastern Trop-
ical Pacific and the Arabian Sea) generates a strong isotopic enrichment in
the residual NO
3
−
, as shown schematically in Fig. 1. In contrast, sedimentary
denitrification typically goes to completion within the sediment, resulting in no
expression of the isotope effect associated with the denitrification process itself
[8, 9]. In other words, sedimentary denitrification is effectively invisible from
an isotopic standpoint (Fig. 2), removing combined nitrogen from the ocean
