Environmental Engineering Reference
In-Depth Information
global ocean appears to play a dominant role in regulating the oceanic content
of combined nitrogen, and therefore oceanic productivity.
The earliest estimates of the rate of pelagic denitrification were derived by
coupling measurements of the spatial distribution of NO
3
−
in an OMZ to circu-
lation models to provide an estimate of the absolute rate of NO
3
−
disappearance
along the mean flow path [15, 18, 42]. More recently, a variety of geochemical
[25] and isotopic [10] budgeting approaches have been used to constrain the
rate of denitrification in different OMZs. Direct tracer-based measurements
have not been widely used to quantify denitrification in the field to date. This
likely reflects the practical difficulties of maintaining suboxic conditions while
preparing and incubating experimental samples, and the scaling problems asso-
ciated with relating experimental results to the OMZ as a whole. Nonetheless,
tracer experiments have played a critical role in elucidating the role of “alter-
native” denitrification pathways in marine systems, particularly in sedimentary
contexts [19, 33].
The stable isotopes of nitrogen provide a natural analog to tracer experi-
ments and can often be used to elucidate the role of different processes in the
nitrogen cycle. For example, the biomass and detritus formed through biolog-
ical production in the upper ocean acquire an isotopic signature that reflects
the source(s) of N supporting productivity and/or the subsequent processing of
that organic matter. Within the water column, the remineralization of sinking
particles leads to characteristic patterns of variation in the natural abundance
of
15
N with depth [e.g., 5, 38, 41, 56]. Although deep water NO
3
acts as an
important, and often dominant, isotopic end-member for primary production in
the surface ocean, the isotopic composition of sinking organic matter may also
be affected by N
2
-fixation in the surface layer. The
15
N composition of partic-
ulate matter at depth therefore provides an integrative measure of the sources
of N fueling primary production at the surface. This signal can propagate to the
bottom, where it can enter the sedimentary record, providing information on
long-term variation in critical oceanic N cycle processes [3, 4, 17, 23, 27, 31].
1.1 Isotopic Fractionation
Most biological transformations of nitrogen in the ocean are accompanied by
significant kinetic isotopic fractionation, which leads to predictable alterations
in the natural abundance of the stable isotopes of N. The degree of isotopic
fractionation reflects the different reaction rates for molecules containing the
two isotopes of nitrogen and is commonly expressed as a fractionation factor,
α:
14
k
/
15
k
α
=
(1)
