NITROGEN ISOTOPES IN THE OCEAN
D. M. Sigman and K. L. Karsh, Princeton University,
Princeton, NJ, USA
K. L. Casciotti, Woods Hole Oceanographic
Institution, Woods Hole, MA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
Nitrogen has two stable isotopes,
14 N and
15 N
(atomic masses of 14 and 15, respectively).
14 N is the
more abundant of the two, comprising 99.63% of
the nitrogen found in nature. Physical, chemical, and
biological processes discriminate between the two
isotopes, leading to subtle but measurable differences
in the ratio of
15
N to
14 N among different forms of
nitrogen found in the marine environment.
Nitrogen is a central component of marine biomass and one of the major nutrients required by all
phytoplankton. In this sense, biologically available
(or ‘fixed’, i.e., non-N 2 ) N is representative of the
fundamental patterns of biogeochemical cycling in
the ocean. However, N differs from other nutrients in
that its oceanic sources and sinks are dominantly
internal and biological, with marine N 2 fixation
supplying much of the fixed N in the ocean and
marine denitrification removing it. The N isotopes
provide a means of studying both the input/output
budget of oceanic fixed N and its cycling within the
ocean. In this overview, we outline the isotope systematics of N cycle processes and their impacts on
the isotopic composition of the major N reservoirs in
the ocean. This information provides a starting point
for considering the wide range of questions in ocean
sciences to which the N isotopes can be applied.
Terms and Units
Mass spectrometry can measure precisely the ratio of
the N isotopes relative to a N reference containing a
constant isotopic ratio. The universal reference for N
isotopes is atmospheric N 2 , with an
15
N/
14
N ratio of
0.367 65%70.000 81%. Natural samples exhibit
small deviations from the standard ratio, which are
expressed in d notation (in units of per mil, %):
d
15 Nð%Þ ¼
ð
15 N=
14 NÞ sample
ð
15 N=
14 NÞ standard
À 1
!
 1000 ½1
In this notation, the d
15
N of atmospheric N 2 is 0%.
Special terms are also used to characterize the
amplitude of isotopic fractionation caused by a given
process. Isotope fractionation results from both
equilibrium processes (‘equilibrium fractionation’)
and unidirectional reactions (‘kinetic fractionation’).
Nitrogen isotope variations in the ocean are typically
dominated by kinetic fractionation associated with
the conversions of N from one form to another. The
kinetic isotope effect, e, of a given reaction is defined
by the ratio of rates with which the two N isotopes
are converted from reactant to product:
eð%Þ ¼ ð
14 k=
15 k À 1Þ Â 1000
½2
where
14 k and
15
k are the rate coefficients of the
reaction for
14
N- and
15
N-containing reactant, respectively. For e{1000%, e is approximated by the
difference in d
15
N between the reactant and its instantaneous product. That is, if a reaction has an e of
5%, then the d
15 N of the product N generated at any
given time will be B5% lower than the d
15
N of the
reactant N at that time.
Measurements
The isotopic analysis of N relies on the generation of
a stable gas as the analyte for isotope ratio mass
spectrometry. Online combustion to N 2 is currently
the standard method for the preparation of a N
sample for isotopic analysis. With ‘off-the-shelf’
technology, a typical sample size requirement is
1–2 mmol N per analysis. Gas chromatography followed by combustion to N 2 is improving as a technique for specific organic compounds, amino acids in
particular, although the polarity of many N compounds remains a challenge. Liquid chromatography
is also being explored. There are standard methods
of collection for most bulk forms of particulate N
(PN) in the ocean. Shallow and deep samples of
suspended PN are filtered onto glass fiber filters.
Sinking PN is collected by sediment traps. Zooplankton can be picked from filtered samples or net
tows, and particulates can be separated into size
classes. In the case of dissolved forms of N, the
species of interest must be converted selectively to a
gas or other extractable form for collection. Since the
1970s, the d
15 N values of marine nitrate (NO 3
À ),
nitrite (NO 2
À ), and ammonium (NH 4
þ ) have been
analyzed by conversion to ammonia gas and collection of the cationic ammonium form for
549
D. M. Sigman and K. L. Karsh, Princeton University,
Princeton, NJ, USA
K. L. Casciotti, Woods Hole Oceanographic
Institution, Woods Hole, MA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
Nitrogen has two stable isotopes,
14 N and
15 N
(atomic masses of 14 and 15, respectively).
14 N is the
more abundant of the two, comprising 99.63% of
the nitrogen found in nature. Physical, chemical, and
biological processes discriminate between the two
isotopes, leading to subtle but measurable differences
in the ratio of
15
N to
14 N among different forms of
nitrogen found in the marine environment.
Nitrogen is a central component of marine biomass and one of the major nutrients required by all
phytoplankton. In this sense, biologically available
(or ‘fixed’, i.e., non-N 2 ) N is representative of the
fundamental patterns of biogeochemical cycling in
the ocean. However, N differs from other nutrients in
that its oceanic sources and sinks are dominantly
internal and biological, with marine N 2 fixation
supplying much of the fixed N in the ocean and
marine denitrification removing it. The N isotopes
provide a means of studying both the input/output
budget of oceanic fixed N and its cycling within the
ocean. In this overview, we outline the isotope systematics of N cycle processes and their impacts on
the isotopic composition of the major N reservoirs in
the ocean. This information provides a starting point
for considering the wide range of questions in ocean
sciences to which the N isotopes can be applied.
Terms and Units
Mass spectrometry can measure precisely the ratio of
the N isotopes relative to a N reference containing a
constant isotopic ratio. The universal reference for N
isotopes is atmospheric N 2 , with an
15
N/
14
N ratio of
0.367 65%70.000 81%. Natural samples exhibit
small deviations from the standard ratio, which are
expressed in d notation (in units of per mil, %):
d
15 Nð%Þ ¼
ð
15 N=
14 NÞ sample
ð
15 N=
14 NÞ standard
À 1
!
 1000 ½1
In this notation, the d
15
N of atmospheric N 2 is 0%.
Special terms are also used to characterize the
amplitude of isotopic fractionation caused by a given
process. Isotope fractionation results from both
equilibrium processes (‘equilibrium fractionation’)
and unidirectional reactions (‘kinetic fractionation’).
Nitrogen isotope variations in the ocean are typically
dominated by kinetic fractionation associated with
the conversions of N from one form to another. The
kinetic isotope effect, e, of a given reaction is defined
by the ratio of rates with which the two N isotopes
are converted from reactant to product:
eð%Þ ¼ ð
14 k=
15 k À 1Þ Â 1000
½2
where
14 k and
15
k are the rate coefficients of the
reaction for
14
N- and
15
N-containing reactant, respectively. For e{1000%, e is approximated by the
difference in d
15
N between the reactant and its instantaneous product. That is, if a reaction has an e of
5%, then the d
15 N of the product N generated at any
given time will be B5% lower than the d
15
N of the
reactant N at that time.
Measurements
The isotopic analysis of N relies on the generation of
a stable gas as the analyte for isotope ratio mass
spectrometry. Online combustion to N 2 is currently
the standard method for the preparation of a N
sample for isotopic analysis. With ‘off-the-shelf’
technology, a typical sample size requirement is
1–2 mmol N per analysis. Gas chromatography followed by combustion to N 2 is improving as a technique for specific organic compounds, amino acids in
particular, although the polarity of many N compounds remains a challenge. Liquid chromatography
is also being explored. There are standard methods
of collection for most bulk forms of particulate N
(PN) in the ocean. Shallow and deep samples of
suspended PN are filtered onto glass fiber filters.
Sinking PN is collected by sediment traps. Zooplankton can be picked from filtered samples or net
tows, and particulates can be separated into size
classes. In the case of dissolved forms of N, the
species of interest must be converted selectively to a
gas or other extractable form for collection. Since the
1970s, the d
15 N values of marine nitrate (NO 3
À ),
nitrite (NO 2
À ), and ammonium (NH 4
þ ) have been
analyzed by conversion to ammonia gas and collection of the cationic ammonium form for
549