353
δ 13 C of these tests. Again, the fractionation of
δ 13 C during precipitation has only a minor effect
on the carbon isotope composition of the solid
phase, the major effect is determined by the
δ 13 C ΣCO2 of the pore water which itself is controlled by the degradation of organic matter.
10.5 Geochemical Influences
on 15 N / 14 N Ratios
10.5.1 δ
δ δ
δ δ 15 N in Marine Ecosystems
Principles of Fractionation
The use of nitrogen isotopes in marine sediments
is a relatively new tool in the field of paleoceanography. The usefulness of this proxy lies in its
recording changes of nutrient dynamics in the
water column. Major biological transformations
of nitrogen in marine systems include the utilization of the dissolved forms of inorganic nitrogen
(NO 2
- , NO 3
- , NH 4
+ ) by phytoplankton, the consumption of phytoplankton by grazers, and the
remineralization of organic nitrogen by animals
and bacteria. In addition, various marine prokaryotes are able to carry out a suite of reactions
that move nitrogen in and out of the major
inorganic pools. These include nitrogen fixation
and dissimilatory transformations of nitrogen,
such as nitrification, denitrification, and anaerobic ammonium oxidation (anammox) which has
recently been recognized as a major sink for fixed
inorganic nitrogen in oxygen minimum zones (e.g.
Kuypers et al. 2003, 2005). The marine nitrogen
cycle involves multiple inorganic and organic
pools which are coupled by rapid biological
transformations of nitrogen (cf. Chap. 6).
Since most marine autotrophs require combined nitrogen as a substrate for growth, the
isotopic composition of dissolved inorganic nitrogen acts as a sort of master variable in setting the baseline isotopic composition of marine
plankton. A number of biological processes may
alter the isotopic composition of the marine
pool of inorganic nitrogen. The most important
are nitrogen fixation, nitrification, denitrification, and anaerobic ammonium oxidation, all
of which may lead to a net transfer between the
oceanic pool of combined nitrogen and the atmosphere. The basic biochemical reactions are
the following:
Nitrogen fixation:
N 2 atm. ⇒ organic N
(10.14)
It is well known that symbiontic bacteria in the
roots of plants, but also bacteria and algae in the
ocean can fix nitrogen. The high energy needed to
break the triple bond of the nitrogen gas molecule
makes natural nitrogen fixation a process feasable
by only few organisms.
Ammonification:
N org ⇒ NH 4
+
(10.15)
Nitrification:
NH 4
+ + 3/2 O 2 ⇒ NO 2
- + H 2 O + 2 H +
(10.16)
NO 2
- + 1/2 O 2 ⇒ NO 3
-
(10.17)
Although ammonium is the direct product of
decomposition of nitrogenous organic compounds (deamination), most inorganic nitrogen in
the ocean occurs in the form of nitrate. Conversion of ammonia into nitrate is carried out by
nitrifying organism.
Denitrification:
5 (CH 2 O) + 4 NO 3
- + 4 H + ⇒
5 CO 2 + 7 H 2 O + 2 N 2
(10.18)
or Anammox:
NH 4
+ + NO 2
- ⇒ N 2 + 2 H 2 O
(10.19)
After oxygen is exhausted, nitrate can serve as
electron acceptor in the degradation of organic
matter. Denitrification results in the conversion of
nitrate to nitrogen gas and therefore balances the
biological fixation of nitrogen. If there were no
denitrification or anammox, atmospheric nitrogen
would be exhausted in less than 100 million years.
Denitrification and anaerobic ammonium oxidation
occur in stratified or stagnant water masses of the
ocean (e.g. within oxygen minimum layers) and in
the sea bed.
The main processes involved in the biological
utilization of nitrogen are all associated with
kinetic fractionation effects, nevertheless they
exhibit different fractionation factors. The isotope effect of nitrogen fixation (α = 1.000 to
1.004) is small relative to the effects of bacterial
nitrification, denitrification, or anammox (α = 1.02
to 1.04) (see Montoya et al. 1994: Table 1). The
results by Miyake and Wada (1971) indicate that
little overall isotope fractionation occurs in the
10.5
Geochemical Influences on 15N/14N Ratios
δ 13 C of these tests. Again, the fractionation of
δ 13 C during precipitation has only a minor effect
on the carbon isotope composition of the solid
phase, the major effect is determined by the
δ 13 C ΣCO2 of the pore water which itself is controlled by the degradation of organic matter.
10.5 Geochemical Influences
on 15 N / 14 N Ratios
10.5.1 δ
δ δ
δ δ 15 N in Marine Ecosystems
Principles of Fractionation
The use of nitrogen isotopes in marine sediments
is a relatively new tool in the field of paleoceanography. The usefulness of this proxy lies in its
recording changes of nutrient dynamics in the
water column. Major biological transformations
of nitrogen in marine systems include the utilization of the dissolved forms of inorganic nitrogen
(NO 2
- , NO 3
- , NH 4
+ ) by phytoplankton, the consumption of phytoplankton by grazers, and the
remineralization of organic nitrogen by animals
and bacteria. In addition, various marine prokaryotes are able to carry out a suite of reactions
that move nitrogen in and out of the major
inorganic pools. These include nitrogen fixation
and dissimilatory transformations of nitrogen,
such as nitrification, denitrification, and anaerobic ammonium oxidation (anammox) which has
recently been recognized as a major sink for fixed
inorganic nitrogen in oxygen minimum zones (e.g.
Kuypers et al. 2003, 2005). The marine nitrogen
cycle involves multiple inorganic and organic
pools which are coupled by rapid biological
transformations of nitrogen (cf. Chap. 6).
Since most marine autotrophs require combined nitrogen as a substrate for growth, the
isotopic composition of dissolved inorganic nitrogen acts as a sort of master variable in setting the baseline isotopic composition of marine
plankton. A number of biological processes may
alter the isotopic composition of the marine
pool of inorganic nitrogen. The most important
are nitrogen fixation, nitrification, denitrification, and anaerobic ammonium oxidation, all
of which may lead to a net transfer between the
oceanic pool of combined nitrogen and the atmosphere. The basic biochemical reactions are
the following:
Nitrogen fixation:
N 2 atm. ⇒ organic N
(10.14)
It is well known that symbiontic bacteria in the
roots of plants, but also bacteria and algae in the
ocean can fix nitrogen. The high energy needed to
break the triple bond of the nitrogen gas molecule
makes natural nitrogen fixation a process feasable
by only few organisms.
Ammonification:
N org ⇒ NH 4
+
(10.15)
Nitrification:
NH 4
+ + 3/2 O 2 ⇒ NO 2
- + H 2 O + 2 H +
(10.16)
NO 2
- + 1/2 O 2 ⇒ NO 3
-
(10.17)
Although ammonium is the direct product of
decomposition of nitrogenous organic compounds (deamination), most inorganic nitrogen in
the ocean occurs in the form of nitrate. Conversion of ammonia into nitrate is carried out by
nitrifying organism.
Denitrification:
5 (CH 2 O) + 4 NO 3
- + 4 H + ⇒
5 CO 2 + 7 H 2 O + 2 N 2
(10.18)
or Anammox:
NH 4
+ + NO 2
- ⇒ N 2 + 2 H 2 O
(10.19)
After oxygen is exhausted, nitrate can serve as
electron acceptor in the degradation of organic
matter. Denitrification results in the conversion of
nitrate to nitrogen gas and therefore balances the
biological fixation of nitrogen. If there were no
denitrification or anammox, atmospheric nitrogen
would be exhausted in less than 100 million years.
Denitrification and anaerobic ammonium oxidation
occur in stratified or stagnant water masses of the
ocean (e.g. within oxygen minimum layers) and in
the sea bed.
The main processes involved in the biological
utilization of nitrogen are all associated with
kinetic fractionation effects, nevertheless they
exhibit different fractionation factors. The isotope effect of nitrogen fixation (α = 1.000 to
1.004) is small relative to the effects of bacterial
nitrification, denitrification, or anammox (α = 1.02
to 1.04) (see Montoya et al. 1994: Table 1). The
results by Miyake and Wada (1971) indicate that
little overall isotope fractionation occurs in the
10.5
Geochemical Influences on 15N/14N Ratios