229
Koike and Sørensen 1988; Hynes and Knowles
1978; Seitzinger 1988).
The application of acetylene and nitrogen
labeling methods usually results in the determination of lower denitrification rates when compared to total nitrogen fluxes (Seitzinger et al.
1993; cf. Section 6.5). Apart from the above
restrictions inherent to the methods themselves,
the recently published concept of Luther et al.
(1997) provides an additional explanation for the
observed discrepancy. They tested the thermodynamic properties of several redox reactions and
found evidence for a catalytic short circuit for the
coupled process of nitrification-denitrification
within the oxic zone (Eq. 6.12.). Organic nitrogen
and ammonia released during oxic respiration are
therefore oxidized to N 2 by MnO 2 , instead of being
further oxidized to NO 3 .
2 NH 3 + 3 MnO 2 + 6 H
+
→ 3 Mn
2+
+ N 2 + 6 H 2 O
(6.12)
This process may outweigh nitrification in
manganese-rich surface sediments and circumvents denitrification. Elevated N 2 fluxes without
increasing denitrification rates and even N 2
production in oxidized sediments as observed by
Seitzinger (1988) may be explained by this
process.
6.5
Significance and Quantitative
Approaches
After the description of the general biogeochemical processes controlling the distribution of
oxygen and nitrate in marine sediments, including
the possibilities and limitations of determining
these inorganic compounds in the deep-sea, the
following section will give an overview of the
dimensions of their fluxes and their distribution in
different marine environments.
6.5.1
Estimation of Global Rates and
Fluxes
The basic mechanism inducing microbial activity
is the supply of organic matter to the seafloor and
this is generally coupled to surface water productivity. Most of the highly productive areas in the
global ocean are adjacent to the continents, so
that we can expect a decrease of respiration intensity from the coastal marine environments over
the continental shelves and slopes into the deepsea. This becomes evident when we look at the
data compiled by Middelburg et al. (1993) which
indicate that 83% mineralization and 87% burial in
marine sediments occurs in the coastal zone
occupying only ~9% of the total ocean area. This
means that the sediments with the highest respiration rates also have the highest burial efficiency
in marine environments. For a more detailed
discussion of this subject see also reviews by
(Henrichs and Reeburgh 1987; Henrichs 1992;
Canfield 1993). As shown in Figure 6.3 the organic
matter supply is not only a function of productivity, but also of water depth generally amplifying
this gradient between shallow and deep water
environments.
Fluxes of oxygen and nitrate, therefore, vary
over several orders of magnitude between oligotrophic open ocean areas and continental shelf and
slope areas. This is about 50 to 6,000 mmol m
-2
yr
-1
for oxygen and -600 to 380 mmol m
-2
yr
-1
for nitrate
(e.g. Devol and Christensen 1993; Glud et al. 1994;
Berelson et al. 1994; Hammond et al. 1996; Luther
et al. 1997; Hensen et al. 1998; Wenzhöfer and Glud
2002) where negative nitrate fluxes indicate fluxes
into the sediment. The above minimum and
maximum values do not permit differentiation
between total and diffusive or in situ and ex situ
fluxes. Figure 6.18 reveals the distribution of
nitrate and phosphate fluxes released from
sediments below 1,000 m water depth in the South
Atlantic (Hensen et al. 1998) based on about 180
ex-situ concentration profiles. Averaged fluxes
vary between 10 – 180 mmol m
-2
yr
-1
for nitrate and
(-2)-11 mmol m
-2
yr
-1
for phosphate (where
negative values are considered to be artifacts due
to decompression and warming, cf. 6.3.2.2). Based
on this compilation the total annual release for the
whole area (about one tenth of the global deep
ocean) is about 1.6 10
12
mol NO 3 yr
-1
and 3.5
10
10
mol PO 4 yr
-1
.
Global denitrification in marine sediments has
been estimated by Middelburg et al. (1996a) to be
about 1.64 – 2.03⋅10
13
mol N yr
-1
with a contribution
of 0.71⋅10
13
mol N yr
-1
of shelf sediments. This
model-based re-estimation produced values which
are up to 3 to 20 times higher than those previously estimated by a number of authors in the
mid-1980s. The predicted denitrification rates,
however, are in the range of those derived from
literature, and the total contribution to organic
6.5
Significance and Quantitative Approaches
Koike and Sørensen 1988; Hynes and Knowles
1978; Seitzinger 1988).
The application of acetylene and nitrogen
labeling methods usually results in the determination of lower denitrification rates when compared to total nitrogen fluxes (Seitzinger et al.
1993; cf. Section 6.5). Apart from the above
restrictions inherent to the methods themselves,
the recently published concept of Luther et al.
(1997) provides an additional explanation for the
observed discrepancy. They tested the thermodynamic properties of several redox reactions and
found evidence for a catalytic short circuit for the
coupled process of nitrification-denitrification
within the oxic zone (Eq. 6.12.). Organic nitrogen
and ammonia released during oxic respiration are
therefore oxidized to N 2 by MnO 2 , instead of being
further oxidized to NO 3 .
2 NH 3 + 3 MnO 2 + 6 H
+
→ 3 Mn
2+
+ N 2 + 6 H 2 O
(6.12)
This process may outweigh nitrification in
manganese-rich surface sediments and circumvents denitrification. Elevated N 2 fluxes without
increasing denitrification rates and even N 2
production in oxidized sediments as observed by
Seitzinger (1988) may be explained by this
process.
6.5
Significance and Quantitative
Approaches
After the description of the general biogeochemical processes controlling the distribution of
oxygen and nitrate in marine sediments, including
the possibilities and limitations of determining
these inorganic compounds in the deep-sea, the
following section will give an overview of the
dimensions of their fluxes and their distribution in
different marine environments.
6.5.1
Estimation of Global Rates and
Fluxes
The basic mechanism inducing microbial activity
is the supply of organic matter to the seafloor and
this is generally coupled to surface water productivity. Most of the highly productive areas in the
global ocean are adjacent to the continents, so
that we can expect a decrease of respiration intensity from the coastal marine environments over
the continental shelves and slopes into the deepsea. This becomes evident when we look at the
data compiled by Middelburg et al. (1993) which
indicate that 83% mineralization and 87% burial in
marine sediments occurs in the coastal zone
occupying only ~9% of the total ocean area. This
means that the sediments with the highest respiration rates also have the highest burial efficiency
in marine environments. For a more detailed
discussion of this subject see also reviews by
(Henrichs and Reeburgh 1987; Henrichs 1992;
Canfield 1993). As shown in Figure 6.3 the organic
matter supply is not only a function of productivity, but also of water depth generally amplifying
this gradient between shallow and deep water
environments.
Fluxes of oxygen and nitrate, therefore, vary
over several orders of magnitude between oligotrophic open ocean areas and continental shelf and
slope areas. This is about 50 to 6,000 mmol m
-2
yr
-1
for oxygen and -600 to 380 mmol m
-2
yr
-1
for nitrate
(e.g. Devol and Christensen 1993; Glud et al. 1994;
Berelson et al. 1994; Hammond et al. 1996; Luther
et al. 1997; Hensen et al. 1998; Wenzhöfer and Glud
2002) where negative nitrate fluxes indicate fluxes
into the sediment. The above minimum and
maximum values do not permit differentiation
between total and diffusive or in situ and ex situ
fluxes. Figure 6.18 reveals the distribution of
nitrate and phosphate fluxes released from
sediments below 1,000 m water depth in the South
Atlantic (Hensen et al. 1998) based on about 180
ex-situ concentration profiles. Averaged fluxes
vary between 10 – 180 mmol m
-2
yr
-1
for nitrate and
(-2)-11 mmol m
-2
yr
-1
for phosphate (where
negative values are considered to be artifacts due
to decompression and warming, cf. 6.3.2.2). Based
on this compilation the total annual release for the
whole area (about one tenth of the global deep
ocean) is about 1.6 10
12
mol NO 3 yr
-1
and 3.5
10
10
mol PO 4 yr
-1
.
Global denitrification in marine sediments has
been estimated by Middelburg et al. (1996a) to be
about 1.64 – 2.03⋅10
13
mol N yr
-1
with a contribution
of 0.71⋅10
13
mol N yr
-1
of shelf sediments. This
model-based re-estimation produced values which
are up to 3 to 20 times higher than those previously estimated by a number of authors in the
mid-1980s. The predicted denitrification rates,
however, are in the range of those derived from
literature, and the total contribution to organic
6.5
Significance and Quantitative Approaches