6
Benthic Cycling of Oxygen, Nitrogen and Phosphorus
210
Redfield et al. (1963) suggested an overall C/N/P/
O 2 ratio of 106/16/1/138 (see Eq. 6.1, cf. Fig 3.11).
However, although widely used, subsequent
investigations have put this formulation into
question. Deviating ratios were formulated as 140/
16/1/172 (Takahashi et al. 1985) or 117/16/1/170
(Anderson and Sarmiento 1994) implying that
there is still debate on the general validity of the
use of one “Redfield ratio” for all ocean basins
and all water depths. Instead, C/N/P ratios seem to
be subject to regional variation.
Oxygen depth profiles show an opposite trend
to alkalinity and nitrate resulting from low
mineralization rates in the deep ocean waters and
input of oxygen-rich water masses by advective
transport. Figure 6.4 shows a meridional transect
of oxygen concentration through the Atlantic
Ocean compiled by Reid (1994). The most prominent pattern is the southward flow of oxygenrich North Atlantic Deep Water raising the oxygen
concentration along its flow path into the
equatorial South Atlantic at water depths of
3,000 – 4,000 m. Less prominent, but still
significant, is also the northward flow of oxygenrich Antarctic Intermediate Water which is marked
by elevated oxygen concentrations on a southnorth path (50°S – 20°S) between 0 – 1,000 m
water depth. The distribution of oxygen in ocean
water is therefore strongly dependent on largescale circulation patterns. The same is valid for
the distribution of nutrients: Vertical concentration profiles are always a mixture of in situ
decomposition and advective transport processes.
The global deep-water circulation pattern shows a
general flow path from the North Atlantic to the
North Pacific and Indian Oceans. As a result,
“older” deep waters in the Pacific and Indian
Oceans are depleted of oxygen and enriched in
nutrients (Fig. 6.2) and CO 2 (Broecker and Peng
1982; Kennett 1982; Chester 1990; see Chapter 9).
When studying the early diagenesis of deepsea sediments, it is very important to consider all
features of the oceanic environment and consequently of the composition of the overlying deepocean water, since it determines the availability of
any solute, i.e. oxygen or nitrate, as possible oxidants for organic carbon mineralization. As a
consequence of intense deep water mixing and
limited supply of degradable material to the
sediments within the big central gyres, oxidant
limitation is the exceptional case, which has,
however, not always been the case in earth
history (cf. Section 4.1).
6.3
The Role of Oxygen, Nitrogen
and Phosphorus in Marine
Sediments
To estimate the role of oxygen and nitrate we have
to describe the general processes occurring close
to the sediment water interface, the methods how
to measure concentrations, fluxes, and consumption rates, and how to relate them to organic matter degradation and other processes. Subsequently,
we will show examples from case studies to
characterize the magnitude of fluxes and different
environments in deep-sea sediments. The early
diagenetic processes at the sediment-water interface are of special interest in global biogeochemical cycles because it is decided at this
separation line between ocean water and sediment
if any substance is recycled or buried for a long
period of time in a geological sense.
6.3.1
Respiration and Redox Processes
6.3.1.1 Nitrification and Denitrification
In principle, the sequence of oxidants is determined by the energy yield for the microorganisms. When oxygen and nitrate are depleted
reduction of Mn and Fe (oxo)hydroxides and
sulfate as well as methane fermentation follow in
the sequence with decreasing yield of energy
(Froelich et al. 1979; cf. Section 3.2.5 and
subsequent chapters). This sequence is generally valid, even though numerous studies have
identified an overlap of carbon oxidation pathways within the sediment resulting from competition between microbial populations (Canfield
1993) and the presence of microenvironments
(e.g. Jørgensen 1977; cf. Chapters 7, 8, 12).
The general equations of coupled oxic respiration and nitrification (6.1) and denitrification
(6.2) describing the “top” of the diagenetic
sequence are given as:
Oxic respiration and nitrification
(CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138 O 2 →
106 CO 2 + 16 HNO 3 + H 3 PO 4 + 122 H 2 O
∆G
0
= - 3190 kJ mol
-1
(6.1)
Denitrification
(CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 94.4 HNO 3 →
106 CO 2 + 55.2 N 2 + H 3 PO 4 + 177.2 H 2 O
∆G
0
= - 3030 kJ mol
-1
(6.2)
Benthic Cycling of Oxygen, Nitrogen and Phosphorus
210
Redfield et al. (1963) suggested an overall C/N/P/
O 2 ratio of 106/16/1/138 (see Eq. 6.1, cf. Fig 3.11).
However, although widely used, subsequent
investigations have put this formulation into
question. Deviating ratios were formulated as 140/
16/1/172 (Takahashi et al. 1985) or 117/16/1/170
(Anderson and Sarmiento 1994) implying that
there is still debate on the general validity of the
use of one “Redfield ratio” for all ocean basins
and all water depths. Instead, C/N/P ratios seem to
be subject to regional variation.
Oxygen depth profiles show an opposite trend
to alkalinity and nitrate resulting from low
mineralization rates in the deep ocean waters and
input of oxygen-rich water masses by advective
transport. Figure 6.4 shows a meridional transect
of oxygen concentration through the Atlantic
Ocean compiled by Reid (1994). The most prominent pattern is the southward flow of oxygenrich North Atlantic Deep Water raising the oxygen
concentration along its flow path into the
equatorial South Atlantic at water depths of
3,000 – 4,000 m. Less prominent, but still
significant, is also the northward flow of oxygenrich Antarctic Intermediate Water which is marked
by elevated oxygen concentrations on a southnorth path (50°S – 20°S) between 0 – 1,000 m
water depth. The distribution of oxygen in ocean
water is therefore strongly dependent on largescale circulation patterns. The same is valid for
the distribution of nutrients: Vertical concentration profiles are always a mixture of in situ
decomposition and advective transport processes.
The global deep-water circulation pattern shows a
general flow path from the North Atlantic to the
North Pacific and Indian Oceans. As a result,
“older” deep waters in the Pacific and Indian
Oceans are depleted of oxygen and enriched in
nutrients (Fig. 6.2) and CO 2 (Broecker and Peng
1982; Kennett 1982; Chester 1990; see Chapter 9).
When studying the early diagenesis of deepsea sediments, it is very important to consider all
features of the oceanic environment and consequently of the composition of the overlying deepocean water, since it determines the availability of
any solute, i.e. oxygen or nitrate, as possible oxidants for organic carbon mineralization. As a
consequence of intense deep water mixing and
limited supply of degradable material to the
sediments within the big central gyres, oxidant
limitation is the exceptional case, which has,
however, not always been the case in earth
history (cf. Section 4.1).
6.3
The Role of Oxygen, Nitrogen
and Phosphorus in Marine
Sediments
To estimate the role of oxygen and nitrate we have
to describe the general processes occurring close
to the sediment water interface, the methods how
to measure concentrations, fluxes, and consumption rates, and how to relate them to organic matter degradation and other processes. Subsequently,
we will show examples from case studies to
characterize the magnitude of fluxes and different
environments in deep-sea sediments. The early
diagenetic processes at the sediment-water interface are of special interest in global biogeochemical cycles because it is decided at this
separation line between ocean water and sediment
if any substance is recycled or buried for a long
period of time in a geological sense.
6.3.1
Respiration and Redox Processes
6.3.1.1 Nitrification and Denitrification
In principle, the sequence of oxidants is determined by the energy yield for the microorganisms. When oxygen and nitrate are depleted
reduction of Mn and Fe (oxo)hydroxides and
sulfate as well as methane fermentation follow in
the sequence with decreasing yield of energy
(Froelich et al. 1979; cf. Section 3.2.5 and
subsequent chapters). This sequence is generally valid, even though numerous studies have
identified an overlap of carbon oxidation pathways within the sediment resulting from competition between microbial populations (Canfield
1993) and the presence of microenvironments
(e.g. Jørgensen 1977; cf. Chapters 7, 8, 12).
The general equations of coupled oxic respiration and nitrification (6.1) and denitrification
(6.2) describing the “top” of the diagenetic
sequence are given as:
Oxic respiration and nitrification
(CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138 O 2 →
106 CO 2 + 16 HNO 3 + H 3 PO 4 + 122 H 2 O
∆G
0
= - 3190 kJ mol
-1
(6.1)
Denitrification
(CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 94.4 HNO 3 →
106 CO 2 + 55.2 N 2 + H 3 PO 4 + 177.2 H 2 O
∆G
0
= - 3030 kJ mol
-1
(6.2)