207
6.1
Introduction
All particles settling at the sea floor continuously
undergo diagenetic alteration due to physical and
chemical processes in the sediment (e.g. particle
mixing, compaction, redox reactions). Marine
sediments are the primary long-term repository of
organic matter and the systematic analysis of the
control mechanisms and processes modifying the
original input signal is of key importance for the
understanding and the reconstruction of
biogeochemical cycles in the ocean. This chapter
mainly focuses on processes occurring at the
transition zone between sediments and bottom
water where fresh, bio-available organic material is
subject to intense bacterially mediated oxidation.
Oxygen and nitrate are treated together because
they are thermodynamically the most favorable
electron acceptors in the diagenetic sequence of
organic matter decomposition and their pathways
are coupled through oxidation of reduced nitrogen
species (nitrification) in oxic systems (cf. Section
3.2.3). Generally, their involvement in the
biogeochemical cycles of the ocean is much more
complex than only seen from this point of view
and therefore, the combined examination of both
parameters is for reasons of convenience and
follows the general concept of this book. Both
oxygen and nitrate pathways are very important
and inherent for the understanding of the oceanic
carbon cycle. Oxygen is introduced into surface
waters by photosynthesis and, even more
important, by exchange with the atmosphere.
Conversely, it is consumed in the course of the
degradation of organic matter. The latter occurs
throughout the water column and in the sediments
resulting in the release of carbon dioxide, nitrate,
and phosphate. Nitrate itself is then used as the
“next” suitable electron acceptor for organic
matter degradation in environments where oxygen
availability is limited, such as oxygen minimum
zones or below the oxygen penetration depth in
the sediments. Nitrate and phosphate are the most
important limiting nutrients for driving primary
productivity in the surface water. There is,
however, a still ongoing debate on whether one or
the other is the limiting nutrient on different time
and spatial scales. The arguments are often called
the “biologist’s view” (nitrogen limitation) and the
“geochemist’s view” (phosphorus regulation),
because they implicate two different perspectives
on looking at the oceanic biogeochemical cycles.
The typical geochemist would argue that over
long time scales nitrogen fixers (converting N 2 to
organic nitrogen) may be able to balance the
nitrate deficit by using the huge reservoir of
dissolved N 2 and allowing certain levels of
productivity even if nitrate becomes exhausted
relative to phosphorus. The supply of phosphorus depends solely on the riverine input and the
remineralisation in the water column and in the
sediments. Once it is exhausted, there is no other
means of replenishing the reservoir. The view of
the biologist might be supported by the fact that
in certain areas of the global ocean small residues
of phosphorus exist while nitrate is undetectable
and the inorganic dissolved N:P ratio of the global
ocean is often below 16:1. A sort of compromising
theory has been proposed by Tyrrell (1999), which
differentiates between nitrogen as the “proximate
limiting nutrient” (with regard to immediate growth
rates) and phosphate as the “ultimate limiting
nutrient” (with regard to the whole system
productivity over long timescales). However,
since this is not the final conclusion and a more
6
Benthic Cycling of Oxygen, Nitrogen
and Phosphorus
CHRISTIAN HENSEN, MATTHIAS ZABEL AND HEIDE N. SCHULZ
6.1
Introduction
All particles settling at the sea floor continuously
undergo diagenetic alteration due to physical and
chemical processes in the sediment (e.g. particle
mixing, compaction, redox reactions). Marine
sediments are the primary long-term repository of
organic matter and the systematic analysis of the
control mechanisms and processes modifying the
original input signal is of key importance for the
understanding and the reconstruction of
biogeochemical cycles in the ocean. This chapter
mainly focuses on processes occurring at the
transition zone between sediments and bottom
water where fresh, bio-available organic material is
subject to intense bacterially mediated oxidation.
Oxygen and nitrate are treated together because
they are thermodynamically the most favorable
electron acceptors in the diagenetic sequence of
organic matter decomposition and their pathways
are coupled through oxidation of reduced nitrogen
species (nitrification) in oxic systems (cf. Section
3.2.3). Generally, their involvement in the
biogeochemical cycles of the ocean is much more
complex than only seen from this point of view
and therefore, the combined examination of both
parameters is for reasons of convenience and
follows the general concept of this book. Both
oxygen and nitrate pathways are very important
and inherent for the understanding of the oceanic
carbon cycle. Oxygen is introduced into surface
waters by photosynthesis and, even more
important, by exchange with the atmosphere.
Conversely, it is consumed in the course of the
degradation of organic matter. The latter occurs
throughout the water column and in the sediments
resulting in the release of carbon dioxide, nitrate,
and phosphate. Nitrate itself is then used as the
“next” suitable electron acceptor for organic
matter degradation in environments where oxygen
availability is limited, such as oxygen minimum
zones or below the oxygen penetration depth in
the sediments. Nitrate and phosphate are the most
important limiting nutrients for driving primary
productivity in the surface water. There is,
however, a still ongoing debate on whether one or
the other is the limiting nutrient on different time
and spatial scales. The arguments are often called
the “biologist’s view” (nitrogen limitation) and the
“geochemist’s view” (phosphorus regulation),
because they implicate two different perspectives
on looking at the oceanic biogeochemical cycles.
The typical geochemist would argue that over
long time scales nitrogen fixers (converting N 2 to
organic nitrogen) may be able to balance the
nitrate deficit by using the huge reservoir of
dissolved N 2 and allowing certain levels of
productivity even if nitrate becomes exhausted
relative to phosphorus. The supply of phosphorus depends solely on the riverine input and the
remineralisation in the water column and in the
sediments. Once it is exhausted, there is no other
means of replenishing the reservoir. The view of
the biologist might be supported by the fact that
in certain areas of the global ocean small residues
of phosphorus exist while nitrate is undetectable
and the inorganic dissolved N:P ratio of the global
ocean is often below 16:1. A sort of compromising
theory has been proposed by Tyrrell (1999), which
differentiates between nitrogen as the “proximate
limiting nutrient” (with regard to immediate growth
rates) and phosphate as the “ultimate limiting
nutrient” (with regard to the whole system
productivity over long timescales). However,
since this is not the final conclusion and a more
6
Benthic Cycling of Oxygen, Nitrogen
and Phosphorus
CHRISTIAN HENSEN, MATTHIAS ZABEL AND HEIDE N. SCHULZ