6
Benthic Cycling of Oxygen, Nitrogen and Phosphorus
234
upper slope surface sediments are partly depleted
in organic carbon (Hensen et al. 2000; cf. section
12.3.2). This is indicated by the good correlation
of nitrate fluxes with organic carbon content in the
surface sediments (Fig. 6.25b).
The above examples have shown that there are
large discrepancies between benthic biogeochemical processes in areas of high and low productivity, but there are no simple relationships or
master variables to correlate benthic fluxes and
mineralization rates with primary productivity or
sediment parameters. However, the quantitative
coupling between these processes will be a main
objective of future research (cf. Chapter 12).
6.6
Summary
In this chapter we have summarized the general
availability of oxygen, nitrate and phosphate in
the oceans and aspects of their significance in the
biogeochemical cycles of marine sediments. Oxic
respiration is by far the most important pathway
of organic carbon in deep-sea sediments and
significantly determines the recycling of organic
matter introduced onto the sediment surface. Even
if some uncertainty remains, benthic oxygen fluxes
reveal probably a reasonable approximation to the
total carbon oxidation rate under deep-marine
conditions. However, benthic fluxes vary considerably in the various ocean basins and the interactions between all parameters controlling benthic
biogeochemical activities are not yet completely
understood. Above, we showed some examples in
which benthic activity deviates from the generally
expected situation as suggested by export fluxes
from the surface waters. We further elaborate
these concepts in Chapter 12 where we summarize
existent approaches for the regionalization of
benthic fluxes and present methods how to access
mineralization processes in surface sediments of
the deep-sea globally and arrive at a definition of
benthic biogeochemical provinces.
Acknowledgements
This is contribution No 0332 of the Research Center
Ocean Margins (RCOM) which is financed by the
Deutsche Forschungsgemeinschaft (DFG) at Bremen
University, Germany.
6.7
Problems
Problem 1
Explain why oxygen consumption can be used as
a measure for the sum of mineralization processes
occurring in the sediment. Why is this - strictly
speaking - not correct, but provides feasible
results? In which areas of the seafloor would this
method fail?
Problem 2
Explain the terms carbon limited and oxidant
limited and give examples, where you would
expect carbon- and oxidant-limited diagenesis.
Problem 3
Denitrification is a major pathway of carbon
degradation in sediments of shallow-marine
continental margin areas. Why is it so important
for biogeochemical element cycles in the ocean,
particularly in terms of regulation of nutrient
levels and primary productivity?
Problem 4
Would you expect a higher (than Redfield) C org. /P org.
ratio in marine sediments under oxic or anoxic
bottom water conditions?
Problem 5
Under which environmental conditions would you
expect anammox (anaerobic ammonia oxidation) to
be an important process?
Benthic Cycling of Oxygen, Nitrogen and Phosphorus
234
upper slope surface sediments are partly depleted
in organic carbon (Hensen et al. 2000; cf. section
12.3.2). This is indicated by the good correlation
of nitrate fluxes with organic carbon content in the
surface sediments (Fig. 6.25b).
The above examples have shown that there are
large discrepancies between benthic biogeochemical processes in areas of high and low productivity, but there are no simple relationships or
master variables to correlate benthic fluxes and
mineralization rates with primary productivity or
sediment parameters. However, the quantitative
coupling between these processes will be a main
objective of future research (cf. Chapter 12).
6.6
Summary
In this chapter we have summarized the general
availability of oxygen, nitrate and phosphate in
the oceans and aspects of their significance in the
biogeochemical cycles of marine sediments. Oxic
respiration is by far the most important pathway
of organic carbon in deep-sea sediments and
significantly determines the recycling of organic
matter introduced onto the sediment surface. Even
if some uncertainty remains, benthic oxygen fluxes
reveal probably a reasonable approximation to the
total carbon oxidation rate under deep-marine
conditions. However, benthic fluxes vary considerably in the various ocean basins and the interactions between all parameters controlling benthic
biogeochemical activities are not yet completely
understood. Above, we showed some examples in
which benthic activity deviates from the generally
expected situation as suggested by export fluxes
from the surface waters. We further elaborate
these concepts in Chapter 12 where we summarize
existent approaches for the regionalization of
benthic fluxes and present methods how to access
mineralization processes in surface sediments of
the deep-sea globally and arrive at a definition of
benthic biogeochemical provinces.
Acknowledgements
This is contribution No 0332 of the Research Center
Ocean Margins (RCOM) which is financed by the
Deutsche Forschungsgemeinschaft (DFG) at Bremen
University, Germany.
6.7
Problems
Problem 1
Explain why oxygen consumption can be used as
a measure for the sum of mineralization processes
occurring in the sediment. Why is this - strictly
speaking - not correct, but provides feasible
results? In which areas of the seafloor would this
method fail?
Problem 2
Explain the terms carbon limited and oxidant
limited and give examples, where you would
expect carbon- and oxidant-limited diagenesis.
Problem 3
Denitrification is a major pathway of carbon
degradation in sediments of shallow-marine
continental margin areas. Why is it so important
for biogeochemical element cycles in the ocean,
particularly in terms of regulation of nutrient
levels and primary productivity?
Problem 4
Would you expect a higher (than Redfield) C org. /P org.
ratio in marine sediments under oxic or anoxic
bottom water conditions?
Problem 5
Under which environmental conditions would you
expect anammox (anaerobic ammonia oxidation) to
be an important process?