4
Organic Matter: The Driving Force for Early Diagenesis
134
of the organic matter discharged by rivers appears
to be soil-derived (Meybeck 1982; Hedges et al.
1986a, b) and highly degraded (Ittekott 1988; Hedges
et al. 1994).
4.3
Early Diagenesis
4.3.1
The Organic Carbon Content
of Marine Sediments
The content of organic carbon in marine surface
sediments from different environments of the presentday oceans varies over several orders of magnitude
depending on the extent of supply of organic matter,
preservation conditions and dilution by mineral matter.
The results of organic carbon measurements are
usually expressed as TOC (total organic carbon) or
C org values in percent of dry sediment. Romankevich
(1984) compiled data from a great number of analyses
of ocean bottom sediments by various authors and
from all over the world. The TOC values range from
0.01 % to more than 10 % C org in a few cases. A statistical
evaluation of data from the early phase of deep-sea
drilling (Deep Sea Drilling Project Legs 1-31) showed
deep-sea sediments to have a mean organic carbon
content of 0.3 %, with a median value of 0.1 % (McIver
1975). However, the range of samples was certainly not
representative, and the organic carbon contents are
probably biased toward higher values. For example, in
the vast abyssal plains and other deep-water regions
far away from the continents, an organic carbon content
as high as 0.05 % is the exception rather than the rule.
In contrast to this, nearshore sediments on continental shelves and slopes usually have considerably
higher organic carbon contents. Typical hemipelagic
sediments on outer shelves and continental slopes
range between 0.3 and 1 % C org . Sediments within the
oxygen-minimum zone of upwelling areas contain
several percent of organic carbon with exceptional
values exceeding 10 % C org where upwelling is very
intense, e.g. off Peru and southwest Africa, and where
the oxygen-minimum zone extends into the shallow
waters of the shelf. Still, generalizations are difficult to
make because sedimentation conditions are highly
variable in space.
Generalizations are also difficult to make with respect
to variation of organic carbon content with time. For
long periods of the geological record the present-day
conditions of organic carbon burial can be projected
to the past. There were times, however, mostly relatively
Fig. 4.5 Three independent approaches used to quantify organic matter degradation in anoxic coastal sediments: (A)
carbon budget based on measurement of recycled (J out ; CO 2 and methane) and calculation of buried (J bur ) carbon fluxes;
(B) kinetic modeling of the concentration/depth distribution of organic carbon (G; G 0 and G ∞ are organic carbon
contents at the top and the bottom of the studied depth interval, respectively); (C) calculated organic carbon
remineralization based on modeled or measured rates of sulfate reduction and methanogenesis (CH 2 O stands for organic
matter) (after Martens and Klump 1984).
Organic Matter: The Driving Force for Early Diagenesis
134
of the organic matter discharged by rivers appears
to be soil-derived (Meybeck 1982; Hedges et al.
1986a, b) and highly degraded (Ittekott 1988; Hedges
et al. 1994).
4.3
Early Diagenesis
4.3.1
The Organic Carbon Content
of Marine Sediments
The content of organic carbon in marine surface
sediments from different environments of the presentday oceans varies over several orders of magnitude
depending on the extent of supply of organic matter,
preservation conditions and dilution by mineral matter.
The results of organic carbon measurements are
usually expressed as TOC (total organic carbon) or
C org values in percent of dry sediment. Romankevich
(1984) compiled data from a great number of analyses
of ocean bottom sediments by various authors and
from all over the world. The TOC values range from
0.01 % to more than 10 % C org in a few cases. A statistical
evaluation of data from the early phase of deep-sea
drilling (Deep Sea Drilling Project Legs 1-31) showed
deep-sea sediments to have a mean organic carbon
content of 0.3 %, with a median value of 0.1 % (McIver
1975). However, the range of samples was certainly not
representative, and the organic carbon contents are
probably biased toward higher values. For example, in
the vast abyssal plains and other deep-water regions
far away from the continents, an organic carbon content
as high as 0.05 % is the exception rather than the rule.
In contrast to this, nearshore sediments on continental shelves and slopes usually have considerably
higher organic carbon contents. Typical hemipelagic
sediments on outer shelves and continental slopes
range between 0.3 and 1 % C org . Sediments within the
oxygen-minimum zone of upwelling areas contain
several percent of organic carbon with exceptional
values exceeding 10 % C org where upwelling is very
intense, e.g. off Peru and southwest Africa, and where
the oxygen-minimum zone extends into the shallow
waters of the shelf. Still, generalizations are difficult to
make because sedimentation conditions are highly
variable in space.
Generalizations are also difficult to make with respect
to variation of organic carbon content with time. For
long periods of the geological record the present-day
conditions of organic carbon burial can be projected
to the past. There were times, however, mostly relatively
Fig. 4.5 Three independent approaches used to quantify organic matter degradation in anoxic coastal sediments: (A)
carbon budget based on measurement of recycled (J out ; CO 2 and methane) and calculation of buried (J bur ) carbon fluxes;
(B) kinetic modeling of the concentration/depth distribution of organic carbon (G; G 0 and G ∞ are organic carbon
contents at the top and the bottom of the studied depth interval, respectively); (C) calculated organic carbon
remineralization based on modeled or measured rates of sulfate reduction and methanogenesis (CH 2 O stands for organic
matter) (after Martens and Klump 1984).