135
short and often termed an event, when high amounts
of organic matter were preserved, not only in shallow
epicontinental seas, but also in deep-sea sediments.
Examples are the Jurassic and particularly Cretaceous
black shales of the Atlantic and Pacific Oceans with
extreme organic carbon contents of 20-30 % and more
(e.g. Herbin et al. 1986; Dumitrescu and Brassell 2005).
Specific oceanographic conditions prevailed during the
younger geological past in semi-enclosed basins like
the Mediterranean Sea. Plio-Pleistocene sapropels in
the eastern Mediterranean Sea were deposited at
regular time intervals due to climatic changes induced
by orbital forces, in this case the 23,000 year cycle of
precession of the Earth’s axis. Some of the Mediterranean sapropels, recovered during Leg 160 of the
international Ocean Drilling Program, yielded more than
30 % C org (Emeis et al. 1996).
The range of organic carbon contents in sediments
and the associated variation in conditions for organic
matter preservation imply that the amount of biogenic
information incorporated in sediments as organic matter
may vary drastically. In the same way, the extent to
which the preserved organic matter is representative
of the ecosystem in the water column above, may be
vastly different. It is not surprising then that organic
geochemists have preferentially investigated sediments
with high organic carbon contents particularly when
emphasis was on the formation of fossils fuels
(petroleum or natural gas) or on molecular organic
geochemical analysis which – at least in its early days
– required relatively large amounts of material. It has to
be kept in mind that this bias has certainly also
influenced the choice of examples given throughout
this chapter, although attempts are made to contrast
case studies representing different environmental
conditions in the oceanic realm.
Within a sediment, the organic carbon content
decreases with increasing depth due to mostly
microbiological remineralization, but possibly also due
to abiological oxidation, during early (and later)
diagenesis. The entire process takes place in a complex
redox system where organic matter is the electron donor
and a variety of substrates are electron acceptors. In
other words, whenever organic matter is destroyed or
altered by oxidation, a reaction partner has to be
reduced. In an extended investigation of the biogeochemical cycling in an organic-matter-rich coastal
marine basin, Martens and Klump (1984) schematically
illustrated three independent approaches to quantify
organic matter degradation in sediments with anoxic
surface layers (Fig. 4.5). These involve (a) a mass
balance of incoming, recycled and buried carbon fluxes,
(b) kinetic modeling of the concentration/depth
distribution of organic carbon and (c) measurement of
degradation rates in the sediment column. The redox
zones in the example given in Fig. 4.5 are restricted to a
depositional environment with anoxic surface sediment
and comprise only sulfate reduction and methanogenesis. In the case of oxic conditions in the upper
sediment layer, there would be additional oxygen,
nitrate, Mn(IV) and Fe(III) reduction zones (Froelich et
al. 1979; cf. also Fig. 4.6, where these zones are
indicated, and Chap. 5).
Martens et al. (1992) applied the approaches in Fig.
4.5 to study the composition and fate of organic matter
in coastal sediments of Cape Lookout Bight
2
. In
separate studies it had previously been established
that organic matter was mostly supplied from backbarrier island lagoons and marshes landward of the
bight at a steady rate. Furthermore, the organic matter
was extensively physically and biologically recycled
in the lagoon before it ultimately accumulated in the
sediments. Thus, systematic downcore decreases in
amount of labile organic matter had to result from early
diagenesis rather than variations of supply. The authors
tried to answer the question of what fraction of the
incoming particulate organic carbon (POC) is remineralized during early diagenesis under the conditions
described by solving the simple mass balance equation.
POC input = POC remineralized + POC buried
(4.6)
In their experience it has proven easiest to measure
fluxes resulting from POC remineralization and burial
and then to calculate POC input by adding these fluxes
together. Numerical values of the fluxes are given in
Figure 4.6. In this model, the incoming POC is either
remineralized to CO 2 , CH 4 and DOC (dissolved organic
carbon) or buried. The CO 2 , CH 4 and DOC produced
during remineralization are either lost to the water
column via sediment-water chemical exchange or buried
as carbonate and dissolved components of sediment
pore waters. Using
210
Pb-based sedimentation rates, the
POC burial rate was found to be 117±19 mol C m
-2
yr
-1
.
Sediment-water chemical exchange accounts for losses
of 40.6±6.6 mol C m
-2
yr
-1
as CO 2 , CH 4 and DOC, whereas
7.0±1.1 mol C m
-2
yr
-1
of these species, including
4.3
Early Diagenesis
2 Cape Lookout Bight (North Carolina, U.S.A.) is an
“end member” environment with respect to sedimentation rate (10 cm per year!), organic matter composition, domination of anoxic degradation processes and
direct ebullition of methane gas, i.e. not typical for
present-day open-ocean marine sediments.
short and often termed an event, when high amounts
of organic matter were preserved, not only in shallow
epicontinental seas, but also in deep-sea sediments.
Examples are the Jurassic and particularly Cretaceous
black shales of the Atlantic and Pacific Oceans with
extreme organic carbon contents of 20-30 % and more
(e.g. Herbin et al. 1986; Dumitrescu and Brassell 2005).
Specific oceanographic conditions prevailed during the
younger geological past in semi-enclosed basins like
the Mediterranean Sea. Plio-Pleistocene sapropels in
the eastern Mediterranean Sea were deposited at
regular time intervals due to climatic changes induced
by orbital forces, in this case the 23,000 year cycle of
precession of the Earth’s axis. Some of the Mediterranean sapropels, recovered during Leg 160 of the
international Ocean Drilling Program, yielded more than
30 % C org (Emeis et al. 1996).
The range of organic carbon contents in sediments
and the associated variation in conditions for organic
matter preservation imply that the amount of biogenic
information incorporated in sediments as organic matter
may vary drastically. In the same way, the extent to
which the preserved organic matter is representative
of the ecosystem in the water column above, may be
vastly different. It is not surprising then that organic
geochemists have preferentially investigated sediments
with high organic carbon contents particularly when
emphasis was on the formation of fossils fuels
(petroleum or natural gas) or on molecular organic
geochemical analysis which – at least in its early days
– required relatively large amounts of material. It has to
be kept in mind that this bias has certainly also
influenced the choice of examples given throughout
this chapter, although attempts are made to contrast
case studies representing different environmental
conditions in the oceanic realm.
Within a sediment, the organic carbon content
decreases with increasing depth due to mostly
microbiological remineralization, but possibly also due
to abiological oxidation, during early (and later)
diagenesis. The entire process takes place in a complex
redox system where organic matter is the electron donor
and a variety of substrates are electron acceptors. In
other words, whenever organic matter is destroyed or
altered by oxidation, a reaction partner has to be
reduced. In an extended investigation of the biogeochemical cycling in an organic-matter-rich coastal
marine basin, Martens and Klump (1984) schematically
illustrated three independent approaches to quantify
organic matter degradation in sediments with anoxic
surface layers (Fig. 4.5). These involve (a) a mass
balance of incoming, recycled and buried carbon fluxes,
(b) kinetic modeling of the concentration/depth
distribution of organic carbon and (c) measurement of
degradation rates in the sediment column. The redox
zones in the example given in Fig. 4.5 are restricted to a
depositional environment with anoxic surface sediment
and comprise only sulfate reduction and methanogenesis. In the case of oxic conditions in the upper
sediment layer, there would be additional oxygen,
nitrate, Mn(IV) and Fe(III) reduction zones (Froelich et
al. 1979; cf. also Fig. 4.6, where these zones are
indicated, and Chap. 5).
Martens et al. (1992) applied the approaches in Fig.
4.5 to study the composition and fate of organic matter
in coastal sediments of Cape Lookout Bight
2
. In
separate studies it had previously been established
that organic matter was mostly supplied from backbarrier island lagoons and marshes landward of the
bight at a steady rate. Furthermore, the organic matter
was extensively physically and biologically recycled
in the lagoon before it ultimately accumulated in the
sediments. Thus, systematic downcore decreases in
amount of labile organic matter had to result from early
diagenesis rather than variations of supply. The authors
tried to answer the question of what fraction of the
incoming particulate organic carbon (POC) is remineralized during early diagenesis under the conditions
described by solving the simple mass balance equation.
POC input = POC remineralized + POC buried
(4.6)
In their experience it has proven easiest to measure
fluxes resulting from POC remineralization and burial
and then to calculate POC input by adding these fluxes
together. Numerical values of the fluxes are given in
Figure 4.6. In this model, the incoming POC is either
remineralized to CO 2 , CH 4 and DOC (dissolved organic
carbon) or buried. The CO 2 , CH 4 and DOC produced
during remineralization are either lost to the water
column via sediment-water chemical exchange or buried
as carbonate and dissolved components of sediment
pore waters. Using
210
Pb-based sedimentation rates, the
POC burial rate was found to be 117±19 mol C m
-2
yr
-1
.
Sediment-water chemical exchange accounts for losses
of 40.6±6.6 mol C m
-2
yr
-1
as CO 2 , CH 4 and DOC, whereas
7.0±1.1 mol C m
-2
yr
-1
of these species, including
4.3
Early Diagenesis
2 Cape Lookout Bight (North Carolina, U.S.A.) is an
“end member” environment with respect to sedimentation rate (10 cm per year!), organic matter composition, domination of anoxic degradation processes and
direct ebullition of methane gas, i.e. not typical for
present-day open-ocean marine sediments.