133
matter is different in these two cases. Turbidity currents
may expose the (fossil) organic matter to an oxygenrich water mass causing further degradation during
resettling. Compact slump masses may transport
significant amounts of labile organic matter from an
initial depositional setting, favorable for organic matter
preservation (e.g. in an oxygen-minimum zone), to an
oxygen-rich deep-water environment. There is no
enhanced mineralization in this case due to the
undisturbed embedding of organic matter in the
sediment matrix (cf., e.g., Cornford et al. 1979; Rullkötter
et al. 1982).
Redeposition may occur almost synsedimentarily,
i.e. the redeposited allochthonous sediment will differ
only very little in age from the underlying autochthonous sediment. The organic matter content of both
may reflect more or less contemporaneous primary
productivity with the only exception that the remains
of shallow(er)-water species are relocated to a deepwater environment. This may be more evident in the
mineral fossils than in the organic matter assemblage,
however. Alternatively, redeposition may occur a long
time after initial sedimentation took place. Deep-sea
drilling on the Northwest African continental margin,
for example, has recovered extended Miocene sediment
series which evidently contained slumps a few
centimeters to several meters thick and of various age,
from Tertiary to Middle Cretaceous (von Rad et al. 1979;
Hinz et al. 1984). Investigation of the organic matter on
a molecular level clearly demonstrated the correspondence between slump clasts embedded in the
Lower Miocene sequence and underlying autochthonous Cenomanian series, whereas paleontological
analysis showed a difference between outer shelf/upper
slope species in the slumps and pelagic species in the
autochthonous Cenomanian sediment (Rullkötter et al.
1984).
The second main source of allochthonous organic
matter in marine sediments are the continents. Wind,
rivers and glaciers transport large amounts of landderived organic matter into the ocean (e.g.
Romankevitch 1984; Hedges and Keil 1995; Hedges et
al. 1997). Again, two principal types of organic matter
have to be distinguished: (1) fresh or (in geological
terms) recently biosynthesized land plant material and
(2) organic matter contained in older sediments that
were weathered and eroded on the continent in areas
ranging from mountains to coastal swamps. Organic
matter in older sedimentary rocks that are being eroded
may have had an extended history of geothermal
heating at great burial depth into the stages of oil or
bituminous coal formation and beyond. This organic
matter carries a distinct signal of geochemical maturation that can easily be detected by bulk (e.g. atomic
composition, pyrolysis yields, vitrinite reflectance; see
Tissot and Welte 1984) and molecular geochemical
parameters (e.g. compound ratios of specific geochemical fossils or biomarkers; see Peters et al. 2005). Due
to its advanced level of diagenetic alteration, even peat
can be distinguished from fresh organic matter in
marine sediments. Only when sediments have been
buried to a depth corresponding to the temperature
which the eroded organic matter earlier experienced do
both fresh and prematured organic matter continue
diagenesis or maturation at the same rate. Geochemical
reactions are virtually stopped (i.e. reaction rates
become very low) as soon as sediments (and their
organic matter contents) in the course of tectonic uplift
are cooled to a temperature of about 15° C lower than
their previous maximum temperature. During transport
to the ocean, oxidation of organic matter eroded on
land – and of terrestrial organic matter in general – has
an effect similar to maturation. The product is a highly
refractory, inert material , which in organic petrography
is termed inertinite and which is easily recognizable
under the microscope by its high reflectance (see
Taylor et al. 1998 for details). Nevertheless, a substantial
fraction of the terrigenous organic matter is reactive
and metabolizable in the ocean (Hedges et al. 1997).
Wind-driven dust and aerosols carry terrigenous
organic matter over long distances into the oceans and
are estimated to contribute a total annual amount of
3.2·10
8
t carbon each year (Romankevitch 1984). Entire
organoclasts, like pollen and spores, are blown offshore
as are lipids from the waxy coatings of plant cuticles
adsorbed to mineral grains (e.g. Rommerskirchen et al.
2003 and references therein). This wind-blown terrestrial
material may comprise the bulk of the organic matter in
open-ocean sediments where very little of the primary
marine organic matter reaches the deep ocean floor.
The higher resistance of terrestrial organic matter
toward oxidation has been invoked to explain this
selective enrichment. Summerhayes (1981) estimated
that most organic-matter-rich sediments in the Atlantic
Ocean, including the Cretaceous and Jurassic black
shales, contain a ‘background level’ of 1 % terrestrial
organic matter in total dry sediment.
Not all of this terrigenous material is brought into
the ocean by winds. The most important contributors
are the rivers draining into the ocean. Each year rivers
transport approximately 0.4·10
9
t of dissolved and
particulate organic carbon from continents to oceans
(Schlesinger and Melack 1981). About 60 % of the river
run-off derives from forested catchments, and the ratio of
the contribution of tropical to temperate and boreal forests
is about two to one (Schlesinger and Melack 1981). Much
4.2
Organic Matter Accumulation in Sediments
matter is different in these two cases. Turbidity currents
may expose the (fossil) organic matter to an oxygenrich water mass causing further degradation during
resettling. Compact slump masses may transport
significant amounts of labile organic matter from an
initial depositional setting, favorable for organic matter
preservation (e.g. in an oxygen-minimum zone), to an
oxygen-rich deep-water environment. There is no
enhanced mineralization in this case due to the
undisturbed embedding of organic matter in the
sediment matrix (cf., e.g., Cornford et al. 1979; Rullkötter
et al. 1982).
Redeposition may occur almost synsedimentarily,
i.e. the redeposited allochthonous sediment will differ
only very little in age from the underlying autochthonous sediment. The organic matter content of both
may reflect more or less contemporaneous primary
productivity with the only exception that the remains
of shallow(er)-water species are relocated to a deepwater environment. This may be more evident in the
mineral fossils than in the organic matter assemblage,
however. Alternatively, redeposition may occur a long
time after initial sedimentation took place. Deep-sea
drilling on the Northwest African continental margin,
for example, has recovered extended Miocene sediment
series which evidently contained slumps a few
centimeters to several meters thick and of various age,
from Tertiary to Middle Cretaceous (von Rad et al. 1979;
Hinz et al. 1984). Investigation of the organic matter on
a molecular level clearly demonstrated the correspondence between slump clasts embedded in the
Lower Miocene sequence and underlying autochthonous Cenomanian series, whereas paleontological
analysis showed a difference between outer shelf/upper
slope species in the slumps and pelagic species in the
autochthonous Cenomanian sediment (Rullkötter et al.
1984).
The second main source of allochthonous organic
matter in marine sediments are the continents. Wind,
rivers and glaciers transport large amounts of landderived organic matter into the ocean (e.g.
Romankevitch 1984; Hedges and Keil 1995; Hedges et
al. 1997). Again, two principal types of organic matter
have to be distinguished: (1) fresh or (in geological
terms) recently biosynthesized land plant material and
(2) organic matter contained in older sediments that
were weathered and eroded on the continent in areas
ranging from mountains to coastal swamps. Organic
matter in older sedimentary rocks that are being eroded
may have had an extended history of geothermal
heating at great burial depth into the stages of oil or
bituminous coal formation and beyond. This organic
matter carries a distinct signal of geochemical maturation that can easily be detected by bulk (e.g. atomic
composition, pyrolysis yields, vitrinite reflectance; see
Tissot and Welte 1984) and molecular geochemical
parameters (e.g. compound ratios of specific geochemical fossils or biomarkers; see Peters et al. 2005). Due
to its advanced level of diagenetic alteration, even peat
can be distinguished from fresh organic matter in
marine sediments. Only when sediments have been
buried to a depth corresponding to the temperature
which the eroded organic matter earlier experienced do
both fresh and prematured organic matter continue
diagenesis or maturation at the same rate. Geochemical
reactions are virtually stopped (i.e. reaction rates
become very low) as soon as sediments (and their
organic matter contents) in the course of tectonic uplift
are cooled to a temperature of about 15° C lower than
their previous maximum temperature. During transport
to the ocean, oxidation of organic matter eroded on
land – and of terrestrial organic matter in general – has
an effect similar to maturation. The product is a highly
refractory, inert material , which in organic petrography
is termed inertinite and which is easily recognizable
under the microscope by its high reflectance (see
Taylor et al. 1998 for details). Nevertheless, a substantial
fraction of the terrigenous organic matter is reactive
and metabolizable in the ocean (Hedges et al. 1997).
Wind-driven dust and aerosols carry terrigenous
organic matter over long distances into the oceans and
are estimated to contribute a total annual amount of
3.2·10
8
t carbon each year (Romankevitch 1984). Entire
organoclasts, like pollen and spores, are blown offshore
as are lipids from the waxy coatings of plant cuticles
adsorbed to mineral grains (e.g. Rommerskirchen et al.
2003 and references therein). This wind-blown terrestrial
material may comprise the bulk of the organic matter in
open-ocean sediments where very little of the primary
marine organic matter reaches the deep ocean floor.
The higher resistance of terrestrial organic matter
toward oxidation has been invoked to explain this
selective enrichment. Summerhayes (1981) estimated
that most organic-matter-rich sediments in the Atlantic
Ocean, including the Cretaceous and Jurassic black
shales, contain a ‘background level’ of 1 % terrestrial
organic matter in total dry sediment.
Not all of this terrigenous material is brought into
the ocean by winds. The most important contributors
are the rivers draining into the ocean. Each year rivers
transport approximately 0.4·10
9
t of dissolved and
particulate organic carbon from continents to oceans
(Schlesinger and Melack 1981). About 60 % of the river
run-off derives from forested catchments, and the ratio of
the contribution of tropical to temperate and boreal forests
is about two to one (Schlesinger and Melack 1981). Much
4.2
Organic Matter Accumulation in Sediments