4
Organic Matter: The Driving Force for Early Diagenesis
128
settling toward the ocean bottom and consuming the
dissolved oxygen.
The productivity model (Fig. 4.2B) is based on
high primary bioproductivity in the photic zone of
the ocean as it presently occurs in areas of coastal
upwelling primarily on the western continental
margins, along the equator and as a monsoon-driven
phenomenon in the Arabian Sea and along the Oman
and Somali coasts. Upwelling brings high amounts of
nutrients to the surface, which stimulate phytoplanktonic growth (e.g. Suess and Thiede 1983;
Thiede and Suess 1983; Summerhayes et al. 1992). On
continental margins, the formation of oxygen-depleted
water masses (oxygen minimum zones) usually implies
that they impinge on the ocean bottom where they
create depositional conditions similar to those in a
stagnant basin. To which extent the reduction in
oxygen concentration at the sediment/water interface
enhances, or is required for, the preservation of
organic matter in the sediments, is the main subject of
debate between the proponents of the productivity
and the preservation models (e.g. Pedersen and
Calvert 1990, 1991; Demaison 1991). For the geological
past, e.g. the Cretaceous, it is also conceivable that
the equable climate on our planet led to a more
sluggish circulation of ocean water worldwide. The
lack of turnover in the water column may have caused
the development of anoxic bottom water conditions
in some parts of the ocean that may explain the
formation of Cretaceous black shales almost synchronously in different areas (Sinninghe Damsté and
Köster 1998 and references therein). Also, transgression as a consequence of eustatic sealevel rise
may have enhanced accumulation of organic matter
in shelf areas (Wenger and Baker 1986) whereas times
of regression may have promoted organic matter
accumulation in prograding delta fans in deeper water.
A detailed discussion of organic matter accumulation
in different oceanic settings, including deep marine
silled basins, progradational submarine fans, upwelling areas, anoxic continental shelves and fluviodeltaic
systems was provided by Littke et al. (1997a).
4
.2 .2
P
rimary P rodu c
tion of O
rg anic
M atter and Ex p ort to the O
c ean
B
ottom
The total annual primary production by photosynthetic
planktonic organisms in the modern world oceans has
been estimated to be in the order of 30-50·10
9
tons of
carbon (Berger et al. 1989; Hedges and Keil 1995).
Oceanic carbon fixation is not evenly distributed, but
displays zones of higher activity on continental margins
(several hundred g C org m
-2
yr
-1
), whereas the central
ocean gyres are mostly characterized by low primary
production (about 25 g C org m
-2
yr
-1
; e.g. Romankevitch
1984; Berger 1989). Due to plate tectonics and the
variable distribution of land masses together with
climatic developments, the global amount and
distribution of organic matter production in the ocean
is likely to have undergone significant changes with
geological time.
Of the total biomass newly formed in the photic
zone of the ocean, only a very small portion reaches
the underlying seafloor and is ultimately buried in the
sediment (for reviews of water column processes see
Emerson and Hedges 1988; Wakeham and Lee 1989).
Most of the organic matter enters the biological food
web in the surface waters and is respired or used for
new heterotrophic biomass production. Because of this
intense recycling, it is difficult to determine the organic
matter flux at different levels of the photic zone.
Oceanographers and biogeochemists usually consider
only the flux of organic matter through the lower
boundary of the photic zone and term it ‘new’
production (Fig. 4.3). It equals 100 % of export
production (see below) and is not to be confused with
net photosynthesis which is gross photosynthesis
minus algal respiration. Below this boundary, the
content of organic matter in the water column decreases
due to consumption in the food web and to microbiological and chemical degradation as observed from
the analysis of material in sediment traps deployed at
different water depths.
The water depth-dependent flux is termed export
production (Fig. 4.3). Export production decreases
rapidly just below the photic zone. Then, there is mostly
a quasi-linear, slower decline at greater water depths
until the organic matter reaches the benthic boundary
(nepheloid) layer close to the sediment/water interface
where the activity of epibenthic organisms enhances
organic matter consumption again. This enhanced
consumption continues in the upper sediment layer
where burrowing organisms depend on the supply from
the water column. Organic matter degradation eventually extends deeply into the sediment pile as became
evident from the detection of a so-called deep biosphere at several hundred meters below the seafloor
(e.g. Parkes et al. 1994). However, the rate of organic
matter degradation apparently decreases significantly
with increasing depth of burial. Overall, it is estimated
that only 1 to 0.01 % of the primary production is buried
deeply in marine sediments (cf. Fig. 12.1). The fraction
strongly depends on a number of parameters including
level of primary productivity, water depth, (probably)
oxygen content in the water column and surface
Organic Matter: The Driving Force for Early Diagenesis
128
settling toward the ocean bottom and consuming the
dissolved oxygen.
The productivity model (Fig. 4.2B) is based on
high primary bioproductivity in the photic zone of
the ocean as it presently occurs in areas of coastal
upwelling primarily on the western continental
margins, along the equator and as a monsoon-driven
phenomenon in the Arabian Sea and along the Oman
and Somali coasts. Upwelling brings high amounts of
nutrients to the surface, which stimulate phytoplanktonic growth (e.g. Suess and Thiede 1983;
Thiede and Suess 1983; Summerhayes et al. 1992). On
continental margins, the formation of oxygen-depleted
water masses (oxygen minimum zones) usually implies
that they impinge on the ocean bottom where they
create depositional conditions similar to those in a
stagnant basin. To which extent the reduction in
oxygen concentration at the sediment/water interface
enhances, or is required for, the preservation of
organic matter in the sediments, is the main subject of
debate between the proponents of the productivity
and the preservation models (e.g. Pedersen and
Calvert 1990, 1991; Demaison 1991). For the geological
past, e.g. the Cretaceous, it is also conceivable that
the equable climate on our planet led to a more
sluggish circulation of ocean water worldwide. The
lack of turnover in the water column may have caused
the development of anoxic bottom water conditions
in some parts of the ocean that may explain the
formation of Cretaceous black shales almost synchronously in different areas (Sinninghe Damsté and
Köster 1998 and references therein). Also, transgression as a consequence of eustatic sealevel rise
may have enhanced accumulation of organic matter
in shelf areas (Wenger and Baker 1986) whereas times
of regression may have promoted organic matter
accumulation in prograding delta fans in deeper water.
A detailed discussion of organic matter accumulation
in different oceanic settings, including deep marine
silled basins, progradational submarine fans, upwelling areas, anoxic continental shelves and fluviodeltaic
systems was provided by Littke et al. (1997a).
4
.2 .2
P
rimary P rodu c
tion of O
rg anic
M atter and Ex p ort to the O
c ean
B
ottom
The total annual primary production by photosynthetic
planktonic organisms in the modern world oceans has
been estimated to be in the order of 30-50·10
9
tons of
carbon (Berger et al. 1989; Hedges and Keil 1995).
Oceanic carbon fixation is not evenly distributed, but
displays zones of higher activity on continental margins
(several hundred g C org m
-2
yr
-1
), whereas the central
ocean gyres are mostly characterized by low primary
production (about 25 g C org m
-2
yr
-1
; e.g. Romankevitch
1984; Berger 1989). Due to plate tectonics and the
variable distribution of land masses together with
climatic developments, the global amount and
distribution of organic matter production in the ocean
is likely to have undergone significant changes with
geological time.
Of the total biomass newly formed in the photic
zone of the ocean, only a very small portion reaches
the underlying seafloor and is ultimately buried in the
sediment (for reviews of water column processes see
Emerson and Hedges 1988; Wakeham and Lee 1989).
Most of the organic matter enters the biological food
web in the surface waters and is respired or used for
new heterotrophic biomass production. Because of this
intense recycling, it is difficult to determine the organic
matter flux at different levels of the photic zone.
Oceanographers and biogeochemists usually consider
only the flux of organic matter through the lower
boundary of the photic zone and term it ‘new’
production (Fig. 4.3). It equals 100 % of export
production (see below) and is not to be confused with
net photosynthesis which is gross photosynthesis
minus algal respiration. Below this boundary, the
content of organic matter in the water column decreases
due to consumption in the food web and to microbiological and chemical degradation as observed from
the analysis of material in sediment traps deployed at
different water depths.
The water depth-dependent flux is termed export
production (Fig. 4.3). Export production decreases
rapidly just below the photic zone. Then, there is mostly
a quasi-linear, slower decline at greater water depths
until the organic matter reaches the benthic boundary
(nepheloid) layer close to the sediment/water interface
where the activity of epibenthic organisms enhances
organic matter consumption again. This enhanced
consumption continues in the upper sediment layer
where burrowing organisms depend on the supply from
the water column. Organic matter degradation eventually extends deeply into the sediment pile as became
evident from the detection of a so-called deep biosphere at several hundred meters below the seafloor
(e.g. Parkes et al. 1994). However, the rate of organic
matter degradation apparently decreases significantly
with increasing depth of burial. Overall, it is estimated
that only 1 to 0.01 % of the primary production is buried
deeply in marine sediments (cf. Fig. 12.1). The fraction
strongly depends on a number of parameters including
level of primary productivity, water depth, (probably)
oxygen content in the water column and surface