468
Specific studies on settling dead foraminifers have
shown that the number of tests devoid of organic matter
increases rapidly with depth; below 1000 m most of the
tests contain none. From rnany trap studies it is now possible to make estimates for the downward vertical carbon
flux below the photic zone (export production) as a TImction of either primary production, PP, or new production,
NP, and water depth, z (Fig. 10.5d ande). Data from different regions reveal the same tendency. However, such calculations do not take into account terrestrial organic matter
which may be transmitted into the sea in greatly varying
quantities.
Adsorbed and Dissolved Organic Matter
Organic matter (OM) accumulates on the sea floor
either as pure organic particulate material, or in combination with bio genie skeletal parts, fecal pellets,
peloids, and inorganic minerals, especially day minerals. Dissolved organic compounds in sea water normally attain concentrations on the order of 0.5 g C/m 3
which are usually much higher than those of particulate organic matter. But only a fraction of this pool
can reach the sediment surface after it has been adsorbed onto the surfaces of various particles settling
through the water column to the sea floor. Some of
the dissolved organic compounds is consumed by
plankton organisms. Thus, the contribution of dissolved organic matter to the total vertical carbon flux
is included in the data measured by the free-floating
sediment traps.
The larger the specific surface area of the partic1es, e.g.
c1ay minerals, the more OM can be adsorbed, commonly as
monolayer-equivalent coatings (e.g. Hedges and Keil
1995). This coatings largely resist . mineralization as they
pass through oxygenated water and surface sediments.
Strong oxygen attack leads to organic loadings lower than
monolayer equivalent; oxygen-depleted environments (dark
sediments) allow loadings higher than monolayer equivalent. Fine-grained partic1es and sediments therefore have
generally a high capacity to preserve and store OM; the
highest organic carbon contents commonly cortelate with
the finest grain sizes.
Under specific conditions, particularly in front of deltas
or in estuaries where fresh water mixes with sea water, organic compounds and c1ay minerals can tlocculate and
form organomineral aggregates which slowly settle to the
sea floor.
Vertical OC Flux through the Water Column
In recent years considerable efforts have been made
to quantify the vertical carbon flux in different parts
ofthe modem oceans. Primary (total) production and
Chapter 10 Sedimentation Rates and Organic Matter
new production at the 100 m water depth appear
dosely related (Fig. 10.5d). If primary production is
low, most of it is consumed by heterotrophie organisrns already in the photic zone, and little is left to
sink as export production into deeper waters. However, as soon as primary production increases to 100
g/m 2 /a) and more, a large part of it is left over as new
(and export) production. On the average, new production at 100 m constitutes about 20% of the primary production (Berger et al. 1989). It amounts to
about 6 to 7 x 10 9 t/a in the modern oceans.
Export production decreases regularly with depth
(Fig. 10.5e), a pattern which can be found in all
present-day ocean basins where the deep water oxygen content is at least 50 to I 00 ~mol/kg (0.8 to 1.6
g/m 3 ). Commonly, 75-85% of the new production
leaving the upper 100 m of water are decomposed
within the underlying 500-1000 m ofwater.
For further information see, e.g., Suess (1980), Betzer et al.
(1984), Martin et al. (1987), Samthein et al. (1988), Berger
et al. (1989) and references listed above.
In the geological past, primary, new, and export production may have been significantly different from the modem
situation. During the last glacial maximum, for example,
the lowered sea level and increased temperature gradients
between equator and poles promoted thermohaline ocean
circulation. This led to accelerated mineralization of OM
and nutrient supply for enhanced primary and new production. In times of more equal global climate, organic production may have been reduced but OM preservation enhanced in relation to the present-day scenario.
Regional Variation in Export Production
Similarly to primary and new production, export production of organic carbon arriving on the sea floor
varies considerably in the different regions of the
present-day oceans.
It is only about 0.5 g/m 2 /a in the deep ocean (water
depths of 5000 m), but some 15 g/m 2 /a in nonupwelling coastal waters (water depths of 250 m).
Thus, a ratio of 1:3 in prirnary production is magnified to 1 :30 on the sea floor. Consequently, the carbon flux to the deep-sea floor is less than that to the
continental shelf floors, although the shelves comprise only about 8% of the total surface area of the
oceans. Below zones of coastal upwelling, the organic carbon fallout on the sea floor at depths of 250
m is ab out 40 gim 2 /a (Table 10.2).
Specific studies on settling dead foraminifers have
shown that the number of tests devoid of organic matter
increases rapidly with depth; below 1000 m most of the
tests contain none. From rnany trap studies it is now possible to make estimates for the downward vertical carbon
flux below the photic zone (export production) as a TImction of either primary production, PP, or new production,
NP, and water depth, z (Fig. 10.5d ande). Data from different regions reveal the same tendency. However, such calculations do not take into account terrestrial organic matter
which may be transmitted into the sea in greatly varying
quantities.
Adsorbed and Dissolved Organic Matter
Organic matter (OM) accumulates on the sea floor
either as pure organic particulate material, or in combination with bio genie skeletal parts, fecal pellets,
peloids, and inorganic minerals, especially day minerals. Dissolved organic compounds in sea water normally attain concentrations on the order of 0.5 g C/m 3
which are usually much higher than those of particulate organic matter. But only a fraction of this pool
can reach the sediment surface after it has been adsorbed onto the surfaces of various particles settling
through the water column to the sea floor. Some of
the dissolved organic compounds is consumed by
plankton organisms. Thus, the contribution of dissolved organic matter to the total vertical carbon flux
is included in the data measured by the free-floating
sediment traps.
The larger the specific surface area of the partic1es, e.g.
c1ay minerals, the more OM can be adsorbed, commonly as
monolayer-equivalent coatings (e.g. Hedges and Keil
1995). This coatings largely resist . mineralization as they
pass through oxygenated water and surface sediments.
Strong oxygen attack leads to organic loadings lower than
monolayer equivalent; oxygen-depleted environments (dark
sediments) allow loadings higher than monolayer equivalent. Fine-grained partic1es and sediments therefore have
generally a high capacity to preserve and store OM; the
highest organic carbon contents commonly cortelate with
the finest grain sizes.
Under specific conditions, particularly in front of deltas
or in estuaries where fresh water mixes with sea water, organic compounds and c1ay minerals can tlocculate and
form organomineral aggregates which slowly settle to the
sea floor.
Vertical OC Flux through the Water Column
In recent years considerable efforts have been made
to quantify the vertical carbon flux in different parts
ofthe modem oceans. Primary (total) production and
Chapter 10 Sedimentation Rates and Organic Matter
new production at the 100 m water depth appear
dosely related (Fig. 10.5d). If primary production is
low, most of it is consumed by heterotrophie organisrns already in the photic zone, and little is left to
sink as export production into deeper waters. However, as soon as primary production increases to 100
g/m 2 /a) and more, a large part of it is left over as new
(and export) production. On the average, new production at 100 m constitutes about 20% of the primary production (Berger et al. 1989). It amounts to
about 6 to 7 x 10 9 t/a in the modern oceans.
Export production decreases regularly with depth
(Fig. 10.5e), a pattern which can be found in all
present-day ocean basins where the deep water oxygen content is at least 50 to I 00 ~mol/kg (0.8 to 1.6
g/m 3 ). Commonly, 75-85% of the new production
leaving the upper 100 m of water are decomposed
within the underlying 500-1000 m ofwater.
For further information see, e.g., Suess (1980), Betzer et al.
(1984), Martin et al. (1987), Samthein et al. (1988), Berger
et al. (1989) and references listed above.
In the geological past, primary, new, and export production may have been significantly different from the modem
situation. During the last glacial maximum, for example,
the lowered sea level and increased temperature gradients
between equator and poles promoted thermohaline ocean
circulation. This led to accelerated mineralization of OM
and nutrient supply for enhanced primary and new production. In times of more equal global climate, organic production may have been reduced but OM preservation enhanced in relation to the present-day scenario.
Regional Variation in Export Production
Similarly to primary and new production, export production of organic carbon arriving on the sea floor
varies considerably in the different regions of the
present-day oceans.
It is only about 0.5 g/m 2 /a in the deep ocean (water
depths of 5000 m), but some 15 g/m 2 /a in nonupwelling coastal waters (water depths of 250 m).
Thus, a ratio of 1:3 in prirnary production is magnified to 1 :30 on the sea floor. Consequently, the carbon flux to the deep-sea floor is less than that to the
continental shelf floors, although the shelves comprise only about 8% of the total surface area of the
oceans. Below zones of coastal upwelling, the organic carbon fallout on the sea floor at depths of 250
m is ab out 40 gim 2 /a (Table 10.2).