5.3 Hemipelagic and Pelagic Sediments
tain abundant fish debris (bone beds) and a relatively high
amount of phosphorus due to the accumulation of abundant
fecal pellets and coprolites. Several trace metals, particularly U, Mo, Cd, Zn, and Ni occur in considerably higher
concentrations than in normal deep-sea sediments (e.g.,
Baturin 1983; Brumsack 1986a,b).
The sedimentation rates in coastal upwelling zones are
largely controlled by terrigenous influx. In the Peru
upwelling region, the mean sedimentation rates of late
Pleistocene sediments (drilIed at water depths of 250 and
450 m) are about 7 and 17 cmlka, respectively (Wefer et al.
1990); older sediments accumulated more rapidly.
The Plio-Pleistocene sediments of the Angola-Benguela
upwelling system off Southwest Africa and those of the
Congo Fan show glacial-interglacial precession cycles
(Berger et al. 1998). These are produced by current-induced changes in biogenic productivity (mostly opaline
silica).
Coastal upwelling was certainly also an important
process in ancient marine environments. The formation of many black shales is now ascribed to this
mechanism not only in the Cenozoic and Mesozoic,
but also in older times (Parrish et al. 1983; Parrish
1987; Pedersen and Calvert 1990. Most of these
black shales occur along west-facing coasts, in settings similar to the present-day major centers of
upwelling. In the completely different paleogeographie situation in the Paleozoic, however, it can be
assumed that coastal upwelling also operated along
east-west trending coastlines.
5.3.5 Siliceous Sediments
Biogenie Production of Opaline Silica
Skeletons of opaline silica are produced in the modem oceans by several groups of organisms (diatoms,
radiolaria, silicoflagellates, and sponges) in large
quantities. The radiolarians re ach a weight percentage of ab out 60 to more than 95% of the silica production in tropical waters, but ;:; 1 % in the Antarctic.
In addition, the radiolaria have a much higher potential for being preserved in marine sediments than the
other groups. It is assumed that the importance ofthe
radiolarians in the formation of siliceous sediments
was even greater in pre-Cenozoic times.
By number, diatoms are the most abundant siliceous organisms in the modem oceans. In tropical regions they make
up between 30 and 75% of the total amount of opal-concentrating organisms, in Antarctic waters 99% (Lisitzin
1972; Blueford 1989). The most abundant group by
weight, however, are the radiolarians. On the average, 1 mg
of silica corresponds to 2000 radiolarian skeletons, but 100
000 diatom cells and 250 000 dinoflagellates equal that
weight (thus yielding a weight ratio of 250:5: I).
As with carbonates, the organisms extract much more
silica from ocean waters than is added by rivers and
197
voIcanic activity. Consequently, the sea water near
the surface is extremely undersaturated (it normally
contains ab out 1 mg/l Si02), particularly with respect
to opaline silica. The solubi1ity of the tiny opaline
tests is around 110 mg/I, in contrast to that of quartz
which is on1y about 5 mg/l. Therefore, most of the
siliceous tests of microorganisms are already dissolved within the upper water co1urnn, before they
reach deeper waters and the sea floor, which have
higher silicon concentrations. But even here, a great
part of the remaining tests is still dissolved within the
uppermost sediment layer (Hein and Obradovic
1989). For average conditions in the present oceans,
it is assumed that perhaps 1 % of the originally produced biogenic silica is stored in the sediment. For
that reason, there are large areas on the sea floor
where bio genie silica is completely absent or can be
neg1ected as an important constituent of deep-sea
sediments.
Preservation of Opaline Silica
The possibility of preserving siliceous skeletal remains is enhanced if they settle comparatively fast
through the water colurnn. This is true for relatively
robust skeletons, such as those of radiolaria, and of
course in shallower waters, e.g. in zones of coastal
upwelling. Particularly favorable for the preservation
of rnicroorganisms is their incorporation into fecal
pellets, which protect the skeletons from being dissolved. The most effective way to increase the preservation of this material is, however, high productivity in zones of upwelling.
It has been repeatedly pointed out that submarine volcanic
activity, releasing substantial amounts of silica into the
ocean, may locally enhance biogenic silica production and
reduce its dissolution in sea water. However, one has to
bear in mind that the input of silica alone (e.g., by volcani sm) into surface waters cannot enhance fertility, unless
other nutrients such as nitrogen and phosphorus are also
available in sufficient quantities.
In order to have high concentrations of opaline si1ica
in sediments, the preserved part of the primary production must not be diluted by terrigenous material
or carbonate. Consequently we can expect highly
siliceous or almost pure biosiliceous sediments only
in areas which meet the following conditions:
(1) High primary productivity.
(2) Very low influx of terrigenous material.
(3) Biogenic carbonate production must not substantially exceed carbonate dissolution, i.e. the lysocline
and CCD are so high that most of the carbonate is
dissolved in the water colurnn or within the sediment.
tain abundant fish debris (bone beds) and a relatively high
amount of phosphorus due to the accumulation of abundant
fecal pellets and coprolites. Several trace metals, particularly U, Mo, Cd, Zn, and Ni occur in considerably higher
concentrations than in normal deep-sea sediments (e.g.,
Baturin 1983; Brumsack 1986a,b).
The sedimentation rates in coastal upwelling zones are
largely controlled by terrigenous influx. In the Peru
upwelling region, the mean sedimentation rates of late
Pleistocene sediments (drilIed at water depths of 250 and
450 m) are about 7 and 17 cmlka, respectively (Wefer et al.
1990); older sediments accumulated more rapidly.
The Plio-Pleistocene sediments of the Angola-Benguela
upwelling system off Southwest Africa and those of the
Congo Fan show glacial-interglacial precession cycles
(Berger et al. 1998). These are produced by current-induced changes in biogenic productivity (mostly opaline
silica).
Coastal upwelling was certainly also an important
process in ancient marine environments. The formation of many black shales is now ascribed to this
mechanism not only in the Cenozoic and Mesozoic,
but also in older times (Parrish et al. 1983; Parrish
1987; Pedersen and Calvert 1990. Most of these
black shales occur along west-facing coasts, in settings similar to the present-day major centers of
upwelling. In the completely different paleogeographie situation in the Paleozoic, however, it can be
assumed that coastal upwelling also operated along
east-west trending coastlines.
5.3.5 Siliceous Sediments
Biogenie Production of Opaline Silica
Skeletons of opaline silica are produced in the modem oceans by several groups of organisms (diatoms,
radiolaria, silicoflagellates, and sponges) in large
quantities. The radiolarians re ach a weight percentage of ab out 60 to more than 95% of the silica production in tropical waters, but ;:; 1 % in the Antarctic.
In addition, the radiolaria have a much higher potential for being preserved in marine sediments than the
other groups. It is assumed that the importance ofthe
radiolarians in the formation of siliceous sediments
was even greater in pre-Cenozoic times.
By number, diatoms are the most abundant siliceous organisms in the modem oceans. In tropical regions they make
up between 30 and 75% of the total amount of opal-concentrating organisms, in Antarctic waters 99% (Lisitzin
1972; Blueford 1989). The most abundant group by
weight, however, are the radiolarians. On the average, 1 mg
of silica corresponds to 2000 radiolarian skeletons, but 100
000 diatom cells and 250 000 dinoflagellates equal that
weight (thus yielding a weight ratio of 250:5: I).
As with carbonates, the organisms extract much more
silica from ocean waters than is added by rivers and
197
voIcanic activity. Consequently, the sea water near
the surface is extremely undersaturated (it normally
contains ab out 1 mg/l Si02), particularly with respect
to opaline silica. The solubi1ity of the tiny opaline
tests is around 110 mg/I, in contrast to that of quartz
which is on1y about 5 mg/l. Therefore, most of the
siliceous tests of microorganisms are already dissolved within the upper water co1urnn, before they
reach deeper waters and the sea floor, which have
higher silicon concentrations. But even here, a great
part of the remaining tests is still dissolved within the
uppermost sediment layer (Hein and Obradovic
1989). For average conditions in the present oceans,
it is assumed that perhaps 1 % of the originally produced biogenic silica is stored in the sediment. For
that reason, there are large areas on the sea floor
where bio genie silica is completely absent or can be
neg1ected as an important constituent of deep-sea
sediments.
Preservation of Opaline Silica
The possibility of preserving siliceous skeletal remains is enhanced if they settle comparatively fast
through the water colurnn. This is true for relatively
robust skeletons, such as those of radiolaria, and of
course in shallower waters, e.g. in zones of coastal
upwelling. Particularly favorable for the preservation
of rnicroorganisms is their incorporation into fecal
pellets, which protect the skeletons from being dissolved. The most effective way to increase the preservation of this material is, however, high productivity in zones of upwelling.
It has been repeatedly pointed out that submarine volcanic
activity, releasing substantial amounts of silica into the
ocean, may locally enhance biogenic silica production and
reduce its dissolution in sea water. However, one has to
bear in mind that the input of silica alone (e.g., by volcani sm) into surface waters cannot enhance fertility, unless
other nutrients such as nitrogen and phosphorus are also
available in sufficient quantities.
In order to have high concentrations of opaline si1ica
in sediments, the preserved part of the primary production must not be diluted by terrigenous material
or carbonate. Consequently we can expect highly
siliceous or almost pure biosiliceous sediments only
in areas which meet the following conditions:
(1) High primary productivity.
(2) Very low influx of terrigenous material.
(3) Biogenic carbonate production must not substantially exceed carbonate dissolution, i.e. the lysocline
and CCD are so high that most of the carbonate is
dissolved in the water colurnn or within the sediment.