5.3 Hemipelagic and Pelagic Sediments
of organic compounds, which were produced in surface
waters, but were decomposed at, or near the sea bottom.
Other sources may be submarine exhalations and normal
bottom currents containing the metals in very low concentrations, either in solution or adsorbed to suspended partieIes. In addition, the composition of the nodules is influenced by diagenesis, particularly their variable Mn/Fe ratios.
In recent years, these widely distributed nodules and
particularly their contents in Co, Ni, and Cu have aroused
considerable economic interest. Tests were run by several
groups for underwater mining of these enormous ore deposits. These activities, ineluding our present knowledge
on the formation and geochemistry of manganese nodules,
have been summarized in a special volume (Halbach et al.
1988). In ancient sediments, mangane se nodules formed in
the deep sea ar~ relatively rare (Cronan et al. 1991). Ifthey
occur on top of paleo-seamounts or in former basinal settings, they resemble lithologically, mineralogically, and
geochemically the modern ferromanganese deposits. In
many cases, however, the manganese nodules have either
been subducted with their accompanying sediments and
transformed by metamorphism, or they have been dissolved
as mentioned above.
5.3.4 Sediments in Zones ofUpwelling
General Aspects
In zones of coastal and equatorial upwelling, where
nutrient-rich waters come to the surface, the productivity of phytoplankton (e.g., coccoliths and diatoms )
and zooplankton (e.g., forarninifers) is significantly
enhanced (Fig. 5.4a). The availability of abundant
food attracts larger organisms (e.g., fish) which feed
on the micro-organism. Although most of the primary
production is used up and rnineralized in the water
colurrm, the overall sedimentation rate of biogenic
material increases considerably in zones ofupwelling
as compared to areas of normal fertility. Therefore,
deep-sea sediments associated with equatorial
upwelling usually still contain biogenic carbonate
and silica and more organic matter than their
neighboring sediments. This means that the CCD and
possibly also the lysocline drop below the sea floor
(Fig. 5.4a).
Coastal Upwelling and Its Sediments
Preservation of organic matter, biogenic silica, and
carbonate is particularly significant in zones of
coastal upwelling where the nutrient supply from
intermediate waters is concentrated in rather narrow
zones on the upper slope and outer shelf along continental margins (Fig. 5.1e).
The modern oceans display four major areas of year-round
coastal upwelling as part of the eastern boundary currents
in the Atlantic and Pacific Oceans (Thiede and Suess
195
1983): off northwest and southwest Africa, western North
America, and northwestern South America. There are also
areas of seasonal upwelling associated with prevailing
wind regimes, for example in the northern Indian Ocean,
off northwest Australia, and in the Gulf of California.
The lower part of the water colurrm below coastal
upwelling is often characterized by a pronounced oxygen minimum zone. Oxygen demand for total mineralization of the large quantities of organic matter produced in surface waters is higher than oxygen supply
in the relatively short (shallow) water colurrm. In addition, the particulate organic matter needs less time
to sink through the water colurrm and reach the sea
floor than in the deep ocean.
Thus, oxygen transfer from outside the zone of
coastal upwelling cannot compensate for rapid oxygen consumption below the area of high biogenic
productivity. Consequently, benthic li fe at the sea
bottom is reduced or entirely e;tbsent, the sediment
can become laminated, and a large part of the organic
matter reaching the sea floor is preserved. The centers of upwelling are, however, not fixed to a certain
location, but may move along the coast and seaward.
Sirnilarly, the intensity of upwelling varies with time,
because it is controlled by fluctuations in the wind
and oceanic current patterns. For example, rapid oxygen depletion by a sudden plankton bloom can cause
a mass mortality of fish and other organisms
(Brongersma-Sanders 1957).
Black shales originating from coastal upwelling
usually display frequent changes from anoxic to lowoxygen conditions, as can be seen by the appearance
or disappearance of a particular ichnofauna (Savrda
and Bottjer 1987, 1989). Sirnilarly, the occurrence of
molluscan shell beds in black shales indicates an interval, in which the upper boundary of the oxygen
minimum zone dropped to the sea floor (Schneider
and Wefer 1990). The fauna preserved in sediments
below upwelling zones is generally characterized by
a limited number of species which indicate a cooler
environment than in adjacent regions. Due to a high
productivity of diatoms and/or radiolaria, sediments
from upwelling zones are often rich in siliceous layers or chert nodules.
Diatoms prevail in cool, high-Iatitude waters where the
production and preservation of carbonate are low. In warm,
lower latitude regions, the carbonate content in upwelling
sediments can become rather high and attain those values
typical of marls, and sometimes even those of marly limestones. During diagenesis, dolomite may form, a process
which is favored by the decomposition of organic matter
(Lippmann 1973; Suess et al. 1987).
The organic matter in such sediments usually reaches a
few percent, but sometimes also up to 10% by dry weight.
It is entircly or predominantly of marine planktonic and
bacterial origin (Summerhayes 1983; ten Haven et al.
1990), which is an important indicator for the type of black
shale. The sediments in zones ofupwelling mayaiso con-
of organic compounds, which were produced in surface
waters, but were decomposed at, or near the sea bottom.
Other sources may be submarine exhalations and normal
bottom currents containing the metals in very low concentrations, either in solution or adsorbed to suspended partieIes. In addition, the composition of the nodules is influenced by diagenesis, particularly their variable Mn/Fe ratios.
In recent years, these widely distributed nodules and
particularly their contents in Co, Ni, and Cu have aroused
considerable economic interest. Tests were run by several
groups for underwater mining of these enormous ore deposits. These activities, ineluding our present knowledge
on the formation and geochemistry of manganese nodules,
have been summarized in a special volume (Halbach et al.
1988). In ancient sediments, mangane se nodules formed in
the deep sea ar~ relatively rare (Cronan et al. 1991). Ifthey
occur on top of paleo-seamounts or in former basinal settings, they resemble lithologically, mineralogically, and
geochemically the modern ferromanganese deposits. In
many cases, however, the manganese nodules have either
been subducted with their accompanying sediments and
transformed by metamorphism, or they have been dissolved
as mentioned above.
5.3.4 Sediments in Zones ofUpwelling
General Aspects
In zones of coastal and equatorial upwelling, where
nutrient-rich waters come to the surface, the productivity of phytoplankton (e.g., coccoliths and diatoms )
and zooplankton (e.g., forarninifers) is significantly
enhanced (Fig. 5.4a). The availability of abundant
food attracts larger organisms (e.g., fish) which feed
on the micro-organism. Although most of the primary
production is used up and rnineralized in the water
colurrm, the overall sedimentation rate of biogenic
material increases considerably in zones ofupwelling
as compared to areas of normal fertility. Therefore,
deep-sea sediments associated with equatorial
upwelling usually still contain biogenic carbonate
and silica and more organic matter than their
neighboring sediments. This means that the CCD and
possibly also the lysocline drop below the sea floor
(Fig. 5.4a).
Coastal Upwelling and Its Sediments
Preservation of organic matter, biogenic silica, and
carbonate is particularly significant in zones of
coastal upwelling where the nutrient supply from
intermediate waters is concentrated in rather narrow
zones on the upper slope and outer shelf along continental margins (Fig. 5.1e).
The modern oceans display four major areas of year-round
coastal upwelling as part of the eastern boundary currents
in the Atlantic and Pacific Oceans (Thiede and Suess
195
1983): off northwest and southwest Africa, western North
America, and northwestern South America. There are also
areas of seasonal upwelling associated with prevailing
wind regimes, for example in the northern Indian Ocean,
off northwest Australia, and in the Gulf of California.
The lower part of the water colurrm below coastal
upwelling is often characterized by a pronounced oxygen minimum zone. Oxygen demand for total mineralization of the large quantities of organic matter produced in surface waters is higher than oxygen supply
in the relatively short (shallow) water colurrm. In addition, the particulate organic matter needs less time
to sink through the water colurrm and reach the sea
floor than in the deep ocean.
Thus, oxygen transfer from outside the zone of
coastal upwelling cannot compensate for rapid oxygen consumption below the area of high biogenic
productivity. Consequently, benthic li fe at the sea
bottom is reduced or entirely e;tbsent, the sediment
can become laminated, and a large part of the organic
matter reaching the sea floor is preserved. The centers of upwelling are, however, not fixed to a certain
location, but may move along the coast and seaward.
Sirnilarly, the intensity of upwelling varies with time,
because it is controlled by fluctuations in the wind
and oceanic current patterns. For example, rapid oxygen depletion by a sudden plankton bloom can cause
a mass mortality of fish and other organisms
(Brongersma-Sanders 1957).
Black shales originating from coastal upwelling
usually display frequent changes from anoxic to lowoxygen conditions, as can be seen by the appearance
or disappearance of a particular ichnofauna (Savrda
and Bottjer 1987, 1989). Sirnilarly, the occurrence of
molluscan shell beds in black shales indicates an interval, in which the upper boundary of the oxygen
minimum zone dropped to the sea floor (Schneider
and Wefer 1990). The fauna preserved in sediments
below upwelling zones is generally characterized by
a limited number of species which indicate a cooler
environment than in adjacent regions. Due to a high
productivity of diatoms and/or radiolaria, sediments
from upwelling zones are often rich in siliceous layers or chert nodules.
Diatoms prevail in cool, high-Iatitude waters where the
production and preservation of carbonate are low. In warm,
lower latitude regions, the carbonate content in upwelling
sediments can become rather high and attain those values
typical of marls, and sometimes even those of marly limestones. During diagenesis, dolomite may form, a process
which is favored by the decomposition of organic matter
(Lippmann 1973; Suess et al. 1987).
The organic matter in such sediments usually reaches a
few percent, but sometimes also up to 10% by dry weight.
It is entircly or predominantly of marine planktonic and
bacterial origin (Summerhayes 1983; ten Haven et al.
1990), which is an important indicator for the type of black
shale. The sediments in zones ofupwelling mayaiso con-