194
the result that far fewer pelagic deep-sea carbonates were
preserved than during other periods.
The carbonate system of the oceans is also closely
linked with the carbon dioxide content of the atmosphere.
Warm periods in the Earth's history appear to have been
associated with high CO2 partial pressures in the atmosphere. Wether and in which way this influenced carbonate
dissolution in the oceans is a topic which is beyond the
intention of this book.
In the transitional zones between full carbonate preservation above the lysoc1ine, carbonate-bearing sediments between the lysoc1ine and the CCD, and siliceous red c1ay (see below), a fluctuating CCD can
generate an altemating succession of these two bed
types (Fig. 5Ab and c, cf. Sect. 7.9).
Adjacent seas with narrow openings to the world
oceans and therefore limited exchange of water
masses (Chap. 4, Fig. 4.2) are characterized by their
own carbonate budget and a CCD which may deviate
significantly from that ofthe major ocean basins.
Such smaller basins are controlled mainly by the climate
and river input of neighboring continents. A high influx of
calcium hydrogen carbonate and limited carbonate production, as frequently observed in humid, mid-Iatitude zones,
cause the CCD to drop to or below the sea bed. Dissolution
of calcium carbonate can then only take place within the
sediment as a result of organic matter decomposition (e.g.,
in the Baltic Sea, cf. Sect. 4.2). Similarly, adjacent basins
surrounded by hot, arid regions will not necessarily develop a high CCD, because their carbonate production is
usually kept low due to a shortage of nutrients, if their water exchange with the open ocean is limited. Thus, a CCD
appears to be absent in most of these basins, i.e., it lies
below their basin floors (e.g., in the Red Sea).
5.3.3 Red Clay and Manganese Nodules
Red Deep-Sea Clay
The water masses of large oceanic gyres (Fig. 5.1e)
have little exchange with nutrient-rich coastal waters
or intermediate waters welling up in equatorial regions. The planktonic productivity of the central
ocean basins is therefore low, and carbonate is dissolved below the CCD. Organic matter is mineralized
in the water colunm or at the sea floor where it is
exposed for a long time to oxygenated bottom water.
These basins receive little terrigenous input (a major
part of it being eolian dust), and their bottom waters
are well oxygenated.
Under these conditions, the so-called red clay is
deposited at an extremely slow rate (Fig. 5Aa and d).
In a fresh state, this material has a reddish brown
color, because it contains finely dispersed iron
oxyhydroxides as pigment.
The clay mineral assemblage of red clays is controlled
mainly by their source areas and therefore varies in space
Chapter 5 Oceanic Sediments
and time. Illite, smectite, kaolinite, and chlorite are major
constituents in wide regions. Eolian dust is assumed to
contribute a large portion of such red clays, and even some
cosmic dust (spherules of nickel-iron, minerals of
chondrites ) has been discovered (Bryant and Bennett
1988).
In the transition zone between highly and poorly fertile surface waters, biogenic carbonate may already
dissolve in the water colunm, whereas part of the siliceous remains n::aches the sea bottom. In this case,
red c1ays contain considerable proportions of
radiolaria (predominantly living in warm equatorial
waters) or diatorns (mainly from colder waters).
Manganese Nodules
In large areas of the present oceans, particularly in
the central Pacific, the red c1ay is covered by manganese nodules which mayaiso contain various
amounts of iron and relatively high quantities of
nickel, cobalt, copper, molybdenum, and other trace
metals (e.g., Seibold 1978; Calvert and Piper 1984;
Halbach et al. 1988). The nodules are concentrated
on the sea bed and mostly become sparse some tens
of cm below the sediment-water interface.
For this reason it has been postulated that the nodules dissolve under reducing conditions at some
depth below the sea floor and that Mn migrates by
diffusion to the interface and precipitates at the contact with sea water. If the chemical environment in
interstitial waters remains oxidizing, the nodules are
preserved. Manganese nodules do not form at the sea
floor below an oxygen minimum zone, e.g. along
continental slopes. Then the sediment can become
depleted of Mn (e.g. Dickens and Owens 1995). It
has also been noted that bacteria play a role in the
fixation of manganese.
Most nodules are flat at their base and show concentric
growth structures. Their growth rates vary from relatively
high values (several mmlka) to very low rates (several
mmlMa). The low values are even lower than the sedimentation rate of pelagic red clay (0.5 to about 5 mmlka,
Andreyev and Kulikov 1987; Halbach et al. 1988). No relationship was found between the growth rate of nodules and
their host sediments.
The finding that most manganese nodules have a higher
age than the surrounding surface sediment leads to the conclusion that they "migrate" upward with the sea bed. This
may be accomplished either by continuous dissolution and
reprecipitation, by burrowing organisms lifting the nodules,
or by bottom currents, which are occasionally strong
enough to move and turn over porous nodules. Bottom currents also prevent continuous sediment accumulation and
thus are responsible for the very low average sedimentation
rates characteristic of deep-sea sediments rich in manganese nodules.
The source of manganese, iron, and the other metals is a
matter of debate. They may derive from the metal content
the result that far fewer pelagic deep-sea carbonates were
preserved than during other periods.
The carbonate system of the oceans is also closely
linked with the carbon dioxide content of the atmosphere.
Warm periods in the Earth's history appear to have been
associated with high CO2 partial pressures in the atmosphere. Wether and in which way this influenced carbonate
dissolution in the oceans is a topic which is beyond the
intention of this book.
In the transitional zones between full carbonate preservation above the lysoc1ine, carbonate-bearing sediments between the lysoc1ine and the CCD, and siliceous red c1ay (see below), a fluctuating CCD can
generate an altemating succession of these two bed
types (Fig. 5Ab and c, cf. Sect. 7.9).
Adjacent seas with narrow openings to the world
oceans and therefore limited exchange of water
masses (Chap. 4, Fig. 4.2) are characterized by their
own carbonate budget and a CCD which may deviate
significantly from that ofthe major ocean basins.
Such smaller basins are controlled mainly by the climate
and river input of neighboring continents. A high influx of
calcium hydrogen carbonate and limited carbonate production, as frequently observed in humid, mid-Iatitude zones,
cause the CCD to drop to or below the sea bed. Dissolution
of calcium carbonate can then only take place within the
sediment as a result of organic matter decomposition (e.g.,
in the Baltic Sea, cf. Sect. 4.2). Similarly, adjacent basins
surrounded by hot, arid regions will not necessarily develop a high CCD, because their carbonate production is
usually kept low due to a shortage of nutrients, if their water exchange with the open ocean is limited. Thus, a CCD
appears to be absent in most of these basins, i.e., it lies
below their basin floors (e.g., in the Red Sea).
5.3.3 Red Clay and Manganese Nodules
Red Deep-Sea Clay
The water masses of large oceanic gyres (Fig. 5.1e)
have little exchange with nutrient-rich coastal waters
or intermediate waters welling up in equatorial regions. The planktonic productivity of the central
ocean basins is therefore low, and carbonate is dissolved below the CCD. Organic matter is mineralized
in the water colunm or at the sea floor where it is
exposed for a long time to oxygenated bottom water.
These basins receive little terrigenous input (a major
part of it being eolian dust), and their bottom waters
are well oxygenated.
Under these conditions, the so-called red clay is
deposited at an extremely slow rate (Fig. 5Aa and d).
In a fresh state, this material has a reddish brown
color, because it contains finely dispersed iron
oxyhydroxides as pigment.
The clay mineral assemblage of red clays is controlled
mainly by their source areas and therefore varies in space
Chapter 5 Oceanic Sediments
and time. Illite, smectite, kaolinite, and chlorite are major
constituents in wide regions. Eolian dust is assumed to
contribute a large portion of such red clays, and even some
cosmic dust (spherules of nickel-iron, minerals of
chondrites ) has been discovered (Bryant and Bennett
1988).
In the transition zone between highly and poorly fertile surface waters, biogenic carbonate may already
dissolve in the water colunm, whereas part of the siliceous remains n::aches the sea bottom. In this case,
red c1ays contain considerable proportions of
radiolaria (predominantly living in warm equatorial
waters) or diatorns (mainly from colder waters).
Manganese Nodules
In large areas of the present oceans, particularly in
the central Pacific, the red c1ay is covered by manganese nodules which mayaiso contain various
amounts of iron and relatively high quantities of
nickel, cobalt, copper, molybdenum, and other trace
metals (e.g., Seibold 1978; Calvert and Piper 1984;
Halbach et al. 1988). The nodules are concentrated
on the sea bed and mostly become sparse some tens
of cm below the sediment-water interface.
For this reason it has been postulated that the nodules dissolve under reducing conditions at some
depth below the sea floor and that Mn migrates by
diffusion to the interface and precipitates at the contact with sea water. If the chemical environment in
interstitial waters remains oxidizing, the nodules are
preserved. Manganese nodules do not form at the sea
floor below an oxygen minimum zone, e.g. along
continental slopes. Then the sediment can become
depleted of Mn (e.g. Dickens and Owens 1995). It
has also been noted that bacteria play a role in the
fixation of manganese.
Most nodules are flat at their base and show concentric
growth structures. Their growth rates vary from relatively
high values (several mmlka) to very low rates (several
mmlMa). The low values are even lower than the sedimentation rate of pelagic red clay (0.5 to about 5 mmlka,
Andreyev and Kulikov 1987; Halbach et al. 1988). No relationship was found between the growth rate of nodules and
their host sediments.
The finding that most manganese nodules have a higher
age than the surrounding surface sediment leads to the conclusion that they "migrate" upward with the sea bed. This
may be accomplished either by continuous dissolution and
reprecipitation, by burrowing organisms lifting the nodules,
or by bottom currents, which are occasionally strong
enough to move and turn over porous nodules. Bottom currents also prevent continuous sediment accumulation and
thus are responsible for the very low average sedimentation
rates characteristic of deep-sea sediments rich in manganese nodules.
The source of manganese, iron, and the other metals is a
matter of debate. They may derive from the metal content