190
follow the comparatively simple rules found for
cohesionless materials such as sand and gravel. Clay
and fine silt-sized particles stick together due to van
der Waals forces or electrostatic attractions which
are large relative to the small weight of the single
particles. In addition, they may be bonded by organic
secretions of bacteria, algae, diatorns, and other
micro-organisrns living on top of or within the sediment. The combined result of all these processes is
described and measured as cohesion.
Fecal pellets, consisting mainly of inorganic, fine-grained
partieles bound together by organic compounds, are abundant on widely extended areas of the sea floor. Furthermore, the same assemblage of partic1es may vary in water
content or degree of compaction. Consequently, the same
general type of mud may display quite different values of
cohesion and thus require lower or higher critical current
stresses for its erosion, which are difficult to predict. Small
partieles and aggregates colliding in the water column can
adhere to one another and form larger aggregates (Gibbs
1985; Kineke and Stemberg 1989). Fully flocculated suspensions settle much faster than their individual partic1es
do in a disaggregated state. In most marine environments,
such aggregates have diameters greater than 1 mm, but
under higher fluid shear c10se to the sea bed they break up
into sizes :>,100 f.1m, unless they have strong biological
binding as, for example, fecal pellets. In this broken-up
state the aggregates are deposited on smooth or rough beds,
where some sorting between larger, more stable aggregates
and smaller aggregates may take place.
Thus, clay aggregates and fecal pellets need higher
current velocities to be transported over long distances than individual clay particles. To erode cohesive aggregates, current velocities higher than those
required for clean coarse silts or sands are necessary,
even if the grain fabric of the aggregated mud is disturbed by burrowing organisms or other discontinuities. Erosion usually does not occur particle by particle, but in the form of variously sized aggregates.
Apart from floating suspensions emanating from
river mouths, silts and clays entering the sea as river
load are sorted out by wave action and move offshore
via nepheloid plumes associated with ocean currents
(Fig. 5.3). Depending on the stratification of the water masses and the density of the suspensions, such
turbid plumes may migrate near the surface, at middepth, or they reach the deep sea where they drop
most of their suspended material. The finest-grained
particles or aggregates can be kept in suspension by
weak bottom currents for a long time. They form the
so-called nepheloid layer in many parts of the modem oceans. The thickness of this layer is on the order
of 100 to 1000 m, but the concentration of suspended
matter is very low.
Eolian dust. Although fine-grained terrigenous particles are distributed in a number of different ways
(including gravity mass flows as described below)
Chapter 5 Oceanic Sediments
over relatively long distances, large areas in the
oceans receive very little river-borne terrigenous
mud. In this regions, a great portion of the
terrigenous component sterns from dustfall from the
stratosphere, where fine-grained particles are rapidly
distributed around the globe (cf. Sect. 4.3.5). Windborne dust seems to be, for example, the main constituent of red deep-sea clay. The contribution of
eolian dust to the modem pelagic deep-sea sediments
is particularly_ significant in ocean regions on the lee
side of continental deserts.
Frequently addressed modem examples are the eastem
Atlantic off West Africa where trade winds carry Saharan
dust into the ocean, or the Arabian Sea which is affected by
southwest monsoons delivering dust from Asian deserts to
the sea (e.g. Sirocko et al. 1993). The central North Pacific
experienced high dustfall since about 3.6 Ma B.P., which is
related to the deposition of huge amounts of loess in China
(cf. Sect. 2.3). A change to dryer c1imate in central Asia,
associated with the buildup of ice -sheets in the northem
hemisphere and the uplift of the northem portion of the
Tibetan Plateau, may have caused this drastic increase of
dustfall (Rea et al. 1998).
The dust, transported over wide distances, is characterized by a narrow grain-size distribution (with a distinct
mode at about 2 f.1m) and relatively high contents of hematite and goethite (Rea and Hovan 1995; Balsam et al.
1995). Fine-grained plant debris indicative of desert environment mayaIso be present.
The biogenie component of the hemipelagic and pelagic sediments is produced either in surface waters
(plankton) or on the sea floor (benthos). In the deep
sea, the latter contribution to the sediment mass is of
minor importance, although it may be a significant
indicator of ancient water depth. Primary planktonic
production varies significantly in different parts of
the oceans (cf. Sect. 10.3.2); in addition, a great part
or, in certain regions, virtually all of the organic and
skeletal remains is mineralized or dissolved on its
way to the sea floor. Thus, the ultimate contribution
of biogenic components to the deep-sea sediments of
an individual basin may differ greatly in space and
time. Generally, the following situations can be distinguished:
(1) Strong dilution of the autochthonous biogenic
component by allochthonous terrigenous material.
The resulting sediments are hemipelagic silty clays or
clayey silts, characterized by relatively high sedimentation rates (often greater than ~5 cm/ka) and low
biogenic carbonate andlor opaline silica contents.
Many fine-grained sediments on gentle continental
slopes belong to this category, but they mayaiso contain redeposited material (see below).
(2) The biogenic component is moderately diluted by
terrigenous material, creating hemipelagic calcareous
andlor siliceous silty clays or clayey silts. Their sedimentation rates often range from 2 to 4 cm/ka.
follow the comparatively simple rules found for
cohesionless materials such as sand and gravel. Clay
and fine silt-sized particles stick together due to van
der Waals forces or electrostatic attractions which
are large relative to the small weight of the single
particles. In addition, they may be bonded by organic
secretions of bacteria, algae, diatorns, and other
micro-organisrns living on top of or within the sediment. The combined result of all these processes is
described and measured as cohesion.
Fecal pellets, consisting mainly of inorganic, fine-grained
partieles bound together by organic compounds, are abundant on widely extended areas of the sea floor. Furthermore, the same assemblage of partic1es may vary in water
content or degree of compaction. Consequently, the same
general type of mud may display quite different values of
cohesion and thus require lower or higher critical current
stresses for its erosion, which are difficult to predict. Small
partieles and aggregates colliding in the water column can
adhere to one another and form larger aggregates (Gibbs
1985; Kineke and Stemberg 1989). Fully flocculated suspensions settle much faster than their individual partic1es
do in a disaggregated state. In most marine environments,
such aggregates have diameters greater than 1 mm, but
under higher fluid shear c10se to the sea bed they break up
into sizes :>,100 f.1m, unless they have strong biological
binding as, for example, fecal pellets. In this broken-up
state the aggregates are deposited on smooth or rough beds,
where some sorting between larger, more stable aggregates
and smaller aggregates may take place.
Thus, clay aggregates and fecal pellets need higher
current velocities to be transported over long distances than individual clay particles. To erode cohesive aggregates, current velocities higher than those
required for clean coarse silts or sands are necessary,
even if the grain fabric of the aggregated mud is disturbed by burrowing organisms or other discontinuities. Erosion usually does not occur particle by particle, but in the form of variously sized aggregates.
Apart from floating suspensions emanating from
river mouths, silts and clays entering the sea as river
load are sorted out by wave action and move offshore
via nepheloid plumes associated with ocean currents
(Fig. 5.3). Depending on the stratification of the water masses and the density of the suspensions, such
turbid plumes may migrate near the surface, at middepth, or they reach the deep sea where they drop
most of their suspended material. The finest-grained
particles or aggregates can be kept in suspension by
weak bottom currents for a long time. They form the
so-called nepheloid layer in many parts of the modem oceans. The thickness of this layer is on the order
of 100 to 1000 m, but the concentration of suspended
matter is very low.
Eolian dust. Although fine-grained terrigenous particles are distributed in a number of different ways
(including gravity mass flows as described below)
Chapter 5 Oceanic Sediments
over relatively long distances, large areas in the
oceans receive very little river-borne terrigenous
mud. In this regions, a great portion of the
terrigenous component sterns from dustfall from the
stratosphere, where fine-grained particles are rapidly
distributed around the globe (cf. Sect. 4.3.5). Windborne dust seems to be, for example, the main constituent of red deep-sea clay. The contribution of
eolian dust to the modem pelagic deep-sea sediments
is particularly_ significant in ocean regions on the lee
side of continental deserts.
Frequently addressed modem examples are the eastem
Atlantic off West Africa where trade winds carry Saharan
dust into the ocean, or the Arabian Sea which is affected by
southwest monsoons delivering dust from Asian deserts to
the sea (e.g. Sirocko et al. 1993). The central North Pacific
experienced high dustfall since about 3.6 Ma B.P., which is
related to the deposition of huge amounts of loess in China
(cf. Sect. 2.3). A change to dryer c1imate in central Asia,
associated with the buildup of ice -sheets in the northem
hemisphere and the uplift of the northem portion of the
Tibetan Plateau, may have caused this drastic increase of
dustfall (Rea et al. 1998).
The dust, transported over wide distances, is characterized by a narrow grain-size distribution (with a distinct
mode at about 2 f.1m) and relatively high contents of hematite and goethite (Rea and Hovan 1995; Balsam et al.
1995). Fine-grained plant debris indicative of desert environment mayaIso be present.
The biogenie component of the hemipelagic and pelagic sediments is produced either in surface waters
(plankton) or on the sea floor (benthos). In the deep
sea, the latter contribution to the sediment mass is of
minor importance, although it may be a significant
indicator of ancient water depth. Primary planktonic
production varies significantly in different parts of
the oceans (cf. Sect. 10.3.2); in addition, a great part
or, in certain regions, virtually all of the organic and
skeletal remains is mineralized or dissolved on its
way to the sea floor. Thus, the ultimate contribution
of biogenic components to the deep-sea sediments of
an individual basin may differ greatly in space and
time. Generally, the following situations can be distinguished:
(1) Strong dilution of the autochthonous biogenic
component by allochthonous terrigenous material.
The resulting sediments are hemipelagic silty clays or
clayey silts, characterized by relatively high sedimentation rates (often greater than ~5 cm/ka) and low
biogenic carbonate andlor opaline silica contents.
Many fine-grained sediments on gentle continental
slopes belong to this category, but they mayaiso contain redeposited material (see below).
(2) The biogenic component is moderately diluted by
terrigenous material, creating hemipelagic calcareous
andlor siliceous silty clays or clayey silts. Their sedimentation rates often range from 2 to 4 cm/ka.