5
somewhat more than 7 % by eolian transport, and
less than 7 % is due to the activity of icebergs.
A distinctly less, but still not insignificant proportion of lithogenous sediment is formed by
volcanic activity which is quite often coupled with
processes of active subduction at the continental
plate boundaries. A large proportion of pyroclastic
fragments becomes wind-dispersed over large
areas, whereafter they are usually retraced in
oceanic sediments as finely distributed volcanic
glass. Yet, the formation of single, distinct, cmthick tephra layers might also occur in the deep
sea where they represent genuine isochronous
markers which can be used for correlation
purposes and the time calibration of stratigraphic
units. Layers of ash deposits in the eastern Mediterranean are prominent examples indicative of the
eruption of the volcanic island Ischia in prehistoric times of more than 25,000 years ago, and
Santorin about 3500 years ago.
Locally, there may be a frequent occurrence of
tephra layers and significant concentrations of
finely dispersed volcaniclastic material in deepsea sediments especially in the proximal zones of
volcanic activity, like in marginal zones of the
modern Pacific Ocean. In a recent evaluation of tephra input into the Pacific Ocean sediments based
on DSDP and ODP data Straub and Schmincke
(1998) estimate that the minimum proportion of
volcanic tephra corresponds to 23 vol.% of the existing Pacific oceanic sediments.
Lithogenous detrital components of marine
sediments, despite all regional variability, include
only few basic minerals (Table 1.1). With the exception of quartz, complete weathering, particularly the chemical weathering of metamorphic and
igneous rock, leads to the formation of clay
minerals. Consequently, this group represents,
apart from the remaining quartz, the most important mineral constituent in sediments; clay minerals make up nearly 50 % of the entire terrigenous
sediment. To a lesser degree, terrigenous detritus
contains unweathered minerals, like feldspars.
Furthermore, there are mica, non-biogenous calcite, dolomite in low quantities, as well as accessory heavy minerals, for instance, amphibole, pyroxene, apatite, disthene, garnet, rutile, anatase,
zirconium, tourmaline, but they altogether seldom
comprise more than 1 % of the sediment. Basically,
each mineral found in continental rock - apart from
their usually extreme low concentrations - may
also be found in the oceanic sediments. The
percentage in which the various minerals are
present in a sediment markedly depends on the
grain-size distribution.
The clay minerals are of special importance inasmuch as they not only constitute the largest
proportion of fine-grained and non-biogenous
sediment, but they also have the special geochemical property of absorbing and easily giving
off ions, a property which affords more detailed
observation. Clay minerals result foremost from
the weathering of primary, rock forming aluminous
silicates, like feldspar, hornblende and pyroxene,
or even volcanic glass. Kaolinite, chlorite, illite,
and smectite which represent the four most
important groups of clay minerals are formed
partly under very different conditions of weathering. Consequently, the analysis of their qualitative
and quantitative distribution will enable us to
draw essential conclusions on origin and transport, weathering and hydrolysis, and therefore on
climate conditions of the rock’s source region
(Biscay 1965; Chamley 1989). The extremely finegranular structure of clay minerals, which is likely
to produce an active surface of 30 m 2 g -1 sediment,
as well as their ability to absorb ions internally
within the crystal structure, or bind them
superficially by means of reversible adsorption, as
well as their capacity to temporarily bind larger
amounts of water, all these properties are
fundamental for us to consider clay sediments as
a very active and effectively working “geochemical factory”.
Clay minerals constitute a large part of the family of phyllosilicates. Their crystal structure is
characterized by alternation of flat, parallel sheets,
or layers of extreme thinness. For this reason clay
minerals are called layer silicates. Two basic types
of layers, or sheets make up any given clay
mineral. One type of layer consists of tetrahedral
sheets in which one silicon atom is surrounded by
four oxygen atoms in tetrahedral configuration.
The second type of layer is composed of
octahedron sheets in which aluminum or magnesium is surrounded by hydroxyl groups and oxygen in a 6-fold coordinated arrangement (Fig. 1.4).
Depending on the clay mineral under study, there
is still enough space for other cations possessing
a larger ionic radius, like potassium, sodium,
calcium, or iron to fit in the gaps between the
octahedrons and tetrahedrons. Some clay minerals
- the so-called expanding or swelling clays - have
a special property which allows them to incorporate hydrated cations into their structure. This
process is reversible; the water changes the
1.2
Sources and Components of Marine Sediments
somewhat more than 7 % by eolian transport, and
less than 7 % is due to the activity of icebergs.
A distinctly less, but still not insignificant proportion of lithogenous sediment is formed by
volcanic activity which is quite often coupled with
processes of active subduction at the continental
plate boundaries. A large proportion of pyroclastic
fragments becomes wind-dispersed over large
areas, whereafter they are usually retraced in
oceanic sediments as finely distributed volcanic
glass. Yet, the formation of single, distinct, cmthick tephra layers might also occur in the deep
sea where they represent genuine isochronous
markers which can be used for correlation
purposes and the time calibration of stratigraphic
units. Layers of ash deposits in the eastern Mediterranean are prominent examples indicative of the
eruption of the volcanic island Ischia in prehistoric times of more than 25,000 years ago, and
Santorin about 3500 years ago.
Locally, there may be a frequent occurrence of
tephra layers and significant concentrations of
finely dispersed volcaniclastic material in deepsea sediments especially in the proximal zones of
volcanic activity, like in marginal zones of the
modern Pacific Ocean. In a recent evaluation of tephra input into the Pacific Ocean sediments based
on DSDP and ODP data Straub and Schmincke
(1998) estimate that the minimum proportion of
volcanic tephra corresponds to 23 vol.% of the existing Pacific oceanic sediments.
Lithogenous detrital components of marine
sediments, despite all regional variability, include
only few basic minerals (Table 1.1). With the exception of quartz, complete weathering, particularly the chemical weathering of metamorphic and
igneous rock, leads to the formation of clay
minerals. Consequently, this group represents,
apart from the remaining quartz, the most important mineral constituent in sediments; clay minerals make up nearly 50 % of the entire terrigenous
sediment. To a lesser degree, terrigenous detritus
contains unweathered minerals, like feldspars.
Furthermore, there are mica, non-biogenous calcite, dolomite in low quantities, as well as accessory heavy minerals, for instance, amphibole, pyroxene, apatite, disthene, garnet, rutile, anatase,
zirconium, tourmaline, but they altogether seldom
comprise more than 1 % of the sediment. Basically,
each mineral found in continental rock - apart from
their usually extreme low concentrations - may
also be found in the oceanic sediments. The
percentage in which the various minerals are
present in a sediment markedly depends on the
grain-size distribution.
The clay minerals are of special importance inasmuch as they not only constitute the largest
proportion of fine-grained and non-biogenous
sediment, but they also have the special geochemical property of absorbing and easily giving
off ions, a property which affords more detailed
observation. Clay minerals result foremost from
the weathering of primary, rock forming aluminous
silicates, like feldspar, hornblende and pyroxene,
or even volcanic glass. Kaolinite, chlorite, illite,
and smectite which represent the four most
important groups of clay minerals are formed
partly under very different conditions of weathering. Consequently, the analysis of their qualitative
and quantitative distribution will enable us to
draw essential conclusions on origin and transport, weathering and hydrolysis, and therefore on
climate conditions of the rock’s source region
(Biscay 1965; Chamley 1989). The extremely finegranular structure of clay minerals, which is likely
to produce an active surface of 30 m 2 g -1 sediment,
as well as their ability to absorb ions internally
within the crystal structure, or bind them
superficially by means of reversible adsorption, as
well as their capacity to temporarily bind larger
amounts of water, all these properties are
fundamental for us to consider clay sediments as
a very active and effectively working “geochemical factory”.
Clay minerals constitute a large part of the family of phyllosilicates. Their crystal structure is
characterized by alternation of flat, parallel sheets,
or layers of extreme thinness. For this reason clay
minerals are called layer silicates. Two basic types
of layers, or sheets make up any given clay
mineral. One type of layer consists of tetrahedral
sheets in which one silicon atom is surrounded by
four oxygen atoms in tetrahedral configuration.
The second type of layer is composed of
octahedron sheets in which aluminum or magnesium is surrounded by hydroxyl groups and oxygen in a 6-fold coordinated arrangement (Fig. 1.4).
Depending on the clay mineral under study, there
is still enough space for other cations possessing
a larger ionic radius, like potassium, sodium,
calcium, or iron to fit in the gaps between the
octahedrons and tetrahedrons. Some clay minerals
- the so-called expanding or swelling clays - have
a special property which allows them to incorporate hydrated cations into their structure. This
process is reversible; the water changes the
1.2
Sources and Components of Marine Sediments