1
The Solid Phase of Marine Sediments
10
tute the major proportion of the massive
structures of coral reefs. Together with calcifying
green algae, foraminifera, and mollusks, these organisms participate in a highly productive ecosystem. Here, coarse-grained calcareous sands and
gravel are essentially composed of various bioclasts attributable to the reef structure, of lime-secreting algae, mollusks, echinoderms and large
foraminifera.
Fine-grained calcareous mud is produced by
green algae and benthic foraminifera as well as by
the mechanical abrasion of shells of the
macrobenthos. Considerable amounts of sediment
are formed by bioerosion, through the action of
boring, grazing and browsing, and predating organisms. Not all the details have been elucidated
as to which measure the chemical and biological
decomposition of the organic matter in biogenic
hard materials might lead to the formation of primary skeletal chrystalites on the micrometer scale,
and consequently contribute to the fine-grained
calcareous mud formation.
It is obvious that the various calcareousshelled groups, especially of those organisms who
secrete aragonite and Mg-calcite, contribute significantly to the sediment formation in the shallow
seas, whereas greater deposits of biogenic opal
are rather absent in the shallow shelf seas. The
isostatically over-deepened shelf region of
Antarctica, where locally a significant accumulation of siliceous sponge oozes occurs, however,
makes a remarkable exception. The relatively low
opal concentration in recent shelf deposits does
not result from an eventual dilution with terrigenous material. The reason is rather that recent
tropical shallow waters have low silicate concentrations, from which it follows that diatoms and
sponges are only capable of forming slightly
silicified skeletons that quickly remineralize in
markedly silicate-deficient waters.
With an increasing distance from the coastal
areas, out toward the open ocean, the relevance of
planktonic shells and tests in the formation of
sediments increases as well (Fig. 1.6). Planktonic
lime-secreting algae and silica-secreting algae,
coccolithophorids and diatoms that dwell as primary producers in the photic zone which thickness measures approximately 100 m, as well as the
calcareous foraminifers and siliceous radiolarians,
and silicoflagellates (Table 1.2), are the producers
of the by far most widespread and essential deepsea sediments: the calcareous and siliceous
biogenic oozes. Apart from the groups mentioned,
planktonic mollusks, the aragonite-shelled
pteropods, and some calcareous cysts forming
dinoflagellates, also contribute to a considerable
degree to sediment formation.
1.2.3
Hydrogenous Sediments
Hydrogenous sediments may be widely distributed, but as to their recent quantity they are relatively insignificant. They will be briefly mentioned
in this context merely for reasons of being
complete. According to Elderfield (1976), hydrogenous sediments can be subdivided into “precipitates”, primary inorganic components which have
precipitated directly from seawater, like sodium
chloride, and “halmyrolysates”, secondary
components which are the reaction products of
sediment particles with seawater, formed subsequent to in-situ weathering, but prior to diagenesis. Of these, manganese nodules give an example. In the scope of this book, these components are not conceived as being part of the “primary” solid phase sediment, but as “secondary”
authigenic formations which only emerge in the
course of diagenesis, as for instance some clay
minerals like glauconite, zeolite, hydroxides of iron
and manganese etc. In the subsequent Chapters
11 and 13 some aspects of these new formations
will be more thoroughly discussed.
The distinction between detrital and newly
formed, authigenic clay minerals is basically difficult to make on account of the small grain size and
their amalgamation with quite similar detrital
material. Yet it has been ascertained that the by far
largest clay mineral proportion – probably more
than 90 % – located in recent to subrecent sediments is of detrital origin (Chamley 1989; Hillier
1995). There are essentially three ways for
smectites to be formed, which demand specific
conditions as they are confined to local areas. Alterations produced in volcanic material is one way,
especially by means of hydration of basaltic and
volcanic glasses. This process is referred to as
palagonitization. The probably best studied
smectite formation consists in the vents of hydrothermal solutions and their admixture with seawater at the mid-oceanic mountain ranges. Should
authigenic clay minerals form merely in recent surface sediments in very small amounts, their frequency during diagenesis (burial diagenesis), will
demonstrate a distinct elevation. However, this
aspect will not be considered any further beyond
this point.
The Solid Phase of Marine Sediments
10
tute the major proportion of the massive
structures of coral reefs. Together with calcifying
green algae, foraminifera, and mollusks, these organisms participate in a highly productive ecosystem. Here, coarse-grained calcareous sands and
gravel are essentially composed of various bioclasts attributable to the reef structure, of lime-secreting algae, mollusks, echinoderms and large
foraminifera.
Fine-grained calcareous mud is produced by
green algae and benthic foraminifera as well as by
the mechanical abrasion of shells of the
macrobenthos. Considerable amounts of sediment
are formed by bioerosion, through the action of
boring, grazing and browsing, and predating organisms. Not all the details have been elucidated
as to which measure the chemical and biological
decomposition of the organic matter in biogenic
hard materials might lead to the formation of primary skeletal chrystalites on the micrometer scale,
and consequently contribute to the fine-grained
calcareous mud formation.
It is obvious that the various calcareousshelled groups, especially of those organisms who
secrete aragonite and Mg-calcite, contribute significantly to the sediment formation in the shallow
seas, whereas greater deposits of biogenic opal
are rather absent in the shallow shelf seas. The
isostatically over-deepened shelf region of
Antarctica, where locally a significant accumulation of siliceous sponge oozes occurs, however,
makes a remarkable exception. The relatively low
opal concentration in recent shelf deposits does
not result from an eventual dilution with terrigenous material. The reason is rather that recent
tropical shallow waters have low silicate concentrations, from which it follows that diatoms and
sponges are only capable of forming slightly
silicified skeletons that quickly remineralize in
markedly silicate-deficient waters.
With an increasing distance from the coastal
areas, out toward the open ocean, the relevance of
planktonic shells and tests in the formation of
sediments increases as well (Fig. 1.6). Planktonic
lime-secreting algae and silica-secreting algae,
coccolithophorids and diatoms that dwell as primary producers in the photic zone which thickness measures approximately 100 m, as well as the
calcareous foraminifers and siliceous radiolarians,
and silicoflagellates (Table 1.2), are the producers
of the by far most widespread and essential deepsea sediments: the calcareous and siliceous
biogenic oozes. Apart from the groups mentioned,
planktonic mollusks, the aragonite-shelled
pteropods, and some calcareous cysts forming
dinoflagellates, also contribute to a considerable
degree to sediment formation.
1.2.3
Hydrogenous Sediments
Hydrogenous sediments may be widely distributed, but as to their recent quantity they are relatively insignificant. They will be briefly mentioned
in this context merely for reasons of being
complete. According to Elderfield (1976), hydrogenous sediments can be subdivided into “precipitates”, primary inorganic components which have
precipitated directly from seawater, like sodium
chloride, and “halmyrolysates”, secondary
components which are the reaction products of
sediment particles with seawater, formed subsequent to in-situ weathering, but prior to diagenesis. Of these, manganese nodules give an example. In the scope of this book, these components are not conceived as being part of the “primary” solid phase sediment, but as “secondary”
authigenic formations which only emerge in the
course of diagenesis, as for instance some clay
minerals like glauconite, zeolite, hydroxides of iron
and manganese etc. In the subsequent Chapters
11 and 13 some aspects of these new formations
will be more thoroughly discussed.
The distinction between detrital and newly
formed, authigenic clay minerals is basically difficult to make on account of the small grain size and
their amalgamation with quite similar detrital
material. Yet it has been ascertained that the by far
largest clay mineral proportion – probably more
than 90 % – located in recent to subrecent sediments is of detrital origin (Chamley 1989; Hillier
1995). There are essentially three ways for
smectites to be formed, which demand specific
conditions as they are confined to local areas. Alterations produced in volcanic material is one way,
especially by means of hydration of basaltic and
volcanic glasses. This process is referred to as
palagonitization. The probably best studied
smectite formation consists in the vents of hydrothermal solutions and their admixture with seawater at the mid-oceanic mountain ranges. Should
authigenic clay minerals form merely in recent surface sediments in very small amounts, their frequency during diagenesis (burial diagenesis), will
demonstrate a distinct elevation. However, this
aspect will not be considered any further beyond
this point.