234 Deep-Sea Sediments - Patterns, Processes, and Stratigraphic Methods
waters and differences in concentration between these waters and the overlying seawater. By far the greatest reflux is from the uppermost part of the sediment column,
where recently arrived shells and frustules are being dissolved. However, the process
of dissolution and redelivery to seawater apparently continues downward into the
section, and includes sediments as old as 10 million years (Fig. 8.16).
It is possible and even probable that some of the silica released to the interstitial
waters reacts within sediments to form new minerals. However, near the sediment
surface most of the silica re-enters the ocean water. Thus, "old" bottom water, which
has been in contact with the sea floor for a long time, is silicate-rich. The reverse is
true for "young" bottom water which has arrived from the surface only recently.
Therefore, concentrations of dissolved silica are high in the deep North Pacific ("old"
water), and are low in the deep North Atlantic ("young" water).
From this overall distribution of dissolved silica in deep ocean waters, we can
draw an obvious conclusion. The reason that silica concentrations in deep water are
relatively low cannot be the uptake, if any, of dissolved silica by clay minerals. If it
were, the "old" waters should be the more depleted in silica. The reason for the low
concentrations must be that the deep water remembers its depleted condition at the
surface (from the silica extraction by diatoms) and that it did not have time to saturate
itself with respect to the actively dissolving opaline shells.
8.6.4 Deep-Sea Cherts. The discovery of chert within deep-sea sediments has both
fascinated geologists and frustated them in efforts to drill and recover complete
sections. The formation of deep-sea chert (siliceous sediments cemented by cryptocrystalline and microcrystalline quartz) appears to proceed from mobilization and
reprecipitation of opal, generating a disordered cristobalite (= fibrous quartz) which
eventually alters toward a quartzitic rock with mostly quartz-replaced and quartzfilled fossils as diagenesis progresses. Recrystallization may proceed at various rates
and somewhat divergent patterns, depending on the original sediment present.
If siliceous fossils and/or silica-rich volcanic glass are to be available as a source
for later production of chert, the following conditions appear necessary: (1) a sufficiently high supply of opal and low supply of dilutant; (2) silica-rich bottom water;
(3) a reasonably high burial rate; and (4) chemical conditions favorable for the preservation of siliceous shells.
During diagenesis, the opaline skeletons dissolve and volcanic material, if any,
releases silica during devitrification, and the silica-rich interstitial solutions migrate
along bedding planes or fractures and vertically to areas of precipitation in nearby
permeable lenses or layers. Why precipitation should be favored in some places and
mobilization in others in the same sediment is poorly understood in detail.
In the western equatorial Pacific and in many other areas of the global ocean,
massive chert beds first appear in upper Eocene sediments, when drilling down into
the sea floor (Fig. 8.16; "Ontong Java Series"). At that level, sound waves are
strongly reflected, due to a sudden change in impedance (product of density and
sound velocity). Interstitial waters tend to lose dissolved silicate to precipitation at
this horizon (Fig. 8.16, right). Note that there are other strong reflectors as well: the
"Drake Series" (marking the beginning of the Neogene) presumably derives from the
waters and differences in concentration between these waters and the overlying seawater. By far the greatest reflux is from the uppermost part of the sediment column,
where recently arrived shells and frustules are being dissolved. However, the process
of dissolution and redelivery to seawater apparently continues downward into the
section, and includes sediments as old as 10 million years (Fig. 8.16).
It is possible and even probable that some of the silica released to the interstitial
waters reacts within sediments to form new minerals. However, near the sediment
surface most of the silica re-enters the ocean water. Thus, "old" bottom water, which
has been in contact with the sea floor for a long time, is silicate-rich. The reverse is
true for "young" bottom water which has arrived from the surface only recently.
Therefore, concentrations of dissolved silica are high in the deep North Pacific ("old"
water), and are low in the deep North Atlantic ("young" water).
From this overall distribution of dissolved silica in deep ocean waters, we can
draw an obvious conclusion. The reason that silica concentrations in deep water are
relatively low cannot be the uptake, if any, of dissolved silica by clay minerals. If it
were, the "old" waters should be the more depleted in silica. The reason for the low
concentrations must be that the deep water remembers its depleted condition at the
surface (from the silica extraction by diatoms) and that it did not have time to saturate
itself with respect to the actively dissolving opaline shells.
8.6.4 Deep-Sea Cherts. The discovery of chert within deep-sea sediments has both
fascinated geologists and frustated them in efforts to drill and recover complete
sections. The formation of deep-sea chert (siliceous sediments cemented by cryptocrystalline and microcrystalline quartz) appears to proceed from mobilization and
reprecipitation of opal, generating a disordered cristobalite (= fibrous quartz) which
eventually alters toward a quartzitic rock with mostly quartz-replaced and quartzfilled fossils as diagenesis progresses. Recrystallization may proceed at various rates
and somewhat divergent patterns, depending on the original sediment present.
If siliceous fossils and/or silica-rich volcanic glass are to be available as a source
for later production of chert, the following conditions appear necessary: (1) a sufficiently high supply of opal and low supply of dilutant; (2) silica-rich bottom water;
(3) a reasonably high burial rate; and (4) chemical conditions favorable for the preservation of siliceous shells.
During diagenesis, the opaline skeletons dissolve and volcanic material, if any,
releases silica during devitrification, and the silica-rich interstitial solutions migrate
along bedding planes or fractures and vertically to areas of precipitation in nearby
permeable lenses or layers. Why precipitation should be favored in some places and
mobilization in others in the same sediment is poorly understood in detail.
In the western equatorial Pacific and in many other areas of the global ocean,
massive chert beds first appear in upper Eocene sediments, when drilling down into
the sea floor (Fig. 8.16; "Ontong Java Series"). At that level, sound waves are
strongly reflected, due to a sudden change in impedance (product of density and
sound velocity). Interstitial waters tend to lose dissolved silicate to precipitation at
this horizon (Fig. 8.16, right). Note that there are other strong reflectors as well: the
"Drake Series" (marking the beginning of the Neogene) presumably derives from the