Siliceous Ooze
231
resources, or from economic disruption caused by climatic change or other environmental problems (even neglecting the possibility of war). At that point, atmospheric
C02 will start decreasing as the alkalinity of the ocean rises from the dissolution of
carbonate.
The C02 problem reminds us of the extent to which we are now exploiting the
natural cycles of our planet for human benefit - yet a large part of a growing world
population remains without adequate food and shelter.
8.6 Siliceous Ooze
8.6.1 Composition and Distribution. In considering the siliceous deposits of the
deep sea, many of the geochemical questions reappear which were raised earlier in
connection with "red clay" and calcareous ooze. What are the contributions from
continental weathering, submarine alteration, volcanic and hydrothermal emanations?
What mechanisms control the concentrations of dissolved matter in seawater? That is,
what controls the state of saturation?
Are biogenous particles the sole sink for such dissolved matter, or is there uptake
by "upgrading" of clays? What is the rate at which redissolution on the sea floor
supplies dissolved matter to the overlying waters?
First, let us take a brief look at the distribution, production, and dissolution patterns of the siliceous deposits.
We have already encountered the constituents of such deposits: remains of diatoms, silicoflagellates, radiolarians, and sponge spicules, all of which are made of
opal, a hydrated form of amorphous silicon dioxide. Diatom oozes are typical for
high latitudes, diatom muds for pericontinental regions, and radiolarian oozes for
equatorial areas (Figs. 8.2 and 8.4). Both diatom and radiolarian ooze, of course, are
mixtures of various kinds of sediments, with one or the other siliceous form being
dominant (Fig. 8.14). The siliceous deposits occur in areas of high fertility; that is, in
regions with relatively high phosphate values in surface waters (Fig. 8.15). This
overall correspondence between fertility patterns and silica-rich deposits can be considerably modified by redeposition processes within individual regions. The silica
frustules are light and easily transported, and the activity of benthic animals, which
tends to resuspend fine sediment, it especially pronounced in fertile areas. Thus,
aided by bottom currents and gravity, siliceous frustules tend to accumulate in local
and regional depressions.
8.6.2 Controlling Factors. In analogy to other kinds of deposits, the concentration of
siliceous fossils in the sediment is a function of (1) the rate of production of siliceous
organisms in the overlying waters. (2) the degree of dilution by terrigenous, volcanic,
and calcareous particles, and (3) the extent of dissolution of the siliceous skeletons,
most of which apparently occurs shortly after deposition.
The first variable, production of siliceous shells, attains its maximum in coastal
regions (Sect. 4.3.3; also Figs. 8.2 and 8.15). This leads to the formation of a silica
ring around each ocean basin. Silica helts are provided by the latitudinally arranged
231
resources, or from economic disruption caused by climatic change or other environmental problems (even neglecting the possibility of war). At that point, atmospheric
C02 will start decreasing as the alkalinity of the ocean rises from the dissolution of
carbonate.
The C02 problem reminds us of the extent to which we are now exploiting the
natural cycles of our planet for human benefit - yet a large part of a growing world
population remains without adequate food and shelter.
8.6 Siliceous Ooze
8.6.1 Composition and Distribution. In considering the siliceous deposits of the
deep sea, many of the geochemical questions reappear which were raised earlier in
connection with "red clay" and calcareous ooze. What are the contributions from
continental weathering, submarine alteration, volcanic and hydrothermal emanations?
What mechanisms control the concentrations of dissolved matter in seawater? That is,
what controls the state of saturation?
Are biogenous particles the sole sink for such dissolved matter, or is there uptake
by "upgrading" of clays? What is the rate at which redissolution on the sea floor
supplies dissolved matter to the overlying waters?
First, let us take a brief look at the distribution, production, and dissolution patterns of the siliceous deposits.
We have already encountered the constituents of such deposits: remains of diatoms, silicoflagellates, radiolarians, and sponge spicules, all of which are made of
opal, a hydrated form of amorphous silicon dioxide. Diatom oozes are typical for
high latitudes, diatom muds for pericontinental regions, and radiolarian oozes for
equatorial areas (Figs. 8.2 and 8.4). Both diatom and radiolarian ooze, of course, are
mixtures of various kinds of sediments, with one or the other siliceous form being
dominant (Fig. 8.14). The siliceous deposits occur in areas of high fertility; that is, in
regions with relatively high phosphate values in surface waters (Fig. 8.15). This
overall correspondence between fertility patterns and silica-rich deposits can be considerably modified by redeposition processes within individual regions. The silica
frustules are light and easily transported, and the activity of benthic animals, which
tends to resuspend fine sediment, it especially pronounced in fertile areas. Thus,
aided by bottom currents and gravity, siliceous frustules tend to accumulate in local
and regional depressions.
8.6.2 Controlling Factors. In analogy to other kinds of deposits, the concentration of
siliceous fossils in the sediment is a function of (1) the rate of production of siliceous
organisms in the overlying waters. (2) the degree of dilution by terrigenous, volcanic,
and calcareous particles, and (3) the extent of dissolution of the siliceous skeletons,
most of which apparently occurs shortly after deposition.
The first variable, production of siliceous shells, attains its maximum in coastal
regions (Sect. 4.3.3; also Figs. 8.2 and 8.15). This leads to the formation of a silica
ring around each ocean basin. Silica helts are provided by the latitudinally arranged