Siliceous Ooze 233
oceanic divergences which are the result of atmospheric circulation. The regions of
divergence have nutrient-rich surface waters, hence there is sufficient silica available
to make robust siliceous shells. Also, such areas are rich in grazing zooplankton,
which pack the siliceous frustules into fecal pellets, thus accelerating delivery to the
sea floor (Figs. 6.2).
To obtain an estimate of the amount of silica precipitated in the upper waters, one
might multiply the measured amount of organic production with the ratio of solid
silica to organic matter found in suspension. This yields only a rough estimate, of
course.
A typical fixation rate of about 200 g SiOz/m z is suggested, with a range from less
than 100 g (central gyres) to more than SOD g (Antarctic). Of this fixation, only I
g/m 2 /yr, that is, 0.5 %, can be incorporated into sediments if the river input is the only
source of silica. Twice that (that is, 1 % of fixation) can be sedimented if we assume
an equal contribution of silica from seawater-basalt reactions, especially at the hydrothermally active ridge crests. The global map of silica flux (Fig. 8.15) suggests
that 2 g/mz/yr (= 0.2 glcm 2 ka) is indeed a reasonable average value.
The second factor, the degree of solution of the siliceous material, reflects the ratio
between accumulation rates of nonsiliceous and siliceous particles. As concerns dilution of silica by carbonate, one might expect that a high supply of calcareous shells
would be accompanied by an equally high supply of siliceous shells, because both
siliceous and calcareous plankton depend on productivity of upper waters. This is not
generally the case, however. Indeed, there is a distinct negative correlation between
silica and calcite distributional patterns. This has been ascribed to opposing chemical
requirements for preservation. We have seen already that increasing productivity,
from some point on, leads to decreasing preservation of calcite, but to increasing
accumulation of silica. A similarly opposing trend is indicated for depth relationships,
with silica corrosion being greatest in upper waters (due to elevated temperature), that
of carbonate being greatest at depth.
The third factor controlling the abundance of siliceous fossils is the extent of
dissolution. The preservation of siliceous shells is rather closely correlated with their
abundance in sediments. A positive correlation between abundance and preservation
may be ascribed to an increased supply of easily dissolved diatom hash in fertile
areas, which will "buffer" interstitial waters for the more robust skeletons, and to an
otherwise favorable chemical environment in organic-rich sediments with slightly
acidic interstitial waters. In general, silico-flagellates and diatoms tend to dissolve
well before radiolarians and sponge spicules, and the following dissolution sequence
can be established (from least to most resistant): (1) silicoflagellates, (2) diatoms, (3)
delicate radiolarians, (4) robust radiolarians, (5) sponge spicules. Because of the
overriding importance of Antarctic opal deposition, the preservation of opal in the
rest of ocean must to a large degree depend on how much the Antarctic Ocean is able
to extract for deposition on its own sea floor.
8.6.3 Geochemical Implications. The dissolution of opaline skeletons within the
surficial sediment layer of the sea floor delivers silica to the deep waters. This flux
from the sediment to the water is evident from concentration gradients in interstitial
oceanic divergences which are the result of atmospheric circulation. The regions of
divergence have nutrient-rich surface waters, hence there is sufficient silica available
to make robust siliceous shells. Also, such areas are rich in grazing zooplankton,
which pack the siliceous frustules into fecal pellets, thus accelerating delivery to the
sea floor (Figs. 6.2).
To obtain an estimate of the amount of silica precipitated in the upper waters, one
might multiply the measured amount of organic production with the ratio of solid
silica to organic matter found in suspension. This yields only a rough estimate, of
course.
A typical fixation rate of about 200 g SiOz/m z is suggested, with a range from less
than 100 g (central gyres) to more than SOD g (Antarctic). Of this fixation, only I
g/m 2 /yr, that is, 0.5 %, can be incorporated into sediments if the river input is the only
source of silica. Twice that (that is, 1 % of fixation) can be sedimented if we assume
an equal contribution of silica from seawater-basalt reactions, especially at the hydrothermally active ridge crests. The global map of silica flux (Fig. 8.15) suggests
that 2 g/mz/yr (= 0.2 glcm 2 ka) is indeed a reasonable average value.
The second factor, the degree of solution of the siliceous material, reflects the ratio
between accumulation rates of nonsiliceous and siliceous particles. As concerns dilution of silica by carbonate, one might expect that a high supply of calcareous shells
would be accompanied by an equally high supply of siliceous shells, because both
siliceous and calcareous plankton depend on productivity of upper waters. This is not
generally the case, however. Indeed, there is a distinct negative correlation between
silica and calcite distributional patterns. This has been ascribed to opposing chemical
requirements for preservation. We have seen already that increasing productivity,
from some point on, leads to decreasing preservation of calcite, but to increasing
accumulation of silica. A similarly opposing trend is indicated for depth relationships,
with silica corrosion being greatest in upper waters (due to elevated temperature), that
of carbonate being greatest at depth.
The third factor controlling the abundance of siliceous fossils is the extent of
dissolution. The preservation of siliceous shells is rather closely correlated with their
abundance in sediments. A positive correlation between abundance and preservation
may be ascribed to an increased supply of easily dissolved diatom hash in fertile
areas, which will "buffer" interstitial waters for the more robust skeletons, and to an
otherwise favorable chemical environment in organic-rich sediments with slightly
acidic interstitial waters. In general, silico-flagellates and diatoms tend to dissolve
well before radiolarians and sponge spicules, and the following dissolution sequence
can be established (from least to most resistant): (1) silicoflagellates, (2) diatoms, (3)
delicate radiolarians, (4) robust radiolarians, (5) sponge spicules. Because of the
overriding importance of Antarctic opal deposition, the preservation of opal in the
rest of ocean must to a large degree depend on how much the Antarctic Ocean is able
to extract for deposition on its own sea floor.
8.6.3 Geochemical Implications. The dissolution of opaline skeletons within the
surficial sediment layer of the sea floor delivers silica to the deep waters. This flux
from the sediment to the water is evident from concentration gradients in interstitial