belt surrounding Antarctica. These sediments typically contain B60% biogenic silica by weight (the
majority of which are the skeletons of diatoms).
Most of these sediments, however, accumulate at
rates of only a few centimeters per thousand years, so
their silica burial rate is quite small (o0.008 mol Si
cm
À2 y
À1 ), accounting for 1 Â 10
12 mol y
À1 of silica
burial or less than 20% of the total burial in the
marine environment. Beneath the Polar Front (corresponding to the northern 200–300 km of the
Southern Ocean siliceous belt), however, the sediment accumulation rates increase dramatically to
values as high as 50 Â 10
À3 cm y
À1 . Regional averages can be as high as 19 Â 10
À3 cm y
À1 , yielding a
silica burial rate of 0.08 mol Si cm
À2 y
À1 . Unfortunately, many of the siliceous deposits beneath the
Polar Front occur in areas of very rugged bottom
topography (because of the submarine Antarctic
Ridge), where sediments are focused into the deeper
basins from the flanks of the oceanic ridge crests.
This distribution of accumulation rates would not
create a bias if all of the sedimentary environments
are sampled equally. However, it is more likely to
collect sediment cores in the deep basins, where the
deposits are thicker and accumulating more rapidly,
than it is on the flanks where the sediment coverage
is thinner. The effects of this sediment focusing can
be assessed by measuring the amount in the seabed of
a naturally occurring, particle-reactive radioisotope,
thorium-230 (
230 Th). If there were no sediment focusing, the amount of excess
230 Th in the sediments
would equal the production from its parent, uranium-234, in the overlying water column. In some
Polar Front Antarctic cores there is 12 times more
excess
230
Th in the sediment column than produced
in the waters above, indicating that sediment focusing is active. Initial estimates of the biogenic silica
accumulation beneath the Polar Front were as high
as 3 Â 10
12 mol Si y
À1
, but tracer-corrected values
are on the order of 1 Â 10
12 mol Si y
À1
.
There are other high-latitude areas accumulating
substantial amounts of biogenic silica, including the
Bering Sea, the Sea of Okhotsk, and much of the
North Pacific Ocean; however, the accumulation
rates are not as high as in the Southern Ocean (see
Table 1). The high rate of silica burial in the highlatitude sediments may be attributed in part to the
facts that cold waters occurring at the surface and at
depth retard the rate of silica dissolution and that
many of the diatom species in high latitudes have
more robust skeletons than do their counterparts in
lower latitudes. Moderately high silica production
rates and elevated silica preservation efficiencies
(approximately double the world average) combine
to yield high-latitude siliceous deposits accounting
for approximately one-third of the world’s biogenic
silica burial.
If the focusing-corrected biogenic silica accumulation rates are correct for the Polar Front, then a large
sink for biogenic silica (B1–2 Â 10
12 mol Si y
À1
) needs
to be identified in order to maintain agreement between the sources and sinks in the marine silica budget. Continental margin sediments are a likely regime
because these environments have fairly high surface
productivity (much of which is diatomaceous), a
relatively shallow water column (resulting in reduced
water column regeneration as compared to the deep
sea), rapid sediment accumulation rates (10–
100 Â 10
À3 cm y
À1
) and abundant aluminosilicate
minerals (see Biogenic Silica Preservation below). The
amount of marine organic matter buried in shelf and
upper slope deposits is on the order of 3 Â 10
12 mol C
y
À1
. When this flux is multiplied by the silica/organic
carbon mole ratio (Si/C org ) of sediments in productive
continental margin settings (Si/C org ¼ 0.6), the result
suggests that these nearshore depositional environments can account for sufficient biogenic silica
burial (1.8–2.8 Â 10
12 mol Si y
À1
) to bring the silica
budget into near balance (i.e., within the errors of
calculation).
Biogenic Silica Preservation
As mentioned earlier, all ocean waters are undersaturated with respect to biogenic silica. Surface
waters may be more than two orders of magnitude
undersaturated, whereas bottom waters are 5–15fold undersaturated. The solubility of biogenic silica
is greater in warm surface waters than in colder deep
waters, which, coupled with the increasing silicate
concentration with depth in most ocean basins, diminishes the silicate/silica disequilibrium (or corrosiveness of the water) as particles sink into the deep
sea. This disequilibrium drives silica regeneration in
oceanic waters along with other factors and processes such as particle residence time in the water
column, organic and inorganic surface coatings,
particle chemistry, particle aggregation, fecal pellet
formation, as well as particle surface area. Recycling
of biogenic silica occurs via inorganic dissolution;
however, the organic coating that siliceous biota use
to cover their skeletons (inhibiting dissolution)
must be removed by microbial or zooplankton
grazing prior to dissolution. This association is
highlighted by the fact that bacterial assemblages can
accelerate the dissolution of biogenic silica in the
water column.
An important aspect of biogenic silica dissolution
pertains to surface chemistry and clay-mineral
MARINE SILICA CYCLE 537
majority of which are the skeletons of diatoms).
Most of these sediments, however, accumulate at
rates of only a few centimeters per thousand years, so
their silica burial rate is quite small (o0.008 mol Si
cm
À2 y
À1 ), accounting for 1 Â 10
12 mol y
À1 of silica
burial or less than 20% of the total burial in the
marine environment. Beneath the Polar Front (corresponding to the northern 200–300 km of the
Southern Ocean siliceous belt), however, the sediment accumulation rates increase dramatically to
values as high as 50 Â 10
À3 cm y
À1 . Regional averages can be as high as 19 Â 10
À3 cm y
À1 , yielding a
silica burial rate of 0.08 mol Si cm
À2 y
À1 . Unfortunately, many of the siliceous deposits beneath the
Polar Front occur in areas of very rugged bottom
topography (because of the submarine Antarctic
Ridge), where sediments are focused into the deeper
basins from the flanks of the oceanic ridge crests.
This distribution of accumulation rates would not
create a bias if all of the sedimentary environments
are sampled equally. However, it is more likely to
collect sediment cores in the deep basins, where the
deposits are thicker and accumulating more rapidly,
than it is on the flanks where the sediment coverage
is thinner. The effects of this sediment focusing can
be assessed by measuring the amount in the seabed of
a naturally occurring, particle-reactive radioisotope,
thorium-230 (
230 Th). If there were no sediment focusing, the amount of excess
230 Th in the sediments
would equal the production from its parent, uranium-234, in the overlying water column. In some
Polar Front Antarctic cores there is 12 times more
excess
230
Th in the sediment column than produced
in the waters above, indicating that sediment focusing is active. Initial estimates of the biogenic silica
accumulation beneath the Polar Front were as high
as 3 Â 10
12 mol Si y
À1
, but tracer-corrected values
are on the order of 1 Â 10
12 mol Si y
À1
.
There are other high-latitude areas accumulating
substantial amounts of biogenic silica, including the
Bering Sea, the Sea of Okhotsk, and much of the
North Pacific Ocean; however, the accumulation
rates are not as high as in the Southern Ocean (see
Table 1). The high rate of silica burial in the highlatitude sediments may be attributed in part to the
facts that cold waters occurring at the surface and at
depth retard the rate of silica dissolution and that
many of the diatom species in high latitudes have
more robust skeletons than do their counterparts in
lower latitudes. Moderately high silica production
rates and elevated silica preservation efficiencies
(approximately double the world average) combine
to yield high-latitude siliceous deposits accounting
for approximately one-third of the world’s biogenic
silica burial.
If the focusing-corrected biogenic silica accumulation rates are correct for the Polar Front, then a large
sink for biogenic silica (B1–2 Â 10
12 mol Si y
À1
) needs
to be identified in order to maintain agreement between the sources and sinks in the marine silica budget. Continental margin sediments are a likely regime
because these environments have fairly high surface
productivity (much of which is diatomaceous), a
relatively shallow water column (resulting in reduced
water column regeneration as compared to the deep
sea), rapid sediment accumulation rates (10–
100 Â 10
À3 cm y
À1
) and abundant aluminosilicate
minerals (see Biogenic Silica Preservation below). The
amount of marine organic matter buried in shelf and
upper slope deposits is on the order of 3 Â 10
12 mol C
y
À1
. When this flux is multiplied by the silica/organic
carbon mole ratio (Si/C org ) of sediments in productive
continental margin settings (Si/C org ¼ 0.6), the result
suggests that these nearshore depositional environments can account for sufficient biogenic silica
burial (1.8–2.8 Â 10
12 mol Si y
À1
) to bring the silica
budget into near balance (i.e., within the errors of
calculation).
Biogenic Silica Preservation
As mentioned earlier, all ocean waters are undersaturated with respect to biogenic silica. Surface
waters may be more than two orders of magnitude
undersaturated, whereas bottom waters are 5–15fold undersaturated. The solubility of biogenic silica
is greater in warm surface waters than in colder deep
waters, which, coupled with the increasing silicate
concentration with depth in most ocean basins, diminishes the silicate/silica disequilibrium (or corrosiveness of the water) as particles sink into the deep
sea. This disequilibrium drives silica regeneration in
oceanic waters along with other factors and processes such as particle residence time in the water
column, organic and inorganic surface coatings,
particle chemistry, particle aggregation, fecal pellet
formation, as well as particle surface area. Recycling
of biogenic silica occurs via inorganic dissolution;
however, the organic coating that siliceous biota use
to cover their skeletons (inhibiting dissolution)
must be removed by microbial or zooplankton
grazing prior to dissolution. This association is
highlighted by the fact that bacterial assemblages can
accelerate the dissolution of biogenic silica in the
water column.
An important aspect of biogenic silica dissolution
pertains to surface chemistry and clay-mineral
MARINE SILICA CYCLE 537