formation on the surface of siliceous tests. Incorporation of aluminum in the initial skeleton as
well as aluminosilicate formation on skeletal surfaces
during settling and burial greatly decrease the solubility of biogenic silica, in some cases by as much as a
factor of 5–10. It appears that some of this ‘armoring’ of siliceous skeletons occurs up in the water
column (possibly in aggregates or fecal pellets), although some aluminosilicate formation may occur in
flocs just above the seabed as well as deeper in the
sediment column. The occurrence of clay minerals on
skeletal surfaces has been documented using a variety of instruments (e.g., the scanning electron
microscope). The nature of the settling particles also
affects dissolution rates in the water column. If siliceous skeletons settle individually, they settle so
slowly (a timescale of years to decades) that most
particles dissolve before reaching the seabed. However, if the siliceous skeletons aggregate or are
packaged into a fecal pellet by zooplankton, sinking
velocities can be enhanced by several orders of
magnitude, favoring preservation during passage
through the water column. Siliceous tests that have
high surface areas (lots of protruding spines and ornate surface structures) also are prone to high dissolution rates and low preservation in the water
column relative to species that have more robust
skeletons and more compact structures.
Very few studies have documented silica production rates in surface waters, established the vertical fluxes of silica in the water column, and then
also examined regeneration and burial rates in the
seabed. One place that all of these measurements
have been made is in the Ross Sea, Antarctica. In this
high-latitude environment, approximately one-third
of the biogenic silica produced in surface waters is
exported from the euphotic zone, with most of this
material (27% of production) making it to the seabed some 500–900 m below. Seabed preservation
efficiencies (silica burial rate divided by silica rain
rate to the seafloor) vary from 1% to 86%, depending primarily on sediment accumulation rate,
but average 22% for the shelf as a whole. Consequently, the overall preservation rate (water column and seabed) is estimated to be B6% in the Ross
Sea. On a global basis, approximately 3% of the
biogenic silica produced in surface waters is buried in
the seabed. The total preservation efficiencies for
different ocean basins vary, with the Atlantic and
Indian Oceans having values on the order of 0.4–
0.8% and the Pacific and Southern Oceans having
values of approximately 5–10%.
Sediment accumulation rate can make a large
difference in seabed preservation efficiency. In the
Ross Sea, for example, increasing the sediment
accumulation rate from 1–2 to 16 Â 10
À3 cm y
À1
,
increases the seabed preservation efficiency from 1–
5% up to 50–60%. In most slowly accumulating
deep-sea sediments (rates of 2 Â 10
À3 cm y
À1 or less),
nearly all of the biogenic silica deposited on the
seafloor dissolves prior to long-term burial. Increasing the sedimentation rate decreases the time that
siliceous particles are exposed to the corrosive
oceanic bottom waters, by burying them in the seabed where silicate, aluminum, and cation concentrations are high, favoring aluminosilicate formation
and preservation. Consequently, continental margin
sediments with accumulation rates of 10–100 Â 10
À3
cm y
À1 are deposits expected to have high preservation efficiencies for biogenic silica and are believed
to be an important burial site for this biogenic phase.
Estuaries extend across the river–ocean boundary
and are generally regions of high nutrient flux and
rapid sediment burial (0.1–10 cm y
À1
). They commonly exhibit extensive diatom production in surface waters, but may not account for substantial
biogenic silica burial because of extensive dissolution
in the water column. For example, on the Amazon
shelf approximately 20% of the world’s river water
mixes with ocean water and silicate dynamics have
been studied in detail. Although nutrient concentrations are highest in the low-salinity regions of the
Amazon mixing zone, biological nutrient uptake is
limited because light cannot penetrate more than a
few centimeters into the water column as a result of
the high turbidity in the river (primarily from natural
weathering of the Andes Mountains). After the terrigenous particles have flocculated in the river–ocean
mixing zone, light is able to penetrate the warm
surface waters, leading to some of the highest biogenic silica production rates in the world. However,
resuspension on the shelf, zooplankton grazing, and
high water temperatures lead to fairly efficient recycling in the water column and nearly all of the
dissolved silicate coming down the river makes it out
to the open ocean. The Amazon shelf seabed does
appear to exhibit clay-mineral formation (primarily
through replacement of dissolving diatoms), but the
burial fluxes are expected to be small relative to the
offshore transport of silicate and biogenic silica.
Biogenic Silica in Marine Sediments
As mentioned above, the primary biota that construct siliceous skeletons are diatoms, radiolaria,
silicoflagellates, and siliceous sponges. Diatoms are
marine algae. These phytoplankton account for 20–
40% of the primary production in the ocean and an
even greater percentage of the export production
538 MARINE SILICA CYCLE
well as aluminosilicate formation on skeletal surfaces
during settling and burial greatly decrease the solubility of biogenic silica, in some cases by as much as a
factor of 5–10. It appears that some of this ‘armoring’ of siliceous skeletons occurs up in the water
column (possibly in aggregates or fecal pellets), although some aluminosilicate formation may occur in
flocs just above the seabed as well as deeper in the
sediment column. The occurrence of clay minerals on
skeletal surfaces has been documented using a variety of instruments (e.g., the scanning electron
microscope). The nature of the settling particles also
affects dissolution rates in the water column. If siliceous skeletons settle individually, they settle so
slowly (a timescale of years to decades) that most
particles dissolve before reaching the seabed. However, if the siliceous skeletons aggregate or are
packaged into a fecal pellet by zooplankton, sinking
velocities can be enhanced by several orders of
magnitude, favoring preservation during passage
through the water column. Siliceous tests that have
high surface areas (lots of protruding spines and ornate surface structures) also are prone to high dissolution rates and low preservation in the water
column relative to species that have more robust
skeletons and more compact structures.
Very few studies have documented silica production rates in surface waters, established the vertical fluxes of silica in the water column, and then
also examined regeneration and burial rates in the
seabed. One place that all of these measurements
have been made is in the Ross Sea, Antarctica. In this
high-latitude environment, approximately one-third
of the biogenic silica produced in surface waters is
exported from the euphotic zone, with most of this
material (27% of production) making it to the seabed some 500–900 m below. Seabed preservation
efficiencies (silica burial rate divided by silica rain
rate to the seafloor) vary from 1% to 86%, depending primarily on sediment accumulation rate,
but average 22% for the shelf as a whole. Consequently, the overall preservation rate (water column and seabed) is estimated to be B6% in the Ross
Sea. On a global basis, approximately 3% of the
biogenic silica produced in surface waters is buried in
the seabed. The total preservation efficiencies for
different ocean basins vary, with the Atlantic and
Indian Oceans having values on the order of 0.4–
0.8% and the Pacific and Southern Oceans having
values of approximately 5–10%.
Sediment accumulation rate can make a large
difference in seabed preservation efficiency. In the
Ross Sea, for example, increasing the sediment
accumulation rate from 1–2 to 16 Â 10
À3 cm y
À1
,
increases the seabed preservation efficiency from 1–
5% up to 50–60%. In most slowly accumulating
deep-sea sediments (rates of 2 Â 10
À3 cm y
À1 or less),
nearly all of the biogenic silica deposited on the
seafloor dissolves prior to long-term burial. Increasing the sedimentation rate decreases the time that
siliceous particles are exposed to the corrosive
oceanic bottom waters, by burying them in the seabed where silicate, aluminum, and cation concentrations are high, favoring aluminosilicate formation
and preservation. Consequently, continental margin
sediments with accumulation rates of 10–100 Â 10
À3
cm y
À1 are deposits expected to have high preservation efficiencies for biogenic silica and are believed
to be an important burial site for this biogenic phase.
Estuaries extend across the river–ocean boundary
and are generally regions of high nutrient flux and
rapid sediment burial (0.1–10 cm y
À1
). They commonly exhibit extensive diatom production in surface waters, but may not account for substantial
biogenic silica burial because of extensive dissolution
in the water column. For example, on the Amazon
shelf approximately 20% of the world’s river water
mixes with ocean water and silicate dynamics have
been studied in detail. Although nutrient concentrations are highest in the low-salinity regions of the
Amazon mixing zone, biological nutrient uptake is
limited because light cannot penetrate more than a
few centimeters into the water column as a result of
the high turbidity in the river (primarily from natural
weathering of the Andes Mountains). After the terrigenous particles have flocculated in the river–ocean
mixing zone, light is able to penetrate the warm
surface waters, leading to some of the highest biogenic silica production rates in the world. However,
resuspension on the shelf, zooplankton grazing, and
high water temperatures lead to fairly efficient recycling in the water column and nearly all of the
dissolved silicate coming down the river makes it out
to the open ocean. The Amazon shelf seabed does
appear to exhibit clay-mineral formation (primarily
through replacement of dissolving diatoms), but the
burial fluxes are expected to be small relative to the
offshore transport of silicate and biogenic silica.
Biogenic Silica in Marine Sediments
As mentioned above, the primary biota that construct siliceous skeletons are diatoms, radiolaria,
silicoflagellates, and siliceous sponges. Diatoms are
marine algae. These phytoplankton account for 20–
40% of the primary production in the ocean and an
even greater percentage of the export production
538 MARINE SILICA CYCLE