MARINE SILICA CYCLE
D. J. DeMaster, North Carolina State University,
Raleigh, NC, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Silicate, or silicic acid (H 4 SiO 4 ), is a very important
nutrient in the ocean. Unlike the other major nutrients such as phosphate, nitrate, or ammonium,
which are needed by almost all marine plankton,
silicate is an essential chemical requirement only for
certain biota such as diatoms, radiolaria, silicoflagellates, and siliceous sponges. The dissolved silicate in the ocean is converted by these various plants
and animals into particulate silica (SiO 2 ), which
serves primarily as structural material (i.e., the biota’s hard parts). The reason silicate cycling has received significant scientific attention is that some
researchers believe that diatoms (one of the silicasecreting biota) are one of the dominant phytoplankton responsible for export production from the
surface ocean (Dugdale et al., 1995). Export production (sometimes called new production) is the
transport of particulate material from the euphotic
zone (where photosynthesis occurs) down into the
deep ocean. The relevance of this process can be
appreciated because it takes dissolved inorganic
carbon from surface ocean waters, where it is exchanging with carbon dioxide in the atmosphere,
turns it into particulate organic matter, and then
transports it to depth, where most of it is regenerated
back into the dissolved form. This process, known as
the ‘biological pump’, along with deep-ocean circulation is responsible for the transfer of inorganic
carbon into the deep ocean, where it is unable to
exchange with the atmosphere for hundreds or even
thousands of years. Consequently, silicate and silica
play an important role in the global carbon cycle,
which affects the world’s climate through greenhouse
feedback mechanisms. In addition, the accumulation
of biogenic silica on the ocean floor can tell us where
in the ocean export production has occurred
on timescales ranging from hundreds to millions
of years, which in turn reveals important information concerning ocean circulation and nutrient
distributions.
Basic Concepts
In understanding the cycling of silicate in the oceans,
the concept of mean oceanic residence time is commonly used. Mean oceanic residence time is defined
as in eqn. [1].
amount of dissolved material in a reservoir
ð
Þ
steady-state flux into or out of the reservoir
ð
Þ
½1
For silicate there are approximately 7 Â 10
16 moles
of dissolved silicon in ocean water. (One mole is
equal to 6 Â 10
23 molecules of a substance, which in
the case of silicic acid has a mass of approximately
96 g.) As described later, the various sources of silicate to the ocean supply approximately 7 Â 10
12 mol
y
À1
, which is approximately equal to our best estimates of the removal rate. Most scientists believe
that there has been a reasonably good balance between supply and removal of silicate from the oceans
on thousand-year timescales because there is little
evidence in the oceanic sedimentary record of massive abiological precipitation of silica (indicating
enhanced silicate concentrations relative to today),
nor is there any evidence in the fossil record over the
past several hundred million years that siliceous
biota have been absent for any extended period (indicating extremely low silicate levels). Dividing the
amount of dissolved silicate in the ocean by the
supply/removal rate yields a mean oceanic residence
time of approximately 10 000 years. Basically, what
this means is that an atom of dissolved silicon supplied to the ocean will remain on average in the
water column or surface seabed (being transformed
between dissolved and particulate material as part of
the silicate cycle) for approximately 10 000 years
before it is permanently removed from the oceanic
system via long-term burial in the seabed.
Distribution of Silicate in the Marine
Environment
Because of biological activity, surface waters
throughout most of the marine realm are depleted in
dissolved silicate, reaching values as low as a few
micromoles per liter (mmol l
À1 ). When the siliceous
biota die, their skeletons settle through the water
column, where more than 90% of the silica is regenerated via inorganic dissolution. This process
enriches the deep water in silicate, causing oceanic
bottom waters to have as much as 10–100 times
533
D. J. DeMaster, North Carolina State University,
Raleigh, NC, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Silicate, or silicic acid (H 4 SiO 4 ), is a very important
nutrient in the ocean. Unlike the other major nutrients such as phosphate, nitrate, or ammonium,
which are needed by almost all marine plankton,
silicate is an essential chemical requirement only for
certain biota such as diatoms, radiolaria, silicoflagellates, and siliceous sponges. The dissolved silicate in the ocean is converted by these various plants
and animals into particulate silica (SiO 2 ), which
serves primarily as structural material (i.e., the biota’s hard parts). The reason silicate cycling has received significant scientific attention is that some
researchers believe that diatoms (one of the silicasecreting biota) are one of the dominant phytoplankton responsible for export production from the
surface ocean (Dugdale et al., 1995). Export production (sometimes called new production) is the
transport of particulate material from the euphotic
zone (where photosynthesis occurs) down into the
deep ocean. The relevance of this process can be
appreciated because it takes dissolved inorganic
carbon from surface ocean waters, where it is exchanging with carbon dioxide in the atmosphere,
turns it into particulate organic matter, and then
transports it to depth, where most of it is regenerated
back into the dissolved form. This process, known as
the ‘biological pump’, along with deep-ocean circulation is responsible for the transfer of inorganic
carbon into the deep ocean, where it is unable to
exchange with the atmosphere for hundreds or even
thousands of years. Consequently, silicate and silica
play an important role in the global carbon cycle,
which affects the world’s climate through greenhouse
feedback mechanisms. In addition, the accumulation
of biogenic silica on the ocean floor can tell us where
in the ocean export production has occurred
on timescales ranging from hundreds to millions
of years, which in turn reveals important information concerning ocean circulation and nutrient
distributions.
Basic Concepts
In understanding the cycling of silicate in the oceans,
the concept of mean oceanic residence time is commonly used. Mean oceanic residence time is defined
as in eqn. [1].
amount of dissolved material in a reservoir
ð
Þ
steady-state flux into or out of the reservoir
ð
Þ
½1
For silicate there are approximately 7 Â 10
16 moles
of dissolved silicon in ocean water. (One mole is
equal to 6 Â 10
23 molecules of a substance, which in
the case of silicic acid has a mass of approximately
96 g.) As described later, the various sources of silicate to the ocean supply approximately 7 Â 10
12 mol
y
À1
, which is approximately equal to our best estimates of the removal rate. Most scientists believe
that there has been a reasonably good balance between supply and removal of silicate from the oceans
on thousand-year timescales because there is little
evidence in the oceanic sedimentary record of massive abiological precipitation of silica (indicating
enhanced silicate concentrations relative to today),
nor is there any evidence in the fossil record over the
past several hundred million years that siliceous
biota have been absent for any extended period (indicating extremely low silicate levels). Dividing the
amount of dissolved silicate in the ocean by the
supply/removal rate yields a mean oceanic residence
time of approximately 10 000 years. Basically, what
this means is that an atom of dissolved silicon supplied to the ocean will remain on average in the
water column or surface seabed (being transformed
between dissolved and particulate material as part of
the silicate cycle) for approximately 10 000 years
before it is permanently removed from the oceanic
system via long-term burial in the seabed.
Distribution of Silicate in the Marine
Environment
Because of biological activity, surface waters
throughout most of the marine realm are depleted in
dissolved silicate, reaching values as low as a few
micromoles per liter (mmol l
À1 ). When the siliceous
biota die, their skeletons settle through the water
column, where more than 90% of the silica is regenerated via inorganic dissolution. This process
enriches the deep water in silicate, causing oceanic
bottom waters to have as much as 10–100 times
533