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Silicon...
...represents 27% of the lithosphere and is an important component of
the marine biogenic matter that accumulates in coastal and abyssal sediments.
Dissolved Si in seawater occurs mostly (95%) as the undissociated monomeric
silicic acid Si(OH)4. On a time scale of 10,000 years or less, the mean
concentration of silicic acid in the world ocean is assumed to be constant.
As for its distribution in the world ocean, there are marked regional
differences. In surface waters of the central gyres the concentrations
are usually < 2µM, but in the Antarctic, concentrations are as
high 80 to 100 muM in surface waters during winter. Deep and bottom waters
are usually silicic acid-rich, with concentrations varying from 10 to
40 µM in the North Atlantic to 100 to 160 µM in the Antarctic,
and 140 to 180 µM in the North Pacific. In the modern ocean, the
distribution of silicic acid in the different water masses is governed
by complex interactions among physical, chemical, geological, and biological
processes. The silicic acid content is determined by the balance between
geological and biological cycles of Si.
The surface reservoir...
...(Figure 1) receives silicic acid inputs from the lithosphere both
directly, via chemical weathering of the continental crust, and indirectly,
through eolian transport. Through both high- and low-temperature weathering
of the oceanic crust and of deposits of siliceous minerals, the ocean's
deep reservoir also receives silicic acid inputs from the lithosphere.
Except in the Atlantic Ocean, this deep reservoir is oversaturated with
respect to lithogenic silica (at low temperature the equilibrium solubility
in ocean water ranges from about 100 µM for quartz to 220 µM
for montmorillonite), but it is undersaturated with respect to biogenic
(or amorphous) silica (at deep-sea temperatures the equilbrium solubility
is about 1,000 µM).
The transfer of silicic acid...
...from the marine hydrosphere to the biosphere initiates the biological
cycle of Si. It is also a way to link the cycle of this element to that
of carbon. Marine organisms such as diatoms, silicoflagellates, and radiolarians
build up their skeletons by taking up silicic acid from seawater. When
these organisms are dead the biogenic silica accumulated in them becomes
dissolved. The portion of biogenic silica that escapes dissolution, either
in the surface or in the deep reservoir (Figure 1), settles downward,
eventually reaching the sediment. The transformation of opal (amorphous
biogenic silica) deposits into sediments through diagenetic processes
is a way for silica to re-enter the geological cycle. Within the sediments
silica is a labil component: Some of the silicic acid produced from seabed
dissolution diffuses into the overlying water mass and another portion
is mobilized and involved in the formation of alumino-silicate mineral
phases. The biogenic silica that is preserved eventually crystallizes
- primarily as chert.
The different processes...
...involved in the biogeochemical cycle of Si occur over a wide range
of time scales. The building-up of diatom frustules takes a few hours
to a few days. A few days to a few months are necessary for the settling
of diatom skeletons to the sea floor of continental margins or abysses.
At the sediment-water interface opal remains for a few months to a few
hundred years. At any of these steps biogenic silica continues to dissolve,
and the recycled silicic acid is transported toward the surface layer.
Deeper within the sediments, on the time scale of 106 to 109 years, the
dissolution of silica debris and diagenetic processes continue. On the
time-scale of <104 years, the biogeochemical cycle of Si and its budget
are affected by inputs of silicic acid from (1) rivers [FR(gross)]; (2)
atmospheric depositions (FA); (3) sea floor weathering (FW); and (4) hydrothermal
activity (FH). Silicon is lost through the accumulation of biogenic silica
in abyssal (FB) and coastal (Fest) sediments. Biogenic silica production
[F(Pgross)] occurs within the surface reservoir. Some Si (FE) is exported
to the deep reservoir where a fraction (FS) ultimately reaches the sediment-water
interface, and an even smaller fraction accumulates in the seabed (FB)
because of seabed dissolution (FD). At steady state the input fluxes balance
the output fluxes.
Multiproxy approach...
...Opal cannot be used alone to reconstruct past surface ocean productivity
for two reasons: (1) Proxies of nutrient utilization or particle flux
can be used to help reconstruct opal fluxes themselves; (2) given the
decoupling between the cycles of carbon and silicon, opal can provide
information on paleoproductivity due to siliceous phytoplankton, but other
proxies of export production (e.g., organic carbon) are also required
to provide information on the non-siliceous fraction of the primary production.
Combining opal with these proxies in a multiproxy approach is the best
way to reconstruct past surface ocean history. The multiproxy approach
also reduces the uncertainties that arise when a single proxy is used.
Each of these proxies discussed at the OPALEO workshops in June 1996 in
Brest/France and in February 1997 in Corvallis/USA is subject to uncertainties,
mainly because we lack a complete understanding of these processes that
control their cycles in the modern ocean. Temporal and regional variations
in opal preservation, dependency of authigenic uranium accumulation rates
on bottom water oxygen concentration, and differential affinity of mineral
phases for Pa are examples of mechanisms that complicate quantitative
interpretations of sedimentary records in terms of paleoproductivity.
Comparison of several of these records at a given location should help
provide a clearer picture of past changes in oceanic productivity. The
overall goal of this research is to combine several properly calibrated
proxies whose cycles in the modern ocean are sufficiently well understood,
which will reduce uncertainties about how to interpret their records.
References
- Olivier Ragueneau, Aude Leynaert, Paul Tréguer (1996) Opal
studied as a marker of paleoproductivity. EOS transactions, AGU 77(49):491-493
- Paul Tréguer, David M. Nelson, Aleido J. Van Bennekom, David
J. DeMaster, Aude Leynaert, Bernard Quéguiner (1995) The silica
balance in the world ocean: A reestimate. Science 268:375-379
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