Silica distribution in the World Ocean: In the water column and in the bottom sedimentsThis text is taken from Lisitzin (1972, pages 149-158, and slightly modified). The reader is referred to this paper for proper reading and full information about cited references (not included in this text) and the many detailed tables and illustrations.
Opal content of the surface water of the World Ocean ranges between 0.13µg/L and 1086 µg/L. As a rule, the amount of suspended biogenous silica generally is 5 to 50 times less than that of dissolved silica. Only in the Antarctic is the difference only 3 to 4 times. Thus, the organisms succeed in converting only an insignificant part of dissolved silica into particulate silica.
Silica in the surface water suspensions:
As microscopic studies show, the most abundant siliceous organisms in suspension are diatomaceous algae, the main producers of oceanic organic matter. The quantitative distribution pattern of suspended diatoms calculated in millions of cells per 1 gram of suspension. The siliceous suspension belts coincide with the diatom suspension belts, which points to the dominant role of diatoms.
Map showing annual production of biogenic silica in the plankton of the world oceans (g/m2/yr) (modified from Lisitzin 1971 and de Wever et al. 1994).
The second major producers of suspended amorphous silica are radiolarians. They occur from the Arctic to the Antarctic, being most abundant in the equatorial zone. In waters of the Equatorial Pacific there are about 16 000 specimens/m3, whereas in the Antarctic waters there are much fewer, only tens or hundreds of specimens per m3. The Antarctic Convergence is a distinct boundary at which both the quantitative distribution and the qualitative composition of radiolarians change sharply.
Amorphous silica contents of modern sediments range from fractions of 1 % to the maximum value of 72 % at Station 275 of the OB cruise at 52°45'S latitude, 62°19'E longitude in a water depth of 4746 m. Three major belts of modern silica accumulation can be distinguished:
The northern belt differs from the southern one by lower silica concentrations and by areal discontinuity. It embraces the northern part of the Pacific including the Bering, Okhotsk and Japan seas.
Silica concentrations in the sediments of the northern Pacific usually do not exceed 10 or 20 % and only rarely are as high as 30 %.
Silica content is substantially higher in the sediments of the Bering Sea, attaining up to 37 % and even higher in the Sea of Okhotsk, where values are as much as 56 %. In the Japan Sea, siliceous sediments occur only in the northern portion where maximum values exceed 20 %. Silica content of bottom sediments in the Yellow, East China and South China seas is no more than 2 or 3 %. In the northern Atlantic Ocean, the silica belt is interrupted due to the influence of the Gulf Stream, and sediments usually contain less than 3 % amorphous silica. Only at several stations are values as high as 5 to 7 %. The Mediterranean Sea sediments contain a maximum of about 3 % and commonly contain less than 1 %. Bottom sediments of the Arctic Ocean and Arctic seas usually contain no more than 0.5 % and only up to 4 to 7 % in the marginal parts. This is attributable both to the ice cover which hampers photosynthesis as well as to an abundant terrigenous supply from the Siberian rivers.
The equatorial belt is made up of separate patches of varying area. Correlations between the silica distribution maps and the maps of sediment type and bathymetry indicate that, unlike the northern and southern belts, the distribution of silica is closely related to depth, occurring only at greater than critical depths where diluent calcium carbonate is dissolved (the carbonate-compensation depth or CCD). The critical depth ranges from 4800 to 5300 m in the equatorial zone of the Pacific, from 5500 to 5600 m in the Atlantic and from 5000 to 5500 m in the Indian Ocean. The extreme boundaries of separate patches of siliceous sediment delineate a zone stretching from 20°N latitude to 20°S latitude and gravitating (i.e. decreasing) toward the Equator. This zone is best pronounced in the Indian and Pacific oceans and has not been observed in the Atlantic Ocean.
Two more patches of siliceous sediments should be noted which are not included in the belts of silica accumulation. One of the patches has been found in the Gulf of California, where maximum amorphous silica content is up to 65 % and the other lies off southwest Africa near the month of the Orange River, where maximum values are more than 50 %. These siliceous areas are associated with divergences off the western coasts of the continents and are also marked by sharp increases in primary production, high phytoplankton biomass, and great amounts of total suspension and of suspended siliceous frustules.
Radiolarians are of the greatest importance in modern sedimentation of the equatorial zone (Lisitzin 1972, Figure 147). In the temperate zone their contents are an order of magnitude less. However, radiolarians are found as far poleward as the Antarctic coasts.
Maximum number of silicoflagellates in bottom sediments, in amounts of 0.6 to 1.3 X 106/g, have been recorded in the equatorial zone. In the arid zones they range in quantity from 0 to 0.4 X 106/g, and toward the humid northern and southern silica accumulation belts numbers again increase up to 0.1 to 5.5 X 106/g with lower values on the shelf of 0.06 to 0.19 X 106/g. Thus, within all the three belts of modern silica accumulation greater contents of all major siliceous organisms are observed both in suspensions and in bottom sediments. In the equatorial zone radiolarians are particularly prominent in a number of places and are even more significant than diatoms which are predominant in all the other zones.
Thus the global surface water belts which were proved to be characterized by high suspended silica in the photosynthetic zone are also silica-rich throughout the entire water column and in the underlying sediments. Sediment cores for the entire past 11,000 years of the Holocene reveal the same situation. However, the near bottom layers of the ocean are reached by only an insignificant amount, 1/10th to 1/100th of the silica bonded by organisms into frustules at the surface. The remaining part of the silica is dissolved and re-enters the geochemical cycle.
Within all the silica belts most of the silica appears to be concentrated in the size fraction finer than 10 µm, the fraction which includes the fine diatom detritus. The numbers of frustules in suspension and in bottom sediments represent only insignificant fractions of their original quantities, which are mostly reduced into fragments.
Amounts of frustules preserved generally decrease with increasing depth if the bottom plane is imagined to shift from 500 to 5000 m depths. Unlike calcium carbonate, no critical depth for silica exists, and siliceous remains occur in bottom sediments down to maximum oceanic depths. All other things being equal, an increase in depth results in a decrease in the total amount of opal frustules due to dissolution, as well as in decreases in median particle sizes and species variety. The influence of depth is especially marked for neritic assemblages in which fine siliceous forms prevail, and is less marked in the oceanic suite in which the preservation of frustules is higher.
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