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Silica distribution in the World Ocean: In the water column and in the bottom sediments

This 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.

General:
Opal content of the surface water of the World Ocean ranges between 0.13g/L and 1086 g/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.

Microscopic examination of suspensions also shows that the basic role in modern silica accumulation belongs to diatom algae, which comprise more than 70 %, and sometimes over 90 % of the suspended silica. Radiolarians are second in importance to diatoms. Silicoflagellates, which occur in amounts of 0 to 3000 cells/m3 in temperate and cold waters, are the third most important silica-formers. Waters of the northern and southern belts of siliceous suspensions are chiefly diatomaceous with an admixture of radiolarians. Within the equatorial belt radiolarians sharply increase. All the three groups of siliceous organisms are encountered in minimum quantities in the arid zones of the ocean.

A rather constant relationship between the weight of a siliceous diatom frustule and the weight of the algal protoplasm can be expressed as the ratio of amorphous silica to organic carbon. The mean value of this ratio for diatom algae is 2.3 and for 50 analyses of Antarctic diatomaceous suspensions the average ratio is 1.85. This makes it possible to determine the annual quantity of amorphous silica bonded by diatoms into opal frustules in different parts of the ocean (Lisitzin 1972, Fig. 142). Suspension studies delineate silica accumulation in a southern, a northern and an equatorial zone. In the equatorial zone diatoms convert 5 to 10 times less dissolved silica into suspended opal per year as in the southern belt.

Silica in the deep water suspensions:
As planktonic organisms die their skeletons descend to the bottom. Settling velocities of the dead siliceous organisms are determined mainly by their sizes and, to a lesser extent, by water density. Currents and density boundaries are also of great importance.

The settling velocities of diatom frustules of different species and sizes determine not only the residence time of particles in the water column and hence the possibility of their dissolution, but also the possibility of their eventual deposition in the bottom sediments of any grain size composition. Observations of the settling velocities of suspended opal and opal in bottom sediments during thousands of grain size analyses yielded values which range within rather wide limits corresponding to the settling velocities of 1- to 50- m quartz spheres: 0.053 X 10-3 to 133 X 10-3 cm/sec at 5C. Most siliceous material settles at rates equivalent to those of 1- to 50- m quartz spheres. This account for the fact that the coarsest and heaviest frustules reach a 5000 m depth in 30 to 100 days and the finest fractions require many decades to settle to the same depths.

Suspension studies also show that only an insignificant part of the frustules descend freely to the ocean floor the way mineral particles do. Instead, most enter food chains and are utilized as food by zooplankton. Copepods break coarse and medium-sized diatom frustules into 50- m, particles and bond them into lumps which in turn may be eaten by deep-water plankton and benthos. Thus, the ultimate fate of diatom frustules in the water column is closely related to food chains.

Studies of suspensions at depth indicate that diatom frustules are destroyed most rapidly in the upper 100 m water layer. Due to the dissolution of the thin frustules, deep-water suspensions and bottom sediments appear enriched in the forms with coarse frustules.

Silicoflagellate skeletons are usually well preserved in the water column; however, their rare occurrence in surface waters predetermines their minor role in deep water suspensions. Radiolarians and, to a smaller extent, silicoflagellates are very well preserved in deep water suspensions and reach the bottom without appreciable loss.

As diatom frustules settle to the ocean bottom their specific composition in suspensions undergoes a general decrease. Coarse siliceous forms of the oceanic suites of the Northern and Southern hemispheres are best preserved; 70 percent and sometimes even 100 percent of the initial number of species in the near-bottom suspension as well as a decrease in the frustule numbers is attributable to the dissolution processes which first affect the fine siliceous forms.

Distribution of siliceous microfossils in surface bottom sediments:
To compile a map of amorphous silica distribution in the bottom sediments of the World Ocean (Lisitzin 1972, Fig. 145) new bathymetric maps of the seas and oceans as well as new maps of sediment types drawn at the Institute of Oceanology of the USSR Academy of Sciences were used. The basic zones of modern silica accumulation and regions low in silica can be shown quite reliably from the results of over 2000 analyses.


Map showing amorphous silica distribution in the surface sediment layer (in % of dry sediment) (modified from Lisitzin 1971 and de Wever et al. 1994).

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 5245'S latitude, 6219'E longitude in a water depth of 4746 m. Three major belts of modern silica accumulation can be distinguished:

  1. A southern belt encompassing the globe almost uninterruptedly in the Southern Hemisphere.
  2. A northern belt in the Pacific Ocean, Sea of Okhotsk, Bering and Japan seas, absent in the Atlantic and Indian oceans.
  3. An equatorial or, more precisely, a near-equatorial belt which is well defined in the Pacific and Indian oceans and less distinct in the Atlantic Ocean.
The southern belt is characterized by large width and by the highest silica content. More than three fourths of all oceanic silica accumulates here. The siliceous sediment belt is 900 to 2000 km wide between the 10 % amorphous silica isopleths, its northern boundary coinciding with the Convergence and with the middle boundary of iceberg distribution. Occurrence of diatom oozes extends southward into the south polar circle and these sediments even appear on the Antarctic shelf. Silica concentrations are distributed unevenly within this belt and are never found on underwater rises or on steep scarps.

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 20N latitude to 20S 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 m, 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.

References:
Lisitzin, A. P., 1971. Distribution of siliceous microfossils in suspension and in bottom sediments. The Micropalaeontology of Oceans, Funnell, B. M. and Riedel, W. R. [eds.], Cambridge University Press, Cambridge, UK, 173-195.

Lisitzin, A.P., 1972. Sedimentation in the world ocean. Society of economic paleontologist and mineralogists. Special publication no. 17, 218 pp.

de Wever, P., Azma, J. and E. Fourcade, 1994. Radiolarians and Radiolarites: primary production, diagenesis and paleogeography. Bulletin des Centres de Recherches Exploration-Production, Elf Aquitaine, 18(1): 315-379.


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