The silica cycle during the last ice age

Palaeogeography, Palaeoclimatology, Palaeoecology, 50 (1985): 2 4 1 - - 2 7 0

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Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands

THE SILICA CYCLE D U R I N G THE LAST ICE AGE

A L E X A N D E R P. L I S I T Z I N

Institute of Oceanology, U.S.S.R. Academy of Sciences, 23 Krasikova, 11 7218 Moscow

(U.S.S.R.) (Received J a n u a r y 17, 1985)

ABSTRACT Lisitzin, A. P., 1985. T h e silica cycle during the last ice age. Palaeogeogr., Palaeoclimatol., Palaeoecol., 50: 241--270. Qualitative and quantitative investigations of the silica geochemistry in the ocean and its cycles and changes during the Q u a t e r n a r y have been carried o u t based o n m o r e t h a n 4000 sample determinations. The present Si cycle within the ocean balance and annual silica a c c u m u l a t i o n were determined. The main part of silica has accumulated on the ocean b o t t o m in three silica a c c u m u l a t i o n belts: southern, equatorial and northern. The changes of the belts were s y n c h r o n o u s for the n o r t h e r n and the southern hemispheres and were caused by climatic changes c o n n e c t e d w i t h glaciation. INTRODUCTION

Silicon is one of the biogenic elements of the ocean, of which distribution and geochemical cycle in the ocean, in organisms and b o t t o m sediments, are closely connected with the cycles of other biogenic elements such as P, N~ and C. Changes in Si accumulation during the Quaternary can be due to factors like variations in its input to the ocean, ocean morphology changes, changes in the bioextraction and biofiltration systems and changes in the w a y of transportation of organic remains down to the b o t t o m (e.g. by pellets). METHODS, DATA AND INVESTIGATION REGIONS

The silica geochemistry during the Quaternary has been studied during almost the last thirty years in the U.S.S.R. according to a united program using c o m m o n methods. The studies include analyses of seawater, suspension, organisms, b o t t o m sediments and interstitial water from almost all parts of the world ocean. The remains of siliceous organisms were thoroughly studied in more than ten thousand suspension samples, taken from the ocean surface d o w n to the m a x i m u m of its depth. In total the Si concentration has been studied at 27,000 ocean stations. The contents of dissolved and suspended silica in the river waters, particularly in the U.S.S.R. rivers, and in estuaries have been studied as well. The 0031-0182/85]$03.30

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transportation of silica with the aerosols to the ocean was investigated in some hundred samples. For the determination of the total Si contents x-ray fluorescence, quantitative spectrochemical as well as atomic-absorption m e t h o d s have been used. The mineral composition was determined b y the quantitative x-ray m e t h o d s {clay minerals, quartz, feldspars and others), and for silty materials a microscope was used (Z. N. Gorbunova and V. V. Serova). The a m o r p h o u s silica c o n t e n t (opal) in suspension and b o t t o m sediments was determined by the double sodium extraction m e t h o d (Eggiman et al., 1979). More than 3000 opal determinations were carried o u t of the surface layer of the b o t t o m sediments, and in cores of Quaternary age determination of a b o u t 4000 samples. Determinations of amorphous silica contents in relation to Si O2 t o t / A1 2 0 3 w e r e carried out simultaneously in order to check other data from most of the stations. Thus, a combination of three main m e t h o d s for the quantitative and qualitative amorphous silica determinations was constantly used. The determinations b y the x-ray m e t h o d in a number of cases gave us results, which are very difficult to reproduce. The input of endogenic silica and the forms of it together with studies of authigenous minerals were investigated in a number of special Legs to the East Pacific Rise (Legs 8 and 14 b y R/V Dmitry Mendeleev), to the triple junction of the Indian ocean rift (Leg 25 b y R/V Dmitry Mendeleev): during the work with the submersibles in the Red Sea (PIKAR expedition), at the Reikjanes Ridge (North Atlantic), and in the Tadgura rift (the Gulf of Aden). During the investigations carried o u t by the underwater craft Pisces -totally more than 100 divings -- we were lucky to sample the bedrock, sediments, organisms and water from hydrotherms. Sedimentation rates and absolute masses were determined on the basis of a b o u t 2000 '4C and 21°Pb determinations carried o u t under the guidance of V.M. Kupzov. Stratigraphical interpretations of the Quaternary sections of more than 1000 cores were established by diatomaceous, foraminiferal, coccolithophoridic, radiolarian and pollen analyses. We also used paleomagnetic dating m e t h o d s and age determinations by the fission track, Io--Pa and Io--Th methods. Oxygen isotope analyses were carried o u t on samples of certain important cores and tephrachronology gave further pieces of information. Thus, qualitative and quantitative investigations of the silica geochemistry in the ocean and its cycles and changes during the Quaternary have been carried out. THE INPUT OF SILICA FROM LAND AND FROM R I F T ZONES

During recent times as well as during the Quaternary Period we have two main silica sources: (a) products of weathering from the land (exogenous silica); (b) input from active ridges and from underwater volcanos i.e. the release of silica b y the reactions b e t w e e n h o t lava and seawater (endogenous silica).

243 The solubility of silica is 100--200 mg/1 at temperatures of 10--20°C and pH 6--8. However, the concentration of dissolved silica in river waters is far less or 6--10 mg/1 (average 6 mg/1), and in seawater still lower or 0.5--3 mg/1 (average 2.1 mg/1). The seawater is, thus, highly undersaturated with respect to silica. Small quantities (about 0.3% of the total) occur in suspension. Therefore, the main mechanism of silica transformation in seawater from dissolved forms to its final removal to b o t t o m sediments is biogenic. Only in the immediate nearness of the active ridges, and also in some depressions, separated from the ocean by the brine strata (the Red Sea), silicon can form authigenous minerals (smectite, nontronite and others). The total annual input of particulate matter from the land to the ocean is 22.1 X 109 tons; 25.5--25.8% or 4.73 X 109 tons of the suspended matters are Si. The main part of it is supplied by the rivers (18.5 X 109 tons/yr.); eolian transportation (1.5 X 109 tons/yr.) and coastal abrasion (about 0.5 X 109 tons/yr.) are less important (Lisitzin, 1974, 1978). These data of the sediment input do n o t differ much from other estimations: 14.5--15.5 X 109 tons/yr. (Milliman and Meade, 1983); 15.5 X 109 tons/yr. (Martin and Meybeck, 1979); 12.7 X 109 (Lopatin, 1950) and 18.3 X 109 tons (Holeman, 1968). Only a small fraction (70--90 X 106 tons) is trapped in the "sea-river" barrier. The total discharge of dissolved substances b y the river waters is estimated at 3.2 X 109 tons (Alekin, 1966), or 4.0 X 109 tons according to Martin and Meybeck (1979). More than 60% of the dissolved substances in the river water consist of elements belonging to the carbonate system. The discharge of dissolved silica was estimated at 324 X 106 tons/yr, by Bruevich (1953), at 462 X 106 tons/yr, by Livingstone (1963), at 203 X 106 tons by Martin and M e y b e c k (1979) and at 195 X 106 tons by Alekin (1979). This amount is 22 times less than river supply of Si in suspended form We take the value of the influx of dissolved Si as 220 X 106 tons/yr, or 6 X 10 -6 g Si/g river water. Some 20--30 X 106 tons are trapped in the 'sea-river" barrier while ca. 200 X 106 tons Si flow o u t in the oceans. Thus, the river input of silica to the ocean is up to 95.6% in suspended forms, most of which (more than 92%) accumulates in the river mouths (estuaries) or at the shelfs, and does n o t penetrate into the pelagic area. For the pelagic parts the dissolved silica is the most important. A b o u t 3/4 of the input of Si to the ocean in dissolved and suspended forms come from the equatorial zone, which occupies only a b o u t 1/4. (26.1%) of the continents. The arid zones do not supply the ocean with any dissolved silica (Lisitzina, 1975). The dominant role of the low-latitude humic zone in the silica discharge has an important bearing in the paleogeography and paleoceanology: during ages when the land area expanded in the equatorial zones, or due to the climatic improvements the influx of silica to the ocean should have increased significantly. If all data a b o u t the silica concentration in the river suspension is given as SiO:, its average contents will be 54.55% (Lisitzin, 1978), which is very

244 close to the average SiO2 contents in the continental clays and shales: 55.09% (based on 10,746 samples). The average SiO2 figure for the platform sedimentary rocks is 49.21% and for the geosynclines 50.0%. The SiO2 contents of the whole continent is considered as 49.82% (Ronov and Jaroschevsky, 1976). Therefore, we can say that the average SiO2 contents in the river suspension exactly reflects the average content of SiO2 of the continental rocks. The ratio of SIO2/A1203 in the world river suspension is 3.48. It is 3.48 also in the continental clays and shales, 4.5 in sedimentary rocks of the platform and 3.65 in the ancient sedimentary rocks from geosynclines. Thus, aluminosilicate material in river-transported suspended materials corresponds well with the average material of the continental sedimentary rocks (the ratio for the platforms seems t o o high). The supply of the endogenic (hydrothermal) silica is a function of the length of the global ridge system or more precisely the amount of hot basalt annually erupted there. From this lava Si is leached b y the seawater. The amount o f volcanic products yearly formed in active ridges is ca. 15 km 3, within the ocean islands area ca. 1 km 3 (3 × 109 tons) and from underwater volcanos ca. 0.015 km 3. Thus, the rift zones are the main source of endogenous 9~ The annual silica supply from the rift zones can be calculated from the 3He output. The hydrothermal c o m p o n e n t of silica of the active ridges is quantitatively connected with the 3He flux (Craig et al., 1975; L u p t o n et al., 1977; Weiss et al., 1977; Jenkins et al., 1978; Coriiss et al., 1979; E d m o n d et al., 1979; and others). According to this m e t h o d a b o u t 70 × 109 tons of silica is transferred from the hydrotherms in the rift zones up into the sea water. Higher figures, up to 190 × 109 tons/yr, are given by E d m o n d et al. (1979). A great amount of silica is released to the b o t t o m waters by underwater weathering of basalts (halmyrolysis) b y the cold b o t t o m water. Some 80 × 109 tons of silica is released in this w a y (Hart, 1973), b u t this value seems rather overstated (Aumento et al., 1979). These two (hot and cold) sources at the ocean b o t t o m together give a b o u t 150--190 × 109 tons. Summing up the discussion, we can say that the present influx of dissolved silica from the land to the sea is 200 × 109 tons/yr, and the endogenic sources give 70--190 × 109 tons. To these amounts should be added the quantity supplied from the glacial weatherings which is a b o u t 20 × 109 tons (Hurd, 1971, 1973; Hurd and Birdwishtell, 1983). Thus, the input from land is the most important source of silica for the ocean; it can be up to three times larger than the endogenic sources. PROCESSES AT THE RIVER--SEA BOUNDARY Many geochemists are of the opinion that m o s t silica from the river water is removed b y coagulation and sorption at the river-sea boundary. Therefore the main part of the dissolved silica would n o t enter the oceanic pelagic

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zone. It was also stated that the extraction of biogenic silica mirrored the primary production in the estuaries (Wollast and De Broen, 1971). However, this opinion seems erroneous. Silica occurs in river and ocean waters in the form of monomeric silicon. The absence of its polymers makes the coagulation during mixing with the river waters rather difficult (unlike A1, Fe and the dissolved organic matter DOM). In a number of works the negative correlation between dissolved silica c o n t e n t and salinity in estuaries is noted, i.e. silica behaves in a conservative way. There has not been any significant removal of Si from the solution during its mixing with the seawaters (Burton et al., 1970; Boyle et al., 1974). The biogenic removal of the dissolved silica is most significant in areas of extreme high primary productivity, such as estuaries. However, the peak of the dissolved and suspended material for most world rivers is connected with floods, i.e. very short-lived events, when p h y t o p l a n k t o n does not have enough time to develop and to use the biogenic elements delivered by the rivers. Usually the productivity is limited to the low water conditions and increases after floods. It must be noted, that the very high rate of organic production in estuaries, is to a great extent a result of macrophytes. Their production can significantly exceed the p h y t o p l a n k t o n production and the silica-extracting diatoms in particular (Mann, 1972). In spite of the widely spread opinion, the main part of the suspended silica is entered into the sea and deposited there, n o t in form of sands and silts, b u t as a c o m p o n e n t in clay minerals. A b o u t 6?0 of the annual input of suspended matter from rivers to the ocean are in the rudaceous fractions (gravel and pebble), about 26% in the sandy-aleuritic material, deposited near the shores, and 67% in clay minerals (Lisitzin, 1978). The supply to the oceans of silica in suspended form, its granulometry and its mineralogy (detrital and clayey minerals) vary between the climatic zones. Detrital material b e y o n d the shelf's margins is characteristic for the glacial zones, where it is d r o p p e d from ice and ice-bergs. Sands and aleurites occur particularly at intermediate latitudes, along the margins of formerly glaciated areas. Most silica in suspended form occurs in montmorillonite (smectite), kaolinite and illite. Their occurrence in ocean suspensions and in b o t t o m sediments mirrors the climatic zones (Rateev et al., 1968; Lisitzin, 1978; Gorbunova and Lisitzin, 1979; Lisitzin and Gorbunova, 1983). While moving from the high-latitude (glacial) zones to the equatorial ones, the intensity of the weathering processes on land increases, and consequently also the o u t f l o w of clay minerals and dissolved elements, while the amount of detrital materials decreases (Lisitzin, 1972, pp. 39--46). As the latitudinal transportation in the World Ocean is approximately ten times stronger than the meridional one, the "climatic mark" of detrital and clay minerals in ocean sediments survives in the main climatic zones of the World Ocean (Lisitzin, 1974, 1977, 1978). In conclusion it can be stated that only 20--30% of the dissolved silica

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are trapped at the river--sea boundary. However, less than 10% of the particulate matter reach b e y o n d the shelf. Furthermore, the input of the dissolved and suspended silica to the ocean from the continents is climatically controlled. Of the whole continental input of silica 88% enter from the humid zones, and the equatorial zone contributes with the largest mass of it, namely 76%. S I L I C A D I S T R I B U T I O N IN T H E O C E A N W A T E R S

The average concentration of dissolved silica in seawater is supposed to be 2.13 rag/l, with a m a x i m u m reaching up to 12--27 mg/1. The average suspension c o n t e n t in the oceanic waters can be assumed as 0.1 mg/1, the average Si contents of which are 3% (Lisitzin and Gordeev, 1979). The suspended matter in rivers consist to 25.5% of Si. The principal form of silica in the ocean is in a dissolved state. During the photosynthesis planktonic organisms concentrate silica 700 times or more. As appears from Fig.1 vast oceanic areas contain less Si than 10 ~g-at/1 in the uppermost 100 m of the waters. There are especially low contents in the Arctic and the North Atlantic oceans. There are three global belts of silica accumulation, two at moderate latitude, and one equatorial. The silica concentration increases 10--15 times from the water surface d o w n to the benthic layers in the same w a y as for other biogenic elements (N, P). It is established that there exists a relationship between the concentrations of carbon, silica, nitrogen and phosphorus in the ocean, a certain ratio which is preserved during the formation of the organic matter b y plankton and during its mineralization (Sverdrup et al., 1942; Richards, 1965). Enhanced silica contents in the waters of the rift zone can usually only be traced along a short distance from the rift, because 0

.0

Fig. 1. The distribution of dissolved silica in the u p p e r m o s t 100 m of the sea ( n o r t h e r n winter), in ug-at Si 1-1.

~ig.2. The p r o d u c t i o n of biogenic silica in g / m 2 yr -1. The m a p is based o n the primary p r o d u c t i o n as well as the relationship b e t w e e n Lmorphous silica and organic carbon in s u s p e n d e d matter: 1 = < 1 0 0 ; 2 = 1 0 0 - - 2 5 0 ; 3 = 2 5 0 - - 5 0 0 ; 4 = > 5 0 0 (Lisitzin, 1 9 7 4 ) .

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248 most silica discharged here is quickly used to form authigenous minerals. THE SI CYCLE WITHIN THE OCEAN Most organic matter in the ocean is produced by the siliceous planktonic diatoms. In the moderate zone t h e y compose 90--98% of the plankton, and in the equatorial up to 50--60%. For the World Ocean diatoms account for a b o u t 70--80% of the primary production, a b o u t 20% are produced b y dinoflagellates while ca. 1% is formed by the blue-green algae. Judging from m y studies of the Antarctic suspended material the ratio of amorphous SiO2/organic C is 2.3 (Lisitzin, 1964), which corresponds well to the relation in diatoms. The size of the organic production is b y different authors estimated at 20--35 × 109 tons which corresponds to an annual extraction of 46--80 X 109 tons of amorphous SiO2 from the seawater. If comparing these values with the annual input of dissolved silica to the ocean, 0.2 × 109 tons, we conclude that annual influx of Si only constitutes 0.4-0.8% o f the total SiO2 involved in the annual biological cycle. The amount o f silica involved in the biological cycle appears to be only 0.5--1% of the total c o n t e n t in the ocean waters. This value is the same as for the other biogenous c o m p o n e n t s as well (P, N, C and others). Since the river input should be equal to their accumulation in the sediment only a small fraction of their c o n t e n t in the seawater is trapped yearly. On an average only a b o u t 2% o f the plankton (remains) p r o d u c e d at the surface are settled d o w n at the floor. One fifth of that is silicon (Lisitzin, 1974). Direct calculations of diatoms on microslides of suspensions from the ocean depth, and also observations from sedimentation traps suggest that siliceous organisms (diatoms, silicoflagellates, radiolarians) are quickly dissolved during the settling (Kozlova, 1964; Kozlova and Mukhina, 1966; and others). The largest part, for diatoms up to 90% or more, is dissolved in the upper 1000 m and particularly in the upper 100 m. Only in shallow waters as in the upweUing areas, diatom remains settle down as single particles at the b o t t o m . Radiolarians, which seldom occur in pellets, usually settle d o w n in the form of separate skeletons. In the equaorial Atlantic the flux of radiolarian skeleton is from 16 up to 24 × 103 X m -2 day -1 (Takahashi and Honjo, 1983). In the arid ocean zones the radiolarian skeleton flu~ in the near-bottom areas is only from 0.5--1 to 2--6 × 103/day (Berger, 1968; Honjo, 1980), i.e. 10% less, or more than in the fertile equatorial belt. According to the observations from the sedimentary traps (Wiebe et al., 1976; Honjo, 1978, 1980; Honjo and Roman, 1978; Hinga et al., 1979; Collier and D y m o n d , 1980; Takahashi and Honjo, 1981, 1983; Suess and Muller, 1984) the main part of the material flow in the ocean is in (the form of) pellets at the size of 200 ~m and more. Their average settling rate is a b o u t 100 m/day. Pellets have an external cover, persistant to dissolving (pellicle). Because of the pellets and their persistent cover also unstable ma-

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t/t Fig.3. Q u a n t i t a t i v e d i s t r i b u t i o n o f d i a t o m s in s u s p e n d e d m a t t e r f r o m t h e s u r f a c e w a t e r s o f t h e A n t a r c t i c s e c t o r o f t h e I n d i a n a n d Pacific O c e a n s ( t h e s o u t h e r n b e l t o f silica a c c u m u l a t i o n ) . ( K o z l o v a , 1 9 6 4 ) . U p p e r m a p : in m i l l i o n s o f cells p e r g r a m o f s u s p e n s i o n : 1 = <6.25; 2 = 6.25--12.5; 3 = 12.5--25; 4 = 25--50; 5 = 50--100; 6 = 100--200; 7 = 2 0 0 - - 4 0 0 ; 8 = > 4 0 0 ( m a x i m u m 5 8 3 ) . L o w e r m a p : in m i l l i o n s o f cells p e r m 3 o f w a t e r : 1 = <:6.25; 2 = 6 . 2 5 - - 1 2 . 5 ; 3 = 12.5--25; 4 = 25--50; 5 = 5 0 - - 1 0 0 ; 6 = 1 0 0 - - 2 0 0 ; 7 = 200--400; 8 = 400--800; 9 = 800--1000.

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terial is settled down in the oceans. According to the observations from sedimentary traps in the Sargasso Sea siliceous organic remains dissolved at a rate of 11% daily, and in the Pacific Ocean at about 13% daily (Berger, 1968; Honjo and Erez, 1978). THE PRESENT

ACCUMULATION

OF SILICA

The distribution of opal in bottom sediments according to the chemical analysis data is shown in Fig.4, a distribution checked by analysis of bottom sediments. Also the distribution of ooze and detrital (minerals) containing the main part of the abiogenic SiO2 was studied in detail (Lisitzin, 1977, 1978; Gorbunova and Lisitzin, 1979; Murdmaa et al., 1979; Serova et al., 1979; Lisitzin and Gorbunova, 1983). The distribution of diatoms, radiolarians and silicoflagellates in the upper layer of the sediments is given in Figs.5--6. These maps are based on detailed microscopic investigations. The opal content in the surficial sediments varies from less than 1% in oligotrophic areas up to 72% off the Antarctic and 74% in upwelling zones. There are three belts of silica accumulation: (1) The south, encompassing the Antarctic continent. (2) The north, only developed in the north Pacific Ocean, in the Bering, Okhotsk and Japan Seas. No siliceous sediment occurs in the Atlantic Ocean due to the low Si content in the photosynthetic zone. (3) The equatorial (more exactly the near-equatorial) is clearly expressed in the sediments of the Pacific and Indian Oceans, while very weakly in the Atlantic Ocean. The north and the south belts consist of diatom oozes, while in the equatorial belts radiolarians contribute greatly to the siliceous ooze. The silica accumulation zones at high latitudes are limited by the light factor and the ice cover (Lisitzin, 1966, 1977, 1978, 1983). The mean content of amorphous SiO2 (opal) in oceanic sediments is 5.0-7.5%. It has been possible to divide the siliceous sediments into different types (Fig.8). High amorphous SIO2/A1203 ratios are recorded in upwellings (they are not shown in the map scales). The dissolution of siliceous organic remains is not restricted only to the upper layer of bottom sediments. It also proceeds further down in the sedimentary sequence. Our analyses of the interstitial waters give evidence of that. We also found that the flux of Si up from the upper sediment layer F i g . 4 . A m o r p h o u s silica d i s t r i b u t i o n in t h e s e d i m e n t s s u r f a c e o f t h e o c e a n s a c c o r d i n g t o m o r e t h a n 3 0 0 0 d e t e r m i n a t i o n s ( L i s i t z i n , 1 9 7 8 ) . P e r c e n t a g e a m o r p h o u s silica in carbonate-free sediment: 1 = <1%; 2 = 1--5%; 3 = 5--10%; 4 = 10--30%; 5 = 30--50%;6 = 5 0 - - 7 0 % ; 7 = > 7 0 % ; 8 = s t a t i o n s ; 9 = b o u n d a r i e s o f c l i m a t i c z o n e s ; 10 = u p w e l l i n g s ; 11 = active r i d g e s . I n s e t m a p s : A . A m o r p h o u s silica d i s t r i b u t i o n in t h e B a l t i c S e a ( E m e l y a n o v et al., 1 9 7 6 ) : 1 = < 1 % ; 2 = 1 - - 2 % ; 3 = 2 - - 3 % ; 4 = > 3 % o f t h e s e d i m e n t . B. A m o r p h o u s silica d i s t r i b u t i o n in t h e S e a o f O k h o t s k s e d i m e n t s ( B e z r u k o v , 1 9 6 0 ) : 1 = < 1 0 % ; 2 = 1 0 - - 2 0 % ; 3 = 2 0 - - 3 0 % ; 4 = 3 0 - - 4 0 % ; 5 = 4 0 - - 5 0 % ; 6 = > 5 0 % . C. A m o r p h o u s silica dist r i b u t i o n in t h e P e r u a n d B a u e r B a s i n s : 1 = < 1 0 % ; 2 = 1 0 - - 3 0 % ; 3 = > 3 0 % o f t h e sedim e n t . D. A m o r p h o u s silica d i s t r i b u t i o n in t h e z o n e o f s o u t h w e s t A f r i c a ( t o t h e S o f the river K u n e s e ) . T h e s a m e s i g n s as o n t h e b a s i c m a p . E. P h o s p h a t e s in t h e P a c i f i c Ocean (PO4--P) (mg/at/l: 1 = <0.25; 2 = 0.25--0.5; 3 = 0.5--1.5; 4 = 1.5--2.0; 5 = >2.0.

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253

to the b o t t o m water is directly proportional to the accumulation rate of the opal in sediments. The highest recycling rates are found in the upwelling areas. As to the low-productive arid and glacial zones the absolute mass of this upward flux from the abyssal floor amounts to a b o u t 2 ~mol cm -2 yr -1., and in the silica accumulation belts 10--20 ~mol cm -2 yr -1. (according to data of Fanning and Pilson, 1973; Boyle et al., 1974; Edmond, 1974; Schink et al., 1974; Bruland et al., 1978; E d m o n d et al., 1979; Collier and D y m o n d , 1980; Nelson and Gordon, 1982). These data agree well with those arrived at in the microscopic investigations of diatoms, radiolarians and silicoflagellates carried o u t at our cores. Specific interstitial water zones can be found (Murdmaa et al., 1979; Lisitzina and Butuzova, 1981; Fig.9). The diagenetic transformation processes are driven by the organic matter. We have already noted, that the organic matter is p r o d u c e d at the ocean surface by p h y t o p l a n k t o n and because of the pellet flow it reaches considerable depths. The preservation of organic matter generally decreases with depths. THE D I S T R I B U T I O N OF SILICA

During the glaciation the influx of dissolved silica from land did n o t change considerably. This is due to the fact that at present a b o u t 2/3 of its quantity originate from the equatorial zone where the conditions did not change much during the Quaternary Period. Thus, the main silica source was conserved. There is not any recorded significant change in the spreading rate of the plates during the Pleistocene, suggesting that the rate of endogenic silica input did not change much during this epoch. One of the main sources of additional silica may have been the erosion of sediments in estuaries and shelves, the interstitial waters of which are very rich in silica (usually the silica concentration in interstitial waters is 10--30 times higher than in the above-bottom water). During the drop in the sea level to ca. --120 m during the last glaciation, not only the estuaries were exposed, b u t also large parts of the shelf surfaces. If the level dropped to --100 m, a b o t t o m area about 13 × 106 km 2, or 3.8% of the ocean surface, would have been exposed. In the period of the sea-level lowering, soft deposits of shelves and estuaries have been eroded and redeposited at successively greater depths, finally d o w n to the continental rise (Lisitzin, 1978, 1983). This a m o u n t is larger, since 70--90% of the annual discharge Fig.5. Q u a n t i t a t i v e d i s t r i b u t i o n o f d i a t o m f r u s t u l e s in t h e s u r f a c e l a y e r o f b o t t o m sedim e n t s (in m i l l i o n s p e r g r a m d r y s e d i m e n t ) . ( F r o m K o z l o v a a n d M u k h i n a , 1 9 6 6 ; J o u z e et al., 1 9 6 9 a n d o t h e r s ) : 1 = a b s e n t ; 2 = 0 . 1 - - 1 ; 3 = 1 - - 5 ; 4 = 5 - - 1 5 ; 5 = 1 5 - - 4 5 ; 6 = 4 5 - - 1 0 0 ; 7 = > 100. O t h e r d e s i g n a t i o n s : 8 = E t h m o d i s c u s o u s - o o z e s . O n t h e i n s e t m a p s : A. T h e G u l f o f C a l i f o r n i a (Calvert, 1 9 6 6 ) . D i a t o m s f r u s t u l e s p e r c e n t a g e o f t h e f r a c t i o n 0 . 2 5 - - 0 . 0 6 2 r a m : 1 = < 1 0 % ; 2 = 1 0 - - 2 5 % ; 3 = 2 5 - - 5 0 % ; 4 = > 5 0 % . B. T h e u p w e l l i n g n e a r a n d o f f P e r u a n d Chile ( J o u z e , 1 9 7 2 ) . T h e c o n t e n t o f d i a t o m s in m i l l i o n s o f f r u s t u l e s p e r g r a m o f d r y s e d i m e n t : 1 = < 1 0 ; 2 = 1 0 - - 2 0 ; 3 --- 2 0 - 5 0 ; 4 = > 5 0 ; 5 = - - 4 0 0 m c o n t o u r .

254

~ C

255

of the particulate matter to the oceans is deposited in estuaries and on the shelf. Migrations of the colossal masses of the loose sediment from the inner (interglacial) shelf area to the outer (glacial) area resulted in numerous landslides, turbidites, etc. This reworking led to a release of silica from the interstitial waters into the waters above. Thus a silica reserve, accumulated during the interglacials in the interstitial waters of the shallow sediments, was mobilized. While the sea level dropped the river m o u t h s occurred successively further o u t on the continental slope. This movement reduced the filter activity of estuaries which now traps 10--30% of dissolved silica. So, the lowering of the sea level down to --120--150 m caused the larger influx of the dissolved silica from the rivers by reducing the filtering processes at the river/sea barrier and mobilizing silica from rewashed sediments. Therefore, the silica discharge (and other biogenic elements) into the ocean waters increased during early parts of glaciations compared with the m o d e m discharge. (Broecker, 1982.) The increase of the influx of biogenic elements to the ocean (eutrophication) increased the primary production and the sedimentation as well. As we know, also at the present (interglacial) stage there is a huge reserve of silica in the abyssal ocean waters, which is mobilized during the intensifying of the vertical circulation. The intensity of the vertical circulation in the oceans increases as the thermal contrasts intensify. Data about the paleotemperature determinations in the tropical and equatorial zones show that during the last glaciation (18,000 B.P.) temperatures in the equatorial and tropical zones did n o t drop more than 1--3°C, while the temperature dropped more at higher latitudes (Fig.9). These changes led to the abrupt increase of the vertical circulation. Evidence of this can be obtained from the analysis of the silica accumulation both in the areas of siliceous oozes and in the upwelling zones. The upwelling areas of the Atlantic existed continuously during glacials as well as during interglacial ages (Barash, 1974, 1984). During the last (Weichselian) maximum, at 18,000 B.P., the upwelling of the Canary and Benguela areas were more powerful than at the present (interglacial) time. The upwelling zones migrated to the equator and reached 14°N and 17°S. Their areas increased to a great extent as well, and judging from the temperature drop, the updrift of abyssal waters greatly increased. In the Canary upwelling zone the temperature decrease was ca. 4.5°C. In the Benguela upwelling area this temperature drop was even more, 8.5°C in comparison with the m o d e m one. Thus, the intensity of the silica accumulation in the upwellings was increased during the fall of temperature and decreased during the rising of temperature. The same conclusions were arrived at also regarding the upweUing areas of the Pacific. F i g . 6 . N u m b e r s o f r a d i o l a r i a n s k e l e t o n s in t h o u s a n d s p e r g r a m d r y s u r f a c e s e d i m e n t (Kruglikova, 1969 and others): 1 = <1; 2 = 1--10; 3 = 10-100; 4 = >100.

257 TABLE I Areas (in 103 k m 2) o f siliceous t y p e s o f t h e World Ocean T y p e o f silica sediment

Diatom ooze M o d e r a t e silica (30%--50% SiO 2 a m o r p h . )

Pacific Ocean

~

Atlantic Ocean 3436

Indian Ocean 693

5848

Strongly silica

World Ocean

22,293 3951

8401

no

2492

10,170

11,586

32,463

(> 50% SiO 2 a m o r p h . )

Radiolarian--diatom and diatom--radiolarian ooze v

7678 13,526

7351

The boundaries of the high latitude silica belts coincide with the convergence b o u n d a r y and the outer limit of ice rafted materials in the b o t t o m sediments (i.e. with the ice and icebergs boundary), and the 10°C isotherm (Globigerina pachyderma zone). These connections were maintained in the geological past as well. The distribution of the ice rafted material can give information about extreme boundaries o f ice-berg expansion in the Quaternary. The author (Lisitzin, 1974) determined the absolute masses of opal accumulation for the last 0,7 × 106 yrs. At the map (Fig.9) the three belts of m o d e r n silica accumulation are distinguished, as well as (not pointed out at the map because of the scale) the near-shore upwellings. High absolute masses are established also in adjacent seas included in the northern belt as Bering, Okhotsk and Japan Seas. In the silica accumulation belts most large absolute masses (more than 0.1 g/cm 2 × 103 yrs.) are noted practically at the same latitudes. Absolute mass 1 g/cm 2 X 103 yrs. is the 1 Strachov unit (1 S) (Lisitzin, 1978). The map significantly differs from the map of Bostrbm (1973). The southern belt is the most pronounced one as it is not divided by continents. It has absolute mass values of more than 0.1 S with m a x i m u m values up to 1--1.5 S which is considerably lower than the values 4.2 S arrived at by DeMaster (1979). For the northern belt absolute mass values of 0.1--0.5 S are characteristic and for the significant areas 0.01--0.1 S. In the arid zones absolute masses are below 0.01 S. In the Bering Sea absolute masses of the silica accumulation for the shelf are 0.2--1 S, 2 . 0 - 2 . 7 S, for abyssal parts and for the continental slope basement up to 3.0 S is arrived at with a m a x i m u m value o f 5 S in a fjord in Providence Bay. Fig.7. The d i s t r i b u t i o n o f t o t a l silica c o n t e n t in surface s e d i m e n t s in p e r c e n t a g e o f dry material, c a l c u l a t e d o n c a r b o n a t e - f r e e basis (Lisitzin, 1978): 1 = < 30%; 2 = 30--50%; 3 = 5 0 - - 7 0 % ; 4 = > 7 0 % ; 5 = m i d - o c e a n i c ridges; 6 = climatic boundaries.

~ "

60 °

60 °

,

0o

0o

60 o

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60 °

~

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120 °

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,

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_

",.~; ~.~-,,-+;,~ ,.,~,,~,,

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120 °

O0

259

Silica accumulation rates in upweUing areas are much larger. In the Californian upwelling area (silica accumulation area 76,000 km2), high sedimentation rates are combined with the huge silica content. Absolute masses vary from 0.6 up to 174 S (De Master, 1981), with an average value of 50 S (Calvert, 1966; Emelyanov, 1973). An average value of 69 S is given for the Californian, Peru and the Southeast African upwelling regions. The annual silica accumulation in the Californian upwelling area is estimated at 15 X 106 tons (Calvert, 1966) or 10 X 106 tons (De Master, 1981). The annual silica accumulation off the Southwest African upweUing area (ca. 16,000 k m : ) is of a size of 1--10 X 1 0 6 tons. The m a x i m u m opal cont e n t there is up to 75%. For the Peru upwelling ( ~ 1 0 6 k m 2) the silica deposition can be 30--50 X 106 tons annually (6 X 1 0 6 tons according to De Master, 1981). Thus, the aforementioned investigated upwelling areas accumulate about 50--100 X 106 tons of silica annually. If we consider the less known upwelling regions as well (Arabian Sea, Australia, Somali and others), then the total a m o u n t of silica accumulation in upwelling areas must be 1.5--2 times larger. To sum up the discussion we can state that the total accumulation of silica in the pelagic zones is equal to 172 X 1 0 6 tons/yr., 45 X 1 0 6 tons are accumulated in the Atlantic, 62 X 1 0 6 tons in the Indian Ocean and 65 X 1 0 6 tons in the Pacific. The accumulations in the upwelling zones axe 50-100 X 1 0 6 tons/yr. All the figures are mean values for the last 700,000 yrs. From this it is obvious how important the upwelling areas are for the Si accumulation. The main zone of silica accumulation of the Earth is the southern belt off the Antarctic. The annual supply of silica from the land and from endogenic sources comes to 200 + 70 = 270--300 X 1 0 6 t o n s , while the accumulation of silica in the pelagic zone and upwellings is 172 + (50--100) = 222--272. The remaining part of the silica is trapped in estuaries and shelves (sorption and co-precipitation) and is used in the formation of pelagic authigenous minerals (Fe-smectites, zeolites and others). It is worth mentioning, t h a t there is no sharp increase of absolute masses near the largest river mouths. Neither can mid-oceanic ridges be revealed by the areal distribution of absolute masses of silica accumulation. The predominating biogenous processes effectively mask the supply of endogenie silica. Fig.8. T h e d i s t r i b u t i o n o f siliceous s e d i m e n t s in t h e oceans. R a d i o l a r i a n o o z e s ( P e t r u s h e v s k a y a , 1 9 6 7 , 1 9 7 5 ; Kurglikova, 1 9 6 9 , 1 9 7 5 ; Hays, 1 9 7 0 ; a n d o t h e r s ) : 1 = s o u t h e r n b o u n d a r y o f t h e b o r e a l c o m p l e x in t h e Pacific O c e a n ; 2 = b o u n d a r y o f t h e tropical c o m p l e x (in t h e n o r t h e r n p a r t o f t h e Pacific O c e a n ) ; 3 = b o u n d a r y for t h e s u b t r o p i c a l p a r t o f t h e t r o p i c a l c o m p l e x ; 4 = b o u n d a r y for t h e e q u a t o r i a l part. O t h e r z o n a l b o u n d aries in t h e Oceans: 5 = n o r t h e r n b o u n d a r y o f t h e boreal c o m p l e x ; 6 = b o u n d a r y o f t h e t r o p i c a l ; 7 = e q u a t o r i a l b o u n d a r y ; 8 = b o u n d a r y o f A n t a r c t i c ice; 9 = b o u n d a r y b e t w e e n c o o l / t e m p e r a t e - z o n e s . D i a t o m s a n d d i a t o m ( - - r a d i o l a r i a n ) o o z e s a n d m u d s (Kozlova, 1 9 6 8 ; J o u z e e t al., 1 9 6 9 ; a n d o t h e r s ) : 1 0 = a r c t o - b o r e a l z o n e ; 1 1 = boreal ( - - t e m p e r a t e ) zone; 12 = subtropical zone; 13 = tropical zone; 14 = equatorial zone; 15 = subantarctic ( - - t e m p e r a t e ) z o n e ; 16 = A n t a r c t i c z o n e ; 1 7 = d i a t o m s absent.

IFig. 9. A b s o l u t e m a s s e s o f a m o r p h o u s SiO~ in t h e s u r f a c e s e d i m e n t s ( g / c m 2 1 0 -3 y r s . ) : 1 = < 0 . 0 1 ; 2 = 0 . 0 1 - - 0 . 0 5 ; 3 = 0 . 0 5 - - 0 . 1 ( o n l y in t h e e q u a t o r i a l b e l t ) ; 4 = 0 . 1 - - 0 . 5 ; 5 = > 0 . 5 ; 6 = s t a t i o n s ; 7 = c l i m a t i c b o u n d a r y ; 8 = a c t i v e ridges. D e t a i l e d m a p s c a n be f o u n d for C e n t r a l Pacific ( B o s t r S m , 1 9 7 3 ) , a n d C e n t r a l A t l a n t i c ( S c h i n k e t al., 1 9 7 4 ) .

O

b~

261 THE HISTORY OF SILICA ACCUMULATION BELTS DURING THE QUATERNARY PERIOD

From the previous discussion about recent silica accumulation in the World Ocean, we arrive at the conclusion that the main part of the silica is removed from the seawater by biological processes and is accumulated at the b o t t o m in three silica accumulation belts, the most important of which is the Southern Belt. While analyzing the history of silica accumulation the study of the "sensitive zones", i.e. the outer parts of each silica accumulation belt, is of special significance. Silica accumulation has never occurred in central parts of arid zones, and in the central parts of the zones of glacial marine sediments. Therefore, the study of the cores obtained in a meridional section is of great interest. By these analyses the movements of aforementioned belts in lateral direction can be established. Siliceous deposits of the high latitude belts n o w grade towards the poles into glacial marine sediments, and towards the equator into calcareous oozes or red clays. The boundaries are shifted towards the equator during the times of temperature decrease and towards the poles during the rise in temperature. The south silica accumulation belt

Figure 10 shows the principal scheme for the meridional movements of the south silica-accumulation belt during the late Quaternary. This is based 1150 km 40"S

AC

I 50"

[ 60"

"~/V)'/// d,'///.4e /// /Z2

o

Hotocene W u r m II

so Wurm I /

/

7O-8(

/

100

RissWurm

Riss ///////×/ o/ /o/ z e FOI r a m ira - ~ / //////,i////

~,~ Glacial :bmarlne I sed ments

120 150

MmdelRuss

17o

MmdeL 200

Gunz Mmdel 250 260

5unz 320

Fig.10. T h e m o v e m e n t s o f the s o u t h silica-accumulation belt during the Late Quaternary according to eight s e d i m e n t cores f r o m 40°S to 70°S (a--h).

262 on silica analyses in the cores of Soviet Antarctic expeditions (Lisitzin, 1966, 1978). As appears from the figure, shifts towards the S and N, indicating rise and fall in temperature, can be found. According to our observation in the Southern Hemisphere the width of the silica accumulation belt changes practically nothing with time. Its average width is 900--1200 miles between the isolines for 10% of amorphous SiO2 in the sediment. The northern b o u n d a r y of this belt corresponds to the Antarctic convergence (polar front), and also to the o u t e r line of the average iceberg expansion and to the line where the average annual temperature of the air and surface waters is + 10°C Thus, the b o u n d a r y has a great paleoceanological and paleoclimatological implication. This sediment b o u n d a r y does n o t exactly follow the parallel, b u t deviates in a number of places b y the influence of the underwater relief (absence of siliceous sediments at the rises). As appears from Fig.10 glacial marine sediment has continuously been accumulated off the Antarctic continent at least from the Gfinz--Mindel Interglacial. The b o u n d a r y b e t w e e n glacial marine sediments and siliceous sediments has migrated N--S with the climatic changes. The north silica accumulation belt

The northern belt of diatom oozes is divided in parts b y continental masses. A corresponding zone is rather weakly expressed in the Atlantic Ocean. The northern part of the Indian Ocean does n o t reach high latitudes and therefore this siliceous belt is only developed in the North Pacific Ocean, including the Far East Seas. The seas of the Far East (Bering Sea, Sea of Okhotsk, Japan Sea) are studied b y the Soviet marine geologists in detail. The lithological--stratigraphical investigations show, that the conditions here correspond to the conditions of the southern parts of the southern belt (section types a--g in Fig.10). In contrast to the conditions in the southern belt the ice age ice caps in the north at adjacent lands were abruptly reduced and even disappeared during interglacial time, while the continental Antarctic ice sheet was permanently close by. Some of the Alaskan glaciers, going o u t to the ocean shores, are the relics of continental glaciations. The nearness to sources of terrigenous materials give high rates of accumulation, much greater than in the southern belt. This factor makes the Far East seas especially important for the study of the silica accumulation history, because o f the high resolution of their st~atigraphies. Of great importance were transgressions and regressions during which the large shelves of these seas were principally exposed and thereby the supply of detrital material to the pelagic zone changed. Silica accumulation in the northern belt began after the last glaciation and a sharp change of conditions was noted. The upper horizon o f the siliceous sediments of the Bering Sea and the Sea of Okhotsk are siliceous with percentages of opaline silica at the surface up to 30--50%. There is an increasing content of ice-rafted material corresponding to the decreasing

263 temperature after the Holocene climatic o p t i m u m (Lisitzin, 1978). The b o u n d a r y in abyssal cores to the subjacent terrigenous (non-siliceous) clayey muds is distinct. The m u d contains numerous layers of volcanic ash (II-horizon, i.e. Wiirm with an interstadial). The amorphous silica c o n t e n t in the sediments of this m u d is no more t h a n 3--5%, even in the places, where the percentage in the surface sediment is 30--50. The layers of volcanic ash, usually black, but in some cores also brownish or white, have a thickness of up to 0.3- -0.5 m. The ash is widely spread in the abyssal part of the Bering Sea. This indicates a strong increase in activity of existing and extinct volcanos during the late Wiirmian times. The good preservation of the thick lava flows and volcanic cones at adjacent land indicates their comparatively y o u n g age. The third characteristic feature of the sediments of the II-layer is their content of stony material, observed by us practically in all the cores from the abyssal parts of the Bering Sea and the Sea of Okhotsk at the horizon from 4 to 5 m. At last, this layer has very low content of calcium carbonate. According to our diatom analysis there is a sharp increase of cold-water forms in the II-layer and an increase of freshwater diatoms as well. Also Tertiary diatoms or rock fragments with high diatom c o n t e n t occur. According to all indications this horizon corresponds to the last glaciation. The most important feature is t h a t the siliceous belt disappeared during the last glaciation in the Far East Seas and the northern part of the Pacific Ocean. The silica accumulation was depressed by the large a m o u n t of the ice-rafted detrital material. The siliceous belt was shifted S of these regions. This period coincides with a time o f increasing volcanic activity. Below the II-horizon of cores from the Far East Seas, a low-siliceous sedim e n t occurs, with a thickness of 2--7 m. These beds, the III-horizon (Riss-Wiirm), consist of gray clayey muds, with low contents of opaline silica but where several samples contain up to 10% of CaCO3. The last indicates a rise o f temperature. No intercalations of gravelly-pebbled material occur in the abyssal sediment and ash layers are almost absent. The IV-horizon (Riss) in cores from the abyssal part of the Bering Sea is characterized by a new drop in the c o n t e n t of silica. A great a m o u n t of volcanic ash layers appear in the terrigenous clayey muds. This was, as stated above, the same as in the II-horizon. The stony material in the abyssal sediments occurs only in the northern part of the Bering Sea. The c o n t e n t of calcium carbonate is very low. This (Rissian) glaciation in the Bering Sea was less intensive and prolonged compared with the Wiirmian (II-horizon) in the Bering Sea area. The V-horizon (Mindel--Riss) was not penetrated at all stations. The amorphous silica c o n t e n t of this bed is up to 10--15%. The CaCO3 content increases in the southern part of the sea to 10%. The thickness of this layer is 10--20 m, and the lower boundary of this V-horizon has n o t been reached y e t by I.P.O.D. This generalized stratigraphy can be visualized by an interesting core from the Sea of Okhotsk, located in the zone with amorphous

264 silica content between 30--40% (Bezrukov, 1960). This core is from the abyssal part of the Sea of Okhotsk (47°26'N, 150°16'E; depth = 3355 m; length of core = 2750 cm) and can be divided into five parts which differ both lithologically and regarding the diatom flora. Horizon I consists of clayey diatom-mud with up to 35% of amorphous silica. Oceanic types of diatoms dominate (layer 0--185 cm). Horizon I consists of clayey diatom-mud with up to 35% of amorphous silica content of a b o u t only 3.5%. Neritic diatoms dominate with large admixtures of sublittoral and freshwater species (layer 185--590 cm). The III-horizon, 590--1050 cm, is c o m p o s e d o f clayey and silty--clayey muds with silty intercalations. The amorphous silica content increases to 5%, and in some interbeds even to more than 10%. The sporadic diatoms are oceanic species. Thus, b o t h in the Bering Sea and the Sea of Okhotsk the glacial ages are recorded b y a sharp increase of the supply of terrigenous material and ashes. A coeval drop in the silica content occurs. A similar pattern (Bezrukov, 1960) is observed in the northern part of the Japan Sea as well. The m a x i m u m values of the absolute silica accumulation, 3--8 g/cm 2 × 103 yrs., were recorded in the peripheral parts of the abyssal basins. A still higher accumulation rate, 5 g/cm 2 X 103 yrs., is found in the Bay of Providence. For most of the shelf area of the Bering Sea a low silica accumulation, 1--2 g/cm 2 X 103 yrs., is typical and a much lower one for the older horizon. Absolute silica accumulation rates are several times larger for the late Wiirmian (11,000--10,000 B.P.), layer. During the Wiirm proper the silica accumulation was much lower than in the Holocene. The absolute silica accumulation rates in the abyssal basins were then 0.04--0.13 g/cm 2 X 103 yrs. This is 10--40 times less than in the Holocene. This can be explained b y the hard ice conditions of that period, when all the water area of the Bering Sea was covered with ice during the major part of the year and resembles the present C h u k o t s k o y e or the East Siberian Seas of the Arctic Ocean or the Weddell Sea in the Antarctic. The absolute silica accumulation rates in the period of the third layer deposition (Riss--Wiirm) were 2--6 times higher than in the Wiirm b u t 5--10 times less than in the Holocene stage. Evidently, despite the rise o f temperature, the ice conditions in the Bering Sea remained very severe even during that interglacial. The diatoms play the predominant role in the silica accumulation in the northern belt during the whole Quaternary. All other siliceous organisms had played rather a small role during the Quaternary. Other forms of amorphous silica supply (gels, volcanic silica and others) into the Far East Seas and the northern part of the Pacific, described b y other authors had no importance. These conclusions are based on detailed investigation of more than 1000 cores from the Bering Sea, the Sea of Okhotsk and the Japan Sea, and also from the northern part of the Pacific Ocean.

265 The northern silica belt has oscillated N and S during times. Maximal changes are observed at the latitudes about 50°N, just as in the southern hemisphere ( F i g . l l ) .

The equatorial silica accumulation belt The equatorial silica belt is distinguished clearly in the Pacific, it is weak in the Indian Ocean, and more weakly in the Atlantic. Radiolarians play a larger role here, in some places the d o m i n a n t role in the siliceous ooze. In the cores from the equatorial zone there are significant changes of the radiolarian content: horizons of pure radiolarian sediments alternate with fossilfree red abyssal clays. The radiolarian belt shifted towards the N during the glacial period (Fig.11). The radiolarian sediments represent the typical lowlatitude pelagic deposits (from 20°N to 29°S) at the depths exceeding the critical depths of calcium carbonates (>4700 m). Radiolarian ooze does not occur at other places in the Pacific. The diatom oozes in the equatorial zone can be divided into two varieties: Ethmodiscus ooze and tropical diatom-ooze w i t h o u t Ethmodiscus. There is also widely spread mixed radiolarian--diatom ooze. The equatorial diatom-ooze zone has sometimes during the Pleistocene expanded (during the glaciations) with a simultaneous increase in the diatom content. Sometimes the diatomic zone has narrowed. During the last periods the concentration of amorphous SiO2 decreased. Thus, the silica accumulation in the equatorial zone was pulsing during the Quaternary Period, just as the south siliceous belt: periods of comparatively intense silica accumulation were replaced by periods of sharp weakening, as it can be observed now (Fig.11). The dominating silica producers during the Quaternary Period have been the diatoms with radiolarians at the second place. Silicoflagellates and siliceous sponges never formed any pelagic siliceous sediments of their own. However, spicules are only of local importance as on the shelf. Thus, out of the four main modern siliceous organisms, diatoms and radiolarians are the most important and the diatoms have during the Quaternary accumulated a hundred times more silica than the radiolarians did. Most data of the silica accumulation rates for the first horizon are within the size of 0.01--0.001 g/cm 2 X 103 yrs. (or less), that is an order lower than the rates in the northern part of the Pacific. In some basins in the western part of the Pacific where Ethmodiscus oozes are spread, the silica accumulation rate sharply increases. Here one can meet rates of several grams and even more than 10 g/cm 2 X 103 yrs., which is typical for the mobile Ethmodiscus-sediments flowing down into depressions. Radiolarian remains are more frequent in the eastern part of the equatorial zone, and at some stations the amorphous silica c o n t e n t increases as well. The absolute silica accumulation rates for the second horizon (Wfirm) are higher at the equator than for the uppermost (Holocene) horizon.

300 400 500 600 700 800 900 1000 1100 1200

50 °

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Fig. 11. T h e a b s o l u t e m a s s e s o f b i o g e n i c silica in the Q u a t e r n a r y P e r i o d at a m e r i d i o n a l s e c t i o n a c r o s s t he P a cif ic O c e a n ( B o g d a n o v , :personal c o m m u n i c a t i o n w i t h th e a u t h o r ' s a l t e r n a t i o n s ) . A b s o l u t e opal m a s s e s in m g / c m = X 1 0 3 yrs.: 1 -- < 1; 2 -- 1 - - 5 ; 3 = 5 - - 1 0 , 4 =

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267 CONCLUSIONS

The changes in the diatom thanatocoenosis at a meridional section through the Pacific Ocean ( F i g . l l ) and also the changes of the absolute masses of silica accumulation at the section through the ocean together with the foraminiferal analyses and paleotemperature determinations allow us to conclude that the history of the northern silica accumulation belt was very close to the history of the southern belt. The changes were synchronous and caused by the general climatic changes, connected with the glaciation. The silica accumulation change during the late Quaternary was not uniform. In some zones of the ocean the glaciation causes a sharp decrease and even the stop of the silica accumulation (the Bering Sea, the Sea of Okhotsk, the northern parts of the Pacific Ocean), and in the other areas, caused a significant increase of it (equatorial zone), and in the third area it caused only a shift in the meridional direction of the belt (the southern belt). Therefore we cannot speak about a total increase or decrease during the fall or rise of temperature in the past. It is necessary to find out the connections between the concrete climatic and the physical-geographical zones. REFERENCES Alekin, O.A., 1966. Chemistry of the Ocean. Gidrometeoizdat, Leningrad (in Russian), 245 pp. Alekin, O.A., 1979. Chemical Composition of the River Runoff Dissolved Matters. Nauka, Moscow, pp. 51--56 (in Russian). Aumento, F., Mitchell, W. S. and Fratta, M., 1979. Interaction between sea water and oceanic layer two as a function of time and depth. 1 -- Field evidence. Can. Mineral., 14: 269--290. Barash, M. S., 1974. Migrations of climatic belts of the Atlantic ocean during the late Quaternary. Dokl. Acad. Sci., U.S.S.R., 216(5): 1158--1160 (in Russian). Barash, M.S., 1984. Quaternary paleooceanology of the Atlantic ocean. Thesis. Inst. Oceanol., Acad. Sci., Moscow, 40 pp. (in Russian). Berger, W. H., 1968. Radiolarian skeletons solution at depth. Science, 159: 1237--1238. Bezrukov, P. L., 1960. Bottom sediments of the Sea of Okhotsk. Inst. Oceanol., Acad. Sci., Moscow, 3 2 : 1 5 - - 9 5 (in Russian). BostrSm, K., 1973. The origin and fate of ferromanganoan active ridge sediments. Acta Univ. Stockh. Contrib. Geol., 27(2): 149--273. Boyle, E. A., Edmond, J. M. and Sholkovitz, E. R., 1977. The mechanism of iron removal in estuaries. Geochim. Cosmochim. Acta., 41(9): 1313--1324. Broecker, W. S., 1982. Glacial to interglacial changes in ocean chemistry. Progr. Oceanol., 11: 151--197. Bruevich, S. V., 1953. Geochemistry of silicon in the sea. Izv. Acad. Sci. U.S.S.R., Ser. Geol., 4 : 6 7 - - 7 9 (in Russian). Bruland, K. W., Knauer, G. A. and Martin, J. M., 1978. Zinc in north-east Pacific water. Nature, 271: 5647. Burton, I. D., Letherland, T. M. and Liss, P. S., 1970. Reactivity of dissolved silicon in some natural waters. Limnol. Oceanogr., 15(3): 473--476. Calvert, S. E., 1966. Accumulation of diatomaceous silica in the sediments of the Gulf of California. Geol. Soc. Am. Bull., 77: 569--596. Collier, R. W. and Dymond, J., 1980. Sediment trap experimentation of the Galapagos spreading center, Equatorial Pacific. Science, 209: 801--802.

268 Corliss, S.B., Dymond, J., Gordon, L.I., Edmond, J.M., Von Herzen, R.P., Ballard, R.D., Green, K., Williams, D., Bainbridge, A. and Crane, K., 1979. Submarine thermal. springs on the Galapagos Rift. Science, 203: 1073--1083. Craig, H., Clark, W.B. and Beg, M. A., 1975. Excess He 3 in deep water on the East Pacific Rise. Earth Planet. Sci. Lett., 26(2): 125--132. DeMaster, D.J., 1981. The supply and accumulation of silica in marine environment. Geochim. Cosmochim. Acta, 45: 1715--1732. Edmond, J. M., 1974. On the dissolution of carbonate and silicate in the deep ocean. Deep-Sea Res., 21: 455--480. Edmond, J. M., Measures, C., McDuff, R. E., Chan, L. H., Collier, R., Grant, B., Gordon, C. I. and Corliss, J. B., 1979. Ridge crest hydrothermal activity and the balances of major and minor elements in the ocean: the Galapagos data. Earth Planet. Sci. Lett., 46(1): 1--18. Eggiman, D.W., Manheim, E . T . and Betzer, P. K., 1979. Dissolution and analysis of amorphous silica in marine sediments. J. Sediment. Petrol., 50(1): 215--225. Emelyanov, E.M., 1973. Mud distribution and composition of the southwest Africa shelf. Inst. Oceanol., Acad. Sci., Moscow, 95 : 211--238 (in Russian). Emelyanov, E. M., Lisitzin, A. P. and Ilyin, A. B., 1976. The types of b o t t o m sediments of the Atlantic ocean. Pravda, Kaliningrad, 578 pp. (in Russian). Fanning, K. A. and Prison, M. E. Q., 1973. The lack of inorganic removal of dissolved silica during river-ocean mixing. Geochim. Cosmochim. Acta, 37: 2405--2415. Gorbunova, Z.N. and Lisitzin, A.P., 1979. Clayey minerals. In: P.L. Bezrukov (Editor), Sedimentation and Magnetism of the Ocean. Nauka, Moscow, pp. 214--230 (in Russian). Hart, R., 1973. A model for chemical exchange in the basalt--seawater system of oceanic layer II. Can. J. Earth Sci., 10: 799--816. Hays, J. D., 1970. Stratigraphy and evolutionary trends of Radiolaria in North Pacific deep sea sediments. Geol. Soc. Am. Mere., 126: 185--218. Hinga, K. K., Sieburth, J. and Heeth, Q. R., 1979. The supply and use of organic material at the deep-sca floor. J. Magn. Reson., 37: 557--579. Holeman, J.N., 1968. Sediment yield of major rivers of the world. Water Resour. Res., 4: 737--747. Honjo, S., 1978. Sedimentation o f materials in the Sargasso Sea of a 5367 m deep station. J. Mar. Res., 36: 469--492. Honjo, S., 1980. Material fluxes and modes of sedimentation in the mesopelagic and bathypelagic zones. J. Mar. Res., 38: 53--97. Honjo, S. and Erez, J., 1978. Dissolution rates of calcium carbonate in the deep ocean: an in-situ experiment in the North Atlantic. Earth Planet. Sci. Lett., 40: 287--300. Honjo, S. and Roman, M. R., 1978. Marine copepod fecal pellets: production, preservation and sedimentation. J. Mar. Res., 36(1): 45--47. Hurd, D. C., 1971. Factors affecting solution rate of biogenic opal in seawater. Earth Planet. Sci. Lett., 15: 411--417. Hurd, D.C., 1973. Interactions of biogenic opal, sediment and seawater in the Central Equatorial Pacific. Geochim. Cosmochim. Acta, 37: 2257--2282. Hurd, D. C. and Birdwhistell, S., 1983. On producing a more general model for biogenic silica dissolution. Am. J. Sci., 283: 1--28. Jenkins, W. J., Edmond, J. M. and Corliss, I. B., 1978. Excess He 3 and He 4 in Galapagos submarine hydrothermal waters. Nature, 272 : 156--158. Jouze, A. P., 1972. Diatoms in the surface sediment layer of the Chilian--Peruvian region of the Pacific Ocean. Oceanology, XII: 831--842 (in Russian). Jouze, A.P., Mukhina, V. V. and Kozlova, O. G., 1969. Diatoms and silicoflagellates in the surface layer of sediments. In: P.L. Bezrukov (Editor), The Pacific Ocean, VIII. Nauka, Moscow, pp. 7--48 (in Russian). Kozlova, O. G., 1964. Diatom Algae in the Indian and Pacific Sectors of the Antarctic. Acad. Sci., Moscow, 168 pp. (in Russian).

269 Kozlova, O. G., 1968. Diatom algae in the suspended matter from the western part of the Bering Sea. Oceanol. Res. Attic., 1 8 : 1 9 3 - - 2 0 3 (in Russian). Kozlova, O. G. and Mukhina, V. V., 1966. Diatoms and silicoflagellates in the suspension and in the bottom sediments of the Pacific Ocean. In: N.M. Strakhov (Editor), Geochemistry of Silica. Nauka, Moscow, pp. 192--219 (in Russian). Kruglikova, S.B., 1969. Radiolarians in the surface layer of sediments. In: P.L. Bezrukov (Editor), The Pacific Ocean, VIII. Nauka, Moscow, pp. 48--73 (in Russian). Kruglikova, S.B., 1975. Radiolarians in the surface sediments of the Sea of Okhotsk. Oceanology, XV: 116--123 (in Russian). Lisitzin, A.P., 1964. Distribution and chemical composition of suspended matters in waters of the Indian Ocean. Resultat. Issled. MGG (IGY Res. Data) Ser. Oceanol., 4(10): 135 pp. (in Russian). Lisitzin, A.P., 1966. Main regularities in the distribution of recent siliceous sediments and their relations with climatic zonality. In: N. M. Strakhov (Editor), Geochemistry of Silica. Nauka, Moscow, pp. 90--192 (in Russian). Lisitzin, A.P., 1972. Sedimentation in the World Ocean. Banta Press, Tulsa, Okla., 218 pp. Lisitzin, A. P., 1974. Ocean's Sedimentation. Nauka, Moscow, 435 pp. (in Russian). Lisitzin, A.P., 1977. Terrigenous sedimentation, climatic zonality and correlation of terrigenous and biogenic sedimentation in the oceans. Lithol. miner, deposits, 6: 3--22 (in Russian). Lisitzin, A. P., 1978. The Processes of Ocean Sedimentation. Nauka, Moscow, 390 pp. (in Russian). Lisitzin, A. P., 1983. Flows of material and energy in the ocean and their biogeochemical significance. In: A. S. Monin and A. P. Lisitzin (Editors), Biochemistry of the Ocean. Nauka, Moscow, pp. 201--274 (in Russian). Lisitzin, A. P. and Gorbunova, Z. N., 1983. Clayey minerals and climatic zonality. In: D.E. Gerchanovich (Editor), Climatic Zonality and Sedimentation. Nauka, Moscow, pp. 160--171 (in Russian). Lisitzin, A. P. and Gordeev, V. V., 1974. About chemical composition of suspension and water of rivers and oceans. Lithol. Miner. Deposits, 3 : 38--58. Lisitzina, N.A., 1975. The supply of sedimentary material into the ocean from the weathering crusts of various climatic zones. In: A.V. Peive (Editor), Problems of Lithology and Geochemistry of Sedimentary Rocks and Ores. Nauka, Moscow, pp. 67--85 (in Russian). Lisitzina, N. A. and Butuzova, G. Y., 1981. Zonality of the diagenetic mineral formation in the World Ocean sediments. In: P.P. Timofeev and V.N. Cholodov (Editors), Lithology at the New Stage of Geological Knowledge Development. Nauka, Moscow, pp. 254--268 (in Russian). Livingstone, D. A., 1963. Data of geochemistry. In: Chemical Composition of Rivers and Lakes, 6th ed. U.S. Geol. Surv. Prof. Pap., 4 4 0 , 4 4 0 pp. Lopatin, G. V., 1950. Erozia y stok nanosov. Priroda, 7. Lupton, J. E., Weiss, R. F. and Craig, H., 1977. Mantle helium in hydrothermal plumes in the Galapagos Rift. Nature, 267 : 603--604. Mann, K. H., 1972. Macrophyte production and detritus food chains in coastal waters. Mem. Inst. Ital. Microbiol., 29: 353--383. Martin, J.M. and Meybeck, M., 1979. Elemental mass-balance of material carried by major world rivers. Mar. Chem., 7 : 173--206. Milliman, J. and Meade, R. H., 1983. World-wide delivery of river sediments to the oceans. J. Geol., 91(1): 1--21. Murdmaa, I. O., Lisitzina, N. A., Butuzova, G. Y. and Lisitzin, A. P., 1979. Authigenous minerals. In: P.L. Bezrukov (Editor), Sedimentation and Magmatism of the Ocean. Nauka, Moscow, pp. 231--250 (in Russian). Nelson, D. M. and Gordon, L. I., 1982. Production and pelagic dissolution of biogenic silica in the Southern Ocean. Geochim. Cosmochim. Acta, 46: 491--501.

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