Provenance and accumulation rates of opaline silica, Al, Ti, Fe, Mn, Cu, Ni and Co in Pacific pelagic sediments

Chemical Geology, 11 (1973) 1 2 3 - 1 4 8 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

PROVENANCE AND ACCUMULATION RATES OF OPALINE SILICA, AI, Ti, Fe, Mn, Cu, Ni AND Co IN PACIFIC PELAGIC SEDIMENTS*

K. BOSq?ROM, T. KRAEMER and S. GARTNER Rosenstiel School of Marine and Atmospheric Science, University of Miami, MiamL Fla. (U.S.A.) (Accepted for publication January 8, 1973)

ABSTRACT Bostr6m, K., Kraemer, T. and Gartner, S., 1973. Provenance and accumulation rates of opaline silica, A1, Ti, Fe, Mn, Cu, Ni and Co in Pacific pelagic sediments. Chem. Geol., 11:123-149. Accumulation rates and chemical compositions have been obtained for pelagic sediments for 73 locations in the Pacific and for 11 in the Indian Ocean. The data for the Pacific show that many elements accumulate rapidly close to the continents and slowly in the central part of the ocean. This pattern is interrupted by two major zones of relatively high accumulation rates, one along the Equator and one along the East Pacific Rise. Deposition of opaline silica is almost completely restricted to areas of known high biological productivities at the Equator and at very high latitudes. Cu and Ni show stronger tendencies than Fe and Mn to precipitate with opaline silica. The highest accumulation rates of Fe and Mn in the open Pacific occur along the East Pacific Rise, to some extent also Cu and Ni are enriched there due to volcanic processes. A1 and Ti show high accumulation rates only close to the continents; these elements appear to be almost completely terrigenous. Provenance studies of minerogen fractions, using the relations between Fe, Ti, A1 and Mn, show that significant quantities of basaltic matter (oceanic crust) are incorporated into the sediments only ~in areas of very low total sedimentation rates in the vicinity of oceanic island groups such as Polynesia and Hawaii, whereas hydrothermal processes act as a major sediment source only on the East Pacific Rise. Sediments in the north as well as in the southernmost part of the Pacific are nearly entirely terrigenous. A balance estimate of the minerogen fraction of the deep-sea sediments from the open Pacific suggests that between 75 and 95% of all sediments in the Pacific are terrigenous and that submarine weathering (and release of basaltic debris in general) and submarine exhalations each account for only a small fraction of the sediments. INTRODUCTION T e r r i g e n o u s a n d b i o g e n o u s m a t t e r as well as l i t h o g e n o u s c o n s t i t u e n t s derived f r o m basaltic debris, a n d a u t h i g e n i c c o n s t i t u e n t s f o r m e d f r o m s e a w a t e r o c c u r a b u n d a n t l y in t h e s e d i m e n t s o f t h e oceans. Less clear is t h e role p l a y e d b y s e d i m e n t s derived f r o m volcanic e m a n a t i o n s o f s o m e f o r m . S u c h v o l c a n i c d e p o s i t s p r o b a b l y f o r m r a p i d l y a n d t h e r e f o r e m u s t b e o f local origin. T h e y are f o u n d e x t e n s i v e l y o n s p r e a d i n g ridges ( B o s t r 6 m et al., 1 9 6 9 , 1 9 7 2 a , b ; B o s t r 6 m , 1 9 7 0 ; B e n d e r et al., 1971). It is u n c l e a r , h o w e v e r , w h a t *Contribution No.1604 from the Rosenstiel School of Marine and Atmospheric Science, University of Miami, Fla. 33149, U.S.A.

124

K. BOSTROM T. KRAEMERAND S. GARTNER

elements besides iron and manganese are of local origin. A large number of chemical analyses have been made of Indo-Pacific sediments (Bostr6m et al., 1972b); sediments in the Atlantic are more terrigenous and are therefore less suited for the present discussion. The results (op. cit.) clearly suggest that Fe, Mri, V, As, B, Cd, Hg, Ba, P and U are to varying extents derived from submarine volcanism and that A1 and Ti are almost exclusively terrigenous. For some elements like Cu, Co, Ni, Sc, Zr, Y and La it is unclear to what extent they derive from submarine volcanism. To illuminate the behavior and the source regions of these elements better, their accumulation rates have to be better known than they are at present. Ideally this would require that we have available a large number of sediment cores that are both well dated and well analyzed. This is not the case and is not likely to happen soon in view of the work required. However, enough determinations of the sedimentation rates and the geochemistryof Indo-Pacific sediments have now been made to permit a preliminary series of accumulation rate maps to be drawn. In spite of some uncertainties involved we believe such maps have merit as early guides for directing future research. ACCUMULATIONRATES OF SEDIMENTS Several studies of sedimentation rates have been made during the last ten years. The results of such studies vary considerably. Heath et al. (1970) found that deposition rates could vary with a factor of ten over a short distance; likewise many cores sampled by Parker (1967) show sedimentation rates appreciably different from rates determined by Ku et al. (1968). Such differences may be biased, however, because the purpose of the study may dictate whether attenuated or expanded sedimentary sections are chosen. Thus, for instance, biostratigraphic large scale variations may be best investigated on cores with compressed stratigraphies (= low sedimentation rate) whereas a detailed search for geomagnetic variations may require cores with relatively expanded yet undisturbed stratigraphies (= high sedimentation rate)..Areas with zero deposition rates may be found anywhere in the ocean, even close to the continents. For certain areas it is difficult to construct a meaningful average in sedimentation rate at the present time because this would require a very detailed bottom physiographic and sediment chronological study. Alternatively it is possible to determine for various regions a probable maximum rate of undisturbed sedimentation, that is, sedimentation not due to turbidity current or slumping. This procedure is followed here. Effects due to winnowing and laterai migration along the ocean floor are not excluded by this procedure. Such processes are at present difficult to rule out even in areas where the sedimentary record may appear undisturbed. For that reason they must be well understood and kept in mind when accumulation rates are discussed. Winnowing processes are selective, that is, finer particles are winnowed downslope to a larger extent than coarse matter. Carbonate particles (-- foram tests, etc.) may well settle where clay-size matter is removed. This is well illustrated by sediments on top of seamounts which may be very sandy with little lutite-size sediment, whereas at the foot of the seamount much fine

CHEMICALCOMPOSITIONIN PELAGIC SEDIMENTS

125

sediment may be present. On laterally extensive elevations such as the Magellan Rise, winnowing mostly seems to occur close to the outer slopes, while the sediments on the central part of the plateau are less affected (Winterer et al., 1971b). Although winnowing is an ocean-wide phenomenon it probably tends to bias the data in some areas more than in others; currents are more apt to effect sedimentary processes in narrow passages or over shallow extended ridges than in deep basins. For this reason the sedimentation rates for some sediments capping abyssal hills on the East Pacific Rise must be considered as minimum values (Bostr6m et al., 1973a). Our accumulation-rate data derive from many sources. The data all represent deposition rates for Quaternary deposits; for older deposits too few data points are available at present to draw a synoptic map. Several mutually consistent values stem from Ku et al. (1968), Bender et al. (1970) and the Initial Reports of the Deep-Sea Drilling Project (DSDP) (McManus et al., 1970; Fischer et al., 1971 ; Winterer et al., 1971a; Tracey et al., 197l ; Hays et al., 1972). Some anomalous DSDP points that are obviously due to tt~rbidite deposition have been excluded. Several long cores have been dated in our laboratory (S.G.); 4 from the Pacific, 3 from the Antarctic Ocean south of Australia and 6 from the Indian Ocean. We have used an age of 350,000 years BP for the Pseudoemiliania lacunosa extinction, which gives a good fit to the chronology by Broecker and Ku (1969) for the Caribbean. This estimate gives a conservative value; data by Emiliani (1966) and Emiliani et al. (1969) would yield higher accumulation rates. The deposition rates obtained in the 4 Pacific cores agree well with deposition rates for neighboring cores as determined by Blackman (1966), Bender et al. (1971 ), Kraemer ( 1971 ). The glacial-interglacial sequences for many equatorial cores were determined by Arrhenius (1952) and Hays et al. (1969) by various methods. Comparison of their results with corresponding glacial-interglacial sequences in the Caribbean using the oxygen isotope curves by Emiliani (1966) and the chronology by Broecker and Ku (1969) yield accumulation rates that agree well with rates from other dated cores in this region of the Pacific. We consider the data coverage reasonably good for the north and equatorial Pacific and the high rates over the East Pacific Rise at about 15°S appear to be well substantiated (see for instance Bostr6m et al., 1973a). For the Antarctic Ocean, however, we would prefer to have more data. Our data for the Indian Ocean are insufficient because more complicated bottom physiography of the Indian Ocean necessitates very closely spaced data points which must be spaced in an informative w~y around various spreading or dead ridges, at various distance from land and within and outside of the high productivity areas. Also, chemical analyses must be available for those cores. Although the data are sparse they suggest that the Indian Ocean in many ways may well resemble the Pacific. In spite of the many difficulties discussed above, certain trends in the Quaternary accumulation rates (on a carbonate free basis) are apparent (see Fig. 1). Thus, sedimentation rates are high close to the continents and become progressively smaller towards the central parts of the Pacific. This general pattern is interrupted by two bands of moderately high accumulation rates; one under the Equator and another one on the East Pacific Rise.

126 ;

K. BOSTROM, T. KRAEMER AND S. GARTNER

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-

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+ ?

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~

~

)~-

>5

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Fig. 1. Accumulation rates of the carbonate-free fraction in pelagic sediments in the Indo-Pacific area. Heavy black line denotes the approximative location of the spreading Indo-Pacific Ridge system. Crosshatched thin lines represent inactive ridges (as the 90°E- and Nazca ridges), or ridges where the length of spreading axis is small compared to that of fracture zones (as the Chile and the southwest Indian Ocean ridges). Dashed lines show major fracture zone areas (not all indicated). Solid dots represent data points that have been used for the accumulation rate maps in Fig.3, 6, 8, 9 and 12; crosses represent data points for which no chemical analyses were available. In addition, to sources quoted in the text this figure is based on data from Romankevich et al. (1964), GoodeU et al. (1968), Holmes et al. (1968), Gartner (1969), and Glass (1969).

GEOCHEMICAL DATA The Indo-Pacific pelagic sediments have been extensively studied during the last 15 years; a compilation of data has been presented elsewhere (Bostr6m et al., 1972b). Enough data now exist for surface sediments so that synoptic geochemical maps can be made (op. cit.). Such maps, however, are not isochronous and this implies a problem for our accumulation rate determinations for various constituents. Older sediments are frequently exposed in areas of very low deposition rates; often they tend to be somewhat richer in iron and manganese and poorer in carbonate content as compared with quaternary sediments (Goldberg and Arrhenius, 1958; Arrhenius, 1963; Bostr6m et al., 1972a,b). This means that not all analyses of surface sediments from such areas represent quaternary deposits for which the deposition rates are determined. We have not been able to fully circumvent this problem but some factors offset the errors introduced by these difficulties: (1) most data points involve an average of several, in some cases 1 0 - 2 0 , different analyses, increasing the chance that a fair fraction of quaternary sediments are represented in this average; (2) only when there are large ranges in age do the compositions tend to vary

CHEMICALCOMPOSITIONIN PELAGICSEDIMENTS

127

considerably (Bostr6m et al., 1972a,b); such strong chemical variations between closely spaced surface samples are unusual among the many hundred analyses we have compiled (Bostr6m et al., 1972b); (3) the absolute accumulation rates in the areas under discussion are usually so exceptionally low that the accumulation rates for almost all constituents appear as striking minima in our maps; the only uncertainty left by our procedure is how small the accumulation rates really are in these areas, not that they are areas of minimal accumulation rates. To obtain the accumulation rates the following procedure and assumptions have been used. The in situ, density of dry, urrcompressed surface sediment is usually about 0.75 g/cm3: with depth this density usually increases to close to 1.0. This is indicated by several physical properties studies reported in the Initial Reports of the Deep-Sea Drilling Project (see, for instance, vol.III-IX). Similar estimates were used by Turekian (1965), Bender et al. (1970) and Kraemer (1971); their values range between 0.70--0.75 g/cm 3. This means that an accumulation rate of 10 mm carbonate-free fraction per 1,000 years is equivalent to an accumulation of 750 mg/cm 2 per 1,000 years of this component. The uncertainty in the density means an error less than + 20% in the accumulation rates, judging from our own studies of the cores from DSDP leg III (see Bostr6m et al., 1972a). Whenever possible, we have. used the geochemical data for the same core oll which the rate of deposition was made; this is the case for 8 long cores on the East Pacific Rise, 3 long cores on the Southeast Indian Ridge, 6 long cores in the Indian Ocean and some other cores distributed in the Pacific; for several widely distributed dated cores we also had analyses of the top section. For more than half the'data points, however, an averaging procedure was used in which analyses for cores less than 200 nautical miles from the dated core were applied; usually this involved many analyses which were then averaged and in 9 cases well-analyzed but undated long cores were used. A consequence of this procedure has been that many published sedimentation rates have not been used since no analyses existed for nearby sediments, these "unused" data points are given as crosses in Fig. 1. The correctness of the averaging procedure described here is indicated by the remarkably good agreement between accumulation rates derived by this procedure and by exacter methods involving complete studies on individual cores, several of them reported by other authors (Heath et al., 1970; Bender et al., 1971 ; Kraemer, 1971 ). RESULTS A N D DISCUSSION

We have calculated the accumulation rates for opaline silica, A1, Fe, Mn, Ti, Cu, Co and Ni. The accumulation rates for opaline silica were determined using the assumption that any silica exceeding alumina with a factor of 3 would be biogenous. This assumption is based on the fact that SIO2/A1203 equals about 3 in the average solid crust. This procedure does not produce very accurate rates of accumulation for opaline silica in areas of low biological productivity, but in such areas opaline silica is of little consequence anyhow and the errors in the estimate thus mean very little; of more interest is that the rates

128

K. BOSTROM, T. KRAEMER AND S. GARTNER

Fig.2. Distribution of SiO2 in pelagic sediments (on a carbonate, salt, and organic matter free basis). 0PAL*NESLCA Acc. rate

/

30°N

~ •.

mg/cmz 1000 yrs

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60 ° S

160°E

i

160 °

120 °

Fig.3. Accumulation rates for opaline silica in Pacific pelagic sediments.

800W

CHEMICAL COMPOSITIONIN PELAGIC SEDIMENTS

129

obtained from high productivity regions are likely to be reasonable. Interestingly enough this procedure is little affected by the eolian deposition of quartz in, for instance, the Northeast Pacific. The explanation for this is that eolian material is very well mixed, having an Si/A1 ratio very close to continental crustal abundances. Opaline silica shows a pronounced latitudinal pattern as would be expected (see Fig.2 and 3). In mid-latitudinal areas less than 5 mg/cm 2 per 1,000 years of opaline silica is accumulating whereas along the Equator and at high latitudes rates much greater than 150 mg/cm 2 per 1,000 years are common. This is in full agreement with previous studies, such as Murray and Renard (1898), Phillipi (1910), Lisitsin (1966), and Nigrini (1967). G.O.S. Arrhenius and E. Bonatti (in Arrhenius, 1.966) found by X-ray diffraction that in equatorial regions opaline silica constitutes almost 100% of many samples (on a carbonatefree basis). Our own X-ray studies of some selected samples confirm this observation. Of

\ ,.o

%

Fig.4. Distribution of A1 in pelagic sediments on a minerogen basis (= MB). Note that very low A1 values only are found on ridges with well-developedspreading. Ti has a similar distribution pattern. interest is that opaline silica shows no maximum on the mid-latitudinal part of the East Pacific Rise contrary to some other elements. The agreement with known high productivity regions as indicated by Fleming (1957) and by Bogorov (1967) is excellent. The data for the Indian Ocean also suggest a high productivity region at about 0°-20°S, again in excellent agreement with data in Bogorov (1967) and Nigrini (1967). The distribution patterns for aluminum and titanium are very similar (see Fig.4), the sediments in the central parts of the oceans in general having low concentrations of these elements. The lowest values are without any regional exception found on the spreading oceanic ridges as discussed elsewhere (Bostr6m et al., 1969). The A1/Ti ratio, although it varies within restricted limits, shows some very distinct latitudinal variations (see Fig.5).

130

K. BOSTROM, T. KRAEMER AND S. GARTNER

Such patterns, on the whole, can be explained only by surface controlled processes such as winds and currents. The AI/Ti ratio also indicates basaltic contributions in areas of low sedimentation rates as will be discussed below.

Fig.5. The relation AI/Ti in pelagic sediments. Note low ratios close to oceanic islands and seamount chains in areas of low sedimentation rates. The accumulation rates for AI (Fig.6) show a very pronounced minimum on the East Pacific Rise; the distribution of accumulation rates for Ti (not shown here) is very similar. There is a weak tendency for the accumulation rates to reach a maximum along the equator; possibly this is due to very far reaching transport of terrigenous matter by surface currents. Iron and manganese show similar distributions and accumulation patterns ( F i g . 7 - 9 ) . Close to (within 1,500 km of) the continents much iron arrives in detrital matter, which produces similar accumulation rate patterns for A1 and Fe in these areas, but in the open sea the accumulation rates of Fe covary more with those of Mn. The very pronounced maxima on the East Pacific Rise have been noticed earlier (Bostr6m, 1970; Bender et al., 1971 ; Kraemer, 1971). It is of interest that a lesser maximum occurs along the equator as well for both Fe and Mn. This will be discussed in more detail below. Copper, nickel and cobalt show several similarities in their distribution patterns (Bostr6m et al., 1972b) as well as in their accumulation rates. We are therefore mainly showing the data for copper ( F i g . 1 0 - 1 2 ) . These elements show a tendency to accumulate rapidly along the East Pacific Rise and 'along the Equator but are less strongly tied to the Rise than Fe and Mn and more strongly tied to the Equator region than the latter two elements. Cobalt and nickel also tend to be

CHEMICALCOMPOSITION IN PELAGIC SEDIMENTS

131 q'~'~R,.--At

Acc. rate

mg/cmJlOOOyrs

<' L--ZJ 2-5

l

IIIIIIIII II I II i

~,~

o

%

30* ~ ~

600S - - - - - - - - - -

160*E 160" 120* 80*W Fig.6. Accumulation rates for AI in the Pacific. Note tendency to form a local maximum along the Equator. /,

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,

~ iv

/l~

vt

,

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/

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Fig.7. Distribution of Fe (MB) in pelagic sediment; compare also with Fig.4.

>25

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132

K. BOSTROM, T. KRAEMER AND S. GARTNER

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~ mg/cm'1OOOyr, ~-

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10-30

30°N

0°

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160='E

160"

120=

80*W

Fig.8. Accumulation rates for Fe in the Pacific.

concentrated more in sediments on the flanks of the East Pacific Rise than copper (compare Fig. 10 and 11).

Provenance of the minerogen fractions in Pacific sediments The relationships Fe/Ti and All(A1 + Fe + Mn) have been used to detect whether the minerogen fraction of a given sediment contains much continental or oceanic detritus, or whether it is largely of submarine exhalative origin (Bostr6m, 1970; Bostr6m et al., 1972a). We have here further refined this procedure (op. cit.). Using average values for Fe, Ti, A1 and Mn in continental crust, oceanic crust and active ridge sediments we have derived a nomogram, showing the relations between Fe/Ti and A1/(A1 + Fe + Mn) in any given mixture of these source constituents. The same chemical data which were used to derive the accumulation rates were used also to deduce the provenance of the sediments. The results are shown in Fig.13. The North Pacific, except for an area around Hawaii, is totally dominated by terrigenous sediments; the same is true for the southernmost Pacific. A large

CHEMICAL COMPOSITION IN PELAGIC SEDIMENTS

13 3

Mn Accorate

mg/cm~1000 yrs

<1~ 1-3~ 3"10 10-30 >30

30=N

O*

30 °

60°S

160°E

160°

1200

Fig.9. Accumulation rates for Mn in the Pacific. Compare with Fig.8.

30*N

Fig. 10. Distribution pattern of Cu (MB) in pelagic sediments.

800W

134

K. BOSTROM, T. KRAEMER AND S. GARTNER

q

Co(MB)inppm > ~ 0 0 ~

r

Fig.11. Distribution pattern of Co (MB) in pelagic sediments. Corresponding pattern for Ni is very similar, but with some tendency to approach that of Cu in Fig. 10.

Cu Acc. rate mg/cm=l 000 000yrs < 25

I

I

50-100 ,00-200

30"N

~k\\

> 200

0*

30 =

60"S

160"E 160 ° 120" 80*W Fig.12. Accumulation rates for Cu in the Pacific. Corresponding patterns for Ni and Co are similar, except that the high values over the East Pacific Rise are less well developed than for Cu.

CHEMICAL COMPOSITIONIN PELAGIC SEDIMENTS

135

f

PROVENANCE Active ridge Soklrce

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Fig,. 13. Provenance relations in Pacific pelagic sediments. Areas without any shading are 9 0 - 1 0 0 % terrigenous.

region between 5°N and 45°S in the Pacific is influenced by basaltic and hydrothermal sources. (The concepts basaltic and hydrothermal sources as used here refer to essentially particulate and solid contrary to essentially dissolved volcanic products. Chemically the differences could be slight, but as shown here it is indeed pronounced.) In areas of fairly low rate of sedimentation and usually close to various oceanic islands, such as Hawaii, Polynesia, or Sala y Gomez, basaltic matter is'abundantly found. Several submarine volcanoes may also have contributed directly to the sediments; it is interesting to note (Fig.5) that the major A1/Ti minimum at about 25°S in the Pacific coincides with a chain of seamounts running westward from the southwest end of the Nazca Ridge over to 120°W and then in a west-northwest direction parallel to the Society Islands. Submarine exhalations are a major source of sedimentary matter along the East Pacific Rise where up to 100% of the sediment may be accounted for by this source, and where the 80-60% region is quite extensive as Fig.13 and Table I show. In Table I is given an areal breakdown of the relative proportions of terrigenous, basaltic and exhalative constituents in Pacific

136

K. BOSTR(iM, T. KRAEMER AND S. GARTNER

deep-sea sediments. (It should be noted here that these exhalations obviously could be of basaltic origin; however, the amount of required leaching of basalt to form these sediments is considerable. The origin of these emanations is further discussed in Bostr6m, 1973.) In Table I (footnote) is presented an estimate of the provenance relations for the total open Pacific (that is, for areas more than about 1,000 km from nearest major land mass). Alternative I implies a continuous sediment cover, while alternative II requires that most sediments in areas with basaltic and exhalative contributions only cover 10% of total bottom surface. Since sediment cover even on the East Pacific Rise appears to be more extensive than that, it is likely that the best estimate is somewhere in between these two extreme alternatives. Since the Pacific has a relatively high proportion of low accumulation areas and active ridges one would expect that in the World Ocean as a whole basaltic and exhalative matter play an even lesser role than is suggested in alternative II (Table I). It should be kept in mind that these provenance relations only pertain to AI, Ti, Fe and, to a lesser degree, to Mn, which have been used to derive the data in Fig.13 and Table I; for opaline silica, Cu, Ni and Co the provenance relations are less obvious and will be discussed more below. Manganese is somewhat ambiguous because of its mobility and because it is not a major constituent in continental or solid oceanic crust. These results are in good agreement with those of Goldberg et al. (1958) and E1 Wakeel et al. (1961) but contrast with some findings by Murray and Renard (1898). The latter authors were probably mislead by the erroneously high Fe/Al relations reported in their work. According to the patterns in Fig.3, 6, 8, 9 and 12 a certain fraction of Fe, Mn, and Cu (as well as Ni and Co) is associated with the high deposition rate area under the equator. A minor portion of these constituents probably is linked to the lutite fraction judging from the similarityof their accumulation rates with that of Al (see Fig.6). Even more pronounced is the resemblance of the above patterns with the opaline silica pattern. The main problem to be solved is whether this resemblance is due to biological deposition of Fe, Mn, Cu, Ni and Co with opaline silica and other organic debris, or whether it is associated with post depositional adsorption of these constituents on already settled opaline silica tests. The relations on a carbonate-free basis between Fe, Mn, Cu and opaline silica as shown in Fig.14 and 15 suggest that opaline silica accounts for some deposition only of Fe and Mn, but possibly accounts for somewhat more Cu. Thus the correlations F e - M n and F e - C u are more pronounced than the correlations Fe-opaline silica, M n opaline silica or Cu-opaline silica. In view of the more complete sediment cover in the equatorial region compared to the patchy sediment cover along the ridge, for instance, it is probable that the quantities of Fe, Mn, Cu, Ni and Co that come down as a result of biological processes are more significant than these graphs would suggest. This impression is reinforced if we study the relations between the accumulation rates of these constituents (see Fig.16 and 17). Although none of the highest accumulation rates of Fe, Mn and Cu are associated with high rates of opaline silica, several of their medium high accumulation rates are, We conclude, therefore, that Cu, Ni and Co are most likely partly being deposited due to biological processes in the equatorial region, probably a little Fe and Mn deposits there as well due to the same processes. These results partly agree with those by Revelle

CHEMICAL COMPOSITION IN PELAGIC SEDIMENTS

137

TABLE I Provenance of Pacific Ocean deep-sea sediments* Range

Average

Area in 10~ km 2

% of investigated area

Estimated % of the total area of the deep Pacific

A. (% continental influence) 95-100 80-95 40-80 10-40 0-10

- 100 87 60 25 5

43.2 21.7 25.3 5.0 3.1 98.3

44.1 22.0 25.7 5.1 3.1 100

58 22 15 3 2 100

B. (% submarine exhalative influence) 80-100 40-80 10-40

90 60 25

1.2 11.9 16.6

1.2 12.1 16.8

0.72 7.2 10

7.5 22 31

14.0 3.5 1.2

14.3 3.6 1.2

8.5 2.1 0.72

C. (% basaltic influence) 0-15 15-30 > 30

*Relations between the various components in the total mass of open Pacific deep-sea sediments (beyond - 1,000-1,500 km from continents): Alt.I: Alt.II: Continental source (continental crust) 76% 94% Basaltic source (oceanic crust) 10% 4.0% Volcanic exhalations (active ridge source) 14% 2.0% et al. (1955). It should be noticed that this discussion, based on accumulation rates, does not suffer from the restrictions imposed by the closed-set conditions which complicate discussions of the element relations in ordinary sets of chemical data. Thus, the negative relations between Fe and opaline silica in Fig.14 and 15 could be due partly to the "closed-set restrictions" but this is hardly the case with the relations depicted in Fig. 16 and 17. The data in Fig.18 furthermore reveal a close correlation between Cu and Ni on the one hand with manganese. Our data suggest (see Fig.6 and 8) that A1 and Fe accumulate approximately in the ratios 1 . 0 - 1 . 5 over large areas of the equatorial Pacific, except over the East Pacific Rise. This range is close to the A1/Fe ratio of 1.7 found in shales (see Table II). This suggests that a considerable fraction of the trace metal content in the sediments is present as adsorbate on clay-particles, which in their turn are captured by filter feeders. Because of the rapid turnover of matter in the biomass under the equator it appears likely that this biological clay-"cloud" could remain in suspension for considerable periods, permitting

138

K. BOSTROM, T. KRAEMER AND S. GARTNER

+4-

c "- 4 m ta. (3 ÷

3

OPALINE SILICA ( C F B )

+ 4,4-

in %

+

+

+

+

<20 20 - 3 0

• 0

•

4- 4-+ +

+4- +

• w r

0 0

4-

I*

5

30-40

•

40-50

•

50-70

• t

18 F e ( C F B ) in %

>70

t

I

't5

20

Fig.14. Scatter plot, showing the relations b~etween Fe, Mn and opaline silica (CFB) in Indo-Pacific sediments; all sediments rich in opaline silica are comparatively poor in Fe and Mn. extensive lateral transport of the terrigenous debris. However, there is somewhat more iron accumulating in this way than one would expect from the shale value for A1/Fe. This excess of iron is probably present as a ferric hydroxide which would be another trace element scavenger in organisms as suggested by Turekian and Chan (1971). Data in Table II support these conclusions. A1, Fe, Ti, and surprisingly often also Mn are present in organisms in approximately the same ratios as in average shale. In only 3 cases does the Mn/Fe ratio in organisms exceed the shale ratio by more than a factor of 2; sometimes it is even less than the shale ratio. The same conclusion pertains to most available Ti/Fe or Ti/A1 ratios. Cu and Co are always considerably enriched over the shale values as are also Ni and Zn in all cases but one. This is in good agreement with the data displayed for Cu in Fig. 17. Another biogenous constituent that we have ignored so far is calcium carbonate. It could be argued that the tendency for much Fe, Mn, Cu, Ni and Co to be deposited on the ridge is due to scavenging by calcite depositing organisms or other biogen constituents

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140

K. BOSTROM, T. KRAEMER A N D S. G A R T N E R

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CHEMICALCOMPOSITIONIN PELAGICSEDIMENTS

141

maximum in the crestal areas. This rules out the possibility that Fe or Mn are associated to any significant extent with biological productivity in this part of the ocean; furthermore, all evidence (Fleming, 1957; Bogorov, 1967; and our own (K.B.) plankton studies in the East Pacific) indicate that the surface waters over large areas of the East Pacific Rise can be considered virtual deserts. It has been suggested that some Cu, Ni and Co is brought down by calcite depositing organisms, but this is at present hard to test. The distribution pattern of calcite is not very similar to those for Cu, Co or Ni, and a map of the rate of accumulation of calcite would have only restricted value due to the overriding effects of the calcite dissolving processes at and below the compensation depth. More likely a map, showing the amount of calcite tests sinking downwards from the top 200 m of the Pacific Ocean would show striking resemblance .with the corresponding map for opaline silica in Fig.3. Biological precipitation is therefore most probably not the source for most of the Fe and Mn that accumulates in the southern mid-latitudinal areas close to the East Pacific Rise, and is hard to prove for Cu, Co and Ni. It appears more likely that the major constituents Fe and Mn are due to submarine volcanism. Cu, Ni and Co are probably delivered by submarine volcanism as well, but the quantitative significance of this source is difficult to assess at present. Adsorption of Cu, Ni and Co out of sea water on colloidal ferromanganoan hydroxides could certainly

Fig. 19. Distribution of CaCO3 in pelagic sediments. account for much of the Cu, Ni and Co found in the East Pacific. If the ferromanganoan hydroxides are extracting Cu, Ni and Co only out of water, however, one would anticipate a considerable lowering of the concentrations of these elements yet, Pacific waters appear unusually rich in these elements (Schutz et al., 1965). The transport of Cu, Co and Ni to

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CHEMICAL COMPOSITIONIN PELAGIC SEDIMENTS

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the ocean floor to some extent can be explained as due to sinking dead and disintegrating organisms which would leave their scavenged trace elements behind, but this process can not explain the high values for these elements in surface waters. Furthermore, it still remains to explain then why Cu, Ni and Co fail to accumulate in the North Pacific where the same biological enrichment processes are operating; the small mass of slowly growing manganese nodules does not explain this mass balance deficit, as we will show below. Too little is still known about the distribution of many trace elements in the ocean waters to permit many more conclusions. Summing up the available arguments for and against, however, it appears that some of the Cu, Ni and Co in South Pacific sediments indeed must be of volcanic origin;the same origin was suggested by Schutz et al. (1965) for much of the Co and Ni in Pacific waters. These conclusions are supported by further evidence discussed in connection with the geochemistry of Ba (see Bostr6m et al., 1973b). The well-known relation between basic magmatism and ores of Cu, Ni and Co renders further support to this interpretation;it would be surprising indeed if ascending hydrothermal solutions would contain only Fe and Mn and nothing else. This conclusion harmonizes with the observation by Baturin (1971) of copper and iron sulfides in MidIndian Ocean magmatic rocks. The minimum in the absolute Cu, Ni and Co values over the ridge (Fig.10 and 11) suggests that these constituents are to a less extent of volcanic origin than Fe and Mn. Otherwise one would expect the efficient scavenging by the freshly formed iron and manganese hydroxides to extract all Cu, Ni and Co that surface with them. Another possibility is that they have longer residence times in the bottom waters than Fe and Mn do. We conclude therefore that in addition to the continentally derived Cu, Co and Ni that deposit in the South Pacific, partly deposited there by biological processes, there are also significant quantities of these elements that are contributed by submarine volcanism. So far we have focused our attention on accumulation rates of elements as recorded in soft unconsolidated and non-concretionary sediments; with non-concretionary sediments we here understand sediments with micronodules smaller than 2 mm or so in diameter. Locally, considerable amounts of authigenic matter are precipitating as nodules and pavements on the ocean floor. To what extent do the accumulation rates in such deposits change the overall pictures as shown for A1, Fe, Mn and Cu in Fig.6, 8, 9 and 12? Manganese nodules and slabs show a frequency distribution (Menard, 1964) that is approximately reciprocal to the sedimentation rate patterns in Fig.1. This is in good agreement with what one would expect, only in areas of very slow sediment accumulation could the slowly growing nodules have a chance to develop. Of interest to notice is the paucity of nodules in the equatorial region, confirming the presence of mid-oceanic deposition rates that are higher than usual. Menard (1964) derived his results by bottom photography; later more detailed studies (not yet published) at the Lamont-Doherty Geological Observatory confirm his results. In the East Pacific (K. Bostr6m, E. Bonatti, J. Honnorez, unpublished results) the frequency of slabs and pavements increases with distance from the spreading ridge; on the very crest of the ridge pavement is usually thin or absent. Similar findings have been reported by Menard (1960) for the Pacific and by

144

K. BOSTRiDM,T. KRAEMER AND S. GARTNER

A um en t o (1969) for the Atlantic Ocean. This distribution pattern also suggests that the total mass of nodules is quite small, and may well account for less than 1% of the total sediment mass in the Pacific. Where they occur nodules may cover from a few to about 100% of the ocean floor. We will here use 33% as an average. Pavement coverage may be almost complete but since the areas of exposed rock are c o m m o n l y much below 30% o f the ocean floor far from spreading center this does not change the following results much. Growth rates of nodules vary, but there is little disagreement on the general order.of magnitude, 2 - 1 1 mm per 1,000,000 years (Bender et al., 1966; Ku et al., 1967; Somayajulu et al., 1971). The amount of accreting nodule surface per cm 2 of ocean floor is 1.3 cm2 with 33% coverage, and 4 cm 2 with 100% coverage, assuming nodules to be roughly spherical. Density estimates of nodules vary considerably. We have collated 74 density values from Pettersson (1943), Bender et al. (1966), Ku et al. (1967) and, most importantly, from Mero (1962, and personal communication); the values range between 1.9 and 3.2 g/cm 3, with 2.45 as the median and 2.40 as the mode. Using the value 2.45 g/cm 3 and assuming an average composition of Pacific nodules as given by Bender (1972) we obtain the values in Table Ill. The data in Table III show that accumulation rates in nodules of A1, Ti and Fe always TABLE III Composition of manganese nodules and slabs and accumulation rates of some elements in such deposits and adjacent sediments 1

A1 Ti Mn Fe Ni Cu

3.2 0.67 19.3 11.7 0.66 0.39

2

2

a

b

0.072 0.015 0.43 0.26 0.015 0.0088

0.43 0.090 2.60 1.58 0.089 0.098

3

:

c 0.078 0.016 0.47 0.29 0.016 0.0096

4.1-18 0.19-0.88 0.25-0.99 2.5-8.7 0.011-0.038 0.012-0.054

1 = Composition of average Pacific nodules and slabs, in % (Bender, 1972). 2 = Accumulation rates for concretions in mg/cm2 per 1,000 years; note that the surface unit refers to the ocean floor, not the surface of a nodule, which is 4 times larger than the surface of the ocean floor it covers; density of concretions assumed to be 2.45 g/cm 3. a = spherical nodules growing all around with a rate of 7 mm/1,000,000 years and covering 33% of the ocean floor. b = spherical nodules, growing all around with a rate of 14 mm/1,000,000 years and covering ~ 100% of the ocean floor. c = oxide slab, growing on top surface only at a rate of 10 mm/1,000,000 years, and covering ~ 100% of the ocean floor. 3 = Common accumulation rates in mg/cm2 per 1,000 years for unconsolidated sediments from the areas of slow accumulation in the North Pacific. Data from this work.

CHEMICALCOMPOSITIONIN PELAGICSEDIMENTS

145

are much below corresponding values for adjacent sediment, and only under very extreme conditions do accumulation rates of Mn, Ni and Cu in nodules exceed those found in adjacent sediments. These figures lead to the conclusion that formation of nodules and slabs are of limited significance in sediment budget considerations; this conclusion is also intuitively near at hand in view of the small mass of nodules in the ocean as compared to the total mass of unconsolidated sediments. It should be realized that this discussion suffers from an incomplete knowledge of the micronodule distribution. Our analyses of several wet-sieved samples from the equatorial region (at about 130°W) and from the East Pacific Rise (15°S) show that most Fe and Mn is present in the less than 37/~ fraction; that is, micronodules account for at most very little authigenic matter in these sediments. In other areas, on the other hand, such as some calcareous oozes from south of Australia or in the Atlantic Ocean, the frequency of micronodules is high as can be easily demonstrated already on unseparated samples. However, although micronodule distribution thus is incompletely studied we would like to point out, that we have never observed micronodules in sediments with the very highest accumulation rates on the East Pacific Rise. This means that the overall patterns as shown in Fig.6, 8, 9 and 12 will only be modified but not basically altered by studies of micronodule distribution. Other authigenic constituents than those discussed in detail above play even smaller roles in the deep-sea element budget. It is of interest to notice that such constituents are again concentrated to areas of low sedimentation rates. Bonatti (1963) studied the distribution of zeolites in the deep sea; their distribution patterns indicate a strong preference for areas of slow deposition. This leads to the interesting conclusion that phillipsite may be a stable phase on the ocean floor, but its formation has virtually no significance for the element budgets discussed here. The results obtained here have implications on some data processing techniques. As has been advocated elsewhere (Landergren, 1964; Bostr6m et al., 1971, 1972a) a straightforward recalculation of analyses to a carbonate-free basis does not remove all obscuring disturbances before the final interpretation. Deduction of all SiO2> 3A1203 has been used to remove the influence of opaline silica (op. cit.). As we can see above, this procedure implies only minor errors for Fe and Mn patterns, as well as for other elements that are associated with opaline silica only to a minor extent. For Cu, Co and Ni, on the other hand, there is little doubt that these elements are to some extent harbored in a biogen fraction. Recalculation of such constituents on a minerogen basis must, therefore, lead to somewhat high values in areas of high productivity. This can be remedied if assessments first are made of Cu and Ni content of opaline silica and deducting these quantities before recalculating to a minerogen basis. Another observation of interest is the relation between A1 and other elements. A consequence of the lutite-veil theory (Kuenen, 1950) is that the quantities of AI present in sediments would be a good indicator of the amount of terrigeneous debris present in the sediments (Arrhenius, 1952; Landergren, 1964). This concept was later extended using the ratio of a given element versus aluminum as an indicator of the accumulation

146

K. BOSTROM, T. KRAEMER AND S. GARTNER

rate of the element in question (for details, see Arrhenius, 1966). Although our data here suggest that this procedure has to be used with some caution it nevertheless appears to be a good first approximation when only incomplete data are available for a certain area. ACKNOWLEDGEMENTS We want to thank Dr. J. Mero, who generously put his collection of primary data on manganese nodules at our disposal. This research was supported by the National Science Foundation Grants NSF-GA-1356, GA-15 248 and GA-29212. REFERENCES Arrhenius, G.O.S., 1952. Sediment cores from the East Pacific. Rept. Swed. Deep-Sea Exped., 5: 227 pp. Arrhenius, G.O.S., 1963. Pelagic sediments. In: M. Hill (Editor), The Sea. Interscience Publishers, New York, N.Y., 3: 655. Arrhenius, G.O.S., 1966. Sedimentary record of long-period phenomena. In: P.M. Hurley (Editor), Advances in Earth Science. M.I.T. Press, Cambridge, Mass., pp. 155-174. Aumento, F., 1969. The Mid-Atlantic Ridge near 45°N, V. Fission track and ferro-manganese chronology. Can. J. Earth Sci., 6: 1431-1440. Baturin, G.N., 1971. Deep-sea ore sediments of hydrothermal origin. In: L.A. Zenkevich (Editor), The History of the World Ocean. Nauka, Moscow, pp.259-277. Bender, M.L., 1972. Manganese nodules. In: R.W. Fairbridge (Editor), Encyclopedia of Geochemistry and Environmental Sciences. Van Nostrand, Reinhold, New York, N.Y., pp.673-677. Bender, M.L., Ku, T.L. and Broecker, W.S., 1966. Manganese nodules: their evolution. Science, 151: 325 -328. Bender, M.L., l(u, T.L. and Broecker, W.S., 1970. Accumulation rates of manganese in pelagic sediments and nodules. Earth Planet. Sci. Letters, 8: 143-148. Bender, M., Broecker, W., Gornitz, V., Middel, U., Kay, R., Sun, S.S. and Biscaye, P., 1971. Geochemistry of three cores from the East Pacific Rise.Earth Planet. Sci. Letters, 12: 425-433. Blackman, A., 1966.'Pleistocene Stratigraphy of Cores from the Sou theast Pacific Ocean. Ph.D. Dissertation, Univ. of California, San Diego, Calif., 200 pp. Bogorov, V.G., 1967. Biological transformation and exchange of energy and matter in the ocean. Okeanologia, 7: 649-665. Bonatti, E., 1963. Zeolites in Pacific pelagic sediments. Trans. N. Y. Acad. Sci., 25: 938-948. Bostr6m, K., 1970. Submarine volcanism as a source for iron. Earth Planet. Sci. Letters, 9: 348-354. Bostr6m, K., 1973. The origin and fate of ferro-manganoan active ridge sediments. Acta Univ. Stockh., in press. Bostr6m, K. and Peterson, M.N.A., 1969. Origin of aluminum poor ferro-manganoan sediments in areas of high heat flow on the East Pacific Rise. Mar. Geol., 7: 427-447. Bostr6m, K. and Fisher D.E., 1971. Volcanogenic uranium, vanadium and iron in Indian Ocean sediments. Earth Planet. ScL Letters, 11: 95-98. Bostr6m, K., Joensuu, O., Valdes, S. and Riera, M., 1972a. Geochemical history of South Atlantic Ocean sediments since Late Cretaceous. Marine Geol., 12: 85-121. Bostr6m, K., Farquharson, B. and Eyl, W., 1972b. Submarine hot springs as a source of active ridge sediments. Chem. Geol., 10: 189-203. Bostr6ml K., Joensuu, O., Kraemer, T., Rydell, H.S., Valdes, S., Gartner, S. and Taylor, G., 1973a. New finds of exhalative deposits on the East Pacific Rise. Submitted to: Geol. Fisren. Stockh. Forhandl. Bostr6m, K., Joensuu, O., Moore, C., Bostr6m, B., Dalziel, M. andHorowitz, A., 1973b. The marine geochemistry of Ba. Lithos, 6: 159-174.

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Broecker, W.S. and Ku, T.L., 1969. Caribbean cores P6304-8 and P6304-9: new analysis of absolute chronology. Science, 166: 4 0 4 - 4 0 6 . El Wakeel, S.K. and Riley, J.P., 1961. Chemical and mineralogical studies of deep-sea sediments. Geochim. Cosmochim. Acta, 25: 110-146. Emiliani, C., 1955. Mineralogy and chemical composition of the tests of certain pelagic foraminifera. Micropaleontology, 1 : 377-380. Emiliani, C., 1966. Paleotemperature analysis of Caribbean cores P6304-8 and P6304-9 and a generalized temperature curve for the last 425,000 years. J. Geol., 74: 109-126. Emiliani, C. and Rona, E., 1969. Caribbean cores P6304-8 and P6304-9: new analysis of absolute chronology. A reply. Science, 166: 1551-1552. Fischer, A.G. et al., 1971. Initial Reports o f the Deep-Sea Drilling Profect, Vol. VI. Government Printing Office, Washington, D.C., 1329 pp. Fleming, R.H., 1957. General features of the Oceans. In: J.W. Hedgpeth (Editor), Geol. Soc. Am. Mem., 67, Vol.1, pp.87-108. Gartner, S., 1969. Correlation of Neogene planktonic foraminifer and calcareous nannofossil zones. Trans. Gulf Coast Ass. Geol. Soc., 19: 5 8 5 - 5 9 9 . Glass, B.P., 1969. Reworking of deep-sea sediments as indicated by the vertical dispersion of the Australian and Ivory Coast microtectite horizons. Earth Planet. ScL Letters, 6 : 4 0 9 - 4 1 5 . Goldberg, E.D. and Arrhenius, G.O.S., 1958. Chemistry of Pacific pelagic sediments. Geochim. Cosmochim. Acta, 13: 153-212. GoodeU, H.G., Watkins, N.D., Mather, T.T. and Koster, S., 1968. The Antarctic glacial history recorded in sediments of the Southern Ocean. Palaeogeogr., Palaeoclimatol., Palaeoecol., 5 : 4 1 - 6 2 . Hays, J.D., Salto, T., Opdyke, N.D. and Buckle, L.H., 1969. Pliocene-Pleistocene sediments of the Equatorial Pacific. Geol. Soc. Am. Bull., 80: 1481-1514. Hays, J.D. et al., 1972. Initial Reports o f the Deep-Sea Drilling Pro]ect, Vol.[X. Government Printing Office, Washington, D.C., 1205 pp. Heath, G.R., Moore, T.C., Somayajulu, B.L.K. and Cronan, D.S., 1970. Sediment budget in a deep-sea core from the central Equatorial Pacific. J. Mar. Res., 28: 225-234. Holmes, C.W., Osmund, J.K. and GoodeU, H.G., 1968. The geochronologies of foraminiferal ooze deposits in the Southern Ocean. Earth Planet. Sci. Letters, 4: 368-374. Kraemer, T.F., 1971. Rates o f Accumulation of lron, Manganese, and Certain Trace Elements on the East Pacific Rise. M.S. Thesis, Florida State University, Tallahassee, Fla., 94 pp. Krinsley, D., 1960. Trace elements in the tests of planktonic Foraminifera. Micropaleontology, 6: 297-300. Krinsley, D. and Bieri, R., 1959. Changes in the chemical composition of pteropod shells after deposition on the sea floor. J. Paleontol., 33: 6 8 2 - 6 8 4 . Ku, T.L. and Broecker, W.S., 1967. Uranium, thorium and protactinium in a manganese nodule. Earth Planet. ScL Letters, 2: 317-320. Ku, T.L., Broecker, W.S. and Opdyke, N., 1968. Comparison of sedimentation rates measured by paleomagnetic and the ionium methods of age determination. Earth Planet. Scl. Letters, 4: 1-16. Kuenen, Ph.H., 1950. Marine Geology. Wiley, New York, N.Y., 568 pp. Kwiecinski, B., 1972. On the organic matter and trace element distribution of the sediments in the Gulf of Panama. Submitted to: Geol. FOr. Stockh. FOrhandl. Kwiecinski, B. and Castrejon, R., 1972. On the organic matter and trace element distribution in the plankton in the Gulf of Panama. Submitted to: Geol. F6r. Stockh. FOrhandl. Landergren, S., 1964. On the geochemistry of deep-sea sediments. Rept. Swed. Deep-Sea Exped., VoL X, Spec. Invest., 5: 5 7 - 1 5 4 . Lisitsin, A.P., 1966. Main regularities in the distribution of recent siliceous sediments and their relations with climatic zonality. In: Geochemistry o f Silica. Commission on Sedimentary Rocks at the Earth Sciences Department. Nauka, Moscow, pp.90-191. McManus, D.A. et al., 1970. Initial Reports of the Deep-Sea Drilling Project, Vol. V. Government Printing Office, Washington, D.C., 827 pp. Menard, H.W., 1960. Consolidated slabs on the floor of the Eastern Pacific. Deep-Sea Res., 7: 35-41. Menard, H.W., 1964. Marine Geology of the Pacific. McGraw-HiU, New York, N.Y., 271 pp.

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