Acta. 1976. Vol. 40. pp. 425 lo 434. Pergamon
in Great Britain
Authigenic transition elements in Pacific pelagic clays S. KRISHNASWAMI Physical Research Laboratory, Ahmedabad-380 009, India (Received 11 February 1975; accepted in revised form 16 October 1975) Abstract-The
concentrations of SC, Ti, Fe, Mn. Co, Ni, Cu, La, Th and U have been measured in several Pacific pelagic clays having widely different accumulation rates, 0.49.0 mm/lo’ yr. The authigenic fractions and deposition rates of these elements have been estimated from the measured concentrations using various models. The results show that in Pacific clays about 90% Mn, 80”/, Co and Ni and 50% Cu are authigenic whereas the major fraction (290%) of SC, Ti, Fe, La, Th and U are of detrital origin. Anticorrelation between the clay accumulation rates and the concentrations of Mn, Co, Ni and Cu is observed. This suggests a uniform authigenic deposition of these elements superimposed on varying amounts of detrital materials. The concentrations of SC, Ti and Th are almost independent of sedimentation rates, indicating that their authigenic deposition is small compared to their detrital contribution. Comparison of the authigenic deposition and river input rates shows that Mn, Co and Ni are accumulating in excess of their supply by factors of 2-10, whereas the converse is true for Cu and U. Additional sources to account for the budgetary discrepancies of Mn, Co and Ni are discussed, with particular reference to in situ leaching of detrital phases transported to the oceans via rivers.
To DELINEATEthe geochemical cycle and balance of various elements in the marine environment it is essential to have a better knowledge of their authigenie concentrations and deposition rates on the ocean floor. Though various techniques to estimate the detrital and authigenic fractions of deep-sea sediments have been reported (GOLDBERG and ARRHJNUS, 1958; CHESIXR and HUGHES, 1967), available data on the authigenic deposition rates of elements are not abundant (BENDERet al., 1966; KRISHNASWAMI and LAL, 1972). Hence it was thought worthwhile to study systematically the distribution of a select group of elements in radiometrically dated Pacific pelagic clays, with a view to quantitatively estimate their authigenic deposition rates. The elements measured are Mn, Co, Ni, Cu, U, SC, Ti, Fe, La and Th. The selection of these elements was based on the observation that some of them (Mn, Co, Ni, Cu and U) exhibit marine budgetary discrepancies (B~STROU, 1967; TUREKIAN, 1968; RILEY and CHESTER, 1971) while for the others (SC, Ti, Fe, La and Th) no data on the authigenic deposition rates were available. The techniques of measurement and the models to estimate the authigenic concentrations are discussed. Based on the authigenic concentrations an attempt has been made to evaluate their marine geochemical balance.
The relevant details of the sediment and nodule samples analysed are given in Table l(a) and (b), respectively. Since
the extent of incorporation or rejection of different elements by biogenic phases in marine sediments are not well understood, only clays were studied in this work. [The CaCO, conceptrations of all the cores listed in Table l(a) is
426 Table 1 (a). Physical data and sedimentation deep-sea sediments studied Water
rates of the
Sedimentation rate*(mm/lO’y) (C) (a) 01
TF-1 TF-2 TF-3 TF-4
176Y. 9’W NOVA-IU-13’
165O45’W CAP-6 BG++
(m) E-17 -36
I (b). Details of manganese growth rates
114O15’W DWHG -49++
119°41. 3’W MSN-14x+
116°41. 3’W CAP-SOBG++
21°2?.2’N 126’43’ W
124’25. VW v-21-71**
117°31. 4’W FAN -BG-7++
42’30’N 16Z”06. 2’W
’ TIFR collection. + + SIO collection, and ** LDGO collection. *All sedimentation rates are based on “‘Th chronology.
(a) GOLDBERG and KOIDE (1962). (b)/ KU et al. (1968). The sedimentation rates of all cores excluding V-18-258 and V-21-71 are recalculated values from the results of GOLDBERG and KOIDE(1962). The major correction factor is due to shortening of open barrel cores compared to piston cores. (c) AMINet al. (1975).
** LDGO collection, + TIFR collection and + + SIO collection. USGS reference rocks are similar to those reported in the literature (FLANAGAN, 1969; FLEISCHER, 1969). Duplicate measurements of the concentrations in the reference rocks and marine samples were in agreement within 210%. The average concentrations of SC, Ti. Fe, Mn, Co, Ni, Cu, La, Th and U in Pacific pelagic clays based on the present measurements are given in Table 5. The deduced averages are in moderate agreement with the data of CRONAN (1969) and KRISHNASWAMI and LAL (1972).
Table 2. Results of analysis of USGS reference samples Co”ce”tratton
W-l Present Reported values* work
Fe(74 model 303 atomic absorption spectrophotometer. Along with the samples USGS reference rocks and reagent blanks were also measured. RESULTS The results of the measurements of USGS rocks, sediments and nodules are given in Tables 2, 3 and 4, respectively. The measured concentrations of various elements in
G-2 Present Rapotied rork “*“e&l* 3. 1 210 1. 6
ECR - 1 Present Reported aork vpluo~: 26.6
9. 3 36-37 30
* From FLANAGAN (1969) and FLEISCHER (1969). Results of measurements directly comparable to the techniques used in this study are given.
Authigenic transition elements in Pacific pelagic clays Table 3. Elemental concentrations
of Pacific pelagic clays
Depth Concentration of elementsInppm.dry weight;unlessstated
analyzed SC I___\
*Th and U data compiled from GOLDBERG and KOIDE (1962),Ku (1966),Ku et al. (1968) and AMIN et al. (1975). The reported *32Th values by La Jolla group were multiplied by 15 to correct for HCI leaching efficiency (GOLDBERG and KOIDE, 1962).
DISCUSSION Estimation of authigenic deposition rates The main aim of the present investigation is to estimate the authigenic deposition rates of various elements on the ocean floor, with a view to evaluate their marine geochemical balance. Since in the past
the main emphasis has been in studying the distribution of elements among the various components of deep-sea sediments, available data on the authigenic deposition rates are sparse. A systematic pattern between the clay accumulation rates and the concentrations of various elements in pelagic clays has emerged from the results
Table 4. Chemical composition of radiometrically dated Pacific manganese nodules Nodulecode sc
Concentrations inppm dry weight,unlessstated ?vlo La Th* Ni Cu Mn(k) Fe(X) Co
DH-2 V21-D2 ZETBS-3D
NM-Not measured. *Data for U and Th concentrations are compiled from BHAT et al. (1973) and Ku and BROECKER (1969).For nodules 2P-50 and 2P-52 are determined in the present study. ** Represents weight loss of air dried nodule samples at 110°C for 2 hr.
S. KRISHNASWAMI Table 5. Average concentration
reliable results for those elements whose abundances in shales or near-shore sediments are small compared to that in pelagic clays. This case is realistic only for Mn. Co and Cu where the correction is less than Y,,. For Ni and SC. where C,C,, _ Z--3. the estimates may be only semi-quantitative. For Fe. Ti, La. Th and U where C, _ C, no reliable estimates of authigenie concentrations can be made. Since the chemical composition of the shales and the shelf sediments are similar, the deduced authigenic cmcentrations by both these models follow the same pattern.
of SC, Ti. Mn. Fe, Co, Ni, Cu, La, Th and U in Pacific sediments Concentration
of element(ppm. wt) Reported averages** (a)
Fe (% wt)
115 12.3 2.2
Average of 20 clay sediments. **(a) CRONAN (1969) and (b) KRISHNAS-
WAMIand LAL (1972). +For estimating the mean Ti concentration the result of DWBG-30 is excluded (Table 3).
of this work. A model for the deposition of elements (henceforth designated as ‘constant flux model’) has evolved from this pattern. Based on this model the authigenic deposition rates and detrital fractions of elements can be evaluated directly. Additionally, the observed distribution of various elements in pelagic clays and the occurrence of deposits like manganese nodules can be understood in terms of this model. The reported methods to estimate the authigenic and detrital fractions of deep-sea sediments are based on a twocomponent system for pelagic clays. Thus the total concentration of any element, C,, in the sediment is the sum of the contributions from these two fractions, i.e. c, = c, + Cd.
where C is the concentration of the element and the suffixes t. a, and d represent total, authigenic and detrital phases. The problem is to ascertain the contributions of either C, or Cd to the total measured value, C,. For the sake of completeness the assumptions of the commonly adopted approaches for estimating the authigenic concentrations and the deduced results using them are discussed briefly below. The results of the ‘constant flux model’ and its implications are presented subsequently.
GOLDBERG and ARKHI-~\;I~Is(195X) observed that in the dissolution rate (g,‘m’ hr) of marine deposits (manganese nodule, calcite and apatite) in chelating agents like EDTA was faster than that of terrestrial minerals (feldspar, goethytc. glass and rutile). Using similar techniques. CHISTEK and Hucrq~s (1967) and CHESTER and MESSIHA-HANNA (1970) have estimated the lithogenous fraction of several Atlantic deep-sea sediments. The estimation of the authigenic fraction of elements using this technique is based on their preferential dissolution in selected chemicals. In this work, the leaching of the samples was done by boiling them under reflux with ammonium salt of EDTA at pH = X. The samples were repeatedly boiled to ensure complete leaching. The results of these measurements (Table 7) show that a large fraction of (3 70”,,) of Mn. Co and Ni in the Pacific clays can be preferentially dissolved in EDTA. The deduced authigenic fractions of various elements (Table 7) follow the same pattern as that observed for the North Pacific and Atlantic sediments measured using similar technique (CHESTER and HI~GHES. 1969: CHESTERand
Table 6. Estimated authigenic concentrations of element based on the shale and shelf sediment correction methods Element
Fe (% wt)
13 4600 4. 72
In this approach the value of Cd is obtained by assuming the composition of the detrital fraction of sediments to be the same as that of shales (BENDER et ul., 1966; KRISHNASWAMIand LAL, 1972) or of sediments collected from near-shore regions. Table 6 gives the estimated authigenic concentrations based on this model. This approach gives
C’uncentration (ppm. wt) Authigernc;:* Near shore sediments (b) (a)
Pat ifir* sedments
il. 5 4085 4.8
68 156 10
74 178 11.5
From Table 5. The data for shales and near-shore sediare from TUREKIAX and WEDEPOHL(1961) and
(1973). ** Using relation 1. (al and (b) represent the estimates based on shale and shelf sediment correction methods. respectively. The values in paranthesis could be in large error since they represent the difference of two approximately equal numbers.
Authigenic transition elements in Pacific pelagic clays Table 7. Average EDTA leachable concentrations ments in Pacific clays* Element
%Element leachable in EDTA
Fe (% wt)
*Based on the analyses of 10 cores: MSN-96, DWHG-49, DWHG-52, DWBG-30, V-18-258, CAP-SBG, NOVA-18, NOVA-10 and V-21-71 (Table la). MESSIHA-HANNA,
Table 8. Comparison of the estimated authigenic concentrations of elements based on commonly adopted methods* Estimated authigenicconcentration(ppm. wt)
Mean concentration CUDI& .. . wt) Authigenic
Excluding Cu, the measured authigenic concentrations follow the same trend as that of the shale model. The detrital concentration of Cu deduced by the leaching technique is higher by about a factor of 2-3 than that estimated by the shale model, an observation similar to that of Ck~sTER and HUGHES (1969). The cause for the observed discrepancy is not clear. It is likely that the non-leachable fraction of Cu in deep-sea clays is of eolian or biogenous origin. Manganese nodule model The authigenic concentrations of elements are estimated using this model on the assumption that their authigenic precipitation rates on the pelagic clays and manganese nodules are the same (KRISHNASWAMI and LAL, 1972). The observation that the authigenic accumulation rates of Mn, Cu, Ni, Co. and of the radioisotopes “Be and 230Th in both sediments and nodules are similar within a factor of 2-3 (BENDER et al.. 1970; SOMAYAJULU et al., 1971; KRISHNASWAMI and LAL, 1972; BHAT et al., 1973) supports this assumption. In this model the authigenic concentrations are calculated using the fbllowing relation (KRISHNASWAMI and LAL, 1972; KRISHNASWAMI, 1973):
where S is the clay accumulation rate, p is the density, (subscripts s and n refer to sediments and nodules, respectively), E is the fraction of time for which the nodules grow on an average, G is time averaged growth rate of nodules and& is the geometry factor for the nodule exposure. The value of R is calculated to be about 10, using the following numerical values for the various parameters. S = 2-3 mm/lo3 yr, ps = 0.5 g/cm3, E = 0.1, G = 3 mm/lo6 yr, pn = 2.5g/cm3 and fw = 2. Considering the uncertainties in the values of p, K and fw the calculated ratio of 10 is considered to be in agreement with the mean observed ratio of 25 (range l&43) for elements with different degrees of chemical reactivities, Mn, Co, Ni,
* (i) Shale method, (ii) shelf sediment method, (iii) leaching method and (iv) nodule method. The estimated authigenie concentration of SC, Ti and Fe by methods (i) and (ii) could be in large error (see text for method of calculation).
Cu, “Be and Th. In view of this agreement, the authigenie concentration of various elements have been calculated using a value of 25 for R. The estimated values are givenjn Table 8.
Comparison of the estimated authigenic concentrations based on various models The authigenic concentrations of Mn, Co, Ni and Cu are easily estimated by all the methods. The deduced values for Mn, Co, Ni, Cu, Fe and Ti by the various methods are in modest agreement. For SC, La, Th and U. only the manganese nodule method could give estimates of the authigenic concentrations. Though the estimates for Mn, Co, Ni and Cu by this method are in agreement with the results of other models, it is likely that some of these elements may be preferentially incorporated into the nodule matrix. It is therefore important to confirm the deductions obtained by this method by some independent approach. The most probable authigenic concentrations for the elements studied are also listed in Table 8. The estimates for SC, Ti, La, Th and U are based mainly on the nodule method, for the others it is the average of all the techniques. Constant Jlux model This model is based on the assumption of uniform authigenic deposition rate of an element over the ocean floor. An anticorrelation between the authigenie concentration, C,, and sedimentation rate, S, is expected, if the authigenic deposition rate of an element is uniform, i.e.:
c, = g s
In such case the total concentration, ment would be:
C,, in the sedi-
where K is the authigenic deposition rate (g/cm” yr), ps is the in situ density of the sediment (g/cm3) and S is the sedimentation rate (cm/yr). The assumption of this model is expected to hold good for elements which are homogeneously distributed in the ocean, i.e. whose residence times are comparable to or more than the oceanic mixing times. Mn. Co, Ni. Cu and U should fall in this category. For SC. Ti, La and Th. which are chemically more reactive and have short residence times, the assumption of uniform deposition may be questionable. The available data on the vertical and geographical distribution of these elements in sea water are sparse and hence it is dificult to make a critical judgement on the nature of their distribution. BENDER et u[. (1966) observed an almost constant accumulation rate for Mn in cores in which both the Mn concentrations and the sedimentation rates vary over an order of magnitude. Later work (BENDER et ul., 1970) on different types of pelagic sediments (red clays, calcareous and siliceous oozes) showed more scatter in the accumulation rate of Mn. It is likely that part of the scatter in their estimates may be due to the assumption that calcite and opal are Mn-free diluents (BLACK, 1965; BOSTROMand FISHER, 1972). Additional uncertainty in their estimates may result from the difficulty in assigning sedimentation rates by the magnetic reversal technique in which there is no accurate way of compensating for the loss of core tops.
The simplest correlation is expected for 130Th which is uniformly produced in the water by the decay of uranium and precipitated to the ocean floor. The terrigenous supply of 230Th is small compared to its in situ production from sea water, since its concentration in rivers (soluble and particulate) is very low (MOORE, 1967; SCOTT, 1968). Figure 1 shows the variation of the surface 13’Th concentration in sediments as a function of the inverse of sedimentation rate (only cores which have an integrated Z30Th activity of 2800 dpm/cm” and which show an undisturbed exponential decay of 230Th with depth are plotted). The data for Fig. 1 have been compiled from GOLDBERG and KOIDE (1962, 19631, GOLDBERG er (11. (1963.1964). GOLDBERGand GRIFFIN (1964), Ku (1966), Ku et ul. (1968) and AMIN et al. (1975). The 230Th concentrations published by the La Jolla group have been multiplied by a factor of I.5 to correct for incomplete recovery of thorium isotopes in the acid leaching procedure (GOLDBERGand KOIDE. 1962). It is evident from Fig. 1 that the surface 230Th concentration is anticorrelated with the sedimentation rate as expected from relation (2). It must be mentioned here that even if the sedimentation rates as recalculated by Ku et al. (1968) are used the anticorrelation pattern is still obtained. The precipitation rates of 13’Th in the Pacific, Atlantic and Indian oceans have been calculated from the slope of the line in Fig. 1 and using a value of 0.5 g/cm3 for in situ density. The estimated values range between 8.9-13.7 and 12%13.5 dpm/cm’ lo3 yr for the Pacific and Atlantic oceans. respectively, depending on the sedimentation rates used. The higher deposition rates are associated with the revised sedimentation rates of Ku et al.
Fig. 1. Plot of surface 230Th concentration (dpm/g) vs inverse sedimentation rate (lo3 yr/cm) for cores from the Pacific, Atlantic and Indian oceans. Only cores showing exponential decay of 230Th with depth and having an integrated 230Th activity of 2 8OOdpm/cm’ are plotted. The anticorrelation between clay accumulation rate and surface 230Th concentration is clearly evident. The *30Th deposition rate deduced from thee slope of the line range between 8 and 13 dpm/cm’ 10” yr.
Authigenic transition elements in Pacific pelagic clays
A “‘OO20 RATE
Fig. 2. Scatter diagrams of the concentrations of Fe, Ni, Cu, Mn, Co, SC, Ti and Th in dated Pacific pelagic clays against inverse sedimentation rates. For plotting the revised sedimeentation rates of Ku et al. (1968) have been used. The lines drawn are least square fits for the data points. As is evident, the concentrations of Mn, Co, Ni, Cu and Fe are anticorrelated with the sedimentation rates. The concentrations of SC,Ti and Th are almost independent of sedimentation rates. The deduced authigenic deposition rates and the detrital component of these elements in pelagic clays are given in Table 9.
(1968). The calculated deposition rate of 230Th on the ocean floor is in agreement with its production rate, 8-9dpm/cn? lo3 yr. The variation of the concentrations of Mn, Ni, Cu, Co, Fe, Ti, Th and SC, in 18 cores as a function of sedimentation rate is plotted in Fig. 2. The results show that the concentrations of Mn, Co, Ni, Fe and Cu are anticorrelated with the sedimentation rate. This is consistent with the model of uniform authigenie deposition superimposed on a background of detrital input. The concentrations of SC, Ti and Th are almost independent of sedimentation rates. The latter observation is expected for those elements whose authigenic concentrations are small compared to their detrital contributions. It is apparent from the data (Fig. 2) that in sediments with clay accumulation rates exceeding about 0.5 g/cm2 lo3 yr (corresponding to about 10 mm/lo3 yr) the authigenic concentrations of elements in them would be small, i.e. C, N Cd. In such cases the elemental abundances in sediments would be the same as that in the detrital component (shales) and would be independent of sedimentation rate. However, the total deposition rate of the element would be positively correlated with the sedimentation rate. The results of TUREKIAN and SHLJTZ(1965) who observed a positive correlation
between trace element and clay accumulation rates in Atlantic calcareous cores fall in this category. The other extreme, i.e. deposition of purely authigenic material would result when the detrital sedimentation rate, S, is very small. The occurrence of ferromanganese minerals in regions of low detrital sedimentation is consistent with this expectation. The authigenic deposition rates and detrital concentrations of these elements are estimated from the slope and intercept of the best fit lines in Fig. 2. These calculations are carried out using the sedimentation rates reported by various groups as well as using the revised rates of Ku et al. (1968) (Table 9). In general, for all elements studied, higher authigenic deposition rates and lower detrital concentrations are deduced using the revised sedimentation rates (Table 9). Marine geochemical balance of transition elements
Table 10 gives the calculated authigenic deposition ,rates of elements in pelagic clays along with their supply rates via rivers. The best estimates of authigenic deposition rates given in Table 10 is based mainly on the constant flux model. The results (Table 10) show that Mn, Co and Ni are depositing in excess of their supply by factors of 2-10 whereas the removal rate of Cu and U are
RENARD(1891) suggested submarine volcanism as a source of elements to the oceans. Though a considerable amount of work has been reported on this subDetrital concentration Au thigenic precipitation ject, no quantitative estimates for input rates of eleElement (ppm. wt) unless stated rate ( fl g/J 103 yj* ments by submarine volcanism are available. The imib) (a) (a) (b) portance of eolian dust as source of trace elements to the deep-sea sediments has been discussed recently Mn 2067 240 650 (WINDOM, 1970; CHESTERand JOHNSON,1971). Based Fe@ wt) 4.33 3.8 640 1500 on the analyses of airborne dust over the Atlantic CO 46 31 2.3 5 and Pacific thev conclude that the concentrations of Ni 92 51 5.1 12.6 Mn. Co and NE in the dust material are not adequate cu 224 169 5.1 12.8 to account for their supply to pelagic clays. La 30.5 24.4 0. 9 1.9 Recently the significance of river suspended load SC 22.1 21 -+ 0. 07 as a source of trace metals to the oceans has been Ti 5262 5291 recognized. The major fraction of solids introduced Th 11.6 11.3 to the oceans are weathered materials from the continents. Elements present in these solids include those (a) and (b) refer to estimates deduced using the original in the lattice positions of the minerals and those and revised sedimentation rates (see text). which are retained on their surfaces by adsorptionion * Calculated from the slope of the least square lines (Fig. exchange processes, The available data on the elemen‘I. tal abundances in river suspended materials (T~IKEK+ In these cases the deposition rates could not be estimated since slopes were negative (Fig. 2). IA~Vand SCOTT, 1967; MARTIN rf ~rl.. 1973) suggest that many of the transition metals are enriched in them compared to shales and the detritai fraction of about an order of magnitude less than their input. deep-sea sediments. The fate of these adsorbed eleBased on the results of three Atlantic cores, TUREKIAN ments in the marine environment is not well under(1968) pointed out that the deposition rates of Co stood. They may be transported and deposited with and Ba are in excess of their stream supply, whereas the detritus, or may get desorbed in the estuarine for Ag its input is an order of ma~itude more than regions. its removal. The deductions of the present work are Attempts to understand the behaviour of these similar to those of TUREKIAN(1968). adsorbed elements while in contact with sea water The origin of ‘excess’ Mn, Co and Ni on the ocean have been carried out by KHARKARrr ul. (1968) and floor is intriguing. Various sources. e.g. submarine MARTINrt 111.(1973). Their results indicate that some volcanism, river detritus, eolian dust and postof the adsorbed trace elements in river detritus are depositional migration have been suggested to explain desorbed on contact with sea water. The fraction of the origin of these elements on the ocean Roor (ARRthe element reieased varies depending on the chemical HENWS et ul., 1964: CHESTER,1965; TUREKIAN,1965; reactivity of the element and the nature of the adsorRILEY and CHESTER,1971). Murray in MURRAYand bent. Further, the fraction of leached elements reaching the open ocean areas is uncertain. and probably Table IO. Estimated authigenic deposition and supply is governed by the organic productivity in the estuarrates of elements ine regions. To evaluate the significance of this source, experiments were carried out to estimate the amount Authigenic deposition rate* (#g/cm’ 103y) River+ I input Best estimate of Mn, Co, Ni, 12(j3[3aand U isotopes leached from Element II river suspended sediments by sea water. The leaching experiments were done using sediments collected from SC 0.05 - (0.07) 0.05 0. 04 Indian rivers and lakes. The samples were powdered Ti 30 _ 30 30 and separated into two size fractions (1) less than IJO Mn 700 240 (650) 500 70 pm and (2) less than 10 ,rlm. The feaching was done Fe 750 640 (1500) 800 6700@ either by passing the sea water through a column CO 8 2.3(5.0) 5 2 of the sediment or by stirring a suspension of the Ni 18 5.7(12.6) 10 3 mud in sea water. For each experiment about 50 g cu 22 J.l(l2.6) 8 70 La 1 0.9( 1.9) 1 2 sediment was used. The experiment was continued till Th 0.2 0.2 1 the concentrations of elements in leachates were close ti 0.04 0.04 I to or less than their detection limits. [For details see KRISHNASWAM~ (1973f.f * I. From the most probable authigenic concentration The input of Mn, Co. Ni, Ra and U to the oceans (Table 9) and a mean clay accumulation rate of 0.1 g/cm2 via ‘dissolved supply” and ‘in situ leaching’ is given 10’ yr, II. from the constant flux model. The values in in Table I I. The data in Table 11 suggests that there parenthesis are based on revised sedimentation rates. is a definite supply of trace elements to the ocean + From TUREKIAN (1969). by in s&r leaching of river-borne suspended matter. 11, Upper limit. Table 9. Deduced detrital concentrations and authigenic deposition rates of elements based on constant flux model
Authigenic transition elements in Pacific pelagic clays Table 11. Input of Mn, Co, Ni, U and Ra by in situ leaching of river suspended material Element
Leachableamount* (microgram/g sediment)
Input(/&!/cm2 1037) In situ leaching
0.3 (0.1-O. 7)
0.1 (0.06-O. 3)
of Physical Research Laboratory, Ahmedabad. Thanks are due to Profs. RAMA,B. L. K. S~MAYAJULU,N. BHANDARI, RICER HART and Mr. S. K. BHATTACHARYAfor helpful discussions and criticisms. The experimental work was carried out while the author was a member of the Tata Institute of Fundamental Research, Bombay. The help given by the various members of the Geophysics Group, TIFR during the different phases of the work is thankfully acknowledged.
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The Clarke Company. The measurements of SC, Ti, Fe, Mn, Co, Ni, Cu, La, Th and U in dated Pacific pelagic clays and manganese nodules have led to the following conclusions : (i) Mn, Co, Ni and Cu are enriched by factors of 3-10 in pelagic clays compared to shales and nearshore sediments. The concentrations of SC, Ti, Fe, La, Th and U in clays, shales and near-shore sediments are similar. A major fraction of the ‘excess’ Mn, Co, Ni and Cu can be preferentially leached. (ii) The authigenic fraction of these elements in pelagic clays have been estimated using different models. The results indicate that Mn, Co, Ni and Cu are primarily of authigenic origin, whereas the concentrations of SC, Ti, Fe, La, Th and U in deepsea sediments are controlled by detrital materials. (iii) The authigenic precipitation rates of Mn, Co, Ni and Cu on the Pacific ocean floor are found to be nearly uniform, thus their variation in absolute concentrations in sediments probably reflect the local sedimentation rates. The authigenic precipitation rates of the elements, SC, Ti, La and Th are small compared to their detrital component. The data also suggest that in sediments whose clay accumulation rates exceed 0.5 g/cm2 lo3 yr the authigenic concentration of tiny element in them would be small.
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