M A N G A N E S E I N T H E S H E L F S E D I M E N T S O F F E A $ T C O A S T O F I N D I A

BY M. SUBBA RAO

(Department oJ Geology, Andhra University; Waltair, India)

Received Octolcer 8, 19f2

(Communicated by Prof. P. iN. Ganapati, r.^.sc.)

ABSTRACT

The distributional pattern of manganese in the shelf sediments off east coast of India has been determined. It is found that sediments from the immediate vicinity of river confluences, those of finer texture, and also those that are brownish in colour are generally more enriched in manganese. Manganese content decreases in a direction that is sea- ward from the coast and away from river confluences. Ah inverse relationship between the manganese content and the carbonate content of these sediments has been established, the richer the sediment in carbo- nate content the less the manganese concentration.

INTRODUCTION

SEVERAL millions of tons of material ate car¡ every day Ÿ the oc~ans both in suspension and solution, a n d a great part of it is precipitated from sea-water by organic and inorganic processes and incorporated largely into the sediments forming on the sea floor. Although the insular shelves of the continental masses ate known to be the major depositional sites a great deal of attention has been paid to the study of the chemistry of sediments of the deep oceanic areas where sedimentation takes place rather slowly. However, there have been quite a few who have studied the migratory and depositional eharacteristics in the shelf sediments, m o d e r n a s well ~s ancient, of certain major and trace elements in an attempt to emphasize their significance as possible environmental indicators. The author has taken u p a programme of studying the nature and distribution of several elements in the sediments of the shelf fringing the east coast of India (Fig. 1) in relation to their source and environmental factors that influence their distributional patterns within the neritic regime. The present paper describes the distribution in the shelf sediments of manganese whose origin in the deep sea sediments has been a matter of ~ e a t debate.

274

Manganese In the Shelf Sediments Off East Coast of India 275

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F I o , 1, Goacral location o f tho Continental ihr along tke ea~t r o f India.

E X P E R I M E N T A L

The experimental procedure is largely based on a method given in Sil~cate Analysis (Groves, 1951). The manganese content has been esti- mated colorimetrically, using Dr. B. Lange's Model IV Colorimeter. The sediments under study range compositionally from highly calcareous to

276 M. StmBA RAO

purely clastic and texturally from colloidal clays to pure sands through all intermediate gradations. When a sample contains appreciable amounts of detrital mineral grains of sand-size, the latter are removed by carefully crushing the material and sieving it several times through a 240-mesh B.S.S. sieve, It is to be remembered that the manganese content of such total samples wiU be less than the value determined on the material passing a 240-mesh B.S.S. sieve. On the other hand, the highly calcareous samples are well ground and manganese determinations are carried out on such ground- mate¡ In this case the manganese content reported on a CaCO3-free basis would be higher than the determined value on the ground sample. The results presented here represent, therefore, only a partial picture. Nonetheless, they give an approximate indica',ion of how the precipit.~ted manganese is distributed in the sediments.

RESULTS

One hundred samples distributed over the entire length and breadth of the shelf from ' Swatch of No Ground ' to the north to Madras to the south have been analysed for their manganese concentrations. On the basis of these determilmtions (Tables I and II), the following observations ate warranted regarding the dist¡252 pattern of manganese in the shelf sediments:

1. Manganese is found in its highest concentration (0"199£ MnO) in a sample from a depth of 8 fathoms opposite the Langulya river confluence about 60 miles north-east of Visakhapatnam, and in its lowest concentration (0.0189/o MnO) in a sample from a depth of 70 fathoms off Visakhapatnam.

2. Sediments from the immediate vicinity of river confluences are generally the most enriched in manganese as compared to those f rom areas far removed from the deltas: thus, the Pennar sediments contain 0.097-0.1119£ the Krislana and Godavari sediments 0.129-0.157~ MnO, and the Ganges sediments 0"085-0-0979£ MnO. In contrast, the Mahanadi sediments contain only 0.0719£ MnO, this low value being probably due to inadequate number of samples on which determinations for manganese have been made. Most of these sediments are either silty clays or clayey silts and clastie in composition. These river-derived sediments r more manganese than those of the Mississippi river whose manganese content averages 0 .06~ MnO for 235 silt samples.

3. The calcareous sediments, particularly the ca.lcareous sands occurring on the outer shelf at depths of more than 50 fathoms, are the poorest in their manganese content (0.018~ MnO).

Manganese in the Shelf Sediments 0.8" East Coast of India

TA~L~ I

Manganese content in shelf sediments off east coast of India

277

Sedal No.

Stafion No.

Depth in fathoms

Nature of Sediment MnO

49

518 751

+ 7,8ra ~~ 7 60 ~ ~

9 66 10 747

11 556 12 560 13 301 14 302 15 303 16 304 < 17 306 ~ 18 307 19 356 20 358 21 359 ' 22 361 23 362 24 36+

365 26 367 .~ 27 368 > 28 360 29 375 30 382 31 384

32 91 33 94 34 95 35 96 36 07 37 98 38 99 39 lOO ~ 40 126E <

42 43 ,101 44 740

46 ~ 47 310 N 48 311 49 312 $0 313 51 314 52 315 53 316 54 317

10 39 15

105

8 100 45.5 21 53.5 150

1"7 3.2

12.5 23 27 32 48 78 22.6 22.6 22.6 21.6 21.6 20.6 22.6 18.6 22 28.6 41.6 37.5 39

22 4O 44 48.5 53.5 58-5 70 84 97

382 100

23.5 22 76 16 24 30 33 40.5 48 58 104 151

Sand Sand-silt-clay

Sand Sandy cia),

Silty clay t !

Sandy clay Silty clay

9~

~t

Silty clay

Sand-silt~:lay Silty clay

I r

I r

Clayey sand Silty clay

g,

,D

ti

11

Clayey sand Sand

Silty clay , p

w,

Sand-silt-clay

Silty clay Sand-~ilt-clay Chyey sand

,1

Sand

Clayey sand Silty clay

~p

Clayey sand

Silty clay i 1

Sar.d.silt-clay Clayey sand

Sand Sand-silt-clay

SilW clay

0 "066 0 �9 053 0.036 0.043

0.111 0"081 0"083 0-097 0"114 0" 129

0.150 0.146 0 '093 0"088 0"074 0"070 0.047 0"067 0.084 0 �9 085 0.085 0"087 0-085 0-098 0.089 0.151 0.151 0.136 0.0�91 0.07S 0.066

0.079 0.054 O.044 0.044 O.030 O. 022 0-018 0.019 0.O28 0-O52 0.059 0.000 0.080 0.025 0-099 o.070 O'O70 (}'071 o-065 o- 044 o.025 0.044 0.040

278 M. SUBBA ~ 0

TABLE I (Contd.)

Serial Stafion I)epth in Nature of ~b No. No. fathoml Sediment MnO

103 811 104 ~, 57 106 ~ 58 106 Z~ 50 107 �9 60 386

61 388 62 390 63 392 64 395 65 401 66 405 .~ 67 4117 68 4o8 69 411 7O 413 ~ 71 415 72 417 .~l 73 724 74 725 ,B 75 727 ~ 76 728 77 729 " 7s 7~ ~ 79 ~�91 80 736 81 737 82 738

83 653 84 654 85 665 86 656 87 ~ "~ ss 89 661

90 108 91 109 92 l lO

94 117 95 121 ~ 96 124 97 125

31.5 60

I00 lOO

18.5 8

11.5 25 22 22 21 49 36

51.6 50 30

7.5 34 45 12 20 33 19 30 38 74

100

23 30 55

I~8 88 55 35

25 29 10.5 11.6

100 63"5 50 63

Sand-silt-clay Silty chy

ii

Sand Sllty clay

Clayey sand Jp

i i

~p

Sand~iit-elay Sand

~v

Clayey sand ~J

Clayey silt Sand- s|lt-elay

Ciayvy silt Sand-silt-clay Clayey sand

Sand I r

t i

Silty clay I I

Silty clay l i

Clayey sand Sand-silt-clay Sandy clay

Clayey silt Sandy silt

~9

Clayey silt Silty cia},

Clayey silt

0.059 0.057 0"053 0.057 0.138 0.194

0.076 0"078 0.103 0.079 O. 161 0.052 0.O61 0.063 0.034 0.047 0"087 0 "070 0-079 0.089 0.146 0.099 0.083 0.1)83 0.059 0.044 0.071 0.049

0.084 0.076 0.054 0.065 0.044 0.057 0.065

0.071 0.068 0.092 0-085 0.093 0.085 0.097 0.093

* Langulya r iver confluence is about 60 miles north-east o f Visakhapatnam.

4. In a given area, there is a seaward fall in manganese concentration in the sediments. Referring to Fig. 2, ir can be seen that in the Visakhapatnam area where the sediments compositionally and texturally fall into several types (details will be given in a forthcoming paper), there is a seaward fall in manganese cortcerttration down to the depth of 70 fathoms. From about

Manganese in the Shelf Sediments Off East Coast of India

TABLE II

Variation o]manganese with silt-clav and calcium carbonate

279

SI. No.

Station Depth in ~, % No. fathoms MnO CaCOa Silt-clay

Visakhapatnam Area

1 309 16 0.099 4.0 39.0 2 91 22 0-079 8.0 71.3 3 740 22 0-080 6.0 71-0 4 101 23.5 0.090 8.5 75.6 5 310 24 0"070 8"0 72"0 6 311 30 0.070 8.5 62.4 7 312 33 0.071 11.0 78.3 8 94 40 0.054 19.5 39" 8 9 313 40.5 0"065 34.0 40.9

10 95 44 0.044 33.0 34- 0 11 314 48 0.044 59.5 32.4 12 96 48.5 0.044 51 "5 41 "9 13 97 53.5 0-030 61.5 31.6 14 315 58 0.025 74.0 15 8 15 98 58.5 0-022 69.5 22" 7 16 99 70 0.018 73"5 17.6 17 743 76 0-025 60.0 33.9 18 100 84 0.019 66.0 22.6 19 126 E 97 0"028 68-0 37.9 20 316 104 0.044 31 "0 57.6 21 317 156 0.049 12"5 68.3 22 605 382 0.052 7.5 70.0 23 720 100 0" 059 6.5 72.5

Pentakota Area

24 301 12" 5 O" 093 19" 0 47- 9 25 302 23 0-088 10"0 92" 8 26 303 27 0"074 9"0 95"6 27 304 32 0"070 10'0 96"4 28 306 48 0"045 48"0 38" 6 29 307 78 0"057 22"0 81" 5 30 364 20.6 0"098 7.5 41.4 31 365 22.6 0.089 8.5 17.8

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280 M. Stm,A RAo

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V ISAKHAP, ATNAM SECTION

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DF..PTH iN F A T H O M S

~ENTAKOTA SECTION

Fto . 2. Variation of manganeg ,,Atl" CaCOj and lilt-clay.

that depth it slowly increases t,o about 0.0719/o MnO at the shelf edge and remains probably about the same on the continental slope and on the deep floor of Bay of Bengal (in its northern half). A similar trend*is discernible in the case of the Pentakota area too. Ir, both the areas, the manganese curve follows the trend of the curve for the fine terrigenous material and reacts inversely with that of the calcium carbonate suggesting that manganese deposition is closely controlled by that of terrigenous detritus brought from the landward end. It is to be noted that, despite textural similarities, the near-shore silty clays at depths of 20-30 fathoms contain more manganese (0.080~ MnO) than the silty clays oeeupying the continental slope (0"0569£ MnO).

5. Generally speaking, the brown-eoloured sediments, espeeially those that do have a surficial brown layer, are richer in manganese than those of other hues.

DISCUSSION

The manganese eontent of deep sea deposits has been att¡ to differ- ent agencies: voleanie eruptions, submarine weathering, baeterial precipi- tation, biological extraetion, and ehemicaI precipitation. But in the case of continental shelf deposits accumulating not far from the land mass the sourcr of manganese lir undoubtedly in the adjaeent land mass. From a study of dissolved and exehangeable manganese eontent in river and otean muds, Murata (1939) eoneluded that manganese is carried to the sea both in solution �91 a s a eonstituent of the suspended sedimentary debris, The Ganges,

Manganese in the Shelf Sediments Off East Coast of India 281

Mahanadi, Godavari and Krishna a n d a number of small rivers and streams debouch into Bay of Bengal and their waters and their load of colloidal particles which adsorb ions of heavy metals as iron and manganese (Mason, 1958) constitute the main souree of manganese in the shelf sediments.

During weathering, manganese is dissolved mainly as bicarbonate, Mn(I-ICO3)2. Divalent manganese is readily oxidized to the quadrivalent state when the solution is brought into contaet with atmospheric or dissolved oxygen in lakes or in shallow near-shore waters in the sea. pH is another important factor in controlling the migration of manganese and the colloidal hydrates of manganese oxide precipitate at a pH ----- 6. When river-waters of low pH meet with sea-water of normal salip.ity the dissolved and exchangeable manganese is precipitated as Mn(OH)4 or finely divided MnO2. According to Rankama and Sahama (1950), the precipitation often takes place in fresh- waters. Most of the remaining manganese is removed in brackish water at the river mouths in the sea. Manganese, thus brought down to the sea floor, will be preserved as long as the s~diments are in contact with the oxy- genated waters. The degree of development of the brown-coloured oxidized film on the surface of the sediments is ah index of intensity of oxidation processes at the sea bottom. The presence of carbon dioxide is also helpful in th~t it exerts a depressing effect on the bottom living fauna and hence conserve the supply of oxygen for oxidation purposes.

The high concentration of manganese in the Godavari and Krishna deltaic areas may be primarily due to a good source of it in the river waters. These two rivers drain the Deccan traps in their upper reaches. The chemical composition of different traps show that manganese content va¡ from 0.22% MnO for lower traps to 0 .11~ MnO for upper traps, averaging 0.16~0 MnO (Krishnan, 1960). The traps, on weathering, give rise to either a deep brown to rich red soil of to ' r egur ' which are rich in lime, magnesia, iron, and alkalies. An important tributary of the Godavari, the Pranhita, drains the Bhandara area in Central India containing manganiferous deposits. Drain- ing the Khondalitic formations, some important tributaries join these ¡ The Khondalitic suite of rocks is characterised by mineral content fairly rich in manganese. Chemical analyses of the Khondalites showed MnO content ranging from 0.06-0.36% while altered Khondalites and laterites derived from them contained no manganese (Krishnan, 1934). Obviously the South Indian rivers carry to the sea in their waters considerable quantities of manganese derived from the soils formed of the above formations. This situation is quite in contrast with that obtaining in the drainage basins of the Gangetic system of rivers.

282 M. SUSBA RAO

In the coastal waters away from river confluentes salinity var.ies from about 10~o in October to 35~o in April-May. The low saline waters result from the influx into the Bay of river waters in the monsoonic periods and these waters are mainly restricted to the coastal areas. In this arca strong salinity gradients also are observed. On several occasions the surficial brown layer was observed to persist in samples to depths of about 45 fathoms. The mixing of the river waters with those of the open Bay with the resulting change in the oxidation potential, and the favourable hydrodynamic conditions for the deposition of the finest silt-clay particles have resulted in the silty clay forming at depths of 20-30 fathoms with more manganese probably as ad- sorbed on the colloidal particles. Further, these sediments do not seem to be supporting benthonic fauna in abundance, which means conservation of oxygen for oxidation purposes, thereby fixing the manganese.

The silty clays of the continental slope are not so rich as those of the coastal areas even though they are texturally far more favourable than the latter. This is understandable for they are removed from the coast by a considerable distance. The presence of glauconitic material, richness in organic matter of the sediments (Subba Rao, 1960), and absence of the surticial brown layer may all point to a slow deposition rate and a reducing environ- ment prevailing on the continental slope. Part of manganese precipitated there may possibly be redissolved at the bottom under the reducing conditions (Rankama and Sahama, 1950).

The poorest zone of manganese concentration which corresponds to the zone of high calcium carbonate is characterised by low content of terrigenous material of fine grain size (Subba Rao, 1958). The deposition of terrigenous material in this zone is taking place very slowly (Subba Rao, in press). Oolites which are the major constituents of the calcareous sediments do not contain any manganese (Poornachandra Rao, 1955). It is likely that whatever manganese that is precipitated at suitable points might remain in suspension and its deposition is prevented by strong currents that prevail over this part of the shelf.

" T h e late of manganese can be predicted even from the colour of bottom sediments. Green and blue or black muds (indicating presence of ferrous iron, sulphides or humic substances) will suggest leaching of manganese. Red clays (indicating oxidation to ferric iron) wiU be indicative of flxation and reprecipitation of manganese also (Goldschmidt, 1954)". The shelf sediments can be broadly divided into five groups on the basis of their colour: (1) those across the shelf from 15-50 fathoms which ate grey, greeaish-grey

Manganese in the Shelf Sedirnents Off East Coast of India 283

in eolour, with a brown layer at the top, (2) those from 70-100 fathoms depth, greenish-grey and dark grey in colour, (3) those from more than 100 fathoms depth which are bluish-grey at the bottom and grey at the top, (4) the Gangetic sediments which are light greenish-grey when wet and light ash grey when dry, and (5) the Godavari and Krishna sediments having a brown top layer and dark grey or black layer at the bottom. The first and last groups of sediments are richer in manganese. The Gangetic sediments come next in the concentration of manganese followed by sediments of the continental slope. Last comes the sediments of the second category which have been shown to contain calcium carbonate in abundante.

SUMMARY AND CONCLUSION

On the basis of determinations of manganese content in 97 sediment samples, the distributional pattern of manganese in the shelf sediments off east coast of India has been evaluated. It is found in its highest, concentration (0.19~ MnO) in a sample from a depth of 8 fathoms opposite the Langulya river confluence, about 60 miles north-east of Visakhapatnam, andin its lowest concentration (0.018~ MnO) in a sample from a depth of 70 fathoms off Visakhapatnam. The source of manganese in these sediments has been traced to the rocks in the drainage basins of various rivers emptying their waters and sediment load ioto the Bay. The black cotton soils and laterific soils derived from and overlying the Decean Traps and rocks belonging to the Dharwars and Sausar Series containing manganese deposits which have a widespread in the drainage basins of the South Indian rivers contribute significant quantities of manganese to these river waters. A general correla- tion between manganese concentration and grain size has been established, the highest eoncentrations being found in silty clays which indicates that ir is mostly ¡ by the colloidal particles of the finest detritus. But the manganese eontent shows a reduction in a direction that is seaward and away from river eonfluences. Ah antipathic relationship between manganese eontent and calcium earbonate content in these sediments has been established. The contact of two water masses of different origin and of different composition with the resulª change in the oxidation potential, and the movement of these water masses eontrolled by waves and currents llave been emphasized in determining the distributional pattern of manganese in the shelf region. Finally, of several elements that ate contributed by the source rocks, manganese, if rigorously worked out on the lines of Keith and Degens (1959), appears to be one of the few that hold out promise to be potential environ- mental indicators in the neritic regime off east coast of India.

A6

284 M. SunaA RAO

ACKNOWLEDGEMENTS

The author wishes to express his grateful thanks to the late Prof. C. Mahadevan for his keen interest in the work during its progress.

REFERENCES

Goldschmidt, V.M. . .

Groves, A .W. ..

Keith, M. L. and Degr E. T.

Krishnan, M. S.

Mason, B.

Murata, K. J.

Poornachandra Rao, M.

Rankama, K. and Sahama: Th. G.

Subba Rao, M.

Geochemistry, Oxford University Press, London, 1954, 637-42.

Silicate Analysis, George Allen & Unwin Ltd., Lendcn, 1951.

Researches in Geochemiatry, John Wiley & Sons, Irc.. l~tw York, 1959, 38-61.

. . "Lateritisation of Khondalites," Rec. GeoL St~rro', India. 1934, 68, 395.

. . Geology of India and Burma.. Higginbothams (Private) Lid., Madras, 1960, 475.

. . Principles of Geochemistry, John Wilcy & Sons, New York, 1956, 169.

. . Amer. Jour. Sci., 1939, 237, 725-35.

. . "Some aspects of marine geolog~ in certain parts ct ]/ay of Bengal," Doctoral Thesis submitted to the Andhra Uaiversity, Waltair, 1955 (unpublisLed).

Geochemistry, The University of Chicago Prr Chicago: 1950, 647-52.

.. Jour. Sed. Petrol., 1958. 28, 274-85.

.. Bull. Amer. Assoc. Petrol. Geol., 1960, 44, 1705-13.