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Biogeochemistry of the Surficial Sediments of the Western and EasternContinental Shelves of IndiaAuthor(s): Josia Jacob, N. Chandramohanakumar, K. A. Jayaraj, T. V. Raveendran, K. K. Balachandran,Thresiamma Joseph, Maheswari Nair, C. T. Achuthankutty, K. K. C. Nair, Rejomon George, and ZeenaP. RaviSource: Journal of Coastal Research, Number 245:1240-1248. 2008.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/06-0794.1URL: http://www.bioone.org/doi/full/10.2112/06-0794.1

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Journal of Coastal Research 24 5 1240–1248 West Palm Beach, Florida September 2008

Biogeochemistry of the Surficial Sediments of the Westernand Eastern Continental Shelves of IndiaJosia Jacob†, N. Chandramohanakumar§, K.A. Jayaraj‡, T.V. Raveendran‡, K.K. Balachandran‡, ThresiammaJoseph‡, Maheswari Nair‡, C.T. Achuthankutty†, K.K.C. Nair†, Rejomon George†, and Zeena P. Ravi§

†National Institute of OceanographyRegional CentreKochi–682 018Indiajosiajacob@yahoo.co.in

‡National Institute of OceanographyRegional CentreKochi–682 018India

§Department of Chemical OceanographySchool of Marine SciencesCochin University of Science and

TechnologyKochi–682 016India

ABSTRACT

JACOB, J.; CHANDRAMOHANAKUMAR, N.; JAYARAJ, K.A.; RAVEENDRAN, T.V.; BALACHANDRAN, K.K.; JO-SEPH, T.; NAIR, M.; ACHUTHANKUTTY, C.T.; NAIR, K.K.C.; GEORGE, R., and RAVI, Z.P., 2008. Biogeochemistryof the surficial sediments of the western and eastern continental shelves of India. Journal of Coastal Research, 24(5),1240–1248. West Palm Beach (Florida), ISSN 0749-0208.

Even though continental margins cover only about 7–9% of the world ocean area, they are important sites for organiccarbon accumulation and hence play a major role in biogeochemical carbon cycling. Surface sediments from the west-ern and eastern continental shelves of India were studied for their total organic matter content and the labile con-stituents such as total carbohydrates and proteins to get a better understanding of the hydrological and biogeochemicaldifferences of the two regions on the nature of organic matter reaching the surface sediments. The sampling alongthe west coast was done during September–October 2003, when upwelling signatures were found to be still persistingalong the shelf as evidenced by the low-oxygenated, nutrient-rich surface waters. The eastern shelf was sampledduring November–December 2002. The shelf was found to be oxygen saturated and oligotrophic during the season.The surface sediments of the west coast contained a greater quantity of total organic matter when compared to theeast coast, but the percentages of labile constituents of total organic matter in the sediments were found to be higherin the east coast by a factor of three as compared to the west coast. The differences in the productivity patterns andthe hydrographical conditions were found to exert a major influence on the quantity and composition of organic matterin the surface sediments of the western and eastern continental shelves of India.

ADDITIONAL INDEX WORDS: Surface sediments, total organic matter, total carbohydrates, proteins, western andeastern continental shelves of India.

INTRODUCTION

The characteristics of organic matter deposited in surfacesediments of marine environments are being widely used inthe correlation of several oceanographic processes, such assurface ocean productivity, input of land-derived materialsinto the ocean, dynamics of water masses, oxireduction po-tential, and sedimentation rates (MICHEL et al., 1999). Mostof the organic carbon annually reaching the ocean floor isconsidered to be deposited on the continental margins. Pres-ent-day continental margins are viewed as potentially themost important sites of organic deposition in the ocean be-cause a greater proportion of total primary production occursover them and a greater proportion of it reaches the bottomthan does the open ocean because of the shallower waterdepths. The preservation of organic matter is greater on somehighly productive shelves than on others, perhaps because ofa reduced availability of oxygen in low-energy depositionalenvironments or of rapid burial of organic matter by mineralparticles in regions where sedimentation rate is high. Organ-

DOI:10.2112/06-0794.1 received 7 November 2006; accepted in revi-sion 23 May 2007.

ic matter preservation is of global importance because it rep-resents a significant sink of organic carbon, which gets ‘‘lost’’from the biosphere through burial and hence is a potentialcontrol on CO2 concentration in the atmosphere (POCKLING-TON, LEONARD, and CREWE, 1991).

Even though the Arabian Sea and Bay of Bengal are twinseas bordering the Indian landmass, they represent two hy-drologically and biogeochemically contrasting areas of signif-icance to global cycling of carbon and nitrogen (DILEEP KU-MAR et al., 1998). Both regions are strongly affected by sea-sonal changes associated with the monsoon, but the Bay ofBengal is traditionally considered to be less productive com-pared to the Arabian Sea (GAUNS et al., 2005; MADHUPRATAP

et al., 2003; PRASANNA KUMAR et al., 2002). Many authorshave studied the distribution of organic matter in the sedi-ments of the western and eastern continental shelves of India(BHOSLE and DHOPLE, 1988; BHOSLE, DHARGALKAR, andBRAGANCA, 1978; MURTHY, REDDY, and VARADACHARI,1969; PAROPKARI, 1979; RAO, 1960). The present study aimsto compare the quantity and the labile composition of theorganic matter in the surface sediments along both regions.The study will provide a better understanding of the effectsof different productivity patterns and hydrographic condi-

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Figure 1. Map showing the station positions. Dotted lines give a roughestimate of the outer boundary of the continental shelf.

tions on the nature and composition of organic matter of thesurface sediments of the continental shelf of the Arabian Seaand Bay of Bengal.

STUDY AREA

The western continental shelf of India is a prominent sub-marine topographic feature stretching all over the westernboundary from Cape Comorin (8� N) in the south to Kutch(22� N) in the north. The shelf is wider in the northern regionand narrows progressively to the south. Except for the north-ern part where the rivers Narmada and Tapti drain into theArabian Sea, the general continental drainage all along thecoast is characterised by small rivers and streams (DILLI,1986). It is recognised as one of the most productive conti-nental margins as a result of the biogeochemical responsesto upwelling caused by the SW monsoon.

The eastern shelf is narrower than the western shelf. The100 fathom isobath is considered the outer limit of the shelf(SIDDIQUE, 1967). The shelf at the mouths of the rivers re-ceives a large part of its sediment supplies from the Ganges,Brahmaputra, and Mahanadi rivers in the north; Godavariand Krishna rivers in the central region; and comparablysmaller rivers such as Pennar and Cauvery in the south(RAO, 1985). The uniqueness of the shelf results from thesemajor rivers introducing 1.4 � 109 tonnes of suspended sed-iments into the region. The shelf is narrowest at Godavariand Krishna deltas and widest at the Ganges–Brahmaputradelta. The topography of the shelf is monotonous except forsome canyons cutting deeply into the shelf (SIDDIQUE, 1967).The east coast is considered to be less productive than thewest coast (MADHUPRATAP et al., 2003). Although the river-ine flux may bring in nutrients, these are thought to be lostto the deep because of its narrow shelf (QASIM, 1977; RAD-HAKRISHNA et al., 1978; SEN GUPTHA, DE SOUZA, and JO-SEPH, 1977). However, it is a cyclone-prone region and theseepisodic events are likely to churn up the area, injecting nu-trients to the shallow euphotic zone and thereby enhancingproduction in the upper layers (MADHU et al., 2002).

METHODS

Sampling

Fifteen surface sediment samples were collected from thewestern continental shelf of India during September–October2003 and nine samples were taken from the eastern conti-nental shelf of India during November–December 2002 on-board Fishery and Oceanographic Research Vessel SagarSampada (Figure 1). Along the west coast, sampling wasdone from eight latitudes—one sample from the inner shelf(�50-m water depth) and the other representing the outershelf (�150-m water depth) except at 8� N, where only theouter shelf sample could be obtained. Along the east coast,sampling was done from six latitudes. Except along 17� and19� N, one sample was collected from the inner shelf and an-other from the outer shelf. The sediment samples were col-lected using Smith McIntyre grab having an area of 0.1 m2.The vessel was stopped during the sampling, and the grabwas supplemented with external lead weights. Sampling was

repeated until the grab was completely filled to retrieve in-tact surface sediment layers. The surface sediment layers (0–1 cm) were immediately stored at �20�C. Before analysis,samples were thawed, air-dried, and ground to a fine powder.Water samples were collected from standard depths, using aSeabird conductivity–temperature–depth rosette system fit-ted with a 1.7-L Niskin sampler.

Laboratory Analysis

The sediment textural characteristics (sand, silt, and clay)were determined by pipette analysis (KRUMBEIN and PETTI

JOHN, 1938) after removing the inorganic carbonates using10% HCl and organic matter using 15% H2O2. Organic carbonwas determined according to GAUDETTE and FLIGHT (1974).The analysis was done in triplicates, and the standard devi-ation was found to be 4.2%. The amount of total organic mat-ter (TOM) was obtained by multiplying the organic carbonvalues with 1.8 (MULLER, SUESS, and UNGERER, 1986). Totalcarbohydrates (TCHOs) were analysed according to GER-CHAKOV and HATCHER (1972), after the hydrolysis of the sed-iment with 1N H2SO4, and expressed as glucose equivalents.The standard deviation of the triplicate analysis was foundto be 4.4%. Proteins (PRTs) were analysed according to LOW-RY et al. (1951) as modified by RICE (1982) to account for thereactivity of phenolic compounds, after extraction with 1NNaOH for 30 minutes. The PRT concentrations are given asbovine serum albumin equivalents. Triplicate analysis of thesamples gave a standard deviation of 5.7%. Sediment sam-ples previously treated in a muffle furnace at 450�C for 6hours were used as the blanks for all the biochemical anal-ysis. Analysis for the dissolved oxygen in the water sampleswas done by the Winkler’s method, and the dissolved inor-ganic nutrients (nitrate, nitrite, and phosphate) were esti-mated with a SKALAR 4-channel Autoanalyser onboard fol-lowing standard procedures (GRASSHOFF, EHRHARDT, andKREMBLING, 1983).

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Figure 2. Distribution of dissolved oxygen and nutrients along the inner shelf of the west coast.

Data Analysis

Pearson correlation was carried out to test correlationsamong organic matter, its labile constituents (PRT andTCHO), and the texture of the sediment samples.

RESULTS

Hydrographic Conditions along the Shelf

The distributions of dissolved oxygen and dissolved inor-ganic nutrients (nitrate, phosphate, silicate, and nitrite)along the western continental shelf are shown in Figures 2and 3. During the season, the 13–17� N were characterisedby low-oxygenated (�170 �M) and nutrient-rich surface wa-ters (NO3 � 2 �M and PO4 � 0.8 �M). The 170-�M contourof dissolved oxygen and 2-�M contour of NO3 found about 18m along 10� N was traced up to 8 m along 15� N but was onlyabout 35 m along 21� N. The 0.8-�M contour of PO4 foundabout 18 m at 10� N was found to surface at 13–17� N. Theprevalence of nutrient-rich waters along the shelf during theseason suggests the persistence of upwelling signatures alongthe coast even during the late phase of summer monsoon. Theentire shelf was characterised by oxygen-deficient (�20 �M)subsurface waters (�20 m), but much lower values (�10 �M)were observed at the intermediate depths along the innershelf (20–30 m between 13 and 15� N) and the outer shelf(100–150 m between 15 and 17� N) (Figures 2 and 3). Intensedenitrification was also observed in these waters, as evi-denced by a nitrite concentration of �4.5 �M. The acute ox-ygen deficiency may be due to the increased oxygen utilisa-tion for the decomposition of the organic matter from the sur-face layers, resulting from the enhanced production due toupwelling.

In comparison to the western shelf, the surface waters (upto 40 m) of the eastern shelf were oligotrophic, with very low

nutrient content (NO3 � 2 �M and PO4 � 0.8 �M) and sat-urated with oxygen (�200 �M) (Figures 4 and 5). The entireinner shelf was oxygen saturated, but the intermediate wa-ters (below 100 m deep) of the outer shelf was oxygen defi-cient (�30 �M), as it may be the outer limit of the permanentoxygen minimum zone (Figure 5).

Along the west coast, the dissolved oxygen content of thebottom waters along the outer shelf was in the range 8 to 25�M and that along the inner shelf was in the range 7 to 70�M (Figure 6a). The dissolved oxygen content of the bottomwaters was the lowest along 11�, 13�, 15�, and 17� N. Oxygen-minimum conditions (�10 �M) were especially observed inthe outer shelf along 17� and 19� N. The bottom temperaturealong the outer shelf ranged from 16.2 to 23.6�C and alongthe inner shelf ranged from 16.3 to 25.3�C (Figure 6b). Thetemperature of the bottom waters of the outer shelf off 13�,15�, 17�, and 19� N were found to be the lowest (16.2–16.7�C).

The dissolved oxygen concentrations of the bottom watersalong the outer shelf of the east coast were low (11–33 �M)compared to the inner shelf (130–195 �M) (Figure 6c). Thebottom temperatures along the outer shelf were in the range15.5 to 26.2�C, and those along the inner shelf were in therange 27.9 to 28.6�C (Figure 6d). The temperature (15.5�C)and dissolved oxygen (16 �M) were the lowest at the outershelf of 13� N. The organic matter was found to be the highestat this station.

Sediment Characteristics

Along the west coast, the southern transects (south of 13�N) were dominated by coarse-grained particles (sand 63%, silt22%, and clay 15%) when compared to the northern transects,which were mainly dominated by fine-grained particles (sand18%, silt 61%, and clay 21%) except at the outer shelf of 21�N (Figures 7a and 7b). In general, the shelf of the east coast

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Figure 3. Distribution of dissolved oxygen and nutrients along the outer shelf of the west coast.

Figure 4. Distribution of dissolved oxygen and nutrients along the inner shelf of the east coast.

was dominated by silt followed by sand (Figures 7c and 7d).The average percentage composition of sand, silt, and clayalong the shelf was 32, 53, and 15, respectively. Sand wasdominant along the southernmost transect (11� N), but fine-grained silt and clay dominated the stations north of the 13�N transect along the entire shelf.

TOM along the western shelf ranged between 7.2 and 65.5mg/g (22.8 17.4), whereas along the eastern shelf it was in

the range 4.9 to 21.5 mg/g (10.5 5.6) (Figure 8a). Along thewest coast, maximum TOM was found at 15� N and 17� N,both along the inner shelf and the outer shelf, while TOMwas maximum at 13� N along the east coast. TCHOs alongthe west coast varied between 1.08 and 9.88 mg/g (2.74 2.30). As observed for TOM, TCHO was concentrated along13 to 17� N. Along the east coast, TCHO was in the range1.28 to 4.43 mg/g (2.72 0.95) with the maximum observed

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Figure 5. Distribution of dissolved oxygen and nutrients along the outer shelf of the east coast.

Figure 6. Distribution of (a) dissolved oxygen and (b) temperature in thebottom waters of the west coast and (c) dissolved oxygen and (d) temper-ature in the bottom waters of the east coast of India.

at 13� N (Figure 8b). PRT ranged between 0.09 and 1.02 mg/g (35 0.26) along the west coast, whereas along the eastcoast it ranged between 0.17 and 0.55 mg/g (0.34 0.13)(Figure 8c). Following the same trend as that of TOM andTCHO, the labile component PRT was also found to be con-centrated along 13 and 17� N in the west coast and 13� Nalong the east coast. The west coast sediments contained a

greater quantity of TOM, but the percentage of labile com-ponents (TCHO and PRT) in the east coast was found to begreater (14.7–60.3%) than that in the west coast (8.3–22.6%)(Figure 9).

Significant correlations of TOM with TCHO (r 0.803, n 16, p � 0.01) and PRT (r 0.948, n 16, p � 0.01) wereobserved for the west coast sediments, while rather poor cor-relations were observed between TOM with TCHO (r 0.516, n 9) and PRT (r 0.456, n 9) along the east coast.But significant correlation was observed for TCHO and PRTin the sediments along both the west coast (r 0.859, n 16, p � 0.01) and the east coast (r 0.887, n 16, p � 0.01).TOM, TCHO, and PRT were significantly correlated with clay(r 0.724, p � 0.01; r 0.517, p � 0.05; r 0.638, p �0.05, respectively) along the west coast, but no significantcorrelation of organic matter with texture was found alongthe east coast.

DISCUSSION

Along the west coast, 13–17� N were characterised by up-welling as evident from the low-oxygenated, nutrient-richsurface waters. Enhanced biological production with relative-ly colder, nutrient-rich waters advecting to the mixed layersis found in the region until October (BANZON et al., 2004;JOHANNESSEN, SUBBARAJU, and BLINDHEIM, 1987; GERSON,2005). The subsurface waters were suboxic, with very low dis-solved oxygen (DO � 5 �M) and NO2 (�4 �M), which indi-cates the high oxygen demand in the area. NAQVI et al. (2000)have reported severe oxygen deficiency leading to anoxiaalong the entire west coast during the same season. Such acondition is also reported in other upwelling areas of theworld, such as off Namibia (CALVERT and PRICE, 1971) andPeru (CODISPOTI and PACKARD, 1980). The lowest tempera-

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Figure 7. Distribution of sand, silt, and clay along the (a) inner shelf and the (b) outer shelf of the west coast and (c) inner shelf and (d) outer shelf ofthe east coast of India.

ture was also observed along the outer shelf of these latitudes(�15�C). TOM and the labile components (TCHO and PRT)were found to be the maximum in the surface sedimentsalong these latitudes. The hydrographic conditions of theoverlying waters, such as high production and low dissolvedoxygen and temperatures were found to be favourable for theaccumulation of organic matter in the surface sediments ofthe west coast during the highly productive upwelling season.The surface waters of the eastern shelf were found to be ol-igotrophic and saturated with oxygen during the study peri-od.

Even though the sediments of the western shelf contained

greater TOM, the surface sediments along the east coast con-tained greater percentages of TCHO and PRT by a factor ofthree than the west coast. Rivers from the Indian subconti-nent alone contribute �1387 � 106 tonnes/y�1 of suspendedsediments to the east coast (SUBRAMANIAN, 1993). The influ-ences of these rivers is not limited to the estuaries and deltas;it reaches far into the open Bay of Bengal. These terrigeneousmaterials exert a major influence on the biogeochemical andhydrological characteristics of the bay. The ballasting effectsof the lithogenic materials introduced by the rivers acceleratethe removal of newly fixed organic matter from the watercolumn to the sediments. In addition, this mineral organic

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Figure 8. Distribution of (a) TOM, (b) TCHO, and (c) PRT along the west and east coasts of India.

association leads to an increase in sedimentation rates andsubsequently to better preservation of organic matter (DI-LEEP KUMAR et al., 1998; ITTEKOT et al., 1992). From theresults of the studies on the respiration rates in the subsur-face waters of the northern Indian Ocean, much lower elec-tron transport system activities were observed in the waters

of the Bay of Bengal than in the Arabian Sea; hence the par-ticulate organic matter undergoes a lesser degree of oxidationin the water column through its incorporation into rapidlysinking organic matter (NAQVI et al., 1996). The bulk of or-ganic matter transported by the rivers is highly degraded andderived from the soils upstream and from floodplains (ITTEK-

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Figure 9. Percentage composition of TCHO and PRT in the sedimentsof (a) inner shelf and (b) outer shelf of the west coast and (c) inner shelfand (d) outer shelf of the east coast of India.

OT and ZHANG, 1989; ITTEKOT et al., 1985). This may lead tothe dilution of organic matter in the sediments, as the TOMin the east coast was found to be low. Moreover, the surfaceproduction along the shelf is less when compared to the west-ern shelf. But the abundance of labile organic matter wasfound to be greater along the east coast than along the westcoast. Rapid sedimentation promotes the burial of fresh,highly reactive organic matter, while in areas of slow sedi-mentation, much of the organic matter that reaches the sea-floor decomposes before it can become deeply buried, even inoxygen-depleted environments (SCHUFFERT et al., 1994). Thecontribution of hydrolysable amino acids and carbohydratesto total organic carbon in the surficial sediments is found tobe low (�10%) along the Oman margin, while organic matterin Peru margin sediments is mostly labile (ALAGARSAMY,2003). Along the Oman margin, most of the organic materialreaching the sea bottom is already degraded. Hence, alongthe east coast, the lithogenic input may lead to an increasein the sedimentation rates and subsequently to a better pres-ervation of organic matter in the surface sediments, as evi-denced by a greater percentage of labile organic matter alongthe east coast compared to the west coast.

CONCLUSION

The present study was done to compare the nature anddistribution of the organic matter in the shelf sedimentsalong the west and east coasts of the Indian landmass, whichare characterised by entirely different hydrographical fea-tures, such as differences in the productivity of the overlyingwater column, input of terrestrial materials, settling rate,and remineralisation of organic matter in the water column.

During the period investigated, the west coast was charac-terised by upwelling and associated anoxia in the subsurfacelayers, which forms an ideal situation for organic matter ac-cumulation, whereas the east coast was oligotrophic and ox-ygen saturated. Even though TOM in the surficial sedimentsof the west coast was high during the most productive up-welling period, the labile components (TCHO and PRT) werefound to be greater in the east coast. The east coast sedi-ments were found to preserve a greater portion of labile or-ganic matter than those in the west coast. The differences inthe hydrographical characteristics of the western and easterncontinental shelves were found to influence the quantity andnature of organic matter along the regions. The ecologicalimportance of this on the biogeochemistry of the regionshould to subjected to further studies.

ACKNOWLEDGMENTS

We would like to thank the director of the National Insti-tute of Oceanography (NIO), Goa, for providing facilities forthe study. This investigation was carried out under the pro-gramme ‘‘Environment and Productivity Patterns along theEEZ of India,’’ funded by the Department of Ocean Develop-ment, government of India, New Delhi. We thank Dr. DanielaUnger, Centre for Tropical Marine Ecology, Bremen, Ger-many, for the critical review, which has helped to increasethe quality of the paper considerably. J. Jacob thanks theCouncil of Scientific and Industrial Research, India, for thefinancial assistance. This is NIO contribution number 4291.

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