Geochemical evaluation of sea surface sediments along the continentalshelf, south east coast of India

V Nisha & H Achyuthan

Department of Geology, Anna University, Chennai 600 025, India[E-mail: nishakeshav22@gmail.com]

Received 21 March 2012; revised 12 September 2012

Present study consists geochemical variations and spatial distribution of trace elements (Fe, Mn, Cr, Co, Cu, Ni,Pb and Zn) of the ocean floor sediments collected along the continental shelf off East coast Tamil Nadu (Chennai toVelanganni) during the 2009 southwest monsoon period. Results indicate sand dominance with abundant calcareousdebris and shell fragments. Normalisation of trace metals with Al signifies a geogenic sediment origin. Enrichmentfactor (EF) of trace metals namely Co, Cr, Cu, Ni, Pb, Mn and Zn exhibit an increasing trend Pb>Cr>Co>Zn>Ni>Cu>Mnindicating high values of Pb, Cr, Co and Zn due to the anthropogenic source. However, contamination factor (CF) andpollution load index (PLI) point less than 1 indicating negligible contamination during the 2009 southwest monsoonperiod.

[Keywords: Southwest monsoon, Continental shelf, Ocean surface sediments, Trace elements, Texture, Geochemistry,Anthropogenic sources]

Indian Journal of Geo-Marine SciencesVol. 43(2), February 2014, pp. 241-251

Introduction

Geochemical studies of surface sediments alongthe continental shelves have been extensively carriedout in the last few decades due to the growingawareness of coastal pollution and its impact on theecosystem1-3. East coast of Tamil Nadu receives largevolumes of sediments discharged through severalmajor south easterly flowing peninsular rivers, thelargest being the Cauvery and the Vaigai river systemand several shorter streams. It is reported that about90% of the suspended particulate matter carried byrivers settles in coastal areas and estuaries4,5. Theirinput reveals physical and chemical erosion of theterrain over which they flow and is strongly dependenton the monsoon intensity, especially the southwestor summer monsoon and northeast rains6. Once themetals are released from the weathered sedimentaryenvironment, they are transferred to the sedimentsthrough adsorption onto the suspended matter suchas mud, organics, chelates and subsequentsedimentation7. The adsorption and sedimentationprocesses of metals mainly depends on the sedimentcomposition including grain size, sediment texture,carbonate content, level of organic matter, Fe-Mnoxyhydroxides as scavengers8 etc.

Plethora of literature on the geochemistry of theArabian Sea floor sediments9-15 has been carried outand it has been mainly focused on the clay mineralogy,foraminifera analyses to understand Quaternarypaleoclimate shifts and elemental distribution for theirprovenance. Similar such work has been carried outalong the east coast Tamil Nadu, Bay of Bengal,however with an emphasis on environmentalaspects1618. However, till date no detailed geochemicalwork has been conducted on the continental shelf floorsediments supporting sea water column greater than50 m-150 m along the east coast off Tamil Nadu. Inthis paper we present an geochemical evaluation ofthe sea surficial sediment along the continental shelf,east coast of Tamil Nadu from Chennai to Velanganniand a purpose to characterise the behaviour of themajor (Al, Ca, Fe, Ti, K, Mg and Na) and traceelements (Fe, Co, Cr, Cu, M, Ni, Pb, As and Zn).

Materials and Methods

Along the east coast Tamil Nadu, the seawardboundary of the Indian eastern continental shelf liesat ~200 m depth. Both the inner shelf and continentalslope (Fig. 1) are covered by classic sediments19. Theouter shelf is covered by calcareous relict sedimentsand off the river mouths (e.g. Cooum, Koratallaiyar,

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Adyar, Palar, Ponnaiyar, Kollidam, Velar, Vettar,Uppanar and the Cauvery) the shelf is covered by fine-grained terrigenous sediments. The southeast coastalzone receives ~65% of its rainfall from the NEmonsoon, the remainder (~35%) occurring during SWmonsoon period. Coastal and the continental slopebetween Chennai and Nagapattinam receive theintense Northeast monsoon (NEM) during Octoberto December and less intense SW rains during Juneto September. Bathymetry is shallow to deep and thetopography of the seafloor in the abyssal plane offthe east coast of India is fairly even20.

Surface samples on the seafloor were collectedfrom a number of locations (Fig.1) at various watercolumn depths between 50 m -150 m duringSeptember 2009 (during SW monsoon) along thecontinental shelf from Chennai N 13º04.3109’E80º29.0144’ to Velanganni N 11º29’94": E 79º58’29"using the research vessel ‘Sagar Paschimi’ ofNational Institute of Ocean Technology, Chennai.Sediment grab samples were collected using Van Veengrab sampling apparatus and the sediment sampleswere stored in pre-labelled plastic bags and wererefrigerated at -4°C in the laboratory and further, thesamples were analysed for sediment texture, calciumcarbonate, organic carbon (OC) and trace metalcontent. Grain size sediment texture was determinedfollowing the procedure of Carver (1971)21. Calciumcarbonate (CaCO

3) content in sediments was

determined following (Loring and Rantala 1992)22.Organic carbon was analysed by exothermic heatingand oxidation using potassium dichromate, with 0.5 Nferrous ammonium sulphate solutions23. For the tracemetal analysis, the sediments were dried at 50

oC in

an oven, homogenised and powdered using an agatemortar. The powdered subsamples were used formajor and trace metal analyses determination usingX-Ray Fluorescence Spectro Xpose powder methodat the Department of Geology, University of Poona,Pune. Statistical parameter such as correlation matrixwas applied to determine elemental interrelationships.In this study all the trace elements analysed werenormalised by Al as it compensates for variation ingrain size and composition. Enrichment Factor (EF)24,Contamination Factor (CF)25 and Pollution LoadIndex (PLI)26 was calculated for all the major and traceelements. Enrichment cactor (EF) was calculated foreach metal, using global average shale values24

adopting the formula,

EF = (Metal conc/Al conc) sample

(Metal conc /Al conc) shale

To understand the level of contamination andaccumulation of metals, contamination factor wascalculated using (Hokanson1980) 25 formula

Contamination Factor (CF) = Cmetal

/ CShale Value

Where Cmetal

is average metal concentration andC

shale value is the world average concentrations of the

considered elements reported for shale (Wedephol1995) 25. The level of pollution was calculated usingpollution load index formula (Tomlinson et al. 1980)26.

where CFn = contamination factor and n = number of

metals.

Results

Textural sediment characteristics

The relative abundance and texturalcharacteristics of sand, silt and clay content of thecontinental shelf floor sediment samples collected atvarious water column depths are presented in theTable 1. Silty sand grains dominate along the eastcoast continental shelf region with sand contentranging from 56.5% to 98.4%. At shallow depths ofwater column (75 m) the maximum sand% was foundat Marakkanam (90%), Cuddalore (97.4%), andPondicherry (85.4 %). At 150 m deeper water columnlevels, the maximum sand content was found atPondicherry (94.7%) and Chennai (92.27%). Claycontent is less than 1% (Fig. 2).

The calcium carbonate content was observed tobe higher in the central sites along the east coast

1 2 3 nP O LLU T IO N LO A D IN D E X (PL I) = n (C F X C F X C F X ......X C F )

Fig.1 Sampling locations from Chennai to Velanganni, Eastcoast Tamil nadu.

243NISHA & ACHYUTHAN: GEOCHEMICAL EVALUATION OF SEA SURFACE SEDIMENTS

Table.1 The textural types, CaCO3 and Organic matter content along the continental shelf of east coast Tamilnadu.

Stations Sample Latitude Longitude Sand % Mud % Sediment CaCO3 % OM%

numbers (Silt + Type Clay)

1 Chennai 75m N 13º04’31" E 80º29’01" 76.484 23.516 Silty sand. 2.4 9.1

2 Chennai 150m N 13º04’10" E 80º33’82" 92.276 7.724 Silty Sand. 2.8 8.2

3 Marakkanamn75m N13°05’4" E 80°22’57.26"90.016 9.984 Silty sand. 3 1.67

4 Marakkanamn150 m N 12º46’82" E 80º28’98" 64.188 35.812 Silty sand. 2.6 1.18

5 Pondicherry 50m N 12º47’14" E 80º37’16" 79.574 20.426 Silty sand. 1.8 2.93

6 Pondicherry 75m N 12º46’00" E 80º21’95" 85.48 14.52 Silty sand. 1.6 2.71

7 Pondicherry 150m N 12º34’44" E 80º30’33" 94.726 5.274 sand 1.8 2.67

8 Chidambaram 50m N 12º34’60" E 80º87’50" 75.628 24.372 Silty sand. 2.4 1.8

9 Chidambaram 75m N 12º14’09" E 80º15’19" 97.386 2.614 sand 1.8 2.1

10 Chidambaram 150m N 12º14’22" E 80º19’57" 72.496 27.504 Silty sand. 2

11 Tarangampadi 50m N 12º14’10" E 80º21' 41" 88.644 11.356 sand 4.6 0.7

12 Tarangampadi 75m N 12º14’32" E 80º10’11" 64.18 35.82 Silty sand. 2.3 2.1

13 Tarangampadi 150m N 11º53’39" E 80º02’87" 81.544 18.456 Silty sand. 2.2 2.32

14 Cuddalore 50m N 11º53’57" E 80º05’19" 98.372 1.628 sand 3.8 1.1

15 Cuddalore 75m N 11º53’55" E 79º57’47" 97.478 2.522 sand 3.8 1.1

16 Cuddalore 150m N 11º42’37" E 79º59’06" 79.666 20.334 Silty sand. 2.6 1.9

17 Nagapattinam 75m N 11º42’58" E 80º01’52" 56.462 43.538 Silty sand. 3 1.5

18 Nagapattinam 150m N 11º42’54" E 79º54’0" 60 40 Silty sand. 1.6 2.2

19 Velanganni 75m N 11º01’13" E 79º59’11" 71.91 28.09 Silty sand. 2 2

20 Velanganni 150m N 11º29’94" E 79º58’29" 74.136 25.864 Silty sand. 4 1

Fig.2 Texture, Organic matter and CaCO3 % at various

depths along the study area

continental slope of Tamil Nadu during the SWmonsoon period (Fig.3). At few stations such asChennai, Cuddallore, Tarangampadi, the CaCO

3

content was comparatively higher. Organic mattercontent was found to be lower in the southern regionsand it exhibited a higher concentration in the northernregions including Chennai during the SW monsoonperiod. Surfer diagrams show that the organic matterand CaCO

3 are inversely correlated (Fig. 4a-b).

Geochemistry and the spatial distribution of majoroxides and Trace metals

Surfer 8.0 software was applied to present thedistribution patterns of the major oxides (Fig.4c-g);(SiO

2, K

2O, Al

2O

3, CaO and Fe

2O

3). Spatial distribution

of SiO2 in the sea floor grab sediments reveal that the

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stations at 50 m seawater column depth (Fig. 5). Atstations 1, 4, 5 and 6, the average concentration oftrace metals (except Mn) shows marginally highercontent (Fig. 6a-e) at 150 m seawater column depth.Elements like Cr, Cu and Ni display quiet similarpattern of distribution and these elements are used asmarkers of metal industry contribution30. Elevatedvalues of Cu is exhibited at Tarangampadi and Chennai(41.9 ppm, 41.5 ppm) indicating anthropogenic origin,and controlled by the sediments, organic matter andgrain size31 distribution.

Correlation coefficient between metalconcentration shows that all the metals correlatesignificantly well with other parameters except sand,mud and CaCO

3. The results of the correlation matrix

(R2) are represented in the (Table 2). Minor metalssuch as Cr, Co, Ni, Co exhibit mutual coherency andalso show strong affinity towards Fe and Mn (R >+8).The strong correlation relationship of Cr with Al(r = 0.86), Cu with Ni (r = 0.9), Co with Pb (r=0.6)and Mn with Al (r = 0.86) exhibits their similarityand existence between them and localizedcontamination32. Fe has strong positive correlationwith all the other metals (r = 0.5 to 0.8) such as Mn,Ni, Zn, Co, Cr (0.59-0.7) suggesting importantassociation between oxides-Oxyhydroxides of Fe-Mnand other elements 33.

In the present study, normalisation was carriedout using Al content to calculate Enrichment Factor(EF) which is calculated using metal/Al values of theUpper Continental Crust and has been graphicallyrepresented (Fig. 6a-h). Al represents the quality of

Fig.3 Organic matter and CaCO3 % at various depths along

the study area

Fig.4 (a-g) Surfer plots showing the distribution of majoroxides, CaCO

3 and organic matter along the continental

shelf, Tamil nadu Trace metal concentrations of sea surfacesediments below a water column of 50m.

detritus quartz is not evenly distributed along thecontinental shelf and this is probably due to waveenergy. Distribution pattern shows that except CaO,all other oxides exhibit a higher concentrationtowards the southern region along the continentalshelf (Fig. 4 f).

Trace metal data indicate a higher concentrationat Chidamparam, Tarangambadi and Cuddalore

Fig.5 Trace metal concentrations of sea surface sedimentsbelow a water column of 50m.

24

5N

ISH

A &

AC

HY

UT

HA

N: G

EO

CH

EM

ICA

L EVA

LUA

TIO

N O

F S

EA

SU

RFA

CE

SE

DIM

EN

TS

Table.2 Correlation matrix of total trace and major elements in the sea surface sediments during the south west monsoon along east coast Tamilnadu.

Sand Silt Clay CaCO3 OM Fe Mn Cr Cu Ni Co Pb Zn Si Al Ca Mg Na K PSand 1.0

Silt -1.0 1.0

Clay -0.3 0.3 1.0

CacO3 0.3 -0.1 0.0 1.0

OM 0.1 -0.1 -0.2 -0.3 1.0

Fe 0.0 0.0 -0.2 0.1 0.1 1.0

Mn 0.2 0.0 -0.1 0.3 -0.1 0.7 1.0

Cr 0.0 0.0 0.0 -0.1 0.0 0.6 0.5 1.0

Cu -0.2 0.2 0.0 -0.2 0.4 0.6 0.2 0.6 1.0

Ni -0.2 0.2 0.0 -0.1 0.2 0.7 0.3 0.8 0.9 1.0

Co -0.1 0.1 0.1 -0.1 0.2 0.5 0.2 0.8 0.7 0.8 1.0

Pb -0.1 0.1 0.2 -0.2 0.1 0.5 0.1 0.5 0.7 0.7 0.6 1.0

Zn -0.1 0.1 -0.1 -0.2 0.2 0.7 0.4 0.6 0.7 0.8 0.5 0.7 1.0

Si 0.1 -0.1 0.0 0.0 0.2 0.7 0.6 0.5 0.3 0.4 0.3 0.3 0.4 1.0

Al 0.3 -0.1 -0.1 0.2 0.1 0.8 0.9 0.9 0.3 0.4 0.3 0.2 0.4 0.7 1.0

Ca -0.3 0.3 0.1 0.0 -0.2 -0.3 -0.4 -0.1 0.0 0.0 0.1 0.1 0.0 -0.5 -0.3 1.0

Mg -0.2 0.1 0.0 0.1 0.0 -0.1 -0.5 -0.3 0.2 0.0 0.0 0.1 0.1 -0.6 -0.4 0.5 1.0

Na 0.3 -0.2 -0.2 0.3 0.0 0.7 0.8 0.6 0.2 0.4 0.3 0.1 0.3 0.6 0.9 -0.2 -0.4 1.0

K 0.3 -0.2 -0.1 0.2 0.2 0.7 0.9 0.5 0.2 0.3 0.3 0.1 0.3 0.7 1.0 -0.3 -0.4 0.9 1.0

P 0.4 -0.2 0.0 0.2 0.0 0.4 0.7 0.7 0.1 0.2 0.3 -0.1 0.1 0.3 0.8 0.0 -0.1 0.8 0.8 1.0

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alumina silicates, which is the most important carrierof adsorbed metals in the coastal shore27-29.Theabundance of major elements and their ratios relativeto Al is found to be higher at Chennai and Cuddalore.Fe/Al ratio showed a maximum of 5.81 atTarangampadi. Estimated values of almost allelements were high at Chennai (Fig. 7a-h). The majorconcentrations of Al normalised metals can besummarised in the decreasing order asCa>Si>Mg>Fe>Na>K>Ti>P. The Enrichment Factorof the trace elements along the east coast continentalshelf is presented in (Table 3). The enrichment of thetrace metals over the crustal values is in the followingorder Pb>Cr>Co>Zn>Ni>Cu>Mn.

Contamination Factor (CF) and Pollution Load Index(PLI)

The values of CF for almost all trace metals inall the stations were CF < 1 (low contamination).Contamination factor of Pb, Cr and Co werecharacterized as 1 < CF > 3 (moderate contamination).CF levels in all the stations existed in the orderPb>Co>Cr>Fe>Zn>Ni>Cu (Table 4). PLI value islow and is <1 in all the stations compared to the globalshale values and hence contamination is insignificantalong the east coast continental shelf of Tamil Nadu.

Discussion

The maximum percentage of sand content wasfound at Cuddalore (98.4%) area and a minimum wasrecorded at Nagapattinam (56.5 %) during the 2009SW monsoon. Probably the inflow of the riverineinflux changed the sediment nature of the continentalshelf during this SW monsoon period. High sandcontent in the sediments indicates a stronger waveenergy regime accentuated by the local bathymetrythat inhibited sedimentation of finer grainedparticles34. The increase in the sand with minoramounts of clay content as observed in several stationsis also due to the influence of littoral drift and thecurrents from the abrasion zones that transport finersediments once they reach the coastal tract35.

The elevated CaCO3 content is of biogenic origin

and the inorganic CaCO3 input may also be from the

adjacent land mass where Tertiary limestone andcalcareous sandstones extend in the southern36-37.Relatively moderate values of carbonate content atthe depth of the various sea water column may bealso due to the sturdy current causing non-depositionof terrigenous material38 (Fig. 3). Sea surface sediment

Fig.6 (a-e) Trace metal concentration at 150m seawatercolumn depths various stations along east coast Tamilnadu

247NISHA & ACHYUTHAN: GEOCHEMICAL EVALUATION OF SEA SURFACE SEDIMENTS

Table.3 Enrichment Factor (EF) in the sea surface sediments.

Stations Zn Cu Cr Ni Co Fe PbChennai 75m 0.6 0.7 0.7 0.6 1.8 6.2 1.7Chennai 150m 0.9 1.2 1.1 1.1 1.3 7.4 1.1

Marakkanamn75m 1.5 0.3 1.3 0.2 2.0 33.7 6.6

Marakkanamn150 m 0.9 0.8 0.1 0.6 1.0 13.2 2.4Pondicherry 50m 2.9 0.1 0.5 0.4 0.4 11.6 1.9

Pondicherry 75m 0.6 0.6 0.3 0.4 0.2 8.0 1.2

Pondicherry 150m 0.9 1.1 1.3 0.9 2.1 6.4 2.0Chidambaram 50m 1.0 0.7 1.8 1.1 2.4 11.2 1.6

Chidambaram 75m 0.8 0.2 2.1 0.7 1.2 11.2 1.4

Chidambaram 150m 1.0 1.1 1.5 1.3 1.4 5.2 1.4Tarangampadi 50m 0.8 0.6 1.7 0.8 1.1 9.3 1.4

Tarangampadi 75m 1.9 2.1 3.0 2.5 2.9 17.7 2.8

Tarangampadi 150m 1.2 0.9 2.4 1.2 1.6 11.0 2.0Cuddalore 50m 0.3 0.1 0.7 0.2 0.5 10.5 0.4

Cuddalore 75m 1.4 1.4 2.4 1.4 3.9 17.4 5.4

Cuddalore 150m 1.4 0.9 0.3 0.6 0.2 11.9 2.7Nagapattinam 75m 0.5 0.4 0.5 0.4 0.2 11.9 1.1

Nagapattinam 150m 1.5 1.3 3.6 1.5 3.2 12.1 3.1

Velanganni 75m 1.0 0.9 2.4 1.0 2.2 8.0 2.1Velanganni 150m 1.2 1.0 2.0 0.9 2.9 11.7 1.8

Fig.7 (a-h) Major elements normalised by Al of the sea surface sediments along the continental shelf, Tamil nadu.

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samples off Pondicherry coast reveal low CaCO3

content (1.6 to 1.8%) and this is attributed to the finenature of sediments that are non-biogenic in origin.Generally, it is observed that there is no preferentialtrend in the distribution of CaCO

3 and OM content at

these depths (50 m, 75 m and 150 m). The variationsin the organic matter values in all the stations indicatesthat they are moderately influenced by the minor inputand deposition of organic debris from local industrialsources. This is largely due to a higher terrestrial andanthropogenic input from the minor rivers that draininto the Bay of Bengal39.

Co, Pb and Zn are elements with similardistribution pattern and are known markers of paintindustry40. The striking feature of the trace metalconcentration in the study area along the east coastTamil Nadu indicate the higher concentration(especially at stations 1, 4, 5 and 6) owing to theanthropogenic activities and influxes from thewaterways and rivers. Moreover, it is also due to therecent development of major industries (along thecoastal tract and offshore drilling) and minor harbouractivities where heavy movement of naval vessels sail

throughout the year for regular surveillance in thecoastal region41-44. In addition, many distributaries ofCauvery drain in to continental shelf through thedeltaic agricultural belts39. These inferences are wellsupported by the high values of Cr, Pb, Zn, Cu, whichare important chemical components of the fertilizersused in agricultural and irrigational activities. Thegeochemical elements delivered to the continentalslopes are not only anthropogenic but also reflectnatural flux of elements from the catchment areas.45

Na and Mg concentrations were low during the2009 SW monsoon period. Fe showed a higherconcentration of 5.8% at Tarangampadi which couldbe attributed from detritus grains supplied throughadjacent rivers. A slight increase in the P content withthe organic carbon suggests that P is partly controlledby the organic supply45. Significant fraction of tracemetals are found co-precipitated with or adsorbed onto Fe and Mn geochemical phases controlling the tracemetals in the sediments. These characters are due tothe large surficial area of the trace metals, extensivecation exchange capacity and wide spreadavailability46. Notably, the correlation and geochemical

Table .4 Contamination Factors of the sea surface sediments along the continental shelf, East coast Tamil nadu.Stations Zn Cu Cr Ni Co Fe PbChennai 75m 0.13 2.89 1.04 2.97 1.32 2.42 1.09Chennai 150m 0.11 7.76 2.62 9.31 1.55 5.34 1.14Marakkanamn150m 0.12 0.19 0.28 0.16 0.23 1.30 0.67Marakkanamn75m 0.05 1.13 0.03 0.97 0.25 1.16 0.54Pondicherry 50m 0.05 0.26 0.38 1.26 0.17 2.59 0.68Pondicherry 75m 0.06 2.07 0.35 1.88 0.17 2.92 0.71Pondicherry 150m 0.11 3.28 1.43 3.66 1.25 2.62 1.03Chidambaram 50m 0.07 3.17 3.16 6.74 2.19 5.97 1.28Chidambaram 75m 0.06 0.87 2.84 3.32 0.83 2.98 0.92Chidambaram 150m 0.06 7.69 3.87 12.00 2.01 5.81 1.69Tarangampadi 50m 0.12 2.65 2.61 4.68 0.90 4.18 0.97Tarangampadi 75m 0.05 7.69 3.87 12.00 2.01 4.89 1.69Tarangampadi 150m 0.16 2.65 2.61 4.68 0.90 2.07 0.97Cuddalore 50m 0.03 1.48 3.18 3.88 1.08 5.47 0.81Cuddalore 75m 0.04 2.06 1.28 2.62 1.08 2.10 1.32Cuddalore 150m 0.04 3.35 0.46 2.83 0.17 4.30 1.75Nagapattinam 75m 0.13 1.46 0.63 1.84 0.17 4.30 0.69Nagapattinam 150m 0.09 3.15 3.16 4.78 1.50 2.58 1.26Velanganni 75m 0.07 3.15 3.16 4.78 1.50 2.92 1.26Velanganni 150m 0.13 1.96 1.41 2.41 1.08 2.35 0.59

Average 0.45 0.35 0.64 0.40 0.64 0.72 0.76

249NISHA & ACHYUTHAN: GEOCHEMICAL EVALUATION OF SEA SURFACE SEDIMENTS

association of trace metals reveal a significant sourceof contamination that reflect a common origin ofsimilar nature existing probably from the industrialeffluents32. Similar observations were also put forwardby Muthu Raj and Jayaprakash (2007)47.

As aluminium is mostly land derived, element/Al ratio gives information about the terrigenousfraction48. The higher Al content could be due to thecontribution from detritus mineral grains suppliedthrough various rivers. For the metals that correlatewith Aluminum, the relationship provides usefulmethod to normalize the relatively large range ofobserved natural metal levels in these sediments48.Recent studies have shown a significant seawaterscavenged component of Al in marine sediments49. Italso represents the quantity of alumina silicates, whichare the most important carrier phase of adsorbedmetals50. The high Ca/Al ratio in the present studycould be due to the precipitation and extraction ofCaCO

3 separated by organisms in warm water in

tropical regions51. Higher Ti/Al along the Chidambaramand Tarangampadi coastal tract indicates majortransition to dominance of carbonate dilution overinsoluble residue. Relatively low P/Al ratio suggestthat organic contribution of P in the sediment samplesis of minor importance and that detritus phases mainlycontrol the P content 45.

An EF value < 1 suggests that trace metals may beentirely from crustal materials or natural weatheringprocesses52. An EF greater than unity indicates asignificant portion of heavy metal delivered from non-crustal materials, or from unnatural sources. In thepresent study the enrichment of the trace metals overthe crustal values is in the following orderPb>Cr>Co>Zn>Ni>Cu>Mn indicating that the highvalues are due to anthropogenic contributions 39. Forexample the high proportion of Pb could be due tothe contamination by off shore drilling, atmosphericdeposition of finer particles, domestic effluentdischarges and use of paints. Higher Cr, Co, Zn couldbe due to the corrosion of iron and chrome alloy whichare used by a number of industries53-55. The result oftrace metals with other coastal industrialised regionsalong the southeast coast is presented (Table 3).

Contamination Factor (CF) is used to evaluatethe source metal contamination and alsoanthropogenic inputs. Metals such as Ni, Cr, Zn, etc.

are used in the marine contamination studies forevaluating anthropogenic activities56,57. In this studycontamination factor values are characterized asfollows: CF<1 (low contamination), 1 < CF > 3(moderate contamination), 3 < CF > 6 (considerablecontamination), 6 < CF- (very high contamination)following (Hokanson 1980). The average CF valuesfor all metals based on global shale values in the studyareas show low to moderate contamination (Table 4).The calculated PLI (<1) values in this study alsosupport the CF values indicating low insignificantpollution along the continental shelf. It presents acumulative indication of the overall level of heavymetal toxicity in a particular sample/station. Ourresults clearly point that the metals accumulated inthe marine continental shelf sediments along the eastcoast Tamil Nadu are influenced by anthropogenicand urban sources that include fertilizers, municipalsewage, and industrial effluents. This baselineelemental relationship provides a tool to assess andmonitor the sediment contaminants along thecontinental shelf of east coast India. However, in orderto corroborate this preliminary geochemical study,further analysis of large number sediment samplescollected from sampling points covering larger areaand deeper sea water column depths of the continentalshelf along the east coast is required. Moreover,further studies also demands a detailed investigationon the metal accumulation and their speciation relatedto sediment texture and grain size.

Conclusion

Grain size is largely silty sand with minor claycontent and this is due to the accretionary nature ofthe coastal area. Major sources for the carbonatecontent are shell fragments and calcareous tests ofbiogenic origin. High concentration of organic matterin the sea surface sediments indicates higherproductivity during SWM. Elevated concentration oftrace metals was observed at Cuddalore,Tarangampadi and Nagapattinam. The EnrichmentFactor (EF), Contamination Factor (CF) and PollutionLoad Index (PLI) indicates a strong impact ofanthropogenic activities around Cuddalore,Tarangampadi, Nagapattinam and Velanganni.However, detailed geochemical evaluation revealslow insignificant pollution along the entire east coastTamil Nadu.

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Acknowledgement

Authors thank the National Institute of OceanTechnology, Chennai, for providing facilities to carryout this work. This work was carried out under theISRO GBP project, Bangalore.

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