Clay minerals and Sr–Nd isotopes of the sediments along the western margin of India and their...

Clay minerals and Sr^Nd isotopes of the sediments alongthe western margin of India and their implication for

sediment provenance

Pratima M. Kessarkar a, V. Purnachandra Rao a;�, S.M. Ahmad b,G. Anil Babu b

a National Institute of Oceanography, Dona Paula, Goa 403 004, Indiab National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India

Received 19 July 2002; accepted 25 July 2003

Abstract

Mineralogy and Sr^Nd isotopic composition of the 6 2-Wm fraction of the Holocene and late Pleistocenesediments from 12 gravity cores collected along the western margin of India were used to delineate the provenanceand understand the response of isotopes with respect to the source rock composition. Holocene sediments showdistinct clay mineral associations representing three major sources: illite, chlorite-rich sediments from the Indus,smectite-dominated sediments from the Deccan Traps, and kaolinite and smectite-enriched sediments from gneissicrocks. The mixing of Indus-derived sediment with the Deccan Traps-derived sediment or the Deccan Traps-derivedsediment with gneissic rock-derived sediment occurs in the deeper water cores. Sr isotopes are very sensitive toweathering and laterization. Weathering does not influence Nd isotopes. The 87Sr/86Sr ratios of the Holocenesediments in shallow water cores are higher and fluctuated widely than those in deeper water cores. ONd correlates wellwith the clay mineral associations, indicating source rock composition or mixing of source sediments. Kaolinite-enriched sediments coupled with high Sr ratios and low ONd on the SW margin of India suggest intense chemicalweathering in the hinterland. The influence of the sediment from the Ganges^Brahmaputra, transported by the Bay ofBengal waters during the NE monsoon surface current or SW monsoon underwater current, on the composition ofthe sediments of the SW Indian margin is insignificant. The late Pleistocene sediments reflect a decrease of hydrolyticprocesses resulting in decrease of smectite and Sr isotopic ratio. Clay minerals and ONd values suggest that theprovenance and transport pathways of the late Pleistocene sediments are the same as those of the Holocene for eachregion.< 2003 Elsevier B.V. All rights reserved.

Keywords: strontium; neodymium; isotopes; clay minerals ; o¡ western India; source area

1. Introduction

Detrital ¢ne-grained sediments are abundantboth on the inner shelf and continental slope ofthe west coast of India. They occur largely as clay

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minerals, weathered from the rocks of the hinter-land and are primarily transported by rivers. Theparent rocks in the drainage basins of rivers have,however, been extensively lateritised (see below).As very large geochemical fractionation occursduring laterisation, it is important to understandhow the weathering products of the rocks respondwith respect to the geochemical signatures of par-ent rock and/or altered continental crust. The Sr^Nd isotope signatures have been extensively usedto characterise the provenance of detrital sedi-ments. These isotopes are primarily controlledby the age of the rocks (Biscaye et al., 1974;Goldstein et al., 1984; Grousset et al., 1988; Rev-el et al., 1996a). The sediments of the rivers drain-ing older crust have relatively more radiogenic Srand non-radiogenic Nd than rivers draining youn-ger crust. Sr is more mobile than Nd and can beremoved easily from soils during weathering. Srisotopic composition is also controlled by the de-gree of chemical weathering of source rocks andstrongly a¡ected by the weathering of authigenicprecipitates such as carbonates and phosphates inthe drainage basin (Goldstein and Jacobsen,1987). Several researchers studied the use of Sr^Nd isotopes with regard to their behaviour in dif-ferent size fractions of the terrigenous sediments(Dasch, 1969; Biscaye and Dasch, 1971; Groussetet al., 1988; Revel et al., 1996a; Innocent et al.,2000), determining the sources of sedimentarysupplies and tracing bottom currents (Innocentet al., 1997a; Revel et al., 1996b; Fagel et al.,1997) and their application in evaluating the cli-mate during glacial and interglacial times (Revelet al., 1996b; Henry et al., 1997; Innocent et al.,1997a; Stille and Shields, 1997; Colin et al., 1999;Walter et al., 2000; Tutken et al., 2002). Investi-gations on size fractions of the sediments indicatethat as the grain size decreases 87Sr/86Sr ratiosincrease and ONd becomes more radiogenic(Dasch, 1969; Biscaye and Dasch, 1971; Revelet al., 1996b; Innocent et al., 1997a, 1997b,2000). Despite variations of isotope compositionwith grain size, Walter et al. (2000) suggested thatthe Sr isotopes are useful to distinguish betweenmaterials from di¡erent sources when data fromthe same grain size fractions are considered. Herewe report a reconnaissance study of mineralogy

and Sr^Nd isotopic composition of the 6 2-Wmfraction of the sediments in 12 gravity cores alongthe western margin of India (Fig. 1A). The pur-pose of the paper is: (a) to identify the prove-nance and transport pathways of the ¢ne-grainedsediments, and (b) to understand the Sr and Ndisotope composition of the 6 2-Wm fraction of thesediments with respect to the composition of theparent rock and its alteration.

2. Previous studies

Regional studies on clay minerals along thewestern margin of India are rare (Stewart et al.,1986; Mattait et al., 1973; Nair et al., 1982a,b;Rao, 1991; Rao and Rao, 1995; Chauhan andGujar, 1996; Thamban et al., 2002). On the basisof di¡erent clay mineral assemblages or clay min-eral ratios (smectite (S)/illite (I) and kaolinite (K)/chlorite (Ch)) of the sediments and major rocktypes in the drainage basins of the rivers, threemajor sources of sediments have been identi¢edalong the continental margin (Mattait et al.,1973; Nair et al., 1982b): (1) Indus-derived sedi-ments, (2) Deccan Traps-derived sediments, and(3) gneissic rocks-derived sediments. The prove-nance of the sediments along the continental shelfis distinct (Nair et al., 1982b), but on the conti-nental slope of NW India the Indus-derived sedi-ments are admixed with Deccan Traps-derivedsediments (Rao and Rao, 1995). The sedimentson the continental slope between Mormugaoand Cape Comorin have been suggested to derivefrom the gneissic rocks of the hinterland (Raoand Rao, 1995). Chauhan and Gujar (1996), how-ever, reported relatively high illite and chlorite inthe sediments of the SW margin of India andidenti¢ed their source as the waters of theGanges^Brahmaputra rivers, transported by theBay of Bengal and intrusion of the SW coast dur-ing the NE monsoon. Kolla et al. (1976, 1981)reported smectite-rich sediments derived fromthe Deccan Traps in the deeper Arabian Seabased on clay mineral and quartz distribution.Clay/sediment accumulation rates along the west-ern margin of India, using 210Pbex, 137Cs and 14C(AMS) ages (Borole, 1988; Somayajulu et al.,


P.M. Kessarkar et al. /Marine Geology 202 (2003) 55^6956

1999) revealed varied rates of sedimentation(V3^0.04 mm/yr). For example, the sedimenta-tion rates vary from 3.3^1.3 mm/yr in di¡erentcores collected o¡ the Gulf of Kachchh and Sau-rashtra and only 0.24 mm/yr at 280 m depth o¡Mumbai. O¡ Mangalore the rates of sedimenta-tion, measured by 210Pb and 137Cs, varied from6.6^3.4 mm/yr in the inner shelf (at 30 m depth)to 1.1^0.8 mm/yr on the upper slope (300^600 m).On the lower slope (at 1680 m) it is only 0.04 mm/yr (based on 14C measurements) (Somayajulu etal., 1999). O¡ Kochi it is about 1.7^1.6 mm/yr at590 m depth. Sirocko and Lange (1991) reportedclay mineral accumulation rates in the deeperArabian Sea.The Sr^Nd isotopic composition of the sedi-

ments of the western margin of India has notyet been reported. Sirocko (1995) measured theSr^Nd isotopes of the modern sediments of thedeep Arabian Sea. Staubwasser and Sirocko(2001) studied the bulk Nd isotopes in a lami-nated sediment core o¡ Pakistan. Besides, thebulk or clay fraction Nd isotope data from theeastern Indian Ocean (Ben Othman et al., 1989),central and SW Indian Ocean (Dia et al., 1992) orthe Sr and Nd isotopes for sediment cores fromthe central Indian Basin, Bengal Fan and theOcean Drilling Program (ODP) cores of the distalBengal Fan (Rea, 1992; Fagel et al., 1994, 1997;Bouquillon et al., 1990; France-Lanord et al.,1993; Derry and France-Lanord, 1996; Colin etal., 1999) were reported. Sr (Karim and Veizer,2000) and Nd (Clift et al., 2002) isotopes for thesediments of the Indus River, Sr and Nd isotopesof the di¡erent Deccan Traps formations (Light-foot et al., 1990) and Sr and Nd (Peucat et al.,1989) or Nd isotopes (Harris et al., 1994) reportedfor the granitic and gneissic rocks of southernIndia are also shown in Fig. 1B.

3. Geological and oceanographic settings

Rivers are the principal agents of transport ofdetrital sediments into the eastern Arabian Sea.The Indus River is the largest one (2900 kmlength, V966 000 km2 drainage area), bringingenormous amounts of sediment (Fig. 1B; Haq

and Milliman, 1984). It experiences a conspicu-ously wet and dry climate. The Deccan Trapscover an area of 500 000 km2 with a maximumthickness of 1200^1700 m in central and NW In-dia and are overlain by Quaternary sediments incoastal Saurashtra. Several rivers debouch intothe Gulf of Khambat (see Fig. 1A,B). The Sabar-mati River rises in the Aravali mountain rangecomprising Precambrian gneisses and schists and£ows through Quaternary formations before en-tering the Gulf. The rocks in the drainage basin ofthe Mahi River are similar to those of the Sabar-mati, but its tributaries also £ow through theDeccan Traps. A semi-arid climate prevails inSaurashtra and the drainage basins of the Sabar-mati and Mahi with rainfall ranging from 500 toV1000 mm/yr (Sharma et al., 1994). The rocks inthe drainage basin of the Narmada River (1300km long, V93 000 km2 drainage area) are com-plex and comprise of Precambrian gneisses(Banded Gneissic Complex), Vindhyans (Protero-zoic sedimentary formations comprising meta-morphosed limestones, sandstones and quartzites)and Satpuras (Deccan Traps plateau basalts) inthe upper and middle reaches and Quaternary al-luvium in the lower reaches. The Tapti is a mod-erate river (700 km long) largely £owing throughSatpuras. South of Tapti the Western Ghatsmountain range is to be found composed of twomajor rock types: the Deccan Traps basalts be-tween Tapti and Mormugao and Precambriangneisses and schists between Mormugao andCape Comorin. The Western Ghats enjoy a hu-mid tropical climate with a maximum rainfall ofV3000 mm. They are drained by several minorrivers that bring sediment load into the coastalArabian Sea (Fig. 1B). The Ghats stretch alongthe shore between Mormugao and Bhatkal as wellas between Quilon and Cape Comorin, but aresituated V50^80 km away from the coast be-tween Bhatkal and Quilon. Here the coastal re-gion is occupied by Recent alluvium and theWarkala beds (ferruginised sand stones with inter-calated clays) of Tertiary age.Extensive laterisation of the parent rocks is a

characteristic feature in western India. Bauxite-and laterite-derived sedimentary facies are com-mon in coastal Kachchh and Saurashtra (Valeton


P.M. Kessarkar et al. /Marine Geology 202 (2003) 55^69 57

and Wilke, 1993). Two phases of laterisation havebeen reported in the Deccan Traps region (Wid-dowson, 1997): the high-level crest laterites (10^20 m thick) developed upon the post-Deccan lavaplains during the lower and mid-Tertiary next tolow-level laterites developed upon the coastal strip(50^60 km wide) during the mid- and late Terti-ary. Di¡erential upliftment and subsequent ero-sion exposed basalts at places, especially in thenorth at 18‡N. Lateritic pro¢les are capped byiron-rich crusts in the southern Deccan Traps re-gion. The Archaean and Proterozoic rocks ofKarnataka and Kerala display a more complexhistory of laterisation. In this area a series ofstepped lateritic surfaces are recognised, probablyindicating periodic upliftment punctuated by sub-sequent geomorphological adjustment of the baselevel (Gunnell, 1997).Two major sediment types occur on the conti-

nental shelf o¡ western India (Fig. 1A): Recentclayey sediments on the inner shelf and relictsandy sediments on the outer shelf (Rao and Wa-gle, 1997). The sediment on the continental slopeis largely a mixture of terrigenous clays and car-bonates. The western margin of India receives alarge sediment input during the SW monsoon(June^September) when rainfall is high. A south-erly surface current, V150 km wide, developsalong the margin during this season in waterdepths of 50 m on the shelf. An underwater cur-rent of about 40 km wide in the depth intervalbetween 100 and 250 m, characteristic of theBay of Bengal waters, prevails during the SWmonsoon (Shetye et al., 1990). Intense upwellingoccurs in the shelf waters south of Mormugaoduring this season. During the NE monsoon (No-vember^February), the southerly surface currentis replaced by a northerly surface current withsignatures of the Bay of Bengal waters intrudingonto the SW coast of India.

4. Materials and methods

Sediment samples were selected from 12 gravitycores recovered along the western margin of Indiafrom di¡erent water depths (Fig. 1A; Table 1).Holocene sediments (at or near to the core top,

within the top-10-cm interval) and late Pleistocenesediments (sediment interval determined based onAMS or conventional radiocarbon ages or mag-netic susceptibility records of the cores) werechosen for the present study. The samples selectedare abundantly clayey with 20^40% calcium car-bonate. The sediments were made free of calciumcarbonate by adding 0.1 N HCl and excess acidwas removed by washing. Organic matter was re-moved by adding hydrogen peroxide (6 10%).The 6 2-Wm fraction of the sediment was sepa-rated by settling velocity principle and orientedclay slides were prepared by pipetting 1 ml claysolution onto glass slides and allowed them to dryin air. The remaining clay sample was dried, pow-dered and used for Sr and Nd measurements. Theair-dried and ethylene glycol treated (for 1 h at100‡C; Carroll, 1970) clay slides were scannedfrom 2‡ to 22‡ 2a at 1‡ 2a/min on a PhilipsX-ray di¡ractometer using nickel ¢ltered Cu KK radiation. Slow scan X-ray di¡raction from 24‡to 26‡ 2a at 0.5‡ 2a/min was also carried out foreach slide. Clay minerals were identi¢ed and theweighted peak area percentages of various clayminerals, following Biscaye (1965), and their ra-tios (Table 1; Fig. 2A,B) were calculated. Thechanges in the relative abundance of individualclay minerals may signify either an increase inthat component or a decrease in one or more ofthe other components and therefore pose a prob-lem in interpreting the clay mineral concentrationdata (see Sirocko and Lange, 1991). In this paperthe ratios of the major clay minerals are consid-ered and discussed.The powdered 6 2-Wm fractions of the clay

samples were digested with HF^HNO3 acid mix-ture. The Sr and Rare Earth Elements (REE)were then separated using columns packed withCation Exchange Resin (DOWEX 50WX8). Ndwas separated from the REE fraction with col-umns packed with HDEHP-coated te£on. TheSr and Nd isotope ratio measurements were car-ried out on a VG 354 Thermal Ionisation MassSpectrometer in the dynamic triple collector modeat the National Geophysical Research Institute,Hyderabad. The isotopic ratios were correctedfor fractionation with 86Sr/88Sr = 0.1194 andnormalised to SRM 987 standard value of



Fig. 1. (A) Map showing the locations of the gravity cores along the western margin of India. Sur¢cial geology of the continentalshelf and slope is also shown. (B) Geology of western India (Anonymous, 1965). For notations of the di¡erent geological forma-tions, see legend to (A). The 87Sr/86Sr ratios and ONd of the river sediments and geological formations (after Peucat et al., 1989;Lightfoot et al., 1990; Harris et al., 1994; Karim and Veizer, 2000; Clift et al., 2002) are also shown on the map. Abbreviationsof the formations in the Deccan Traps region: B, Bushe; P, Poladpur; A, Ambenali; M, Mahabaleshwar. Further abbreviations:GG, granitic gneiss; CG, charnockite gneiss. The concerned llithologies were discussed in the text in Section 3.



0.710220Q 0.000024 (2 sigma S.D.; n=10). 143Nd/144Nd ratios were corrected for fractionation with146Nd/144Nd=0.7219 and normalised to the LaJolla standard value of 0.511850Q 0.000010 (2 sig-ma S.D; n=8). Blank values obtained for the Srand Nd isotopic measurements are 6 250 and6 25 pg, respectively. Sr isotope determinationswere repeated for some samples (see Table 1).The isotopic ratios of Sr and Nd are shown inTable 1 and Fig. 3A,B.

5. Results

5.1. Clay minerals

Illite (I), smectite (S), chlorite (Ch) and kaolin-ite (K) are present as major clay mineral groups insediments of all the cores. The relative propor-tions of each clay mineral, however, vary signi¢-cantly for each region (Fig. 2A). Illite, smectite,chlorite, and kaolinite, in order of decreasing

Table 1Mineralogy and isotope (Sr^Nd) chemistry of the 6 2-Wm fraction of sediments from the cores of the western margin of India

Core details Weighted peak area percentages Isotope data Remarks/Sampleinterval (cm)

Core No. CruiseNo.

Waterdepth(m)

S I K Ch S/I K/Ch 87Sr/86Sr 143Nd/144Nd ONd

Holocene sedimentsIndus-derived sediments1 SK 148/22 56 34 44 04 18 0.8 0.2 0.7215 0.51187 314.812 SK148/21 1900 31 48 05 16 0.6 0.3 0.7184 0.51191 314.105 SK111/22 3235 48 35 11 06 1.4 1.8 0.7172 0.51189 314.57 mixed sedimentDeccan Traps-derived sediments4 SK148/38 31 76 12 07 05 6.5 1.5 0.7253 0.51199 312.583 SK 148/32 111 55 28 07 10 2.0 0.7 0.7207 0.51173 317.266 SK148/55 500 28 44 17 10 0.6 1.7 0.7150 0.51194 313.717 SK126/50 2650 23 37 24 15 0.6 1.6 0.7167 0.51182 315.98 mixed sedimentGneissic rock-derived sediments8 GC3 355 27 41 20 12 0.7 1.7 0.7216 0.51160 320.299 SK126/39 1940 31 31 24 14 1.0 1.7 0.7175 0.51210 310.5910 GC5 280 36 16 31 17 2.2 1.9 0.7229 0.52269 318.45

0.7202 R11 SK126/16 1420 34 30 24 13 1.1 1.8 0.7195 0.51163 319.6612 SK148/4 1380 37 20 29 14 1.8 2.0 0.7185 0.51162 319.74Late Pleistocene sedimentsIndus-derived sediments1 SK 148/22 56 07 70 05 18 0.1 0.3 0.7200 0.51184 315.41 150^1552 SK148/21 1900 09 61 06 23 0.2 0.3 0.7225 0.51193 313.64 120^1265 SK111/22 3235 38 41 07 14 0.9 0.5 0.7160 0.51175 317.34 270^280

mixed sedimentDeccan Traps-derived sediments4 SK148/38 31 62 21 12 05 3.0 2.4 0.7237 210^2156 SK148/55 500 36 37 16 11 1.0 1.4 0.7131 0.51199 312.62 206^212

0.7134 448^452Gneissic rock-derived sediments8 GC3 355 34 32 19 14 1.1 1.4 0.7229 0.51170 318.20 75^809 SK126/39 1940 29 32 21 19 0.9 1.1 0.7192 0.51178 316.78 360^37010 GC5 280 24 31 27 17 0.8 1.6 0.7188 0.51146 323.02 325^33011 SK126/16 1420 28 37 27 17 0.8 1.6 0.7185 0.51171 318.04 150^160

0.7174 170^180

Abbreviations: S, smectite; I, Illite; K, kaolinite; Ch, Chlorite; R, Repeated measurements.



abundance, are present in Holocene sedimentsfrom the cores (1 and 2) o¡ the Gulf of Kachchhand Indus (Table 1; Fig. 2A). The low S/I (av.0.7) and K/Ch (av. 0.25) ratios are characteristicof the sediments (Fig. 2B). The high illite andchlorite contents o¡ the Indus decrease progres-sively southwards, and their decrease is accompa-nied by an increase of smectite (av. S/I = 2.0) andkaolinite (K/Ch= 0.7) in a shelf core (No. 3) o¡Saurashtra or a deeper water core (No. 5) o¡ theIndus (S/I = 1.4; K/Ch= 1.8) (Table 1; Fig. 2B).The inner shelf core (No. 4) o¡ the Narmada

and Tapti shows abundant smectite (76%) withminor illite (12%), kaolinite (7%) and chlorite(6%) (Table 1; Fig. 2A). Higher kaolinite (17^24% with K/Ch=1.7) and illite (42.5% withS/I = 0.65) contents are characteristic of the sedi-

ments in the cores (Nos. 6 and 7) o¡ Ratnagiri(Fig. 2B). The cores (Nos. 8^12) between Mormu-gao and Cape Comorin show higher kaolinite andsmectite contents. The sediments o¡ Kochi (core10) exhibit the highest kaolinite (31% withK/Ch=1.9) and increased smectite contents(36% with S/I = 2.2; Fig. 2A,B). The illite propor-tions are relatively high in the cores (Nos. 8 and9) o¡ Mormugao and Cape Comorin (No. 11)and low in the core (No. 10) o¡ Kochi (Table1). Gibbsite is present in all the samples southof Mormugao but more prominent in the sampleso¡ Cape Comorin. Although the relative propor-tions of the individual clay minerals vary, theirorder of abundance in the late Pleistocene sedi-ments of the cores is the same as that of the Ho-locene counterparts for each region (Table 1; Fig.

Fig. 2. (A) Clay mineral distribution in the 6 2-Wm fraction of the Holocene sediments o¡ western India. (B) Ratios of smectite(S)/illite (I) and kaolinite (K)/chlorite (Ch) in the Holocene (H) and late Pleistocene (P) sediments. Provenances of the di¡erentsource sediments and mixed sediments along the western margin of India are also shown.



2B). The illite and chlorite contents are muchhigher and the smectite content is lower thanthat of the corresponding Holocene sediments ofthe same core.

5.2. Sr and Nd isotopes

The 87Sr/86Sr ratios for the 6 2-Wm fraction ofthe Holocene sediments varied from 0.7150 to0.7253 with consistently higher values on theSW margin of India (Table 1; Fig. 3A,B). Simi-larly, the ONd values varied from 312.58 to320.29. It tends to be more radiogenic with rela-tively higher values in the NW and central partand less radiogenic with more negative lower val-ues towards the SW margin of India (Table 1;Fig. 3A,B). The 87Sr/86Sr ratios (Fig. 3A) arehigher and varied signi¢cantly in the shallow

water cores (Nos. 1, 3, 4, 8, 10) than those inthe deeper water cores (Nos. 2, 5, 6, 7, 9, 11,12). Although the Sr ratios vary (0.7215^0.7172),the ONd values are nearly the same (314.10 to314.81; Fig. 3B) for the Holocene sediments ofcores 1, 2 and 5. The highest 87Sr/86Sr ratio andthe most radiogenic Nd correspond to the core(No. 4) o¡ the Narmada and Tapti (Fig. 3A).The Sr ratio changes signi¢cantly in cores 3, 4,6 and 7 and appears to be decreasing o¡shore.The ONd values are relatively more radiogenic(312.58 to 316.98) in cores 4, 6 and 7. Amongthe cores (Nos. 8^12) between Mormugao andCape Comorin, the 87Sr/86Sr ratio is relativelyhigh (0.7229) in core 10 o¡ Kochi (Fig. 3A,B)and low (av. 0.7190) in the cores (Nos. 11 and12) o¡ Cape Comorin. Except for core 9, theONd values are more negative (318.45 to 320.29)

Fig. 3. (A) Distribution of Sr^Nd isotopes in the 6 2-Wm fraction of the Holocene (H) and late Pleistocene (P) sediments. (B) Bardiagram showing Sr and Nd isotopes for three major sources. Numbers on top of the bars represent the core numbers.



and lower than any of the northern cores (Nos. 1^7) (see Fig. 3B).The 87Sr/86Sr ratios for the late Pleistocene sedi-

ments of the cores are in general low compared tothose of the Holocene of the same cores andmimic the distribution pattern found in the Holo-cene sediments (Fig. 3A,B). The ONd value o¡ theGulf of Kachchh was slightly negative (315.41)but tends to be more radiogenic in the samples o¡the Indus (313.64). The deeper water core o¡ theIndus (No. 5) showed less radiogenic Nd(317.34). It is 312.62 in the core (No. 6) o¡Ratnagiri. The ONd values varied from 316.78 to323.02 for the slope sediments of the SW marginof India (Fig. 3A,B).

6. Discussion

6.1. Holocene sediments

For the convenience of description, the studyregion is divided into three geographic areasbased on the variations in clay mineral distribu-tion and Sr^Nd isotopic composition of the sedi-ments. These are the Indus^Gulf of Kachchh re-gion, the Saurashtra^Ratnagiri region and theMormugao^Cape Comorin region. These areasrepresent three main sources of sediments. Mixedsediments are found at the boundaries (see Fig.2B).

6.1.1. Indus^Gulf of Kachchh region (cores 1and 2)The illite- and chlorite-dominated sediments o¡

the Gulf of Kachchh and the Indus (Fig. 2A) in-dicate that the sediments are predominantly In-dus-derived. The Indus drains the Precambriangneissic rocks of the Himalayas under glacialweathering conditions in the upper reaches andthe arid soils and recent alluvium of Pakistanand NW India in the lower reaches (Krishnan,1968). The low S/I and K/Ch ratios (Fig. 2B) in-dicate that the mechanical or physical weatheringproducts of the Himalayan rocks are the domi-nant source for these sediments. Konta (1985) re-ported high illite and chlorite contents in the con-tinental detritus derived from the Indus. Several

workers reported illite, smectite, chlorite, and mi-nor kaolinite in the shelf sediments o¡ the Indusand northern part of the Gulf of Kachchh, whichwas attributed to the in£uence of the Indus dis-charge (Nair et al., 1982a; Rao, 1991; Rao andRao, 1995). The Sr isotopic ratios are usuallyhigher for the sediments weathered from oldersilicic rocks. Karim and Veizer (2000) reportedvaried 87Sr/86Sr ratios (0.71253^0.72912) for dif-ferent tributaries of the Indus (see Fig. 1B) andexplained the variations to the fact that these trib-utaries £ow through di¡erent lithologies such assedimentary carbonates and crystalline rocks.Similarly, Clift et al. (2002) reported varied ONd

values of Indus River sediments, which fall down-stream from 38.4 to 315.0 before entering theArabian Sea, and explained the changes by theprogressive mixing of radiogenic into less radio-genic sediments. The ONd value (av. 314.5) re-ported here is lower than that of the Indus Riversuspension (312.2; Goldstein et al., 1984), shelfsediments (311.0) o¡ Pakistan (Staubwasser andSirocko, 2001), and Indus Fan (312.5; Clift et al.,2002), but within the values reported for the In-dus sediments (Clift et al., 2002). The ONd valuesand Sr ratios may therefore indicate the isotopiccomposition of mixed sediments contributed bythe Himalayan rocks with the sediment £ux de-rived from the lower reaches.The material brought by the Indus is largely

transported to the deep Arabian Sea by turbiditycurrents (Haq and Milliman, 1984) and onto theshelf east of the Indus by the southerly currentdeveloped during the SW monsoon. Aeoliandust, characterised by high palygorskite and se-piolite and relatively low 87Sr/86Sr (0.705) andhigh ONd (36), is abundant in the western ArabianSea (Prell et al., 1989). The contrasting clay min-erals and Sr and Nd values in the sediments o¡the Indus and Gulf of Kachchh imply that theaeolian £ux did not reach the sediments of theNE Arabian Sea.

6.1.2. Saurashtra^Ratnagiri region (cores 3, 4and 6)The hinterland of the region comprises abun-

dant Deccan Traps basalts, which can be dividedinto four stratigraphic subdivisions (Fig. 1B). The



lower basalts (Bushe and Poladpur Formations)are the largest and show evidence of contamina-tion with upper crustal material. Lightfoot et al.(1990) reported higher Sr ratios (0.714^0.716) andless radiogenic Nd values (316.35 to 315.4) forthese basalts. On the other hand, the upper ba-salts (Ambenali and Mahabaleshwar Formations)show lower contamination and/or mobilisation ofmaterial from the mantle lithosphere and containlow Sr ratios (0.704^0.706) and high radiogenicNd (7.06 to 36.59) (Lightfoot et al., 1990).Higher smectite and relatively low illite and

chlorite (Fig. 2A) values are characteristic of theclay minerals in the cores (Nos. 3 and 4) and arecompatible with the observation that smectite de-rived from the weathering of the basic volcanicrocks under a semi-arid climate. The higher Srratios (Fig. 3A) than those of the Deccan Trapsbasalts (see Fig. 1B), however, need explanation.The rise of Sr ratios could be due to: (1) additionof high Sr component, (2) preferential weatheringand removal of unradiogenic Sr, and (3) enrich-ment of radiogenic Sr in low size fractions (Borgand Banner, 1996). In contrast to Nd, Sr is moremobile and can be removed easily from soils dur-ing weathering (Goldstein and Jacobsen, 1987).As Narmada drains heterogenous geological for-mations of di¡erent age (Fig. 1B) this river mayhave supplied sediments with high Sr ratios. It isalso likely that the soils in the drainage basin ofthe river have undergone rapid and varying de-grees of weathering during which the unradiogen-ic Sr is removed from the soils. Most importantly,bauxites and thick lateritic crusts cover the basal-tic formations. Innocent et al. (1997b) measuredSr isotopes in lateritic sequences and associatedwaters and found that the major part of the initialSr is removed from basalts during the early stagesof weathering and that the Sr ratios are higher inthe increasingly weathered facies and their relatedwaters. They further reported that the Nd isotopiccomposition does not change signi¢cantly be-tween the fresh basalts and the weathering prod-ucts. The 87Sr/86Sr ratio also increases with de-creasing grain size (Dasch, 1969; Biscaye andDasch, 1971; Revel et al., 1996b; Innocent etal., 1997a, 2000). The smallest grain size of smec-tite compared to other clay minerals (Whitehouse

et al., 1960) and its abundance (up to 76%) in thisregion may have been a factor for the enrichmentof the Sr ratio. We therefore suggest that the highSr ratios are due to the combined e¡ect of sedi-ment mixing, lateritic weathering and grain size.On the other hand, the Nd isotopes may be due tothe mixing of sediments derived from the largerBushe Formation with those from the drainagebasins of the rivers.The ONd value (317.26) in core 3 o¡ Saurashtra

is lower than the reported values for the DeccanTraps (see Fig. 1B). It may have derived fromolder rocks or a mixture of sediments from di¡er-ent sources. Because of the southern position ofthe core (see Fig. 1A), we interpret that the lowONd may represent a local input of detrital sour-ces. For example, the catchment areas of the Sa-barmati and Mahi Rivers lie in a mountain rangethat contains Precambrian gneisses and schists.Mesozoic formations are also found in the Sau-rashtra region (see Fig. 1B) The weathering prod-ucts of these formations under semi-arid condi-tions would contain low ONd values. Therefore,the low Nd isotopic composition (ONd =317.26)is assumed to result from mixing between the olderoded crust and the relatively younger DeccanTraps and subsequent transportion to the conti-nental slope. This implies cross shelf transport ofdetrital clay from the hinterland. On the basis ofclay mineral distribution of several sur¢cial sedi-ments o¡ Saurashtra, Rao (1991) and Rao andRao (1995) suggested that the Indus-borne sedi-ments were carried by the SW monsoon currentand admixed with clays derived from Saurashtra.This interpretation may still be valid, in view ofthe fact that the Sabarmati and Mahi are minorrivers and may not able to supply and distributetheir £ux around the continental slope o¡ Sau-rashtra.Despite the fact that core 6 is situated in the

vicinity of the Deccan Traps, it contains low S/Iand high K/Ch ratios (Fig. 2A,B). These are incontrast with those of the core (No. 4) o¡ Nar-mada and Tapti, which contains high S/I and lowK/Ch ratios. However, the isotopic compositionof Sr (0.715) and Nd (313.71) agrees well withthose of the Bushe Formation of the DeccanTrap basalts (0.714, 315), implying no major



change in the source area. The increased illite andkaolinite may be due to the fact that the DeccanTraps south of the Tapti River are associated withthe Western Ghat mountain ranges which weatherunder a humid tropical climate and intense rain-fall. The basaltic lava £ows in the coastal regionhave su¡ered substantial erosion during the lowerand mid-Tertiary, prior to laterization (Widdow-son and Cox, 1996). Under these conditions onewould expect kaolinite and illite to be dominantas weathering products. The sediments at the coresite thus represent hinterland £ux. Sirocko (1995)reported similar Sr and Nd isotopic ratios (0.714,314) for the modern sediments of the easternmiddle Arabian Sea and attributed their sourceto the Deccan Traps of central India.

6.1.3. Mormugao^Cape Comorin (cores 8^12)The sediments in this region are characterised

by increased proportions of kaolinite togetherwith minor gibbsite and also show high Sr ratiosassociated with more negative ONd values. Thesesuggest intense chemical weathering (see Millot,1970; Chamley, 1989; Stille and Shields, 1997)of the area’s Precambrian gneisses and schistsunder a humid tropical climate. The Sr isotopiccomposition of the Holocene sediments of thecores (0.7175^0.7229) are close to that of theGranitic Gneiss (0.71788^0.72163) but higherthan that of the Charnockite Gneiss (0.70315^0.71448) from the hinterland (Peucat et al.,1989). However, the ONd values (318.45 to320.29) are within the range of granitic (319.0to 324.8) and charnockite gneiss (318.6 to322.1) (Harris et al., 1994).Illite proportions are relatively higher in the

sediments of the cores (Nos. 8 and 9) o¡ Mormu-gao and Cape Comorin (No. 11) than that of thesediments o¡ Kochi (core 10) (see Fig. 2A). Raoand Rao (1995) made similar observations basedon several sur¢cial sediments. These sedimentsmay not have been transported from the Indus,because illite is associated with higher kaolinite(Fig. 2A) and the Sr and Nd isotopes are di¡erentfrom those of the Indus (Fig. 3A). Chauhan andGujar (1996) reported increased illite and chloritecontents in the sediments of the SW margin ofIndia and attributed their presence to the

Ganges^Brahmaputra Rivers and transportationby the Bay of Bengal waters by the surface cur-rent during the NE monsoon. The 87Sr/86Sr andONd values of the sediments o¡ Cape Comorin (av.0.719; 319.7) are much lower than those of thewestern Bay of Bengal (0.7230^0.7316; 314 to316.2), o¡ the Ganges^Brahmaputra (0.720^0.726; 312.5 to 312.8), or the Ganges River(s 0.735; 315.0) (see Bouquillon et al., 1990;France-Lanord et al., 1993; Derry and France-Lanord, 1996; Colin et al., 1999). The 87Sr/86Srvs. ONd plot of the samples also falls well outsidethe ¢eld of the Ganges or western fan sediments(Fig. 4). These suggest that the sediments trans-ported from the Bay of Bengal by the surfacecurrent during the NE monsoon or the under-water current during the SW monsoon have insig-ni¢cant in£uence on mineralogy and/or Sr andNd isotopic composition of the sediments in thisregion.It is known that illite is quite resistant to chem-

ical and physical weathering. Relatively high illiteo¡ Ratnagiri^Mormugao and Cape Comorin nextto low illite and high kaolinite o¡ Kochi may beexplained based on the position of the WesternGhats and the sedimentary formations lying be-tween the coast and Western Ghats. As the steepcli¡s of the Western Ghats are situated on the

Fig. 4. (A) 87Sr/86Sr ratio vs. ONd plot of the Holocene sedi-ments of the cores (Nos. 1^12) along the western margin ofIndia. Values of the Western Bengal Fan sediments; Ganges-derived sediments, Deccan Traps and Aeolian Dust are alsoshown.



coast o¡ Ratnagiri^Bhatkal and Quilon^CapeComorin (see Fig. 1B), it is likely that residualillite and chlorite were released from the gneissesand schists under intense rainfall conditions andsubsequently transported and deposited onto thecontinental slope. Since the Western Ghats areaway from the coast between Bhatkal and Quilon,the soils between the Ghats and the shore, i.e.alluvium and the Warkala beds, may have beensubjected to active hydrolysis and drainage andpreferentially released large kaolinite and radio-genic Sr. Kaolinites are able to trap Sr from sur-¢cial waters and the kaolinitic facies of tropicallateritic soils contain high Sr ratios (Innocent etal., 1997b). Therefore, the thick weathered se-quences (Warkala beds) must be considered asnatural cation exchange layers that play an activerole in the weathering process. The high Sr iso-topic ratio and kaolinite o¡ Kochi agree well withthis hypothesis. It is therefore evident that theillite in the sediments re£ect the petrographic in-£uence of the rocky substrate, whereas the highkaolinite and smectite re£ect the pedogenic in£u-ence of the soils weathered under humid tropicalconditions. Therefore, the relative increase of illiteand/or kaolinite and Sr isotopes are explainedhere based on the mechanical and chemicalweathering processes associated with the rocksand soils of the hinterland rather than the £uxfrom the Bay of Bengal. Sirocko and Lange(1991) reported high accumulation rates of illtein the slope sediments of this region and indicatedsouthern India as the sediment source. We areunable to explain the more radiogenic Nd(310.59) for the Holocene sediments o¡ Manga-lore (Fig. 3A). Harris et al. (1994) reported highNd values (310.6 to 315.4) for the Palghat^Cauvery Shear Zone (PCSZ; Fig. 1B) lying justsouth of Mangalore. It is likely that the sedimentscontributed from this zone were deposited at thesite.

6.1.4. Mixed sediments (cores 5 and 7)Core 5 contains higher smectite and kaolinite,

lower illite and chlorite and lower Sr ratios thancore 1 o¡ the Indus. The ONd value (314.57) issame as that of the Indus (314.81) (Fig. 3A,B)and falls within the range of values reported for

the Deccan Traps (see Fig. 1B). This implies thatthe sediments at core 5 are of mixed type trans-ported from the Indus by the SW monsoon cur-rent and from the Deccan Traps by cross shelftransport of sediments.At core 7, illite and kaolinite increase, the Sr

ratio increases while the ONd value decreases com-pared to that of core 6 (Fig. 3B). The Sr and Ndvalues are intermediate between those of cores 6and 8. This indicates probable mixing of DeccanTraps £ux with that of the gneissic rocks. Themajor sources of sediments and their mixing areshown in Fig. 2B. This is consistent with our ear-lier observations (Rao and Rao, 1995).

6.2. Late Pleistocene sediments

The decrease of smectite and Sr ratios in thesesediments (Table 1) and their corresponding in-crease in the Holocene sediments (Fig. 2A) mostprobably suggest an overall decrease of hydrolyticprocesses and the increase of direct rock erosionduring the late Pleistocene. Under such conditionsone would expect the release of increased concen-trations of primary minerals such as illite andchlorite by physical/mechanical weathering. Theproportions of clay minerals o¡ the Indus sedi-ments during the late Pleistocene very well re£ectthe same (Table 1). The Holocene sediments aremore ¢ne-grained than those of the late Pleisto-cene and higher Sr ratios are usually associatedwith ¢ner sediments. Despite variations in abun-dance of individual clay minerals, the distributionof major clay minerals and the ONd values in thesesediments are similar to those of the Holocene inthe same core. This suggests that there is neither amajor change in sediment sources nor one in thetransport pathways in the respective regions. TheONd value of the late Pleistocene sediments o¡Mangalore is close to that of the gneissic rocksof the hinterland suggesting that the hinterland£ux is predominant during that time.

7. Conclusions

(1) The clay mineral assemblages and/or ratioscorrelate well with the Nd isotopic composition.



Nd is a better tracer. It either re£ects the sourcerock characteristics or mixing of source sedi-ments.(2) Sr isotopes are very sensitive to the sedimen-

tary cycle. Sr is an ambiguous tracer. The Sr iso-topes are higher and vary widely in shallow watercores than in deep water cores.(3) The isotopic and mineralogical composition

of the sediments of the SW margin of Indialargely represent hinterland £ux and are not in£u-enced by the sediment transported from the Bayof Bengal waters during the NE monsoon surfacecurrent or the SW monsoon underwater current.(4) The characteristics of the late Pleistocene

sediments re£ect a decrease in hydrolysis that re-sulted in a decrease of smectite content and Srratio. The Nd isotopes of sediment cores fromthe SW margin of India will be studied in future.

Acknowledgements

We wish to thank the Directors of the NationalInstitute of Oceanography, Goa, and the NationalGeophysical Research Institute, Hyderabad, fortheir encouragement and the use of their facilities.P.M.K. wishes to thank the CSIR, New Delhi, forproviding a Senior Research Fellowship. Wethank the referees (Dr. N. Fagel, an anonymousreviewer and Professor Chamley) for their criticaland constructive comments on a previous draft ofthis paper. Mr. Girish Prabhu assisted with theX-ray analysis. This work is NIO contributionNo. 3843.

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