molybdenum isotopes in two indian estuaries: mixing...

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Geochimica et Cosmochimica Acta 141 (2014) 407–422

Molybdenum isotopes in two Indian estuaries:Mixing characteristics and input to oceans

Waliur Rahaman ⇑, Vineet Goswami, Sunil K. Singh, Vinai K. Rai

Physical Research Laboratory, Ahmedabad, India

Received 2 October 2013; accepted in revised form 27 June 2014; Available online 14 July 2014

Abstract

The distributions of dissolved and particulate Mo and their isotope composition (d98Mo) have been measured in theNarmada and the Tapi estuaries draining into the Arabian Sea. During monsoon, the d98Mo of dissolved Mo in the Narmadaestuary ranges from 0.49& to 2.19& in the salinity range 0–17.2 practical salinity unit (psu) quite similar to that in the Tapiestuary, 0.99–2.36&, in the salinity range 0–20.3 psu. Mo concentration in suspended sediments of the Narmada estuarycollected during monsoon average 512 ± 44 ng/g (range 459–602 ng/g) similar to that measured in one sample from the Tapiestuary 560 ng/g Mo. d98Mo of particulate Mo in the Narmada ranges from �0.21& to 0.48& with an average�0.03 ± 0.2&. Dissolved Mo in the Narmada and the Tapi rivers display isotopically heavier Mo compared to that in basalts,the major lithology of their drainage. This could result from a variety of processes, preferential weathering of Mo richsulphide minerals dispersed in the basalts, preferential removal of isotopically lighter Mo during transport or contributionfrom marine cyclic salts supplied via rain or chemical weathering of organic rich shales in the basins.

The distribution of d98Mo in the Narmada and the Tapi estuaries with salinity does not follow the theoretical mixing linebetween river and seawater endmembers suggesting its non-conservative behavior. Particulate Mo and d98Mo show concom-itant increase with salinity in the Narmada estuary indicating loss of dissolved Mo by adsorption onto Fe–Mn oxyhydroxide.Balancing the Mo budget along the course of these estuaries using inverse model suggests that in the Narmada estuary therecould be loss up to 8% of the dissolved Mo and that in the Tapi supply from anthropogenic sources could be up to 27%. Theresults obtained in this study bring out the processes modifying riverine input of Mo and its d98Mo in the estuaries, oxic sinkin the Narmada and anthropogenic input in the Tapi. Repetitive adsorption and desorption of Mo in the Narmada estuarycan modify the supply of dissolved Mo and its d98Mo relative to riverine supply by up to 40%, this can significantly impact theMo isotope budget of the oceans. In contrast, in the Tapi estuary there is enhancement in the dissolved supply of Mo relativeto that from river due to anthropogenic input of Mo. The investigations in these two estuaries underscore the importance ofsolute particle interactions and anthropogenic input in determining the Mo flux and its d98Mo to the open Arabian Sea.� 2014 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Molybdenum (Mo) and its isotopes (d98Mo) are beingused as a potential proxy to trace paleo-redox condition

http://dx.doi.org/10.1016/j.gca.2014.06.027

0016-7037/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Present Address: National Centre forAntarctic & Ocean Research, Goa, India.

E-mail address: [email protected] (W. Rahaman).

of oceans and constrain past variations in the spatial extentof ocean anoxia (Barling et al., 2001; Anbar and Knoll,2002; Siebert et al., 2003; Arnold et al., 2004; Anbar,2004; Pearce et al., 2008; Kendall et al., 2009, 2011; Dahlet al., 2010; Duan et al., 2010; Voegelin et al., 2010;Cazaja et al., 2012). Quantitative interpretation of Mo iso-tope record in marine sedimentary deposits to reconstructredox conditions of ancient oceans requires data on Moinput and its isotope composition to the oceans. The


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408 W. Rahaman et al. / Geochimica et Cosmochimica Acta 141 (2014) 407–422

dominant source of Mo to the oceans is rivers, with minorcontribution from hydrothermal sources (Morford andEmerson, 1999; McManus et al., 2002, 2006). The isotopecomposition of Mo supplied by global rivers is not wellconstrained; earlier it was assumed that rivers deliver Moto the oceans with an isotopic composition [d98Mo,expressed as d98Mo = [(98/95Mosample/

98/95Mostandard) � 1)� 1000] close to that of average crustal rocks, �0&

(Barling et al., 2001; Arnold et al., 2004; Nagler et al.,2005; Siebert et al., 2006). This assumption however camein doubt as a few recent measurements of Mo isotopes inglobal rivers with significant water discharge displayedlarge variations in its isotope composition, from 0.15& to2.40& (Archer and Vance, 2008; Pearce et al., 2010;Neubert et al., 2011; Voegelin et al., 2012). In addition tothe abundance of Mo and its isotope composition in rivers,another key factor that will influence their delivery to theopen oceans is their geochemical behaviour in estuaries.In this context, a few available studies exhibit divergentbehaviour of Mo in estuaries. Recently, Rahaman et al.(2010) in their investigations of estuaries from Indiareported removal of dissolved Mo in the Hooghly and theMandovi estuaries and gain in the Tapi estuary. Mo iso-topes behave conservatively in the Itchen estuary, SE Eng-land (Archer and Vance, 2008) whereas its behaviour isnon-conservative in the Borgarfjorður estuary with addi-tion of lighter isotopes (Pearce et al., 2010). The processesresponsible for the loss and/or gain of Mo in these estuar-ies, however, are not well understood. Pearce et al. (2010)hypothesized the release of lighter isotopes from particu-lates in the Borgarfjorður estuary for its non-conservativebehaviour.

The reported divergent behavior of Mo and its isotopesin estuaries makes it important to investigate their behav-iour in other estuaries characterized by different geologicaland environmental settings to obtain more representativedata on their riverine flux to the oceans. This informationis critical to evaluate the contemporary isotope budget ofMo and its isotopes in the oceans and their application asa proxy to determine the spatial extent of paleo-redox con-ditions of the oceans and their variability.

This work focuses on the study of Mo isotopes in twoIndian estuaries; the Narmada and the Tapi draining intothe Arabian Sea. The goal of the present study is to inves-tigate Mo isotope distribution in these two tropical estuar-ies and to understand the processes controlling theirabundances. The results of such a study provide better con-straints on the isotope composition of Mo supplied to theopen Arabian Sea from these rivers. This study representsthe first set of measurements of Mo isotopes in Indian riversand estuaries.

2. STUDY AREA

The Narmada and the Tapi estuaries selected for thisstudy (Fig. 1a and b) enter the Arabian Sea through theGulf of Cambay near Bharuch and Surat cities, respectively(Fig. 1b).

The Narmada originates from the Amarkantak in theVindhyan Mountains in Madhya Pradesh (MP). It drains

through the Deccan basalts and the Vindhyans comprisedof carbonates, shales and sandstones. Sporadic occurrencesof black/gray shales are reported in the Vindhyan (Guptaand Chakrapani, 2007). The Tapi originates at Multai inBetal district of MP and drains through Deccan basaltsand alluvial deposits before draining into the Gulf of Cam-bay. The major source of water to these rivers is monsoonrains, in addition they also receive significant contributionfrom groundwater (Gupta and Chakrapani, 2007; Guptaet al., 2011). A number of hydrothermal springs have beenreported along the Narmada and the Tapi lineament(Minissale et al., 2000; Sharma and Subramanian, 2008).The annual discharge of the Narmada �47.3 � 109 m3, is�2.5 times that of the Tapi �18.9 � 109 m3 (Alagarsamyand Zhang, 2005). Both these rivers have dams/reservoirsalong their courses. Large scale chemical, pharmaceutical,petrochemical and steel industries are situated along theNarmada and the Tapi rivers and their estuaries(Rahaman and Singh, 2010).

3. SAMPLING

Samples for this study are from the archives of earliercollections that were analyzed for Re, U, Sr and 87Sr/86Sr,the results of which are already published (Rahaman andSingh, 2010; Rahaman et al., 2010 and Rahaman andSingh, 2012). Briefly, water samples were collected fromthese two estuaries during one tidal cycle (low tide) alongthe salinity gradient. Separate aliquots of 10 ‘ water sam-ples from different salinities were collected for suspendedparticulate matter. The Narmada estuary was sampled dur-ing both the monsoon and non-monsoon seasons whereasthe Tapi was sampled only during the monsoon. The Nar-mada estuary could not be sampled beyond salinity of17.2 psu during monsoon because of inadequate facilitiesfor sample collection in the highly turbulent sea conditions.The Arabian Sea samples were collected on-board FORVSagar Sampada (cruise # SS 256). Salinity, pH and temper-ature were measured at site during collection. All samplesfor trace elements and their isotope analyses were filteredthrough 0.2 lm nylon filters, the filtered water acidified topH �2 with HNO3 and stored in precleaned high-densitypolypropylene bottles till the time of measurement.

3.1. Analysis

Mo isotope composition in dissolved and particulatesamples was measured following the double spike method(Siebert et al., 2001). The filtered and acidified watersamples were spiked with 97Mo–100Mo double-spikesolution, allowed to equilibrate before processing for Moseparation. In case of particulates, about �100–200 mg ofashed (at 550 �C) samples were digested with HF-HCland HNO3-HCl after addition of the double-spike. Mowas separated and purified from water and sediment sam-ples following the protocol of Pearce et al. (2009) using2 ml of Bio-Rad AG� 1-X8 anion exchange resin in a10 ml Poly-Prep� column. After separation and purificationof Mo from the samples, Mo isotopes were measured at thePhysical Research Laboratory, Ahmedabad using a

Fig. 1. (a) and (b) Location maps of the Narmada and the Tapi estuaries. Both these estuaries enter the Arabian Sea through the Gulf ofCambay.

W. Rahaman et al. / Geochimica et Cosmochimica Acta 141 (2014) 407–422 409

Thermo Neptune MC-ICPMS attached with ElementalScientific Apex-Q. Intensity of all the seven Mo isotopes92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo and 100Mo weremeasured. In addition, intensities of isobaric interferenceson Mo isotopes 91Zr and 99Ru were also monitored duringthe same cycle of measurements. Data reduction was per-formed off-line based on three-dimension geometric solu-tion scheme of Siebert et al. (2001) written in Matlabroutine. In addition, raw data were also processed usingthe available online Matlab code (http://www.john-rudge.com/doublespike/; Rudge et al., 2009). Both theMatlab routines of data reduction yielded same results.The d98Mo values were calculated by normalizing withthe in-house Johnson-Matthey Company (JMC) SpecPure�

Mo plasma standard (Lot # 21614). Double spike and thenormalizing standard (JMC) were calibrated following theprocedure by Siebert et al. (2001). Appropriate double spikeand standard solution mixtures of pure Mo were measuredseveral times to precisely determine the 98Mo/95Mo ratio ofthe standard. The internal precision (2SD) of these mea-surements calculated from 100 cycles was better than�0.1&. The external precision based on several replicateanalyses is 0.1& (2SD). Repeat measurements (n = 8) of

a seawater sample collected from the Bay of Bengal (cruiseSS-259, station 0811, water depth 300 m, Singh et al., 2011)yielded d98Mo of 2.38 ± 0.10& (2SD); these data providean assessment of external reproducibility. The averaged98Mo in six seawater samples in the salinity range 34.8–36.0 psu from the Arabian Sea and the Bay of Bengal is2.35 ± 0.20& (2SD, Table 1), in agreement within the ana-lytical uncertainty with that published for seawater (Siebertet al., 2003; Pearce et al., 2009; Nakagawa et al., 2012). Thehigher variance in our data probably is a result of poolingup of results from two different basins, the Bay of Bengaland the Arabian Sea having different salinities. The totalprocedural blank typically ranged from 3 to 6 ng for Mowhich is <1% of the total Mo analysed for majority ofthe samples and therefore, unlikely to have any discernableeffect on their measured Mo isotope composition. In case ofa few river samples having low Mo concentration, the blankaccounts for about 5%, however, no correction has beenmade for the same in these samples.

Mo concentration in water samples (Table 2a) was mea-sured earlier by quadrupole ICP-MS (Thermo-X series) andthe results published elsewhere (Rahaman et al., 2010). Moconcentration in the particulate matter was determined in

http://www.johnrudge.com/doublespike/

http://www.johnrudge.com/doublespike/

Table 1Dissolved Mo and d98Mo in seawater.

Sample Depth Salinity [Mo] d98Mo Errorm nmol/kg (2r)

Arabian Sea, 2010

SS256-2 m 2 35.1 113a 2.44 0.09SS256-6 m 6 36.0 116a 2.34 0.09

Bay of Bengal, 2008

SS259_300 m 300 35.0 112b 2.44 0.10SS259_2000 m 2000 34.8 113b 2.29 0.17

Arabian Sea, 2007

GA07-1 Surface � 108c 2.39 0.09GA07-3 Surface � 109c 2.18 0.09Average 110 ± 2 2.35 ± 0.20 0.20

a Goswami et al. (2012).b Singh et al. (2011).c Rahaman and Singh (2012).


this study based on the isotope dilution technique. The pre-cision of Mo concentration measurements in both the dis-solved and particulate phases are better than ±5% asdetermined based on several replicate measurements.

4. RESULT AND DISCUSSION

The dissolved and particulate Mo concentrations mea-sured in the estuaries along with salinity, pH and alkalinityare given in Tables 2a and 2b. The pH in the Narmada estu-ary remains nearly constant at �8 from river water to sea-water (Fig. 2a); the exception being the high pH (8.83) ofthe most upstream river water sample (NE07-1) duringthe non-monsoon (Table 2a). The Tapi estuary shows sig-nificant variations in pH (Fig. 2c), from 7.5 to 8.0. Alkalin-ity ranges from �2000 to 3000 leq/l in these estuaries(Table 2a, Fig. 2b and d). The alkalinity profiles in theNarmada estuary show conservative mixing during boththe seasons (Fig. 2b) whereas non-conservative mixing isobserved in the Tapi estuary (Fig. 2d); it shows a humpin the 4–12 psu salinity range (Rahaman et al., 2010).

4.1. Mo in rivers and estuaries

Dissolved Mo concentration in the Narmada and theTapi river water endmembers during monsoon ranges from4.3 to 6.1 nmol/kg (Table 2a).The dissolved Mo during themonsoon in these rivers is likely to be dominated by contri-butions from weathering of various lithologies exposed intheir drainage basins. Basalts are the dominant lithologyof both the Narmada and the Tapi river basins. There isno data on Mo abundance in Deccan basalts, however bas-alts from other regions are reported to contain 1–5 lg Mo/g(Fitton, 1995; Yamaguchi, 2002). If the Deccan basalts alsohave Mo in the same range, then the concentration valuesabove indicate that to achieve �5 nmol/kg dissolved Moin rivers �0.1–0.5 g basalts would have to completelyrelease its Mo, a process that would also release Na, K,Mg and Ca to the rivers. It can be estimated based on theabundances of these cations in Deccan basalts (Das et al.,2005) that �10–50 mg/l of (Na + K + Mg + Ca) would be

released to rivers if the chemical weathering of Mo andthese cations are congruent. The measured sum of these cat-ions supplied by basalt weathering to the Narmada andKrishna rivers are �15 mg/l (Das et al., 2005; Guptaet al., 2011). This result seems to indicate that Deccanbasalt weathering can be an important source of dissolvedMo to the Narmada and the Tapi rivers if its concentrationsin these basalts is �5 lg/g and the weathering of Na, K,Mg, Ca, and Mo are congruent. If however, the Mo con-centration in the Deccan basalts is much lower, similar tothat measured in suspended matter of these rivers(�0.5 lg/g, Table 2b) it would predict the need for a dom-inant additional source for Mo to the rivers; this can bepreferential weathering of Mo enriched sulphide mineralsdispersed in basalts (Voegelin et al., 2012) and/or weather-ing of organic rich shales, a constituent of the Vindhyansediments that occupy about a third of the Narmada basin(Gupta et al., 2011). Sen (2001) has reported occurrence ofsulphide minerals in the Deccan basalts. Oxidative weather-ing of sulphide minerals would release Mo from them, withan isotopic composition that would depend on their d98Mo.The d98Mo of sulphides is different from their host basaltsand crustal rocks in general, their weathering can supplydissolved Mo to rivers with d98Mo different from the crustalrocks (Voegelin et al., 2012).

The results of Mo distribution in the Narmada and theTapi estuaries reported by Rahaman et al. (2010) are repro-duced in Table 2a and Figs. 3 and 4. The Mo distributionbased on dissolved Mo vs. salinity plot shows conservativemixing (r2 = 0.99) during both the seasons in the Narmadaestuary (Rahaman et al., 2010). In contrast, in the Tapiestuary, Mo increases with salinity (Fig. 4a) with a ‘bulge’in the salinity range 3.9–11.5 psu. This ‘bulge’ representsgain of Mo and suggests that it has an additional sourcein this salinity range. The gain ranges from 4% to 32% withrespect to conservative mixing (Fig. 4a). The Mo bulgecoincides with the observed increase in alkalinity in thesame salinity range indicating that the additional sourcefor Mo and CO3

�/HCO3� may be coupled. A potential addi-

tional source can be anthropogenic input (Rahaman et al.,2010). Re and U, two other redox sensitive elements also

Table 2aDissolved Mo and d98Mo in the Narmada and the Tapi Estuaries.

Sample Latitude Longitude Salinity Alkalinity pH [Mo] d98Mo Error (2r)(lEq/‘) nmol/kg (&)

Narmada (Pre-monsoon, March 2007)

NE07-1 21�40.810 72�54.570 4.8 2947 8.08 20.9 1.26 0.09NE07-2 21�41.260 72�52.600 9.3 2962 7.92 36.9 1.98 0.09NE07-3 21�40.580 72�50.390 11.8 2909 7.98 40.0 1.95 0.10NE07-4 21�39.200 72�47.420 15.9 2814 7.93 50.1 2.26 0.09NE07-5 21�38.800 72�45.340 18.2 2807 7.96 54.0 2.09 0.09NE07-6 21�40.750 72�42.050 20.0 2754 8.00 60.7 2.14 0.11NE07-7 21�40.710 72�49.830 21.3 2744 7.95 62.7 2.37 0.09NE07-8 21�39.460 72�35.890 22.1 2776 8.03 66.9 2.28 0.11NE07-9 21�39.360 72�34.510 23.9 2738 8.02 70.2 2.38 0.09NE07-10 21�39.620 72�33.280 25.3 2668 7.98 75.0 2.45 0.09NE07-11 21�38.730 72�33.020 25.5 2686 8.11 73.6 2.25 0.09NE07-12 21�38.040 72�32.700 27.2 2699 7.9 78.3 2.36 0.09NE07-13 21�38.00 72�32.70 29.0 2653 8.08 82.9 2.34 0.09NE07-14 21�37.940 72�32.050 30.2 2654 8.10 86.6 2.30 0.13NE07-15 21�37.940 72�32.050 31.1 2635 8.08 89.6 2.27 0.09

Narmada (Monsoon, July 2007)

NEM07-1 21�40.590 72�55.580 0–<0.1 2395 8.83 4.3 0.49 0.12NEM07-2 21�39.080 72�46.640 0.1 2385 7.98 8.7 0.94 0.12NEM07-3 21�40.070 72�37.120 1.3 2410 8.06 10.7 0.95 0.09NEM07-4 21�39.580 72�36.340 2.2 2405 7.99 12.9 1.05 0.09NEM07-5 21�39.420 72�35.490 3.4 2309 8.00 16.2 1.53 0.18NEM07-6 21�39.150 72�34.280 5.1 2270 8.03 21.6 1.64 0.09NEM07-7 21�38.550 72�33.180 6.0 2322 8.04 23.7 1.64 0.09NEM07-8 21�38.390 72�33.260 8.0 2259 8.00 28.9 1.80 0.09NEM07-9 21�39.270 72�35.260 9.8 2219 8.00 34.2 1.94 0.09NEM07-10 21�38.520 72�33.20 12.1 2222 8.03 46.9 2.01 0.09NEM07-11 21�38.810 72�33.940 14.1 2180 8.03 51.2 2.20 0.09NEM07-12 21�38.050 72�33.190 15.5 2155 8.02 49.6 2.14 0.09NEM07-13 21�38.550 72�32.930 17.2 2146 8.25 53.8 2.19 0.09

Tapi (Monsoon, July2007)

TPM07-1 21�10.560 72�46.740 0–0.2 2848 7.57 6.1 0.99 0.12TPM07-2 21�8.90 72�45.80 1.1 2942 7.66 9.6 1.28 0.10TPM07-3 21�8.520 72�43.610 2.1 2937 7.73 12.4 1.49 0.10TPM07-4 21�8.860 72�42.260 3.9 3008 7.80 20.3 1.58 0.09TPM07-5 21�9.150 72�4.730 4.9 3085 7.74 25.1 1.56 0.09TPM07-6 21�9.30 72�40.190 5.8 3030 7.84 31.8 1.49 0.09TPM07-7 21�7.970 72�39.580 7.8 2941 7.90 37.9 1.81 0.09TPM07-8 21�7.050 72�39.740 9.8 2809 7.92 42.4 1.91 0.09TPM07-9 21�5.790 72�39.980 11.5 2700 – 44.0 2.12 0.09TPM07-10 21�4.970 72�40.640 13.1 2710 8.02 47.2 2.23 0.09TPM07-11 21�4.410 72�40.710 14.9 2685 8.01 54.3 2.36 0.09TPM07-12 21�3.60 72�40.680 17.4 2715 7.99 61.2 2.30 0.10TPM07-13 21�3.16/ 72�40.350 20.3 2530 8.02 70.5 –

Salinity, pH, alkalinity are taken from Rahaman and Singh (2010). Mo concentration data are taken from Rahaman et al. (2010).


measured in the same samples (Rahaman and Singh, 2010;Rahaman et al., 2010), however, displayed conservativemixing in the Tapi estuary suggesting Mo specific contam-ination; a likely source being the steel and lime industriessituated along the Tapi estuary (Rahaman et al., 2010).Effluents from these plants get discharged into this estuaryat mid salinities around the location of Mo-alkalinityhump. Industrial effluents discharged into rivers and estuar-ies are known to increase their heavy metal concentrations(Chakraborty et al., 2014); however, their impact on Moisotope systematics is not yet known.

Mo concentration in suspended sediments of theNarmada estuary collected during monsoon ranges from459 to 602 ng/g with an average of 512 ± 44 ng/g(Table 2b). One sample of suspended matter from the Tapihas 560 ng Mo/g similar to that measured in the Narmadasuspended matter (Table 2b). Basalts, which are the mainlithology of the Narmada catchment, generally contain1–5 lg/g Mo (Voegelin et al., 2012). The Mo content inthe suspended matter are lower compared to that in basalts,the major lithology of the basin. This can arise due to sig-nificant loss of Mo from basalts during their chemical

Table 2bMo and d98Mo in the suspended matter from the Narmada and theTapi Estuaries.

Sample Salinity d98Mo Error [Mo]& ±2r ng/g

Narmada monsoon

NEM2 0.1 �0.21 0.06 459NEM3 1.3 �0.12 0.04 459NEM5 3.4 �0.06 0.03 506NEM6 5.1 �0.11 0.04 469NEM8 8.0 0.08 0.04 539NEM10 12.1 0.48 0.07 537NEM11 14.1 �0.19 0.05 515NEM12 15.5 0.08 0.05 602NEM13 17.2 �0.13 0.12 512NEM13R 17.2 �0.11 0.07 520

Tapi estuary

TPM10 13.1 0.34 0.10 560

[Mo] represents ng of Mo in one gram of sediment air dried at100 �C.

Fig. 2. The distribution of pH and alkalinity vs. salinity in the estuarieuniform distribution excluding the sample, NE07-1 during non-monsoonlinearly increasing trend with salinity. Alkalinity profiles in the Narmadanon-monsoon seasons whereas it shows non-conservative mixing in the T(5–13 psu).


weathering and/or because the Mo abundance in Deccanbasalts of the basins is much lower. In the Narmada estuaryparticulates, Mo seems to increase with salinity (Fig. 5a),similarly Mo/Al of particulates in the Narmada collectedduring non-monsoon (Rahaman et al., 2010) also seemsto display an increasing trend (Fig. 5b) albeit significantscatter. This trend seems to indicate that in this estuary sizesorting of particles may not be a significant factor in deter-mining their Mo content. The increase in the Mo concentra-tion of the particles with salinity can be explained in termsof its adsorption from dissolved phase on Fe–Mn oxyhy-droxides. Relative to the particulate sample at salinity 0.1psu in the Narmada estuary, the gain in Mo in other sam-ples range from 10 to 143 ng/g. Considering that the Mogain is from the dissolved phase in estuary waters and theparticulate concentration estimated in these waters is�1.5 g/l during monsoon, it can be estimated that adsorp-tive loss of Mo from water is in the range of 0.7–4.4% withan average of �2.7% of its dissolved concentration. Theestimated loss is difficult to unambiguously detect as it iswithin the analytical uncertainty of measurements.

s. The pH – salinity profiles in the Narmada estuary show almostwhich has significantly higher pH. The pH profile in the Tapi showsestuary show conservative mixing during both the monsoon and the

api estuary with systematic gain in alkalinity in the lower salinity

Fig. 3. Scatter plots of dissolved Mo vs. salinity. The data show conservative mixing in the Narmada estuary during both the monsoon andthe non-monsoon seasons. Mixing plot of 1/Mo vs. d98Mo indicates conservative mixing within the salinity range of 4.8–31.1 psu in theNarmada estuary during non-monsoon whereas it shows non-conservative mixing during monsoon. Theoretical mixing lines (dotted lines) aredrawn separately based on the two samples (NEM07-1, NEM07-2) and seawater. This clearly shows that most of the points fall below thetheoretical conservative mixing line even if NEM07-02 is considered as riverine endmember. This could be a result of adsorption of heavierMo isotopes from dissolved to particulate phase or influx of SGD. Dissolved Mo data from Rahaman et al. (2010).


4.2. d98Mo in rivers and estuaries

The isotope composition of Mo (d98Mo) was measuredin both the dissolved and particulate phases of the Narmad-a and the Tapi estuaries. The dissolved d98Mo in the Nar-mada estuary ranges from 1.26& to 2.45& in the salinityrange 4.8–31 psu during non-monsoon whereas duringmonsoon it ranges from 0.49& to 2.19& within the salinityrange 0–17.2 psu (Table 2a, Fig. 3b and d). The d98Mo val-ues in the Tapi estuary is also in similar bracket, varyingfrom 1.28& to 2.36& in the salinity range 1.1–20.3 psu(Table 2, Fig. 4b). The d98Mo measured in nine particulatesamples from the Narmada estuary collected during mon-soon, ranges from �0.21& to 0.48& with an average�0.03 ± 0.2& (Table 2b). Similar to Mo content, d98Moalso shows an increasing trend in particulate matter from�0.21& to 0.48& between the salinity range 0.1 to12.1 psu, the d98Mo subsequently decreases to �0.19&

(Fig. 6). One particulate sample from the Tapi estuary(TPM-10) yielded a value of 0.34 ± 0.05& for d98Mo(Table 2b) within the range observed for the Narmadaestuary.

d98Mo of the Narmada river water (salinity < 0.1 psu) is0.49& during monsoon whereas in the Tapi it is marginallyheavier (0.99&). These riverine values are within the range(0.2–2.4&) reported for other global rivers (Archer andVance, 2008; Pearce et al., 2010; Neubert et al., 2011;Voegelin et al., 2012). The average d98Mo value for basaltsis �0.0& (Siebert et al., 2003; Pearce et al., 2009; Voegelinet al., 2012). If the Narmada and the Tapi rivers derive theirdissolved Mo entirely from basalts, they are expected tohave d98Mo values same as that of basalts, �0& assumingcongruent weathering of its minerals. The Mo isotope com-position of the Narmada and the Tapi river waters are,however isotopically heavier than that in basalts. Theenrichment in heavier isotopes of Mo in river waters canresult from: (i) Dissolution of sulphide minerals: preferen-tial dissolution (incongruent weathering) of sulphide miner-als dispersed in the Deccan basalts that results in the releaseof isotopically heavier Mo (Voegelin et al., 2012). There-fore, incongruent weathering of sulphide minerals couldbe a potential process responsible for isotopically heavierMo in the river water. Such sulphide minerals have beenshown to be enriched in Re and Os (Roy-Barman et al.,

Fig. 4. (a) Scatter plots of salinity vs. Mo in the Tapi estuary. The Mo profile shows a hump in the salinity range of �4–12 psu in this estuary.Effluent discharge from steel plant situated in the vicinity of this estuary is a likely source for the Mo “bulge” (Rahaman et al., 2010). Thetheoretical conservative mixing line is based on data in the salinity 0.0–0.1 psu and 13.1–17.4 psu (i.e. excluding the bulge). (b) Mixing plot of1/Mo vs. d98Mo in the Tapi estuary. The profile show departure from theoretical conservative mixing line drawn based on the river andseawater endmembers.


1998) and has been reported as a potential source fordissolved Re to rivers draining the Deccan Basalts(Rahaman and Singh, 2012). (ii) Weathering of blackshales: In addition to basalts, the Narmada basin also hasalluvium and the Vindhyan sediments with sporadic occur-rences of black shales (Gupta and Chakrapani, 2007). Blackshales generally contain isotopically heavier Mo, withd98Mo in the range of �0.46 to 2.14& (Siebert et al.,2003; Arnold et al., 2004; Duan et al., 2010; Dahl et al.,2010; Kendall et al., 2011). The overlap in d98Mo valuesbetween black shales and the Narmada river water hintsat the possibility of black shales being an additional sourceof dissolved Mo to the Narmada, especially to account forits heavy Mo isotopes. However, the lighter d98Mo in par-ticulate matter of the Narmada river raises doubts aboutthe importance of black shales as a source of Mo. In con-trast to the Narmada, the isotopically heavier Mo (d98Mo�1&) in the Tapi river water relative to Mo in rocks inits drainage basin is difficult to be explained in terms ofblack shale weathering as they are not reported in its catch-ment. Further, (iii) Preferential uptake of lighter Mo

isotopes from dissolved phase onto Fe–Mn oxyhydroxides:Mo can be adsorbed on secondary phases mainly clays andoxides (Goldberg et al., 1996) and Fe–Mn oxyhydroxides, aprocess that can cause Mo isotope fractionation (Barlingand Anbar 2004; Goldberg et al., 2009). In oxic near neutralriver waters, Fe–Mn oxy-hydroxides can precipitate ontoparticulates during their transport. The Fe–Mn oxy-hydroxide coatings can scavenge Mo from aqueous phasea process that can fractionate Mo isotopes, the lighter iso-topes being scavenged preferentially onto the oxide phasesleaving the river water relatively heavier in Mo isotopes(Archer and Vance, 2008; Voegelin et al., 2012). This prop-osition, however, has met with challenges for the Icelandicrivers (Pearce et al., 2010). (iv) Contribution of heavier Moisotopes from groundwater: Isotopically heavier Mo in theNarmada and the Tapi can result from groundwater inputwith isotopically heavier Mo, a mechanism suggested byPearce et al. (2010) for the Icelandic rivers. Pertinent to this,it is important to mention that during lean flow, groundwa-ter contribute up to �6% of the total water discharge atGarudeshwar, final outflow of the Narmada river (Gupta

Fig. 5. (a) Mo concentration in particles vs. salinity in theNarmada estuary during monsoon. The data show an overallincreasing trend with salinity. (b) Molar ratio of Mo/Al vs. salinityin the Narmada estuary during the non-monsoon season(Rahaman et al., 2010). The data seem to show increasing trendalbeit significant scatter. Al Normalization minimizes the impact ofvariations resulting from grain size sorting.

Fig. 6. Scatter plot of Mo concentration and d98Mo in particles vs.salinity. The data show overall increase of Mo concentration andd98Mo with salinity indicating loss of dissolved Mo resulting fromits uptake on Fe–Mn oxyhydroxides.


et al., 2011). (v) Marine cyclic salts: Isotopically heavier Moin the Narmada and the Tapi rivers can be a result of itscontribution through marine cyclic salts. Pearce et al.(2010) based on snow melt data demonstrated that in someof the tributaries of the Icelandic rivers with lower Mo con-tent and isotopically heavier Mo, rainwater could be signif-icant contributor to their Mo budget. The observedconcentrations of Mo in the Narmada and the Tapi riversare low enough to be impacted by marine cyclic salt contri-bution, however it is difficult to confirm the validity of thishypothesis due to lack of Mo data in rain water from theseriver basins. Mo concentration in rainwater over Japan var-ies over one order of magnitude with values as high as1.2 nM (Shijo et al., 1996). If such high Mo concentrationsare typical of rains over the Narmada and the Tapi basinsand assuming �50% evapotranspiration (Gupta et al.,2011), rainwater Mo may contribute up to 30–50% of dis-solved Mo budget. The supply of seawater derived Movia rain can also explain the higher value of d98Mo in riversrelative to that in the source rocks of their basins. In thiscontext, it is pertinent to mention here that Singh et al.(2013) interpreted their results on boron abundances inthese rivers in terms of significant contribution from rains.

The presently available data on Mo and its isotope compo-sition in various sources are inadequate to choose amongthe various alternatives.

4.3. Behaviour of d98Mo in the Narmada and the Tapi

estuaries

The mixing plots for dissolved Mo isotopes,1/Mo-d98Mo, in these estuaries are presented in Figs. (3band d and 4b). The theoretical conservative mixing linesfor two endmember mixing of river and seawater are alsoplotted in these figures; the comparison shows that mea-sured data in these estuaries deviate from the theoreticalmixing trends during monsoon. In these mixing plots, theseawater endmember values for Mo and d98Mo are takento be the average of the six seawater samples from theArabian Sea and the Bay of Bengal measured in this study;110 ± 2 nmol/kg and 2.35 ± 0.20&, respectively (Table 1).These values are similar to those reported for the globaloceans (Morris, 1975; Quinby-Hunt and Turekian, 1983;Collier, 1985; Siebert et al., 2003; Pearce et al., 2010;Nakagawa et al., 2012). The 1/Mo-d98Mo plot of theNarmada estuary during non-monsoon, however, showsconservative mixing (r2 = 0.90, n = 15) in the salinity range4.8–31.1 psu. Data for Mo and d98Mo in the lower salinityregion (64.8 psu) are not available due to lack of samplesand hence the nature of Mo isotopes distribution in thislow salinity range is not known. The d98Mo distributionin the lower salinity range including river water, however,shows non-conservative mixing during monsoon (Fig. 3d)with many of the data points falling below the theoreticalconservative mixing line (TML). Pearce et al. (2010) alsoreported similar results for the Borgarfjorður estuary whichwas explained in terms of release of lighter Mo isotopesfrom colloidal and particulate fractions due to change insalinity. A closer look at the data in Fig. 3c shows thatthe sample at <0.1 psu salinity deviates from the conserva-tive mixing line; its Mo concentration of 4.3 nmol/kg at<0.1 psu increases to 8.7 nmol/kg at 0.1 psu salinity, an


observation that can be explained in terms of release ofadsorbed Mo from Fe–Mn hydroxide coatings of riverineparticles. Such a process can also account for the observeddecrease in dissolved d98Mo in the estuary. This hypothesis,however, does not seem to be consistent with the abun-dance of Mo in particles along the salinity gradient in theestuary, which seem to show an overall increasing trend.Such a trend is an indication of uptake of Mo from dis-solved to particulate phase. Added to this is the finding thatthe d98Mo also increases with salinity in the range 0.1–12.1 psu (Fig. 6). The concomitant increase in both Moconcentration and its d98Mo in particles seem to indicatethat dissolved Mo is being adsorbed onto particulates inthe estuary. Removal of dissolved Mo, though is reportedfor some of the estuaries such as seasonally anoxic Chesa-peake Bay (Scheiderich et al., 2010) and the Mandovi andthe Hooghly estuaries from the Arabian Sea and the Bayof Bengal (Rahaman et al., 2010) such a Mo loss is notdiscernible in the dissolved Mo vs. salinity profile in theNarmada estuary. This, as discussed earlier, can be becauseof the low fractional loss of Mo, 0.7–4.3% that is difficult tobe clearly discerned in the Mo vs. salinity plot given theanalytical uncertainties in Mo measurements. Mass balancecalculation based on the measured Mo concentration andits d98Mo in particles in the estuary and those in the partic-ulate sample at salinity 0.1 psu yields loss of dissolved Mowith d98Mo in the range of �0.03 to 4.5& with an averageof 1.96 ± 1.83&. This suggests that 0.7–4.3% of dissolvedMo is being adsorbed onto particles with d98Mo1.96 ± 1.83& in the Narmada estuary during monsoon.Such a loss of Mo on Fe-Mn hydroxide yieldsD98/95Modissolved-Fe-Mn oxyhydroxide in range of �2.8 to

Fig. 7. Salinity vs. d98Mo (dissolved and particulate) plot in the Narmamechanisms which modify dissolved and particulate Mo and d98Mo in thisotopes from particulate due to change in salinity in the beginning of theexplained in terms of Mo loss associated with Fe–Mn oxyhydroxide precparticulate d98Mo based on the first sample (NEM2) which is expectedassociated with Fe–Mn oxyhydroxide corresponding to the dissolved MoD98/95Mowater-Fe-Mn oxyhydroxide = 0.6–2.9& (Barling and Anbar, 2004; Gcircles represent the precipitated Mo based on D98/95Mowater-Fe-Mn oxyhydro

will also follow increasing trend similar to that of water from which theyparticulate (ab dotted line) via Fe-Mn oxyhydroxide precipitation tend tobased on the bulk particulate d98Mo. Slope of this trend is determined bya0 b0 dotted line represents linear regression line based on particulate d98

+2.3&. In Fig. 7, a model is presented to explain thisobservation. Laboratory experiments have shown thatD98/95Modissolved-Fe-Mn oxyhydroxide is in the range of 2.4–2.9and 0.6–2.6& for adsorption on Mn and Fe oxyhydroxidesrespectively (Barling and Anbar, 2004; Goldberg et al.,2009). Therefore, removal of Mo by adsorption ontoFe–Mn oxyhydroxides would have a constant offsetbetween 0.6& and 2.9& with respect to estuary water;the offset being dependent on the type of oxyhydroxideminerals being precipitated onto estuary sediments. Basedon this constant offset, the trend of d98Mo in the absorbedMo from the Narmada estuary water is estimated andshown in Fig. 7. The Mo isotope composition of the river-ine particulates, however is a resultant composition ofd98Mo inherited from the source rock and d98Mo of theadsorbed Mo which increases with salinity. However, thediverging results i.e. release of lighter isotopes from partic-ulates responsible for lowering the d98Mo of the lower salin-ity zone and dissolved Mo loss based on increase inparticulate Mo concentration with salinity in the Narmadaestuary need to be investigated in more detail through sys-tematic study of vertical profile samples of water andparticulate from the estuary.

The 1/Mo-d98Mo plot for the Tapi estuary also showsnon-conservative mixing, however in this case both Moand d98Mo show consistent behaviour with depletion ofd98Mo and gain of Mo and alkalinity in the salinity range3.9–11.5 psu (Fig. 4b). This behaviour as discussed in Sec-tion 4.1, can be attributed to input of anthropogenic Moto this estuary. The non-conservative behaviour of Moand its isotopes as evidenced from mixing plots of salinityvs. Mo and 1/Mo vs. d98Mo, suggest that in addition to

da estuary. In this plot, a model is presented to explain plausiblee estuary. Dark ellipse shows the location of desorption of lighter

estuary. The overall increase of particulate Mo and d98Mo could beipitation into particulate. The ab dotted line represent base line ofto be unaffected by Mo gain. The open circles represent the Mofrom which they get precipitated based on the fractionation factoroldberg et al., 2009). Two dotted lines passing through the open

xide = 0.6& (top line) and 2.9& (bottom). Therefore, these two linesprecipitate. Addition of Mo with increasing d98Mo to the riverinefollow increasing trend which is expected to be similar to the trendthe extent of this process and mass balance between these phases.

Mo data.


river and the seawater endmembers, anthropogenic input ofMo with its own characteristic d98Mo is an importantsource to the Tapi estuary.

4.3.1. The budget of dissolved Mo in the Narmada and the

Tapi estuaries

The results of Mo distribution and its d98Mo in theNarmada and the Tapi estuaries show that dissolved Mowith isotopically heavier Mo is being lost from dissolvedphase to particulates in the Narmada, in contrast, in theTapi where there is addition of Mo from anthropogenicsource to the dissolved phase. In the following section,an attempt is made to estimate the fractional loss of Moand its d98Mo in the Narmada estuary and the quantumof anthropogenic Mo input to the Tapi estuary based oninverse model. In addition to the magnitude of gain/lossof Mo, the model also provides the estimates of Mocontribution from river and sea waters to these estuariesalong their salinity gradients.

The inverse model has been successfully used to appor-tion the contribution of cations from different sources toriver waters (Negrel et al., 1993; Gaillardet et al., 1999;Tripathy and Singh, 2010), to quantify the SGD in the Nar-mada estuary (Rahaman and Singh, 2012) and to estimatethe contribution of Nd from particles to the Bay of Bengalwaters (Singh et al., 2012). The details of the inverse mod-eling approach and calculations made in this work are thesame as that followed by Rahaman and Singh (2012). Themodel is based on mass balance equations for Mo andd98Mo in the estuary and a-priori values of these parametersfor the endmembers i.e. seawater, river water and loss viaadsorption/gain via anthropogenic input of dissolved Moin the Narmada/Tapi estuaries (Table 3a and b). The equa-tions used are

Salinitym ¼Xn

i¼1

Salinityi � fi ðiÞ

Mom ¼Xn

i¼1

Moi � fi �Moloss=gain ðiiÞ

Table 3Salinity, Mo and d98Mo in seawater, river waters and particulate release

Component Salinity (psu)

(a) Narmada estuaryA-priori values

Seawater 35.0 ± 0.1River water 0.0 ± 0.1Mo loss –A-posteriori values

Seawater 35.0 ± 0.1River water 0.0 ± 0.0

(b) Tapi estuaryA-priori values

Seawater 35.0 ± 0.1River water 0.20 ± 0.1Anthropogenic –A-posteriori values

Seawater 35.0 ± 0.1River water 0.19 ± 0.10

d98Mo� �

m�Mom ¼

Xn

i�1

d98Mo� �

i�Moi � fi

� d98Mo� �

loss=gain�Moloss=gain ðiiiÞ

Xn

i¼1

fi ¼ 1 ðivÞ

where i = 1, 2 and represents the two endmembers, seawa-ter and river water, loss/gain denotes the loss of dissolvedMo in the Narmada estuary and its gain from anthropo-genic sources in the Tapi estuary, respectively and the sub-script m represents the measured values, f represents waterfractions. Moloss term will be subtracted in case of the Nar-mada whereas Mogain term will be added in Eqs. (ii) and(iii). Eqs. i, ii, iii, iv represent budgets of salinity, Mo con-centration, d98Mo and water. The best possible set of solu-tions for these model parameters and their associateduncertainties were obtained from iterative solutions ofEqs. i, ii, iii, iv using non-linear weighted fit following theQuasi-Newton method (Tarantola, 2005; Tripathy andSingh, 2010; Rahaman and Singh, 2012). Mo concentra-tions and d98Mo of the two endmembers, seawater andriver water are known and are given in Table 3a and b.River water endmember values are the same as those mea-sured in samples from these two rivers collected duringmonsoon. The a-priori values for calculating the fractionsof water from these two endmembers along the estuariesare assumed to be 50 ± 50%. The a-priori value of d98Mobeing lost from dissolved phase is 1.96 ± 1.83&, same asthat estimated for adsorbed Mo in particulates and thevalue for Mo loss from water to the particles is taken as2.7 ± 1.5% of dissolved Mo based on increase in Mo con-centration in estuarine particulates (Section 4.3). Calcula-tions show that the model results are not very sensitive tothese a-priori values and hence any uncertainty associatedwith them will not result in ambiguous model output.

4.3.1.1. Mo and d98Mo loss from the Narmada estuary. Thecalculated a-posteriori values of Moloss associated d98Mo

endmembers.

[Mo](nmol/kg) d98Mo (&)

110 ± 2.0 2.40 ± 0.0.024.3 ± 0.1 0.49 ± 0.122.7% ± 1.5% of Mom 1.96 ± 1.83

116.4 ± 1.3 2.39 ± 0.024.4 ± 0.1 0.4 ± 0.1

110.0 ± 2.0 2.40 ± 0.026.1 ± 0.1 1.0 ± 0.14.0 ± 4.0 1.2 ± 0.6

110.8 ± 1.8 2.40 ± 0.026.1 ± 0.1 1.1 ± 0.1

Table 4Estimated contribution of Mo loss (in Narmada estuary) and Moexcess (in Tapi estuary) and their isotope compositions.

Sample Moloss d98Moloss

nmol/kg &

NEM07-2 0.2 ± 0.1 2.4 ± 2.2NEM07-3 0.3 ± 0.1 2.8 ± 2.6NEM07-4 0.3 ± 0.2 3.0 ± 2.8NEM07-5 0.4 ± 0.2 2.1 ± 2.0NEM07-6 0.5 ± 0.3 2.4 ± 2.2NEM07-7 0.6 ± 0.3 2.5 ± 2.3NEM07-8 0.8 ± 0.4 2.4 ± 2.2NEM07-9 1.1 ± 0.5 2.3 ± 2.1NEM07-10 0.7 ± 0.4 2.1 ± 2.0NEM07-11 0.9 ± 0.5 2.0 ± 1.9NEM07-12 3.2 ± 1.1 2.4 ± 2.0NEM07-13 4.5 ± 1.2 2.4 ± 1.9

Mogain d98Mogain

nmol/kg &

TPM07-2 1.0 ± 0.7 1.4 ± 0.6TPM07-3 1.1 ± 0.7 1.4 ± 0.6TPM07-4 2.9 ± 1.0 1.5 ± 0.6TPM07-5 4.6 ± 1.1 1.5 ± 0.6TPM07-6 8.6 ± 1.1 1.6 ± 0.6TPM07-7 8.7 ± 1.2 1.9 ± 0.7TPM07-8 7.2 ± 1.2 1.7 ± 0.7TPM07-9 3.7 ± 1.2 1.5 ± 0.7TPM07-10 2.4 ± 1.2 1.4 ± 0.6TPM07-11 3.8 ± 1.3 1.5 ± 0.7TPM07-12 3.3 ± 1.4 1.4 ± 0.6

Table 5Estimated contribution of Mo from seawater, river water and Mo loss a

Sample Salinity (psu) Mo budg

River wa

Narmada estuary

NEM07-2 0.1 49.2 ± 1.NEM07-3 1.3 39.2 ± 0.NEM07-4 2.2 31.8 ± 0.NEM07-5 3.4 24.5 ± 0.NEM07-6 5.1 17.4 ± 0.NEM07-7 6.0 15.5 ± 0.NEM07-8 8.0 11.9 ± 0.NEM07-9 9.8 9.4 ± 0.2NEM07-10 12.1 5.9 ± 0.2NEM07-11 14.1 5.1 ± 0.1NEM07-12 15.5 5.1 ± 0.1NEM07-13 17.2 4.3 ± 0.1

Tapi estuary River wa

TPM07-2 1.1 61.6 ± 1.TPM07-3 2.1 46.4 ± 0.TPM07-4 3.9 26.7 ± 0.TPM07-5 4.9 20.8 ± 0.TPM07-6 5.8 16.0 ± 0.TPM07-7 7.8 12.5 ± 0.TPM07-8 9.8 10.3 ± 0.TPM07-9 11.5 9.3 ± 0.2TPM07-10 13.1 8.1 ± 0.2TPM07-11 14.9 6.4 ± 0.2TPM07-12 17.4 5.0 ± 0.1


and the contribution of Mo from the two endmembers, sea-water and river water, to the estuary samples are given inTables 3a, 4 and 5. The magnitude of Moloss and d98Moloss

from individual samples derived from the inverse model aregiven in (Tables 3 and 4) ranges from 1.7% to 8.3% of thetotal Mo with d98Moloss 2.40 ± 0.58& (Table 4). The Moloss

is present all along the estuary indicating removal of isoto-pically heavier dissolved Mo throughout estuary.

It is interesting to note that the Mo loss in the Narmadaestuary is �2% in most of the samples similar to the loss(�2.7%) estimated earlier based on uptake in particulates(Section 4.3). Two samples at salinities of 15.5 and17.2 psu show higher Mo loss of 5.9% and 7.9%, respec-tively. The budget of dissolved Mo, in terms of fractionalcontributions from river water and seawater and Mo lossare plotted with respect to salinity which shows Mo lossin the Narmada estuary ranges from 2% to 8% in the entiresalinity range (Fig. 8a). The results obtained in this studyon dissolved and particulate Mo concentration andd98Mo in the Narmada estuary bring out the importanceof estuarine processes of adsorption/or desorption in mod-ifying the riverine Mo isotope composition before its finalinput to the open ocean.

4.3.1.2. Anthropogenic Mo and d 98Mo in the Tapi estuary.

Mo can be contributed to fluvial system through anthropo-genic activities (Colodner et al., 1995; Peucker-Ehrenbrinket al., 1995; Chappaz et al., 2008; Chappaz et al., 2012).Chappaz et al. (2008) studied Mo isotope composition intwo sediment cores from the Lakes Tantare and Vose in

nd anthropogenic sources in the Narmada and Tapi estuary.

et (%)

ter Seawater Mo loss

1 49.1 ± 3.8 2.2 ± 1.29 60.8 ± 3.5 2.4 ± 1.37 69.0 ± 3.2 2.4 ± 1.36 76.9 ± 2.9 2.4 ± 1.34 83.9 ± 2.6 2.3 ± 1.24 86.5 ± 2.6 2.5 ± 1.33 91.3 ± 2.4 2.9 ± 1.5

94.1 ± 2.3 3.1 ± 1.692.6 ± 1.9 1.5 ± 0.895.1 ± 1.8 1.7 ± 0.9102.2 ± 2.2 6.4 ± 2.2104.9 ± 2.2 8.3 ± 2.3

ter Seawater Anthropogenic

1 29.2 ± 26.2 10.6 ± 7.39 45.8 ± 13.8 8.9 ± 6.05 59.2 ± 8.3 14.2 ± 5.04 60.7 ± 6.8 18.4 ± 4.33 56.9 ± 5.9 27.1 ± 3.63 64.4 ± 4.5 23.0 ± 3.22 72.6 ± 3.7 16.9 ± 2.9

82.2 ± 3.2 8.4 ± 2.887.0 ± 2.8 5.0 ± 2.586.5 ± 2.7 7.0 ± 2.589.6 ± 2.4 5.3 ± 2.3

Fig. 8. Salinity vs. Mo fractions (%) of river, seawater, Mo lossand anthropogenic sources. (a) Mo loss in the Narmada estuary isup to 2–8% in the entire salinity range. (b) The anthropogenic Mofraction profile shows highest value �28% at salinity 6–10 psuwhich indicates point source of anthropogenic Mo in this region ofthe estuary.

Fig. 9. (a) Comparison of the estimated fractions of River waterand Seawater (a) and Mogain (b) in Tapi estuary for differenta-priori values of d98Mogain. Changing the a-priori d98Mogain

(anthropogenic sourced) from 1.2& to 0.0&, fractions of theRiver water and seawater and the Mogain converges to values whichare same within uncertainties.


Qubec, Eastern Canada and reported dramatic shift in Moisotope composition since 20th century resulting from sig-nificant input of anthropogenic Mo through industrialeffluents and atmospheric deposition. Rahaman (2011)reported substantial input of anthropogenic Mo from steelindustry to the Tapi estuary. It is important to characterizeMo and its isotope composition of the anthropogenicinputs to the Tapi estuary as well as the coastal areas ofthe Gulf of Cambay and elsewhere as anthropogenic Mocan modify the natural input of Mo and its isotopecomposition.

The magnitude of anthropogenic supplied Mo and itsd98Mo to the Tapi estuary has been estimated using theinverse model. In absence of d98Mo measurement of efflu-ents of steel plants situated on the banks of the Tapi estu-ary, the a-priori value of d98Mo for the anthropogenicinput is assumed to be 1.2 ± 0.0.6& (Table 3b) based onthe assumption that this Mo is sourced from Mo ores whichgenerally has d98Mo in the range, as �0.4& to +2.0&

(Hannah et al., 2007; Mathur et al., 2010; Chappaz et al.,2012). The a-priori value for Mo input from for the anthro-pogenic sources is estimated to range from 0 to 8 nmol/kgbased on the deviation of Mo distribution with respect to

TML in estuary waters (Fig. 4a); the mean of this range4.0 ± 4.0 nmol/kg is used as the a-priori value for the input.The anthropogenic Mo and d98Mo contributed to eachsample in the estuary estimated based on the inverse modelis presented in Table 4. The results show that the contribu-tion of anthropogenic Mo ranges from �5% to 27% withmaximum in the salinity range 6–10 psu (Fig. 4b). The a-

posteriori value of anthropogenic d98Mo is �1.52 ± 0.33&

for the entire salinity range (Table 4); this value is withinthe range reported for Mo ores. Mo contributions fromriver and seawater endmembers are estimated (Table 5)and plotted with respect to salinity (Fig. 8b). The anthropo-genic Mo fraction profile shows the highest value �27% atsalinity 6–10 psu which indicates point source of anthropo-genic Mo in this region of the estuary

Model results of Mo contributed by anthropogenicsources have only a limited sensitivity to the assumed


d98Mo of the anthropogenic source. To assess the modelsensitivity to the assumed d98Mogain along with its riverineand seawater components has also been estimated assumingd98Mo of 0.0&. The output of both the runs (with d98Mo ofanthropogenic sources taken as 0 and 1.2&) is similarwithin uncertainties (Fig. 9).

4.3.2. Implications of the Narmada/Tapi results to oceanic

budgets

Riverine supply is the major source of Mo to the globaloceans. The results of this study shows that both in theNarmada and the Tapi estuaries the riverine supply ofMo and its d98Mo get modified in the estuaries. Modelcalculations of Mo distribution during the Narmada estu-ary though show that the Mo loss is only marginal (�3%),however, repetitive adsorption/desorption of Mo in theestuary can lower the d98Mo by as much as 42% in thelow salinity samples. In contrast, there is net addition ofMo to the Tapi estuary from industrial effluents. Theseresults highlight the impact of the solute-particle interac-tions and anthropogenic supply in determining the behav-iour of Mo and its isotopes in estuaries and their ultimateflux to the open oceans. If the non-conservative behaviourof Mo and d98Mo observed in this work is prevalent inglobal estuaries, the estimate of Mo flux and its d98Moto open oceans from land based on river water data canbe in doubt.

5. CONCLUSIONS

The Narmada and the Tapi estuaries draining into theArabian Sea have been investigated for dissolved and par-ticulate Mo isotope distribution with salinity. The Mo iso-tope composition of the Narmada and the Tapi rivers areheavier than the parent rocks (basalts) in their drainagebasins. This enrichment in heavier isotope could be eitherdue to preferential dissolution of sulphide minerals dis-persed in the Deccan basalts or removal of lighter Mo ontoFe–Mn oxyhydroxides or supply of Mo through marinecyclic salts or chemical weathering of Vindhyan sedimentsparticularly black shales in them. The distributions ofd98Mo with salinity demonstrates non-conservative mixingbetween river and seawater endmembers in both estuaries.In the Narmada estuary there is loss of isotopically heavierdissolved Mo consistent with observations of concomitantincrease in both Mo and d98Mo in particulates with salinitywhereas in the Tapi there is addition of Mo via anthropo-genic sources. The Mo loss in the Narmada can result fromits uptake on Fe–Mn hydroxides. Inverse model is used toestimate the magnitude of Mo loss and anthropogenic inputof Mo and their isotope compositions in the Narmada andthe Tapi estuaries respectively. These calculations yield Moloss up to 8% of the dissolved Mo in the Narmada estuarywhereas the anthropogenic addition of Mo to the Tapi canbe up to 27 ± 4% of dissolved Mo.

This study highlights the processes responsible for thenon-conservative behaviour of Mo in the two estuariesand provide estimate of the extent of modification of river-ine dissolved Mo flux and its isotope composition to openArabian Sea.

ACKNOWLEDGEMENTS

We thank S. Krishnaswami for his suggestions and commentson the manuscript; J.P. Bhavsar, Gyana Ranjan Tripathy, SatinderPal Singh and Jayati Chatterjee for help in sampling. The Ministryof Earth Sciences, Government of India provided partial financialsupport and logistics for this work under the GEOTRACES pro-gramme. We thank Derek Vance and three anonymous reviewersfor their comments and suggestions that have helped to improvethe manuscript.

REFERENCES

Alagarsamy R. and Zhang J. (2005) Comparative studies on tracemetal geochemistry in Indian and Chinese rivers. Curr. Sci. 89,299–309.

Anbar A. D. and Knoll A. H. (2002) Proterozoic ocean chemistryand evolution: A bioinorganic bridge? Science 297, 1137–1142.

Anbar A. D. (2004) Molybdenum stable isotopes: Observations,interpretations and directions. Rev. Mineral. Geochem. 55, 429–454.

Archer C. and Vance D. (2008) The isotopic signature of the globalriverine molybdenum flux and anoxia in the ancient oceans.Nature Geo. Sci. 1, 597–600.

Arnold G. L., Anbar A. D., Barling J. and Lyons T. W. (2004)Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans. Science 304, 87–90.

Barling J. and Anbar A. D. (2004) Molybdenum isotope fraction-ation during adsorption by manganese oxides. Earth Planet.

Sci. Lett. 217, 315–329.Barling J., Arnold G. L. and Anbar A. D. (2001) Natural mass

dependent variations in the isotopic composition of molybde-num. Earth Planet. Sci. Lett. 193, 447–457.

Cazaja A. D., Johnson C. M., Roden E. E., Beard B. L., VoegelinA. R., Nagler Th. F., Beukes N. J. and Wille M. (2012)Evidence for free oxygen in the Neoarchean ocean based oncoupled iron-molybdenum isotope fractionation. Geochim.

Cosmochim. Acta 86, 118–137.Chappaz A., Gobeil C. and Tessier A. (2008) Geochemical and

anthropogenic enrichments of Mo in sediments from perenni-ally oxic and seasonally anoxic lakes in Eastern Canada.Geochim. Cosmochim. Acta 72, 170–184.

Chappaz A., Lyons T. W., Gwyneth Gordon W. and Anbar A. D.(2012) Isotopic fingerprints of anthropogenic molybdenum inlake sediments. Environ. Sci. Technol. 46, 10934–10940.

Chakraborty P., Ramteke D., Chakraborty S. and Nagender NathB. (2014) Changes in metal contamination levels in estuarinesediments around India – An assessment. Mar. Pollut. Bull. 78,15–25.

Colodner D., Edmond J. and Boyle E. (1995) Rhenium in the BlackSea: Comparison with molybdenum and uranium. Earth Planet.

Sci. Lett. 131, 1–15.Collier R. W. (1985) Molybdenum in the Northeast Pacific Ocean.

Limnol. Oceanogr. 30, 1351–1354.Dahl T. W., Hammarlund E. U., Anbar A. D., Bond D. P. G., Gill

B. C., Gordon G. W., Knoll A. H., Nielsen A. T., Schovsbo N.H. and Canfield D. E. (2010) Devonian rise in atmosphericoxygen correlated to the radiations of terrestrial plants andlarge predatory fish. Proc. Natl. Acad. Sci. 107, 17911–17915.

Das A., Krishnaswami S., Sarin M. M. and Pande K. (2005)Chemical weathering in the Krishna Basin and Western Ghatsof the Deccan Traps, India: Rates of basalt weathering andtheir controls. Geochim. Cosmochim. Acta 69, 2067–2084.

Duan Y., Anbar A. D., Arnold G. L., Lyons T. W., Gordon G. W.and Kendall B. (2010) Molybdenum isotope evidence for mild

http://refhub.elsevier.com/S0016-7037(14)00442-6/h0005





















































environmental oxygenation before the Great Oxidation Event.Geochim. Cosmochim. Acta 74, 6655–6668.

Fitton J. G. (1995) Coupled molybdenum and niobium depletion incontinental Basalts. Earth Planet. Sci. Lett. 136, 715–721.

Gaillardet J., Dupre B., Louvat P. and Allegre C. J. (1999) Globalsilicate weathering and CO2 consumption rates deduced fromthe chemistry of large rivers. Chem. Geol. 159, 3–30.

Goldberg S., Forster H. S. and Godfrey C. L. (1996) Molybdenumadsorption on oxides, clay minerals, and soils. Soil Sci. Soc.

Am. J. 60, 425–432.Goldberg T., Archer C., Vance D. and Poulton S. W. (2009) Mo

isotope fractionation during adsorption to Fe (oxyhydr) oxides.Geochim. Cosmochim. Acta 73, 6502–6516.

Goswami V., Singh S. K. and Bhushan R. (2012) Dissolved redoxsensitive elements, Re, U and Mo in intense denitrification zoneof the Arabian Sea. Chem. Geol. 291, 256–268.

Gupta H. and Chakrapani G. J. (2007) Temporal and spatialvariations in water flow and sediment load in the Narmadariver. Curr. Sci. 92, 689-684.

Gupta H., Chakrapani G. J., Selvaraj K. and Shuh-Ji Ka (2011)The fluvial geochemistry, contributions of silicate, carbonateand saline–alkaline components to chemical weathering fluxand controlling parameters: Narmada River (Deccan Traps),India. Geochim. Cosmochim. Acta 75, 800–824.

Hannah J. L., Stein H. J., Wieser M. E., de Laeter J. R. and VarnerM. D. (2007) Molybdenum isotope variations in molybdenite:Vapor transport and Rayleigh fractionation of Mo. Geology 35,703–706.

Kendall B., Creaser R. A., Gordon G. W. and Anbar A. D. (2009)Re–Os and Mo isotope systematics of black shales from theMiddle Proterozoic Velkerri and Wollogorang Formations,McArthur Basin, northern Australia. Geochim. Cosmochim.

Acta 73, 2534–2558.Kendall B., Gordon G. W., Poulton S. W. and Anbar A. D. (2011)

Molybdenum isotope constraints on the extent of late Paleo-proterozoic ocean euxinia. Earth Planet. Sci. Lett. 307, 450–460.

Mathur R., Brantley S., Anbar A., Munizaga F., Maksaev V.,Newberry R., Vervoort J. and Hart G. (2010) Variation of Moisotopes from molybdenite in high-temperature hydrothermalore deposits. Miner. Deposita 45, 43–50.

McManus J., Nagler T. F., Siebert C., Wheat C. G. and HammondD. E. (2002) Oceanic molybdenum isotope fractionation:Diagenesis and hydrothermal ridge-flank alteration. Geochem.

Geophys. Geosys. 3, 1–9.McManus J., Berelson W. M., Severmann S., Poulson R. L.,

Hammond D. E., Klinkhammer G. P. and Holm C. (2006)Molybdenum and uranium geochemistry in continental marginsediments: Paleoproxy potential. Geochim. Cosmochim. Acta 70,4643–4662.

Morford J. L. and Emerson S. (1999) The geochemistry of redoxsensitive trace metals in sediments. Geochim. Cosmochim. Acta

63, 1735–1750.Morris A. W. (1975) Dissolved molybdenum and vanadium in

Northeast Atlantic Ocean. Deep-Sea Res. 22, 49–54.Minissale A., Vaselli O., Chandrasekharam D., Magro G., Tassi F.

and Casiglia A. (2000) Origin and evolution of intracratonicthermal fluids from central-western peninsular India. Earth

Planet. Sci. Lett. 181, 377–394.Nagler T. F., Siebert C., Luschen H. and Bottcher M. E. (2005)

Sedimentary Mo isotope record across the Holocene freshbrackish water transition of the Black Sea. Chem. Geol. 219,283–295.

Nakagawa Y., Takano S., Firdaus M. L., Norisuye K., Hirata T.,Vance D. and Sohrin Y. (2012) The molybdenum isotopiccomposition of the modern ocean. Geochem. J. 46, 131–141.

Negrel P., Allegre C. J., Dupre B. and Lewin E. (1993) Erosionsources determined by inversion of major and trace elementratios and strontium isotopic ratios in river water: The CongoBasin case. Earth Planet. Sci. Lett. 120, 59–76.

Neubert N., Heri A. R., Voegelin A. R., Nagler T. F., SchluneggerF. and Villa I. M. (2011) The molybdenum isotopic composi-tion in river water: constraints from small catchments. Earth

Planet. Sci. Lett. 304, 180–190.Pearce C. R., Cohen A. S., Coe A. L. and Burton K. W. (2008)

Molybdenum isotope evidence for global ocean anoxia coupledwith perturbations to the carbon cycle during the EarlyJurassic. Geology 36, 231–234.

Pearce C. R., Cohen A. S. and Parkinson I. J. (2009) Quantitativeseparation of molybdenum and rhenium from geologicalmaterials for isotopic analysis by MC-ICP-MS. Geostand.

Geoanal. Res. 33, 219–229.Pearce C. R., Burton K. W., Pogge von Strandman P. A. E., James

R. H. and Gıslason S. R. (2010) Molybdenum isotopebehaviour accompanying weathering and riverine transport ina basaltic terrain. Earth Planet. Sci. Lett. 295, 104–114.

Peucker-Ehrenbrink B., Ravizza G. and Hofmann A. W. (1995)The marine 187Os/186Os record of the past 80 million years.Earth Planet. Sci. Lett. 130, 155–167.

Quinby-Hunt M. S. and Turekian K. K. (1983) Distribution ofelements in sea water. Eos, Trans. AGU 64, 130–131.

Rahaman W. and Singh S. K. (2010) Rhenium in rivers andestuaries of India: Sources, transport and behaviour. Mar.

Chem. 118, 1–10.Rahaman W., Singh S. K. and Raghav S. (2010) Dissolved Mo and

U in rivers and estuaries of India: Implication to geochemistryof redox sensitive elements and their marine budgets. Chem.

Geol. 278, 160–172.Rahaman W. (2011) Chemical and isotopic studies of estuaries and

the Ganga Basin Sediments. Ph.D. Thesis, Mohan Lal Sukh-adia University, Udaipur, Rajasthan.

Rahaman W. and Singh S. K. (2012) Sr in Indian rivers andestuaries: Implication to geochemistry and submarine ground-water discharge. Geochim. Cosmochim. Acta 85, 275–288.

Roy-Barman M., Wasserburg G. J., Papanastassiou D. A. andChaussidon M. (1998) Osmium isotopic compositions and Re–Os concentrations in sulfide globules from basaltic glasses.Earth Planet. Sci. Lett. 154, 331–347.

Rudge J. F., Reynolds B. C. and Bourdon B. (2009) The doublespike toolbox. Chem. Geol. 265, 420–431.

Scheiderich K., Helz G. R. and Walker R. J. (2010) Century longrecord of Mo isotopic composition in sediments of a seasonallyanoxic estuary (Chesapeake Bay). Earth Planet. Sci. Lett. 289,189–197.

Sen G. (2001) Generation of Deccan Trap magmas. Proc. Indian

Acad. Sci. (Earth Planet. Sci.) 110, 409–431.Sharma S. K. and Subramanian V. (2008) Hydrochemistry of the

Narmada and Tapi Rivers, India. Hydrol. Process. 22, 3444–3455.

Shijo Y., Suzuki M., Shimizu T., Aratake S. and Uehara N. (1996)Determination of trace amounts of molybdenum in rain waterand snow by graphite-furnace atomic absorption spectrometryafter solvent extraction and micro-volume back-extraction.Anal. Sci. 12, 953–957.

Siebert C., Nagler T. F. and Kramers J. D. (2001) Determination ofmolybdenum isotope fractionation by double-spike multicol-lector inductively coupled plasma mass spectrometry. Geochem.

Geophys. Geosyst. 2, 2000GC000124.Siebert C., Nagler T. F., von Blanckenburg F. and Kramers J.

D. (2003) Molybdenum isotope records as a potential newproxy for paleoceanography. Earth Planet. Sci. Lett. 211,159–171.







































































































































Siebert C., McManus J., Bice A., Poulson R. and Berelson W. M.(2006) Molybdenum isotope signatures in continental marginmarine sediments. Earth Planet. Sci. Lett. 241, 723–733.

Singh S. P., Singh S. K. and Bhushan R. (2011) Behavior ofdissolved redox sensitive elements (U, Mo and Re) in the watercolumn of the Bay of Bengal. Mar. Chem. 126, 76–88.

Singh S. P., Singh S. K., Goswami V., Bhushan R. and Rai V. K.(2012) Spatial distribution of dissolved neodymium and eNd inthe Bay of Bengal: Role of particulate matter and mixing ofwater masses. Geochim. Cosmochim. Acta 94, 38–56.

Singh S. P., Singh S. K. and Bhushan R. (2013) Dissolved boron inthe Tapi, Narmada and the Mandovi estuaries, the westerncoast of India: Evidence for conservative behavior. Estuaries

Coasts. http://dx.doi.org/10.1007/s12237-013-9736-7.Tarantola A. (2005) The least square criterion. In Inverse Problem

Theory and Methods for Model Parameter Estimation. Soc. ForInd. and Appl. Math, Philadelphia, PA, pp. 68–72.

Tripathy G. R. and Singh S. K. (2010) Chemical erosion rates ofriver basins of the Ganga system in the Himalaya: Reanalysis

based on inversion of dissolved major ions, Sr, and 87Sr/86Sr.Geochem. Geophys. Geosyst. 11, Q03013.

Voegelin A. R., Nagler T. F., Beukes N. J. and Lacassie J. P. (2010)Molybdenum isotopes in late Archean carbonate: Implicationsfor early Earth oxygenation. Prec. Res. 182, 70–82.

Voegelin A. R., Nagler T. F., Pettke T., Neubert N., SteinmannM., Pourret O. and Villa I. M. (2012) The impact of igneousbedrock weathering on the Mo isotopic composition of streamwaters: Natural samples and laboratory experiments. Geochim.

Cosmochim. Acta 86, 150–165.Yamaguchi K. E. (2002) Geochemistry of Archean-Paleoprotero-

zoic Black Shales: The Early Evolution of the Atmosphere,Oceans, and Biosphere. Ph. D. thesis, Pennsylvania StateUniversity.

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