Impact of water mass mixing and dust deposition on Nd concentration and εNd of the Arabian Sea...

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Geochimica et Cosmochimica Acta 145 (2014) 30–49

Impact of water mass mixing and dust deposition onNd concentration and eNd of the Arabian Sea water column

Vineet Goswami ⇑, Sunil K. Singh, Ravi Bhushan

Physical Research Laboratory, Ahmedabad, India

Received 22 April 2014; accepted in revised form 2 September 2014; Available online 21 September 2014

Abstract

The concentration and isotopic composition (eNd) of dissolved Nd have been measured in the sub-oxic/denitrifying watercolumn of the eastern Arabian Sea to explore the impact of water mass mixing and release from particles. Dissolved Nd in thenorth-eastern Arabian Sea is more radiogenic (eNd: �7 to �10) compared to its south-eastern part (eNd: �11 to �15) suggest-ing contribution of Nd from the Bay of Bengal (BoB). The vertical profile of Nd typically show higher values in surface waters(15.8–27.8 pmol/kg), followed by minima (12–18 pmol/kg) in the subsurface waters (200–300 m) and a gradual increase withdepth thereafter. The Nd concentration does not seem to show any changes in the sub-oxic layers suggesting no impact of thesub-oxic/denitrifying conditions in this oceanic region on the biogeochemistry of Nd. The enrichment of Nd in surface waterscan be explained either by supply from high Nd low salinity waters from the BoB to the Arabian Sea via the East India Coast-al Current (EICC) or, alternatively the Nd surface excess can be result of release from aeolian dust depositing over the seasurface. Inverse modeling computations suggest that in addition to Nd contributed from water mass mixing, some additionalexcess Nd (Ndxs) is required to balance the Nd inventory in the water column. There is significant Ndxs in the surface waters ofthe north-eastern Arabian Sea with eNd � �6, similar to that of dust depositing over the Arabian Sea. The fractional solubilityof Nd released from the aeolian dust was estimated to be varying from �3% to 17% (for excess Nd inventory per unit area(Ndxs*) of 20 lg m�2 with sNd ranging from 1 to 3 years). This study highlights the significance of aeolian dust deposition incontrolling the abundance and distribution of Nd in the Arabian Sea, the western arm of the northern Indian Ocean, lying inthe proximity to the arid landmass and characterized by high lithogenic aeolian dust deposition.� 2014 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

The distribution of dissolved Nd and its isotopic compo-sition (eNd; Jacobsen and Wasserburg, 1980) serve as tracersto track the source and circulation of water masses bothwithin and between the different ocean basins (Jeandel,1993; Jeandel et al., 1998; Frank, 2002; Goldstein andHemming, 2003; Singh et al., 2012) and to investigate

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⇑ Corresponding author. Tel.: +91 7926314313; fax: +917926314900.

E-mail addresses: [email protected] (V. Goswami), [email protected] (S.K. Singh), [email protected] (R. Bhushan).

particle-water interactions within oceans and at theirboundaries (Tachikawa et al., 1999; Rickli et al., 2009;Singh et al., 2012; Wilson et al., 2012; Pearce et al., 2013).The oceanic residence time of Nd (Tachikawa et al., 1999;Sidall et al., 2008) is comparable to ocean mixing time(�1500 y), allowing its transport through the ocean systembut not long enough to maintain homogeneity in its abun-dance among the oceans. It is this heterogeneity in Nd con-centration and its isotopic composition among variouswater masses that is primarily responsible for promotingNd and eNd as water mass tracers. The variation in143Nd/144Nd of different sources is quite small and this isexpressed as eNd given by the relation:


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V. Goswami et al. / Geochimica et Cosmochimica Acta 145 (2014) 30–49 31

eNd ¼143Nd=144Nd

143Nd=144NdCHUR

� 1

" #� 104 ð1Þ

where 143Nd/144Nd is the measured Nd isotopic composi-tion of the sample and 143Nd/144NdCHUR is the presentday CHUR (CHondritic Uniform Reservoir) value of0.512638 (Jacobsen and Wasserburg, 1980). The oceansacquire their eNd signatures predominantly from dissolvedNd transported by rivers from continents, its supply bydesorption from aeolian dust and riverine particulates andinput from margin sediments. The spatial distribution ofeNd in the oceans is determined by the source function ofNd and its subsequent modification by mixing with differentwater masses and internal biogeochemical cycling (Frank,2002; Goldstein and Hemming, 2003; Lacan et al., 2012).

More recently, Nd isotopic studies in the ocean, how-ever, have underscored the importance of particle-waterinteractions on its distribution (Greaves et al., 1999;Tachikawa et al., 1999; Rickli et al., 2010; Singh et al.,2012; Wilson et al., 2012; Pearce et al., 2013). These inter-actions include those occurring in surface waters with dustparticles depositing from the atmosphere (Greaves et al.,1999; Rickli et al., 2010), in the water column with settlingparticles (Jeandel et al., 1995) and with sediments, particu-larly margin sediments at the sediment–water interface(Singh et al., 2012; Wilson et al., 2012; Pearce et al.,2013). Some of the recent studies on Nd isotopic signaturesof seawater bring out the significance of sediment/particle-water interactions in influencing its distribution (Lacan andJeandel, 2005; Singh et al., 2012; Wilson et al., 2012; Pearceet al., 2013). For example, investigations of dissolved Nd inthe BoB highlight the role of sinking particles in seawater incontributing to its isotopic composition. Singh et al. (2012)show that the huge quantity of particulate matter suppliedby the Ganga–Brahmaputra (G–B) to the BoB significantlyalters the eNd signature of the bay water column. Similarly,there is also evidence that demonstrates the role of Ndreleased from aeolian dust in influencing the isotopic com-position of dissolved Nd in surface waters (Greaves et al.,1999; Tachikawa et al., 1999; Rickli et al., 2010).

The work presented in this paper is on the distributionof dissolved Nd and eNd in the water column of the ArabianSea. This ocean basin is characterized by some uniqueproperties that affect its physical, chemical and biologicalsettings. The high biological productivity associated withupwelling during monsoon (both South-West (SW) andNorth-East (NE) monsoons) produces a thick sub-oxicand denitrifying water column at intermediate depths. Ara-bian Sea receives significant fresh water and particulate dis-charge from the rivers, the Indus, Narmada and Tapi andaeolian dust deposition from adjoining desert regions. Inaddition, this basin also receives supply of low salinitywaters through its south-eastern boundary from the BoBduring the NE monsoon. The particulate flux from riversand atmosphere to the Arabian Sea is reported to be�500 and �100 Tg/y respectively (Milliman et al., 1984;Sirocko and Sarnthein, 1989). However, in recent decades,there has been a significant drop in the sediment flux fromthe Indus river (from �250 to �10 Tg/y) and other rivers,due to the construction of numerous dams along their

courses (Garzanti et al., 2005), reducing the annual sedi-ment flux to the Arabian Sea to �30 Tg/y (Milliman andFarnsworth, 2011). Of the �100 Tg of aeolian dust depos-iting on the Arabian Sea surface annually from the desertsof Oman, Africa and western India, the eastern ArabianSea receives about 30 Tg/y, which further decreasestowards the Indian peninsula (Sirocko and Sarnthein,1989). More recently, based on available aerosol data andmodeling, Maher et al. (2010) estimated that the presentday mass accumulation rates of aeolian dust vary from�10–20 g m�2 y�1 in the central Arabian Sea to �5–10 g m�2 y�1 towards the eastern Arabian Sea.

The concentration of Nd and its isotopic composition inthe Arabian Sea water column would be determined by itssupply from the Indus, Narmada, Tapi and other minor riv-ers, the Nd and eNd of its constituent water masses and theirmixing proportions, and addition/removal of Nd throughparticle-water interactions occurring in the water columnand at the atmosphere-water and sediment–water inter-faces. In this work, efforts are made to investigate the roleof the above processes in determining the dissolved Ndand eNd distributions in the Arabian Sea. This has beenachieved through inverse modeling of the measured Ndconcentration and its isotopic composition.

This study forms a part of our initiative to investigatethe distribution and behavior of selected trace elementsand isotopes in the northern Indian Ocean as a componentof the international GEOTRACES program. The samplesanalyzed for Nd are aliquots of those collected for measur-ing the distribution of dissolved Re, U and Mo, results ofwhich have been published earlier (Goswami et al., 2012a).

2. WATER MASS STRUCTURE OF THE ARABIAN

SEA

The water mass structure of the Arabian Sea has beendiscussed earlier by many workers (Wyrtki, 1973; Shenoiet al., 1993; Dileep Kumar and Li, 1996; You, 2000;Schott and McCreary, 2001). This can be briefly describedas follows. The upper 1000 m water column is composedof three water masses, the Arabian Sea High Salinity Water(ASHSW), The Persian Gulf Water (PGW) and the RedSea Water (RSW). The ASHSW forms due to excess evap-oration over precipitation in the Arabian Sea. On an aver-age, the core of this water is observed between 50 and 100 mdepth. The salinity of this water mass varies from 36.7 to35.3 while its potential temperature is in the range26 ± 2 �C (Shenoi et al., 1993). The PGW forms due tothe spillage of high salinity water from the Persian Gulf intothe Gulf of Oman. Near the formation zone of this watermass, it is characterized by salinity �36.8 and potentialtemperature �18 �C (Shenoi et al., 1993). As this watermass moves towards the south-eastern Arabian Sea, salinitydecreases to a value as low as 35 while the temperature alsodecreases to 12.5 �C (Shenoi et al., 1993). RSW forms dueto spillage of water from the Red Sea into the Gulf of Aden.In the Gulf of Aden, RSW has a salinity of �36 and apotential temperature of �11 �C. On its course in the Ara-bian Sea towards the east, the salinity and potential temper-ature drops to �35 and �9 �C respectively (Shenoi et al.,

32 V. Goswami et al. / Geochimica et Cosmochimica Acta 145 (2014) 30–49

1993). In addition, low salinity surface waters from the BoBenter the Arabian Sea from around the southern tip ofIndia through the East India Coastal Current (EICC) andspreads over its surface; the extent of this spread beingdependent on monsoon intensity, the NE monsoon in par-ticular, and the discharge of various rivers into the BoB.The deeper and bottom waters of the Arabian Sea are com-posed of the denser Modified North Atlantic Deep Water(MNADW) and the Antarctic Bottom Water (AABW)(Wyrtki, 1973; Mantyla and Reid, 1995; Dileep Kumarand Li, 1996). These deep waters in the Arabian Sea arecharacterized by a very weak salinity maximum (Wyrtki,1973). The deeper and bottom water masses in the ArabianSea have salinity of �34.8 and potential temperature of�1.8 �C (Wyrtki, 1973).

The upper water masses viz. ASHSW, PGW and RSWprogress southwards and mix together below 1000 m toform the North Indian Intermediate water (NIIW). TheNIIW, on its southward progression mixes with polarwaters at �2000 m depth to form the North Indian DeepWater (NIDW) (Wyrtki, 1973). The Modified North Atlan-tic Deep water (MNADW) and the Antarctic Bottom water(AABW) make up the deep water masses of the ArabianSea (Dileep Kumar and Li, 1996). The MNADW is formedby mixing of the southward moving North Atlantic DeepWater (NADW) with Weddell Sea Deep Water (WSDP)in the South Atlantic. The AABW occupies the bottommostlayer of the Arabian Sea. In the western Indian Ocean, theMNADW and AABW progress northwards after travelingthrough the Southwest Indian Ridge at �30�S, 60�E(Warren, 1978), the Madagascar and the Mascarene basins.These deeper water masses then pass via the narrow Amir-ante passage (at �10�S, 53�E) (Johnson and Damuth, 1979;Johnson et al., 1991; Dileep Kumar and Li, 1996) to reachthe Somali basin before entering the Arabian Sea via theOwen fracture zone (Mantyla and Reid, 1995). TheMNADW entering the Indian Ocean in the west (andreaching the Arabian Sea) undergoes lesser degree of mix-ing and modification in the circumpolar Antarctic region(Toole and Warren, 1993).

Warm and low salinity surface and intermediate watersof the Pacific origin enter the Indian Ocean at its easternboundary via the Indonesian through flow (Jeandel et al.,1998). Characteristic water masses of the Indonesian Sea,originally derived from the Pacific Ocean are transportedwestwards as the Indonesian Water (IW) and the Indone-sian Intermediate Water (IIW) (Jeandel et al., 1998). Mod-eling the trajectory of these Indonesian waters, Song et al.(2003) and Valsala and Ikeda (2005) suggested that theyalso contribute to the surface layers of the Arabian Sea.

3. MATERIALS AND METHODS

The seawater samples for the present study were col-lected in November, 2008 during the cruise SS-259 ofFORV Sagar Sampada (Goswami et al., 2012a). Water sam-ples for Nd measurements were collected (from depthsranging from �5 m to �100 m above the sediment–waterinterface) using 12 L lever-action type Niskin bottles on a12 position CTD rosette array. Seabird CTD sensors were

used to measure conductivity, temperature and depth.From the 12 L Niskin bottles, �5–10 L of seawater wastransferred to pre-cleaned carboys for eNd measurement.A separate 1 L aliquot from the same Niskin bottle wassub-sampled for measurements of Nd and other trace ele-ment concentrations. A total of three depth profiles werecollected; two from the north-eastern Arabian Sea (stations0802 and 0803) and the third (station 0805) close to thesouthern tip of India. The sampling locations are shownin Fig. 1 (encompassed in red band) and their details aregiven in Table 1. The Chagos-Laccadive ridge (sill depth�1500 m) acts as a barrier for deep water movement andseparates the two north-eastern profiles (0802 and 0803)from the southernmost profile (0805; Fig. 1).

All the three profiles are characterized by a sub-oxiclayer in the depth range of �200 to �1000 m; among these,station 0802 has the lowest dissolved oxygen concentration�4 lM (Goswami et al., 2012a). This profile lies in the coredenitrification zone (Goswami et al., 2012a), with nitriteconcentration exceeding 0.4 lM at several depths in the50–800 m range (Fig. 2). The maximum nitrite (3.3 lM)in the profile was at 230 m followed by a secondary peak(1.6 lM) at 800 m depth (Goswami et al., 2012a). The watercolumn of stations 0803 and 0805 had a hint of nitrite justbelow the surface. These stations also had several hundredmetres thick sub-oxic layer (Fig. 2; Table 2). Nd concentra-tion and eNd was measured through these sub-oxic, denitri-fying layers to learn more about the biogeochemicalbehavior of Nd in them.

3.1. Nd Isotopic composition

For eNd measurement, Nd from the large volume (5–10 L) seawater samples was pre-concentrated on board.The samples were filtered through 0.45 lm Millipore cellu-lose filter and the filtered water collected in pre-cleaned car-boys. The pH of each sample was then adjusted to 3.5 andrare earth elements (REEs) were extracted using SepPakC18 cartridges loaded with di(2-ethyl)hydrogen-phosphate(HDEHP) complexant (Shabani et al., 1992; Jeandelet al., 1998). Subsequently, the cartridges were flushed with5 ml 0.01 N HCl to elute Ba and the REEs were eluted with50 ml 6 N HCl into acid precleaned polypropylene bottles.The REE fraction was brought to the laboratory, dried andredissolved in 2 N HCl and further purified following ionexchange chromatography (AG 50W-X8 cation exchangeand reverse phase chromatography, Lanthanide specificresin column LN-B50-S, Eichrom Company) (Singh et al.,2008; Goswami et al., 2012b).

Nd isotopic composition (143Nd/144Nd) of the sampleswas measured as Nd+ on Isoprobe-T TIMS at the PhysicalResearch Laboratory (PRL) with a Ta-Re-Ta triple fila-ment configuration. The analyses were made in staticmulti-collection mode. Mass fractionation correction forNd was made by normalizing 146Nd/144Nd to 0.7219. Dur-ing the course of analysis, the JNdi-1 Nd standard was mea-sured repeatedly, this yielded a mean value of0.512108 ± 0.000016 (2r, n = 35) for 143Nd/144Nd, wellwithin the recommended value (0.512100; Tanaka et al.,2000). The total procedural blank for Nd was measured

Fig. 1. Sampling location in the Arabian Sea (Black circles) for Nd concentration and isotopic analysis. Three stations 0802, 0803 and 0805were sampled (red band). In addition to these stations, sampling locations of earlier studies in the Arabian Sea are also shown. The red squarerepresents the locations of stations 1507 and 1506 in the Somali basin (Bertram and Elderfield, 1993); 1605 and 1608 in the Arabian Sea(German and Elderfield, 1990). The yellow square marks the location of station 0813 in the northern BoB (Singh et al., 2012). The formationregions/pathways for various water masses (BoBs, RSW, PGW, MNADW, AABW) are also shown (Shenoi et al., 1993; Mantyla and Reid,1995; Singh et al., 2012). The EICC (East India Coastal Current) is a coastal current strengthening from BoB to Arabian Sea during the NEmonsoon. The pathways for the wind direction have been shown by red arrows with dust sources being derived from the Arabian Peninsula,Iran, Pakistan and Thar desert, India (Pease et al., 1998; Kumar et al., 2012). The red band encompassing the stations 0802, 0803 and 0805 inthe Arabian Sea shows the direction/projection of the section for Fig. 6. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

Table 1Details of sampling locations for Nd and eNd measurements.

Cruise Month, Year Station Lat (�N) Long (�E) Water Depth (m) Depth range sampled (m)

SS-259 Nov, 2008 0802 14.42 69.42 4156 5–4040SS-259 Nov, 2008 0803 10.12 71.86 2504 5–2400SS-259 Nov, 2008 0804 6.01 77.48 2350 5–2250


to be �600 pg, similar to the blank levels reported forextraction of Nd by C18 cartridges (Lacan and Jeandel,2004, 2005). The typical precision of eNd measurements is±0.4 units (2r). In addition, to assess the external precisionof the analysis, three sets of duplicate measurements of sea-water were made on different aliquots of samples (Table 3).The results demonstrated that the difference in the eNd val-ues between duplicates averaged 0.2 eNd units (Table 3)within the typical precision (2r) of eNd measurements.

Further, to assess the accuracy and precision of neodym-ium isotope measurements, GEOTRACES intercalibrationreference seawater samples (BATS, 15 m and BATS,2000 m) were measured. Nd isotopic composition of thesesamples measured at PRL is well within the recommendedvalues (van de Flierdt et al., 2012), attesting further to the

accuracy and precision of Nd isotope measurements inour laboratory.

3.2. Nd concentration

For the Nd concentration measurement, a knownweight (�500 g) of filtered (through 0.45 lm) and acidifiedseawater was spiked with a precisely known weight of 150Ndtracer. The REEs were co-precipitated from seawater withFe oxy-hydroxides (using Fe solution which waspre-cleaned by ether extraction). The precipitate was col-lected and re-dissolved in 6 N HCl. It was further driedand taken up in 2 N HCl. The REEs were separated fromthis solution by passing through AG 50W-X8 cationexchange resin.

Fig. 2. The depth distribution of Nd concentration in stations 0805, 0803 and 0802. The top panel shows the upper 1000 m water column. TheNd concentration shows enrichment at the surface, depletion in sub-surface (200–300 m depth) and higher values in deeper waters. Thevertical distribution of Nd in station 1605 (German and Elderfield, 1990) has also been plotted for comparison. The Nd concentrations and itsdistribution in the stations 0802 and 1605 in general and similar; a noticeable difference is the more pronounced enrichment of Nd at thesurface for station 0802. The Nd content of the surface waters in the Stations sampled shows enrichment from north to south with thesouthern station 0805 having high Nd concentration of 27.8 pmol/kg. The shaded portion marks the sub-oxic layer of the water column.


The final solution containing REEs was taken up in 5 mlof 0.4 N HNO3 and measured on an inductively coupledplasma mass spectrometer (Thermo X-Series II Q-ICP-MSat PRL). Typical counts obtained for 144Nd and 150Nd werein excess of 5000 and 20,000 cps respectively, much higherthan the background counts (around 1 cps). The Nd concen-tration was determined by measuring the 150Nd/144Nd ratioafter correcting for the Sm contribution. The instrumentalmass bias was corrected using 146Nd/144Nd ratio (0.7219).The analytical precision (CV%) for Nd concentrationmeasurement determined based on five pairs of duplicateanalyses is ±3.5%. The total procedural blank of Nd forconcentration measurement was �50 pg.

4. RESULTS AND DISCUSSION

The dissolved Nd concentration and eNd measured in thethree profiles are given in Table 2 and shown in Figs. 2 and3. The uncertainty in eNd of the samples is dependent on thesignal strengths and is typically ±0.4 eNd units (2r); in someof the samples, however it is higher (Table 2) due to theirlow signal strength.

4.1. General observations

The water mass structure of the three Arabian Sea pro-files sampled in this study is shown in a h–S plot (Fig. 4).The presence of water masses ASHSW and BoBs is

prominent at or near the surface waters with BoBs beingpronounced in stations 0803 and 0805 as a low salinityintrusion at the top of the h–S diagram (Fig. 4). The pres-ence of PGW and RSW can also be seen in the h–S plot.The deeper water masses, MNADW and AABW in theArabian Sea have very close or overlapping h and S, mak-ing it difficult to differentiate them based only on h and S(Fig. 4).

The concentration-depth profiles of dissolved Nd in allthe three stations measured in this study are similar, withhigher concentration at and near surface, a sub-surfaceminimum in the depth range of 200–400 m followed by asteady near linear increase towards the bottom (Fig. 2).The Nd concentration in surface waters of the three stationsincreases north to south with the highest Nd in surfacewaters of the southern station 0805. The distribution ofNd with depth in station 0805 exhibits marginally highervalues in samples from similar depths compared to stations0802 and 0803 (Table 2). The Nd profiles in stations 0802and 0803 overlap with each other up to a depth of�500 m, they however seem to be higher in deeper watersof station 0803 by �1–2 pmol/kg (Table 2; Fig. 2).

The nature of Nd-depth profiles (Fig. 2) observed in thepresent study is typical of elements that have a supply at ornear air-sea interface, biological uptake/particle scavengingin shallow waters and subsequent release in deeper waters.Potential sources for the high Nd in surface/near surfacewaters of the stations sampled can be advection of Nd rich

Table 2Salinity, potential temperature (h), Dissolved Oxygen, Si, Nd concentrations and eNd in profiles from the eastern Arabian Sea.

Depth (m) Salinitya h (�C) O2, lM Si, lM Ndb, pmol/kg eNd err eNd (2r)

0802 (14.42�N, 69.42�E)

5 36.7 28.9 209.0 0.3 15.8 �7.7 0.465 36.5 26.3 171.0 1.0 14.0 �7.1 0.5

120 35.8 20.0 12.9 13.0 14.3 �8.5 0.4230 35.7 14.6 8.9 26.1 12.5 �11.4 0.5400 35.5 12.3 5.2 35.5 12.7 �8.5 0.3500 35.5 11.7 8.3 41.2 13.5 �9.0 0.3700 35.5 10.3 6.4 60.5 – �8.6 1.8

1000 35.3 8.0 16.5 84.9 20.2 �9.4 0.31500 35.0 5.1 51.6 111.7 16.9 �8.3 2.12100 34.8 2.7 109.1 124.4 17.5 �10.7 0.33000 34.7 1.6 147.3 125.9 23.1 �8.8 0.43600 34.7 1.4 154.5 125.3 25.3 �9.0 0.33900 34.7 1.4 145.9 125.8 24.6 �8.4 0.34040 34.9 1.4 148.9 126.4 34.3 �9.4 0.3

0803 (10.12�N, 71.86�E)

5 35.5 28.9 202.3 0.9 20.1 �7.8 0.735 36.4 28.8 201.6 0.6 15.4 �7.1 0.6

130 35.1 16.4 – 23.2 15.6 – –210 35.2 13.3 36.6 23.2 – �9.9 0.6300 35.2 12.1 27.3 29.1 13.3 �11.2 0.5360 � 11.5 33.5 32.7 13.3 �9.9 0.4465 35.3 10.9 20.3 42.1 14.2 �9.4 0.5600 35.2 10.3 17.1 54.7 14.4 �8.8 0.6900 35.2 8.4 25.8 78.0 16.6 �9.9 0.3

1200 35.0 6.4 43.9 99.6 18.6 – –1600 34.9 4.2 85.1 116.5 19.7 �9.3 0.62400 34.8 1.9 144.5 123.3 22.5 �10.2 0.6

0805 (6.01�N, 77.48�E)

5 32.4 29.1 211.6 0.7 27.8 – –60 35.3 28.0 187.8 1.0 16.5 – –

120 35.0 21.8 49.9 13.1 – �12.1 0.7200 35.0 14.1 20.1 27.6 19.3 �10.7 0.7300 34.9 11.6 19.6 29.8 18.0 �12.4 0.4580 35.0 9.4 24.1 53.4 18.1 � �650 34.8 9.0 – 60.3 19.5 �14.7 1.4845 35.0 7.8 34.8 73.5 21.0 �11.3 0.4

1000 35.0 6.9 45.5 87.7 22.0 �10.9 0.41150 34.9 6.0 52.7 93.4 21.9 – –1450 34.9 4.6 81.0 109.5 22.8 �12.6 0.51850 34.8 3.1 115.1 119.7 23.5 �10.1 0.32250 34.8 2.3 133.2 120.7 25.7 �10.6 0.4

–: Not measured.a Salinity was measured at a much higher precision, but is rounded off in the table.b Error on [Nd] is ±3.5%.

Table 3Replicate analysis of Nd isotopic composition measurements.

Sample 143Nd/144Nd 2r eNd 2r

0802 (3000 m) 0.512187 0.000010 �8.8 0.40802 (3000 mR) 0.512182 0.000011 �8.9 0.40803 (360 m) 0.512133 0.000011 �9.9 0.40803 (360mR) 0.512143 0.000009 �9.7 0.30803 (900m) 0.512132 0.000009 �9.9 0.30803 (900mR) 0.512151 0.000013 �9.5 0.5


waters from the BoB and/or its release from lithogenic aeo-lian dust depositing on the sea surface; these are discussedin more detail in a later section (Section 4.2). The concen-tration of Nd with depth in oceans steadily increases froma minimum in shallow waters to maximum in bottomwaters; this coupled with its interoceanic variation has ledto suggest that Nd has a nutrient like behavior, especiallysilicate (de Baar et al., 1985; Goldstein and Hemming,2003). The Nd profiles measured in this study in their

Fig. 3. The distribution of Nd isotopic composition (eNd) withdepth in stations 0802, 0803 and 0805. The eNd shows a north–south gradient with the southern station (0805) having lessradiogenic values. The eNd of stations 0802 and 0803 are similarto that in station 1507 in the Somali basin (Bertram and Elderfield,1993).

Fig. 4. Measured potential temperature vs salinity (h–S) plots forthe stations sampled. The dashed lines show the density (rh)contours for the water masses. The surface of the plots for profiles0805 and 0803 show lower salinity due to influx of lower salinitywater from the BoB. There is also a significant presence ofASHSW, RSW and PGW water masses in the profiles sampled.The deep water masses MNADW and AABW are clubbed togetherdue to closeness of h and S in them. Fig. 5. Nd–Si relationship in the waters of the Arabian Sea. The

Nd in waters of the Arabian Sea shows linear correlation withdissolved Si in the intermediate waters; 120–2100 m (station 0802),130–2400 m (station 0803) and 120–2250 m (station 0805); how-ever, the Nd-Si relation breaks down in the surface and bottomwaters. The Nd-Si relationship for the station 0802 is almostidentical to that for the nearby station 1605 (German andElderfield, 1990).


entirety that includes surface water maximum however,seem to be more similar to that of dissolved Al having ascavenged type distribution with strong interaction withparticles and residence times that are less than the oceanmixing time (Bruland and Lohan, 2003).

It is observed that regression analysis of depth profiledata of Nd (and other trivalent REEs) and silicate yieldsa significant linear correlation (de Baar et al., 1985;German and Elderfield, 1990; Bertram and Elderfield,1993). The cause of this correlation, however is not wellunderstood; German and Elderfield (1990) propose that itis a result of common oceanic processes regulating their dis-tributions, whereas, Akagi (2013) and Stichel et al. (2012)attribute it to uptake of Nd by diatoms and their dissolu-tion in deep waters. The Nd data measured in this studyin the Arabian Sea also shows a significant linear correla-tion with dissolved Si (Fig. 5) in the depth range 120–2100 m (station 0802), 130–2400 m (station 0803) and120–2250 m (station 0805). The plots in Fig. 5 show thatNd–Si linear trend breaks down near the surface in all thethree profiles and in bottom waters of sta. 0802 where mea-surements were made at depths > 2500 m with “excess” Ndover that would be expected from the Nd–Si linear trends.It is quite intriguing that in station 0802 the Nd–Si


correlation is observed only over a part of the water columnvertical profile covering the depth �100 m to �2200 m;(Fig. 5) and not in deeper waters, a result not consistentwith that would be expected if Nd–Si relationship is solelydue to the dissolution of diatoms tests. The observationshowever, can be reconciled if the Nd–Si correlation in theintermediate water column is dominated by mixing of watermasses and the “excess” Nd in the deeper waters resultsfrom additional supply of Nd by dissolution of Fe–Mnoxy-hydroxides in sediments that is decoupled from Si. TheNd–Si correlation for station 0802 is almost identical to thatfor station the 1605 (German and Elderfield, 1990; Bertramand Elderfield, 1993) (Fig 5), this is expected considering thatthese two stations are from nearby locations (Fig. 1).

German and Elderfield (1990) had reported Nd concen-trations in three depth profiles from the Arabian Sea; theirstation 1605 (14.43�N, 66.92�E) is a few degrees west of ournorthern station 0802 (14.42�N, 69.42�E). Station 1605 islocated within the zone of intense denitrification in the Ara-bian Sea and was sampled during the SW monsoon period(Aug–Sept, 1986). The Nd concentration and its distribu-tion with depth in both these stations are by and large sim-ilar (Fig. 2); the noticeable departure being in surfacewaters where Nd shows a more pronounced enrichmentin station 0802. Temporal variation in surface water mixingcharacteristics can be a cause for this difference. Station1605 was sampled during the tail-end of the SW monsoon,a period of intense surface water mixing that can result indeeper mixed layer (>100 m) in the region (PrasannaKumar and Narvekar, 2005). The impact of this intensemixing would be to dilute the signature of Nd released fromaeolian dust at the air-sea interface by distributing it in theentire mixed layer. In contrast, the sampling for the presentstudy was during November, a relatively quiescent periodof surface water mixing and shallower mixed layer depth�50 m (Prasanna Kumar and Narvekar, 2005). Further,during the SW monsoon period, there would be no supplyof Nd rich BoB surface waters to the Arabian Sea (Schottand McCreary, 2001) unlike in November (period of sam-pling for this work) when the EICC brings high Nd BoBsurface waters to the Arabian Sea (Schott and McCreary,2001) that can enrich the surface layers with high Nd. Inaddition temporal variations in dust fluxes can complementthe water mixing effects. Available information (Pease et al.,1998) seem to indicate that the dust concentrations over theArabian Sea are lower during September compared to thatin November. These three factors acting in tandem in sta-tion 0802 have the potential to contribute to higher Nd nearits surface relative to that in station 1605. In addition sea-sonal variation in dust sources, the leachable Nd in themand in surface water productivity are other factors thatmay influence the surface water distribution of dissolvedNd.

Two other stations, 1608 and 1609 from the Gulf ofOman (German and Elderfield, 1990) also exhibit lower dis-solved Nd in surface samples (10.5 pmol/kg at 1608;12.9 pmol/kg at 1609) compared to that in all the three sta-tions measured in this study (15.8–27.8 pmol/kg; Table 2).This is intriguing considering that the annual aeolian litho-genic dust flux over the western and central Arabian Sea is

higher than that over the eastern Arabian Sea (Pease et al.,1998; Maher et al., 2010). The processes responsible for thisobservation are not clear; it can however be speculated thatone or more of the potential factors mentioned in the aboveparagraph could contribute to the surface enrichment ofNd in the eastern Arabian Sea stations relative to stations1608 and 1609 in the Gulf of Oman.

An initial look at the vertical distribution of eNd (Fig. 3)reveals that it is similar in stations 0802 and 0803 with moreradiogenic values (radiogenic refers to Nd (143Nd) that isproduced from decay of 147Sm) at surface �(�7.7) thatdecreases to �(�10.5) at a depth close to 250 m andremains in the range of �8 to �10 in deeper waters. TheeNd profile in the Somali basin (station 1507; Bertram andElderfield, 1993) also shows more radiogenic eNd in the sur-face waters �(�6) than in intermediate to deep waters [eNd

�(�8) to (�10)] similar to the Arabian Sea profiles 0802and 0803. The southern station 0805 shows less radiogeniceNd values compared to stations 0802 and 0803. The Nd iso-topic composition in surface waters of the southern station0805 could not be measured, however, the sample from120 m is less radiogenic (eNd �12.1; Table 2; Fig 3) thanthat observed for similar depths in the two northern profiles(0802 and 0803). In general, the eNd value of the water col-umn of station 0805 is �2e units less radiogenic than that ofstations 0802 and 0803 (especially in the depth range of �0–1800 m), indicating enhanced supply of less radiogenic Ndto station 0805.

4.2. Nd and eNd in surface waters: lateral transport vs.

atmospheric deposition

The concentration of dissolved Nd shows enrichment inthe surface waters (5 m) relative to that in samples fromdepths of 35–65 m depth (Table 2; Fig. 2). Signatures ofsurface input of Nd to stations 0802 and 0803 are also evi-dent from the eNd distribution. The eNd profiles of stations0802 and 0803 show distinctly more radiogenic values in thesurface �(�7.7) than that in intermediate and deep water�(�8 to �10).

The dissolved Nd in surface waters of the three stationsshows an overall north–south increase, with highest valuesoff the southern tip of India (station 0805; 27.8 pmol/kg)and a strong inverse correlation with salinity (r2 = 0.99).An explanation for these observations can be mixing oflower salinity Nd rich waters of the BoB with the ArabianSea surface waters; the north–south trend in dissolved Ndbeing a result of the northward decreasing contribution ofwaters from the BoB. The above hypothesis is also consis-tent with the surface water circulation of the region duringthe NE monsoon (Nov–Feb) when the EICC is known totransport lower salinity surface waters from the BoB tothe Arabian Sea (Schott and McCreary, 2001; Shankaret al., 2002) around the southern tip of India. Further,the reported higher Nd concentrations in the northernBoB (�19�N–20�N) surface waters with salinity <33& is�45 pmol/kg (Singh et al., 2012) also lends support to theidea that its mixing can enrich Nd in the Arabian Sea sur-face waters. Independent evidence for the transport of BoBwaters to the Arabian Sea comes from 228Ra concentration


which shows that it is significantly higher at locations closeto stations 0805 and 0803; a result that has been attributedto 228Ra supply from the BoB (Nozaki et al., 1998). Othergeochemical proxies such as oxygen isotopic composition ofthe Arabian Sea surface waters is known to be impacted bythe influx of low salinity waters from the BoB and havebeen used to investigate the relative variations in the inten-sity of the SW and NE monsoons in the past (Sarkar et al.,1990; Chodankar et al., 2005; Tiwari et al., 2005). Morerecently, variations in salinity and d18O of contemporaryArabian Sea surface water due to influx of BoB water hasalso been reported (Tiwari et al., 2013).

The mixing hypothesis given above would require thateNd also shows a linear trend with salinity. This requirementhowever could not be tested as eNd in surface waters of sta-tion 0805 could not be measured and uncertainties associ-ated with surface water eNd values of stations 0802 and0803 are high (Table 2). Nonetheless, the observation thateNd in the surface waters of stations 0802 and 0803 whichdiffer in their salinity by 1.2 units are nearly identical raisesconcern about the validity of the mixing hypothesis dis-cussed above to explain the Nd salinity correlation. Fur-ther, the eNd in surface waters of stations 0802 and 0803is �7.7 and �7.8 respectively, which is more radiogenicthan the eNd of surface waters that can be supplied by theBoB (Singh et al., 2012). This is an indication that therecan be ‘other source’ of Nd to the surface waters of the Ara-bian Sea in addition to the BoB. This ‘other source’ can beNd release from aeolian dust.

The release of Nd from aeolian dust depositing on thesea surface has been proposed earlier (Greaves et al.,1999; Tachikawa et al., 1999; Rickli et al., 2009, 2010), amechanism that is also important to Al (Measures andVink, 1999; Schußler et al., 2005) and other trace elementssuch as Fe, Mn (Jickells, 1995) and Pb (Schaule andPatterson, 1981) in various regions of the world oceans.The atmosphere over the Arabian Sea is known to receivesignificant amount of aeolian dust from the semi-arid andarid regions surrounding it, particularly from the desertsof Arabia, Iran Pakistan, Afghanistan and India (Peaseet al., 1998; Tindale and Pease, 1999; Kumar et al., 2012).The Arabian Peninsula (which includes Oman, Yemenand the Rub-Al-Khali) is among the world’s most prolificaeolian dust producing regions and a major source of aeo-lian dust to the Arabian Sea (Pease et al., 1998). In addi-tion, but to a lesser extent, aeolian dust is alsocontributed by the Sabkha complexes along the south-eastern coastline of the Arabian Peninsula and the Thardesert in India (Pease et al., 1998).

The abundance of atmospheric dust over the ArabianSea exhibits both spatial and temporal variations (Peaseet al., 1998; Tindale and Pease, 1999). These variationsare a result of several factors, prominent among them beingthe wind strength and direction and proximity to dustsources (Sirocko and Sarnthein, 1989; Pease et al., 1998;Maher et al., 2010). Records of aeolian dust influx to theArabian Sea are also contained in material collected in sed-iment traps (Ramaswamy and Nair, 1994) and in sedimentcores of the Arabian Sea (Sirocko and Sarnthein, 1989;Clemens and Prell, 1990). Given that aeolian dust could

be an important source of various elements to surfacewaters of the Arabian Sea and other oceanic regions,attempts have been made in this study to assess its role insupplying Nd to the Arabian Sea surface waters throughmodel calculations. The role of water mixing and releaseof Nd from aeolian dust is further discussed in Section 4.4.

4.3. Nd in the sub-oxic zone

The Nd concentration data in station 0802, which is inthe core denitrification zone, does not show any clearly dis-cernible gain/loss in the sub-oxic/denitrifying layer includ-ing at depths corresponding to the peak nitriteconcentration (�230 m); however, there seems to be a spikein dissolved Nd at �1000 m, just below the sub-oxic layer(Fig. 2). Further, analogous to the results of station 0802,there is also no clearly discernible enrichment in dissolvedNd in the sub-oxic layers of the other two stations 0803and 0805, though all the three profiles seem to hint at a pos-sible one point hump at around the oxic/sub-oxic boundary(Fig 2). This observation is somewhat at variance with themarginal enrichment of 10–20% in Nd and other trivalentREEs in a thin layer at the top of OMZ of Station 1605(German and Elderfield, 1990) adjacent to station 0802.More recently 2–3-fold enrichment in Fe, Mn and Cu alsohas been reported (Saager, 1994; Lewis and LutherIII,2000; Moffett et al., 2007) in the OMZ of the ArabianSea underscoring the importance of reductive dissolutionin supply of these metals. These findings make the lack ofenrichment in dissolved Nd in the sub-oxic layers of sta-tions 0802, 0803 and 0805 intriguing the cause(s) for whichis/are unclear; the coarser depth resolution of Nd measure-ments in the present study could be one of them. Morerecently, Grasse et al. (2012) and Jeandel et al. (2013) havereported Nd and eNd results in the eastern equatorial andsoutheast Pacific Ocean covering OMZ of the regions.These results also do not show enrichment in dissolvedNd at depths corresponding to the oxygen minimum. How-ever, to obtain better insights into the behavior of Nd in theOMZ of these regions, higher depth resolution samplingwould be needed. Thus, the available data on dissolvedNd in the sub-oxic layers of the Arabian Sea and the east-ern equatorial Pacific demonstrate that the Nd enrichmentis at best marginal. The behavior of Nd in these oceanicbasins is much different from those reported for the anoxicbasins, the Saanich inlet (German and Elderfield, 1989) andthe Cariaco trench (de Baar et al., 1988) where many foldconcomitant enrichment in Nd (and REEs), Fe and Mn isreported. The pronounced enrichment of REEs, Fe andMn in these anoxic basins is attributed to reductive dissolu-tion of Fe and Mn from particles settling through the OMZand release of associated REEs (de Baar et al., 1988). Thenear absence of or far lower enrichment of Nd and trivalentREEs in the sub-oxic layers of the Arabian Sea thereforewould suggest that in this oceanic region, their regenerationis much less significant than that in anoxic basins. Factorsthat can influence REE regeneration in the sub-oxic layersinclude particle flux with adequate desorbable REEs andthe prevailing redox conditions. The lithogenic flux asdetermined based on sediment trap studies are in the range


of �25 g m�2 y�1 for the Cariaco trench (Smoak et al.,2004) and �150 g m�2 y�1 for the Saanich inlet (Timothy,2001). This compares with values in the range of�7 g m�2 y�1 for the eastern Arabian Sea (Ramaswamyand Nair, 1994). These data if “typical” of these oceanicbasins, would suggest that the particle rain through thewater column of the eastern Arabian Sea is much lower,about an order of magnitude lower than that in the Saanichinlet and the Cariaco trench; data that are consistent withthe much lower regeneration fluxes of Fe, Mn and REEsin the sub-oxic layers of the Arabian Sea.

4.4. Nd and eNd profiles in the Arabian Sea: source waters

and their mixing proportions

The abundance and distribution of dissolved Nd con-centration and its isotopic composition in the ArabianSea is mixture of Nd contributed by various water massesand its supply from aeolian dust, contribution from sinkingparticulate matter and/or from sediment–water interac-tions. In the present work, an inverse modeling approachhas been used to quantify the mixing proportions of varioussources that can yield the observed Nd distributions.

The inverse modeling approach is based on a set of massbalance equations that relate the observed data with themodel parameters. In this approach, the contributions fromvarious water masses are estimated by iteratively solvingthe set of mass balance equations using the observedparameters, (i) potential temperature (h), (ii) salinity (S),(iii) Nd concentration (Nd) and (iv) Nd isotopic composi-tion (eNd).

The solution of the inverse model calculation providesthe best fit values for these properties of different watermasses and computes their fractional contribution to thesamples.

These mass balance equations are as follows:

X m ¼Xn

i¼1

X i � f im ð2Þ

where, X = potential temperature or salinity of the watermass, f is the fraction of ith water mass in sample m.

For Nd concentration and eNd, the mass balance equa-tions are:

Ndm ¼ Ndxsm þ

Xn

i¼1

Ndi � f im ð3Þ

eNdð Þm ¼eNdð Þxs

m �Ndxsm þ

Pni¼1Ndi � eNdð Þi � f im

Ndxsm þ

Pni¼1Ndi � f im

ð4Þ

Eqs. (3) and (4) also include the terms, Ndmxs and

(eNd)mxs. These represent the Nd contribution (and its

eNd) to the water column from sources other than watermass mixing in ‘mth’ sample. This excess Nd contributionarising from sources other than water mass mixing has beentermed as Ndxs and can arise from particle-water interac-tions occurring in the water column and at the atmo-sphere-water and sediment–water interfaces.

In this study, a total of nine water masses (n = 9) havebeen considered as constituents of the Arabian Sea water

column. The details of these water masses are given in theSection 4.4.1. Mass conservation requires that;

Xn

i¼1

f im ¼ 1 ð5Þ

The above Eqs. (2)–(5) can be written as d = C(l), whered and l are the data and model spaces respectively. C is thefunction that relates the data and model spaces. The modelspaces are assumed to follow log-normal probability distri-bution. A non-linear weighted fit of Eqs. (2)–(5) wasobtained using the Quasi-Newton method (Tarantola,2005), which provided the best possible fit for modelparameters and covariance (i.e. uncertainties) associatedwith them.

4.4.1. Source water masses, a priori and a posteriori values of

their properties

The inverse modeling approach to estimate the contribu-tion of various water masses to the profile samples requiresa rough knowledge of the properties; h, S, Nd and eNd toserve as input to the model. These initial data on thesources is designated as the “a priori” values.

The nine water masses in this study are based on avail-able information on the water circulation of the region(Table 4); these are (1) Riverine supply to the ArabianSea; (2) Bay of Bengal surface waters (BoBs); (3) ArabianSea High Salinity Water (ASHSW); (4) Persian Gulf Water(PGW); (5) Red Sea Water (RSW) (6) Indonesian Water(IW); (7) Indonesian Intermediate Water (IIW); (8) Modi-fied North Atlantic Deep Water (MNADW); and (9) Ant-arctic Bottom Water (AABW). The a priori informationon these water masses are given in Table 4.

The a priori values of potential temperature (h) andsalinity (S) for river discharge are taken to be 28 �C and0.1, respectively (Table 4). The Nd concentration (Nd) ofriver supply is assumed to be �7 pmol/kg, based on datafor the Indus River (Goldstein and Jacobsen, 1987) and70% removal of Nd in the estuarine zone (Elderfieldet al., 1990). The Nd isotopic composition (eNd) of the riv-erine end member is assumed to be the same as the fluxweighted Nd isotopic composition of river sediments,�15.7. The latter is calculated from eNd data of sedimentsof the Indus, Narmada, Tapi, Mahi and other rivers flowingthrough peninsular India; these rivers account for themajority of riverine sediment influx into the Arabian Sea(Allegre et al., 2010; Goswami et al., 2012b). This valuecompares well with the measured value of ��15 for theIndus (Clift et al., 2002), the dominant source of riverinesuspended matter to the Arabian Sea.

The a priori value of S for the Bay of Bengal surface waters(BoBs) has been taken from the published salinity data at thesouthern tip of Indian peninsula, which is �30 during themonth of November (Han and McCreary, 2001; Grunseichet al., 2011). The a priori value for Nd concentration isderived from the available data for the northern BoB surfacewaters during the sampling period (Singh et al., 2012); thisvalue is �45 pmol/kg. The Nd content of the BoBs wouldbe modified as it is transported through EICC by mixing withdischarge received from the Krishna, Godavari and otherpeninsular rivers. Thus, Nd concentration of 32 pmol/kg is

Table 4Inverse model parameters: a priori and a posteriori values of the water mass end-members.

Water Mass h (�C) S Nd, pmol/kg eNd

a priori valuesRiverine supply 28.0 ± 0.1 0.1 ± 0.1 6.6 ± 0.2 �15.7 ± 0.4BoBs 28.0 ± 0.2 30.0 ± 0.2 32.0 ± 1.6 �15.0 ± 0.4ASHSW 26.8 ± 0.1 36.6 ± 0.1 10.9 ± 0.4 �7.1 ± 0.4PGW 15.7 ± 0.1 35.9 ± 0.1 10.6 ± 0.4 �8.5 ± 0.4RSW 10.5 ± 0.1 35.5 ± 0.1 12.2 ± 0.4 �9.4 ± 0.4IW 13.5 ± 0.1 34.5 ± 0.1 11.6 ± 0.4 �2.9 ± 0.4IIW 4.5 ± 0.1 34.6 ± 0.1 18.1 ± 0.6 �3.6 ± 0.4MNADW 1.8 ± 0.1 34.8 ± 0.1 19.9 ± 0.7 �10.5 ± 0.4AABW 1.5 ± 0.1 34.7 ± 0.1 27.8 ± 1.0 �8.5 ± 0.4

a posteriori valuesRiverine supply 28.1 ± 0.1 0.1 ± 0.1 6.6 ± 0.2 �15.7 ± 0.4BoBs 28.8 ± 0.2 30.4 ± 0.2 31.5 ± 1.5 �15.2 ± 0.4ASHSW 28.3 ± 0.1 36.9 ± 0.1 10.7 ± 0.4 �6.9 ± 0.4PGW 15.7 ± 0.1 36.1 ± 0.1 10.3 ± 0.4 �9.0 ± 0.4RSW 10.5 ± 0.1 35.7 ± 0.1 11.8 ± 0.4 �10.0 ± 0.4IW 13.5 ± 0.1 34.6 ± 0.1 11.5 ± 0.4 �3.0 ± 0.4IIW 4.4 ± 0.1 34.7 ± 0.1 17.7 ± 0.6 �3.8 ± 0.4MNADW 1.6 ± 0.1 35.1 ± 0.1 18.4 ± 0.6 �10.8 ± 0.4AABW 1.1 ± 0.1 34.8 ± 0.1 25.7 ± 0.7 �8.9 ± 0.3

Data source:Riverine supply: Data derived from (Goldstein and Jacobsen, 1987; Elderfield et al., 1990; Allegre et al., 2010; Goswami et al., 2012b). ReferSection 4.4.1 for more details.BoBs: Data derived from (Han and McCreary, 2001; Grunseich et al., 2011; Singh et al., 2012). Refer Section 4.4.1 for more details.ASHSW: (German and Elderfield, 1990); h, S and Nd from station 1605 (60 m depth), for eNd refer Section 4.4.1.PGW: (German and Elderfield, 1990); h, S and Nd from station 1608 (200 m depth), for eNd refer Section 4.4.1.RSW: (German and Elderfield, 1990); h, S and Nd from station 1605 (700 m depth), for eNd refer Section 4.4.1.IW: (Jeandel et al., 1998); station 20 (200 m depth).IIW: (Jeandel et al., 1998); station 20 (1099 m depth).MNADW: (Bertram and Elderfield, 1993); h, S and Nd from station 1506 (2300 m depth), eNd from station 1507 (3175 m depth).AABW: (Bertram and Elderfield, 1993); station 1506 (4000 m depth).


used as a priori value for the BoBs. The eNd of waters in thenorthern BoB is �15, this value is adopted for a priori eNd

of BoBs; it is assumed that this value is the same for EICCat the southern tip of India as the eNd of sediments fromthe major rivers draining form the peninsular India into theBoB is �(�15) (Ahmad et al., 2009). The lack of Nd andeNd of BoBs near the southern tip of Indian peninsula is alimitation in the choice of their a priori values for modelinput, these however is unlikely to impact the results of theinverse model calculations as it derives set of best fit a poste-

riori values which can closely match the measuredparameters.

The a priori values of h, S and Nd for ASHSW, PGWand RSW are from published data of Bertram andElderfield (1993). There is a dearth of eNd data in the Ara-bian Sea waters. This study is the first reporting of eNd inthe waters of the Arabian Sea therefore; the a priori eNd val-ues for ASHSW, PGW and RSW are taken from data mea-sured in the present study. The eNd for the ASHSW is takento be the same as that at 65 m of station 0802 (eNd = �7.1).For the PGW it is assumed to be the same as that at 400 mof station 0802 (eNd = �8.5), the depth at which PGW has asignificant presence in the Arabian Sea (Shenoi et al., 1993).The eNd of the RSW was also taken from station 0802 in thesample from 1000 m depth (eNd = �9.4). The rationale for

using the a priori information of ASHSW, RSW andPGW from the station 0802 is that the presence of thesewater masses is significant in the eastern Arabian Sea(Shenoi et al., 1993).

The a priori values of the parameters for the IW and IIWare assumed to be the same as that for the Indonesianthrough flow straits (Jeandel et al., 1998). For MNADW;h, S and Nd values are those measured at a depth of2300 m in the profile 1506 in the Somali basin and foreNd, it is the measured value at �3000 m of station 1507(Bertram and Elderfield, 1993). The a priori data forAABW are those from a depth of 4000 m in profile 1506in Somali basin (Bertram and Elderfield, 1993). Both forMNADW and AABW, the properties considered for theinverse model represent values just prior to their entry tothe Arabian Sea via the Somali basin.

The inverse model also requires an initial a priori valueof Ndxs, in the present work, this has been assumed as0.1 pmol/kg with a positive offset of 5 pmol/kg (Ndxs can’tbe negative).

Singh et al. (2012) used a similar inverse model approachto estimate the water mass fraction and Ndxs contributingto the measured Nd and eNd profiles in the BoB. The majordifference in the a priori values used between the two mod-els is that Singh et al. (2012) constrained their h and S far


more tightly (±0.001) compared to those used in the presentstudy where a larger uncertainty in h and S was provided(±0.1). The larger uncertainty used in the present modelallows more leeway to arrive at the best fit values of hand S for source water masses. The models of Singh et al.(2012) and that of the present study have some commonwater masses such as ASHSW, MNADW and AABW.The a priori values used by Singh et al. (2012) for someof the properties of water masses differ from that used inthe present work; for example for the ASHSW, Singhet al. (2012) used the data of station 0802 measured in thisstudy, whereas in the present study the data for ASHSW isfrom the station 1605 in the central Arabian Sea. For theMNADW, Singh et al. (2012) used the data from station1504 in the Madagascar basin (Bertram and Elderfield,1993), whereas in this work it is taken from a much proxi-mal location (near Somali basin; station 1507; Fig. 1) to theArabian Sea. For AABW, Singh et al. (2012) used h and Svalues from the Southern Indian Ocean (�60�S) and Ndand eNd from the South Atlantic Ocean, whereas in thepresent work, these properties of AABW are from theSomali basin. The a priori values used here and the deriveda posteriori numbers are location specific, the choice oflocation however is dependent on available informationon water circulation of the region.

The sensitivity of the model was checked by varying oneof the model parameters and keeping the others constant;the range of variation in the a priori values used for assess-ing the sensitivity were ±1 unit for h and S, ±10% for Ndand ±2 units for eNd for the major water masses BoBs,ASHSW, MNADW and AABW. The model was foundto be more sensitive to changes in the h and S propertiesfor the deeper water masses MNADW and AABW andtherefore to their fractional contributions; this results fromcloseness of these properties in them. For the other changesmade to the a priori input, the model yielded a posteriori

values and water mass fractions that overlap within errors.The inverse model was also run without including the

Ndxs term, but with the same a priori values for otherparameters (Table 4); this effort was made to assess if thewater column Nd data can be fully explained in terms ofwater mixing (i.e. without invoking the need for particle-water interactions and biogeochemical processes). Theresults of this exercise though suggested that the Nd con-centration in the upper layers of the water column can beexplained by having BoBs waters with a much higher Nd(�42 pmol/kg) than the requirement based on the inclusionof Ndxs term in the model (�32 pmol/kg), the model how-ever was not able to balance the Nd budget of deeperwaters. The high Nd requirement in the BoBs though fallsclose to some of the highest Nd concentration in the north-ern BoB surface waters (Singh et al., 2012), it howeverremains to be ascertained if such waters form the dominantcomponent of the EICC that transports the BoB waters tothe Arabian Sea. Such a model however was found to beadequate to account for the physical properties of the watercolumn.

Oxygen can also be used a tracer for distinguishingbetween water masses (Toole and Warren, 1993; Mantylaand Reid, 1995; You, 2000). This parameter, however, is

not conservative in the Arabian Sea as a significant fractionof it gets consumed by biological activity in the intermedi-ate depths leading to the formation of the OMZ. An effortwas made to include oxygen as an additional parameter(along with other parameters h, S, Nd and eNd) in theinverse model. A loss term for oxygen was included in equa-tion to take care of its biological consumption in the watercolumn. The outcome of this modeling exercise resulted inmodel results that were consistent and invariant to the ori-ginal model run with h, S, Nd and eNd as parameters. Inaddition, this model also provided additional informationon loss/consumption of oxygen in the water column. Theseresults based on oxygen parameters are not discussed anymore in this paper.

The a posteriori or the best fit values of h, S, Nd and eNd

of the water mass end members derived from the inversemodel are given in Table 4. The fractional contribution ofthese water masses to the profile samples was also com-puted; the uncertainties in these estimates average �10%.From these data, contour plots of water mass contributionswith depth in the eastern Arabian Sea for the latitudinalrange �14.5�N to �6�N are generated using the ODV soft-ware (Schlitzer, 2010); these are given in Fig. 6.

Singh et al. (2012) also used the inverse model to calcu-late the a posteriori values for the water masses in the BoBin their investigation of Nd budget in the BoB. Here, weshall compare the results of both the models for themajor/dominant water masses; ASHSW, MNADW andAABW. The comparison of these two sets of values showsthat there are some differences between them. This can arisefrom (i) larger uncertainty in h and S a priori values pro-vided in this study and (ii) the derived values are locationspecific, for example for the ASHSW, the values calculatedin this study refer to station 1605 (14.43�N, 66.92�E)whereas that of Singh et al. (2012) is for station 0802(14.42�N, 69.42�E). For ASHSW, the a posteriori valuesfor h and S in both the calculations are similar whereasfor eNd the value derived in the present work is more radio-genic (�6.7), compared to that obtained by Singh et al.(2012), (�8.1). In the case of MNADW, the most signifi-cant difference is seen in the a posteriori values of eNd; thecalculation of Singh et al. (2012) yielded a value of �9.2,compared to the less radiogenic value of �10.8 obtainedin this study. The NADW forms in North Atlantic withan eNd of �13.5 (Piepgras and Wasserburg, 1987). Thiswater mass mixes with the more radiogenic WSDW (eNd =�8.1 ± 0.6) (Jeandel, 1993) and enters the Indian Ocean asMNADW. It is suggested that the MNADW entering theIndian Ocean in the west and reaching the Arabian Seaundergoes lesser degree of mixing and modification in thecircumpolar Antarctic region (Toole and Warren, 1993),this limited mixing may explain the lesser radiogenica posteriori eNd values of �10.8 in the Arabian Sea. In con-trast, the MNADW enters the BoB via the eastern route inthe Indian Ocean after undergoing mixing in circumpolarAntarctic region (Mantyla and Reid, 1995; Dileep Kumarand Li, 1996) resulting in its more radiogenic eNd than thecomponent of MNADW entering via the western route.Thus, the less radiogenic a posteriori eNd and lower Nd ofMNADW in the Arabian Sea relative to that in the BoB

Fig. 6. Lateral and vertical fractional distribution (in%) of various water masses in the Arabian Sea calculated by the inverse model: (a)Riverine supply; (b) (BoBs) Bay of Bengal surface waters; (c) ASHSW (Arabian Sea High Salinity Water); (d) PGW (Persian Gulf Water); (e)RSW (Red Sea Water); (f) IW (Indonesian throughflow surface Water); (g) IIW (Indonesian throughflow Intermediate Water); (h) MNADW(Modified North Atlantic Deep Water) and (i) AABW (Antarctic Bottom Water).


seems consistent with the shorter route taken by thiswater mass in reaching Arabian Sea. For the AABW, thea posteriori values of eNd derived based on both the modelsare identical within errors.

4.4.2. Water mass contribution to various depths of profiles

sampled

The results of water mass contribution to various depthsin the profiles samples can be summarized as follows:


(i) The contribution of river water to the water budget(from the Indus and the west flowing rivers of India)is quite small, less than 2% to the surface waters ofthese profiles in the eastern Arabian Sea (Fig. 6a);consistent with the high salinity of surface waters atstations 0802 and 0803 (>35.5).

(ii) The contribution of BoBs to the eastern Arabian Seashows a decreasing trend from south to north(Fig. 6b); with values decreasing from 77% (�6�N)to 10% (�14.5�N), the maximum value being in thesurface waters of the southernmost station 0805. Thisis consistent with the known surface water circulationof the region (Schott and McCreary, 2001) during thesampling period when the EICC is active and trans-ports BoB waters to the Arabian Sea around thesouthern tip of India. The trend is also consistentwith the strong inverse correlation between Nd andsalinity in these waters.

(iii) In the top �100 m surface waters of the Arabian Sea,ASHSW is the dominant water mass with contribu-tions of �90% at �14.5�N (Fig. 6c). The ASHSWforms in the northern Arabian Sea due to excessevaporation over precipitation; within the core of thiswater mass the salinity can be in excess of 36.7(Shenoi et al., 1993), nearly identical to that in sur-face waters of station 0802.

(iv) The distribution of PGW (Fig. 6d) forming as a resultof spillage of high salinity waters from the PersianGulf into the Gulf of Oman indicates that its presenceis discernible in station 0802 accounting for �25–35%in the 200–600 m depth range; this decreases to �10–20% at station 0805 in the depth range of 200–800 m.Subsequent to its formation, the PGW moves alongthe west coast of India during the NE monsoon; inthis process its composition changes by mixing withlow salinity BoB waters in the region south of 10�Nand along the west coast of India (Prasad et al.,2001). This is evident from the reduced fractional con-tribution of PGW south of 10�N (Fig. 6d). The ana-logue to PGW, the RSW (Shenoi et al., 1993) alsoshows a north–south gradient (Fig. 6e). This watermass accounts for �20–30% of the intermediatewaters (400–700 m) of stations 0802 and 0803(Fig. 6e), which reduces to �10–20% at station0805. The contribution of Indonesian Water (IW)and Indonesian Intermediate Water (IIW) to the sta-tions studied are very minor <10% (Figs. 6f and g).

(v) The MNADW and AABW make up the deeperwaters of the Arabian Sea. The deep water circula-tion between the northern stations (0802 and 0803)and station 0805 is constrained by the Chagos-Lacca-dive ridge (Fig. 1). The distribution of MNADWshows that it accounts for >70% of water fractionat �14.5�N in 1800–2500 m depth range and remainsin the range of 50–60% south of �10�N in the 1200–1800 m depth range. This is similar to the findings ofDileep Kumar and Li (1996), that in the westernIndian Ocean (in the region north of 10�N), theMNADW accounts for >70% abundance in the1500–2500 m depth range.

(vi) Among the three profiles sampled, only at station0802 water samples below 2500 m depth were col-lected. At this station, the AABW is found to bethe most abundant water mass (in excess of 80%)below the depth of 3000 m (Fig. 6i). The contributionof AABW is far more pronounced in the northernparts of transect as they are deeper than the southernpart (Fig. 1). Singh et al. (2012) also observed thatthe contribution of AABW to the deeper waters ofthe BoB is >80%, similar to our finding in the Ara-bian Sea. These results lead to infer that the AABWis the dominant water mass in the deeper waters ofthe northern Indian Ocean.

4.4.3. Estimates of Nd budget, Ndxs and its eNd (eNdxs ) from

the inverse model

The sum of model derived Nd contributions from thenine water masses to the individual samples of the threeprofiles is given in Table 5. A cursory look at the data sug-gest that the estimated Nd contribution from the watermasses is deficient relative to measured values in surfaceand near surface samples of stations 0802 and 0803 andin the deepest sample of station 0802. In addition, in station0805 the deficiency seems to be ubiquitous throughout thedeeper waters (>100 m) of the profile.

As mentioned in Section 4.2, a potential source for theenrichment of Nd in surface waters can be atmosphericinput of lithogenic dust that releases Nd to surface waters.Alternatively, if the BoBs has much higher dissolved Nd(�42 pmol/kg instead of �32 pmol/kg used in the model),it can also explain the surface excess. However, if the BoBsis the source, the eNd in surface waters of stations 0802 and0803 are expected to be �9 and �10, significantly less radio-genic than that measured. This indicates that release fromthe aeolian dust in surface waters is more likely to be thesource for the Ndxs. The eNd

xs derived from the inverse modelcan provide insights into the role of sources in contributingto the estimated Nd enrichment at these stations. Themodel derived eNd

xs component of the surface samples fromthe two stations 0802 and 0803 overlaps within errors,�6.1 ± 2.2 and �5.3 ± 1.9, respectively (Table 5). There isno data on the Nd concentration and eNd of the aeoliandust from this sampling region. However, Sirocko (1995)estimated a eNd of �6 based on the isotopic compositionof potential dust sources and their mixing proportions.The dust sources considered for this estimate are from theOman and Arabian deserts. This estimated eNd value forthe dust is the same within uncertainties as that derivedfor Ndxs from the inverse model. This nearly identical eNd

value hints at the possibility that the Ndxs in surface watersof stations 0802 and 0803 can be of aeolian dust origin.Some of the recent studies on dust composition over theeastern Arabian Sea however, seem to indicate that duringthe NE monsoon, Thar Desert region can be an importantsource of dust supply (Pease et al., 1998; Kumar et al.,2012). Tripathi et al. (2004) have measured eNd in samplesof the Thar desert origin and found values ranging from�13 to �15, far less radiogenic than that estimated bySirocko (1995) and that derived from the inverse model.

Table 5Measured Nd (Ndm), Ndest, Ndxs (in pmol/kg) and eNd

xs in the threevertical profiles sampled.

Depth (m) Ndm Ndest Ndxs eNdxs

0802 (14.42�N, 69.42�E)

5 15.8 13.0 ± 2.0 2.9 ± 1.1 �6.1 ± 2.265 14.0 11.5 ± 2.0 2.5 ± 1.0 �7.2 ± 2.4

120 14.3 13.2 ± 2.0 1.2 ± 1.7 �8.4 ± 3.1230 12.5 13.0 ± 2.1 0.0 ± 0.1 �8.0 ± 3.0400 12.7 13.3 ± 2.1 0.0 ± 0.1 �8.0 ± 3.0500 13.5 13.9 ± 2.1 0.0 ± 0.1 �8.0 ± 3.0700 � 15.5 ± 2.1 0.0 ± 0.1 �8.0 ± 3.0

1000 20.2 16.7 ± 2.1 3.6 ± 2.2 �8.9 ± 2.91500 16.9 17.5 ± 2.0 0.0 ± 0.1 �8.0 ± 3.02100 17.5 18.6 ± 2.0 0.0 ± 0.1 �8.0 ± 3.03000 23.1 24.1 ± 2.0 0.0 ± 0.1 �8.0 ± 3.03600 25.3 24.9 ± 2.0 0.4 ± 1.3 �8.0 ± 3.03900 24.6 25.0 ± 2.0 0.0 ± 0.2 �8.0 ± 3.04040 34.3 25.0 ± 2.0 9.4 ± 1.5 �9.7 ± 1.9

0803 (10.12�N, 71.86�E)

5 20.1 15.7 ± 2.0 4.8 ± 1.5 �5.3 ± 1.935 15.4 12.8 ± 2.0 2.8 ± 1.2 �5.9 ± 2.2

130 15.6 14.4 ± 2.1 1.2 ± 2.7 –210 � 15.1 ± 2.1 0.0 ± 0.5 �8.0 ± 3.0300 13.3 13.9 ± 2.1 0.0 ± 0.1 �8.0 ± 3.0465 14.2 14.5 ± 2.1 0.0 ± 0.1 �8.0 ± 3.0600 14.4 14.8 ± 2.1 0.0 ± 0.1 �8.0 ± 3.0900 16.6 16.6 ± 2.1 0.0 ± 0.3 �8.0 ± 3.0

1200 18.6 18.0 ± 2.1 0.6 ± 2.3 –1600 19.7 19.7 ± 2.0 0.0 ± 0.9 �8.0 ± 3.02400 22.5 22.9 ± 2.0 0.0 ± 0.1 �8.0 ± 3.0

0805 (6.01�N, 77.48�E)

5 27.8 26.8 ± 2.0 0.9 ± 2.2 �60 16.5 14.5 ± 2.0 2.0 ± 3.0 �

120 – 15.8 ± 2.0 0.0 ± 0.2 �8.0 ± 3.0200 19.3 14.9 ± 2.1 4.4 ± 2.8 �12.2 ± 3.7300 18.0 17.3 ± 2.0 0.4 ± 1.9 �8.0 ± 3.0580 18.1 16.5 ± 2.1 1.5 ± 2.6 �650 19.5 15.6 ± 2.1 4.5 ± 1.9 �23.1 ± 6.0845 21.0 16.6 ± 2.1 4.6 ± 2.1 �14.6 ± 3.8

1000 22.0 17.3 ± 2.1 4.9 ± 2.1 �13.0 ± 3.51150 21.9 18.4 ± 2.1 3.5 ± 2.5 –1450 22.8 18.0 ± 2.0 5.1 ± 1.7 �18.5 ± 4.31850 23.5 20.1 ± 2.1 3.4 ± 2.2 �10.1 ± 3.32250 25.7 21.7 ± 2.0 4.2 ± 2.1 �12.4 ± 3.8

The superscripts; m: measured Nd concentration; est: estimated Ndconcentration by summing the contribution from nine water mas-ses; xs: the excess Nd. [Ndxs = Ndm–Ndest].Error on Ndm is ±3.5%.The errors on Ndest, Ndxs and eNd (Ndxs) are propagated from theinverse modeling results.


Thus, if the eNd of dust (�6) is “typical” of these regions, itwould lead us to infer that the dissolved Ndxs in the surfacewaters of eastern Arabian Sea is dominated by the contri-bution from the Oman and Arabian deserts. The contribu-tion from the Thar Desert though could be significantduring the winter season (Pease et al., 1998); its impacton eNd

xs may be limited as Ndxs is integrated over the resi-dence time of Nd in surface waters. The limitation in iden-tifying the dust source over the Arabian Sea during the

sampling period underscores the need for direct eNd mea-surements of aeolian dust over this region.

Another important result on Ndxs is its absence in theintermediate waters of the 0802 and 0803 (Table 5; Fig. 7).This would indicate that there is no discernible signatureof lateral transport of Nd from anoxic/sub-oxic westerncontinental shelf sediments (Naqvi et al., 2000). In the sta-tions 0802 and 0803, the Ndxs is discernible in surface waters(at the air-sea interface), whereas in station 0802 it alsooccurs in bottom waters. This suggests that excess Nd is sup-plied to these stations at air-sea interface and for station0802 it is also derived from the sources at sediment–waterinterface. In contrast, in the southern station 0805, there isno evidence for Ndxs in the surface waters (Table 5) suggest-ing that at this location the Nd enrichment can be explainedin terms of water mixing alone, and that there is no need foran additional source. In this profile, however excluding thesurface waters, Ndxs seems to be present throughoutthe water column. This station located in the proximity ofthe BoB, may be receiving Ndxs from in-situ release fromG-B particles, a mechanism proposed by Singh et al.(2012) to explain the Nd distribution in BoB profiles. Theexcess Nd calculated by Singh et al. (2012) in the more dis-tal/remote sites in the southern BoB shows a significantrelease of Nd from the sinking particles with values in Ndxs

of �3–4 pmol/kg (in southern BoB, �6�N; depth range of0–2000 m), similar to the Ndxs estimated for station 0805.In station 0802, the eNd

xs in near the bottom water (depth4040 m) is -9.7 (Table 5; Fig. 7); this overlaps with that mea-sured for the top (�9.4) of a nearby sediment core (12.8�N,71.7�E) in the Arabian Sea (Goswami et al., 2012b). Thesimilarity in the isotopic composition of Ndxs in the bottomwaters of the station 0802 and that of core top sediments isindicative of underlying sediments being a potential sourceof Ndxs in the near bottom waters.

The importance of atmospheric dust in contributing Ndto surface waters is well recognized (Greaves et al., 1999;Tachikawa et al., 1999; Rickli et al., 2010). The flux ofNd released from dust would depend on the lithogenic dustflux FLITH, Nd concentration in the lithogenic dust (CNd);taken to be Nd concentration in the upper continentalcrust, 26.5 lg g�1; (Taylor and McLennan, 1995) and thefractional solubility of Nd (f) from the dust. The Ndreleased to surface waters would accumulate in the upperwater column over its residence time (sNd). These parame-ters are linked through the following relation.

Ndxs� ¼ FLITH � CNd � f � sNd ð6Þ

In the above relation, Ndxs* is the excess Nd inventoryper unit area (in lg m�2) in the upper layers of stations0802 and 0803. The Ndxs* is calculated from the inversemodel derived Ndxs at 5 and 65 m of station 0802 and 5and 35 m for station 0803 as only these two surface samplesseem to have discernible Ndxs concentration (Table 5). TheNdxs in these samples translates to Ndxs* of 24 ± 10 and20 ± 7 lg m�2 for stations 0802 and 0803 respectively.For further calculation, a working value of 20 lg m�2 hasbeen used for the Ndxs*.

Knowing Ndxs* and using the above relation (Eq. 6) it ispossible to estimate lithogenic dust flux, fractional solubility

Fig. 7. Lateral and vertical distribution of (a) Ndxs and its corresponding (b) eNdxs in the waters of the Arabian Sea computed using the inverse

model. The eNdxs is shown only at those sites where there is a significant (non-zero) presence of Ndxs. The eNd

xs in the surface waters of thestations 0802 and 0803 yields values of �6.1 and �5.3 respectively, similar to the estimated eNd of dust supplied over the Arabian Sea (�6)(Sirocko, 1995). The bottom waters of the station 0802 (4040 m) shows significant Ndxs with eNd

xs of �9.8; hinting at a possible release of Ndfrom bottom sediments. The southern station 0805 in the Arabian Sea shows significant Ndxs throughout the profile with less radiogenic eNd

xs .In the northern station 0802, there is significant Ndxs at a depth of 1000 m.


of Nd (f) or the residence time of Nd in surface layers (sNd)once any two of these parameters are known or can be esti-mated. There are independent estimates of lithogenic dustflux from the atmosphere to the Arabian Sea. These esti-mates are based on the dissolved profile of Al in the surfacewaters of the Arabian Sea (Measures and Vink, 1999;Schußler et al., 2005); abundance of lithogenic elements(Al) in aerosols and their settling velocities (Schußleret al., 2005; Kumar et al., 2008, 2012) and based on availableaerosol data and modeling (Maher et al., 2010). These esti-mates mostly range between �5 and �10 g m�2 y�1.Measures and Vink (1999) estimated the flux of aeolian dustto the eastern Arabian Sea to be �10 g m�2 y�1 from dis-solved Al data assuming a residence time of 2.5 years for dis-solved Al. The lithogenic flux through the water column at�2800 m depth in the eastern Arabian Sea has been deter-mined to be �7 g m�2 y�1 (Ramaswamy and Nair, 1994);this flux which includes both atmospheric and riverine mate-rial should serve as an upper limit for aeolian lithogenic dustflux.

Data on sNd is not available for the Arabian Sea andhence a range of values have been assumed based on resultsof other particle reactive elements in the region and that ofNd in other oceanic areas. Measurement of 210Pb and

234Th-238U in the eastern/central Arabian Sea duringNov-Dec, 1988 (same sampling months as that of the pres-ent study), have yielded residence times of 2–8 months forPb and Th, two particle reactive nuclides (Sarin et al.,1994). Given that both Pb and Th are more particle reactivethan Nd, it is very likely that sNd would be higher.Amakawa et al. (2000) derived a residence time of �1.5–2.6 years for Nd in the south-eastern Indian Ocean by mod-eling its distribution and isotopic composition. Singh et al.(2012) used a residence time of �2.5 years for dissolved Ndin surface waters to estimate the flux of Nd release fromsinking particles in the BoB. Based on the above informa-tion, we have assumed the sNd to range between 1 and3 years for calculations. The general relationship betweenFLITH and fractional solubility (f) in the Arabian Sea forNdxs* of 20 lg m�2 as a function of sNd is shown inFig. 8. For a lithogenic dust flux of 5 to 10 g m�2 y�1

(shaded portion in Fig. 8) and assumed sNd (of 1–3 years),the fractional solubility of Nd range between 3% and 17%(Fig. 8). This compares with the range estimated by Singhet al. (2012), �9–23% for the release of Nd from sinkingG–B particulate matter in BoB but is much higher com-pared to the results of Greaves et al. (1994) who determinedthe fractional solubility to be �2% based on the lab

Fig. 8. Relationship between FLITH and fractional solubility (f)(derived for Ndxs* of 20 lg m�2 estimated from the inverse model)as a function of sNd. For a lithogenic dust flux of 5 to 10 g m�2 y�1

(shaded portion, which marks the range of lithogenic dust fluxestimates for the eastern Arabian Sea) and assumed sNd (of 1–3 years), the fractional solubility of Nd range from 3% to 17%. Redline parallel to x axis marks the lithogenic flux estimate(�7 g m�2 y�1) in the eastern Arabian Sea based on sediment traps(Ramaswamy and Nair, 1994).


experiments. These estimates of fractional solubility arecritically dependent on the estimates of Ndxs*. In the pres-ent study, the entire Ndxs* derived from the model isassumed to be of aeolian origin and therefore is likely tobe an upper limit. Further, the uncertainty associated withNdxs* estimate is �40%; therefore, the estimates on the frac-tional solubility of Nd will also carry similar uncertainty.The present measurements and modeling underscores theneed to have better depth resolution sampling and moreprecise measurement of Nd and eNd to derive quantitativeinformation on Nd sources and their contribution to theNd and eNd distribution in the Arabian Sea.

5. CONCLUSIONS

The distribution of dissolved Nd and eNd has beenmeasured in three vertical profiles from the eastern Ara-bian Sea. The water columns in these stations are charac-terized by a thick sub-oxic intermediate layer of �800 mthickness with one of the stations (0802) in the core deni-trification zone. The Nd concentration with depth in allthree profiles by and large shows a similar trend, with a

near surface maximum, a minimum in the 200 to 400 mdepth range followed by a steady increase with depth.The Nd concentration does not show clearly discernibleincrease in the sub-oxic layers indicating that the Nd bio-geochemistry in this oceanic region is not influenced sig-nificantly by the sub-oxic/denitrifying conditions;however high resolution depth sampling for Nd isrequired to confirm this inference.

The impact of water mass mixing and supply from addi-tional sources to the measured Nd and eNd distribution inthe three profiles was assessed by analyzing the profile datausing an inverse model. The key findings are (i) the surfacewater high Nd concentration (in station 0802 and 0803) canin principle be explained in terms of supply from high Ndlow salinity waters from the BoB to the Arabian Sea viathe EICC. This hypothesis though is consistent with thesouth-north decrease in Nd with a strong inverse correla-tion with salinity, its validity can be confirmed only afterdata on Nd and eNd of EICC waters entering the ArabianSea becomes available. This is particularly important giventhat currently there is paucity of data to assess eNd-salinitytrend in these waters. (ii) alternatively, aeolian dust depos-iting over the sea can serve as source for the Nd surfaceexcess. Model calculations show that the contribution fromthis source can dominate the Ndxs in surface waters of sta-tions 0802 and 0803. The fractional solubility of Nd fromaeolian dust is estimated to be varying from 3% to 17% withNd residence time of 1 to 3 years and lithogenic dust flux inthe range of 5–10 g m�2 y�1, values which are in the rangeof those reported in literature. (iii) In deeper waters of sta-tion 0805, there is ubiquitous release of Nd throughout thewater column, most likely from particle-water interactions.(iv) Among the nine water masses considered, ASHSW,BoBs, MNADW and AABW seem to dominate the Ndand eNd distribution in the water column.

This study brings out the significance of aeolian dustdeposition on the concentration and distribution of Nd inthe Arabian Sea, the western arm of the northern IndianOcean surrounded by arid landmass and characterized byhigh lithogenic aeolian dust deposition. This is in contrastto the BoB, the eastern arm of the northern Indian Ocean,where river supply seems to dictate the Nd concentrationand distribution in the water column.

ACKNOWLEDGEMENTS

We thank the captain and crew of FORV Sagar Sampada cruiseNo. 259 and all the onboard scientific and technical staff for theirsupport and cooperation. Discussions with Prof. S. Krishnaswamiand his comments were helpful and constructive. We acknowledgethe help from the Director and staff of CMLRE for logistic sup-port; J.P. Bhavsar, B. Srinivas, S. P. Singh and J. Chatterjee forassistance in onboard sampling and experiments; S.V.S. Murtyfor providing time on PLANEX Q-ICP-MS and D. Prasad for helpwith Nd concentration measurement. We thank K. Patel and G. R.Tripathy for guidance in computational works. We are thankful tothe Ministry of Earth Sciences (MoES) for the partial financial andlogistic support; this work is a part of PRL’s program on GEO-TRACES. We thank the four reviewers and Dr. Andrew Bowie,associate editor for their extensive and constructive comments thathelped in improving this manuscript significantly.


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Impact of water mass mixing and dust deposition on Nd concentration and εNd of the Arabian Sea...

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