Sedimentary sources and processes in the eastern Arabian Sea: Insights from environmental magnetism, geochemistry and clay mineralogy

Geoscience Frontiers 7 (2016) 253e264

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Sedimentary sources and processes in the eastern Arabian Sea: Insights from environmental magnetism, geochemistry and clay mineralogy Kumar Avinash a, P. John Kurian a, *, Anish Kumar Warrier a, R. Shankar b, T.C. Vineesh a, Rasik Ravindra c a Earth System Science Organization, National Centre for Antarctic and Ocean Research (ESSO-NCAOR), Ministry of Earth Sciences (MoES), Headland Sada, Vasco-da-Gama 403 804, Goa, India b Department of Marine Geology, Mangalore University, Mangalagangotri 574 199, Karnataka, India c Earth System Science Organization, Ministry of Earth Sciences (MoES), Lodhi Road, New Delhi 110 003, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 November 2014 Received in revised form 20 April 2015 Accepted 2 May 2015 Available online 29 May 2015

The spatial distribution patterns of surficial sediment samples from different sedimentary domains (shallow to deep-sea regions) of the eastern Arabian Sea were studied using sediment proxies viz. environmental magnetism, geochemistry, particle size and clay mineralogy. Higher concentrations of magnetic minerals (high clf) were recorded in the deep-water sediments when compared with the shallow water sediments. The magnetic mineralogy of one of the shallow water samples is influenced by the presence of bacterial magnetite as evidenced from the cARM/clf vs. cARM/cfd biplot. However, the other samples are catchment-derived. The high correlation documented for clf, anhysteretic remanent magnetisation (cARM) and isothermal remanent magnetisation (IRM) with Al indicates that the deep-sea surficial sediments are influenced by terrigenous fluxes which have been probably derived from the southern Indian rivers, the Sindhu (the Indus) and the Narmada-Tapti rivers. A lower Mn concentration is recorded in the upper slope sediments from the oxygen minimum zone (OMZ) but a higher Mn/Al ratio is documented in the lower slope and deep-sea sediments. Clay minerals such as illite (24e48.5%), chlorite (14.1e34.9%), smectite (10.6e28.7%) and kaolinite (11.9e27.5%) dominate the sediments of shallow and deep-sea regions and may have been derived from different sources and transported by fluvial and aeolian agents. Organic carbon (OC) data indicate a low concentration in the shallow/shelf region (well oxygenated water conditions) and deeper basins (increased bottom-water oxygen concentration and low sedimentation rate). High OC concentrations were documented in the OMZ (very low bottom-water oxygen concentration with high sedimentation rate). The calcium carbonate concentration of the surface sediments from the continental shelf and slope regions (<1800 m) up to the Chagos-Laccadive Ridge show higher concentrations (average ¼ 58%) when compared to deep basin sediments (average ¼ 44%). Our study demonstrates that particle size as well as magnetic grain size, magnetic minerals and elemental variations are good indicators to distinguish terrigenous from biogenic sediments and to identify sediment provenance. Ó 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Keywords: Magnetic minerals Major elements Organic carbon Calcium carbonate Terrigenous fluxes Eastern Arabian Sea

1. Introduction The Arabian Sea differs from other oceans as it is strongly influenced by the seasonal reversal of winds which blow from

* Corresponding author. Tel.: þ91 832 2525570; fax: þ91 832 2525566. E-mail address: [email protected] (P.J. Kurian). Peer-review under responsibility of China University of Geosciences (Beijing).

the southwest during JuneeSeptember but also from the northeast during NovembereFebruary. This results in large variations in upwelling intensity and primary productivity (Haake et al., 1993; Honjo et al., 1999). Lithogenic material from several sources is delivered to the Arabian Sea via transporting agencies, both fluvial and eolian (Sirocko and Lange, 1991; Dahl et al., 2005). The primary fluvial agencies are the Sindhu (the Indus), the Tapti and the Narmada rivers; the singularly important agency among

http://dx.doi.org/10.1016/j.gsf.2015.05.001 1674-9871/Ó 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NCND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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them is the Sindhu River (Ramaswamy et al., 1991; Prins et al., 2000). The several small rivers that originate in the Sahyadri (the western Ghat) and debouch into the Arabian Sea via the west coast of India also contribute lithogenic sediments (Manjunatha and Shankar, 1992). Eolian sediments are transported from the Thar and the Arabian Deserts (Sirocko and Lange, 1991; Schnetger et al., 2000). Sediments could be derived from natural processes (lithogenic and pedogenic) and/or anthropogenic activities (Karbassi and Shankar, 1994). Sedimentary provenance studies are important as they help in deducing the sources of sediment and the processes involved thereof. In the past, provenance studies have been made using different approaches like environmental magnetism, geochemistry, sedimentology and clay mineralogy (Sirocko and Lange, 1991; Karbassi and Shankar, 1994; Schnetger et al., 2000; Sirocko et al., 2000; Alagarsamy, 2009). Textural, chemical and biological characteristics of marine sediments are strongly influenced by factors such as source-area composition (Paropkari et al., 1980), climatic conditions, length and energy of sediment transport (Borole et al., 1982), primary productivity of overlying waters, dissolved oxygen in bottom waters and redox conditions in the depositional environment (Shankar et al., 1987; Ramaswamy et al., 1991; Schnetger et al., 2000; Sirocko et al., 2000). Hitherto, several investigations have been carried out on surficial sediments from the Arabian Sea. However, they have used only single proxies (Shankar et al., 1987; Prakash Babu et al., 1999; Alagarsamy, 2009; Pattan and Pearce, 2009) and have been restricted to either shallow sea or deep-sea regions. In this paper, we present data on surficial sediments from both shallow and deep-sea regions of the eastern Arabian Sea (Fig. 1) using a multi-proxy approach (rock magnetism, geochemistry, sedimentology and clay mineralogy). Utilising the data that we obtained and those available in the literature for the eastern

Arabian Sea (Prakash Babu et al., 1999; Alagarsamy, 2009; Pattan and Pearce, 2009), we attempted to identify the sources of sediments based on the spatial distribution patterns of magnetic minerals (Karbassi and Shankar, 1994; Alagarsamy, 2009), clay minerals (Sirocko and Lange, 1991), organic carbon, calcium carbonate and geochemical parameters (Schnetger et al., 2000; Sirocko et al., 2000). 2. Area of study The Arabian Sea is surrounded by arid land masses like Africa, the Arabian Peninsula, and the Iran-Makran-Thar desert region to the west and north, and by the coastal highlands of western India to the east (Fig. 1). The area is geographically surrounded by the Owen Ridge (Murray Ridge) on the western side (north-western side), and the Carlsberg Ridge (Chagos-Laccadive Ridge) on the southern side (eastern side). The climate of the Arabian Sea is dominated by seasonal reversal of winds, resulting in large seasonal physical/hydrographic/biological/chemical variations in water column and sediment transport (Nair et al., 1989). The Arabian Sea is strongly influenced by the southwest monsoon (JuneeSeptember) and moderately by the northeast monsoon (DecembereFebruary) and the associated reversal of surface currents. It is well known for upwelling, which is coupled with the intensity of the southwest (SW) monsoon (Naidu, 1998). Summer upwelling along the coasts of Somalia, Arabia and India occurs during the SW monsoon period, leading to high primary productivity. Surface winds during the SW monsoon blow from the SW, which lead to an increase in continental humidity and precipitation over the Indian Peninsula. The study area is located in the eastern part of the Arabian Basin and the western side of the ChagoseLaccadive Ridge. The major rivers draining into the Arabian Sea are the Sindhu, the Narmada

Figure 1. Map showing the locations of sediment samples analysed in this study (red circle) from the eastern Arabian Sea. Also shown are locations of samples used by Prakash Babu (1999; blue triangle) and Pattan and Pearce (2009; yellow star). Tectonic features shown in the map are from Campanile et al. (2007). The bathymetric contours were obtained using GEBCO v 2.0 (General Bathymetric Chart of the Oceans, 2008). Contouring was done using ArcGIS.

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and the Tapti. The Sindhu is the oldest river in the Himalayan region, with the location of its upper reaches remaining stationary within the Indus Suture Zone since the early Eocene (>54.6 Ma; Clift, 2002). It is also the largest source of sediments to the Arabian Sea; its annual sediment discharge has been estimated at 300e675 million tons (Milliman et al., 1984). The Narmada and the Tapti rivers together contribute w60 million tonnes of suspended matter every year (Borole et al., 1982), most of which is distributed on the inner continental shelf of India. The sediments derived from the western Ghat River are less compared to those sourced from the Sindhu and the Narmada-Tapti rivers. Hence, the sedimentation rate in the northern and the western parts of the Arabian Sea is relatively high compared to the southeastern part (Manjunatha and Shankar, 1992). Clay mineral studies of the Arabian Sea sediments suggest that illite (>40%) is the most dominant clay mineral that is principally derived from the Sindhu River (Kolla et al., 1981a; Borole et al., 1982). Smectite-rich sediments derived from the Deccan Trap terrain are locally dominant near the western continental shelf of India (Ramaswamy et al., 1991). Gravity cores were collected on board ORV Sagar Kanya from 2 shallow- and 13 deep-water locations from the eastern Arabian Sea (Table 1; Fig. 1). The top 3 cm of the sediment core were used for this investigation. The sediment samples were labelled and stored in deep-freeze. They were safely transported to the laboratory for further analyses.

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3. Materials and methods 3.1. Environmental magnetic measurements Standard techniques (Walden et al., 1999) were used for rock magnetic measurements at the Environmental Magnetism Laboratory, Mangalore University, India. Approximately 12 g each of bulk samples were tightly packed in 8 cm3 cylindrical, non-magnetic plastic bottles. After magnetic measurements, the samples were oven-dried at w35  C; the dry weight of the samples was used for computing the magnetic parameters values on a mass-specific basis. Magnetic susceptibility was determined at low- and highfrequencies (0.47 and 4.7 kHz; clf and chf) on a Bartington Susceptibility Meter (Model MS2B) with a dual-frequency sensor. The sensor was calibrated using the standard (1% Fe3O4) provided by the manufacturer. From clf and chf measurements, the frequencydependent component of susceptibility (cfd) was calculated. Anhysteretic remanent magnetisation (ARM) was induced in the samples by steadily ramping down a mains frequency alternating field (AF) of 100 mT while the sample was subjected to a steady field of 0.04 mT. An AF demagnetiser and an ARM attachment (both of Molspin make) were used for this purpose. The ARM thus grown was measured on a Molspin spinner fluxgate magnetometer which was calibrated using the calibration sample (a strip of magnetic tape embedded in a wooden cylinder) provided by the manufacturer. The

Table 1 Locations of samples from the eastern Arabian Sea analyzed in this investigation and those analyzed by Prakash Babu et al. (1999) and Pattan and Pearce (2009) along with the parameters analyzed. Sl. No.

Sampling stations (sampling year)

Water depth (m)

Samples analyzed in this study 1 *SK-268/01(2010) 215 2 *SK-257/02 (2009) 1689 3 SK-268/02 (2010) 1850 4 SK-257/01 (2009) 1978 5 SK-257/05 (2009) 1990 6 SK-257/03 (2009) 2270 7 SK-266/01 (2010) 2500 8 SK-221/05 (2005) 2700 9 SK-257/04 (2009) 3200 10 SK-266/01 (2010) 3200 11 SK-257/10 (2009) 3467 12 SK-257/08 (2009) 3839 13 SK-257/09 (2009) 4082 14 SK-257/06 (2009) 4108 15 SK-257/07 (2009) 4239 Samples investigated by Prakash Babu et al. (1999) 16 *RVG-17/372 40 17 *RVG-18/36 75 18 *RVG-18/6 121 19 *RVG-191/5 275 20 *RVG-100/2323 700 21 *RVG-191/6 975 22 *RVG-190/23 1100 23 *SK-44/5 1217 24 *RVG-163/3809 1570 25 *RVG-191/13 1600 26 *SK-44/6 1610 27 *RVG-100/2317 1630 28 *RVG-100/2315 1680 29 RVG-100/2324 1950 30 RVG-100/2314 1960 31 RVG-191/2 2050 32 RVG-163/3808 2118 33 SK-44/7 2297 34 RVG-163/3810 2590 35 SK-44/9 3679 Sample investigated by Pattan and Pearce (2009) 36 SK-129/05 2300 *indicates shallow water stations (<1700 m).

Latitude ( N)

Longitude ( E)

Parameters analyzed

15.995 12.984 15.995 15 14.002 13.998 12 9.009 14.999 11.067 15 14 14 13 13

72.624 70.492 72 71.554 71.997 70.998 70 72.086 70.516 69.3 70 70 69 69 70

Magnetic parameters (clf, chf, cfd and cARM), organic carbon, CaCO3, particle size, clay mineralogy and major elements (Al, Fe, Mn, K, Mg, Cu and Ni)

15.83 15.17 15.86 15.07 13.53 14.85 13.9 15.87 12.24 14.17 15.87 13.54 13.7 13.6 13.73 14.78 11.5 15.86 10 15.87

73.38 73.37 72.79 72.93 72.65 72.71 72.64 72.39 72.26 71.72 71.93 71.94 72.38 72.98 72.7 72.01 71.76 71.41 71.75 70.01

Organic carbon, CaCO3 and Al

9.35

71.983

Organic carbon, CaCO3 and Al

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susceptibility of ARM (cARM) was calculated by dividing the massspecific ARM by the size of the biasing field (0.04 mT ¼ 31.84 A m1; Walden, 1999). Isothermal remanent magnetisation (IRM) was induced in the samples at different field strengths (20, 60, 100, 300, 500 and 1000 mT) using a Molspin pulse magnetiser. IRM at 1000 mT, the maximum field attainable in the lab, was considered as saturation isothermal remanent magnetization (SIRM). The remanence acquired was measured using the Molspin spinner fluxgate magnetometer. “Hard” IRM and inter-parametric ratios like S-ratio, cARM/clf, cARM/SIRM and SIRM/clf were calculated to determine the magnetic mineralogy and grain size (Walden et al., 1999). The details of all the magnetic parameters determined, their units, interpretation and the instruments used are given in Table 2. 3.2. Geochemical analysis For inorganic geochemical analysis, the samples were digested as per standard methods (Johnson and Maxwell, 1981). About 30 mg of finely powdered sample was oven-dried at 110  C and digested completely with 48% v/v HF, conc. HNO3 and conc. HClO4 (all Supra-pure grade) in the ratio of 6:3:1, in pre-weighed Teflon beakers on a hotplate. The crystalline paste formed was dissolved in 10 mL 1:1 HNO3. The solution was made up to 50 mL volume with deionised water from a Millipore unit. Aluminum, Fe, K, Mg, Mn, Ni and Cu were analysed using an atomic absorption spectrophotometer (ThermoFisher Scientific, M Series; Angino and Billings, 1972). Continuous calibration with standard reference material (SRM 2702; NIST) and blank solutions was ensured during measurements (Govindaraju, 1989; Loring and Rantala, 1992). Precision of the analyses was checked by triplicate analysis with respect to SRM 2702 for major and minor elements. Calculated coefficients of variation for the different elements were within w 1e5%. To determine the organic carbon (OC) content, 1 g of finely powdered samples was treated with 2M HCl to remove the

inorganic fraction. The OC content was estimated using a total organic carbon analyzer (Shimadzu TOC-V Series, Model SSM5000A). To measure the total inorganic carbon (CaCO3) content, w100 mg of the untreated, powdered sample was placed in ceramic sample boats and 0.5 mL of 85% phosphoric acid added to remove the organic fraction. The total inorganic carbon content was measured using a TOC analyzer (Schumacher, 2002). The accuracy of organic carbon and carbonate determinations was better than  5%. 3.3. Particle size and clay mineral analyses Particle size analysis was carried out on 5 g samples after removing organic matter and carbonate materials by treating the samples with H2O2 and 20% acetic acid. The sample was wet-sieved through an ASTM test sieve (No. 230; 62 mm pore diameter) to separate the sand fraction. The silt þ clay (<62 mm) fraction was transferred to a 1000-mL measuring cylinder and 1 g of sodium hexametaphosphate (Calgon) added to prevent flocculation. The silt and clay fractions were determined by pipette analysis (Carver, 1971). The sample was stirred and, according to Stokes’ law, silt and clay (<2 mm) fractions were withdrawn from the measuring cylinder using a 20-mL pipette. The 20-mL sample solutions were transferred to pre-weighed beakers and dried in an oven at 100  C. Once dried, the weight was noted down. The weight of the silt and clay fractions obtained were multiplied by 50 and a value of 1 (Calgon correction factor) was subtracted from it to account for the weight of dispersant (sodium hexametaphosphate). The final values are expressed as weight percentage. For clay mineralogy, the clay fraction (<2 mm; 6h 24 min after stirring and pipetting 20 mL for estimation of clay concentration) up to 5 cm depth was withdrawn from the remaining suspended clay solution. About 10e30 mL of 50% MgCl2 solution was added to the clay suspension in order to charge the clay minerals and make them sink by flocculation/agglomeration. Subsequently, excess ions

Table 2 Magnetic measurements, their units, interpretation and instrumentation (after Thompson and Oldfield, 1986; Oldfield, 1991). Magnetic measurements

Units

Interpretation

Instruments used

Low- and high-frequency susceptibility (clf and chf) Frequency-dependent susceptibility (cfd)

10-8 m3 kg-1

Bartington susceptibility meter

Susceptibility of Anhysteretic Remanent Magnetization (cARM)

10-5 m3 kg-1

Isothermal Remanent Magnetisation (IRM) and Saturation Isothermal Remanent Magnetisation (SIRM) cARM/clf

10-5 A m2 kg-1

Proportional to the concentration of magnetic minerals Proportional to the concentration of superparamagnetic grains Proportional to the concentration of magnetic minerals of stable single domain size range Proportional to the concentration of remanence-carrying magnetic minerals

10-8 m3 kg-1

cARM/cfd

cARM/SIRM

10-5 mA-1

SIRM/clf

103 Am-1

S-ratio ¼ IRM300mT/SIRM

HIRM ¼ SIRM e IRM300mT

10-5 A m2 kg-1

Indicative of magnetic grain size. A higher ratio indicates a finer grain size. Also indicates the presence of bacterial magnetite if the ratio is > 40. Indicative of magnetic grain size. A higher ratio indicates a finer grain size. Also indicates the presence of bacterial magnetite if the ratio is > 1000. Indicative of magnetic grain size. A higher ratio suggests a finer grain size. Indicative of magnetic grain size. A higher ratio suggests a coarser grain size. Also indicates the presence of greigite if the ratio is > 30. Relative proportions of ferrimagnetic and anti-ferromagnetic minerals (high ratio ¼ a relatively high proportion of ferrimagnets). Proportional to the concentration of antiferromagnetic minerals like hematite, goethite etc.

Susceptibility meter with a dual-frequency sensor AF-demagnetiser with an ARM attachment and fluxgate magnetometer Pulse magnetizer and fluxgate magnetometer

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were removed by double centrifugation with de-ionised water, and the samples dried at 60  C (Ehrmann et al., 1992; Petschick et al., 1996). Oriented slides were prepared by pipetting 1 mL each of the concentrated clay suspensions on to glass slides, ensuring uniform distribution and avoiding size sorting (Rao and Rao, 1995; Thamban et al., 2002). After air-drying, the slides were used for XRD analysis on a Rigaku-Ultima IV X-ray diffractometer. The slides were scanned from 3 to 30 2Ө at 1.5 2 Ө/min, using nickel-filtered Cu Ka radiation (Das et al., 2013). 4. Results and discussion 4.1. Environmental magnetic and sedimentological data of shallow and deep-sea sediments The environmental magnetic parameters for both shallow and deep-sea sediment samples are plotted with respect to depth in

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Fig. 2. Magnetic susceptibility (clf) is a concentration-dependent parameter and represents the sum of magnetic signals from ferrimagnetic and canted antiferromagnetic minerals. It may also be influenced by the presence of diamagnetic and paramagnetic minerals (Thompson and Oldfield, 1986). The shallow water sediment samples have a lower concentration of magnetic minerals as evident by the low clf values (average ¼ 27.15  108 m3 kg1; Fig. 2) whereas the deep-sea ones register high clf values (average ¼ 47.57  108 m3 kg1; Fig. 2), indicating a relatively high concentration of magnetic minerals. Frequency-dependent susceptibility (cfd) is an indicator of the proportion of ultrafine-grained superparamagnetic (SP) grains (Dearing, 1999a). Values <2% suggest that the samples contain no SP grains whereas those in the range of 2e10% indicate a mixture of SP and coarse magnetic minerals. The cfd values for the shallow and deep-sea samples are low (average ¼ 4.39%), suggesting that sediments from both the depth regimes contain an admixture of SP

Figure 2. Rock magnetic and sedimentological data for surficial sediment samples from different water depths in the eastern Arabian Sea.

258 Table 3 Correlation matrix of rock magnetic, geochemical, clay mineralogical and particle size data along with water depth for deep-water sediment samples (n ¼ 13, ‘r’ values > 0.4 are statistically significant at 95% confidence level). Depth cARM SIRM clf

cARM

1.00 0.35 0.29 0.17 0.30 0.46 0.41 0.44 0.19 e0.07 0.36 0.54 e0.14 e0.38 0.15 0.26 e0.21 e0.10 e0.54 e0.73 e0.12 e0.58 e0.41 e0.02 0.05 0.63 e0.09

1.00 0.99 e0.56 e0.71 0.98 e0.63 e0.04 e0.28 e0.02 0.96 0.66 e0.04 0.03 e0.06 0.06 0.24 0.19 0.33 0.06 e0.61 0.15 e0.23 e0.64 0.64 e0.17 e0.44

1.00 e0.55 e0.74 0.98 e0.69 e0.15 e0.37 e0.05 0.96 0.67 e0.07 0.07 e0.10 0.09 0.25 0.21 0.39 0.13 e0.59 0.23 e0.22 e0.68 0.68 e0.26 e0.49

1.00 0.88 e0.51 0.77 e0.04 e0.26 0.11 e0.56 e0.53 e0.33 e0.31 0.45 0.07 e0.54 e0.09 e0.46 e0.20 0.46 e0.25 0.42 0.71 e0.73 0.34 0.37

1.00 e0.65 0.96 0.24 0.19 0.00 e0.68 e0.46 e0.19 e0.31 0.34 0.06 e0.51 e0.21 e0.69 e0.46 0.45 e0.51 0.24 0.75 e0.75 0.59 0.41

Statistically significant “r” values shown in bold font. Smectite; b Illite; c Kaolinate; d Chlorite.

a

1.00 e0.58 e0.06 e0.30 e0.04 0.95 0.75 e0.09 0.01 e0.10 0.14 0.21 0.19 0.28 e0.01 e0.54 0.12 e0.33 e0.66 0.67 e0.15 e0.46

1.00 0.51 0.40 0.05 e0.61 e0.41 e0.09 e0.36 0.38 0.01 e0.49 e0.25 e0.78 e0.60 0.34 e0.66 0.18 0.77 e0.77 0.74 0.47

1.00 0.80 0.21 e0.10 e0.09 0.26 e0.29 0.30 e0.22 e0.11 e0.22 e0.55 e0.63 e0.19 e0.73 e0.08 0.41 e0.39 0.74 0.40

1.00 e0.07 e0.26 e0.02 0.47 e0.12 e0.01 e0.23 0.12 e0.29 e0.43 e0.47 e0.09 e0.57 e0.19 0.21 e0.19 0.61 0.20

1.00 e0.30 e0.40 e0.28 e0.10 0.43 e0.10 e0.15 0.34 0.02 e0.04 0.23 e0.16 e0.07 0.43 e0.41 0.06 0.44

1.00 0.75 0.04 0.06 e0.19 0.10 0.26 0.08 0.31 0.07 e0.65 0.18 e0.20 e0.73 0.73 e0.18 e0.55

1.00 0.02 0.06 e0.48 0.36 0.16 0.00 0.03 e0.25 e0.32 0.04 e0.67 e0.79 0.81 e0.08 e0.54

a

Smc (%) bIll (%) cKao (%) dChl (%) Fe/Al Mn/Al Mg/Al K/Al

1.00 e0.36 0.02 e0.45 0.73 e0.27 0.32 0.36 e0.09 0.10 0.29 e0.31 0.28 0.29 e0.44

1.00 e0.39 e0.18 e0.01 e0.20 0.08 0.08 e0.43 0.04 e0.24 e0.18 0.19 e0.43 0.06

1.00 e0.61 e0.10 e0.32 e0.19 e0.02 e0.03 e0.25 0.46 0.58 e0.60 0.50 0.34

1.00 e0.42 0.63 e0.12 e0.30 0.43 0.12 e0.43 e0.16 0.19 e0.32 e0.04

Ni/Al Cu/Al Sand Silt (%) Clay (%) (%)

OC

1.00 e0.15 1.00 0.73 0.27 1.00 0.60 0.19 0.88 1.00 e0.14 0.31 0.03 0.05 1.00 0.37 0.32 0.82 0.82 0.26 1.00 0.06 e0.03 0.17 0.51 e0.07 0.17 1.00 e0.57 e0.09 e0.58 e0.35 0.33 e0.48 0.35 1.00 0.55 0.09 0.55 0.30 e0.32 0.45 e0.41 e1.00 1.00 e0.11 e0.45 e0.68 e0.61 e0.13 e0.83 0.08 0.43 e0.42 1.00 e0.56 0.02 e0.42 e0.35 0.34 e0.23 0.00 0.79 e0.77 0.08

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e0.66 e0.24 clf e0.18 cfd (%) e0.35 cARM /SIRM e0.37 IRM20mT e0.25 cARM / clf e0.47 SIRM/ clf e0.46 cARM / cfd e0.10 S-Ratio e0.02 HIRM e0.23 Al e0.06 a Smc (%) 0.02 b Ill (%) 0.33 c Kao (%) e0.30 d Chl (%) 0.00 Fe/Al 0.22 Mn/Al 0.19 Mg/Al 0.46 K/Al 0.50 Ni/Al 0.35 Cu/Al 0.67 Sand (%) e0.17 Silt (%) e0.20 Clay (%) 0.21 OC e0.69 CaCO3 0.17 SIRM

cfd (%) cARM /SIRM IRM20mT cARM / clf SIRM/ clf cARM / cfd S-Ratio HIRM Al

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and coarse magnetic grains. It exhibits a negative correlation with clf (r ¼ e0.55; n ¼ 13; Table 3), suggesting that the SP grains do not control the clf signal. Susceptibility of ARM (cARM) is a concentration-dependent parameter and is biased towards the stable single domain (SSD) magnetic grains (Thompson and Oldfield, 1986). It exhibits a positive correlation with clf (r ¼ 0.29; n ¼ 13; Table 3) for deep-water samples, indicating that SSD grains make a partial contribution to the clf signal. Isothermal remanent magnetization at 20 mT (IRM20mT) is proportional to the concentration of multi-domain (MD) grains of ferrimagnetic minerals (Walden et al., 1999; Xie et al., 1999). High values of IRM20mT indicate that the samples are predominantly made up of MD ferrimagnetic grains. A significant correlation is documented between IRM20mT and clf (r ¼ 0.98; n ¼ 13; Table 3), suggesting that the clf signal in deep-water samples is controlled mainly by ferrimagnetic minerals of MD grain-size. This interpretation is further supported by magnetic grainsize indicators such as cARM/SIRM and cARM/clf (Walden et al., 1999). High (low) values of these ratios indicate a fine (coarse) magnetic grain size (Oldfield, 1991). The ratio values are high for the shallow water sediments, indicating a fine magnetic grain size but low for the deep-water samples (Fig. 2). The ratios, cARM/SIRM and cARM/clf, exhibit a statistically significant negative correlation with clf (r ¼ e0.74 and e0.69 respectively; n ¼ 13), indicating that coarse magnetic grains control the clf signal in the deep-water samples. Rock magnetic data such as S-ratio, ‘hard’ isothermal remanent magnetization (HIRM) and IRM acquisition curves are used to identify the magnetic mineralogy of natural samples (Walden et al., 1999). S-ratio indicates the relative proportions of low-coercivity and high-coercivity magnetic minerals. A value close to unity suggests a high proportion of ferrimagnetic ‘soft’ minerals such as magnetite and titanomagnetite, and low values a high proportion of canted antiferromagnetic ‘hard’ minerals like hematite (also reflected by high HIRM values). In this study, the magnetic mineralogy of both shallow and deep-water samples is dominated by ferrimagnetic ‘soft’ minerals as evident by the high S-ratio values (Fig. 2) and IRM acquisition curves (Fig. 3a). However, more sophisticated techniques such as Curie Balance and X-ray diffractometry (XRD) are warranted to confirm the type of magnetic minerals present (Liu et al., 2007). Magnetic minerals present in sediments may be terrigenous, i.e., derived from the weathering and erosion of continental materials. However, the magnetic signal of terrigenous sediments may be influenced by the presence of bacterial magnetite,

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authigenic minerals like greigite, and dissolution of magnetic minerals (Maher and Thompson, 1999). Magnetotactic bacteria (e.g. Magnetospirillum spp.) are aquatic organisms that produce small magnetite crystals, generally in a narrow, single domain size range, within their body that help them navigate along geomagnetic field lines in search of anaerobic conditions (Bazylinski and Williams, 2007). The presence of bacterial magnetite has been reported from many marine (Oldfield et al., 2003), lacustrine (Pan et al., 2005), estuarine (Oldfield et al., 1989) and soil (Fassbinder and Stanjek, 1993) environments. Bacterial magnetite is said to be present in the sample (Fig. 3b) if the inter-parametric ratio cARM/clf is > 40, cARM/cfd > 1000 (Dearing, 1999b) and cARM/ SIRM > 200  105 m A-1 (Walden et al., 1999). A solitary shallow water sample plots close to the envelope for bacterial magnetite, suggesting that the magnetic mineralogy is influenced by bacterial magnetite. Bacterial magnetite does not seem to influence the magnetic signal of the deep-water sediments as their cARM/clf and cARM/cfd values are less than the threshold values of 40 and 1000 respectively (Fig. 3b). The deep-water samples exhibit increasing trends of clf, IRM20mT and SIRM accompanied by decreasing values of cARM/SIRM and cARM/clf, indicating an increase in the concentration of magnetic minerals and in the magnetic grain size. According to Rao and Wagle (1997), titanomagnetite is the dominant magnetic mineral present in deep-water sediments; it is one of the weathering products brought to the Arabian Sea by the Sindhu and the Narmada-Tapti rivers. Our S-ratio data also suggest the presence of magnetically ‘soft’ minerals like magnetite and titanomagnetite (Fig. 2). Semi-quantitative analysis like SEM-EDS and low-temperature magnetic measurements would further help in establishing the magnetic mineralogy. Sand content is high in the shallow-water samples (w36%; Fig. 2) and decreases to an average of 0.36% for deep-water samples. Correspondingly, the silt and clay percentages are high for the deep-water samples. Magnetic susceptibility exhibits a negative correlation with sand and silt (r ¼ e0.22 and e0.68 respectively; n ¼ 13; Table 3) for deep-water sediments. A significant positive correlation is documented between clf and clay (r ¼ 0.68; n ¼ 13; Table 3), indicating that the magnetic minerals reside in the clay fraction of the deep-water sediments. Sand values are expected to be high in shallow-water sediments (Aprile and Bouvy, 2008) and vice versa. This is because rivers transporting sediments from the hinterland deposit coarse particles near the coast, whereas fine ones are transported farther into the ocean due to their fine size.

Figure 3. (a) IRM acquisition curves for shallow- and deep-water sediment samples; and (b) Bi-logarithmic plot of cARM/clf vs. cARM/cfd (Oldfield, 1994) for the shallow-water and deep-water sediment samples. Note: One sample is influenced by the presence of bacterial magnetite; the rest are catchment-derived.

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4.2. Geochemistry and clay mineralogy of shallow and deep-sea sediments In order to take care of the varying proportion of the clay fraction in the samples, elemental data were normalized to aluminium (Al), as it is one of the ideal indicators of terrigenous material (Engstrom and Wright, 1984; Brown et al., 2000). A statistical analysis (correlation coefficient matrix) of the entire data set was performed to examine the first-order relationships and controls on the chemical composition of deep-sea sediments of the eastern Arabian Sea (Table 3). As there are only two shallow water sediment samples, they were not used for the correlation analysis. The metal/ Al ratio can be effectively used to understand paleoenvironment, which includes weathering of rocks in the catchment, transport processes as well as diagenesis. Elemental ratios and geochemical parameters have generally been used as paleoenvironmental proxies in marine sediments (Zabel et al., 2001). The geochemical results indicate a higher Mn/Al ratio for deepwater sediments (259  10e4 to 730.96  10e4; Fig. 4a) when compared with shallow water sediments (133  10e4to 1051  10e4; Fig. 4a). High Mn/Al ratio values (more than the average shale value of 100  10e4; Wedephol, 1971) in the shelf, shallow and deep-sea sediments indicate the presence of oxygenated waters. This may be due to dissolution of Mn in the Mnreduction zone, its upward diffusion and precipitation as Mn dioxide (Klinkhammer et al., 1982). However, Prakash Babu (1999) reported a low Mn/Al value (100  10-4) for shallow water sediments from the oxygen minimum zone (OMZ) compared to

oxygenated zone. The Cu/Al ratio value varies from 10.49  10e4 to 26.5  10e4 in the deep-water sediments but low in the shallowwater sediment samples (11.06  10e4 to 15  10e4; Fig. 4b). The Cu/Al values in marine sediments are significantly influenced by the lithogenic characteristics of weathered and eroded rocks, as these values, in general, are above the average shale value (Cu/Al: 4.4  10e4; Wedephol, 1971). The ratio values for Fe/Al and Mg/Al are similar except the solitary shallow water sediment sample (water depth ¼ 215 m). The similarity in their spatial distribution suggests that both Fe and Mg are derived from terrigenous sources. The shallow water sample, located close to the coast, shows anomalously high values for both Fe/Al and Mg/Al. It is well known that near river mouths, part of Fe may be precipitated as iron oxide/ hydroxide in the water column and deposited (Slomp et al., 1997). Clay-mineral assemblages have been distinguished in the deep Arabian Sea sediments; they vary significantly depending on the source and intensity of weathering (Nair et al., 1982; Rao and Rao, 1995). Illite-smectite and chlorite-smectite were identified as mixed layer clay minerals in most of the continental slope sediments (Rao and Rao, 1995). Clay mineralogical data obtained in this study indicate that illite (24e48.5%), chlorite (14.1e34.9%), smectite (10.6e28.7%) and kaolinite (11.9e27.5%; Fig. 5) dominate the sediments from both shallow and deep-sea regions. These may have been sourced from different areas and transported by fluvial or aeolian agents (wind-blown dust from the adjacent deserts). Earlier studies by Kolla et al. (1981a), Debrabant et al. (1991) and Sirocko and Lange (1991) suggested that the clay minerals in surficial sediments consist of: (a) illite and chlorite derived from the

Figure 4. Spatial distribution patterns of Mn/Al (a), Cu/Al (b), Ni/Al (c), K/Al (d), Mg/Al (e) and Fe/Al (f) for sediment samples from the eastern Arabian Sea. The bathymetric contours were derived using GEBCO v 2.0 (General Bathymetric Chart of the Oceans, 2008).

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the west-flowing rivers of southern India, the Sindhu (Rao and Rao, 1995) and the Narmada-Tapti rivers (Shankar et al., 1987). Values of Fe/Al and Mg/Al exhibit significant negative correlations with cfd, whereas Mn/Al shows a significant positive correlation. Ni/Al ratio is negatively correlated (at the 1% level) with clf (Table 3). Magnetic susceptibility is a concentration-dependent parameter and indicative of the sum total of magnetic minerals present (ferrimagnetic, antiferromagnetic, paramagnetic and diamagnetic; Thompson and Oldfield, 1986). Fig. 6 shows the scatter plots for a few geochemical parameters that show a good correlation with magnetic susceptibility: clf vs. Fe (r ¼ 0.62) and Al (r ¼ 0.65) with a statistical significance of 95%. Magnetic susceptibility is negatively correlated with organic carbon (r ¼ e0.36) and CaCO3 (%) (r ¼ e0.49) with a statistical significance of <95%. All these data indicate that the magnetic minerals are derived from terrigenous sources. 4.4. Distribution patterns of organic carbon and carbonate contents in shallow and deep-sea sediments Marine sediments are the ultimate sink for organic carbon (OC) because > 90% of the total organic carbon deposited in the oceans gets incorporated in the sediments of continental margins (Hedges and Keil, 1995). The burial of OC in marine sediments is closely linked to the biogeochemical cycling of elements like nitrogen, sulphur, phosphorus, iron and manganese (Berner, 1982; Hartnett et al., 1998). However, OC concentration in marine sediments is controlled by primary productivity (Calvert and Pedersen, 1992), OC flux through the water column (Jahnke, 1990), rate of sedimentation (Gupta et al., 2005; Narayana et al., 2009) and oxygen availability in the depositional environment (Paropkari et al., 1992). The analytical data of OC and CaCO3 (totally 36 surficial sediment Figure 5. Spatial distribution patterns of (a) Smectite, (b) Illite, (c) Kaolinite, and (d) Chlorite for surficial sediments from the eastern Arabian Sea. The bathymetric contours were derived using GEBCO v 2.0 (General Bathymetric Chart of the Oceans, 2008).

Himalayan complex via the Sindhu River and wind-blown dust from the Iran-Makran region delivered via the Gulf of Oman by northwesterly winds; (b) smectite derived from the Deccan Traps of India; and (c) kaolinite derived from the tropical soils of southern India. Sediments of the northern Arabian Sea are rich in chlorite and illite, which may have been contributed by the Sindhu River (Gupta and Hashimi, 1985). They also contain a significant amount of wind-borne dust from the adjacent deserts. The clay-mineral assemblage in these dust plumes consists of illite, smectite, kaolinite, chlorite and palygorskite, in the decreasing order of abundance. The correlation of potassium with illite is significantly high when compared with smectite (Table 3). It is known that potassium in clay-rich sediments is primarily found in clay minerals (mainly illite), K-feldspar and mica. Smectite is a product of chemical weathering of the Deccan basalts; it traps Kþ and thereby contributes to its limited mobility (Das and Krishnaswami, 2006). Smectite-rich sediments off southwestern India are contributed by small rivers. As in situ weathering of basalts from mid-oceanic ridges does not contribute smectite significantly (Sirocko and Lange, 1991), we surmise that smectite is principally derived from the Deccan Trap terrain through the Narmada-Tapti rivers.

4.3. Relationship between environmental magnetic and geochemical parameters Magnetic susceptibility exhibits a modest correlation with Al (r ¼ 0.67; n ¼ 13; Table 3), indicating that the deep-sea sediments contain a significant amount of terrigenous fraction contributed by

Figure 6. Scatter plots of geochemical data of sediment samples from the eastern Arabian Sea. Note the positive correlation between the pairs Fe-clf, Al-clf, and Al-Fe, and the negative correlation between the pairs CaCO3-clf, and organic carbon-clf.

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samples) from shallow and deep-water regions of the eastern Arabian Sea were used to understand their distributional patterns in the different sedimentary domains of the OMZ (Table 1; Fig. 7). The plot of OC vs. water-depth of sampling locations shows a low OC concentration (<1%) in shelf regions (up to 150 m water depth) except for a solitary sample at 40 m water depth, which has a higher OC (2.9%). The deep-water (>1500 m) sediments document a low concentration of OC (<1%; Fig. 7a). However, slope regions (150e1500 m) show higher OC concentrations (2e5%). This observation shows that OC burial in the eastern Arabian Sea is mainly controlled by bottom water oxygen concentration, the rate of sedimentation as well as primary productivity in the surface waters due to upwelling and/or the high degree of preservation in sea-floor sediments. Hence, in the continental shelf and deep basin (>1500 m water depth), some oxidation of organic carbon takes place before its burial and preservation in sediments. Our data further substantiate the results obtained by previous workers (Kolla et al., 1981b; Paropkari et al., 1992). A high OC concentration was documented in the continental slope region (150e1500 m water-depth) where bottom water oxygen content is remarkably low and the sedimentation rate high (Fig. 7a). This has previously been shown by several studies (Qasim, 1977; Paropkari et al., 1992; Calvert et al., 1995). The OC concentration is low in shallow water regions due to the well oxygenated nature of waters and the strong influence of water currents and waves (Agnihotri and Kurian, 2008). However, organic carbon preservation in deep-water sediments from the eastern Arabian Basin is inhibited by the increased bottom-water oxygen concentration and low sedimentation rate. Sediments have similar contents of Al and OC in the oxygen minimum zone (OMZ); the low OC (or high Al) content in the continental shelf sediments may be because of high detrital input. This is shown by the higher negative ‘r’ value for the shallow water sediments. To identify the source and the factors controlling the OC variation, the data were correlated with Al content. There is no significant correlation between the Al and OC contents for all the sediments (r ¼ e0.10, n ¼ 36) as well as for shallow water sediments (r ¼ e0.29, n ¼ 15) and deep-sea sediments (r ¼ e0.41, n ¼ 21). The results indicate mixed sources of OC: primary productivity and terrigenous input.

Figure 7. Depth-wise distribution of organic carbon and CaCO3 (this study and modified from Prakash Babu et al., 1999 and Pattan and Pearce, 2009) for sediment samples from the eastern Arabian Sea. Note that water depth data on the x-axis are not to scale.

The CaCO3 concentration of surficial sediments from the eastern Arabian Sea varies from 4.4 to 90.6% (Fig. 7b). Sediments from the continental shelf and slope (<1800 m) up to the Chagos-Laccadive Ridge show high concentrations (average ¼ 58%) when compared to the values for deep-water sediments (average ¼ 44%). The maximum average carbonate concentration of 66% is recorded by slope sediments (water-depths of 150e1500 m). The carbonate content increases gradually from the continental shelf to the slope region, reflecting the waning terrigenous influx. It is high on the Chagos-Laccadive Ridge although the Ridge is located far away from significant terrigenous influences; this reflects the bathymetric control of carbonate distribution. The high carbonate band in the outer shelf is abruptly interrupted by low CaCO3 values off the Sindhu River (<30%). These low-carbonate sediments appear to extend to deep-sea areas (2000e3000 m water depth) southward and are confined to the easternmost Indus Fan valley (Kolla et al., 1981b). The high concentrations of lithogenic elements corroborate the enhanced supply of terrigenous material that dilutes the contents of CaCO3 and biogenic elements. To identify the source and the factors controlling CaCO3 distribution, depth-wise carbonate data were correlated with the Al content. Shallow and deepsea sediments together exhibit a negative correlation between Al and CaCO3 (r ¼ e0.74, n ¼ 36); the corresponding negative correlations are e0.70 (n ¼ 15) for shallow sea sediments and e0.55 (n ¼ 21) for deep-sea sediments. The negative correlation between CaCO3 and Al indicates that terrestrial input dilutes the CaCO3 content. An earlier study by Kolla et al. (1981b) suggested that the low-carbonate band confined to the easternmost Indus Fan valley may be partially due to the high bottom water turbidity and deposition of sediments, derived from the Sindhu River and from the western region of Peninsular India, by deep turbid layer flows.

5. Conclusions The following inferences were made from the environmental magnetic, geochemical (inorganic and organic), particle size and clay mineralogical studies of surficial sediment samples from the eastern Arabian Sea:  The high cfd in the deep-sea sediments of the eastern Arabian Basin indicates a high concentration of superparamagnetic grains when compared to the sediments from the shelf/ shallow-water depth. The strong positive correlation exhibited by clf, IRM and ARM with Al indicates that the deep-water surficial sediments are influenced by terrigenous input. There is a strong correlation between clf and IRM20mT of the eastern deep Arabian Basin sediments when compared to the coastal/ continental shelf sediments. This indicates that the high clf value of the deep Arabian Basin sediments is due to the presence of a high concentration of ferrimagnetic minerals derived from the Peninsular Indian rivers.  The geochemical results indicate that the Mn/Al ratio is high in the lower slope and deep-sea sediments of the eastern Arabian Sea when compared to sediments in contact with oxygenated waters. Higher values of Cu/Al in the deep-water sediments suggest contributions from lithogenic sources. The spatial variations of Fe/Al and Mg/Al are similar, indicating a detrital origin for both Fe and Mg.  The clay mineralogical study suggests that the illite and chlorite-rich assemblage is mainly derived from the Sindhu River and the Thar Desert whereas smectite and kaolinite with minor illite are derived mostly from the weathering of the Deccan Trap basalts and the Gneissic province of the western Ghats.

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 The organic carbon (OC) content is low in the shallow/shelf (well oxygenated water conditions) and deep basins (increased bottom-water oxygen concentration and low sedimentation rate). A high OC content is recorded in the OMZ, where the bottom water oxygen content is exceptionally low and the sedimentation rate high. The results indicate that the sources of organic carbon in these regions are biogenic productivity and riverine input.  The shelf and Chagos-Laccadive Ridge sediments are characterized by a high CaCO3 content when compared to deep-water sediments. The strong negative correlation between CaCO3 and Al suggests that terrestrial sediment input dilutes the CaCO3 content. Acknowledgements We gratefully acknowledge the Director, National Centre for Antarctic and Ocean Research (NCAOR) for his continuous encouragement and support. We thank Dr. Thamban Meloth for providing the TOC analyser facility, Dr. A.K. Tiwari and Mr. Brijesh for AAS, and Mr. Girish Prabhu (NIO) for XRD analyses. We also thank Dr. Harshavardhana, B.G., Department of Marine Geology, Mangalore University for extending assistance during the magnetic studies. The magnetic instruments used for this investigation were procured through grants (DOD Sanction No. DOD/11-MRDF/1/48/P/94ODII/12-10-96) under a research project (to RS) sponsored by the erstwhile Department of Ocean Development (now Ministry of Earth Sciences), Government of India. We appreciate Dr. C.T. Achuthankutty for improving the presentation of the manuscript. We express our gratitude to Dr. Nick Roberts (Associate Editor) and the anonymous reviewers whose comments enabled us to improve the manuscript. This is NCAOR contribution No. 19/2015. References Agnihotri, R., Kurian, S., 2008. Recently studied sedimentary records from the eastern Arabian Sea: implications to Holocene monsoonal variability. e-Journal Earth Science India 1, 188e207. Alagarsamy, R., 2009. Environmental magnetism and application in the continental shelf sediments of India. Marine Environmental Research 68, 49e58. Angino, E.E., Billings, G.K., 1972. Atomic Absorption Spectrometry in Geology. Elsevier. Aprile, F.M., Bouvy, M., 2008. Distribution and Enrichment of Heavy metals in sediments at the Tapacurá river Basin, Northeastern Brazil. Brazilian Journal of Aquatic Science and Technology 12 (1), 1e8. Bazylinski, D.A., Williams, T.J., 2007. Ecophysiology of magnetotacticbacteria. In: Schüler, D. (Ed.), Magnetoreception and Magnetosomes in Bacteria. Springer, Berlin, Germany, pp. 37e75. Berner, R.A., 1982. Burial of organic carbon and pyrite sulfur in the modern ocean: its geochemical and environmental significance. American Journal of Science 282, 451e473. Borole, D.V., Sarin, M.M., Somayajlulu, B.L.K., 1982. Composition of Narbada and Tapti estuarine particles and adjacent Arabian Sea sediments. Indian Journal of Marine Science 11, 51e62. Brown, E.T., Le Callonnec, L., German, C.R., 2000. Geochemical cycling of redoxsensitive metals in sediments from Lake Malawi: a diagnostic paleotracer for episodic changes in mixing depth. Geochimica et Cosmochimica Acta 64, 3515e3523. Calvert, S.E., Pedersen, T.F., 1992. Productivity, accumulation and preservation of organic matter. In: Whelan, J.K., Farrington, J.W. (Eds.), Recent and Ancient Sediments. Columbia University Press, New York, pp. 231e263. Calvert, S.E., Pedersen, T.F., Divakarnaidu, P.D., von Stackelberg, U., 1995. On the organic carbon maximum on the continental slope of the eastern Arabian Sea. Journal of Marine Research 53, 269e296. Campanile, D., Nambiar, C.G., Bishop, P., Widdowson, M., Brown, R., 2007. Sedimentation record in the Konkan-Kerala Basin: implications for the evolution of the Western Ghats and the Western Indian passive margin. Basin Research 20 (1), 3e22. Carver, R.E., 1971. Procedures in Sedimentary Petrology. Wiley Inter-science, John Wiley and Sons Inc., New York, p. 653. Clift, P.D., 2002. A brief history of the Indus River. In: Clift, P.D., Kroon, D., Craig, J., Gaedicke, C. (Eds.), The Tectonics and Climatic Evolution of the Arabian Sea Region, Geological Society of London Special Publication, vol. 195, pp. 237e258.

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