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Indian Journal of Geo Marine Sciences Vol. 45 (12), December 2016, pp 1623-1630 Distribution of rare earth elements in the inner shelf sediments, off the southwest coast of India C. Jayaprakash1*, R. Sajeev2 & A. Anil Kumar3 2 1 Marine & Coastal Survey Division, Geological Survey of India, Kochi - 682037, India Dept. of Physical Oceanography, Cochin University of Science and Technology, Kochi- 682 016, India 3 Marine & Coastal Survey Division Geological Survey of India, Mangalore- 575001, India *[E-mail: [email protected]] Received 23 September 2016 ; revised 29 November 2016 Shallow part of the seafloor adjoining the coast of Kerala is mostly covered by silty clay and clayey silt, from north of Quilon to Kasaragod. Sand dominate the shallow seafloor south of Quilon as well as the deeper part of inner shelf from Vizhinjam to Ponnani. Concentrations of Rare Earth Elements were determined in the surface sediments within the inner shelf off the coast of Kerala. ΣREE content in surface sediments varies between 9.15 mg. kg-1 and 319.66 mg. kg-1 . Distribution pattern suggests higher concentration of REE in the inner shelf sediments off southern Kerala, between Ponnani and Vizhinjam. Sediments off northern Kerala have lower REE. ΣLREE ranges between 5.61 mg. kg-1 and 306.31 mg. kg-1 and is more in the southern sector. ΣHREE in inner shelf sediments is very low ranging between 2.91 mg. kg-1 and 36.8 mg. kg-1 . REE in coastal sediments off Kerala is closely related to heavy mineral content. LREE is mostly contributed by detrital monazite, apatite and sphene in the fine fractions and HREE by detrital zircon, garnet, amphibole and pyroxene. Sediments from southern and central sectors show enrichment in ΣLREE compared to PAAS. There is a depletion in La, Ce and Pr in the shelf sediments from the northern sector and remaining LREE in this sector show enrichment. HREE is enriched in the central and northern sectors and depleted in the southern sector compared to PAAS. [Keywords: Rare Earth Elements (REE), LREE, HREE, Kerala, Eu, Ce, Marine Sediments, West Coast, India.] Introduction Rare Earth Elements (REE) are a group of 15 elements of the Periodic Table that have similar ionic radii and valency state. The REE are relatively plentiful in the Earth’s crust having an overall crustal abundance of 9.2 mg. kg-1 1. Crustal abundance of individual REE varies widely from Ce, the most abundant at 43 mg. kg-1 to Tm, the least abundant, at 0.28 mg.kg-1 2. REE are usually grouped as Heavy Rare Earth Elements (HREE) and Light Rare Earth Elements (LREE) 3. La, Ce, Pr, Nd, Pm, Sm and Eu are grouped as LREE while Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu form the HREE4. REE have decreasing ionic radii with increasing atomic weight from La (1.016 Å) to Lu (0.85 Å) 5. They are mainly trivalent, with the exception of Ce and Eu which are also stable in the tetra and divalent oxidation states. The decreasing ionic radii from La to Lu cause different fractionation patterns in various phases during geochemical processes6. Due to their chemical coherence, the REE are widely used as tracers of sources and processes controlling trace element distribution in marine sediments7-12 and for determining depositional processes and sediment provenance2. REE present in terrigenous grains are largely unreactive13 and reflect the signature of hinterland source rocks. REE distribution in sediments is largely controlled by scavenging processes by Fe-Mn oxides14-17, redox conditions of the overlying water column18, composition of the terrigenous source19-20 and anthropogenic inputs21. In coastal areas, sediments deposited in zones of oxidation with relatively higher 1624 INDIAN J. MAR. SCI., VOL. 45, NO. 12, DECEMBER 2016 exposure time are found to contain higher concentration of REE. Present study attempts to understand the distribution of REE in surface sediments from the inner shelf off Kerala and compare the role of mineralogy in their enrichment. Materials and Methods The present study used 19 surface sediment samples collected from the inner continental shelf off Kerala for understanding the distribution and enrichment of REE (Fig.1). Since the outer boundary of the inner shelf is vaguely defined, the study area is restricted within the Territorial Water (TW) limit, for exactly delineating the outer boundary. The samples after drying and homogenisation were washed repeatedly with deionised water for salt removal, wet sieved using 230 ASTM mesh and subjected to size analysis22. A portion of the samples was powdered using a pulveriser and analysed for ΣREE at Regional Chemical Lab, Hyderabad using Varian-Agilent 7700X ICP-MS and also at Central Geological Lab, CHQ, Kolkata using Perkin Elmer SCIEX ELAN R DRCTM–e Inductively Coupled Plasma Mass Spectrometry technique. The ICP-MS is calibrated by multi-element standard solution to cover the mass range and generate the mass response curve. Matrix matching with a standard reference material (SRM) was used for calibration. International standard GSD-10 and GSD-12 were used for the analysis. Alkali fusion (mixture of lithium metaborate and lithium tetraborate) is used to decompose the fine powdered sediment samples. A Perkin Elmer ELAN DRC (e) instrument, assembled with a cross flow nebulizer Scott spray chamber was used in this experiment. The ICP-MS, located in a temperature-controlled laboratory (25 ± 2 °C), was allowed to stabilize before optimization procedures were carried out. Oxide ratio (CeO/Ce+) and doubly charged ion ratio (Ba++/Ba+) was found <3% under full optimum condition. Results Simplified sediment distribution map within the inner shelf off Kerala is given in Fig.2. The shallow part of the seafloor adjoining the coast is mostly covered by silty clay and clayey silt, from north of Quilon to Kasaragod. Sand dominate the shallow seafloor south of Quilon as well as in the deeper part of inner shelf from Vizhinjam to Ponnani. Coarse fractions of these terrigenous sediments contain detrital quartz as the principal mineral component with feldspar and heavy minerals such as ilmenite, sillimanite, monazite, rutile, zircon, garnet, magnetite, sphene, biotite, amphiboles and pyroxenes besides shell carbonate insubordinate quantities23-27. Sample locations, sediment type and REE content of the samples are given in Table-1. Fig. 1-Map showing sample location 1625 JAYAPRAKASH et al.: RARE EARTH ELEMENTS IN THE INNER SHELF SEDIMENTS Table 1: The Sample Locations and REE Content of surface sediments. Samp. No G-1 G-2 G-3 G-4 G-5 G-6 Location Chandragiri River mouth KasaragodChandragiri Chandragiri River mouth KasaragodChandragiri Karingote River mouth Karingote River mouth Lat. 12.47 Long. 74.99 12.47 74.97 12.42 74.87 12.40 74.82 12.21 75.11 12.16 74.99 G-7 Cannanore 11.93 75.29 G-8 Cannanore 11.86 75.12 G-9 CalicutKallayi 11.24 75.46 G-10 G-11 G-12 G-13 G-14 G-15 G-16 CalicutKallayi AzhikoduPeriyar PonnaniBharatapuzha PonnaniBharatapuzha AzhikoduPeriyar Alleppy Alleppy Depth(m) Sedi. Type 2.2 Clay 8.0 Clay 30.0 Clay 42.0 Clay La Nd Eu Gd Tb Dy Ho Er Tm Yb Lu ΣREE ΣLREE ΣHREE 21.35 Ce 40.62 Pr 6.90 34.11 2.97 Sm 8.59 11.89 1.81 10.03 1.96 5.25 0.79 4.41 0.66 151.34 114.54 36.80 35.53 70.26 7.76 24.98 1.09 5.87 6.57 0.89 4.93 0.93 2.67 0.50 2.68 0.50 165.16 145.49 19.67 9.21 27.00 2.10 7.53 0.44 1.62 1.56 0.28 1.65 0.32 1.08 0.20 1.28 0.21 54.48 47.90 6.58 32.14 61.54 7.18 21.71 0.99 5.43 6.07 0.83 4.70 0.90 2.59 0.50 2.61 0.50 147.69 128.99 18.70 3.1 Clay 28.9 Clay 8.0 Silty Clay 34.74 66.90 7.53 24.88 1.06 5.62 6.21 0.86 4.80 0.92 2.69 0.50 2.75 0.50 159.96 140.73 19.23 37.5 Clay 32.87 62.13 7.18 22.07 1.06 5.49 6.16 0.85 4.75 0.90 2.58 0.50 2.56 0.50 149.60 130.80 18.80 7.0 Silty Clay 33.25 63.97 7.48 22.55 1.05 5.51 6.25 0.85 4.71 0.91 2.57 0.50 2.58 0.50 152.68 133.81 18.87 31.5 Clay 31.89 59.94 6.98 21.09 0.93 5.16 5.45 0.71 3.96 0.76 2.23 0.50 2.34 0.50 142.44 125.99 16.45 29.5 Clay 44.60 82.86 9.58 35.30 1.26 6.88 6.86 0.88 4.60 0.87 2.46 0.50 2.41 0.50 199.56 180.48 19.08 9.8 Clay 53.87 102.07 11.51 39.73 1.54 8.32 8.03 1.00 5.15 0.95 2.67 0.50 2.61 0.50 238.45 217.04 21.41 31.4 Clay 21.77 40.01 4.02 13.89 0.50 2.63 2.01 0.50 0.77 0.50 0.50 0.50 0.50 0.50 238.45 82.82 5.78 13.5 Clay 49.30 91.86 10.70 37.84 1.44 7.76 7.69 1.00 5.17 0.96 2.68 0.50 2.60 0.50 220.00 198.90 21.10 1.08 2.16 0.43 1.56 0.06 0.32 0.58 0.09 0.79 0.18 0.68 0.13 0.93 0.16 9.15 128.99 3.54 38.80 70.25 6.84 23.45 0.47 3.84 3.15 0.45 2.08 0.37 1.11 0.19 1.21 0.20 152.41 143.65 8.76 11.18 75.63 10.17 76.02 10.68 75.89 10.63 75.78 10.18 76.11 9.52 76.25 15.0 Clay 49.70 93.51 10.93 37.30 1.44 7.60 7.63 0.98 5.20 0.97 2.72 0.50 2.65 0.50 221.63 200.48 21.15 76.14 37.0 Clay 42.32 78.54 9.20 32.01 1.21 6.54 6.61 0.84 4.41 0.84 2.33 0.50 2.34 0.50 221.63 169.82 18.37 9.45 Fig. 2-The sediment distribution map within the inner shelf off Kerala (Modified after Sukumaran et al., 2010 28) Fig.3- Contour Map showing distribution of ΣREE in the surface sediments ΣREE in the inner shelf sediments varies between 9.15 mg. kg-1and 319.66 mg. kg-1 and their distribution is depicted in Fig. 3 as contour map. Sediments between Ponnani and Vizhinjam show higher ΣREE (>200 mg. kg-1 ) whereas low ΣREE is observed in the northern sector between Ponnani and Kasaragod. ΣLREE in surface sediments within the Territorial Waters off Kerala ranges between 5.61 mg. kg-1 and 306.31 mg. kg-1 and its distribution is shown in Fig.4. La ranges between 1.08 mg. kg-1 and 78.57 mg. kg-1 . Sediment sample collected off Kaymkulam shows the highest concentration of La whereas the sample collected from Karingote river mouth, south of Kanhangad, has analyzed the lowest. The sector between Ponnani and Vizhinjam show high 1626 INDIAN J. MAR. SCI., VOL. 45, NO. 12, DECEMBER 2016 concentration of La. Ce content ranges from 2.16 mg. kg-1 to 143.21 mg. kg-1 and like La, it also shows highest concentration off PonnaniVizhinjam sectors. The lowest concentration of Ce is recorded in the offshore segment between Kasaragod and Kanhangad. Karingote river mouth to 0.66 mg. kg-1 off Chandragiri river mouth. Its content in sediments is extremely low all along Kerala coast. Pr, Nd and Sm range from 0.43 mg. kg-1 , 1.56 mg. kg-1 and 0.32 mg. kg-1 in Karingote river mouth to 15.6 mg. kg-1 , 55.68 mg. kg-1 and 10.28 mg. kg-1 respectively in the sediments off Kayamkulam. Both Pr and Nd show higher concentration in the offshore sectors between Ponnani and Vizhinjam. Sm content is higher in sediments off PonnaniVizhinjam sectors. Kasaragod-Kanhangad offshore sector shows the lowest content of Pr, Nd and Sm. Eu is low, ranging from 0.06 mg. kg-1 recorded in the Karingote river mouth to 2.97 mg. kg-1 in the Chandragiri river mouth. Lowest Eu is in the sediments off KasaragodKanhangad sector. Distribution pattern of ΣHREE is shown in Fig.5. Its content is very low and ranges from 2.91 mg. kg-1 to 36.8 mg. kg-1 .Gd and Tb vary from 0.58 mg. kg-1 and 0.09 mg. kg-1 in the Karingote river mouth to 11.89 mg. kg-1 and 1.81 mg. kg-1 in the Chandragiri river mouth respectively. Sediments off PonnaniAlappuzha show relatively higher content of these elements. Lowest Dy, Er and Yb contents of 0.77 mg. kg-1 , 0.5mg. kg-1 and 0.5 mg. kg-1 is from the sediment sample off Bharatapuzha mouth whereas the highest contents of 10.03 mg. kg-1 , 5.25 mg. kg-1 and 4.41 mg. kg-1 are from Chandragiri river mouth. Sectors such as Kasaragod-Kanhangad, Alappuzha-Trivandrum and Ponnani recorded lower Dy, Er and Yb. Ho in the sediments ranges from 0.18 mg. kg-1 in the Karingote river mouth to 1.96 mg. kg-1 in Chandragiri river mouth. Low concentration of Ho is found off Kasaragod-Kanhangad and AlappuzhaTrivandrum sectors. Lowest Tm of 0.13 mg. kg-1 is in the sample from Karingote river mouth and the highest of 0.79 mg. kg-1 is in the Chandragiri river mouth. Yb is highest in Chandragiri sediments (4.41 mg. kg-1 ) and lowest in Bharatapuzha mouth (0.5 mg. kg-1 ). Highest Tm recorded is in the sediments off Someshwara. Lu ranges from 0.16 mg. kg-1 off Fig.4- Contour Map showing distribution of ΣLREE in surface sediments In order to compare the ΣREE in sediments with that of Post Archaean Australian Shale (PAAS), REE data of sediments from Kasaragod, Ponnani and Vizhinjam representing northern, central and southern inner shelf were normalized using PAAS (Fig.6). Discussion The shallow part of the seafloor adjoining the Kerala coast is mostly covered by silty clay and clayey silt, from north of Quilon to Kasaragod. Sand dominate the shallow seafloor south of Quilon as well as from the deeper part of inner shelf from Vizhinjam to Ponnani. REE distribution pattern suggests higher content along southern Kerala between Ponnani and Vizhinjam and low content in sediments off northern Kerala. JAYAPRAKASH et al.: RARE EARTH ELEMENTS IN THE INNER SHELF SEDIMENTS 1627 Detrital monazite derived from the hinterland contribute most of the REE in shallow marine sediments whereas biogenic/authigenic processes control its concentration in deep sea sediments29-30. Monazite can contain about 55 wt.% of ΣREE31. Accessory minerals such as garnet, sphene, apatite, zircon, amphibole and pyroxene are also known to concentrate and fractionate ΣREE32-35. Fig. 5-Contour map showing distribution of ΣHREE in surface sediments The sediments from Vizhinjam (southern sector) and Ponnani (central sector) show enrichment ΣLREE compared to PAAS. There is a depletion in La, Ce and Pr in the shelf sediments from Kasaragod whereas other LREE are higher than PAAS. HREE is enriched in the central and northern sectors and depleted in the sediments off Vizhinjam compared to PAAS. REE/PAAS 10 1 0.1 La Ce Pr Nd Eu Sm Gd Tb Dy Ho Er Tm Yb Lu REE (mg.kg-1) Kasaragod Ponnani Vizhinjam Fig. 6-PAAS normalized REE in the Surface Sediments The inner shelf sediments along the southern coast of Kerala has high concentration of heavy minerals which include monazite, zircon, garnet and sphene23, 25-27, 36-37. These minerals, due to their high distribution coefficient, incorporate REE during crystallization, depending upon the temperature, pressure, composition of the mineral and melt35. Study by Siby Kurian et al (2013)38 suggested high ΣREE in the sediments off Kollam and Kochi which was related to monazite bearing placers. Anil Kumar et al (2016) considered REE in the heavy mineral rich sediments along Kerala coast as the contribution by monazite39. REE are concentrated by clay fraction in fine grained sediments 2 and also by fractionation by authigenic minerals during diagenesis40. Prudencio et al (1989)41 suggested kaolinite as the principal carrier of REE in the clay fraction. Seafloor of the inner shelf of northern Kerala is covered by silty clay and clayey silt containing negligible amount of heavy minerals42-45. Very low ΣREE in this sector could be the resultant of insignificant heavy minerals content in them. The LREE in inner shelf sediments along southern sector is high and is moderate to low in northern sector. High heavy mineral content in the inner shelf sediments along the southern sector is reported by Geological Survey of India (GSI). Ilmenite and sillimanite are dominant in this sector while rutile, zircon, monazite, magnetite, garnet, pyroxenes, biotite and iron oxides occur in subordinate quantities36-37. Heavy minerals are concentrated more in the finer fractions25, 36-37. The northern sector between Ponnani and Kasaragod has extremely low heavy mineral content45. Monazite is a known source of LREE and might have contributed most of the LREE along Kerala coast. Minerals such as apatite and sphene also contain LREE and could be the source of these metals. 1626 1628 INDIAN J. MAR. SCI., VOL. 45, NO. 12, DECEMBER 2016 HREE is very low in shelf sediments off Kerala coast. Mineralogical and chemical data of shale samples indicate that distribution of HREE in the fine fraction is mainly controlled by zircon46. Small percentage of other heavy minerals such as garnet, amphibole and pyroxene also could be a source of HREE35,46. Considering the high concentration of minerals such as garnet, sillimanite, rutile, zircon, mica, hornblende and in the coarse fractions from southern inner shelf and insignificant quantity from northern inner shelf42-45, and congruent variation in HREE with heavy minerals, HREE is suggested to be contributed by the heavy minerals. The pattern of distribution of heavy minerals and REE are mutually complimenting all along Kerala coast. Along southern Kerala, both heavy minerals and REE are high in inner shelf sediments whereas along the northern sector both are present in insignificant quantities. Sediments devoid of heavy minerals are not showing appreciable REE enrichment. Based on this, it is possible to correlate REE in the inner shelf sediments to their heavy mineral content. The study suggests minerals such as monazite, garnet, sphene, apatite, zircon, amphibole and pyroxene as the principle contributors of REE in the inner shelf sediments. Kaolin is present in good amounts in the inner shelf off Kasargod42 which also got the ability to enrich REE41. Considering the dominance of clayey sediments along the northern inner shelf and its ability to enrich REE, a part of REE in this sector also might have contributed by clay which adsorb these trace metals on their surfaces while interacting with seawater. The sediments from southern and central sector show enrichment in ΣLREE whereas a depletion in La, Ce and Pr was observed in the northern sector from Kasaragod than PAAS. HREE is enriched in the central and northern sectors and depleted in the southern sector. Southern inner shelf have higher concentration of heavy minerals compared to that of northern inner shelf. Southern shelf sediments hold considerable concentration of heavy minerals such as monazite, garnet, sphene, apatite, zircon, amphibole and pyroxene which contain fairly high REE. ΣREE vary from 9.15 mg. kg-1 to 319.66 mg. kg-1 , ΣLREE between 5.61 mg. kg-1 and 306.31 mg. kg-1 and ΣHREE between 2.91 mg. kg-1 and 36.8 mg. kg-1 . REE content is high along southern inner shelf than the northern shelf. The southern shelf also has high content of LREE. Heavy minerals such as monazite and sphene contribute most of the LREE present in sediments. High heavy mineral content along the southern sector appears to have caused high LREE content in the shelf sediments. In the norther inner shelf, heavy minerals are found in negligibly low amounts, which is also reflected in the distribution pattern of LREE. HREE in general is very low in the shelf sediments of Kerala coast. Heavy minerals such as zircon, garnet, amphibole and pyroxene are the principle sources of HREE and their content is also low in the shelf sediments, particularly along the norther sector. These heavy minerals would have contributed the HREE present in sediments. REE in sediments are also enriched in the clay fractions. In the surface sediments off Kasaragod, kaolin is a major constituent. Considering the dominance of clay sediments along the northern inner shelf and its ability to enrich REE, a part of REE also might have contributed by clay which adsorb these metals on their surfaces. The sediments from southern sector and central sector show enrichment in ΣLREE compared to PAAS. There is a depletion in La, Ce and Pr in the shelf sediments from the northern sector and remaining LREE in this sector show enrichment. HREE is enriched in the central and northern sectors and depleted in the southern sector compared to PAAS. Conclusion Acknowledgement Shallow part of the seafloor adjoining the coast of Kerala is mostly covered by silty clay and clayey silt, from north of Quilon to Kasaragod. Sand dominate the shallow seafloor south of Quilon as well as the deeper part of inner shelf from Vizhinjam to Ponnani. Authors are highly thankful to Sri. V. Devdas, Deputy Director General & HOD, M&CSD, GSI, Mangalore, for the logistic and technical support extended during this work. JAYAPRAKASH et al.: RARE EARTH ELEMENTS IN THE INNER SHELF SEDIMENTS 1629 1627 References 1. Rudnick, R. L., Heinrich, D. H., and Karl, K. 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