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

C. Jayaprakash1*, R. Sajeev2 & A. Anil Kumar3

1Marine & Coastal Survey Division, Geological Survey of India, - 682037, India 2Dept. of Physical Oceanography, Cochin University of Science and Technology, Kochi- 682 016, India 3Marine & Coastal Survey Division Geological Survey of India, - 575001, India

*[E-mail: [email protected]]

Received 23 September 2016 ; revised 29 November 2016

Shallow part of the seafloor adjoining the coast of is mostly covered by silty clay and clayey silt, from north of Quilon to . Sand dominate the shallow seafloor south of Quilon as well as the deeper part of inner shelf from to . 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 The decreasing ionic radii from La to Lu cause 15 elements of the Periodic Table that have different fractionation patterns in various similar ionic radii and valency state. The REE phases during geochemical processes6. are relatively plentiful in the Earth’s crust having an overall crustal abundance of 9.2 mg. Due to their chemical coherence, the REE -1 1 kg . Crustal abundance of individual REE are widely used as tracers of sources and varies widely from Ce, the most abundant at processes controlling trace element -1 43 mg. kg to Tm, the least abundant, at 0.28 distribution in marine sediments7-12 and for -1 2 mg.kg . determining depositional processes and sediment provenance2. REE present in REE are usually grouped as Heavy Rare terrigenous grains are largely unreactive13 and Earth Elements (HREE) and Light Rare Earth reflect the signature of hinterland source rocks. 3 Elements (LREE) . La, Ce, Pr, Nd, Pm, Sm REE distribution in sediments is largely and Eu are grouped as LREE while Gd, Tb, controlled by scavenging processes by Fe-Mn 4 Dy, Ho, Er, Tm, Yb and Lu form the HREE . oxides14-17, redox conditions of the overlying REE have decreasing ionic radii with water column18, composition of the terrigenous increasing atomic weight from La (1.016 Å) to source19-20 and anthropogenic inputs21. 5 Lu (0.85 Å) . They are mainly trivalent, with the exception of Ce and Eu which are also In coastal areas, sediments deposited in stable in the tetra and divalent oxidation states. zones of oxidation with relatively higher 1624 INDIAN J. MAR. SCI., VOL. 45, NO. 12, DECEMBER 2016

exposure time are found to contain higher A portion of the samples was concentration of REE. Present study attempts powdered using a pulveriser and analysed for to understand the distribution of REE in ΣREE at Regional Chemical Lab, surface sediments from the inner shelf off using Varian-Agilent 7700X ICP-MS and also Kerala and compare the role of mineralogy in at Central Geological Lab, CHQ, their enrichment. using Perkin Elmer SCIEX ELAN R DRCTM–e Inductively Coupled Plasma Mass Spectrometry technique. The ICP-MS is Materials and Methods calibrated by multi-element standard solution to cover the mass range and generate the mass The present study used 19 surface response curve. Matrix matching with a sediment samples collected from the inner standard reference material (SRM) was used continental shelf off Kerala for understanding for calibration. International standard GSD-10 the distribution and enrichment of REE and GSD-12 were used for the analysis. Alkali (Fig.1). Since the outer boundary of the inner fusion (mixture of lithium metaborate and shelf is vaguely defined, the study area is lithium tetraborate) is used to decompose the restricted within the Territorial Water (TW) fine powdered sediment samples. A Perkin limit, for exactly delineating the outer Elmer ELAN DRC (e) instrument, assembled boundary. The samples after drying and with a cross flow nebulizer Scott spray homogenisation were washed repeatedly with chamber was used in this experiment. The deionised water for salt removal, wet sieved ICP-MS, located in a temperature-controlled using 230 ASTM mesh and subjected to size 22 laboratory (25 ± 2 °C), was allowed to analysis . 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

JAYAPRAKASH et al.: RARE EARTH ELEMENTS IN THE INNER SHELF SEDIMENTS 1625

Table 1: The Sample Locations and REE Content of surface sediments. Sedi. Samp. Location Lat. Long. Depth(m) La Ce Pr Nd Eu Sm Gd Tb Dy Ho Er Tm Yb Lu ΣLREE ΣHREE No Type ΣREE Chandragiri 2.2 Clay G-1 12.47 74.99 21.35 40.62 6.90 34.11 2.97 8.59 11.89 1.81 10.03 1.96 5.25 0.79 4.41 0.66 151.34 114.54 36.80 River mouth Kasaragod- 8.0 Clay G-2 12.47 74.97 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 Chandragiri Chandragiri 30.0 Clay G-3 12.42 74.87 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 River mouth Kasaragod- 42.0 Clay G-4 12.40 74.82 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 Chandragiri Karingote 3.1 Clay G-5 12.21 75.11 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 River mouth Karingote 28.9 Clay G-6 12.16 74.99 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 River mouth 8.0 Silty G-7 Cannanore 11.93 75.29 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

G-8 Cannanore 11.86 75.12 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 Calicut- G-9 11.24 75.46 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 Kallayi Clay Calicut- 31.5 Clay G-10 11.18 75.63 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 Kallayi Azhikodu- 29.5 Clay G-11 10.17 76.02 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 Ponnani- 9.8 Clay G-12 10.68 75.89 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 Bharatapuzha Ponnani- 31.4 Clay G-13 10.63 75.78 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 Bharatapuzha Azhikodu- 13.5 Clay G-14 10.18 76.11 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 Periyar G-15 Alleppy 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

G-16 Alleppy 9.45 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

Fig. 2-The sediment distribution map within the inner Fig.3- Contour Map showing distribution of ΣREE in the shelf off Kerala (Modified after Sukumaran et al., 2010 28) surface sediments

ΣREE in the inner shelf sediments varies 5.61 mg. kg-1 and 306.31 mg. kg-1 and its between 9.15 mg. kg-1and 319.66 mg. kg-1 and distribution is shown in Fig.4. La ranges their distribution is depicted in Fig. 3 as between 1.08 mg. kg-1 and 78.57 mg. kg-1 . contour map. Sediments between Ponnani and Vizhinjam show higher ΣREE (>200 mg. kg-1 ) Sediment sample collected off whereas low ΣREE is observed in the northern Kaymkulam shows the highest concentration sector between Ponnani and Kasaragod. of La whereas the sample collected from ΣLREE in surface sediments within the Karingote river mouth, south of , Territorial Waters off Kerala ranges between 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 Karingote river mouth to 0.66 mg. kg-1 off 2.16 mg. kg-1 to 143.21 mg. kg-1 and like La, it mouth. Its content in also shows highest concentration off Ponnani- sediments is extremely low all along Kerala Vizhinjam sectors. The lowest concentration coast. of Ce is recorded in the offshore segment

between Kasaragod and Kanhangad.

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 . Both Pr and Nd show higher concentration in the offshore sectors between Ponnani and Vizhinjam. Sm content is higher in sediments off Ponnani- Vizhinjam 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 Kasaragod- Kanhangad 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 Ponnani- show relatively higher content of these elements. Lowest Dy, Er and Yb Fig.4- Contour Map showing distribution of ΣLREE in surface sediments contents of 0.77 mg. kg-1 , 0.5mg. kg-1 and 0.5 -1 mg. kg is from the sediment sample off In order to compare the ΣREE in sediments Bharatapuzha mouth whereas the highest with that of Post Archaean Australian Shale -1 -1 contents of 10.03 mg. kg , 5.25 mg. kg and (PAAS), REE data of sediments from -1 4.41 mg. kg are from Chandragiri river Kasaragod, Ponnani and Vizhinjam mouth. representing northern, central and southern Sectors such as Kasaragod-Kanhangad, inner shelf were normalized using PAAS Alappuzha-Trivandrum and Ponnani recorded (Fig.6). lower Dy, Er and Yb. Ho in the sediments -1 ranges from 0.18 mg. kg in the Karingote Discussion river mouth to 1.96 mg. kg-1 in Chandragiri river mouth. Low concentration of Ho is found The shallow part of the seafloor adjoining off Kasaragod-Kanhangad and Alappuzha- the Kerala coast is mostly covered by silty clay Trivandrum sectors. and clayey silt, from north of Quilon to -1 Kasaragod. Sand dominate the shallow Lowest Tm of 0.13 mg. kg is in the seafloor south of Quilon as well as from the sample from Karingote river mouth and the -1 deeper part of inner shelf from Vizhinjam to highest of 0.79 mg. kg is in the Chandragiri Ponnani. REE distribution pattern suggests river mouth. Yb is highest in Chandragiri higher content along southern Kerala between sediments (4.41 mg. kg-1 ) and lowest in -1 Ponnani and Vizhinjam and low content in Bharatapuzha mouth (0.5 mg. kg ). Highest sediments off northern Kerala. Tm recorded is in the sediments off Someshwara. Lu ranges from 0.16 mg. kg-1 off 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. 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 and Kochi which was related to monazite bearing placers. Anil Kumar et al (2016) considered REE in the heavy mineral rich sediments along Kerala Fig. 5-Contour map showing distribution of ΣHREE 39 in surface sediments coast as the contribution by monazite . REE are concentrated by clay fraction in fine grained sediments 2 and also by fractionation by authigenic minerals during diagenesis40. The sediments from Vizhinjam (southern Prudencio et al (1989)41 suggested kaolinite as sector) and Ponnani (central sector) show the principal carrier of REE in the clay enrichment ΣLREE compared to PAAS. There fraction. Seafloor of the inner shelf of is a depletion in La, Ce and Pr in the shelf northern Kerala is covered by silty clay and sediments from Kasaragod whereas other clayey silt containing negligible amount of LREE are higher than PAAS. HREE is heavy minerals42-45. Very low ΣREE in this enriched in the central and northern sectors sector could be the resultant of insignificant and depleted in the sediments off Vizhinjam heavy minerals content in them. compared to PAAS. The LREE in inner shelf sediments along

southern sector is high and is moderate to low in northern sector. High heavy mineral content 10 in the inner shelf sediments along the southern sector is reported by Geological Survey of India (GSI). Ilmenite and sillimanite are 1 /PAAS dominant in this sector while rutile, zircon, monazite, magnetite, garnet, pyroxenes, biotite REE and iron oxides occur in subordinate 36-37 0.1 quantities . Heavy minerals are 25, 36-37 La Ce Pr Nd Eu Sm Gd Tb Dy Ho Er Tm Yb Lu concentrated more in the finer fractions . The northern sector between Ponnani and REE (mg.kg-1) Kasaragod has extremely low heavy mineral Kasaragod Ponnani Vizhinjam content45. Monazite is a known source of LREE and might have contributed most of the Fig. 6-PAAS normalized REE in the Surface Sediments LREE along Kerala coast. Minerals such as apatite and sphene also contain LREE and could be the source of these metals.

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HREE is very low in shelf sediments off Southern inner shelf have higher concentration Kerala coast. Mineralogical and chemical data of heavy minerals compared to that of northern of shale samples indicate that distribution of inner shelf. Southern shelf sediments hold HREE in the fine fraction is mainly controlled considerable concentration of heavy minerals by zircon46. Small percentage of other heavy such as monazite, garnet, sphene, apatite, minerals such as garnet, amphibole and zircon, amphibole and pyroxene which contain pyroxene also could be a source of HREE35,46. fairly high REE. ΣREE vary from 9.15 mg. Considering the high concentration of minerals kg-1 to 319.66 mg. kg-1 , ΣLREE between 5.61 such as garnet, sillimanite, rutile, zircon, mica, mg. kg-1 and 306.31 mg. kg-1 and ΣHREE hornblende and in the coarse fractions from between 2.91 mg. kg-1 and 36.8 mg. kg-1 . southern inner shelf and insignificant quantity REE content is high along southern inner shelf from northern inner shelf42-45, and congruent than the northern shelf. The southern shelf also variation in HREE with heavy minerals, HREE has high content of LREE. Heavy minerals is suggested to be contributed by the heavy such as monazite and sphene contribute most minerals. of the LREE present in sediments. High heavy mineral content along the southern sector The pattern of distribution of heavy appears to have caused high LREE content in minerals and REE are mutually complimenting the shelf sediments. In the norther inner shelf, all along Kerala coast. Along southern Kerala, heavy minerals are found in negligibly low both heavy minerals and REE are high in inner amounts, which is also reflected in the shelf sediments whereas along the northern distribution pattern of LREE. HREE in sector both are present in insignificant general is very low in the shelf sediments of quantities. Sediments devoid of heavy Kerala coast. Heavy minerals such as zircon, minerals are not showing appreciable REE garnet, amphibole and pyroxene are the enrichment. Based on this, it is possible to principle sources of HREE and their content is correlate REE in the inner shelf sediments to also low in the shelf sediments, particularly their heavy mineral content. The study along the norther sector. These heavy minerals suggests minerals such as monazite, garnet, would have contributed the HREE present in sphene, apatite, zircon, amphibole and sediments. pyroxene as the principle contributors of REE in the inner shelf sediments. Kaolin is present REE in sediments are also enriched in the in good amounts in the inner shelf off clay fractions. In the surface sediments off Kasargod42 which also got the ability to enrich Kasaragod, kaolin is a major constituent. REE41. Considering the dominance of clayey Considering the dominance of clay sediments sediments along the northern inner shelf and along the northern inner shelf and its ability to its ability to enrich REE, a part of REE in this enrich REE, a part of REE also might have sector also might have contributed by clay contributed by clay which adsorb these metals which adsorb these trace metals on their on their surfaces. The sediments from southern surfaces while interacting with seawater. sector and central sector show enrichment in ΣLREE compared to PAAS. There is a The sediments from southern and central depletion in La, Ce and Pr in the shelf sector show enrichment in ΣLREE whereas a sediments from the northern sector and depletion in La, Ce and Pr was observed in the remaining LREE in this sector show northern sector from Kasaragod than PAAS. enrichment. HREE is enriched in the central HREE is enriched in the central and northern and northern sectors and depleted in the sectors and depleted in the southern sector. southern sector compared to PAAS.

Conclusion Acknowledgement Shallow part of the seafloor adjoining the Authors are highly thankful to Sri. V. Devdas, coast of Kerala is mostly covered by silty clay Deputy Director General & HOD, M&CSD, and clayey silt, from north of Quilon to GSI, Mangalore, for the logistic and technical Kasaragod. Sand dominate the shallow support extended during this work. seafloor south of Quilon as well as the deeper part of inner shelf from Vizhinjam to Ponnani. JAYAPRAKASH et al.: RARE EARTH ELEMENTS IN THE INNER SHELF SEDIMENTS 1629 1627

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