The Rare Earth Elements (REEs) from lanthanum to lutetium are members of Group IlIA in the periodic table and all have very similar chemical and

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1 The Rare Earth Elements (REEs) from lanthanum to lutetium are members of Group IlIA in the periodic table and all have very similar chemical and physical properties. REEs along with certain other trace metals give information about the various mechanisms, by which they have been incorporated into the sediments such as scavenging onto the particles, co-precipitation, ion pairing and redox reactions. Many trace elements can be disassociated or get associated during their transport from the source to the depositional sites due to biogeochemical processes in seawater. But in case of REEs, only subtle variations may take place across the series, and this coherence makes them useful geochemical tracers. To use the REEs as natural tracers we need to understand the extent to which these processes are dominant in different geochemical environments. The REEs in sediments are likely to be influenced by (1) particulate supply from the adjacent landmasses 2) biogenic sedimentation from overlying seawater 3) oxygenation conditions in the water column. In view of the importance of REE studies reported earlier, an attempt has been made to address the behaviour and processes of REEs and trace elements in the Arabian Sea sediments and Andaman Backarc basin sediments. In the present study, surface sediment samples were collected from the western Indian continental shelf to estimate the distribution pattern and geochemical behaviour of REEs, major elements and trace metals. Bottom 217

2 Summary water samples were also taken at each station, and analysed for temperature, salinity and dissolved oxygen, hence the response of redox sensitive elements to varying oxygenation conditions can be addressed. Sediments collected from the Kerala coast were analysed to examine the seasonal variations of REEs and trace elements. In addition, three sediment cores were studied from the Andaman Backarc basin to understand the downcore variation of REEs and other trace elements and also to study their accumulation with respect to other major elements such as Mn, so as to see the signatures of hydrothermal influences through REE fractionation indices. Rare earth elements were normalized with P AAS (Post Archean Australian Shale) values for plotting shale normalized patterns and determining the anomalies if any. Eu and Ce anomalies and fractionation indices were used for explaining REE behaviour in the system. Additionally, elements were normalized with UCC (Upper Continental Crust) values to compare with continental crust composition. Element excess and enrichment factors are other parameters calculated in the present study. Correlation matrix and factor analysis were used for interpretation of data. The results and conclusions of the study can be summarised as follows. 1) 1) ~REE abundance is more for the coastal stations compared to offshore ones indicating REE input through river run-off and terrigenous influence. The study of REEs in the sediments of Kerala coast also supports this observation. Ce is the most abundant element among the REEs and the inconsistent relation between dissolved 218

3 SummanJ oxygen and Ce anomaly is observed for some of the transects, may be due to the influence of high terrigenous sedimentation and along shore currents over the coastal regions. 2) Thorium enrichment and high Eu anomaly for the Kerala coast (especially for the transects Quilon and Cochin) may be due to the presence of heavy minerals in these sediments. U showed enriched concentration in the sediments impinged by oxygen minimum zone. 3) The concentration of Al is observed to be higher in the coastal samples compared to offshore ones and this indicates the terrigenous source of Al in the sediments. The concentration of Fe and Mn are also found to be higher for the coastal stations than the deeper ones. The highest value is observed for the coastal station off Goa, which may be due to the impact from the onshore mining sites of Goa. 4) CaC03 and sand (%) showed higher values for loom stations which is correlated with the relict carbonate band present in the western continental shelf. Organic carbon (%) is found to be higher for the 200m samples which is overlying by suboxic waters, but the present study indicates that the preservation of organic matter depend not only on D.O. of bottom water, but also on other factors such as productivity of water column and sedimentation rates. Clay rich sediments were found to have higher trace metal contents compared to silty and sandy fractions of sediments probably because of stronger adsorption capacity of clay sized fraction for metals. 219

4 Summary 5) VCC normalized patterns showed V enrichment in the sediments under OMZ. Cr also showed enriched pattern, which indicates coastal western margin could be a sink for Cr and its enrichment can be due to intense chemical weathering. 6) In contrast to the water column characteristics, which have shown distinct seasonal variation, REEs and other trace elements including the redox-sensitive elements in the coastal sediments have not significantly responded to the changing bottom water chemistry and seasonal variations, indicating slower response time of sediments vis-a-vis the overlying waters. 7) REE studies, discrimination plots and trace metal ratios of Andaman Backarc sediments suggest hydrothermal source along with some detrital input for these sediments. Shale like flat REE patterns and (LaNb)n ratios on one hand suggest a significant detrital component in the sediment, while the positive Eu anomaly, Mn and Mo enrichment and negative Ce anomalies in few samples and V/Fe ratios close to plume ratios collectively indicate an unquantifiable metalliferous/ hydrothermal component in these sediments. Detrital component in the Andaman sediments could possibly be due to the delivery from Himalayan River such as Irrawadi or Sawleen. The accumulation studies of elements indicate a predominant hydrothermal source for Mn and a mixed detrital-hydrothermal signature for REEs. 220

5 Summary The major objectives of the study were 1) the assessment of distribution of REEs in the western continental margin 2) seasonal variation of REEs in the coastal environment and 3) behaviour and depositional characters of REEs in a different geochemical environment such as Andaman Backarc basin. The present study could generate sufficient data to analyse the distributional character of the REEs in the EEZ of west coast of India, but to establish geochemical character and reactivity of these elements and to use these as provenance indicator further detailed and comprehensive investigations are essential. This is not attempted here, as it is beyond the scope and dimensions of this work. 221

6 REE LREE HREE MREE OMZ EEZ SW NW NE CAB PAAS UCC PSU ppm HFSE LILE DO O.C EF LREE Elex MAR Rare Earth Elements LightREE HeavyREE MiddleREE Oxygen Minimum Zone Exclusive Economic Zone Southwest Northwest Northeast Central Andaman Basin Post Archean Australian Shale Upper Crustal Composition Practical Salinity Unit Parts per million High Field Strength Elements Large Ion Lithophile Elements Dissolved Oxygen Organic Carbon Enrichment Factor Sum REEs Itotal REEs Elemental concentration in excess Mass Accumulation Rate 223