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Rare earth elements of sediments in rivers and estuaries of the east coast of India

The rare earth elements (REE) in the clay fraction of sediments in 15 rivers and their estuaries along the east coast of India were analysed in this study. The total REE content (∑REE) varied from 130.98 to 289.85 μg/g and from 70.89 to 352.61 μg/g in rivers and estuaries respectively. The ∑REEs of estuarine clays (except the Brahmani and Baitarani) was lower than in rivers. The Post-Archean average Australian Shale-normalized REE patterns in rivers and estuaries were similar and categorized into three types. The REE patterns reflect the composition of dominant geological formations in river basins and extent of sediment mixing from different sources during transport. Hydrodynamic conditions controlled the abundance and fractionation of REE in the estuaries. The Sm/Nd ratios of clays were largely controlled by mineral composition and Y/Ho ratios were affected by sedimentary processes in the estuaries.

RARE earth elements (REE), because of their unique and chemically coherent behaviour, are commonly used as geochemical tracers for provenance studies 1,2 . The distribution of REE in natural waters, suspended particulate matter and sediments in rivers and estuaries has been extensively studied [2][3][4][5][6][7][8][9][10] . Significant progress has been achieved in our understanding of REE distribution in the sediments. For example, earlier studies had indicated that 'REE as a group are insoluble elements, exhibit little fractionation during the Earth surface processes' 1 . This was refuted in the subsequent studies and it was suggested that the REE are fractionated and mobilized during weathering in all types of climatic conditions and in estuaries 5,6,[11][12][13][14][15][16] . Moreover, Fe-Mn oxyhydroxides, colloidal material and organic compounds could also be hosts for REE and their presence in sediments could bias total REE concentration [17][18][19][20] . The type of weathering 21 , lithology of the rocks and their susceptibility to erosion 22 also control the degree of REE fractionation in the silt and clay fractions of sediments. Fractionation within REE groups, i.e. among light-REE (LREE), middle-REE (MREE) and heavy-REE (HREE) has been reported in different size fractions of sediments 18,23 , and attributed to the presence and amount of heavy minerals and carbonate fraction in the sediments 8,9,24,25 . In other words, several factors contribute to the mobilization, fractionation and concentration of REE in sediments and therefore place a limit on the use of REE geochemistry for provenance studies.
Several studies have focused on REE distribution in the sediments of large rivers, because they cover extensively large areas and help better understand the global REE cycle 26 . However, it is difficult to identify the sourcerock characteristics from sediments of large rivers, because they drain through different rock types under varied climatic conditions, and REE from different sources are admixed during weathering, erosion, transportation and deposition, resulting in homogenous REE patterns in large rivers of the world 18 . REE were abundantly removed to the sediments in several estuaries 6 , but low REE removal in low-salinity regions (<5) has also been reported 10 . In our earlier study, the composition of peninsular India rivers average clay (PIRAC) 27 was compared with that of world river average clay (WRAC) and other reference crustal sediments from different continents. In the present study, sediments collected in the lower reaches of 15 rivers, low-salinity regions (<5) of their estuaries and two lakes along the east coast of India were analysed in detail for REE geochemistry ( Figure 1). The purpose of the study was to: (a) report REE distribution in clay fraction of sediments in rivers and document that this fraction represents dominant source-rock characteristics in their drainage basins, and (b) verify whether the estuarine sediments enriched in REE, similar to that reported in the literature.

Study area and samples
Several rivers draining peninsular and extra-peninsular India bring suspended and bed-load of sediments to the Bay of Bengal. Figure 1 shows the generalized geological map of India 28 , and location of the rivers and lakes analysed in this study. Table 1 provides details of the major and medium rivers 29 (length, drainage basin area and annual discharge) and geological formations in their drainage basins. The predominant rock formations are the Archean and Pre-Cambrian 30,31 in the drainage basins of the Cauvery, Ponnaiyar, Palar and Pennar rivers, the Deccan Trap volcanic rocks 32,33 in the Krishna and Godavari rivers, the Eastern Ghats gneissic rocks 34,35 (khondalites) consisting of Mn ore deposits for the Nagavali and Vamsadhara rivers, and both igneous and metamorphic rocks and different soils (mixed lithology terrain) [36][37][38] in the Mahanadi, Brahmani, Baitarani, Subarnarekha, Haldia and Hoogly rivers ( Table 1). The sediments in the lower reaches of rivers are usually derived from different geological formations in their drainage basins, and admixed thoroughly during transportation and deposition. In order to meet the objectives of the study, two sediments, one in the lower reaches of 15 rivers and another in the lowsalinity region (<5) of their estuaries and two lakes (one freshwater lake and one brackish-water lake) along the east coast of India were collected, using Peterson grab and with the help of a mechanized boat. A total of 32 sediment samples were collected and dried in an oven at <60°C.

Analytical methods
The sand, silt and clay contents of 32 sediments were determined following Folk 39 . The geochemistry of the clay fraction (<4 μm) of sediments was used to understand the provenance. The <62 μm fraction of the sediments were separated from the total sediments by wet sieving using 230 (ASTM) mesh sieve. This fraction was collected in a measuring glass cylinder, made up to 1000 ml volume with distilled water and stirred vigorously for homogeneity. The <4 μm fraction of the sediments was separated based on the Stoke's settling velocity 39 , dried in oven at 60°C and then powdered. One gram of dried sample powder was used to prepare pellets. Major elements were determined using an X-ray fluorescence spectrometer 'WD-XRF' (model Axios mAX, PAN-Analytical), at the CSIR-National Geophysical Research Institute (CSIR-NGRI), Hyderabad. For trace elements, 50 mg of the homogenized powdered sediment sample was transferred into Teflon beaker and 10 ml of acid mixture (7 : 3 of HF and HNO 3 ) was added to each beaker, kept overnight on a hot plate at 150°C for 48 h. Two drops of HClO 4 were added to the beaker and evaporated at 150°C for ~1 h to near dryness. This procedure was repeated till the sample in the beaker was completely digested. The final residue was dissolved in 10 ml of 1 : 1 water and HNO 3 mixture. Next, 5 ml of 1 ppm Rh 103 solution was added to the beaker as an internal standard and final volume of the sample solution was made up to 250 ml. High-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS; Nu Instruments, UK) at CSIR-NGRI, was used for analysis of the sample solutions. MAG-1 (marine mud), certified reference material (CRM) from the United States Geological Survey was used as calibrating standard for the analysis of sediments and, JSd-1, CRM for stream sediments from the Geological Survey of Japan was used as a cross-check standard for the data. Rh 103 was used as an internal standard for instrument drift correction. Details of instrument parameters and HR-ICP-MS analysis with accuracy and precision achieved are given in Satyanarayanan et al. 40 . Table 2 shows relative standard deviation (RSD) for MAG-1 standard and reported values 41 using HR-ICP-MS. The chemical index of alteration (CIA) of sediments was calculated, following the equation 42 given in Table 3. The REE were subdivided into LREE (La-Nd), MREE (Sm-Ho) and HREE (Er-Lu), following Rollinson 2 . The Ce (Ce/Ce*) and Eu (Eu/Eu*) anomalies were calculated (Table 3) 43 . Carbonate content of the sediments was calculated based on weight loss method, by weighing the sediment sample before and after dissolving carbonate in the sample with 10% HCl, followed by washing for acid removal and drying. The difference in weight was considered as the carbonate content and percentage was calculated. This procedure was repeated three times for each sample and carbonate content (%) of the sample was the mean of three values. Tables 3 and 4 show the major elements, rare earth elements (REE) and trace metals in the clay (<4 μm) fraction of sediments. The total REE content (∑REE) of the clays varied from 130.98 to 289.85 μg/g ( Figure 2; Table  3) in 15 rivers and a freshwater (Kolleru) lake. The average ∑REE of the rivers and lake was slightly lower than that of the WRAC 18 (205.14 μg/g) and Post-Archean average Australian Shale 44 (PAAS: 208.67 μg/g). The ∑REE was higher (230-290 μg/g) than in PAAS in some major (Gautami Godavari, Mahanadi and Brahmani) and medium (Nagavali and Baitarani) rivers ( Figure 2).

∑REE and its relation with light elements and heavy metals
Similarly, ∑REE was lower (130-144 μg/g) than in the upper continental crust 45 (UCC: 148.97 μg/g) in both medium (Palar) and major (Krishna) rivers. The ∑REE of the estuarine clay (70.89-352.61 μg/g) was lower than that of the corresponding river, except in the Brahmani and Baitarani estuaries ( Figure 2 and Table 4).
Scatter plots showed that ∑REE was positively correlated with clay content, both in rivers and estuaries (Figure 3  In estuarine clay, ∑REE showed moderate correlation with Al 2 O 3 (r = 0.52; P > 0.05) and weak correlation with Fe 2 O 3 (r = 0.48; P > 0.1; Figure  3 b-d). The correlation of ∑REE with K 2 O (r = 0.81) was strong and statistically most significant (P < 0.001). The correlations between ∑REE and heavy metals (Zr, Hf, U, Th) were positive and statistically significant (P > 0.001) in river clays and statistically most significant (P < 0.001) in estuarine clay (Figure 4 a-d). The carbonate content was lower in rivers than in estuaries (Tables 3 and 4). ∑REE was moderately correlated (r = 0.58) with carbonate content in river clay, but weak and negatively correlated (r = -0.51) in estuarine clay.
The inter-relationship among REE revealed that ∑LREE was moderately correlated with ∑MREE and ∑HREE (r = 0.45-0.59; P > 0.5 to 0.001; Figure 5  (La/Yb) n was significantly correlated with the ∑LREE/ ∑HREE ratio in river and estuarine clay and reference sediments ( Figure 5 d).

Sm/Nd and Y/Ho ratios
Scatter plots showed linear correlation of Sm with Nd, and Y with Ho ( Figure 6 a and c). The Sm/Nd ratio in river clay was marginally lower than in estuaries (

PAAS-normalized REE patterns, Ce and Eu anomalies
The PAAS-normalized REE patterns of river clay, except the Brahmani, showed MREE enrichment ((Sm/La) n and (Sm/Yb) n ratios varied from 0.95 to 1.24 and from 0.95 to 1.35 respectively; Table 3) over LREE and HREE ( Figure  7). They can be categorized into three types, based on the predominant geological formations in the drainage basins of rivers ( Figure 1 and Table 1). Type-1 REE patterns varied widely among six rivers which drain predominantly the Archean and Pre-Cambrian gneissic formations. For example, the Ponnaiyar and Palar river clay showed LREE (av (La/Yb) n = 1.35) and MREE-enriched and HREE-depleted (av (Lu/La) n ratio = 0.78) REE patterns, whereas the Cauvery and Pennar river clays showed MREE enrichment and near equal proportions of LREE (av (La/Yb) n ratio = 1.06) and HREE (av (Lu/La) n ratio = 1.02). The Nagavali and Vamsadhara river clays showed LREE-depleted (av (La/Yb) n ratio = 0.54) and MREE-and HREE-enriched (av (Lu/La) n ratio = 1.96) patterns. Clays from the Krishna and Godavari rivers and Kolleru Lake showed type-2 REE patterns, characterized by LREE-depletion (av (La/Yb) n ratio = 0.87) and MREE-and HREE-enrichment (av (Lu/La) n ratio = 1.24). These rivers drain predominantly the Deccan Traps. The type-3 REE patterns were MREE-enriched with near equal proportions of LREE (av (La/Yb) n = 0.95) and HREE (av (Lu/La) n = 0.94). Clays from the Mahanadi, Brahmani, Baitarini, Subarnarekha, Haldia and Hoogly rivers showed type-3 patterns. These rivers drain predominantly mixed lithology ( Table 1). The REE patterns in the estuarine clays were nearly identical to that of rivers. Positive Ce and Eu anomalies were present both in river and estuarine clays (Figure 7). Clays from the Chilika Lake, a brackish-water lake, showed type-3 REE patterns ( Figure 7).
The Ce anomaly (Ce/Ce*) values (Figure 8 a) were in the narrow range (1.04 to 1.15) for river clays, except Brahmani (2.31). The average Ce/Ce* value was not different for rivers draining the dominant Archean and    Pre-cambrian crust (1.10), volcanic crust (1.16) and mixed lithology crust (1.10). The Ce/Ce* values ranged from 1.0 to 1.22 in estuaries. The Eu anomaly (Eu/Eu*) values ranged from 1.17 to 1.52 and from 1.13 to 1.51 in the river and estuarine clays respectively (Figure 8 b). The mean Eu/Eu* was highest for rivers draining dominantly the volcanic crust (1.43), followed by metamorphic crust (1.34) and mixed lithology crust (1.23).

Controls on ∑REE of clays
Very high ∑REE values in major and medium rivers ( Figure 2) compared to that of PASS and WRAC imply that the abundance of REE is not dependent on the size of the drainage basin or the amount of detrital material supplied by rivers. It could be due to mineral composition in the clays (Table 1). Su et al. 9 reported that mineral com-position of the sediments is the ultimate control for REE composition. The positive correlation of ∑REE with total clay content (Figure 3 a), and Al 2 O 3 and K 2 O (Figure 3 b) in rivers suggest that the clay fraction is important as it contains abundant minerals, which are better host for REE 43 . Moreover, K-feldspar or K-rich clay also contributed to REE. Yang et al. 24 reported high REE content corresponding to finer grain-size fractions of the sediments. The low ∑REE associated with high clay content in Kolleru Lake (Table 3) indicates that the clay fraction alone is not responsible for variations in ∑REE content. Moderate correlation of ∑REE with Fe 2 O 3 in river clay (Figure 3 c) indicates association of REE with Fe-oxyhydroxides. Iron is known to exist as organically stabilized colloids in river water, and high REE are associated with high Fe in rivers 20 and are also influenced by Fe-oxyhydroxides in the sediments 23,46 .
Strong correlation between ∑REE and heavy metals (Figure 4 a-d) suggests REE contribution from heavy   22,25,47 . High concentration of heavy minerals has been reported in beach and nearshore sediments along the east coast of India; they are being mined at places for REE and radioactive metals and industrial metals 48 . The beaches of Tamil Nadu are enriched in garnet, ilmenite, zircon and magnetite, whereas the beaches of Andhra Pradesh consist of ilmenite, garnet, sillimanite, zircon and monazite 49 . Heavy minerals such as sillimanite, ilmenite, garnet, rutile, hornblende, zircon and monazite are important in both coastal dunes and beaches of Odisha. Charnockites and high-grade metamorphic rocks in the Eastern Ghats are responsible for the enriched concentration of heavy minerals. Therefore, heavy minerals in the finer-grain-size fractions of sediments may have contributed to REE enrichment. Moderate correlation between the carbonate content and ∑REE in rivers suggests probable contribution from carbonate-associated minerals such as phosphorites.
Weathering results in solution of REE from the parent rock. The dissolved REE, REE associated with colloidal phases and particulate matter are transported to the estuaries and then extensively removed because of salt-induced coagulation. Therefore, high REE content is expected and reported in estuarine sediments 5,50 . However, in the present study ∑REE in the estuaries was lower than in the rivers (Figure 2). Moderate to weak correlation of ∑REE with Al 2 O 3 and Fe 2 O 3 in the estuaries (Figure 3 b and c)  suggests that the REE removal processes are not active in low-salinity regions. Researchers have proposed that high pH, low REE concentration and REE complexation with carbonate ions and natural organic ligands stabilize REE in solution, resulting in low REE removal to the sediments in low-salinity regions 10,19 . Negative correlation between ∑REE and carbonate content in estuarine clay suggests that carbonate content dilutes the REE content.
The explanation for the relatively low ∑REE in lowsalinity regions of estuaries is as follows: Almost all rivers of the east coast of India supply high sediment load to the estuaries either during the southwest monsoon (June-September) or the northeast monsoon (November-February). Sediment supply through rivers is extremely low during other months. Therefore, it is possible that the REE-associated clays are quickly transported farther into the sea, because of strong river flow during the monsoons, resulting in less REE removal in low-salinity regions. Very high ∑REE in the estuarine clay of the Brahmani and Baitarani (Table 4) suggests that more REE-associated minerals from deposits in their drainage basins are transported into the estuaries.

PAAS-normalized REE patterns in rivers
The MREE-enriched REE patterns (Figure 7) may have been caused by at least three factors. (i) REE of clay fraction has been analysed in the present study. Bayon et al. 18 reported high MREE and HREE in clay fractions compared to the bulk sediment. (ii) Tropical conditions prevail in the drainage basins of all rivers and therefore chemical weathering is predominant. Nesbitt and Markovics 12 suggested that intense chemical weathering causes MREE-enrichment in river sediments. (iii) Rivers such as the Cauvery drain through phosphate-rich limestones. Weathering of phosphate minerals also causes MREE-enrichment 20 . Strong correlation between ∑MREE and ∑HREE ( Figure 5 c) indicates that MREE and HREE are from the same source. The REE patterns are categorized into three types (Figure 7).

Type-1 REE patterns:
Although the geological formations and weathering conditions in the drainage basins are similar for the medium (Ponnaiyar and Palar) and major (Pennar and Cauvery) rivers, their REE patterns are different (Figure 7). These rivers drain the Archean and Pre-Cambrian formations comprising hornblende-biotite gneisses and granitoid gneisses in the upper reaches. In the lower reaches, the medium rivers drain Cretaceous sandstone and limestone with intercalated clay, whereas the major rivers drain basaltic intrusions intercalated with sandstone and limestone, laterites and alluvium ( Table 1). The CIA values for the clay in medium (79-81) and major (77-81) rivers indicate that the rocks weather under moderate to high degree of chemical weathering. Under these conditions, HREE are preferentially released to the dissolved loads and the residual clay is enriched in LREE. The linear correlation of ∑LREE/∑HREE and (La/Yb) n points to the LREE-enriched sediments ( Figure  5 d). The high (LREE/HREE) n and (La/Yb) n ratios also indicate LREE enrichment (Table 3). LREE-enriched REE patterns were reported for sediments from the Archean metamorphic formations 4 . Bayon et al. 18 suggested that the weathering of granitic/granodioritic soils results in significant LREE enrichment over other REE. The LREE-and MREE-enriched and HREE-depleted REE patterns in the Ponnaiyar and Palar rivers suggest that REE are controlled by the geology of their drainage basins.
The shale-like REE patterns in the Cauvery and Pennar (major) rivers indicate increased HREE concentration compared to the medium rivers (Figure 7). Relatively high smectite in these sediments indicates increasing proportions of swelling clay minerals that favour high cation exchange capacity (CEC) and high REE adsorption.
The HREE released from the rocks in the upper reaches may have been adsorbed onto clays and Fe-oxyhydroxides during long-distance transport, and resulted in REE patterns close to the shale. Chang et al. 16 attributed the fractionation between LREE and HREE to high adsorption affinity of REE to the secondary Fe, Mn and Al minerals in the soils. In other words, the shale-like REE patterns of the Cauvery and Pennar river clay are related to the mixing of upstream sediments with those in the lower reaches during long-distance transportation and sedimentation.
The Nagavali and Vamsadhara rivers drain the Eastern Ghats (mountain ranges) composed of Pre-Cambrian khondalites ( Table 1). The manganese ores are intimately associated with khondalites 34 . High Mn content in the Nagavali river clay (Table 3) attests that the clay is weathered and eroded from these ores and transported short distances, as these Ghats are located close to the shore. Moriyama et al. 35 reported HREE-enriched REE patterns with positive Eu anomaly in Mn ores. The LREE-depleted and HREE-enriched REE patterns in the Nagavali and Vamsadhara river clay could be due to the erosion from ore deposits and preferential weathering of felsic component of the metamorphic rocks, thus representing source-rock characteristics. Wide variations within the type-1 REE patterns imply that the abundance of REE depends on the chemical and mineral composition of rocks in the drainage basin and the extent of sediment mixing from different sources during transport. Su et al. 9 suggested that the REE composition is controlled largely by mineral composition and clay minerals, Mn-Fe hydroxides and secondary phosphate minerals.

Type-2 REE patterns:
The LREE-depleted and MREEand HREE-enriched REE patterns are characteristic of clay from the Krishna and Godavari rivers and Kolleru Lake (Figure 7). The Godavari is the largest river in peninsular India. Vasishta Godavari is a tributary of the Godavari, separated from the main river (Gautami Godavari) in the lower reaches at ~100 km from the Bay of Bengal. The basic volcanic rocks (Deccan Traps) cover 50% of the total drainage basin of the Godavari and, onethird of the drainage basin of the Krishna in the upper reaches (Table 1). Pre-Cambrian granitic and gneissic rocks and deltaic sediments occur in the middle and lower reaches of both rivers (Figure 1 and Table 1). Several studies have reported positive Eu anomaly and HREEenriched REE patterns as weathering products from volcanic rocks 4,18 . Ce anomaly is not the characteristic of basalts. The presence of Ce anomaly indeed shows mixing of volcanic sediments with gneissic rock-derived sediments in the lower reaches. Strong river flow and high sediment load from upstream during monsoon months may have dominated the clay in the lower reaches, favouring LREE-depleted and HREE-enriched patterns. In the Gautami Godavari, the LREE concentration is close to that of HREE (Figure 7). Gautami Godavari being a large/major river, the REE produced in the upper reaches may have been efficiently mixed with clay from the lower reaches during long-distance transport, resulting in shale-like pattern.

Type-3 REE patterns:
The MREE-enriched patterns with near equal proportions of LREE and HREE are typical for the clay from the Mahanadi, Brahmani, Baitarini and Subarnarekha rivers (Figure 7). These rivers drain through igneous and metamorphic rocks, abundant mineral (ilmenite, rutile, zirconium and rare earth minerals) and ore deposits of both (Fe-Mn) oxides and (Cu-Pb) sulphides in the upper reaches, and lateritic, yellow and red soils and deltaic sediments in the middle and lower reaches ( Table 1). The Fe-Mn oxide ores exhibit LREEdepleted and MREE-and HREE-enriched REE patterns with positive Ce and Eu anomalies 35,51,52 . The LREEenriched REE patterns are characteristic of Cu and Zn sulphide deposits, and Pb sulphides exhibit HREEenriched patterns with positive Eu anomaly 53 . Positive correlation of ∑REE with heavy metals (Figure 4) suggests that the REE composition is influenced by heavy minerals, which were reported even in the 0.1-2.0 μm size range of sediments 19,22 . Minerals such as garnet, zircon and xenotime produce HREE-enriched patterns, while monazite and allanite produce LREE-enriched REE patterns 4,23,24 . Strong correlation of ∑REE with K 2 O content (Figure 3 d) suggests REE contribution from Kfeldspars and micas, common in granites and high-grade metamorphic rocks. Alteration of K-feldspar produces LREE-enriched patterns with positive Eu anomaly 43 . High ∑REE and shale-like REE pattern of these four rivers thus demonstrates efficient mixing of clay from different lithologies.
The Haldia and Hoogly river clay exhibits slightly enriched HREE (av (Lu/La) n = 1.22) over LREE (av (La/Yb) n = 0.87; Figure 7). These are medium rivers and tributaries of the Ganges, which drains Pre-Cambrian metamorphic rocks from the Himalaya and sedimentary formations in the lower reaches. The low CIA values for the clay in these rivers (75-77) attest more of physical weathering products than chemical weathering. The clays derived from felsic component of metamorphic rocks and reworked sedimentary formations probably yielded slight HREE-enrichment over LREE.

PAAS-normalized REE patterns in estuarine sediments
The ∑REE of estuarine clay are 3-50% lower than in their rivers ( Figure 2). However, the PAAS-normalized REE patterns of estuarine clay are nearly similar to those of rivers (Figure 7). Indeed, one would expect significant fractionation of REE in the low-salinity regions of estuaries because of the salt-induced coagulation of particulates and changing proportion of colloidal and dissolved REE pools. Elderfield et al. 5 reported increased ∑REE content from ~30% to near quantitative in the lowsalinity regions. The low ∑REE content in the estuarine clay could be due to other factors (high pH, low REE concentration and solution complexation) 19,20 or, as mentioned earlier, strong river flow during monsoon months most probably inhibited estuarine processes and quickly transported river sediments to the continental shelf. In other words, hydrodynamic conditions control REE abundance and fractionation in the estuaries.

Ce and Eu anomalies
Ce in river sediments tends to be unfractionated with respect to other LREE. However, Ce 3+ oxidizes to less soluble Ce 4+ in estuarine and marine water, and is rapidly removed to the sediments by particle scavenging. Therefore, one would expect pronounced negative Ce anomaly in seawater and positive Ce anomaly in the estuarine sediments 6 . Nearly identical Ce anomaly values in the clay of each river and estuary (Figure 8 a), imply that Ce is unfractionated in the low-salinity regions of estuaries. The strong and efficient river flow probably inhibited estuarine processes and therefore there was no significant change in Ce/Ce* in rivers and low-salinity regions of estuaries. Since Ce/Ce* is weakly correlated with Fe 2 O 3 or MnO in both rivers and estuaries, Ce is not associated with Fe-Mn oxyhydroxide phases. High Ce anomaly in the Brahmani River may be from the hinterland heavy minerals enriched in the clay fractions.
Positive Eu anomaly is characteristic of charnokites of southern India 54 , banded-iron formations, Pre-Cambrian gniesses, Fe-Mn-bearing arenites, hydrothermal and volcanic sediments and anthropogenic organic pollutants 55,56 . High Eu anomaly values in rivers draining volcanic rocks, followed by metamorphic rocks and mixed lithology formations are in agreement with the lithological formations in their drainage basins. No significant change in the absolute Eu anomaly in the river and estuarine clay indicates that Eu is not influenced by estuarine processes.

Controls on Sm/Nd ratio of sediments
Strong correlation between Sm and Nd both in river and estuarine clay (Figure 6 a) suggests their similar geochemical behaviour during weathering, and the Sm/Nd ratio is unaffected. Similar Sm/Nd ratios (~0.201) of clay in rivers draining the Archean and Pre-Cambrian crust and mixed lithology (Figure 6 b) and a value close to that of the Ganges River sediments (0.205) indicate mixing of REE from different sources.
The relatively high Sm/Nd ratio of clay from volcanic crust (0.211; Figure 6 b) is in agreement with the fact that this ratio increases linearly with decreasing age 57 . Both Sm and Nd are REE. The REE-bearing minerals respond differently during differential weathering 50,58 . For example, Nd from feldspar is easily weathered during intense chemical weathering. The weathered products such as clay exhibit low Sm/Nd ratio and residual rock/coarser sediments exhibit high Sm/Nd ratio. Thus, the sediments derived from basalts or those with felsic composition show high Sm/Nd ratio. Therefore, the high Sm/Nd ratio represents mineral composition of the sediments. High Sm/Nd ratio of clay from the Nagavali and Vamsadhara (Figure 6 b) may be because of ore minerals in the clay fraction; their original ratio is preserved because the rivers are medium in size and drain mono-lithology.
Since the REE chemistry of <4 μm fraction of sediments alone was studied, the influence of grain size on Sm/Nd ratio cannot be ascertained. The Sm/Nd ratio is 0.198 for major river sediments with high clay content (37-47%) and for medium river sediments with low clay content (<30%). Similarly, high Sm/Nd ratio (0.201-0.223) is associated with the Krishna (a major river) with high clay content (45-55%) and medium rivers with low clay content (<30%). This implies that the Sm/Nd ratio is determined by mineral composition of clay rather than clay content of the sediments.

Y/Ho ratio
Strong positive correlation between Y and Ho ( Figure  6 c) suggests their similar geochemical behaviour. The Y/Ho ratio of clay from the Archean and Pre-Cambrian crust (av 27.2) is close to that of PAAS: (27.4), suggesting that high ratios reflect metamorphic crust. Feng 50 reported high Y values associated with metamorphic rocks, laterites, Fe-Mn deposits and heavy minerals. Lowest Y/Ho ratio (av 26.2) for clay from volcanic rocks ( Figure 6 d) supports the findings of Bau and Zhao 58 that this ratio is lower in basaltic rocks (21.9-26.5) than in granitic rocks . This ratio is ~26.0 in the Pennar and Baitarani river clay (Figure 6 d), and is associated with high Fe and Mn. It suggests greater adsorption of Ho onto organic-rich clayey particulates and/or Fe and Mn particulates leading to low Y/Ho ratios 50,59 . Since Fe-oxyhydroxides have greater affinity for Ho relative to Y, it is likely that Ho is adsorbed on these particulates resulting in low Y/Ho ratio.
Fractionation of Y/Ho ratio usually occurs during weathering and transportation from river to estuary. Strong fractionation of Y/Ho ratio in the estuaries of the Cauvery, Ponnaiyar, Palar, Pennar and Krishna ( Figure  6

REE patterns of clays for major rivers in India and the world
Except the Krishna, other major rivers exhibit slight MREE enrichment with near equal proportions of LREE and HREE, similar to that of shale ( Figure 9). The uniform shale-like REE patterns suggest effective mixing of clay from different sources during erosion, transportation and deposition. On the other hand, sediments in the major tropical rivers of the world exhibit uniform LREEenriched patterns (Figure 9) attributed to efficient mixing of sediments during erosion, transportation and deposition 4,24 . In other words, clay in the Indian major rivers shows slightly enriched MREE and HREE compared to that of major rivers in the world.

Conclusion
The distribution and fractionation of REE in the clay fraction of sediments in 15 rivers, their estuaries and two lakes in the east coast of India indicate the following: (i) ∑REE of the river clay is controlled by the mineral composition of sediments. (ii) The PAAS-normalized REE patterns in river clay are of three types. They are controlled by the dominant geological formations in their drainage basins and extent of sediment mixing during transport and sedimentation. (iii) Low ∑REE in estuarine clay compared to their rivers, and similar REE patterns in both rivers and estuaries suggest that REE removal to the sediments is controlled by hydrodynamic conditions in low-salinity regions of estuaries. (iv) The Sm/Nd ratios of clay are in agreement with the age of major geological formations in the drainage basins and mineral composition of sediments. (v) The Y/Ho ratios in rivers simply reflect source-rock signatures, while in estuaries the lower ratios reflect complexation processes of Ho onto Fe-Mn oxyhydroxides and organic particulates.