The magnesium-to-calcium (Mg/Ca) and strontium-to-calcium (Sr/Ca) ratios in certified reference materials (CRMs) and natural carbonate samples have been analysed using an inductively coupled plasma optical emission spectrometer (ICP-OES) for method validation. The CRMs included ECRM 752-1, a limestone standard and NIST SRM 915a, a pure calcium carbonate standard, while the marine carbonates included planktonic foraminifera ( Globigerinoides ruber ) from the southeastern Arabian Sea and large benthic foraminifera ( Nummulites ) from the Palaeogene Jaisalmer Basin, Rajasthan, India. The study achieved high precision and accurate analysis of Mg/Ca and Sr/Ca ratios. The observed values for ECRM 752-1 were consistent with certified data, exhibiting relative standard deviations (RSD) of 2.15% for Mg/Ca and 2.07% for Sr/Ca. Further, the SST reconstructions from Mg/Ca measurements of G. ruber indicated a temperature increase of approximately 4°C from the Last Glacial Maximum (LGM) to the Holocene. In contrast, the lower Mg/Ca and Sr/Ca ratios in Nummulites suggest post-depositional alteration, emphasising the necessity of considering diagenetic effects before deriving palaeotemperature, particularly for deep geological past samples. The reliability of ICP-OES in generating quality geochemical data for palaeothermometry is emphasised.

ABSTRACT The Himalayan river system (HRS) constitutes a major conduit for continental material transport to the adjacent oceans. The unresolved geological complexity and active tectonics of the Himalaya bring heterogeneity in sediment geochemistry. There is no known certified reference material (CRM) that represents the geochemical heterogeneity of HRS and large tropical rivers (LTR) though they rank first in terms of sediment transport and water discharge. As an ongoing exercise to develop reference material to calibrate the instruments and to validate the geochemical results, a river bedload sediment sample from the Ganga river basin (GBS) which is representative of both HRS and LTR was collected in sufficient quantity and characterized for its geochemical composition using inductively coupled plasma instrumentation techniques (ICP-OES and ICP-MS). The GBS has been analyzed for major, trace, and rare earth elements (REEs) to infer the homogeneity and suitability of geological material for reference material characterization through the test of reproducibility and representativeness. The precision of measurement (% RSD) within and across the four analytical sessions is less than < 7 % for most of the elements measured. The method is validated with respect to precision, limit of detection (LOD), limit of quantification (LOQ), and measurement uncertainties by analyzing CRMs. The concentration values of GBS are reproducible. The GBS composition also showed a good correlation with published datasets from the HRS and LTR. Thus, the GBS sample is developed as a potential matrix matching geological standard for geochemical studies of the Himalayan river sediments as well as other large tropical river systems.

Iron speciation has emerged as a robust proxy for discerning oceanic redox conditions; nonetheless, it is subject to certain limitations. Specifically, the applicability of the degree of pyritization is contingent upon the presence of unequivocal evidence of an anoxic water column and its discriminatory capacity is limited to distinguish between ferruginous (anoxic) and euxinic conditions. This study highlights that through the integration of redox‐sensitive trace metal enrichment data with Fe‐speciation data, the depositional redox conditions for marine sediments can be established with greater certainty. Recently, a set of dedicated geological reference materials (BHW and WHIT) have been developed for validating the Fe‐speciation analytical results for redox reconstruction studies; however, to the best of our knowledge, these reference materials are not characterized for trace and rare earth elements (REEs). In this connection, the BHW (oxic) and WHIT (anoxic) reference materials are measured for major, trace and REEs. After careful statistical considerations for these reference standards, a complete set of trace and REEs is reported. Furthermore, considering BHW and WHIT as oxic and anoxic end‐members, respectively, the utility of trace metal enrichment and Fe‐speciation data in combination has been discussed. The trace and REE concentrations of BHW and WHIT reported in this study will enhance their applicability as a reference material to understand ocean chemistry and the oxidation state of the ancient oceans.

Palaeo-monsoon and palaeoclimate conditions over Southeast Asia are a matter of debate despite notable studies on the continental and oceanic sedimentary record. The present study investigates the environmental magnetic and geochemical records preserved in the deep marine sediments of the northeastern (NE) Arabian Sea to elucidate the erosion history of the western Himalayas and its link with the prevailing hydroclimatic conditions since the late Miocene. For this, the sediment core retrieved during International Ocean Discovery Program (IODP) Expedition 355 at Site U1457 in the NE Arabian Sea has been explored. The results reveal that the hydroclimatic conditions were predominantly arid during the late Miocene, except for humid intervals from 6.1 Ma to 5.6 Ma. Humid climate conditions in the Indus River Basin returned during the mid-Pliocene and continued to the Pleistocene with an intense chemical weathering regime from 1.9 Ma to 1.2 Ma. The dominant sediment source to the NE Arabian Sea at Site U1457 during the late Miocene and the Pliocene was the Indus River, while during the Pleistocene, mixed sediments brought by the Indus River and the Peninsular Indian rivers were observed. The sediment contribution from a chemically less altered mafic source (the Deccan basalts) increased between 1.2 Ma and 0.2 Ma, possibly linked to a weak Indian Summer Monsoon. The summer monsoon wind strength and associated shift in the Inter-Tropical Convergence Zone (ITCZ) influenced the dominant sediment provenance at Site U1457 of the Laxmi Basin.

This study presents the first results on palaeoredox condition in the northern Indian Ocean since the late Miocene using non-traditional stable isotopes of molybdenum (Mo) and tungsten (W). For this, the marine sediment samples were collected at the Site U1457 (67°55.80'E, 17°9.95'N, water depth 3534m) of Laxmi Basin during IODP 355 Arabian Sea monsoon expedition. The redox sensitive trace elements (Mo, W, U, V, Ba, Cd and P) as well as stable molybdenum isotope (δ 98/95 Mo relative to NIST SRM 3134 lot No. 130418) and stable tungsten isotope (δ 186/184 W relative to NIST SRM 3163 lot No. 080331) compositions were used as a palaeoproxy to demarcate the long-term (million-year scale) ocean deoxygenation history. The bulk Mo concentration in the core sediment varied from 0.25 μg g -1 to 3.59 μg g -1 (avg. 0.74 μg g -1 ) which is less than that of the average upper continental crust (UCC) concentration. The W concentration in the sediment varied from 0.49 μg g -1 to 3.67 μg g -1 (avg. 1.75 μg

The changes in precipitation pattern provide an understanding on the hydroclimatic response to global warming during the Anthropocene. The present study investigates sources of precipitation moisture for the Indian Monsoon and the local environmental mechanisms controlling its distribution over the southwest coast of India. This is achieved by the characterization of stable isotope ratios of oxygen (delta O-18) and hydrogen (delta H-2) in rainwater samples collected from a high humid tropical setting (Swarna-Madisal river basin) of the Western Ghats, South India and another station further south (Bakrabail, southern edge of Nethravati river basin). This study contributes to the detailed investigation on rainwater isotopic composition and microclimate characteristics which is lacking in the humid west coast region. The rainwater isotopic composition of coast was close to that of the Arabian Sea water and reflected the first condensate of vapours which were originally formed under fast evaporation at the nearby ocean. In inland location, the higher d-excess values reflect continental moisture recycling. Evapotranspiration has led to higher kinetic fractionation effect in the inland region. The isotopic storm effect during winter monsoon season suggested the rain distribution from saturated air masses formed under deep convective effect in the Bay of Bengal. The overall local meteoric water line (LMWL) in the SwarnaMadisal basin was found to be delta H-2 = {(7.2 x delta O-18) + 7.5}, R-2 = 0.98. Further south, the LMWL of Bakrabail was, delta H-2 = {(8.19 x delta O-18) + 16.1}, R-2 = 0.98 for annual observation and displayed minimal variability for interseasonal slopes (7.73 for summer monsoon, 8.48 for winter monsoon and 8.36 for pre-monsoon) and intercepts (15.6 for summer monsoon, 17.8 for winter monsoon and 15.5 for pre-monsoon). In regions of vegetation dominance and humid climate, the prevailing local air mass masked the rain-out effect of marine air mass as well as the amount effect which support microclimatic settings at the local precipitation sites in southwest India. The time and space variability of regional moisture circulation in controlling atmospheric water balance has been deduced in this study. Thus, the high efficacy of stable isotopes in tracing the manifestation of microclimate in the humid tropics has been demonstrated.

The present study investigates the oxygenation history of the northeastern Arabian Sea since the late Miocene using redox sensitive elemental and metal stable isotopic signatures in the deep-sea sediments. To achieve this, the sediment core samples collected at Site U1457 (67 degrees 55.80'E, 17 degrees 9.95'N, water depth 3534 m) of Laxmi Basin in the northeastern Arabian Sea during the International Ocean Discovery Program (IODP) Expedition 355 were analysed for a suite of elemental (Mo, W, U, V, Ba, Cd and P) and stable molybdenum (Mo) isotope (delta Mo-98/95 relative to NIST SRM 3134 lot No. 130418) as well as stable tungsten (W) isotope (delta W-186/184 relative to NIST SRM 3163 lot No. 080331) composition. Sedimentary delta Mo-98/95 values (-0.70 parts per thousand to +1.18 parts per thousand) at IODP Site U1457 in the northeastern Arabian Sea indicated partial authigenic Mo component. In contrast, the sedimentary 8186/ 184W values (-0.02 parts per thousand to +0.21 parts per thousand) were in the range similar to that of lithogenous material suggesting dominance of detrital composition. The study reveals that the water column in the eastern Arabian Sea was oxic during the late Miocene and Pliocene while oxic to suboxic condition prevailed during the Pleistocene. The study also explores that under oxic to suboxic condition with limited particle shuttling, the W isotopes do not undergo significant fractionation, and their isotope ratios reflect the detrital source signature. This study reports the first results on isotopic compositions of Mo and W in sediments of the northeastern Arabian Sea since the late Miocene to investigate the palaeoredox conditions on a million-year time scale.

•Synoptic scale observation of water isotopes over high elevations of Western Ghats.•Study reports elevation effect on stable isotope ratio of water in the Western Ghats.•Reports role of humid tropical mountains in controlling seasonal monsoon moisture.•Traces moisture source and mixing processes along highlands of the Western Ghats.•Provides insight into processes controlling regional hydrology in the Western Ghats.•Unravels factors affecting local microclimate to seasonal climate in South India.

The Western Ghats form a major mountain belt, next to the Himalayas, in controlling the flux of water and carbon to the northern Indian Ocean. This study attempts to understand the water and carbon cycles in two humid tropical river basins with its streams originating at higher altitudes of the Western Ghats, India. Water and suspended particulate matter (SPM) were collected on a monthly scale during summer monsoon season (June-September) from Swarna and Nethravati rivers draining into the Arabian Sea. For the source apportionment, samples have been measured for stable isotopes of oxygen (δ 18 O) and hydrogen (δ 2 H) in water and stable isotopes of carbon (δ 13 C POC ) in particulate organic matter (POM) at spatial scale from tributaries and main channel of rivers, and runoff water from agricultural land (dominant paddy field) and forest in the downstream region. The association between δ 18 O and deuterium-excess in river water and rain water shows that water in these tropical basins depicts rainout effect of marine source moisture during the onset of summer monsoon. As the monsoon intensifies, the fresher rain water replenishes older water stored previously in sub surface soil layer leading to its flushing into the river during summer monsoon season. Stable carbon isotope ratio and elemental ratio of POM (δ 13 C POC = -27.1 ± 0.4 ‰ and C/N = 8.1 ± 1.7) in two humid tropical river water during summer monsoon season is an admixture of suspended particulates from runoff water of forest (δ 13 C POC = - 27.82 ± 0.4 ‰) and agricultural land (δ 13 C POC = -26.29 ± 0.4 ‰). It is found that δ 13 C POC shows minimal variability with SPM content and C/N ratio within the same organic carbon pool. The study emphasizes the need to consider the agricultural runoff contribution to the rivers while establishing the global elemental budget and observing the global climate change.

Several studies have confirmed the role of chemical weathering in sequestering atmospheric carbon dioxide on a million year time scale [1]. The sequestered carbon dioxide during weathering, is transported as bicarbonates into the ocean and deposited as carbonates, which can be locked up in the ocean bottom for several hundred years. Recent studies have indicated that chemical weathering has intensified in the past few decades on account of land-use/ land-cover changes (eg. deforestation) [2], which has doubled the CO2 uptake by the silicate and carbonate rocks [3]. This can be significant on a decadal to centennial time scale. Hence there is an increasing interest generated to quantify the CO2 drawdown during weathering in the tropical river catchments, which make up 60% of the total water discharge into the world oceans [4]. Very few studies exist from India, that has reported on tropical silicate rock catchments as a potential CO2 sequester. We have attempted to estimate the silicate weathering rate and CO2 sequestration in the Western Ghats through the study of a representative river (Swarna) in the southwestern India that drains a predominantly silicate catchment. We observed very high silicate weathering rate (65.96 tons/km2/yr) which is higher than all the reported west flowing rivers of India draining Deccan basalts [5] and granitic gneiss [6]. We have attributed the high silicate weathering rate to intense monsoonal rainfall (~5000 mm yr-1) in the upper reaches of Swarna river, dominantly silicate lithology and land-use/ land-cover changes. The carbon-dioxide consumption rate (CCR) in the Swarna river is estimated at 6.72 x 105 mol/km2/yr, which is equal to the Deccan rivers, whereas 2.3 times higher than the adjacent Nethravati river, which is also draining a predominant granite-gneissic lithology. The higher CCR in the Swarna catchment could be from the type of rock assemblages and land-use, land-cover changes. Therefore, detailed studies on chemical weathering and associated CO2 sequestration from rivers draining Western Ghats is required for drawing an accurate estimation of carbon dioxide sequestration in the tropical Western Ghats.

The study presents the seasonal and inter-annual monitoring of molybdenum (Mo) distribution and variability in humid tropical riverine and estuarine systems (Nethravati, Gurupur and Mandovi estuaries) of west coast of India. The study investigates the geochemical behaviour of Mo in the river and estuaries, and their ultimate fluxes into the ocean. The riverine flux of dissolved Mo (DMo) to the Nethravati, Gurupur and Mandovi estuaries are 1800molyr−1 (4.88molday−1), 195molyr−1 (0.53molday−1) and 10.5×103molyr−1 (28molday−1) respectively, and the riverine particulate Mo (PMo) flux to Nethravati estuary is 10.8×103molyr−1. The DMo in river (~30 to 40%) is scavenged onto particles under oxidized acidic river water conditions and subsequently released in the estuary, impacting the solute budget of Mo to the sea.In the estuaries, under low salinity conditions, DMo is sequestered onto particles during pre-monsoonal season. The DMo sequestration in the estuary is estimated to be ~2molday−1 in the Nethravati estuary and ~1.9molday−1 in the Mandovi estuary. During this season sequestration in the estuary is higher than the riverine supply, indicating the sequestration of both marine and river borne DMo. However, the mechanisms involved in the removal process are different in these estuaries viz. oxidative adsorption process in the Nethravati-Gurupur estuary and microbial utilization in the Mandovi estuary. The lower salinity region during monsoon and post-monsoon season shows slight excess of DMo, river borne particulate Mo could release up to 3 to 4nmolL−1 by desorption under alkaline higher ionic strength conditions. At higher salinity (>20psu) in both the estuaries and in all the seasons, DMo gain is systematic (~1 to 37nmolL−1). Mo release from river borne particles could contribute only up to 3 to 4nmolL−1, which is not sufficient to balance the observed Mo excess. On the other hand, the reductive Mo remobilization from bottom sediments (Mo=4mgkg−1) during sediment diagenesis and subsequent tidal activity, release up to 28nmolL−1 of DMo to the estuarine water. Mo release to water column is supported by the gradual enrichment of DMo with depth in the estuary. Therefore, diagenetic release of DMo forms the potential source of DMo excess in the estuary.

RATIONALE The small river basins in the narrow stretch of the Arabian Sea coast of southwest India experience high annual rainfall (800-8000 mm), with a higher proportion (85 %) during the summer monsoon period between June and September. This is due to a unique orographic barrier provided by the Western Ghats mountain belt (600-2600 m) for the summer monsoon brought by the southwesterly winds. This study is the first of a kind focusing on the water cycle with an intensive stable isotopes approach (samples of river water, groundwater, rainwater; seasonal and spatial sampling) in this part of the Western Ghats in Karnataka and also in the highest rainfall-receiving region (with places like Agumbe receiving 7000-8000 mm annual rainfall) in South India. In addition, the region lacks sustainable water budgeting due to high demographic pressure and a dry pre-monsoon season as the monsoon is mainly unimodal in this part of India, particularly close to the coast. METHODS The stable isotopic compositions of groundwater, river water and rainwater in two tropical river basins situated approximately 60 km apart, namely the Swarna near Udupi and the Nethravati near Mangalore, were studied from 2010 to 2013. The δ(18)O and δ(2)H values of the water samples were measured by isotope ratio mass spectrometry, and the d-excess values calculated to better understand the dominant source of the water and the influence of evaporation/recycling processes. RESULTS The water in the smaller area basin (Swarna basin) does not show seasonal variability in the δ(18)O values for groundwater and river water, having a similar mean value of -3.1 ‰. The d-excess value remains higher in both wet and dry seasons suggesting strong water vapor recycling along the foothills of the Western Ghats. In contrast, the larger tropical basin (Nethravati basin) displays specific seasonal isotopic variability. The observation of higher d-excess values in winter with lower δ(18)O values suggests an influence of northeast winter monsoon water in the larger basin. CONCLUSIONS The narrow coastal strip to the west of the Western Ghats displays unique water characteristics in both tropical river basins investigated. For the smaller and hilly Swarna basin, the dense vegetation (wet canopies) could largely re-evaporate the (intercepted) rain, leading to no marked seasonal or altitude effect on the water isotope values within the basin. The larger Nethravati basin, which stretches farther into the foothills of the Western Ghats, receives winter monsoon water, and thus exhibits a clear seasonal variability in rainfall moisture sources. The degree of water vapor recycling in these wet tropical basins dominates the isotopic composition in this narrow coastal stretch of South India. An insight into the soil water contribution to the river water and groundwater, even in the rainfall-dependent tropical basins of South India, is provided in this study.

River water composition (major ion and 87Sr/86Sr ratio) was monitored on a monthly basis over a period of three years from a mountainous river (Nethravati River) of southwestern India. The total dissolved solid (TDS) concentration is relatively low (46mgL−1) with silica being the dominant contributor. The basin is characterised by lower dissolved Sr concentration (avg. 150nmolL−1), with radiogenic 87Sr/86Sr isotopic ratios (avg. 0.72041 at outlet). The composition of Sr and 87Sr/86Sr and their correlation with silicate derived cations in the river basin reveal that their dominant source is from the radiogenic silicate rock minerals. Their composition in the stream is controlled by a combination of physical and chemical weathering occurring in the basin. The molar ratio of SiO2/Ca and 87Sr/86Sr isotopic ratio show strong seasonal variation in the river water, i.e., low SiO2/Ca ratio with radiogenic isotopes during non-monsoon and higher SiO2/Ca with less radiogenic isotopes during monsoon season. Whereas, the seasonal variation of Rb/Sr ratio in the stream water is not significant suggesting that change in the mineral phase being involved in the weathering reaction could be unlikely for the observed molar SiO2/Ca and 87Sr/86Sr isotope variation in river water. Therefore, the shift in the stream water chemical composition could be attributed to contribution of ground water which is in contact with the bedrock (weathering front) during non-monsoon and weathering of secondary soil minerals in the regolith layer during monsoon. The secondary soil mineral weathering leads to limited silicate cation and enhanced silica fluxes in the Nethravati river basin.

The Southwestern part of Peninsular India is one among the shield terrains experiencing extreme geological, geomorphological and climatic gradients. Many small rivers originate in the western slope of Western Ghats and flow towards the Arabian Sea. The moisture from the Arabian Sea carried by the southwesterly winds forms the primary source of water for these rivers. These west flowing rivers exhibit characteristic water chemistry distinct from that of the east flowing rivers and the rivers draining the Deccan traps. The lithological heterogeneity and the intensity of weathering have significant effect on the river water as observed by the difference in water chemistry between the Deccan, East flowing and West flowing rivers. The geomorphological settings of the Western Ghats (which brings-in the rainfall over its western slope) induce higher surface runoff which has significant effect on the weathering process and thus, on the water chemistry. However, the slope of the terrain has minimal effect on the chemistry of water in this region. The weathering of sedimentary formations in the plains leads to elevated fluxes of silica and radiogenic strontium. The weathering of bedrock forms the main source of trace elements to the river water in this region. The abundance of trace elements in monsoon dominated terrain is controlled primarily by the discharge. However, the secondary processes namely, redox reactions and oxidative scavenging of surface reactive metals by the oxyhydroxides of Fe and Mn appears to have significant role in determining the geochemical abundance. Unlike the tropical river basins of Africa and South America where the organic carbon complexation plays a vital role in dissolved metal abundance, in the rivers of Southwestern India, the control of organic complexation on the dissolved metal chemistry varies widely between rivers. This could be due to variable abundance of labile fraction of organic carbon in these rivers. The secondary geochemical processes lead to the enrichment of metals in sedimentary phase. However, the enrichments of these metals are within the permissible limits. From the pollution point of view, the metal contaminant studies in these river basins need to consider the rigorous fluvial geochemical redistribution of metals between particulate and dissolved phases to minimize the erroneous attribution of metal sources. The metal isotope tracing of pollution sources could be useful for accurate determination of pollution sources.

The thesis on ‘Geochemical and isotopic studies of the Swarna-Madisal River and its estuary, Southwest coast of India’ investigates the chemical weathering aspects, working of hydrological cycle and biogeochemical cycling of trace elements in the riverine and estuarine environment of the small tropical river basin, Swarna-Madisal basin, Southwest coast of India. The Swarna-Madisal river basin lies on the western part of the Western Ghats and receives higher rainfall mainly during the south-westerly summer monsoon season. The river basin is dominated by the silicate rock lithology with the Peninsular Gneiss covering the major part and the Dharwar Schists in the eastern edge of the basin. The short river length with the steep gradient particularly at the initial stage near the Ghats and higher surface runoff in the basin could support rapid transport of water and sediments to the nearby Arabian Sea. These characteristics feature the Swarna-Madisal river basin as an ideal setting to carry out chemical weathering and biogeochemical studies using advanced geochemical and isotopic tracers. Moreover, no geochemical and isotopic studies have been carried out earlier in the Swarna-Madisal river basin, even as half a million people of Udupi district use it for domestic and irrigational purposes. As large community in this region depends on freshwater of the Swarna River, the study on the geochemistry of this west flowing river becomes essential for better water management and sustainable development. With these views, the geochemical and isotopic studies have been carried out in the Swarna-Madisal river basin. Chemical weathering in river basin forms the key process to study the global climate change on a long term scale due to its association with the carbon sequestration. Water samples from the surface and sub-surface regions were collected for a period of two years to study the chemical weathering process and to quantify silicate weathering and associated carbon-dioxide consumption rates in the Swarna river basin. Physico-chemical aspects, major ion composition and radiogenic strontium isotopic ratio measured in the Swarna river water reflect the dominance of silicate rocks in the river basin. The river water chemistry is found to be least affected by the anthropogenic impact; however, the effect of evaporation is observed on few samples during the pre-monsoon. The delineation of percentage contribution from different sources shows that the dissolved major ion is mainly from the weathering sources. The chemical weathering rate (CWR), silicate weathering rate (SWR) and carbon-dioxide consumption rate (CCR) in the Swarna river

The study presents a 3-year time series data on dissolved trace elements and rare earth elements (REEs) in a monsoon-dominated river basin, the Nethravati River in tropical Southwestern India. The river basin lies on the metamorphic transition boundary which separates the Peninsular Gneiss and Southern Granulitic province belonging to Archean and Tertiary–Quaternary period (Western Dharwar Craton). The basin lithology is mainly composed of granite gneiss, charnockite and metasediment. This study highlights the importance of time series data for better estimation of metal fluxes and to understand the geochemical behaviour of metals in a river basin. The dissolved trace elements show seasonality in the river water metal concentrations forming two distinct groups of metals. First group is composed of heavy metals and minor elements that show higher concentrations during dry season and lesser concentrations during the monsoon season. Second group is composed of metals belonging to lanthanides and actinides with higher concentration in the monsoon and lower concentrations during the dry season. Although the metal concentration of both the groups appears to be controlled by the discharge, there are important biogeochemical processes affecting their concentration. This includes redox reactions (for Fe, Mn, As, Mo, Ba and Ce) and pH-mediated adsorption/desorption reactions (for Ni, Co, Cr, Cu and REEs). The abundance of Fe and Mn oxyhydroxides as a result of redox processes could be driving the geochemical redistribution of metals in the river water. There is a Ce anomaly (Ce/Ce*) at different time periods, both negative and positive, in case of dissolved phase, whereas there is positive anomaly in the particulate and bed sediments. The Ce anomaly correlates with the variations in the dissolved oxygen indicating the redistribution of Ce between particulate and dissolved phase under acidic to neutral pH and lower concentrations of dissolved organic carbon. Unlike other tropical and major world rivers, the effect of organic complexation on metal variability is negligible in the Nethravati River water.