B. Nagender Nath

Influence of Provenance, Weathering, and Sedimentary Processes on the Elemental Ratios of the Fine-Grained Fraction of the Bedload Sediments from the Vembanad Lake and the Adjoining Continental Shelf, Southwest Coast of India

ABSTRACT Geochemical studies, comprising twelve major elements and twenty-four trace elements, including the rare earth elements (REE), have been carried out on the Concentrations normalized to the average upper continental crust (UCC) show that the sediments studied are depleted in Rb, K, Ba, Ta, Hf, Na, and Ca and enriched in Cs, U, Th, REEs, Sc, Fe, Co, and Cr. UCC-normalized patterns are strikingly similar for all the three sediment types, probably because of efficient mixing of sedimentary material during weathering, transport, and deposition. The depletion of Na, K, Ca, and Ba in sediments relative to UCC could be attributed to their mobility during weathering. The depletion of Ta and Hf and enrichment of Th, REE, and Sc are probably related to provenance characteristics, and enrichment of U is attributed to oxidation-reduction processes. Shale-normalized REE values reveal steep HREE patterns [(Tbsample/Tbshale)/(Ybsample/Ybshale)] >> [(Lasample/Lashale)/(Smsample /Smshale)] and flat LREE patterns. The shale-normalized patterns for all the three types of sediments are remarkably similar, with variations in the magnitude of LREE enrichment (Lan/Ybn varies between 1.72 to 2.57 times that of PAAS). LREE enrichment and steep HREE patterns are attributed to felsic source-rock characteristics. Other geochemical characteristics such as high La/Th and low La/Sc and Th/Sc ratios compared to UCC also support a felsic provenance. Eu/Eu* values for all the three types are little more than 1, which is also a characteristic feature of Archean crust. Furthermore, data plotted on discrimination diagrams of La/Th vs. Hf and La-Th-Sc cluster closer to granite and away from basalt and komatiite and are consistent with source-rock geology. Geochemical data have also helped in ascertaining the weathering trends. The chemical index of alteration (CIA) has been used to quantify the degree of weathering. CIA values range between 81 and 94 on a scale of 40-100, indicating a high degree of alteration (fluvial sediments seem to be more altered). On an A-CN-K diagram, the data fall closer to the compositional fields of highly weathered minerals kaolinite, gibbsite, and chlorite. A high degree of weathering is also indicated by other plots such as K2O-Fe2O3-Al2O3 and log K vs. log Rb. The geochemical data are consistent with the studies on clay mineralogy (higher kaolinite) carried out by other authors on the sediments of the same area. Secondary processes seem to affect other geochemical discriminants such as Co, Cr, Ni, and U, as revealed by plots of (1) La/Sc vs. Co/Th and (2) Th-Hf-Co and Co/Th, Cr/Th ratios and makes them unsuitable for determining provenance and tectonic setting.

TRACE AND RARE EARTH ELEMENTAL VARIATION IN ARABIAN SEA SEDIMENTS THROUGH A TRANSECT ACROSS THE OXYGEN MINIMUM ZONE

We have determined the calcium carbonate (CaCO3), organic carbon (Coorg), trace element, and rare earth element (REE) composition of surface sediments collected from a transect on the central western continental shelf and slope of India in the Eastern Arabian Sea. The transect samples across the oxygen minimum zone (OMZ) allows us to compare the relative abundances of trace elements and REEs in the sediments beneath and beyond the OMZ. Shale-normalized REE patterns, Lan/Ybn ratios, and Eu/Eu∗ anamolies indicate that the sediments in the study area are either derived from the adjoining Archaean land masses or from distal Indus source. Sediment deposited in the OMZ have high U values from 3.6 to 8.1 ppm, with their Uexcess (of that can be supplied by continental particles) values ranging between 82–91% of the total U, indicating that the U may be precipitated as U+4 in the reducing conditions of OMZ. Sediments deposited beneath the intense OMZ (<0.2 mL/L) and away from the OMZ (1–2 mL/L) show slight negative Ce anomalies, with no significant differences between these two sets of sediments. The Ce/Ceshale∗ values are poorly related to U and Corg which are indicators of suboxic bottom waters. Normative calculations suggest that two sources, namely, terrestrial and seawater (terrestrial ⪢ seawater) contribute to the total Cc anomaly of the sediments. The Cc anomaly values of the calculated seawater derived component are similar to the anomalies reported for other coastal waters and the oxygenated surface waters of the Arabian Sea and do not show any correspondence to the lowered redox state of the overlying water, probably due to the redirection of dissolved Ce into the oxic deeper water. This data on recently deposited sediments from a known oxygen-poor environment suggest that the Ce/Cesh∗ in high sedimentation areas of continental margin environments may not be a good paleoredox proxy. U/Th ratios above 1.25 noticed in OMZ are in accordance with the earlier usage of this ratio for identifying reducing environment of ancient sediments.

Geochemical evidence of terrigenous influence in deep-sea sediments up to 8°S in the Central Indian Basin

Abstract Thirty-two representative deep-sea sediment samples have been studied from three manganese nodule-bearing environments in the Central Indian Basin: diatomaceous-radiolarian ooze-clay, pelagic brown clay and calcareous ooze. They have been analysed for major and minor elements such as Si, Al, Ca, Mg, Na, K, Fe, Mn, P, Ti, Ba, Cu, Ni, Co, V, Zn and Cr. R-mode factor analysis reveals five important sources for these elements: (1) detrital (loaded with Fe, Ti, Al, Mg, Mn, P and K), (2) combined hydrogenetic-diagenetic (Mn, Ni, Cu, Co and Fe), (3) biogenic (Si), (4) seasalts (Na and Mg) and (5) dissolution residue (Ba). Factor scores, distributional trends of detrital elements, Al, Ti and Fe concentrations and Al Al + Fe + Mn ratios provide new geochemical evidence for the effect of the terrigenous influx from the Ganges-Brahmaputra rivers into the northern part of the basin up to 8°S which inhibits the growth of manganese nodules in this area. In contrast, incorporation of transitional metals into sediments and manganese nodules in the other two areas of the basin is governed by hydrogenetic and diagenetic processes.

Early diagenetic processes affecting nutrients in the pore waters of Central Indian Ocean cores

Abstract Pore-water nutrients, nitrite, nitrate, phosphate, silicate, pH and solid-phase organic carbon were analysed for one core from the Arabian Sea and three cores from the manganese nodule area in the Central Indian Ocean Basin. Possible denitrification and unoxidized organic carbon are observed in the Arabian Sea core, while nitrification with intermediate low nitrate and high organic carbon are found in the mottled zones of the pelagic cores. Mobilization of iron-host oxyhydroxides and peaked profiles of organic carbon are related to a gradual increase in the phosphate profiles. The asymptotic silica values are less than the 100% saturation levels of amorphous silica and they fall outside the stability field of sepiolite.

Changes in metal contamination levels in estuarine sediments around India--an assessment.

This review is the first attempt to comprehend the changes in metal contamination levels in surface estuarine sediments with changing time around India. Contamination factor, geoaccumulation index, pollution load index, effects range low and effects range median analysis were used to evaluate the quality of the estuarine sediments (by using the available literature data). This study suggests that estuarine sediments from the east coast of India were comparatively less contaminated by metals than the west coast. Sediments from those estuaries were found to be more contaminated by metals on which major cities are located. An improvement in estuarine sediment quality (in terms of metal contamination) over time around India was noticed. This study provides managers and decision-makers of environmental protection agency with a better scientific understanding for decision-making in controlling metal pollution in estuarine sediments around India.

The composition and the source of hydrocarbons in sediments taken from the tectonically active Andaman Backarc Basin, Indian Ocean

Abstract Concentrations of total hydrocarbons in sediments of the tectonically active part of the Andaman Basin ranged from 130–720 (with an average of 349) mg/kg in the most active Spreading zone (the Rift Valley) to 43–180 (with an average of 113) mg/kg in the less active Fault zone of the Deep Basin. Average concentrations of total aromatic hydrocarbons ranged from 215 mg/kg in the Spreading zone to 49 mg/kg in the Deep Basin. Compound distributions were dominated by short-chain n -alkanes n -C 13 – n -C 24 (on the average, 77% in the Spreading zone and 64% in the Deep Basin, correspondingly). Distribution of n -alkanes in this region is characterized by a carbon predominance index (CPI 13–35 ) being mainly either below or about 1,0. Substituted homologues of biphenyl, fluorene, naphthalene, phenanthrene, benzofluorenes, chrysene (6-methylchrysene), pyrene and perylene (3-methylperylene) as well as unsubstituted individual polycyclic aromatic hydrocarbon (PAH): pyrene, benzo( a )pyrene, perylene, benzo( g , h , i )perylene, were identified in all samples. Concentrations of substituted homologues of PAH ranged, on the average, from 27 μg/kg in the Spreading zone to 16 μg/kg in the Deep Basin. Concentrations of unsubstituted individual PAH ranged, on the average, from 25 μg/kg in Spreading zone to17 μg/kg in the Deep Basin. A composition of both aliphatic hydrocarbons and PAH in the sediments of the region studied is identical to the composition of non-biological compounds: petroleum products or hydrothermal organic matter. Anthropogenic sources in region studied are of minor importance. From the results obtained, it may be deduced that the hydrocarbons in the sediments of the tectonically active part of the Andaman Basin are mainly due to an alteration of sediment organic matter by emanation processes (hydrothermal and thermal fluxes, Earths outgassing, petroleum show, etc.).

Impact of benthic disturbance on megafauna in Central Indian Basin

Abstract Deep-sea photographs and video data were studied to evaluate the effects of benthic disturbance on megafaunal distribution in the Central Indian Basin. Xenophyophores (41%) and holothurians (30%) are the most abundant taxa, followed by other groups in the area before the disturbance. An overall reduction (32%) in the total megafaunal population after disturbance is direct evidence of the impact on benthic environment. Different groups such as xenophyophores, sea anemones, shrimps, starfish, brittle stars, holothurians and fish show different degrees of reduction (21–48%) in their numbers, depending upon their ability to withstand increased turbidity and sedimentation rates due to disturbance. Faunal groups such as protobranch molluscs, polychaete worms, seafans and squids, observed before the disturbance, were not seen after disturbance, whereas populations of some taxa increased after the disturbance. Increased numbers of mobile taxa could be due to increased levels of organic carbon due to resedimentation, whereas increase in sessile taxa may partly reflect the difficulty in distinguishing live from dead specimens. The impact on faunal assemblages is more severe in the disturbed area than the undisturbed area. Our results indicate that monitoring of megafauna can be used effectively to evaluate the potential impacts of large-scale mining or other disturbance on the seafloor, and may therefore help in developing measures for conservation of the benthic environment.

Cerium anomaly variations in ferromanganese nodules and crusts from the Indian Ocean

Fifty analyses of rare earth elements as well as mineralogical studies have been carried out on a suite of manganese nodules and crusts from the Central Indian Basin and the Western Indian Ocean. The aim was to identify the processes controlling the REE patterns of the phases hosting the REE in the manganese nodules, with an emphasis on an understanding of the Ce anomaly. This has involved separating the encrusting layers and nuclei physically as well as Fe-Mn oxides from the aluminosilicate phase chemically (using a 2 M HCl leach) prior to analysis. The presence of nodule nuclei seems to have little influence (mostly <5% to a maximum of 30%) on the overall magnitude of the Ce anomalies in these nodules. The ratios of concentrations of elements in the acid leachates and the corresponding bulk values yield flat REE patterns indicating that the aluminosilicate phase contributes very little to the Ce anomalies. Interelement relations indicate that the Ce anomalies are largely controlled by the amorphous mineral phase FeOOH.xH2O. The relationship of Fe, Ce anomaly and δ-MnO2 further suggests that Ce is chemisorbed onto iron oxyhydroxides which are epitaxially intergrown with δ-MnO2. The regional distribution of the Ce anomaly values appears to depend on many of the factors responsible for the uptake of other minor metals in nodules and crusts.

Lead distribution in coastal and estuarine sediments around India

This study describes the geochemical distribution of lead (Pb) and identifies the critical factors that significantly control Pb distribution and speciation in coastal and estuarine sediments around India by using published data from the literature. Crustal sources influence the abundance of Pb in coastal sediment from the south-east and central-west coast of India. Parts of north-east, north-west, and south-west coast of India were polluted by Pb. Distribution of Pb in sediments, from the north-east and north-west coasts of India, were controlled by Fe-Mn oxyhydroxide mineral phases of the sediments. However, organic carbon (OC) seemed to be a dominant factor in controlling the distribution of Pb in sediments from the central-east and south-west coasts of India. The outcome of this study may help in decision-making to predict the levels of Pb from natural and anthropogenic sources and to control Pb pollution in coastal and estuarine sediments around India.

Effects of bottom water dissolved oxygen variability on copper and lead fractionation in the sediments across the oxygen minimum zone, western continental margin of India.

This study describes the effect of varying bottom-water oxygen concentration on geochemical fractionation (operational speciation) of Cu and Pb in the underneath sediments across the oxygen minimum zone (Arabian Sea) in the west coast of India. Both, Cu and Pb were redistributed among the different binding phases of the sediments with changing dissolved oxygen level (from oxic to hypoxic and close to suboxic) in the bottom water. The average lability of Cu-sediment complexes gradually decreased (i.e., stability increased) with the decreasing dissolved oxygen concentrations of the bottom water. Decreasing bottom-water oxygen concentration increased Cu association with sedimentary organic matter. However, Pb association with Fe/Mn-oxyhydroxide phases in the sediments gradually decreased with the decreasing dissolved oxygen concentration of the overlying bottom water (due to dissolution of Fe/Mn oxyhydroxide phase). The lability of Pb-sediment complexes increased with the decreasing bottom-water oxygen concentration. This study suggests that bottom-water oxygen concentration is one of the key factors governing stability and lability of Cu and Pb complexes in the underneath sediment. Sedimentary organic matter and Fe/Mn oxyhydroxide binding phases were the major hosting phases for Cu and Pb respectively in the study area. Increasing lability of Pb-complexes in bottom sediments may lead to positive benthic fluxes of Pb at low oxygen environment.