AbstractMethane concentration and its stable carbon isotope ratio studies were carried out in deep waters of ultraslow‐spreading eastern Southwest Indian Ridge (eSWIR), 63.5°–68°E, to identify hydrothermal plumes and associated biogeochemical processes. High methane concentrations (1.0–37.8 nmol/kg) confirm the presence of 11 hydrothermal plumes along the ∼500 km long section of eSWIR. Among these 11 plumes, eight are reporting for the first time. Detailed investigations near 67.67°E; 26.61°S show the presence of three plumes at various depths and their dispersion >100 km along the ridge in east‐west directions. Stable carbon isotope ratios of these plumes show lighter and heavier isotope enrichment (−40‰ to −3‰). Lighter isotope enrichment may indicate that methane could be derived from a combination of abiotic and thermogenic sources and, heavier isotope enrichment is mainly because of microbial methane oxidation. We report strong evidence for the presence of an active hydrothermal field at 67.67°E; 26.61°S in the eSWIR for the first time.

Iron is a limiting nutrient in the marine biogeochemical cycle, and hydrothermal processes at mid-ocean ridges are well-known as one of its sources to the water column. However, a major portion of the hydrothermal iron is precipitated near the source and plays an essential role in oceanic elemental cycling. Here, we carried out a detailed study on the geochemical characteristics of Fe, using a sequential chemical extraction protocol, in a short sediment core collected from the eastern Southwest Indian Ridge (SWIR) to understand the iron association in individual mineral phases. Major and trace and rare-earth element concentrations, positive europium anomaly, and rare-earth fractionation show that the source components in the sediment core are composed of biogenic, local mafic, ultramafic, and hydrothermal origin. Solid-phase Fe speciation results indicate that >60% of Fe is associated with the Fe-oxides phase and indicate the hydrothermal plume particulates settled from the water column. A relatively low concentration of Fe associated with the pyrite and silicate (FeRes) phase suggests an erosion of sulphide and silicate minerals from the nearby vent field. The down-core variation reflects the transformation of primary ferrihydrite to more stable oxide mineral goethite/hematite and, to some extent, the formation of silicate minerals.

While the peninsular Indian rivers deliver considerable loads of detrital sediments to the Bay of Bengal, changes in their depositional patterns with time are relatively less constrained compared to those deposited by Himalayan Rivers. Here, we present a 42 kyr record of detrital sedimentation, studied using whole-core logging, environmental magnetism, and major element geochemistry from the continental margin near Chennai, in the western Bay of Bengal. The variations of magnetic mineral properties and elemental concentrations of detrital sediments combined with the published oxygen isotope record, suggest monsoonal control on detrital input to the margin. Four major periods of changes in sedimentation are identified 1) the lower section of the core representing marine isotope stage (MIS) 3 (42-24 ka) shows varying detrital input, while magnetic minerals are affected by early diagenesis, 2) the period between 24 and 15 ka exhibits increased detrital input as a result of subaerial erosion in southern India during the last glacial sea-level lowstand, 3) the deglacial period from 15 to 10 ka is marked by decreased detrital input and increased productivity, and 4) the Holocene (10-2 ka) period experienced increased detrital input and chemical weathering in the source areas. Although the post-depositional alteration of magnetic minerals in MIS 3 limits the use of environmental magnetism, major elements and & delta;18O record the variations in paleomonsoon and detrital input during this period. The increased detrital input during 24-15 ka, despite the glacial conditions and intensified NE winter monsoon, indicates the increased subaerial erosion of exposed continental shelf during the sea-level lowstand owing to the proximity of the sampling site to the coast. The high magnetic susceptibility (& chi;lf), chemical index of alteration (CIA), Al and Ti content beginning at -10 ka marks the intensification of SW summer monsoon through the Holocene.

Mineral magnetic and geochemical investigations were carried out on a short sediment core collected from the eastern Southwest Indian Ridge (ESWIR) to study the variations in the source components. The magnetic concentration parameter chi arm (representing the concentration of stable single domain (SSD) sized grains), magnetic grain size ratios chi arm/chi lf, chi arm/SIRM, geochemical ratios Fe/Ti, Co/Zn, and Zn/Fe and Ba content serve as hydrothermal indicators, all of which show a distinct top - bottom variability. The magnetomineralogical parameters S-ratio and coercivity (Hc) also follow these variations, suggesting an increased contribution of hydrothermal components in the top 15 cm of the core. Significant enrichments of Fe, Mn, and Mg in the samples, along with depletions of detrital elements Al, Si, Rb, Zr, Hf, and Nb in the element/Upper Continental Crust (UCC) values, show that the majority of the detrital component is locally sourced. Thermomagnetic measurements of samples indicate that the major remanence carrier is magnetite in the core, along with minor contributions from titanomagnetite, hematite, and iron sulfides.

The Late Pleistocene Northern Hemisphere (NH) climate was extremely variable, owing to the abrupt warm Dansgaard–Oeschger (D/O) events and cold Heinrich events in the Greenland and North Atlantic. Although the close coupling between the Indian monsoon and the Northern hemisphere climatic fluctuations has been previously reported from several western and northern Arabian Sea records, these millennial-scale climatic oscillations are not clearly defined in the Eastern Arabian Sea (EAS) possibly because many of the past records are from the continental margin which are vulnerable to sediment slumps and slides. Here, we present a 50 kyr eolian sedimentation record from a bathymetric high off Goa in the EAS, reconstructed using the environmental magnetic parameter S-ratio (a measure of the relative proportions of ferromagnetic and canted antiferromagnetic minerals) and the mass accumulation rate of iron and magnesium (FeMAR and MgMAR). S-ratio record shows striking similarity with the δ18O record from the Greenland GISP2 ice core, indicating the control of the temperature fluctuations of high northern latitudes on the low-latitude climate variability. The cold Heinrich events are marked by lowered S-ratios indicating less eolian input while the D/O interstadials are marked by increased S-ratios illustrating the intensified summer monsoon wind strength and the increased eolian supply during these periods. The mass accumulation rate of Mg and Fe reveals high values during 29–18 kyr reflecting the increased source area aridity and dust transport during the last glacial period. The spectral analysis of the S-ratio record reveals significant periodicities centered at 24 kyr and 1.3 kyr establishing the orbital and northern hemisphere controls on the EAS climate.

This paper presents a short review of some of the notable contributions by scientists from several parts of the world in furthering our understanding of the seafloor hydrothermal activity on the Indian Ocean ridges (IOR). Studies to date on the hydrothermal plumes and vents on the IOR and the marine sediments around the vents highlight the complex nature of hydrothermal activity on the Indian Ocean ridge system which is characterised by varying spreading rates, differences in the host rock system and perceptible variations in the chemical composition of the hydrothermal fluids.

India is industrializing rapidly and with this there comes higher releases of contaminants into the environment. Change in Pb deposition over the last century on the eastern (off Andhra Pradesh) and western (off Karnataka) shelves of India was investigated based on the data extracted from two sediment cores covering the past ~114 and ~145 yrs. The variations of the total Pb content, its enrichment factor, and concentrations of non-residual Pb in both the sediment cores document that there was a gradual increase in anthropogenic Pb input into the coastal sediments of India over the last century. Sediment leachates were used to monitor the increase in anthropogenic Pb input and its Pb isotope composition. The anthropogenic end member composition of the western shelf sediment location (206Pb/207Pb: 1.105; 206Pb/208Pb: 2.149) was significantly less radiogenic than the eastern shelf isotopic composition (206Pb/207Pb: 1.145; 206Pb/208Pb:2.120). A binary mixing model suggests that Pb emitted from the heavy industries (e.g., ore mining, Pb processing and smelting plants) of India has been the major source of anthropogenic Pb to the sediments of western continental shelf. In contrast, the isotopic signatures suggest that coal combustion is responsible for elevated anthropogenic Pb levels in the sediments from the eastern shelf of India.

The vast area, varying climate regimes, diverse source terrain lithology of the mainland India generate complex weathering and erosional patterns and leads to diverse sediment distribution and dispersal on the surrounding continental margins of India. Thus, to delineate the sources, distribution and sediment dispersal pathways around India, 254 sediment cores from the eastern and western continental margins of India were investigated for physical properties using non-destructive, whole core logging technique combined with grain-size analyses. The distribution of magnetic susceptibility, gamma ray density and grain-size traces the sources in the Indian landmass which contribute sediments to the margins and demarcates their extent of influence. The results from the magnetic susceptibility distribution indicate that the dominant influx of modern high susceptibility sediment into the margins of India originates from the weathering of Deccan Basalts and its subsequent transportation by Narmada and Tapti Rivers in the west coast and Krishna and Godavari Rivers in the east coast. Though the Himalayan sources are the second largest supplier of magnetic minerals to the shelf, their contribution, when compared to the Deccan sediments is not significant. The lack of present-day sedimentation in the outer shelf regions of margins of India is evident from the gamma ray density distribution and is confirmed by the discrete sample grain-size analysis. Gamma ray density variations in the margins follow the same pattern as the grain size variations and demarcate the boundary between inner shelf modern clayey sediments and the outer shelf relict carbonate sands. Regional distribution maps of physical properties and grain-size for both the margins of India are presented and geographical limits of each provenance are defined, thus providing a comprehensive picture of sediment distribution and dispersal patterns in the area and highlights the use of magnetic susceptibility as a powerful tool in deciphering the source to sink pathways of sediments.

The paleomonsoonal variations and their impact on the weathering patterns in the source regions of sediments of the Andaman Sea were investigated from a 21 kyr sedimentary record using environmental magnetic, clay mineralogical and geochemical techniques. The sediment provenance of the study area determined from the geochemical discrimination diagrams and available erqd records indicates that the Western Andaman Sea receives a considerable contribution from the Indo-Burman ranges (IBR) and local sources compared to the central and eastern Andaman Sea. The results illustrate a strong Indian Summer Monsoon (ISM) during the late glacial (16-13 kyr), early (10.5-8.5 kyr) and middle Holocene (5-3.5 kyr) and a weak ISM during the Younger Dryas (12.9-11.7 kyr) and late Holocene (3.5-0 kyr) periods. The mineral magnetic grain size parameters (chi(ARM)/SIAM, SIRM/chi(LF), chi(ARM)/chi(LF)) show finer grain sizes during the strong ISM periods indicating the increased chemical weathering while cold and dry periods are marked by an increase in magnetic grain size indicating the shift from chemical to physical weathering in the source regions. The results from our study are in agreement with the global and local records of paleoclimate and weathering and exhibit a close correlation with the solar insolation data suggesting the major role played by the solar forcing on the ISM variability.

A regional solid-phase phosphorus (P) speciation study in the Central Indian Basin (CIB) was carried out to elucidate the spatial distribution of sedimentary P species and P diagenesis in an oligotrophic setting. The results show that P enrichment is in the order of biogenic (P-bio)>authigenic (P-auth)>iron bound (P-Fe)>detrital (P-det)>organic (P-org) P. The higher concentrations of P-bio are found in siliceous oozes, which are attributed to high biological productivity in the overlying waters compared to pelagic clay region. High P-auth and P-det contents with low molar C-org/P-react ratios in the pelagic/red clays indicate the deposition of calcium fluorapatite and refractory material from the atmospheric input. The oxygenated bottom water promotes adsorption of P onto iron oxyhydroxides making the P-Fe an important sink. Remobilization of P within the sediments is limited because of well-oxygenated conditions and an efficient adsorption by iron oxyhydroxides and clay-sized sediments. A twofold increase in P-total and P-species is observed in hydrothermally altered, ferruginous sediments from a seamount flank suggesting an important role of hydrothermal processes in P cycling. The calculated P accumulation rate in the present study ranges between 0.6 and 11.7mol cm( -2) kyr(.)( -1) The burial flux of P for the entire CIB (5.7x10(6)km(2)) is 0.01x10(10)mol P per year which accounts for 0.05% of the global flux. The benthic P fluxes from the seawater to the sediments in the area range between 0.0093 and 0.133mol cm( -2) kyr( -1) indicating that the CIB sediments are an important sink for P. Plain Language Summary The paper presents a comprehensive account of regional distribution of pathways of P accumulation in Central Indian Basin, an abyssal region in the Indian Ocean. As mentioned in the Introduction, the papers dealing with P geochemistry in the deep-sea settings are few though these regions occupy a large oceanic space. Thus, our understanding of diagenetic processes and the pathways of P deposition in the oligotrophic areas is not as good as we know about productive continental margins. Lack of significant diagenetic P regeneration, P enrichment in sediments due to hydrothermal alteration, and a significant atmospheric P deposition are the key findings in the paper. The data also show that Central Indian Basin is an important sink for dissolved P. The paper presents quantitative estimates of P accumulation rates and diffusive fluxes of the study area. Extrapolated basinal fluxes are also presented for a global perspective.

Chromium (Cr) released from industrial units such as tanneries, textile and electroplating industries is detrimental to the surrounding ecosystems and human health. The focus of the present study was to check the Cr(VI) removal efficiency by marine-derived fungi from liquid broth. Amongst the three Cr(VI) tolerant isolates, #NIOSN-SK56-S19 ( Aspergillus sydowii ) showed Cr-removal efficiency of 0.01 mg Cr mg −1 biomass resulting in 26% abatement of total Cr with just 2.8 mg of biomass produced during the growth in 300 ppm Cr(VI). Scanning Electron Microscopy revealed aggregation of mycelial biomass with exopolysaccharide, while Electron Dispersive Spectroscopy showed the presence of Cr 2 O 3 inside the biomass indicating presence of active Cr(VI) removal mechanisms. This was further supported when the Cr(VI) removal was monitored using DPC (1,5-diphenylcarbazide) method. The results of this study point to the potential of marine-derived fungal isolates for Cr(VI) removal.

Results of a study on phosphorus (P) cycling in the Eastern Arabian Sea provides convincing evidence of present-day phosphogenesis in the study area. This finding contrasts with previous reports of the occurrence of only old phosphorites along the Indian margin. Extensive benthic P regeneration is observed in the sediments that are deposited on topographic highs in the western Indian continental margin, which is impinged by a perennial oxygen minimum zone (OMZ). The phosphate flux from the sediments ranged between 1.1 and 22mmolm−2yr−1. Among the P reservoirs, biogenic P (Pbio) is the major pool of P within the upper 30cm of sediment. P released from organic matter decomposition and fish debris dissolution mainly controls benthic P regeneration while the role of redox cycling of iron is weak. The change in authigenic P (Pauth) with depth in the sediment and concomitant decrease in porewater dissolved inorganic phosphate (DIP), solid phase Pbio and organic P (Porg) content collectively indicate the transformation of P from labile phases to an authigenic phase. Molar Corg/Porg and Corg/Preactive ratios also support the notion of ongoing phosphogenesis in the area. The content of carbonate fluroapatite (CFA) is elevated especially in one of the three study sites as a result of in-situ precipitation and a contribution from eolian and water column authigenesis. The estimated burial efficiency of P at the sediment-water interface at two locations is only about 2%, which is much less than in the western Arabian Sea also indicating extensive P regeneration.

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.

Total Hg distributions and its speciation were determined in two sediment cores collected from the western continental marginal high of India. Total Hg content in the sediment was found to gradually increase (by approximately two times) towards the surface in both the cores. It was found that Hg was preferentially bound to sulfide under anoxic condition. However, redox-mediated reactions in the upper part of the core influenced the total Hg content in the sediment cores. This study suggests that probable increase in authigenic and allogenic Hg deposition attributed to the increasing Hg concentration in the surface sediment in the study area.