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Now showing items 1 - 16 of 605771

  • Upwelling intensity modulates N2O concentrations over the western Indian shelf

    Sudheesh, V.   Gupta, G. V. M.   Sudharma, K. V.   Naik, H.   Shenoy, D. M.   Sudhakar, M.   Naqvi, S. W. A.  

    Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continental shelf of India in 2012 revealed high concentrations of 45 +/- 32 nM (off Kochi) and 73 +/- 63 nM (off Mangalore) during the summer monsoon (SM). N2O concentrations increased nonlinearly during the peak of the SM upwelling, when low O-2 (< 25 mu M) conditions prevailed in the water column. Off Kochi, N2O levels fell gradually from the fall intermonsoon (20 +/- 8 nM) to the winter monsoon (8.8 +/- 2 nM) and remained low (9.265.2 nM) through the spring intermonsoon season. The N2O supersaturation off Kochi (574 +/- 720%) was presumably due to its high yield during sediment denitrification, whereas the higher N2O supersaturation observed off Mangalore (1046 +/- 885%) was due to its production during denitrification in both the anoxic water column and the underlying sediments. Such distinctive biogeochemical behavior between these two shelf segments is at first augmented by the natural origin of intense upwelling at Mangalore relative to Kochi wherein suboxic to anoxic oxygen minimum zone waters spread from offshore to the shelf of Mangalore, over which the runoff and terrestrial nutrients supply acts in unison. Following new zonal extrapolation approach, our revised estimate of N2O effluxes from the southwestern Indian shelf (7-14 degrees N) was four times lower (0.019-0.039 Tg y(-1)) than previous estimate. Nevertheless, further studies are needed to refine the N2O effluxes from the entire western Indian shelf to monitor its modification due to expansion/intensification of the coastal low-O-2 zones and also to ascertain its actual N2O contribution to the world oceans.
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  • Lime muds and their genesis off-Northwestern India during the late Quaternary

    Rao, V. Purnachandra   Kumar, A. Anil   Naqvi, S. W. A.   Chivas, Allan R.   Sekar, B.   Kessarkar, Pratima M.  

    Two sediment types were found in five gravity cores collected from water depths between 56 m and 121 m along the northwestern continental margin of India: lime muds were abundant in the lower section while siliciclastic sediments dominated the upper section. Lime mud-dominated sediments in shelf cores contained 60%-75% carbonate, 0.3%-0.6% Sr and terrigenous minerals, whereas those at the shelf break were found to have > 90% carbonate, 0.6%-0.8% Sr and traces of terrigenous minerals. Aragonite needles showing blunt edges, jointed needles and needles wrapped in smooth aragonite cement were found to be common. Stable (O and C) isotopes of lime mud indicate a potentially freshwater contribution for shelf cores and purely marine contribution for those at the shelf break. Calibrated radiocarbon ages of the lime muds ranged from 17.6-11.9 ka in different cores. The results reported here suggest that the lime muds in the shallow shelf are probably reworked from the Gulf of Kachchh, whereas those at the shelf break were biodetrital, initially formed on the carbonate platform during low stands of sea level and then exported. The change in lime mud-dominated to siliciclastic-dominated sediments in the cores may be due to climate change and rapid rise in sea level during the early Holocene.
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  • Marine hypoxia/anoxia as a source of CH(4) and N(2)O

    Naqvi, S. W. A.   Bange, H. W.   Farias, L.   Monteiro, P. M. S.   Scranton, M. I.   Zhang, J.  

    We review here the available information on methane (CH(4)) and nitrous oxide (N(2)O) from major marine, mostly coastal, oxygen (O(2))-deficient zones formed both naturally and as a result of human activities (mainly eutrophication). Concentrations of both gases in subsurface waters are affected by ambient O(2) levels to varying degrees. Organic matter supply to seafloor appears to be the primary factor controlling CH(4) production in sediments and its supply to (and concentration in) overlying waters, with bottom-water O(2)-deficiency exerting only a modulating effect. High (micromolar level) CH(4) accumulation occurs in anoxic (sulphidic) waters of silled basins, such as the Black Sea and Cariaco Basin, and over the highly productive Namibian shelf. In other regions experiencing various degrees of O(2)-deficiency (hypoxia to anoxia), CH(4) concentrations vary from a few to hundreds of nanomolar levels. Since coastal O(2)-deficient zones are generally very productive and are sometimes located close to river mouths and submarine hydrocarbon seeps, it is difficult to differentiate any O(2)-deficiency-induced enhancement from in situ production of CH(4) in the water column and its inputs through freshwater runoff or seepage from sediments. While the role of bottom-water O(2)-deficiency in CH(4) formation appears to be secondary, even when CH(4) accumulates in O(2)-deficient subsurface waters, methanotrophic activity severely restricts its diffusive efflux to the atmosphere. As a result, an intensification or expansion of coastal O(2)-deficient zones will probably not drastically change the present status where emission from the ocean as a whole forms an insignificant term in the atmospheric CH(4) budget. The situation is different for N(2)O, the production of which is greatly enhanced in low-O(2) waters, and although it is lost through denitrification in most suboxic and anoxic environments, the peripheries of such environments offer most suitable conditions for its production, with the exception of enclosed anoxic basins. Most O(2)-deficient systems serve as strong net sources of N(2)O to the atmosphere. This is especially true for coastal upwelling regions with shallow O(2)-deficient zones where a dramatic increase in N(2)O production often occurs in rapidly denitrifying waters. Nitrous oxide emissions from these zones are globally significant, and so their ongoing intensification and expansion is likely to lead to a significant increase in N(2)O emission from the ocean. However, a meaningful quantitative prediction of this increase is not possible at present because of continuing uncertainties concerning the formative pathways to N(2)O as well as insufficient data from key coastal regions.
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  • Historical records of coastal eutrophication-induced hypoxia

    Jorissen, F.   Levin, L. A.   Middelburg, J. J.   Naqvi, S. W. A.   Rabalais, N. N.   Scranton, M.   Zhang, J.  

    Under certain conditions, sediment cores from coastal settings subject to hypoxia can yield records of environmental changes over time scales ranging from decades to millennia, sometimes with a resolution of as little as a few years. A variety of biological and geochemical indicators (proxies) derived from such cores have been used to reconstruct the development of eutrophication and hypoxic conditions over time. Those based on (1) the preserved remains of benthic organisms (mainly foraminiferans and ostracods), (2) sedimentary features (e.g. laminations) and (3) sediment chemistry and mineralogy (e.g. presence of sulphides and redox-sensitive trace elements) reflect conditions at or close to the seafloor. Those based on (4) the preserved remains of planktonic organisms (mainly diatoms and dinoflagellates), (5) pigments and lipid biomarkers derived from prokaryotes and eukaryotes and (6) organic C, N and their stable isotope ratios reflect conditions in the water column. However, the interpretation of these indicators is not straightforward. A central difficulty concerns the fact that hypoxia is strongly correlated with, and often induced by, organic enrichment caused by eutrophication, making it difficult to separate the effects of these phenomena in sediment records. The problem is compounded by the enhanced preservation in anoxic and hypoxic sediments of organic microfossils and biomarkers indicating eutrophication. The use of hypoxia-specific proxies, such as the trace metals molybdenum and rhenium and the bacterial biomarker isorenieratene, together with multi-proxy approaches, may provide a way forward. All proxies of bottom-water hypoxia are basically qualitative; their quantification presents a major challenge to which there is currently no satisfactory solution. Finally, it is important to separate the effects of natural ecosystem variability from anthropogenic effects. Despite these problems, in the absence of historical data for dissolved oxygen concentrations, the analysis of sediment cores can provide plausible reconstructions of the temporal development of human-induced hypoxia, and associated eutrophication, in vulnerable coastal environments.
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  • Recent advances in the biogeochemistry of nitrogen in the ocean

    Naqvi, S. W. A.   Voss, M.   Montoya, J. P.  

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  • Impact of a tropical cyclone on biogeochemistry of the central Arabian Sea

    Naik, Hema   Naqvi, S. W. A.   Suresh, T.   Narvekar, P. V.  

    Remotely sensed data are combined with shipboard measurements to investigate biogeochemical changes caused by a moderate tropical cyclone in the central Arabian Sea in December 1998. The sea surface temperature decreased by similar to 4 degrees C, whereas surface nitrate and chlorophyll concentrations increased by > 5 mu M and up to 4 mg m(-3), respectively, over a large area affected by the cyclone. Nutrient enrichment in the surface layer of the cyclone-affected zone is estimated to have supported a new production of similar to 4.2 Tg C, approximately 5% of the annual organic carbon export to the deep sea ( beyond the continental margin) for the entire Arabian Sea. Entrainment of nitrous oxide from the thermocline led to more than doubling of its concentration in the mixed layer. The cyclone also resulted in an increase in nitrous oxide inventory within the oxygen minimum zone. Our results imply that, should there be an increase in the frequency and intensity of tropical cyclones as a result of global warming, as projected in some recent reports, carbon production and respiration, and redox processes within the oxygen minimum zones, such as the production of nitrous oxide through nitrification/denitrification, and of molecular nitrogen through denitrification/anaerobic ammonium oxidation, may be significantly impacted.
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  • The next generation of iron fertilization experiments in the Southern Ocean

    Smetacek, V.   Naqvi, S. W. A.  

    Of the various macro-engineering schemes proposed to mitigate global warming, ocean iron fertilization (OIF) is one that could be started at short notice on relevant scales. It is based on the reasoning that adding trace amounts of iron to iron-limited phytoplankton of the Southern Ocean will lead to blooms, mass sinking of organic matter and ultimately sequestration of significant amounts of atmospheric carbon dioxide (CO(2)) in the deep sea and sediments. This iron hypothesis, proposed by John Martin in 1990 (Martin 1990 Paleoceanography 5, 1 - 13), has been tested by five mesoscale experiments that provided strong support for its first condition: stimulation of a diatom bloom accompanied by significant CO(2) drawdown. Nevertheless, a number of arguments pertaining to the fate of bloom biomass, the ratio of iron added to carbon sequestered and various side effects of fertilization, continue to cast doubt on its efficacy. The idea is also unpopular with the public because it is perceived as meddling with nature. However, this apparent consensus against OIF is premature because none of the published experiments were specifically designed to test its second condition pertaining to the fate of iron-induced organic carbon. Furthermore, the arguments on side effects are based on worst-case scenarios. These doubts, formulated as hypotheses, need to be tested in the next generation of OIF experiments. We argue that such experiments, if carried out at appropriate scales and localities, will not only show whether the technique will work, but will also reveal a wealth of insights on the structure and functioning of pelagic ecosystems in general and the krill-based Southern Ocean ecosystem, in particular. The outcomes of current models on the efficacy and side effects of OIF differ widely, so data from adequately designed experiments are urgently needed for realistic parametrization. OIF is likely to boost zooplankton stocks, including krill, which could have a positive effect on recovery of the great whale populations. Negative effects of possible commercialization of OIF can be controlled by the establishment of an international body headed by scientists to supervise and monitor its implementation.
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  • Coastal versus open-ocean denitrification in the Arabian Sea

    Naqvi, S. W. A.   Naik, H.   Pratihary, A.   D'Souza, W.   Narvekar, P. V.   Jayakumar, D. A.   Devol, A. H.   Yoshinari, T.   Saino, T.  

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  • A zona incognita surrounds the secondary nitrite maximum in open-ocean oxygen minimum zones

    Banse, K.   Naqvi, S. W. A.   Poster, J. R.  

    An analytic gap exists between the lower limit of detection of oxygen (O-2) by conventional sensors (LOD, similar to 1 - 3 mu M O-2) and the appearance of NO2- from NO3- reduction (similar to 0.05 mu M O-2), which signifies the secondary nitrite maximum. The near-anoxic milieu where precise O-2 measurements could not be made until very recently even by current optodes, favors biogeochemically important redox reactions. The ongoing improvement of optodes might make the gap vanish. Significant O-2 levels near the conventional LOD for O-2, when accompanied by similar to >=3D 0.05 mu M NO2-, must be erroneous and should be deleted from historical data going back to the 1930s.
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  • Spatial variation of phytoplankton pigments along the southwest coast of India

    Pratihary, Anil   Mangesh, Gauns   Naqvi, S. W. A.  

    Phytoplankton composition and abundance were studied along the southwestern Indian coast toward the end of the upwelling season in October 2004. Phytoplankton pigment analyses, complemented by limited microscopic counts, were carried out to determine the community structure. Chlorophyll a was the most abundant of all pigments, followed by fucoxanthin. Zeaxanthin was abundantly found in the southern part of the study region (off Trivandrum), whereas fucoxanthin was the dominant marker pigment in the north (off Goa). The infer-red shift in the community structure from a dominant picoplankton fraction and Prymnesiophytes to diatom-dominated microplankton toward the north is ascribed to differences in the physico-chemical environment. (c) 2006 Elsevier Ltd. All rights reserved.
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  • Benthic fluxes in a tropical estuary and their role in the ecosystem (vol 85, pg 387, 2009)

    Pratihary, Anil K.   Naqvi, S. W. A.   Naik, H.   Thorat, B. R.   Narvenkar, G.   Manjunatha, B. R.   Rao, V. P.  

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  • N-2 production rates limited by nitrite availability in the Bay of Bengal oxygen minimum zone

    Bristow, L. A.   Callbeck, C. M.   Larsen, M.   Altabet, M. A.   Dekaezemacker, J.   Forth, M.   Gauns, M.   Glud, R. N.   Kuypers, M. M. M.   Lavik, G.   Milucka, J.   Naqvi, S. W. A.   Pratihary, A.   Revsbech, N. P.   Thamdrup, B.   Treusch, A. H.   Canfield, D. E.  

    A third or more of the fixed nitrogen lost from the oceans as N-2 is removed by anaerobic microbial processes in open ocean oxygen minimum zones. These zones have expanded over the past decades, and further anthropogenically induced expansion could accelerate nitrogen loss. However, in the Bay of Bengal there has been no indication of nitrogen loss, although oxygen levels are below the detection level of conventional methods (1 to 2 mu M). Here we quantify the abundance of microbial genes associated with N-2 production, measure nitrogen transformations in incubations of sampled seawater with isotopically labelled nitrogen compounds and analyse geochemical signatures of these processes in the water column. We find that the Bay of Bengal supports denitrifier and anammox microbial populations, mediating low, but significant N loss. Yet, unlike other oxygen minimum zones, our measurements using a highly sensitive oxygen sensor demonstrate that the Bay of Bengal has persistent concentrations of oxygen in the 10 to 200 nM range. We propose that this oxygen supports nitrite oxidation, thereby restricting the nitrite available for anammox or denitrification. If these traces of oxygen were removed, nitrogen loss in the Bay of Bengal oxygen minimum zone waters could accelerate to global significance.
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  • Benthic mineralization and nutrient exchange over the inner continental shelf of western India

    Pratihary, A. K.   Naqvi, S. W. A.   Narvenkar, G.   Kurian, S.   Naik, H.   Naik, R.   Manjunatha, B. R.  

    The western Indian continental shelf is one of the most productive coastal systems of the world ocean. This system experiences extreme changes in its oxygen regime, being normoxic from November to May and suboxic (denitrifying)/anoxic from June to October, owing to the biogeochemical response to cyclical monsoonal influence. In order to understand the impact of the seasonally varying oxygen regime on benthic mineralization, nutrient exchange and, in turn, on the shelf ecosystem, we carried out the first ever intact-core incubations during two contrasting seasons - spring intermonsoon and fall intermonsoon (late southwest monsoon) at a 28 m-deep fixed site on the inner shelf off Goa, dominated by fine-grained cohesive sediments. The results showed that incomplete sediment oxygen consumption (SOC) occurred during April as opposed to the complete SOC and subsequent sulfide flux observed in the fall intermonsoon incubations. The sediments acted as a perennial net source of DIN (dissolved inorganic nitrogen i.e. NO3- + NO2- + NH4+), PO43- and SiO44- to the overlying water column. The efflux of DIN increased from 1.4 to 3.74 mmol m(-2) d(-1) from April to October, of which NH4+ flux comprised 59-100 %. During the oxic regime, similar to 75% of diffusing NH4+ appeared to be nitrified (2.55 mmol m(-2) d(-1)), of which similar to 77% remained coupled to benthic denitrification. Consequently, 58% of NH4+ flux was lost in active coupled nitrification-denitrification, resulting in substantial N loss (1.98 mmol m(-2) d(-1)) in the sediments. The continental shelf sediments switched over from being a NO3- source during the oxic regime to a NO3- sink during the anoxic regime. During suboxia, benthic denitrification that is fed by NO3- from the overlying water caused N loss at the rate of 1.04 mmol m(-2) d(-1). Nitrogen loss continued even under sulfidic conditions during October, possibly through the chemolithoautotrophic denitrification, at a potential rate of 3.21 mmol m(-2) d(-1). Phosphate flux increased more than 4-fold during October as compared to April, due to reductive dissolution of Fe-and Mn oxides. The SiO44- flux increased during October apparently due to the higher availability of siliceous ooze from diatom blooms commonly occurring in the monsoon season. Slow oxidation of organic carbon (C-org) under anoxia, lower temperature and reduced benthic faunal activity appeared to decrease benthic mineralization by 25% as suggested by the drop in the C-org oxidation rate from 63.8 mmol C m(-2) d(-1) in April to 47.8 mmol C m(-2) d(-1) in October. This indicated a higher preservation of C-org during the late southwest monsoon. Sediment porosity, C-org content and nutrients did not show significant variations from April to October. Porewaters were found to be enriched with NH4+, PO43- and SiO44- but depleted in NO3- and NO2- in these organic-rich sediments. Significant DIN, PO43- and SiO44- effluxes indicate the potential of benthic input in meeting nutrient demand of the phytoplankton community in this seasonally N-limited shelf system.
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  • Benthic mineralization and nutrient exchange over the inner continental shelf of western India

    Pratihary, A. K.   Naqvi, S. W. A.   Narvenkar, G.   Kurian, S.   Naik, H.   Naik, R.   Manjunatha, B. R.  

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  • Biogeochemistry of methane in a tropical monsoonal estuarine system along the west coast of India

    Araujo, Jesly   Naqvi, S. W. A.   Naik, Hema   Naik, Richita  

    Estuaries are known to be important sources of methane (CH4) to the atmosphere. However, a lack of adequate field studies in understanding the sources/sinks of CH4 in estuaries hampers the global atmospheric budgeting. Therefore, more studies are needed to bridge the gap and improve our understanding of fluxes of CH4 to the atmosphere. We report here results of a year-long study of the Mandovi-Zuari estuarine system of Goa, on the west coast of India. Methane was measured along with the other ancillary parameters such as temperature, salinity and oxygen in the water column of both the estuaries along the salinity gradient. The estuarine waters were supersaturated with CH4, which exhibited significant spatial (along the salinity gradient) and temporal variability. The concentrations varied from 6 to 901 nM (saturation: 300-46,000%) in the Mandovi estuary and from 8 to 1022 nM (saturation: 400-41,000%) in the Zuari estuary from the estuary mouth to the fresh water end. The water-air CH4 fluxes increased towards the freshwater end in both the estuaries. The annual freshwater CH4 input was higher in the Zuari (209 +/- 182 x 10(6) mu mol d(-1)) than in the Mandovi (153 +/- 144 x 10(6) mu mol d(-1)) estuary. The high build-up of CH4 could be due to a combination of supply from mangrove swamps, sedimentary inputs and river runoff. (C) 2017 Elsevier Ltd. All rights reserved.
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  • Seasonal occurrence of anoxygenic photosynthesis in Tillari and Selaulim reservoirs, Western India

    Kurian, S.   Roy, R.   Repeta, D. J.   Gauns, M.   Shenoy, D. M.   Suresh, T.   Sarkar, A.   Narvenkar, G.   Johnson, C. G.   Naqvi, S. W. A.  

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