• S W A Naqvi

      Articles written in Journal of Earth System Science

    • Denitrification processes in the Arabian Sea

      S W A Naqvi

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      Recent information on some consequences of the acute mid-water oxygen deficiency in the Arabian Sea, especially on carbon-nitrogen cycling, is reviewed. An evaluation of published estimates of water column denitrification rate suggests an overall rate in the vicinity of 30Tg Ny-1, but the extent of benthic contribution remains unknown. A decoupling of denitrification from primary production, unique to the Arabian Sea, is revealed by nitrite, electron transport system (ETS) activity and bacterial production data. Results of both enzymatic and microbiological investigations strongly point to a major role of organic carbon other than that sinking from surface layers in supporting denitrification. Although denitrification is associated with an intermediate nepheloid layer, it seems unlikely that the excess carbon comes with particles re-suspended along the continental margins and transported quasi-horizontally into the ocean interior; instead, the particle maximum may directly reflect a higher bacterial abundance. It is proposed that denitrification may be predominantly fuelled by the dissolved organic matter.

    • Nitrogen isotopic studies in the suboxic Arabian Sea

      S W A Naqvi T Yoshinari J A Brandes A H Devol D A Jayakumar P V Narvekar M A Altabet L A Codispoti

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      Measurements of15N/14N in dissolved molecular nitrogen (N2), nitrate (NO3) and nitrous oxide (N2O) and18O/16O in N2O [expressed as δ15N and δ18O, relative to atmospheric N2 and oxygen (O2), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O2 minima observed in the open ocean. Microbially-mediated reduction of NO3 to N2 (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ15N of NO3 increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250–350 m); the δ15N of N2 concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (ε) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ15N of NO3 is observed from ∼ 200m (> 11‰) to 80m (∼ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis of N2O reveals extremely large enrichments of both15N and18O within the OMZ, presumably due to the preferential reduction of lighter N2O to N2. However, isotopically light N2O is observed to accumulate in high concentrations above the OMZ indicating that the N2O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N2O ever found at the sea surface are observed, show moderate depletion of15N, but slight enrichment of18O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly18O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N2O in the atmosphere. Also, the N2O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N2O production.

    • Lime muds and their genesis off-Northwestern India during the late Quaternary

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

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      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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