Chemical oceanography of the Indian Ocean, north of the equator

Gupta, R. Sen; Naqvi, S.W.A.

doi:10.1016/0198-0149(84)90035-9

Cited by 123 publications

(51 citation statements)

References 30 publications

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“…Such a long residence time, in addition to high anammox rates (up to 39 nmol N L −1 day −1 ) measured along the Oman shelf, leads to the conclusion that processes at the basin's boundaries were mainly responsible for the severe NO − 3 deficits seen in the SNM of the whole Arabian Sea . The residence time of nitrite-enriched OMZ waters resulting from these rate measurements of 49 years is within the upper range of earlier estimates, which vary between 1 and 50 years (Naqvi and Shailaja, 1993;Olson et al, 1993;Sen Gupta and Naqvi, 1984). The only study using trichlorofluoromethane for calculating the residence time suggested a mean residence time of 10 years for waters in the OMZ (Olson et al, 1993).…”

Section: T Rixen Et Al: Seasonal and Interannual Variations In The supporting

confidence: 53%

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

et al. 2014

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Abstract. The Arabian Sea plays an important role in the marine nitrogen cycle because of its pronounced mid-water oxygen minimum zone (OMZ) in which bio-available nitrate (NO − 3 ) is reduced to dinitrogen gas (N 2 ). As the nitrogen cycle can respond fast to climate-induced changes in productivity and circulation, the Arabian Sea sediments are an important palaeoclimatic archive. In order to understand seasonal and interannual variations in the nitrogen cycle, nutrient data were obtained from the literature published prior to 1993, evaluated, and compared with data measured during five expeditions carried out in the framework of the Joint Global Ocean Flux Study (JGOFS) in the Arabian Sea in 1995 and during a research cruise of RV Meteor in 2007. The data comparison showed that the area characterized by a pronounced secondary nitrite maximum (SNM) was by 63 % larger in 1995 than a similarly determined estimate based on pre-JGOFS data. This area, referred to as the core of the denitrifying zone, showed strong seasonal and interannual variations driven by the monsoon. During the SW monsoon, the SNM retreated eastward due to the inflow of oxygenenriched Indian Ocean Central Water (ICW). During the NE monsoon, the SNM expanded westward because of the reversal of the current regime. On an interannual timescale, a weaker SW monsoon decreased the inflow of ICW from the equatorial Indian Ocean and increased the accumulation of denitrification tracers by extending the residence time of water in the SNM. This is supported by palaeoclimatic studies showing an enhanced preservation of accumulative denitrification tracers in marine sediments in conjunction with a weakening of the SW monsoon during the late Holocene.

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Section: T Rixen Et Al: Seasonal and Interannual Variations In The supporting

confidence: 53%

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

et al. 2014

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“…1a). Nitrite accumulation is a characteristic of the upper margin of oxygen minimum zones (Cline and Richards, 1972;Sen Gupta and Naqvi, 1984;Codispoti and Christensen, 1985). The ratio of nitrite accumulating to nitrate consumed denotes the inefficiency of suboxic N 2 -production.…”

Section: Stoichiometric Constraintsmentioning

confidence: 99%

Heterotrophic denitrification vs. autotrophic anammox – quantifying collateral effects on the oceanic carbon cycle

Koeve

Kähler

2010

Biogeosciences

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Abstract. The conversion of fixed nitrogen to N 2 in suboxic waters is estimated to contribute roughly a third to total oceanic losses of fixed nitrogen and is hence understood to be of major importance to global oceanic production and, therefore, to the role of the ocean as a sink of atmospheric CO 2 . At present heterotrophic denitrification and autotrophic anammox are considered the dominant sinks of fixed nitrogen. Recently, it has been suggested that the trophic nature of pelagic N 2 -production may have additional, "collateral" effects on the carbon cycle, where heterotrophic denitrification provides a shallow source of CO 2 and autotrophic anammox a shallow sink. Here, we analyse the stoichiometries of nitrogen and associated carbon conversions in marine oxygen minimum zones (OMZ) focusing on heterotrophic denitrification, autotrophic anammox, and dissimilatory nitrate reduction to nitrite and ammonium in order to test this hypothesis quantitatively. For open ocean OMZs the combined effects of these processes turn out to be clearly heterotrophic, even with high shares of the autotrophic anammox reaction in total N 2 -production and including various combinations of dissimilatory processes which provide the substrates to anammox. In such systems, the degree of heterotrophy ( CO 2 : N 2 ), varying between 1.7 and 6.5, is a function of the efficiency of nitrogen conversion. On the contrary, in systems like the Black Sea, where suboxic Nconversions are supported by diffusive fluxes of NH + 4 originating from neighbouring waters with sulphate reduction, much lower values of CO 2 : N 2 can be found. However, accounting for concomitant diffusive fluxes of CO 2 , the ratio approaches higher values similar to those computed for open ocean OMZs. Based on this analysis, we question the significance of collateral effects concerning the trophic nature of suboxic N-conversions on the marine carbon cycle.

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“…The waters derived from the Southern Hemisphere gradually lose O 2 and accumulate products of metabolism (CO 2 , nutrients) during their northward flow. Thus, a lower supply and an enhanced O 2 demand combine to produce very intense O 2 deficiency (Winkler O 2 <0.1 mL L −1 ; ∼4 µM) within a very wide depth range (∼100/150 to 1000 m) in the north, particularly in the Northwestern Indian Ocean (Wyrtki, 1971;Sen Gupta and Naqvi, 1984;Naqvi, 1987).…”

Section: Introductionmentioning

confidence: 99%

Coastal versus open-ocean denitrification in the Arabian Sea

Naqvi

Naik²,

Pratihary³

et al. 2006

Biogeosciences

161

132

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Abstract. The Arabian Sea contains one of the three major open-ocean denitrification zones in the world. In addition, pelagic denitrification also occurs over the inner and midshelf off the west coast of India. The major differences between the two environments are highlighted using the available data. The perennial open-ocean system occupies two orders of magnitude larger volume than the seasonal coastal system, however, the latter offers more extreme conditions (greater nitrate consumption leading to complete anoxia). Unlike the open-ocean system, the coastal system seems to have undergone a change (i.e., it has intensified) over the past few decades presumably due to enhanced nutrient loading from land. The two systems also differ from each other with regard to the modes of nitrous oxide (N 2 O) production: In the open-ocean suboxic zone, an accumulation of secondary nitrite (NO

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Chemical oceanography of the Indian Ocean, north of the equator

Cited by 123 publications

References 30 publications

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

Heterotrophic denitrification vs. autotrophic anammox – quantifying collateral effects on the oceanic carbon cycle

Coastal versus open-ocean denitrification in the Arabian Sea

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