Abstract
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 (NO3−) is reduced to dinitrogen gas (N2). 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 oxygen-enriched 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.
Highlights
The marine nitrogen cycle, which strongly influences the fertility of the ocean and the sequestration of CO2 from the atmosphere, is mainly controlled by nitrogen fixation (N2 → NH3) and the reduction of NO−3 to dinitrogen gas (NO−3 → N2; Brandes and Devol, 2002; Deutsch et al, 2007; Dugdale and Goering, 1967; McElroy, 1983)
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
In order to study recent changes of the nitrogen cycle in the Arabian Sea, we evaluated and compared nutrient data obtained from the literature published prior to 1993 (Naqvi and Shailaja, 1993; Naqvi, 1991) during five expeditions carried out within the framework of US Joint Global Ocean Flux Study (JGOFS) in 1995 (Morrison et al, 1998) and a research cruise of RV Meteor in 2007 (M74/1b; Fig. 1)
Summary
The marine nitrogen cycle, which strongly influences the fertility of the ocean and the sequestration of CO2 from the atmosphere, is mainly controlled by nitrogen fixation (N2 → NH3) and the reduction of NO−3 to dinitrogen gas (NO−3 → N2; Brandes and Devol, 2002; Deutsch et al, 2007; Dugdale and Goering, 1967; McElroy, 1983). The reduction of NO−3 → N2, referred to as denitrification (Eqs. 1–3), is a combination of different microbial processes such as the. NO−3 (Eq 1) and NO−2 reduction (Eq 2), which can be expressed stoichiometrically as follows:. → 106CO2 + 276HNO2 + H3PO4 + 122H2O;. (2) nitrite reduction: 1.75(CH2O)106(NH3)16(H3PO4) + 276HNO2 (2). (1 + 2) denitrification: 2.75(CH2O)106(NH3)16(H3PO4) + 260HNO3
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
