Meridional and Cross‐Shelf Variability of N₂O and CH₄ in the Eastern‐South Atlantic

Abstract

Nitrous oxide (N 2 O) and methane (CH 4 ) are long-lived atmospheric trace gases which account for a large fraction of the total radiative forcing due to well-mixed greenhouse gases (GHGs;Myhre, 2013). Albeit a smaller source of atmospheric N 2 O and CH 4 in comparison with terrestrial ecosystems, marine environments represent a significant and therefore nonnegligible contribution to their overall budget (Ciais et al., 2013). Overall, the trace gas distribution in the ocean is spatially and vertically heterogeneous due to biogeochemical (microbial) and physical (mixing, upwelling, and stratification) processes, acting either as a trace gas source or sink with respect to the atmosphere (Bates et al., 1996;Reeburgh, 2007). N 2 O is produced in intermediate waters via microbially driven nitrification and denitrification under oxic and suboxic conditions, respectively, and exchanged with the atmosphere in surface waters (Bakker et al., 2014). In the nitrification pathway, carried out by ammonia-oxidizing bacteria and archaea, N 2 O is produced as a by-product during the oxidation of NH 4 + to NO 2 − (Löscher et al.,

Unveiling Profound Seasonal and Spatial Dynamics of Greenhouse Gases (CO2, CH4, N2O) in the Northern Benguela Upwelling System Unnamed Article

“…The concentrations of dissolved CH 4 and N 2 O were computed after Sabbaghzadeh et al (2021). To further explore the N 2 O cycle in the BUS, the excess N 2 O (ΔN 2 O) was calculated, representing the difference between the observed N 2 O concentration (N 2 O in situ ) and the equilibrium concentration of N 2 O with the atmosphere (N 2 O eq . )…”

“…This enhancement is more pronounced off Walvis Bay than off Kunene, likely attributable to the extent of the shelf and the presence of organic-rich mud belt (e.g. Van Der Plas, et al, 2007;Sabbaghzadeh et al, 2021). In the winter season in Kunene, two sources of CH 4 within the water column are proposed, including a sedimentary source and an upper source involving in-situ biogenic production of CH 4 by bacteria within the oxygenated layer (Sabbaghzadeh et al, 2021).…”

“…These vials were then sealed with rubber butyl stoppers and poisoned with 200 μL of saturated HgCl 2 solution immediately after. Determination of CH 4 and N 2 O was carried out by means of a dynamic headspace method as outlined in Sabbaghzadeh et al (2021). This method ensures an average precision better than 0.5% for CH 4 and 0.6% for N 2 O by purging 20 ml of the water sample, along with detection limits below 0.3 nmol for both gases.…”

“…This method ensures an average precision better than 0.5% for CH 4 and 0.6% for N 2 O by purging 20 ml of the water sample, along with detection limits below 0.3 nmol for both gases. Measurements were performed by means of a custom-designed purge and trap system integrated with a gas chromatograph (Agilent 7890B) as described previously (Sabbaghzadeh et al 2021). To ensure accuracy and detect drift, different volumes of standard gases (9.26 ppm ± 0.2% CH 4 and 1992 ppb ± 0.2% N 2 O in synthetic air) were measured before and after each set of analysis on a daily basis.…”

“…The BUS is a net source of CH 4 and N 2 O to the atmosphere (see e.g. Arevalo-Martinez et al, 2019;Sabbaghzadeh et al, 2021;Mashifane et al, 2022). CH 4 sources in OMZs are anoxic sediments where microbial anaerobic methanogenesis occurs during organic matter degradation.…”