Oxygen Minimum Zone Contrasts Between the Arabian Sea and the Bay of Bengal Implied by Differences in Remineralization Depth

Abstract

The combination of high primary productivity and weak ventilation in the Arabian Sea (AS) and Bay of Bengal (BoB) generates vast areas of depleted oxygen, known as oxygen minimum zones (OMZs). The AS OMZ is the world's thickest and hosts up to 40% of global denitrification. In contrast, the OMZ in the BoB is weaker and denitrification free. Using a series of model simulations, we show that the deeper remineralization depth (RD) in the BoB, potentially associated with organic matter aggregation with riverine mineral particles, contributes to weaken its OMZ. When the RD is set uniformly across both seas, the model fails to reproduce the observed contrast between the two OMZs, irrespective of the chosen RD. In contrast, when the RD is allowed to vary spatially, the contrasting distributions of oxygen and nitrate are correctly reproduced, and water column denitrification is simulated exclusively in the AS, in agreement with observations.

Oxygen Variability Controls Denitrification in the Bay of Bengal Oxygen Minimum Zone

“…The long‐term observations with profiling floats show that denitrification in the BoB is inhibited by a process that is quite different than the mechanisms previously suggested (Al Azhar et al, ; Bristow et al, ; McCreary et al, ). We find highly variable oxygen concentrations that are frequently above the 0.2 μmol/kg level observed to inhibit denitrification rates by 50% or the 0.9 μmol/kg level that causes a similar reduction in anammox rates (Dalsgaard et al, ).…”

“…Our observations of frequent concentrations above the critical levels for denitrification contrast with the conclusion that oxygen is poised at low levels in the nmol/kg concentration range (Bristow et al, ). McCreary et al () and Al Azhar et al () find that enhanced remineralization of organic carbon in the AS, which is driven by various mechanisms, leads to the lower oxygen and greater denitrification than in the BoB. However, McCreary et al () also note that there is strong seasonality in carbon export but little seasonality in OMZ oxygen concentrations of the BoB.…”

Physical Controls on Oxygen Distribution and Denitrification Potential in the North West Arabian Sea

“…As the PGW spreads below the thermocline, at the same depth as the OMZ's upper boundary, the presence (or absence) of PGW plays a key role in regulating the depth of the oxycline. As the intensity of denitrification has been linked to remineralization depth (Al Azhar et al, ), a deeper suboxic boundary not only reduces suboxic volume of the OMZ but may inhibit denitrification within the depth range where denitrification would otherwise be stronger.…”

<i>p</i>CO<sub>2</sub> variability in the surface waters of the eastern Gulf of Cádiz (SW Iberian Peninsula)

“…The Gulf of Cádiz is part of one of the four major eastern boundary upwelling systems of the world: the North Atlantic upwelling (e.g. Alvarez et al, 2009) that extends from south of Cap-Vert (Senegal) to Cape Finisterre (northwest of Spain). For this reason, the Gulf of Cádiz presents characteristics typical of this system: seasonal variability of a winds system favourable to the coastal upwelling (Fiúza et al, 1982), high biological productivity (Navarro and Ruiz, 2006), a system of fronts and zonal currents (García Lafuente and Ruiz, 2007) and a zone of water exchange between the coastal zone and open ocean (Sánchez et al, 2008).…”

“…Moreover, in the coastal zone another source of CO 2 results from the net production of inorganic carbon derived from the processes of remineralization of the organic matter in the surface sediments originating from the continuous deposition of organic matter through the water column (de Haas et al, 2002;Jahnke et al, 2005). The intensity of this effect decreases towards offshore areas since the influence of primary production and the continental supplies on the deposition of the particulate organic matter are less (Friedl et al, 1998;Burdige, 2007;Al Azhar et al, 2017), which could be related to the greater effect determined by the mixing and biology processes in the coastal areas using the Olsen et al (2008) method. Ferrón et al (2009) quantified the release from the sediment of DIC related to the processes of oxidation of organic matter in the coastal zone (depth <50 m) of the Gulf of Cádiz, between the Guadalquivir and the Bay of Cádiz.…”