of organic matter) during eastward transit of ICW from Somali coast to the west coast of India (McCreary et al., 2013; Acharyya and Panigrahi, 2016; Rixen et al., 2020);
rapid sinking of the large phytoplankton detritus in the upwelling regions of the WAS (Naqvi et al., 2010b) results in minimum respiration, alternatively, the slow sinking rate of organic matter (Hood et al., 2009) produced in the WAS and subsequent eastwardly advection of the organic particulates leads to DO depletion in central and eastern AS;
effective renewal of subsurface water along the western boundary of the AS, by advection from the south, from the Red Sea and from the Persian Gulf (Naqvi et al., 2006). The prolonged ventilation times 7–8 years for RSW and (2–3 years) for PGW in the EAS compared with the ventilation time of (3–4 years and 1–2 years, respectively, in WAS lead to more significant DO depletion in the EAS (Schmidt et al., 2020a)
physical processes such as mesoscale eddies prevailing in the WAS are expected to contribute towards effective oxygen renewal in the water column (McCreary et al., 2013; Schmidt et al., 2020b);
the cross‐shelf transport of organic‐rich sediments from the western continental shelf of India (Somayajulu et al., 1996; Sarma et al., 2020) and subsequent respiration, causing suboxia in the EAS.
the warming of the PGW. Recent observations have seen warming of the PGW (shallow semi‐enclosed sea with an average depth of 35 m) at a rate of two to three times faster than the global average rate, increasing its buoyancy and may lead to poor, intermediate water ventilation (Lachkar et al., 2019).
In addition to the perennial OMZ (pOMZ), the AS experiences seasonal suboxia and denitrification over the inner and mid‐shelf off the west coast of India during and shortly after the southwest monsoon (Sharma, 1978; Shetye et al., 1990; Naqvi et al., 2000, 2006). This coastal oxygen‐deficient zone is separated from the pOMZ in the central AS by the presence of slightly more oxygenated waters of the WIUC that flow poleward (Naqvi et al., 2006). During the peak of upwelling season (in September), almost the entire Indian shelf (and some of the Pakistan shelf) is severely hypoxic having an O2 concentration <0.5 ml l–1 covering an area of about 180 000 km2 (Naqvi et al., 2000). The seasonal suboxia (even anoxia) reported in this region occurs because of coastal upwelling occurring along the western Indian shelf during June to November. Upwelling begins in April, along the southwest coast of India (along with Kerala), and gradually moves northward. The intense DO depletion in both the seasonal OMZ (sOMZ) and pOMZ enhances the deposition of organic matter in the underlying sediments as a result of a combination of high (upwelling‐driven) primary productivity and inefficient degradation during sinking in the oxygen‐depleted water column (Paropkari et al., 1992, 1993; Cowie, 2005). The intense DO depletion has a profound influence on the underlying sediments, with respect to the redox conditions, microbial community, nature and activity of benthic communities, pore water redox processes, and consequently on the diagenetic pathways (e.g. Cowie, 2005).
1.1.2. The Bay of Bengal Oxygen Minimum Zone
The BoB, in the northern Indian Ocean, is the largest bay in the world, bordered by India, Bangladesh, Sri Lanka, and Myanmar. The BoB receives a large amount of freshwater from several river systems, which drain into it. The rivers Ganga‐Brahmaputra (G‐B), Irrawaddy, Godavari, Mahanadi, Krishna, and Kaveri contribute 60% of the total freshwater received by the BoB. Riverine water flux of 2.95 × 1012 m3 year–1 (Sengupta et al., 2006) combined with an excess of precipitation over evaporation results in a salinity stratified water column (Prasanna Kumar et al., 2007). The BoB is one of the major global OMZs. Although the conditions appear to be conducive for denitrification/anammox to occur, there has been no indication of nitrogen loss via denitrification in the BoB. Anammox reaction involves oxidation of NH4 + to N2 gas using NO2 – as the electron acceptor under anoxic conditions. The relatively less intense OMZ in the BoB may be attributed to weaker upwelling along the east coast of India than in Oman and Somalia. McCreary et al. (2013) attributed lower respiration in the BoB to the absence of an advective flux of additional organic matter, as observed in the EAS. The aggregation of organic matter with mineral particles supplied by high river discharge in the BoB increases the sinking speed of the organic detritus, thereby decreasing its remineralization rate and impacting on the OMZ structure and intensity (Al Azhar et al., 2017). In contrast, Sarma et al. (2018) suggested the activities of cyclonic and anticyclonic eddies to be one of the possible reasons for excess DO in the BoB reported by Bristow et al. (2017). The cyclonic eddies bring up nutrient‐rich water, causing enhanced productivity and subsequent DO drawdown as a result of respiration. In contrast, the anticyclonic eddies cause the downwelling of DO‐rich surface water to deeper layers causing the weakening of the OMZ. In aAdition, Sarma and Bhaskar (2018) suggested westward advection of oxygenated waters within the 150– 300 mbsl depth zone driven by anticyclonic eddies originating in the Andaman waters. Recently, however, Bristow et al. (2017) observed extremely low levels of DO in the BoB and hypothesized that future increases in the deposition of nutrients from atmospheric sources combined with the varying intensity of the summer monsoon could enhance organic matter decomposition and O2 consumption leading to anammox and nitrogen loss. The vertical distribution of O2 and NO2 – in the AS (Lam et al., 2011) and BoB (Bristow et al., 2017) is plotted in Figure 1.3.
1.2. PRESERVATION OF ORGANIC MATTER AND SEDIMENT BIOGEOCHEMISTRY
The preservation of organic carbon (OC) is efficient within the sediments underlying oxygen‐depleted waters (Paropkari et al., 1993; Eglinton et al., 1994; Van der Weijden et al., 1999; Hartnett et al., 1998; Hartnett and Devol, 2003; Böning et al., 2004; Jessen et al., 2017, More et al., 2018) because of incomplete breakdown while sinking through the water column as well as diminished bioturbation activity. A more significant proportion of labile organic matter escapes degradation while sinking through hypoxic or oxygen minimum regions of the modern ocean than oxic zones (Van Mooy et al., 2002). The partly degraded (reactive) organic matter fuels microbe‐mediated sediment biogeochemical processes during burial, which results in modification of the pore fluid composition and precipitation/dissolution of inorganic mineral species (Madigan et al., 2000; Middelburg and Levin, 2009) Fernandes et al., 2018). The global distribution pattern of particulate organic carbon flux (Reimers, 2007) is depicted in Figure 1.4.
Figure 1.3 Vertical distribution of oxygen (solid blue circle) and nitrite concentrations (solid red circle) over the Central–northeast Arabian Sea, Omani Shelf and the Bay of Bengal. Redrawn from Lam et al. (2011) and Bristow et al. (2017).
Figure 1.4 Global distribution pattern of particulate carbon flux to the seafloor.
Source: Reimers (2007) with permission from American Chemical Society.
Remineralization of the partly degraded organic matter is strongly influenced by benthic biotic activity controlled by sediment–water interface conditions such as DO concentration, diffusion of CH4/H2S, sedimentation rate, organic matter content, and sediment grain size. The upper and lower boundaries of the ASOMZ have
