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

Abstract

At suboxic oxygen concentrations, key biogeochemical cycles change and denitrification becomes the dominant remineralization pathway. Earth system models predict oxygen loss across most ocean basins in the next century; oxygen minimum zones near suboxia may become suboxic and therefore denitrifying. Using an ocean glider survey and historical data, we show oxygen loss in the Gulf of Oman (from 6-12 to <2 μmol kg À1 ) not represented in climatologies. Because of the nonlinearity between denitrification and oxygen concentration, resolutions of current Earth system models are too coarse to accurately estimate denitrification. We develop a novel physical proxy for oxygen from the glider data and use a high-resolution physical model to show eddy stirring of oxygen across the Gulf of Oman. We use the model to investigate spatial and seasonal differences in the ratio of oxic and suboxic water across the Gulf of Oman and waters exported to the wider Arabian Sea. Plain Language SummaryOxygen is present in the ocean and is required by all marine plants and animals to breathe. In certain regions around the world, oxygen concentrations reach very low levels. These are known as "oxygen minimum zones". When oxygen is absent, chemical cycling of nitrogen, a key nutrient for plant growth, changes dramatically. Computer simulations of ocean oxygen show a decrease in oxygen over the next century and growing oxygen minimum zones. However, these simulations have a difficult time representing small but very important features such as eddies, which impact how oxygen is transported. It is difficult to predict what will happen in the biggest of the world's oxygen minimum zones, the Arabian Sea, as piracy and geopolitical tensions have limited past opportunities for observing these processes. To remedy this, we deployed two remote-controlled submarines, known as a Seagliders, in the Gulf of Oman. These instruments measured a strong decrease of oxygen in the oxygen minimum zone compared to pre-1990 values. We then combined the Seaglider data with a very high resolution computer simulation to determine how oxygen is spread around the northwestern Arabian Sea throughout different seasons and the monsoons.The Arabian Sea is home to the thickest and most intense OMZ worldwide (Figure 1), with suboxic concentrations between 150 and 1,000 m (Bange et al., 2005; Banse et al., 2014; McCreary et al., 2013; Piontkovski QUESTE ET AL. 4143

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

“…For example, the depth of the oxycline in both the AS and BoB is strongly related to sea surface height variations driven by mesoscale eddies (Prakash et al, ). Glider observations in the northwest AS demonstrate the role of eddies in transporting oxygen (Queste et al, ). There is little seasonal variability in the frequency of eddies found in the BoB, but there are large, interannual changes in the eddy number (Chen et al, ).…”

Ventilation of Oxygen to Oxygen Minimum Zone Due to Anticyclonic Eddies in the Bay of Bengal

“…Considering the mean ventilation time of OMZ to be 12 years (Sarma, ), the rate of oxygen input through PGW is estimated to be 0.003 to 0.005 μmol/L/day. Queste et al () suggested that PGW may not bring significant amount of DO to the BoB as it passes through intensely DO‐depleted OMZ of the AS before entering to BoB. Nevertheless, the supply of oxygen through horizontal advection of PGW is an order of magnitude less than that from ACEs (0.07 ± 0.02 μmol/l/d) and bacterial respiration rates (0.02 ± 0.004 μmol/l/d), suggesting that ventilation of oxygen through ACEs is very important in the maintenance of OMZ.…”

Dark carbon fixation in the Arabian Sea oxygen minimum zone contributes to sedimentary organic carbon (SOM)

“…Settings similar to the Arabian Sea, with ≤ 4.5 μmol kg -1 O 2 , occur in the East Pacific and those settings in total occupy an area of 8.45 to 15 × 10 12 m 2 (Karstensen et al, 2008;Hattori, 1983;Paulmier & Ruiz-Pino, 2009), or a volume of 0.46 × 10 15 m 3 with 1.148 × 10 12 m 2 in contact with the sediment (Helly & Levin, 2004). OMZs are expanding in size and in volume (Queste et al, 2018;Stramma et al, 2010), and anammox bacteria play a key role in these settings, particularly because they tolerate higher oxygen concentrations than denitrifiers (Dalsgaard et al, 2014). The labile organic matter added to the sinking carbon can fuel heterotrophic processes, further exacerbating oxygen depletion, and nitrogen loss by denitrification.…”