Oxygen minimum zone of the open Arabian Sea: variability of oxygen and nitrite from daily to decadal time scales
Authors: Banse K. , Naqvi S. W. A. , Narvekar P. V. , Postel J. R. , Jayakumar D. A. 
Published: September 2013

Abstract. The oxygen minimum zone (OMZ) of the Arabian Sea is the thickest of the three oceanic OMZs, which is of global biogeochemical significance because of denitrification in the upper part leading to N2 and N2O production. The residence time of the OMZ water is believed to be less than a decade. The upper few hundred meters of this zone are nearly anoxic but non-sulfidic and still support animal (metazoan) pelagic life, possibly as a result of episodic injections of O2 by physical processes. The very low O2 values obtained with the new STOX sensor in the eastern tropical South Pacific probably also characterize the Arabian Sea OMZ, but there is no apparent reason as to why the temporal trends of the historic data should not hold. We report on discrete measurements of dissolved O2 and NO2-, besides temperature and salinity, made between 1959 and 2004 well below the tops of the sharp pycno- and oxyclines near 150, 200, 300, 400, and 500 m depth. We assemble nearly all O2 determinations (originally, 849 values, 695 in the OMZ) by the visual endpoint detection of the iodometric Winkler procedure, which in our data base yields about 0.04 mL L−1 (∼2 μM) O2 above the endpoint from modern automated titration methods. We find 632 values acceptable (480 from 150 stations in the OMZ). The data are grouped in zonally-paired boxes of 1° lat. and 2° long. centered at 8°, 10°, 12°, 15°, 18°, 20°, and 21° N along 65° E and 67° E. The latitudes of 8–12° N, outside the OMZ, are only treated in passing. The principal results are as follows: (1) an O2 climatology for the upper OMZ reveals a marked seasonality at 200 to 500 m depth with O2 levels during the northeast monsoon and spring intermonsoon season elevated over those during the southwest monsoon season (median difference, 0.08 mL L−1 [3.5 μM]). The medians of the slopes of the seasonal regressions of O2 on year for the NE and SW monsoon seasons are −0.0043 and −0.0019 mL L−1 a−1, respectively (−0.19 and −0.08 μM a−1; n = 10 and 12, differing at p = 0.01); (2) four decades of statistically significant decreases of O2 between 15° and 20° N but a trend to a similar increase near 21° N are observed. The balance of the mechanisms that more or less annually maintain the O2 levels are still uncertain. At least between 300 and 500 m the annual reconstitution of the decrease is inferred to be due to lateral, isopycnal re-supply of O2, while at 200 (250?) m it is diapycnal, most likely by eddies. Similarly, recent models show large vertical advection of O2 well below the pycno-cum-oxycline. The spatial (within drift stations) and temporal (daily) variability in hydrography and chemistry is large also below the principal pycnocline. The seasonal change of hydrography is considerable even at 500 m. There is no trend in the redox environment for a quarter of a century at a GEOSECS station near 20° N. In the entire OMZ the slopes on year within seasons for the quite variable NO2- (taken as an indicator of active denitrification) do not show a clear pattern. Also, future O2 or nutrient budgets for the OMZ should not be based on single cruises or sections obtained during one season only. Steady state cannot be assumed any longer for the intermediate layers of the central Arabian Sea.