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The oxygen minimum zone (OMZ) of the Arabian Sea is the thickest of the threeoceanic OMZ. It is of global biogeochemical significance because ofdenitrification in the upper part leading to N2 and N2O production.The residence time of OMZ water is believed to be less than a decade. Theupper few hundred meters of this zone are nearly anoxic but non-sulfidic andstill support animal (metazoan) pelagic life, possibly as a result ofepisodic injections of O2 by physical processes. We report on discrete measurements of dissolved O2 and NO2–,temperature and salinity made between 1959 and 2004 well below the tops ofthe sharp pycnocline and oxycline near 150, 200, 300, 400, and 500 m depth.We assemble nearly all O2 determinations (originally there were 849values, 695 of which came from the OMZ) by the visual endpoint detection ofthe iodometric Winkler procedure, which in our data base yields about0.04 mL L−1 (~ 2 μM) O2 above the endpoint frommodern automated titration methods. We acknowledge that much lower(nanomolar) O2 values have been measured recently with the STOX(Switchable Trace amount OXygen) sensor in the eastern tropical SouthPacific, and that similar conditions may also prevail in the Arabian Sea OMZ.In spite of the error in O2 measurements at vanishingly low levels, weargue that the temporal trends of the historic data should still hold. We find 632 values acceptable (480 from 150 stations in the OMZ). The dataare grouped in zonally paired boxes of 1° lat. and 2° long.centered at 8, 10, 12, 15, 18, 20, and 21° N along 65 and67° E. The latitudes of 8–12° N, outside the OMZ, aretreated in passing. The principal results are as follows: (1) an O2climatology for the upper OMZ reveals a marked seasonality at 200 to 500 mdepth with O2 levels during the northeast monsoon and springintermonsoon seasons elevated over those during the southwest monsoon season(median difference, 0.08 mL L−1 [~ 3.5 μM]). Themedians of the slopes of the seasonal regressions of O2 on year for eachof 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 15and 20° N but an opposing trend toward an increase near21° N are observed. The mechanisms of the balance that more or lessannually maintain the O2 levels are still uncertain. At least between300 and 500 m, the replenishment is inferred to be due to isopycnalre-supply of O2, while at 200 (or 250?) m it is diapycnal, most likelyby eddies. Similarly, recent models show large vertical advection of O2well below the pycnoclines and oxyclines. The NO2– distribution, taken as an indicator of active NO3–reduction, does not show a trend in the redox environment for a quarter of acentury at a GEOSECS station near 20° N. In the entire OMZ, theregression slopes on year within seasons for the rather variable NO2–do not present a clear pattern but by other measures tended to an increase ofNO2–. Vertical net hauls collect resident animal (metazoan) pelagic life in theNO2– maximum of the OMZ at O2 levels well below the lower limitof the Winkler titration; the extremely low O2 content is inferred fromthe presence of NO2– believed to be produced through microbialNO3– reduction. Instead of the difficult measurement by the STOXsensor, the relation between the very low O2 inferred from presence ofNO2– and mesozooplankton should be studied with 100 to 150 L bottlesrather than nets. The spatial (within drift stations) and temporal (daily) variability inhydrography and chemistry is large also below the principal pycnocline. Theseasonal change of hydrography is considerable even at 500 m depth. FutureO2 or nutrient budgets for the OMZ must not be based on single cruisesor sections obtained during one season only. Steady state cannot be assumedany longer for the intermediate layers of the central Arabian Sea

Oxygen minimum zone of the open Arabian Sea: variability of oxygen and nitrite from daily to decadal timescales