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Abstract Observations of dissolved iron (dFe) in the subtropical North Atlantic revealed remarkable features: While the near‐surface dFe concentration is low despite receiving high dust deposition, the subsurface dFe concentration is high. We test several hypotheses that might explain this feature in an ocean biogeochemistry model with a refined Fe cycling scheme. These hypotheses invoke a stronger lithogenic scavenging rate, rapid biological uptake, and a weaker binding between Fe and a ubiquitous, refractory ligand. While the standard model overestimates the surface dFe concentration, a 10‐time stronger biological uptake run causes a slight reduction in the model surface dFe. A tenfold decrease in the binding strength of the refractory ligand, suggested by recent observations, starts reproducing the observed dFe pattern, with a potential impact for the global nutrient distribution. An extreme value for the lithogenic scavenging rate can also match the model dFe with observations, but this process is still poorly constrained. 

Oxygen inventory of the global ocean has declined in recent decades potentially due to the warming-induced reduction in solubility as well as the circulation and biogeochemical changes associated with ocean warming and increasing stratification. Earth System Models predict continued oxygen decline for this century with profound impacts on marine ecosystem and fisheries. Observational constraint on the rate of oxygen loss is crucial for assessing the ability of models to accurately simulate these changes. There are only a few observational assessments of the global oceanic oxygen inventory reporting a range of oxygen loss. This study develops a gridded data set of dissolved oxygen for the global oceans using optimal interpolation method. The resulting gridded product includes full-depth map of dissolved oxygen as 5-year moving average from 1965 to 2015 with uncertainty estimates. The uncertainty can come from unresolved small-scale and high-frequency variability and mapping errors. The multi-decadal trend of global dissolved oxygen is in the range of −281 to −373 Tmol/decade. This estimate is more conservative than previous works. In this study, the grid points far from the observations are essentially set equal to zero anomaly from the climatology. Calculating global inventory with this approach produces a relatively conservative estimate; thus, the results from this study likely provide a useful lower bound estimate of the global oxygen loss. 

This study investigates the mechanisms of interannual and decadal variability of dissolved oxygen (O₂) in the North Pacific using historical observations and a hindcast simulation using the Community Earth System Model. The simulated variability of upper ocean (200 m) O₂is moderately correlated with observations where sampling density is relatively high. The dominant mode of O₂variability explains 24.8% of the variance and is significantly correlated with the Pacific Decadal Oscillation (PDO) index (r = 0.68). Two primary mechanisms are hypothesized by which the PDO controls upper ocean O₂variability. Vertical movement of isopycnals (“heave”) drives O₂variations in the deep tropics; isopycnal surfaces are depressed in the eastern tropics under the positive (El Niño‐like) phase of PDO, leading to O₂increases in the upper water column. In contrast to the tropics, changes in subduction are the primary control on extratropical O₂variability. These hypotheses are tested by contrasting O₂anomalies with the heave‐induced component of variability calculated from potential density anomalies. Isopycnal heave is the leading control on O₂variability in the tropics, but heave alone cannot fully explain the amplitude of tropical O₂variability, likely indicating reinforcing changes from the biological O₂consumption. Midlatitude O₂variability indeed reflects ocean ventilation downstream of the subduction region where O₂anomalies are correlated with the depth of winter mixed layer. These mechanisms, synchronized with the PDO, yield a basin‐scale pattern of O₂variability that are comparable in magnitude to the projected rates of ocean deoxygenation in this century under “unchecked” emission scenario.