No.266 X-Y plots of Phosphate (P) and Oxygen consumption (AOU)

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• X-Y plots of Phosphate and Oxygen consumption at 3000m depth in entire ocean
  • The consumption of oxygen in organic decomposition is roughly proportional to the regeneration of nutrients. Again, show the reaction equation for organic matter decomposition assuming the Redfield ratio.
    
    

                (CH₂O)₁₀₆(NH₃)₁₆H₃PO₄ + 138O₂
    

    　　　　　　　　　　　　　　　= 106CO₂ + 16HNO₃ + H₃PO₄ + 122H₂O

    Remineralizing 1 mol of PO43- in seawater consumes 138 mol of O2. To examine the relation between remineralization and oxygen consumption, PO₄^3- vs. AOU (oxygen consumption) plot at deep sea (3000m) is shown below.
    

    In deep North Pacific water with well-degraded organic matter, the ratio of AOU increase to phosphate regeneration (⊿AOU/⊿P) is equal to the rate of change assumed from the Redfield ratio (⊿AOU / ⊿PO₄^3- = 138).

    
    

    In contrast, ⊿AOU/⊿P = 92 in the deep Atlantic. It is probably because, at the beginning of the deep circulation, organic matter is abundant with a high P ratio (e.g., lipids), which decompose first, and more P is remineralized into seawater in the deep Atlantic than would be expected from the Redfield ratio.

    
    

    
    

    
    

    
    

    Consider the characteristic of Phosphate vs AOU plots.
    

    Consideration①
    

    What does the phosphate concentration (bold red box in the figure below: 1.2 μmol/L) at the intersection of AOU = 0, which is the extension of the line ΔAOU/ΔP = 138 in North Pacific deep water, mean?　Surface water is in equilibrium with the atmosphere, so AOU = 0. Therefore, the phosphate concentration at the intersection can approximately be regarded as the initial value of the phosphate concentration when the water mass was at the surface. In other words, it is estimated to be the phosphate concentration of the winter surface water in the North Atlantic Arctic region (about 1.2 μmol/L), the starting point of the deep circulation. The concentration in deep water in the North Atlantic high latitudes (Arctic region) is 1.0 μmol/L, so this is a reasonable estimate.

    Consideration②
    

    The plot of Antarctic deep water (bold red dotted line in the figure below) is located off the regression line. Why is this?　In the Antarctic region, sea surface cooling is significant and vertical mixing is very active. Oxygen-rich water in the surface layer (water in which oxygen is not consumed) is transported to the deeper layers, which is probably why the AOU is lower.

    
    

    
    

    
    

    
    
    

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    Contents of "Marine Chemistry" URL
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    X-Y plots of N and P in the entire ocean URL
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