XY plots of DIC and Alk

　Solving a total of six equations for the equilibrium conditions of carbonic acid system components and water, and the definitions of DIC and Alk, the remaining two items can be obtained if two of the four items, DIC, Alk, pCO₂, and pH, are given. Please learn how to calculate them in "Special Practice Courses (in preparation)" as "Developmental Courses".

　After simplifying the equation to Alk = 【HCO₃^-】 + 2【CO₃^2-】, the equation for the carbonic acid system equilibrium is developed.

　The relationship between DIC, Alkalinity (Alk), and pCO₂ under a water temperature of 20°C and a constant salinity of 34 is shown in the figure below.

　For a given value of pCO₂ (from 250 μatm to 50 μatm interval), the relationship between Alk and DIC is represented by the solid black line in the figure.

　For example, if the atmospheric pCO₂ is 400 μatm, the North Atlantic surface layer has an alkalinity (Alk) of 2.3 mmol/L, so the DIC is calculated to be 2.05 mmol/L. That plot is the starting point of arrow (a). The explanation of the figure is summarized below.

The calculation formula followed the formula in Table 8.2.1 of Ocean Biogeochemical Dynamics, Sarmiento and Gruber, Princeton Univ Press, (2006). The figure was drawn based on the calculation results with different Alkalinity and DIC display ranges, referring to Figure 8.3.5.

【Plot shifts due to calcium carbonate dissolution】

　North Atlantic surface water contains high levels of calcium carbonate shells (to be discussed later). When that water is transported to deeper layers, calcium carbonate particles dissolve. For example, in 1 L of seawater, the dissolution of 0.05 mmol of calcium carbonate results in a 0.1 mmol/L increase in alkalinity and a 0.05 mmol/L increase in DIC. In other words, the ratio Alk: DIC = 2: 1 moves through the above figure. The movement of the plot due to the dissolution of calcium carbonate particles is indicated by arrow (a). Once the calcium carbonate particles are formed, the plot moves in the opposite direction.

【Plot movement due to organic decomposition】

　The decomposition of 0.1 mmol (carbon equivalent) of organic carbon in 1 L of seawater raises the DIC by 0.1 mmol/L. In addition to carbon, organic matter also contains nitrogen. These are also mineralized and NO₃^- is regenerated in seawater. Alkalinity = "total charge of strong electrolyte cations" - "total charge of strong electrolyte anions". Therefore, alkalinity decreases due to the regeneration of NO₃^- associated with organic matter decomposition.

　If the C: N ratio = 106: 16 (Redfield ratio) in organic matter, 16 mmol of NO₃^- is regenerated for every 106 mmol of organic carbon. Converting this to the ratio of alkalinity decrease and DIC increase, ΔAlk: ΔDIC = -16: 106. However, the Redfield ratio cannot be used directly as a regeneration ratio for the decomposition of organic matter in the mesopelagic layer. As explained in the organic matter chapter, the Redfield ratio represents the C: N: P ratio in particulate organic matter. For organic matter decomposition in deeper layers, a ratio of ΔAlk: ΔDIC = -16: 117 has been proposed. The ratio is represented by the arrow (b) in the figure.