Long-term and short-term inorganic carbon reservoirs in Aegean seawater – an experimental study

The relevant literature does not explicitly address the fact that there are two fundamentally different inorganic carbon (DIC) reservoirs in seawater; (1) a long-term “background” DIC reservoir that is not in net-transfer equilibrium with the atmosphere, and (2) a short-term “atmospheric” DIC reservoir that is fed by atmospheric pCO2. In addition, we may define a third “anthropogenic” DIC reservoir that quantifies the increase in DIC since industrialization.

We perform experiments to quantify these reservoirs. We equilibrate Aegean seawater with N2-O2 (79:21) gases with variable pCO2 from < 10 to 100,000 µatm, and pure CO2 gas. We quantify electrochemically the changes in pH and, by titration and IR spectroscopy, total alkalinity (TA) and dissolved inorganic carbon (DIC) that occur with variations in pCO2. About 78 % of the Aegean DIC is “background“, introduced into the Aegean sea by the long-term carbon cycle, i.e. riverine input, remineralization of organic carbon, and hydrothermal CO2. In terms of concentration and in the short term, this reservoir is independent of atmospheric pCO2. About 22 % of DIC is atmospheric in origin and is in exchange equilibrium with atmospheric pCO2. The anthropogenic contribution to the atmospheric DIC reservoir is derived by measuring the increase in DIC between 280 (pre-industrial) and 410 µatm (present-day) pCO2 and quantified at around 26 %.

Our experiments also allow projections into the future. It has been suspected that increasing atmospheric pCO2 lowers the CO2 absorption capacity of ocean surface water. Our data confirm this assessment. When the pCO2 increases, the pH and the CO32--concentration fall, and with them the ability of seawater to hydrolyze CO2. Without measures to limit anthropogenic CO2 emissions, the absorption capacity of Aegean seawater in the year 2100 will be only about one half of the absorption capacity of today.

Gäb F. M., Ballhaus C. & Siemens J., in press. Long-term and short-term inorganic carbon reservoirs in Aegean seawater – an experimental study. EGUsphere. Article.

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