26 Jul 2022
26 Jul 2022

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

Fabian Matthias Gäb1, Chris Ballhaus1, and Jan Siemens2,a Fabian Matthias Gäb et al.
  • 1Institut für Geowissenschaften, Universität Bonn, Bonn, 53115, Germany
  • 2Institut für Nutzpflanzenwissenschaften und Ressourcenschutz, Bereich Bodenwissenschaften, Universität Bonn, Bonn, 53115, Germany
  • apresent address: Institut für Bodenkunde und Bodenerhaltung, iFZ, Universität Giessen, Giessen, 35392. Germany

Abstract. 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.

Fabian Matthias Gäb et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-564', Anonymous Referee #1, 28 Aug 2022
  • RC2: 'Comment on egusphere-2022-564', Anonymous Referee #2, 12 Sep 2022
  • RC3: 'Comment on egusphere-2022-564', Anonymous Referee #3, 15 Nov 2022
  • CC1: 'Comment on egusphere-2022-564', Siv K Lauvset, 21 Nov 2022

Fabian Matthias Gäb et al.

Fabian Matthias Gäb et al.


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Short summary
Experiments we conducted show that the carbonate inventory of Aegean seawater is in part not in exchange with atmospheric CO2. This "background" reservoir makes up around 78 % of the total carbon in Aegean Seawater. This means that only 22 % of the dissolved inorganic carbon is in equilibrium with the atmosphere and is thus able to buffer anthropogenic CO2. Based on our results it can be calculated that the buffer capacity for CO2 in Aegean seawater will be reduced by 50 % by the year 2100.