The Acid Competition Hypothesis: a new theory on the controls of the equilibrium concentration of atmospheric CO2

Classical models of the long-term carbonate–silicate cycle set the atmospheric CO2 sink by the rate of base production on the land surface, conserving carbon and treating carbonic acid as the only acid that weathers rock (Urey, 1952; Walker, Hays and Kasting, 1981; Berner, Lasaga and Garrels, 1983); strong acids enter the cycle only as corrections. Here we propose a new framework for the long-term carbonate–silicate cycle that conserves alkalinity instead of carbon. Carbon accounting cannot represent the competition between acids, because CO2 is uncharged: adding or removing it changes dissolved inorganic carbon but leaves the charge balance untouched. We define alkalinity as the charge balance between base cations and strong-acid anions. In that balance sulphate and nitrate appear explicitly, each with a negative sign, so the strong acid that claims base becomes a term we can measure in a water sample. We develop the Acid Competition Hypothesis, which predicts that the addition of strong acid to the Earth surface results in higher equilibrium CO2. We show that the classical carbonate–silicate cycle is the zero-strong-acid case of a planetary titration in which strong and carbonic acid compete for a finite supply of base. We create a column model of the titration in which the acid competition for base reduces to a single parameter, the matched fraction: the extra base that weathering releases per equivalent of strong acid delivered to the weathering front. Its complement, the unmatched fraction, is the share of strong acid that competes with carbonic acid for base. At a matched fraction of one, the acid increases weathering to produce its own base and the equilibrium atmospheric CO2 does not change; the Acid Competition Hypothesis is that real landscapes operate below a matched fraction of one, and it is falsified if the matched fraction reaches one. We report the first estimates of the matched fraction, and they support the Acid Competition Hypothesis (Methods M1). Across sixteen Finnish river-mouth stations the unmatched fraction is 0.91, with a stoichiometric range of 0.89 to 0.93: nine tenths of every equivalent of strong acid displaces CO2 drawdown instead of calling forth new weathering. A global compilation of river chemistry puts the unmatched fraction higher still, at 0.983, with a bootstrap interval of 0.949 to 0.996. Therefore, across the range of the matched fraction in these catchments the addition of strong acid raises equilibrium atmospheric CO2.