Has Seasonal Ocean Calcification Been Overlooked as a Cause of the Increasing Keeling Curve? A Thermal Acidifying Calcification Hypothesis

The Keeling Curve documents a steady rise in partial atmospheric pressure of CO₂ (pCO₂), conventionally attributed to fossil fuel emissions. Here, we propose an alternative cause, the thermal acidifying-calcification (TAC) hypothesis, principles of physical chemistry showing that warmingwill induce calcification in surface seawater, driving seasonal and longer-term increases in atmospheric pCO₂. Using thermodynamic modeling and oceanographic data, we estimate that precipitation of ~10 μmol of of CaCO₃ per kg of surface seawater annually could release sufficient CO₂ to equilibrate locally with the recent 2 ppmv annual rise in atmospheric pCO₂. Seasonal pH and pCO₂ variations at marine stations (e.g., ALOHA, Hawaii) support this mechanism, challenging the dominant terrestrial photosynthesis- respiration explanation for Keeling oscillations. The TAC process can also explain the decreasing trend in 13CO₂ in air because of dilution by the same processes that are driving the longer-term increase in atmospheric pCO₂. To test this hypothesis, we recommend the following program: 1. field validation of CaCO₃ deposition rates, 2. measures of the CO₂ flux and 3. isotopic analysis to test this alternative carbon cycle driver with measures to be taken both in the northern Pacific and in the southern hemisphere on a transect past the Great Barrier Reef.