Ocean Alkalinization deliberately modifies the chemistry of the surface ocean to enhance the uptake of atmospheric CO2. Here we quantify, using idealized Earth system model (ESM) simulations, changes in carbon cycle feedbacks and in the seasonal cycle of the surface ocean carbonate system due to ocean alkalinization. We find that both, carbon-concentration and carbon climate feedback, are enhanced due to the increased sensitivity of the carbonate system to changes in atmospheric CO2 and changes

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... in temperature. While the temperature effect, which decreases ocean carbon uptake, remains small in our model, the carbon concentration feedback enhances the uptake of carbon due to alkalinization by more than 20%. The seasonal cycle of air-sea CO2 fluxes is strongly enhanced due to an increased buffer capacity in an alkalinized ocean. This is independent of the seasonal cycle of pCO2, which is only slightly enhanced. The most significant change in the seasonality of the surface ocean carbonate system is an increased seasonal cycle of the aragonite saturation state, which has the potential to adversely affect ecosystem health.