Role of oceanic abiotic carbonate precipitation in future atmospheric CO₂ regulation

The oceans play a major role in the earth's climate by regulating atmospheric CO₂. While oceanic primary productivity and organic carbon burial sequesters CO₂ from the atmosphere, precipitation of CaCO₃ in the sea returns CO₂ to the atmosphere. Abiotic CaCO₃ precipitation in the form of aragonite is potentially an important feedback mechanism for the global carbon cycle, but this process has not been fully quantified. In a sediment-trap study conducted in the southeastern Mediterranean Sea, one of the fastest warming and most oligotrophic regions in the ocean, we quantify for the first time the flux of inorganic aragonite in the water column. We show that this process is kinetically induced by the warming of surface water and prolonged stratification resulting in a high aragonite saturation state (Ω_Ar ≥ 4). Based on these relations, we estimate that abiotic aragonite calcification may account for 15 ± 3% of the previously reported CO₂ efflux from the sea surface to the atmosphere in the southeastern Mediterranean. Modelled predictions of sea surface temperature and Ω_Ar suggest that this process may weaken in the future ocean, resulting in increased alkalinity and buffering capacity of atmospheric CO₂.