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 (ΩAr ≥ 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 ΩAr suggest that this process may weaken in the future ocean, resulting in increased alkalinity and buffering capacity of atmospheric CO2.