Global Carbonate Chemistry Gradients Reveal a Negative Feedback on Ocean Alkalinity Enhancement

Ocean alkalinity enhancement (OAE) aims to remove CO₂ from the atmosphere and store it as carbonate and bicarbonate in the ocean. A new study in Nature Geoscience examined how such a shift in seawater carbonate chemistry could affect marine phytoplankton species, specifically the balance of calcifying over non-calcifying phytoplankton. Using satellite data, the authors found that oceanic surface gradients similar in magnitude to OAE-induced changes in carbonate chemistry support the proliferation of calcifying over non-calcifying phytoplankton. This positive correlation aligns with the theory that increased substrate availability and pH from OAE facilitates calcium carbonate precipitation and preservation. While this correlation with carbonate chemistry was observed at the global level across biogeochemically-distinct ocean regions, other environmental drivers can dominate on a local scale. Through the removal of alkalinity during calcification, a proliferation of calcifiers directly affects OAE's efficiency as a CO₂ removal strategy.

Reference: Lehmann, N. & Bach, L.T. (2025). Global carbonate chemistry gradients reveal a negative feedback on ocean alkalinity enhancement. Nat. Geosci.. https://doi.org/10.1038/s41561-025-01644-0