Global warming is a main cause for current ocean deoxygenation. A deployment of marine carbon dioxide removal (CDR) for mitigating global warming could therefore also be viewed as a measure for mitigating ocean deoxygenation if, and only if, the respective CDR measure itself does not lead to a larger oxygen loss than the reduction in atmospheric CO2 would prevent. We here review the current state of knowledge regarding the potential impacts of various marine CDR (mCDR) options onto ocean oxygen, a key ocean state variable and an essential element for all higher forms of marine life. Using results from global model simulations, we show that biotic approaches, such as ocean fertilization, macroalgae cultivation and sinking, and placement of organic matter that is prone to remineralization, can lead to a loss in seawater dissolved oxygen that is 4–40 times larger than the oxygen gain that would result from the CDR-induced reduction in global warming only. Biotic approaches also tend to enhance the amplitude of the diel cycle in dissolved oxygen, with possible physiological impacts specifically in shallow-water environments of coastal vegetated ecosystem. In contrast, geochemical approaches, and biotic approaches that avoid remineralization of biomass within the ocean, may be applied in ways that have minimal impacts on dissolved oxygen. We suggest that impacts on marine oxygen should be accounted for in assessing the suitability of mCDR, and that oxygen should be measured prior to, during and after any research-scale or full-scale implementation activity.

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