Ocean alkalinity destruction by anthropogenic seafloor disturbances generates a hidden CO₂ emission

The seafloor is responsible for 40% of the alkalinity input to the ocean, thus contributing to the ocean's capacity to sequester atmospheric CO₂. Anthropogenic seafloor disturbances induced by mobile bottom-contact fishing and dredging influence this natural carbon sink, yet the human impact on the ocean's alkalinity cycle remains poorly quantified. Model simulations show that the combined impact of mobile bottom-contact fishing (e.g., trawling) and dredging reduces natural alkalinity generation by 60 to 220 gigaequivalent year^-1, which is equivalent to a reduction of the natural marine carbon sink by 2 to 8 teragrams CO₂ year^-1. Alkalinity destruction by anthropogenic seafloor disturbance hence generates a hidden CO₂ emission, of which the impact is comparable to the estimated reduction of organic carbon burial by mobile bottom-contact fishing. Our analysis emphasizes that carbon accounting in marine systems should consider the anthropogenic impact on both the organic and inorganic carbon cycles.