Catalogus

Submarine sinkholes are found on carbonate platforms around the world. They are thought to form and grow when groundwater interactions generate conditions corrosive to carbonate minerals. Because their morphology can restrict mixing and water exchange, the effects of biogeochemical processes can accumulate such that the sinkhole water properties considerably diverge from the surrounding ocean. Studies of sinkhole waters can therefore reveal new insights into marine biogeochemical cycles; thus sinkholes can be considered “natural laboratories” where the response of marine ecosystems to environmental variations can be investigated. We conducted the first measurements in recently discovered sinkholes on Luymes Bank, part of Saba Bank in the Caribbean Netherlands. Our measurements revealed a plume of gas bubbles rising from the seafloor in one of the sinkholes, which contained aconstrained body of dense, low-oxygen ([O₂] = 60.2 ± 2.6 µmol kg ⁻¹), acidic (pH_T = 6.24 ± 0.01) seawater that we term the “acid lake”. Here, we investigate the physical and biogeochemical processes that gave rise to and sustain the acid lake, the chemistry of which is dominated by the bubble plume. We determine the provenance and fate of the acid lake's waters, which we deduce must be continuously flowing through. We show that the acid lake is actively dissolving the carbonate platform, so the bubble plume may provide a novel mechanism for submarine sinkhole formation and growth. It is likely that the bubble plume is ephemeral and that other currently non-acidic sinkholes on Luymes Bank have previously experienced acid lake phases. Conditions within the acid lake were too extreme to represent future marine environmental responses to anthropogenic CO₂ emissions on human timescales but may reflect the impact of proposed schemes to mitigate climate change by the deliberate addition of CO₂ and/or alkalinity to seawater. Other Luymes Bank sinkholes did host conditions analogous to projections for the end of the 21st century and could provide a venue for studies on the impacts of anthropogenic CO₂ uptake by the ocean.