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Abstract. 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 a constrained body of dense, low-oxygen ([O2] = 60.2 ± 2.6 µmol kg−1), acidic (pHT = 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.

Major advances have been made recently in understanding of ongoing climate change and predicting its trajectory into the near future. However, only knowledge of past climate change events in the deep history of Earth can inform us about the possible extremes of greenhouse conditions, rates and magnitude of long-term climate change, and their consequences to the ocean and the biosphere. The end of the Triassic period was a time of major greenhouse warming, driven by volcanic emission of CO2 and other gases from eruptions in the Central Atlantic Magmatic Province, one of the largest known igneous provinces. The end-Triassic mass extinction, one of the five most severe crises of the biosphere in the Phanerozoic, is best regarded as the biotic response to a cascade of rapid environmental changes triggered by volcanism. Ocean acidification was likely a major factor driving the selective extinction of calcifying marine organisms. Research in Triassic-Jurassic boundary sections in Hungary has helped elucidate changes in the carbon cycle as recorded in carbon isotope excursions, ocean acidification reflected in changes in carbonate sedimentation, and crises in both marine and terrestrial ecosystems. As anthropogenic increase of atmospheric CO2 is the key forcing mechanism of ongoing climate change, further research into natural high-CO2 events in Earth history, such as at the end-Triassic, will provide valuable insights for climate evolution scenarios. $.1 Past and present climate change, ocean acidification and biodiversity crisesThe Earth system is composed of four major interconnected subsystems, the lithosphere, hydrosphere, atmosphere and biosphere. Geologists use the rock record to understand processes and past changes in climate and environment, both on land and in the ocean, whereas paleontologists study the fossil record to reveal the history