Abstract
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.
Highlights
Submarine sinkholes are voids in carbonate platforms that are open to the surrounding marine waters (Mylroie et al, 1995)
Sinkholes W and E were partially exposed at the edges of Luymes Bank, whereas sinkholes N and S were fully surrounded by the platform
Sinkholes N and S were separated from each other only beneath 187 m, which was the deepest point on an internal ridge (Fig. 1c)
Summary
Submarine sinkholes are voids in carbonate platforms that are open to the surrounding marine waters (Mylroie et al, 1995). Their formation and growth are usually driven by the mixing of groundwaters of different salinities, which can generate conditions corrosive to the carbonate minerals that form the platform (Wigley and Plummer, 1976). Humphreys et al.: Dissolution of a submarine carbonate platform edges of continental shelves can generate new submarine sinkholes (Land et al, 1995). Sinkholes can provide new insights into how marine biogeochemical cycles operate under conditions that differ from the presentday bulk ocean (Yao et al, 2020; Qiao et al, 2020)
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