Dissolution of a submarine carbonate platform by a submerged lake of acidic seawater

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.

First Documentation of Large Submarine Sinkholes on the Ganquan Carbonate Platform in the Xisha Islands, South China Sea

“…ROV Haima 2 imaged corals (Figure 5a,c), flow marks (Figure 5e), and corroded holes (Figure 5f) in the inner wall of submarine sinkhole No. 2, indicating that these submarine sinkholes show strong morphologic similarities (e.g., size, depth, and shape) with those developed on other carbonate platforms worldwide, such as the southern Florida Straits [8][9][10], southern Adriatic Sea [11], Abrolhos Bank of Australia [12,13], and the Bahamas and Caribbean Seas [7,14].…”

“…Their water depths range from 4584 to 4967 m, and their diameters range from 255 to 1819 m, with vertical depths from 30 to 185 m [7]. In addition, more than 21 submarine sinkholes have been discovered on Luymes Bank, which is a part of Saba Bank in the Caribbean Netherlands, and their water depths range from 93 to 187 m. These submarine sinkholes were formed via carbonate platform dissolution caused by acidic seawater from a submerged lake, which is a novel mechanism for submarine sinkhole formation and growth [14].…”