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

[摘要] Submarine sinkholes are found on carbonate platforms around the world. They are thought to form and grow when groundwater interactions generateconditions corrosive to carbonate minerals. Because their morphology can restrict mixing and water exchange, the effects of biogeochemical processescan accumulate such that the sinkhole water properties considerably diverge from the surrounding ocean. Studies of sinkhole waters can thereforereveal new insights into marine biogeochemical cycles; thus sinkholes can be considered “natural laboratories” where the response of marineecosystems 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 ( [O 2 ]   =  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 thatgave 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 acidlake's waters, which we deduce must be continuously flowing through. We show that the acid lake is actively dissolving the carbonate platform, sothe bubble plume may provide a novel mechanism for submarine sinkhole formation and growth. It is likely that the bubble plume is ephemeral and thatother currently non-acidic sinkholes on Luymes Bank have previously experienced acid lake phases. Conditions within the acid lake were tooextreme to represent future marine environmental responses to anthropogenic CO 2 emissions on human timescales but may reflect the impactof proposed schemes to mitigate climate change by the deliberate addition of CO 2 and/or alkalinity to seawater. Other Luymes Banksinkholes did host conditions analogous to projections for the end of the 21st century and could provide a venue for studies on the impacts ofanthropogenic CO 2 uptake by the ocean.

[学科分类] 大气科学