This carbonate mineral precipitation in sediments has been called ‘authigenic carbonate,’ or ‘sedimentary carbonate,’ and is found in the form of cements and disseminated particles within the marine sedimentary pile ( Ussler and Paull, 2008 Higgins et al., 2009). Within marine sediments, changing environmental conditions such as changes in pH and the concentration of dissolved inorganic carbon may allow carbonate minerals to precipitate within the sediment column ( Middelburg et al., 1997 Soetaert et al., 2007 Arndt et al., 2009). Calcium carbonate minerals, largely biogenic but also detrital, are delivered to marine sediments from the water column, however the majority of deep marine sediments are bathed in water that is undersaturated with respect to calcium carbonate minerals ( Burdige, 2006 Burdige et al., 2008). We conclude that carbonate minerals precipitate in sediments with methane, where the anaerobic oxidation of this methane helps promote particularly high saturation states for carbonate minerals.Ĭarbon burial in marine sediments as organic carbon or calcium carbonate minerals represents the largest removal path for carbon from Earth’s surface environment ( Berner and Caldeira, 1997 Berner, 2003 Martin, 2017). We explore how carbonate mineral saturation state changes as a function of calcium concentrations, alkalinity, and pH, and suggest a reason why calcium concentrations are never fully depleted in sedimentary pore fluids.
Our data can be used to highlight sediments where both dolomite formation and dolomitization of previously deposited calcium carbonate minerals is occurring. We also note that sedimentary pore fluids that are high in phosphate concentration are also high in alkalinity, which may indicate suppression of carbonate mineral precipitation in the presence of sedimentary phosphate. Our data compilation shows that the gradient of pore fluid sulfate concentrations correlates strongly with the gradient of alkalinity as well as with the gradient of calcium, and that these correlations improve dramatically in sediments where methane is present. Here we present a global compilation of pore fluid data and compare the sulfate, calcium, phosphate and magnesium concentrations with pore fluid alkalinity to explore the emerging relationships and explore what drives carbonate mineral precipitation in sediments. However, carbonate minerals can also precipitate abiotically within marine sediments, and this carbonate mineral precipitation within sediments has been suggested as a third major, and isotopically distinct, sink in the global carbon cycle, particularly important earlier in Earth history. The vast majority of carbonate minerals in modern marine sediments are biogenic, derived from the skeletal remains of organisms living in the ocean. 2Baltic Sea Center, Stockholm University, Stockholm, Sweden.1Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom.
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