CALCIUM CARBONATE CYCLING IN ANOXIC OCEANS

Systematic changes in carbonate sedimentology over Earth history indicate a shift in the locus of abundant CaCO₃ precipitation from on to above the seafloor (Grotzinger, 1990).  Oxic respiration of organic carbon leads to undersaturation and dissolution of CaCO₃, even in cases where the overlying seawater is supersaturated (Walter & Burton, 1990).  In contrast, anoxic respiration of organic carbon buffers pH, leading to preservation and/or precipitation of CaCO₃ (Ben Yaakov et al., 1977).  Because the oxygen content of sediments together with the flux of organic carbon largely determines whether or not sediments dissolve or precipitate CaCO₃, changes in O₂ and organic carbon cycling over Earth history should be reflected in the sedimentology of carbonate rocks.  Although the evolution of atmospheric O₂ is poorly constrained, this hypothesis is broadly consistent with observations of declining seafloor CaCO₃ precipitation in the Proterozoic and early Phanerozoic.  Because low values of carbonate mineral saturation negatively affect the energetics of CaCO₃ biomineralization, the evolution and episodic demise of skeletal CaCO₃ producers may be related to the effect of changes in the O₂ content of sediments and the cycling of organic carbon on CaCO₃ precipitation on the seafloor.