Chemostatic Modes of the Phanerozoic: Calcite-Dolomite Versus Aragonite Seas and Their Control by Weathering, Carbonate Precipitation, and Basalt-Seawater Reactions

The essential state of the sedimentary rock cycle with respect to chemically precipitated sediments and organic carbon components can be constrained by relatively few observations: these include the total dissolved river load, seawater ionic ratios, the rate of dolomite precipitation in shallow marine sediments, the net rate of organic matter deposition to the seafloor and the ratio of organic C/S contained within those sediments. These constraints are sufficient to compute the remaining fluxes in a comprehensive model describing the compositional history of the atmosphere, ocean, and chemical sediment distributions over the Phanerozoic.

Model coupling between ocean, atmosphere, and sediment reservoirs produce two dominant chemostatic modes characterized by seawater carbonate saturation state, major ion chemistry and atmospheric CO₂: (1) “calcite-dolomite” seas, a state of depressed seawater Mg/Ca and SO₄/Ca ratios but elevated CO₂, continental weathering, and carbonate saturation state favoring elevated primary productivity, organic matter burial, sulfate reduction rates, and dolomite accumulation, versus (2) “aragonite” seas, a state of elevated Mg/Ca and SO₄/Ca ratios but depressed CO₂ and carbonate saturation states in which accumulation rates of organic matter and dolomite are low. Covariant trends in Mg/Ca ratio and seawater carbonate saturation state reflect the balances in calcium and alkalinity between the demands of global CaCO₃ precipitation versus the supply of continental weathering. Hydrothermal reactions between seawater and seafloor basalt, and well as low temperature reactions between seawater and clastic sediments (“reverse weathering”), further modify this balance. These feedbacks produce a system that is remarkably poised, in which transitions between these dominant modes are driven primarily by seafloor spreading and tectonism, but modified and overprinted by irreversible evolutionary and ecosystem changes. Examples include enhanced weathering and terrestrial organic carbon burial by land plant succession, and a shift of the locus of carbonate sediment deposition through radiation of pelagic calcifiers.