2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 7
Presentation Time: 8:00 AM-6:00 PM

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

ARVIDSON, Rolf S., Department of Earth Science MS-126, Rice University, 6100 S Main Street, Houston, TX 77005, GUIDRY, Michael, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road MSB R204, Honolulu, HI 96822 and MACKENZIE, Fred T., Department of Oceanography, University of Hawaii, 1000 Pope Rd, Honolulu, HI 96822, rsa4046@ruf.rice.edu

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.