Linking the lithogenic, atmospheric, and biogenic cycles of silicate, carbonate, and organic carbon in the ocean

Abstract. Geochemical theory describes long term cycling of atmospheric CO₂ between the atmosphere and rocks at the Earth surface in terms of rock weathering and precipitation of sedimentary minerals. Chemical weathering of silicate rocks takes up atmospheric CO₂, releases cations and HCO₃⁻ to water, and precipitates SiO₂, while CaCO₃ precipitation consumes Ca²⁺ and HCO₃⁻ and releases one mole of CO₂ to the atmosphere for each mole of CaCO₃ precipitated. At steady state, according to this theory, the CO₂ uptake and release should equal one another. In contradiction to this theory, carbonate precipitation in the present surface ocean releases only about 0.6 mol of CO₂ per mole of carbonate precipitated. This is a result of the buffer effect described by Ψ, the molar ratio of net CO₂ gas evasion to net CaCO₃ precipitation from seawater in pCO₂ equilibrium with the atmosphere. This asymmetry in CO₂ flux between weathering and precipitation would quickly exhaust atmospheric CO₂, posing a conundrum in the classical weathering and precipitation cycle.

While often treated as a constant, Ψ actually varies as a function of salinity, pCO₂, and temperature. Introduction of organic C reactions into the weathering-precipitation couplet largely reconciles the relationship. ψ in the North Pacific Ocean central gyre rises from 0.6 to 0.9, as a consequence of organic matter oxidation in the water column. ψ records the combined effect of CaCO₃ and organic reactions and storage of dissolved inorganic carbon in the ocean, as well as CO₂ gas exchange between the ocean and atmosphere. Further, in the absence of CaCO₃ reactions, Ψ would rise to 1.0. Similarly, increasing atmospheric pCO₂ over time, which leads to ocean acidification, alters the relationship between organic and inorganic C reactions and carbon storage in the ocean. Thus, the carbon reactions and ψ can cause large variations in oceanic carbon storage with little exchange with the atmosphere.