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

• Main Authors: Smith, S. V., Gattuso, J.-P.
• Format: Other/Unknown Material
• Language: English
• Published: 2018
• Subjects: Pacific; Ocean acidification
• Online Access: https://doi.org/10.5194/bgd-6-6579-2009; https://www.biogeosciences-discuss.net/bg-2009-161/

Geochemical theory describes long term cycling of atmospheric CO 2 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 CO 2 , releases cations and HCO 3 − to water, and precipitates SiO 2 , while CaCO 3 precipitation consumes Ca 2+ and HCO 3 − and releases one mole of CO 2 to the atmosphere for each mole of CaCO 3 precipitated. At steady state, according to this theory, the CO 2 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 CO 2 per mole of carbonate precipitated. This is a result of the buffer effect described by Ψ, the molar ratio of net CO 2 gas evasion to net CaCO 3 precipitation from seawater in p CO 2 equilibrium with the atmosphere. This asymmetry in CO 2 flux between weathering and precipitation would quickly exhaust atmospheric CO 2 , posing a conundrum in the classical weathering and precipitation cycle. While often treated as a constant, Ψ actually varies as a function of salinity, p CO 2 , 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 CaCO 3 and organic reactions and storage of dissolved inorganic carbon in the ocean, as well as CO 2 gas exchange between the ocean and atmosphere. Further, in the absence of CaCO 3 reactions, Ψ would rise to 1.0. Similarly, increasing atmospheric p CO 2 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.