CARBONATE MINERALS AND THE GLOBAL CARBON CYCLE

Fluxes of C between biosphere, ocean, and atmosphere reservoirs are considered to be the most dynamic part of Earth’s C cycle, although because of anthropogenic combustion over the past couple centuries, fossil fuels have become an additional dynamic and critical part of the cycle. Missing from this dynamic system is the largest reservoir of C, lithospheric carbonate minerals, which contains around 10⁸ Pg of C, or approximately 4 times the next largest reservoir of C (kerogen) and between 10³ and 10⁴ times more carbon than the dynamic part of the cycling including fossil fuels, terrestrial biosphere, oceans, and atmosphere. Lithospheric carbonate is not typically considered dynamically involved in the C cycle because atmospheric CO₂, which when dissolved, hydrates to carbonic acid and dissolves carbonate minerals, but is subsequently released back to the atmosphere as carbonate minerals re-precipitate. This cycle results in no net change in atmospheric C concentrations; however, dissolution of carbonate minerals by other acids, including sulfuric acid could generate a net flux of CO₂ to the atmosphere from C previously sequestered in the lithosphere. Several sources of sulfuric acid exist in the environment, including burning of pyrite-bearing coal that forms sulfuric acid rain. Limestone dissolution forms caves from redox reactions of sulfur contributed from hydrocarbon brines. In coastal zones, sulfate may be the primary electron acceptor in organic carbon oxidation, particularly where fresh and salt water mix in water filled sinkholes (blue holes/cenotes) in carbonate platforms. Sulfide oxidation also contributes to the flux of CO₂ to the atmosphere during early weathering from trace contents of terrestrial carbonate in glaciated silicate dominated terrains, with a good example in exposed and subglacial portions of western Greenland. Although an atmospheric source of CO₂ from sulfuric acid dissolution is likely to be small, it represents a net source of CO₂ from terrestrial carbonate minerals to dynamic portions of the global carbon cycle. The exact impact on global carbon cycling from dissolution by acids other than carbonic is currently unknown.