Carbonate minerals in the global carbon cycle

Carbonate minerals constitute Earth's largest C reservoir. This reservoir is considered unimportant to the global C cycle over long periods of time (e.g., > 106 yrs) because it provides balanced sources and sinks of atmospheric CO2 as carbonate minerals precipitate and dissolve from carbonic acid, respectively. However, carbonate mineral reactions may impact the carbon cycle at short time scales (102 to 105 yrs) particularly when atmospheric pCO2 changes by several hundred ppm, as exemplified by the Paleocene-Eocene Thermal Maximum, at transitions between glacial-interglacial times, and over the past century from anthropogenic inputs. Here I review recent literature about mechanisms in which carbonate minerals may impact the global carbon cycle. Carbonate mineral dissolution, which increases dissolved inorganic carbon (DIC) concentrations, may represent a net atmospheric CO2 sink as primary productivity consumes DIC, resulting in organic C burial in fluvial systems of carbonate terrains. Alternatively, carbonate minerals dissolved by sulfuric and nitric acids may represent an atmospheric source of CO2 with no rapid balancing atmospheric sink. Sulfuric acid dissolves carbonate minerals of modern carbonate platform sediments and water-filled sinkholes, and in the deep sea at the base of carbonate platform scarps. In terrestrial landscapes, sulfuric acid dissolves carbonate minerals of karst terrains (hypogene caves) and in comminuted glacial sediments. Nitric acid forms from oxidation of reactive N, which has doubled in the past hundred years as humans fix atmospheric N2. Similar to some reactions involving sulfuric acid, nitric acid dissolution of carbonate minerals represents a net source of atmospheric CO2. This source is concentrated in fertilized agricultural fields in carbonate terrains. Each of these processes represents a small flux relative to major C exchanges, such as volcanic sources and silicate mineral weathering, but collectively they could impact the C cycle at magnitudes similar to these major processes. These impacts may increase as humans further alter elemental cycles of S and N.