CARBON CYCLE 339 Figure The global inorganic carbon cycle of weathering, runoff, carbonate precipitation, and burial Numbers refer to carbon reservoirs (R; in gigatons) and carbon fluxes (F; in gigatons year 1) Reprinted from Kump LR, Kasting JF, and Crane RG (1999) The Earth System Upper Saddle River, NJ: Prentice Hall and therefore the effects of these controls on the longterm global carbon cycle There is no consensus on these issues because several unrelated factors are involved, such as climate (both rainfall and temperature), denudation rates, weathering history, and rock type Through a combination of these factors, certain tropical, humid areas of the world with freshly exposed volcanic rock can exert an outsized influence over the global carbon cycle This is because fresh basalts are particularly susceptible to chemical weathering and are made up predominantly of Ca–Mg silicates The weathering of basalts today may contribute as much as a third of all the silicate-weathering CO2 flux Thus volcanism acts not only as a source for atmospheric CO2, but also as one of the major long-term sinks The products of chemical weathering, including Ca, Mg, and bicarbonate ions, arrive in the oceans in solution; concentrations build up in the oceans until an equilibrium state is reached between input and carbonate precipitation Bicarbonate (HCO3 ) and carbonate (CO23 ) ions, which together make up 99% of all dissolved carbon in seawater, help to regulate seawater pH by transferring protons (or hydrogen ions) between carbonate species (eqn [6]) according to the rules of carbonate equilibria In effect, the carbonate system in seawater is capable of neutralizing a large portion of any extra CO2 added from, say, fossil fuel burning or volcanism CO2 þ H2 O ¼ H2 CO3 ¼ Hþ þ HCO3 ẳ 2Hỵ ỵ CO23 Carbon dioxide$Carbonic acid$Bicarbonate $Carbonate ẵ6