338 CARBON CYCLE returned to the surface environment by tectonic processes at convergent margins (deep-sea trenches), but not for many millions of years, as part of the ‘longterm’ carbon cycle of carbon burial, subduction, uplift, weathering, and carbonate deposition The Long-Term Carbon Cycle Although some carbon reaches Earth from space in the form of impacting comets, most carbon first enters the surface environment as volcanic CO2, originating from deep within Earth’s interior at mid-ocean ridges, convergent margins, and the Great Rift Valley and other terrestrial volcanic provinces (e.g., Kamchatka) Some of this carbon is entirely new to the surface environment, being derived directly from the mantle, but some will be ‘old’ carbon, recycled from the sediment pile by tectonic processes such as metamorphism At convergent plate margins, carbonate sediments can be carried to great depths, up to hundreds of kilometres, riding on subducting ocean lithosphere The high pressures and temperatures at great depths encourage biogenic calcite (calcium carbonate) to react with biogenic and detrital silica (silicon dioxide) to form metamorphic calcium silicate minerals This process releases CO2 (eqn [3]) that may eventually find its way into the atmosphere via hot springs and seeps (Figure 4) CaCO3 ỵ SiO2 ! CaSiO3 þ CO2 ½3 Much study is devoted to estimating the proportion of genuinely new CO2 from the mantle relative to recycled CO2 at convergent margins, but there is currently no consensus If volcanic outgassing were allowed to proceed unchecked, then atmospheric partial pressure of CO2 (pCO2) would rise until an equilibrium concentration was reached, balanced only by escape of CO2 into space Exactly how hostile such an Earth would be can be appreciated by comparison with the furnace-like Venus, Earth’s planetary neighbour and twin Although Venus and Earth are of a similar size and are a similar distance from the sun, Venus has a much thicker atmosphere, made up almost entirely of CO2, over 200 000 times more concentrated than in Earth’s atmosphere There is no reason to suspect that Venus overall is any richer in carbon than Earth is, so Earth must possess powerful mechanisms of CO2 sequestration, unique in the solar system The first step in the permanent removal of CO2 from the atmosphere is chemical weathering (Figure 5), whereby rainwater, made acidic by the dissolution of aqueous CO2, corrodes rock, forming minerals such as silicates and carbonates In the presence of soil, this process is accelerated by plants, which help to concentrate CO2 in the soil to levels generally 10 to 100 times higher than in the atmosphere CO2 used up in this way may then be transported via rivers and groundwater to the oceans, chiefly as bicarbonate anions (HCO3 ), along with the weathered-out major cations such as Si, Al, Ca, Mg, K, Na, and Fe Once in the ocean, some CO2 may be removed permanently from the exogenic system by chemical precipitation as calcium carbonate, mostly in the form of shells and skeletal elements that rain down to the seafloor Because CaCO3 precipitation also releases CO2 back into the water column (eqn [4]), CO2 taken up by the weathering of carbonate rocks does not result in any long-lasting net change to atmospheric CO2 levels In other words, the CO2 taken up during carbonate dissolution is simply returned during carbonate precipitation in the ocean: CaCO3 ỵ CO2 ỵ H2 O ẳ 2HCO3 ỵ Ca2ỵ Carbonate weathering ! ½4 Carbonate precipitation Similarly, CO2 consumed during the weathering of Na and K silicates does not result in any net change in atmospheric CO2 because Na and K carbonate minerals are very soluble and not readily precipitate from seawater Equation [5] shows the net result of Ca–Mg silicate weathering, which is the most important mechanism of permanently removing CO2 from the atmosphere: CaMgịSiO3 ỵ CO2 ! CaMgịCO3 ỵ SiO2 Figure The long term carbon cycle of sedimentation, subduc tion, and outgassing ½5 Much research effort has been expended to better understand the controls on chemical weathering rates