Figure 8.2: Effect of the abiotic form of the silicate weathering feedback on the variation of surface temperature with stellar luminosity. The luminosity in this graph is translated into absorbed stellar radiation per unit surface area of the planet, allowing for a 20% albedo. The pCO2 curve gives the value of pCO2 in equilibrium with a fixed outgassing rate for the case including the weathering thermostat.
Supposing this planet to have enough silicates to support silicate weathering, and supposing that there is enough reservoir of carbonate to support CO2 outgassing from the interior, what do we expect the atmospheric CO2 content to be? Do we have a steam-dominated atmosphere, or would the atmosphere also have significant amounts of CO2 in it? Given the exponential dependence of weathering kinetics, it seems likely that the weathering reaction would be driven to equilibrium for a planet with surface temperatures much above 400K, unless the outgassing rates are enormously greater than those prevailing on Earth today. Thus, one can estimate the CO2 content during a wet runaway simply by looking at the equilibria for the Ebelmen-Urey reactions. The answer depends on temperature and surface mineralogy. At a temperature of 400K there would not be much CO2 in the atmosphere unless the surface was dominated by iron carbonates, in which case one could have around 10bar of CO2. By the time one reaches 600K there could be nearly a hundred bar of CO2 even if the surface is dominated by magnesium carbonates. Calcium carbonates are very stable, however, so the temperature would have to go up to 700K before one had some tens of bars of CO2 in the atmosphere. This is above the critical point for water, at which point the distinction between liquid water and water vapor disappears. This renders the distinction between a dry and wet runaway moot, and raises the interesting (and evidently unresolved) question of how the Ebelmen-Urey reactions behave in the presence of supercritical water. Is it like the dry reaction, like the aqueous reaction, or is it something completely different?
On a planet with oxygenic photosynthesis, the atmospheric CO2 can also be affected by the burial of organic carbon and the release organic carbon by oxidation of the organic carbon pool - an example of oxidative weathering. Burial of organic carbon produced by oxygenic photosynthesis converts CO2 (which is a greenhouse gas) into O2 (which is not). This would cool the planet if it happens rapidly enough that the silicate weathering thermostat can't keep up. On a well-oxygenated planet like the current Earth, organic carbon burial is relatively inefficient, since bacteria have had a few billion years to get very good at extracting energy by oxidizing any organic carbon that may be around. During the Great Oxygenation Event near the dawn of the Protero-zoic, however, it is conceivable that oxygenic photosynthesis took over the planet so quickly that huge amounts of organic carbon were buried, drawing down atmospheric CO2 and precipitating the Makganyene Snowball event. It is hard to imagine circumstances where something like this could happen under heavily oxygenated conditions. Massive release of CO2 by oxidative weathering, on the other hand, can occur under modern oxygenated conditions. It is likely that the CO2 release during the PETM event 55 million years ago came from oxidative weathering of the land carbon pool, and what is the present era of anthropogenic CO2 release other than a form of oxidative weathering due to a particularly exotic form of biology? One ought to worry about whether the resulting warming could trigger an additional PETM-like land carbon release, which would add to the direct anthropogenic CO2 release and compound our climate woes.
The CO2 weathering feedback is but one of many possible climate feedbacks involving at-mopheric reactions with crustal minerals, though to date it is the only one that has been worked out in detail. Other cycles that have been proposed include release of CH4 from organic carbon by methanogenic bacteria on the young Earth, the regulation of SO2 on Venus from dry reactions with surface carbonates, and the regulation of SO2 on Early Mars and Early Earth by aqueous reactions producing sulfite minerals at the surface. On a planet without a substantial oceanic reservoir of water, the exchange of water with hydrated minerals could exert an important influence on atmospheric water vapor content; insofar as water affects the fluidity and melting-point of minerals, this can even feed back on the plate tectonics that affects the recycling of crustal material. N2 doesn't easily form minerals on rocky planets, and so is unlikely to participate in major climate cycles, though it has been suggested that the biological formation of the ammonium ion NH+ allows some drawdown of N into the Earth's mantle; this would somewhat affect the climate via Rayleigh scattering, pressure-broadening and lapse rate effects. On an icy body like
Titan, however, N2 readily forms "minerals" - they just happen to be called "ices" instead. Indeed, cryovolcanism based on various mixed NH3-H2O ices may well play a role in determining the amount of N2 in Titan's atmosphere. The methane cycle on Titan is also likely to involve crustal and interior exchange processes. Photochemistry converts Titan's atmospheric methane to liquid ethane and various tarry sludges on the surface. At the time of writing the search is on for some way of closing the cycle by converting these substances back into methane.
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