Urey cycle

It is well known that the crustal Urey cycle of silicate weathering and metamorphism can function as a dynamic buffer for atmospheric... 

Controls on C02-drawdown In the modern Earth the principal agent of C02-drawdown is through the weathering of silicate rocks as part of what is known as the "Urey Cycle," after its identification by Urey (1952). The chemical reaction which summaries this process is... 

If the Zahnle and Sleep (2002) model for COa-drawdown is correct, and C02 was principally stored in the oceanic crust during the Archaean, this does not necessarily negate the calculations of Kramers (2002), but it does shift the time of C02-drawdown back to perhaps the mid-Archaean. However, a problem with the Zahnle and Sleep (2002) model is that, unlike the Urey cycle, the Archaean oceanic weathering cycle has no inbuilt temperature feedback and needs a "thermostat" to maintain equable surface temperatures on the early Earth. This problem could be solved (just) if there were very high levels of COa in the atmosphere, but the better solution is to include methane as a significant component of the Archaean atmosphere. 

In fact, a negative feedback exists. Reduction of carbonate to organic carbon leaves the divalent cations Mg " " or Ca " " in its wake. An excess of these cations speeds carbonate burial. [The Urey cycle of stabilizing climate is based on this effect (see Ref. [46] for review). Weathering releases more divalent cations when the climate is warm leading to an enhanced CO2 sink.] A dearth of divalent cations retards the burial of carbonate. 

Stable isotope fractionation was predicted and calculated by Urey and coworkers as early as 1932. Bigeleisen (1961) dealt with the statistical mechanics of isotope effects. Hydrogen isotope fractionation is mainly connected with the processes in the hydrosphere, but its equilibrium and kinetic effects were also studied for water of crystallization, ore-forming fluids, as represented by liquid inclusions, and biological cycles. 

A transition between the completion of weathering and the advent of reverse weathering must occur in the crust in order to maintain the acid balance within the rock cycle (e.g., Urey, 1956 Krauskopf, 1979) and to maintain levels of atmospheric and oceanic CO2 within observed secular ranges (e.g., Berner et al, 1983 Berner, 1991). Early work on reverse weathering investigated the degree to which early marine diagenesis... 

On geological time scales, CO2 cycles between rocks, often by way of the ocean and atmosphere. The rock reservoirs include the mantle, continental carbonates, carbon in reduced form mostly in continental shales, and carbon (mostly carbonate) in or on the sea floor. The small volatile reservoir (ocean plus atmosphere) cycles through carbonate rock in a hundred thousand to a million years. Over longer periods free CO2 is dynamically controlled by processes that form carbonates at low temperatures and processes that decompose carbonates at high temperatures by (Urey) reactions of the form... 

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