Sources and Sinks Equal at Steady State

A steady-state carbon dioxide level in the atmosphere/ ocean system is achieved by the equality of the sink fluxes (removal processes) and the sources (supply processes) to this 'pool'. At a given time, the partitioning of carbon between atmosphere and ocean is determined by global temperature (or the corresponding atmospheric pCO2) and either the pH or degree of carbonate saturation of the ocean (i.e., the carbonate and bicarbonate level). Now most of the carbon in the atmosphere/ocean pool is in the ocean (Figure 1) with a ratio of atmospheric to oceanic carbon of 1:50, while earlier in Earth's history, especially in the Early Precambrian, the ratio was likely closer to 1:1, since the speciation of carbon in the ocean shifts from bicarbonate to dissolved carbon dioxide with rising pCO2 and the solubility of carbon dioxide in water is relatively small.

For the atmosphere/ocean 'pool' to remain at steady state on a timescale of 105-106 years or more, the sums of the input fluxes must equal the output fluxes (see Figure 3). F1 corresponds to the flux of carbonate deposited in the ocean, derived from the reaction of atmospheric carbon dioxide with CaMg silicates and carbonates in weathering reactions on land. F2 is the flux of carbon from the chemical weathering of land carbonates alone. Thus (F1 — F2) corresponds to the CaMg silicate weathering sink alone since a flux equal to F2 is deposited as carbonate in the ocean. F3 is the flux of organic carbon into the sedimentary reservoir from the net deposition in the ocean from both terrestrial and oceanic sources. F4 is the carbon flux back into the pool derived from the weathering of organic carbon in exposed sedimentary rock on land (e.g., oxidation of coal). V is the volcanic/ metamorphic C source flux. Then V = (F1 — F2) + (F3 — F4). The evolution of carbon sinks with respect to the atmosphere/ocean reservoir over geologic time remains uncertain despite inferences made from the sedimentary carbon isotopic record. Low-temperature alteration of seafloor basalt by seawater and the

Crustal reservoirs

Crustal reservoirs

Figure 3 Long-term carbon cycle.

Loss to mantle

Figure 3 Long-term carbon cycle.

weathering of NaK silicates have also been proposed as long-term carbon sinks with respect to the atmosphere/ ocean pool but their importance is still unclear. The weathering of CaMg silicates and marine organic carbon deposition likely dominate this sink.

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