Geologic carbon sinks the weathering of carbonate rocks on land

sequestration process as the 'sea-floor CaCO3 neutralization' sink. It should be carefully considered that although the dissolution of sedimentary CaCO3 results in an increase in the total amount of carbon dissolved in the ocean, the proportion of dissolved inorganic

CO2 in the atmosphere dissolves in rainwater to form a weak carbonic acid solution, which dissolves carbonate minerals in rocks exposed at the land surface and mineral grains in soils:

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Fig. 6.8. Model analysis of the role of 'geologic' carbon sinks in the sequestration of fossil fuel CO2. (a) Model-predicted global accumulation rate of CaCO3 in deep-sea sediments. The lighter shaded area under the curve (negative CaCO3 accumulation rate) represents the net erosion of carbonates previously deposited in deep-sea sediments - sea-floor CaCO3 neutralization. The darker shaded region represents periods characterized by a positive accumulation rate of CaCO3, but at a rate lower than the supply by carbonate weathering on land - terrestrial neutralization. (b) Trajectory of atmospheric CO2. The dotted line represents anthropogenic uptake by the ocean only (same as the curve shown in Fig. 6.5 solid line). The dashed line shows the effect of seafloor neutralization (Fig. 6.7a) in addition to the ocean invasion carbon sink. The lighter shaded region thus indicates the reduction of atmospheric CO2 due to sea-floor neutralization alone. The solid line shows the effect of terrestrial neutralization (Fig. 6.7b) in addition to ocean invasion and sea-floor neutralization. The darker shaded region thus indicates the reduction of atmospheric CO2 due to terrestrial neutralization alone. Note that weathering rates are held constant for this experiment, meaning that the ultimate CO2 sequestration mechanism of silicate weathering (Fig. 6.7c) is not 'switched on'. (c) Evolution of the different components of the ocean dissolved inorganic component (DIC) reservoir: CO2(aq), HCO- and CO3-.

The solutes that result from this reaction are carried by rivers to the ocean. All the while that anthropogenic acidification of the ocean is causing carbonate accumulation in the deep ocean to be reduced and then reversed (Fig. 6.8a), the input of solutes derived from carbonate weathering on land continues. The accumulation rate of new marine carbonates is thus slower than the terrestrial weathering rate. The consequence of this is a net removal of CO2 from the atmosphere and transformation into HCO- (Fig. 6.8b). At the same time, the carbonate ion concentration in the ocean increases (the solid CO- inventory line in Fig. 6.8c), raising the saturation state of the ocean (higher W) and increasing carbonate preservation in deep-sea sediments. Eventually, the preservation and burial of CaCO3 in deep-sea sediments will once again balance the weathering input (Fig. 6.8a) and this second 'geologic' sequestration process comes to an end. We will refer to this process as the 'terrestrial CaCO3 neutralization' sink.

How much CO2 can be sequestered by reaction with terrestrial carbonates? The evolution of atmospheric CO2 with this additional process enabled is shown in Fig. 6.8b. Now, even at year 40,000, steady state has not quite been attained and CO2 is still continuing to fall slightly. (A model run of 120,000 years' duration (not shown) reveals that atmospheric CO2 would fall by only another 18 ppm.) At year 40,000, the concentration of CO2 in the atmosphere is 435 ppm, which equates to an atmospheric inventory of 951 Pg C. Thus, terrestrial CaCO3 neutralization has removed 15% of the original 4167 Pg C burn (in addition to the initial ocean invasion and erosion of CaCO3 in deep-sea sediments). Just 344 Pg C (8%) of anthropogenic CO2 is then left in the atmosphere - a finding that is consistent with previous estimates (with a range of 7.4-7.9%, depending on the magnitude of the assumed fossil fuel burn (Archer et al., 1998)).

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