Even the geologic sink is not for ever

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On the longest timescales we actually come full circle - carbonates deposited in marine

Nitrogen Deposition

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Fig. 6.9. Model analysis of the impact of a reduction in marine calcification (Ridgwell and Hargreaves, in press) on the 'geologic' carbon sink and sequestration of fossil fuel CO2. The CO2 trajectory resulting from a combination of ocean invasion, sea-floor neutralization and terrestrial neutralization is shown as a solid line (i.e. the same as the solid line in Fig. 6.8b). The dashed line shows the impact of a reduction in calcification rates in the open ocean.

settings are destined to be weathered (i.e. dissolved) or undergo 'decarbonation', a reaction that proceeds at high temperatures (and pressures) according to:

The carbon that was formerly locked up in the form of CaCO3 is thus released back to the atmosphere and ocean. In the case of carbonates laid down in shallow seas such as limestones or chalks, these can be exposed to weathering during mountainbuilding episodes and as a result of sea-level fall. In contrast, carbonates deposited in deep-sea sediments are only infrequently exposed at the Earth's surface. Instead, the primary recycling of deep-sea CaCO3 occurs through the decarbonation reaction when sea-floor sediments are subducted under (i.e. dragged beneath) continental margins (Caldeira, 1991; Berner, 1999; Edmond and Huh, 2003).

While virtually all fossil fuel CO2 will be sequestered in ocean floor carbonates over the next million years or so, the ultimate fate of this material is subduction followed by decarbonation and the release of stored carbon back to the atmosphere. Fortunately, the sequestered CO2 will not be released all at once, or anytime soon, because the opening and closing of ocean basins and recycling of oceanic crust takes place on 100 million year timescales.

Interestingly, we are currently short-circuiting the 'natural' geologic process of decarbonation through cement manufacture, although limestones are quarried as the reactant rather than deep-sea carbonate oozes. This process is something of a 'double whammy' (two problems at the same time) to the environment because fossil fuels are used to create the high temperatures needed for the CO2-releasing decarbonation reaction to proceed. Annually, ~0.2 Pg C is released as a result of this activity (Houghton et al., 2001), which is about twice the rate of burial of carbonates in the entire deep ocean (~0.1 Pg C/year).

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