Other considerations in the geologic carbonate carbon sink

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The reduction in pH and carbonate ion (CO2-) concentration in the ocean caused by anthropogenic CO2 emissions has another important consequence, in addition to the dissolution of carbonates deposited on the sea floor (Section 6.2.2). This arises because even if surface waters do not quite become undersaturated (W < 1.0), the marine organisms that produce carbonate shells and skeletons will be affected (Royal Society, 2005). If CaCO3 precipitation becomes less thermodynamically favourable, the meta bolic (energy) cost of making shells and skeletons will rise. The result is that organisms will precipitate less carbonate and/or will be disadvantaged in the ecosystem. The implications of this for coral reef ecosystems and associated biodiversity and economic impacts are already being widely recognized (Kleypas et al., 2001; Hughes et al., 2003). There is also increasing evidence that calcifying plankton could also be affected by higher atmospheric CO2 (Bijma et al., 1999; Riebesell et al., 2000; Zondervan et al., 2001; Delille et al., 2005) as well as pteropods, which make aragonite shells (Orr et al., 2005). To understand the implications of this effect for geologic carbon sequestration one must first recognize that the precipitation of CaCO3 by calcifying plankton in the surface ocean and its subsequent removal through gravitational settling raises the partial pressure of CO2 (pCO2) at the surface (see Box 6.1). This acts to reduce the rate of fossil fuel CO2 uptake from the atmosphere. Thus, if carbonate production were to decrease, surface ocean pCO2 would fall and the rate of CO2 invasion into the ocean would increase (Zondervan et al., 2001; Barker et al., 2003; Zeebe and Westbroek, 2003). Secondly, a reduction in the flux of CaCO3 to deep-sea sediments brings forward the year in which the net accumulation of carbonate first becomes negative and 'erosion' starts to occur. This means that neutralization by sea-floor carbonates would have an earlier and potentially more extensive impact compared to the case where CaCO3 production does not change.

These effects are illustrated with the help of the model. Carbonate production in the open ocean is now allowed to decrease in response to anthropogenic acidification and reduced surface carbonate ion concentrations. The result is that atmospheric CO2 is 82 ppm lower in year 3000 compared to the control run (solid line = no calcification change in Fig. 6.9), and 36 ppm lower in year 10,000. Interestingly, the final (steady-state) CO2 concentration is virtually identical (Ridgwell and Hargreaves, in press). The important point is that the maximum CO2 value attained, and thus the maximum

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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.

degree of greenhouse warming, is lower in the presence of CO2-calcification feedback (Ridgwell et al., 2006b). Changes in the production and burial of carbonates in shallow waters (not represented in the model) are likely to drive an additional reduction in the CO2 maximum. However, we have not taken into account in this analysis any impact of reduced carbonate production on the transport of organic matter into the deep ocean - the 'ballast hypothesis' (Armstrong et al., 2002; Klaas and Archer, 2002). If this hypothesis is correct, the amount of CO2 sequestered due to the CO2-calcifica-tion feedback will be less than we have predicted (Barker et al., 2003; Ridgwell, 2003; Heinze, 2004). There are also significant uncertainties as to just how sensitive biogenic calcification is to a reduction in COf- (and saturation state), particularly at the ecosystem (and global) level (Ridgwell et al., 2006b).

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