Abstract
The paired chemical reactions, Ca2+ + 2HCO3 − ↔ CaCO3 + CO2 + H2O, overestimate the ratio of CO2 flux to CaCO3 flux during the precipitation or dissolution of CaCO3 in seawater. This ratio, which has been termed ψ, is about 0.6 in surface seawater at 25°C and at equilibrium with contemporary atmospheric CO2 and increases towards 1.0 as seawater cools and pCO2 increases. These conclusions are based on field observations, laboratory experiments, and equilibrium calculations for the seawater carbonate system. Yet global geochemical modeling indicates that small departures of Ψ from 1.0 would cause dramatic, rapid, and unrealistic change in atmospheric CO2. Ψ can be meaningfully calculated for a water sample whether or not it is in equilibrium with the atmosphere. The analysis presented here demonstrates that the atmospheric CO2 balance can be maintained constant with respect to seawater CaCO3 reactions if one considers the difference between CaCO3 precipitation and burial and differing values for ψ (both <1.0) in regions of precipitation and dissolution within the ocean.
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Acknowledgments
We thank several people for their discussions of this problem over many years. We particularly identify the following supportive skeptics who challenged us to find a reconciliation between the “0.6 rule” and the standard equation for CaCO3 reactions: Bob Berner, Bob Garrels, Fred Mackenzie, and Roland Wollast. Fred Mackenzie, Bob Berner, Louis Legendre, and Dennis Swaney have all provided helpful comments on earlier drafts of this manuscript. The contribution of Michel Frankignoulle is gratefully acknowledged, particularly for his recognition of variation in the coefficient ψ. The manuscript has been greatly improved by the comments of two anonymous reviewers. We dedicate the paper to the memory of John Morse, a friend and respected colleague who was well known (among other things) for his interest in marine carbonate geochemistry. This work is a contribution to the “European Project on Ocean Acidification” (EPOCA), which receives funding from the European Community’s Seventh Framework Programme under grant agreement 211384.
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Smith, S.V., Gattuso, JP. Balancing the Oceanic Calcium Carbonate Cycle: Consequences of Variable Water Column Ψ. Aquat Geochem 17, 327–337 (2011). https://doi.org/10.1007/s10498-010-9109-9
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DOI: https://doi.org/10.1007/s10498-010-9109-9