Parsing the Oceanic Calcium Carbonate Cycle: A Net Atmospheric Carbon Dioxide Source, or a Sink?

Stephen V. Smith

Smith, Stephen V. 2013. Parsing the Oceanic Calcium Carbonate Cycle: A Net Atmospheric Carbon Dioxide Source, or a Sink? Limnol. Oceanogr. e-Books, doi:10:4319/svsmith.2013.978-0-9845591-2-1

ABSTRACT: The “standard” equation used to describe CaCO3 precipitation and dissolution is: Ca2+ + 2HCO3- = CaCO3 + CO2 + H2O.
As written, the equation somewhat overestimates the ratio of CO2 gas flux to CaCO3 precipitation and dissolution considered individually in the contemporary ocean; and it substantially mis-estimates the flux associated with the net reaction (i.e., precipitation minus dissolution).

The equation can be adjusted to reflect this molar flux ratio more accurately:
Ca2+ + (1+Ψ)HCO3- + (1-Ψ)OH- = CaCO3 + ΨCO2 + H2O,
where Ψ is a coefficient related to the HCO3- buffer capacity of seawater and is a function of temperature and pCO2. When the equation is applied to contemporary hydrographic data for the world ocean, it is found that Ψ presently varies from an average of ~0.63 in low to mid-latitude surface waters (the primary sites of CaCO3 production) to ~0.85 below 500 m throughout the ocean (where most dissolution occurs). In addition to this vertical variation, surface ocean values for Ψ have varied substantially over time, primarily in response to variations in atmospheric pCO2.

The CaCO3-generated flux of CO2 between ocean and atmosphere and the direction of flux are strongly affected by the vertical variations in Ψ interacting with amounts and relative proportions of pelagic versus benthic CaCO3 production, export, dissolution, and accumulation.

Most pelagic CaCO3 production is transported downward by particle fallout from production sites high in the water column and dissolves lower in the water column; only a small fraction of pelagic production is incorporated into the sedimentary record. As a consequence of vertically variable Ψ in the sites dominated by CaCO3 production and dissolution, net reactions leading to the accumulation of pelagically derived CaCO3 in the contemporary ocean absorb more CO2 from the atmosphere than is released during production. By contrast, most benthic CaCO3 production accumulates in the sediments, with little dissolution. Therefore net accumulation of benthically derived CaCO3 is a net source of CO2 release to the ocean, and ultimately to the atmosphere.

Net CaCO3 reactions in the contemporary ocean are a slight net source of CO2 release to the atmosphere. As a geochemical consequence of vertically and temporally variable Ψ, as well as the relative contribution of benthic versus planktonic calcifying communities to CaCO3 production and accumulation, the role of gross and net CaCO3 production as a source or a sink of atmospheric CO2 varies over time. Geochemical consequences of these observations and calculations with respect to air-sea CO2 flux are explored for the present and post-modern (increasingly acidified) ocean; for changing shallow benthic habitat area and Ψ over the past 120,000 years (the last glacial-interglacial cycle); and over Phanerozoic geological time (~550 million years).

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