The Role of CaCO₃ Reactions in the Contemporary Oceanic CO₂ Cycle

Abstract

The present analysis adjusts previous estimates of global ocean CaCO₃ production rates substantially upward, to 133 × 10¹² mol yr⁻¹ plankton production and 42 × 10¹² mol yr⁻¹ shelf benthos production. The plankton adjustment is consistent with recent satellite-based estimates; the benthos adjustment includes primarily an upward adjustment of CaCO₃ production on so-called carbonate-poor sedimentary shelves and secondarily pays greater attention to high CaCO₃ mass (calcimass) and turnover of shelf communities on temperate and polar shelves. Estimated CaCO₃ sediment accumulation rates remain about the same as they have been for some years: ~20 × 10¹² mol yr⁻¹ on shelves and 11 × 10¹² mol yr⁻¹ in the deep ocean. The differences between production and accumulation of calcareous materials call for dissolution of ~22 × 10¹² mol yr⁻¹ (~50 %) of shelf benthonic carbonate production and 122 × 10¹² mol yr⁻¹ (>90 %) of planktonic production. Most CaCO₃ production, whether planktonic or benthonic, is assumed to take place in water depths of <100 m, while most dissolution is assumed to occur below this depth. The molar ratio of CO₂ release to CaCO₃ precipitation (CO₂↑/CaCO₃↓) is <1.0 and varies with depth. This ratio, Ψ, is presently about 0.66 in surface seawater and 0.85 in ocean waters deeper than about 1000 m. The net flux of CO₂ associated with CaCO₃ reactions in the global ocean in late preindustrial time is estimated to be an apparent influx from the atmosphere to the ocean, of +7 × 10¹² mol C yr⁻¹, at a time scale of 10²–10³ years. The CaCO₃-mediated influx of CO₂ is approximately offset by CO₂ release from organic C oxidation in the water column. Continuing ocean acidification will have effects on CaCO₃ and organic C metabolic responses to the oceanic inorganic C cycle, although those responses remain poorly quantified.

Appendix: Procedure for Estimating Submerged Reefs from the “Visible Reefs” Data set Available Through the World Resources Institute (http://www.wri.org)

A 7-km geographic buffer was initially applied around the visible reefs. This buffer was sufficient to include most passes between reefs (e.g., reefs around the margins of atolls). The boundaries between adjoining buffered reefs were then “dissolved,” and the ArcMap “union” function was used to join the touching, buffered reefs into individual units. So-called donut holes (empty spaces) within these buffered reefs become filled during the union function. An example of such a donut hole would the lagoons of large atolls.

Finally a “negative buffer” of 6.8 km was applied around the buffered reef features. On the assumption that seaward reef slopes average about 30°, this negative buffer operation effectively “trims” the seaward sides of these buffered reef features to a distance of 200 m seaward of the visible reefs (and an estimated oceanic water depth of about 90 m). It would be desirable to use oceanic bathymetry outside of reefs to adjust the seaward margin of submerged reefs seaward of the visible reefs. However, globally available bathymetric data remain too coarsely resolved to do so at this time.